Effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer

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This study investigates the effects of different heating atmosphere compositions and contents (N 2 , O 2 , CO 2 and H 2 O) on the micro-morphology and phase evolution of the exfoliated and residual oxides on copper billet surface by high-temperature oxidation experiments. The results show that the main phase of the exfoliated oxides is CuO attached to loose Cu 2 O, and the residual nodular oxides are also CuO which directly adhere to copper matrix; water vapour can increase the number and size of Cu 2 O particles on the interface between exfoliated oxide and copper matrix, and effectively reduces the number of residual nodular oxides; the vapour and its decomposition products may effectively eliminate the pores within the oxide layer and at the oxide-matrix interface, while CO 2 may increase the pore size inside the oxide layer. copper billet high-temperature oxidation water vapour carbon dioxide cupric oxide cuprous oxide Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction Copper is one of the most widely used non-ferrous metals, which is an indispensable basic material for economic development and technological progress. Due to the rapid development of electronic information, the consumption of high-precision copper strip is increasing in recent years [ 1 ] . However, the nodular oxides may form on the copper surface in the heating atmosphere during rolling reheating furnace, resulting in quality decline on the product surface [ 2 ] . Copper oxidation at high temperature is generally summarized as oxygen first adsorbed on copper substrate to generate Cu 2 O, and then Cu 2 O continues to react with oxygen to form CuO [ 3 – 5 ] . From the perspective of oxidation thermodynamics and kinetics, the oxidation essence of copper is mutual diffusion of Cu and O. The thickness growth of oxide layer mainly relies on the diffusion of Cu ions to the outside [ 6 ] . However, the actual oxidation is quite complicated due to many influencing factors. Among them, one important factor is metallic properties such as microstructure and purity [ 7 ] . Moreover, the oxides properties play an important role in its further oxidation. For example, the stress of oxide layer may increase due to the different thermal expansion coefficients of CuO and Cu 2 O. This may cause the oxide layer to crack or flake easily when its stress exceeds the bonding strength between oxides and substrate. Then, the direct contact of reaction atmosphere with the copper matrix may accelerate oxidation rate, sometimes resulting in residual nodular oxides on the copper surface [ 8 ] . The fundamental factor affecting copper oxidation is the atmosphere environment which it is exposed. Feng [ 9 ] investigated the oxidation kinetics of copper in pure oxygen and air using thermogravimetric analysis (TGA), who suggested that the oxidation rate of copper in moist air is higher than that in dry air, but the rate in pure oxygen is almost the same as that in dry air. Bridges [ 10 ] et al. proposed that the weight gain of copper is parabolic with respect to time when oxidized in oxygen (0.026–20.4 atm) at 600–1000 ℃. Yongfu Zhu [ 11 ] et al. further investigated the mechanism of copper oxidation in oxygen (0.1 MPa) at 350–1050 ℃, who found that the restrictive step of oxidation rate is the outward diffusion of Cu + in Cu 2 O. Above studies mainly focused on the copper oxidation in oxygen or air, in which the growth mechanism of oxide layer is mostly investigated. However, the heating atmosphere is a multi-component mixture [N 2 , O 2 , CO 2 and water vapour(H 2 O)] in rolling reheating furnace. Each atmosphere composition would play an effect on the copper oxidation, which make part of the oxide layer peel off. Some of the oxides remain on the copper surface and form nodules, which are significantly thicker than the substrate around it. In the subsequent rolling process, the nodules would be rolled into the copper matrix, which increases the milling thickness and thus improves the production cost. This study investigated the micro-morphology and phase evolution of the exfoliated and residual oxides and analyzed the formation mechanism of nodular oxides by controlling the composition and content of atmosphere in the reheating furnace (N 2 , O 2 , CO 2 and water vapor characterized in terms of dew-point temperature). 2. Experimental Procedures 2.1 Materials The experimental samples were copper billet (Cu content of 99.999%) before entering the rolling reheating furnace, which were cut into block with the size 50 mm×30 mm×5 mm. 2.2 Experimental Methodology The oxidation experiment of copper was carried out in a vacuum tube atmosphere furnace ( Fig. 1 ). According to the laboratory equipment capacity and combined with the actual production process (average heat temperature is 940 ℃ and holding time is 3.5 h), the copper heating process curve (shown in Fig. 2 ) was designed. In addition, the corresponding experimental atmosphere was designed based on the composition of the atmosphere in the rolling reheating furnace of copper billets, as shown in Table 1 . In this experiment, the total flow rate of the atmosphere was 2 L/min, in which the contents of N 2 , O 2 and CO 2 were controlled by the corresponding mass flow meters, and the water vapor was regulated by controlling the dew point temperature [ 12 ] . Table 1 Experiment atmosphere composition for high-temperature oxidation (volume fraction, %) Serial number N 2 O 2 CO 2 H 2 O Dew point/℃ P O2 ( T = 25 ℃)/atm P O2 ( T = 940 ℃)/atm 1 69.724 0.441 9.411 20.424 60.700 4.411×10 − 3 4.411×10 − 3 2 90.589 0 9.411 0 \ 2.452×10 − 9 4.722×10 − 7 3 79.576 0 0 20.424 60.700 3.523×10 − 8 8.476×10 − 7 4 70.165 0 9.411 20.424 60.700 3.768×10 − 8 1.320×10 − 6 5 99.559 0.441 0 0 \ 4.410×10 − 3 4.410×10 − 3 6 90.148 0.441 9.411 0 \ 4.410×10 − 3 4.410×10 − 3 7 79.135 0.441 0 20.424 60.700 4.410×10 − 3 4.411×10 − 3 The exfoliated oxide layer and residual oxides of oxidized samples were detected by a small angle diffraction with the X-ray diffractometer (XRD). The surface morphology of residual oxides (SE), the cross-sectional morphology of exfoliated oxide layer (OM), and the cross-sectional morphology of the residual oxide (BSE) were observed using optical microscopy and field emission scanning electron microscopy (Nova 400 Nano SEM), respectively. In addition, the elemental composition and distribution of the residual oxides on the copper surface were observed by an electron probe microanalyzer (EPMA 8050G). 3. Results 3.1 Macroscopic morphology of residual oxides on copper surface with different heating atmosphere The macroscopic morphology of the residual oxides on the copper surface with different heating atmospheres is shown in Fig. 3. There is no external oxide layer is formed on the copper surface when the oxidizing composition in heating atmosphere is CO 2 , H 2 O and CO 2 + H 2 O mixture. However, many small oxides liked white "spots" are generated in above reaction condition, meanwhile the order of its number is CO 2 + H 2 O༞CO 2 ༞H 2 O [Fig. 3(d), (b), (c)]. When the oxidizing composition is O 2 , O 2 + CO 2 mixture and O 2 + CO 2 + H 2 O mixture, the external oxide layer is formed on the copper surface, and part of them peels off. When the oxidizing composition is O 2 + H 2 O, an external oxide layer is formed on the copper surface and it completely peels off [Fig. 3(g)]. The macroscopic morphology of exfoliated oxides is shown in Fig. 3(h), in which the black oxides formed at the atmosphere-copper interface, and the dark-red oxides formed at the interface between substrate and external oxide layer, as shown in Fig. 4(a). 3.2 Phase composition and microscopic morphology of residual oxides on copper surface with different heating atmosphere 3.2.1 Phase and element distribution of residual oxides The phase composition of oxides on copper surface with different heating atmospheres is shown in Fig. 4. When the oxidizing composition is CO 2 + H 2 O mixture, the residual oxide on copper surface is only Cu 2 O. When the oxidizing composition is O 2 , the residual nodular oxides are the mixture of CuO and Cu 2 O, in which CuO accounts for a very high proportion. When the oxidizing composition is O 2 + H 2 O mixture, both the exfoliated and residual oxides on the copper surface are mixture of CuO and Cu 2 O, in which the exfoliated oxide is almost entirely CuO, and the residual oxide is mostly Cu 2 O. When the heating atmosphere is N 2 + O 2 , the microscopic morphology of residual oxides at the oxide-matrix interface is shown in Fig. 5. The black nodule oxide [Fig. 5(a) 1 position] is dense which macroscopic morphology is showed in Fig. 5(b) (1 position). The dark-red oxide [Fig. 5(a) 2 position] distributed near the nodular oxide is a few small particles, which macroscopic morphology is showed in Fig. 5(b) (2 position). This experiment analyzes the elements concentration of O and Cu at oxide-matrix interface by EPMA in order to determine the physical phase composition of the nodular oxides and its their nearby small particles. Figure 6(a) shows the EPMA mapping of the interface position, in which region A and B are not exactly in the same plane due to the presence of nodular oxide. For this reason, the focusing position selected in this experiment is in region B. Based on this, the oxygen concentration in region A could be observed [Fig. 6(a)], which value is extremely low because the Popper peak of O is broader. However, the copper concentration in region A could be not observed [Fig. 6(a)] because the Popper peak of Cu is narrower. Therefore, the mapping of O and Cu elements in region A and region B at position 1,2,3 are locally enlarged. The nodular oxides in region A are dense with large grain size, which phase is mainly CuO as shown in Fig. 6(b). It is in agreement with the results of Sneha Samal [ 13 ] . Figure 6(c) shows that the phase of oxides are exfoliated and the residual is mainly Cu at position 1 in region B. Figure 6(d) shows that the phase exposed after oxides peeling off in the left zone is Cu, and the phase of residual small particles in the right zone is mainly Cu 2 O at position 2 in region B [ 14 ] . Figure 6(e) shows that at position 3 in region B, there are small particles of Cu 2 O on the left side and a bare copper matrix on the right side. The above results show that the residual nodular oxides on the copper billet surface are mainly dense CuO with large grain size; the dark red area near the residual nodular oxides is composed of a mixture of Cu 2 O and Cu, in which the grain size of Cu 2 O is small and the distribution is uneven. 3.2.2 Surface microscopic morphology of residual oxides The microscopic morphology of the residual oxides on copper surface with different heating atmospheres is shown in Fig. 7. When the oxidizing composition is CO 2 , H 2 O and CO 2 + H 2 O mixture, there is no large-sized oxide grains observed on the surface. Meanwhile combined with the XRD results under the condition of CO 2 + H 2 O (Fig. 4), it can be seen that the surface oxides are all Cu 2 O, and their grain sizes are in the order of H 2 O༞CO 2 + H 2 O༞CO 2 [Fig. 7(c), (d), (b)]. When the oxidizing composition is O 2 + H 2 O, the residual oxide on the surface is known to be a mixture of CuO and Cu 2 O from its XRD results. But the large-sized grains of CuO are not observed on the sample surface, which is replaced by small particles of Cu 2 O that are uniformly distributed [Fig. 7(g)]. When the oxidizing composition is O 2 , O 2 + CO 2 and O 2 + CO 2 + H 2 O, it can be seen from its XRD and EPMA results that the residual oxide on the surface is a mixture of CuO and Cu 2 O. At this time, there are both large-sized grains of CuO and small particles of Cu 2 O on sample surface. As seen from Fig. 3, the oxide surface is dark-red when nodular oxide is thin [Fig. 3(a)]. At this time the amount of CuO remaining on the substrate is small, which is cross-distributed with Cu 2 O [Fig. 7(a)]. The oxide surface is black when nodular oxide is thicker [Fig. 3(e), (f)], in which a large area of CuO remains on the sample surface [Fig. 7(e), (f)]. 3.3 Microscopic morphology of oxide layer on copper surface with different heating atmosphere 3.3.1 Microscopic morphology of peeled oxide layer Figure 8 shows the cross-sectional microcosmic morphology of exfoliated oxide layer when the oxidizing composition is O 2 + CO 2 + H 2 O, O 2 , O 2 + CO 2 and O 2 + H 2 O. As seen from the figure, the thickness of exfoliated oxide layers are all around 200 µm, which main phase is CuO combined with the XRD results. Figure 8(b) shows the exfoliated oxide layer is loose when the oxidizing composition is O 2 , in which there is a row of pores parallel to the oxide-matrix interface. When the oxidizing composition is O 2 + CO 2 , the "parallel pores" move down to the external oxide-matrix interface [Fig. 8(c)]. Compared with that in Fig. 8(b), the number and size of pores at this interface are reduced. When the oxidizing atmosphere is O 2 + H 2 O, the external oxide-matrix interface is almost poreless. When the oxidizing composition is O 2 + CO 2 + H 2 O [Fig. 8(a)], there are no "parallel pores" in the external oxide layer. Meanwhile the size and number of pores at oxide-matrix interface are in between compared with Fig. 8(b) and Fig. 8(d). In addition, the number of pores inside the exfoliated oxide layer with each atmosphere is in the order of O 2 + CO 2 > O 2 > O 2 +CO 2 + H 2 O, whereas there are almost no pores inside the external oxide layer under the atmosphere of O 2 + H 2 O. 3.3.2 Sectional microscopic morphology of residual oxides As shown in Fig. 9, the thickness of residual oxides on copper surface is about 35 µm, 60 µm, 250 µm, and 30 µm, respectively when the oxidizing composition is O 2 + CO 2 + H 2 O, O 2 , O 2 + CO 2 and O 2 + H 2 O. When nodular oxides (O 2 + CO 2 + H 2 O, O 2 , O 2 + CO 2 ) are formed on the copper surface, its oxide thickness is significantly greater than the residual oxide thickness when the oxidizing composition is O 2 + H 2 O. Meanwhile, the nodular oxides are tightly connected to the matrix, with only small-sized pores remaining at the connection surface. Comparison with Fig. 8 shows that when the number and size of pores at the external oxide-matrix interface are more and larger [Fig. 8(a)-(c)], the nodular oxides are more likely to remain on the substrate [Fig. 9(a)-(c)]. It is speculated that compared with the oxidizing composition of O 2 + H 2 O, the pores formed in external oxide layer are unevenly distributed when the oxidizing composition is O 2 + CO 2 + H 2 O, O 2 and O 2 + CO 2 . During the peeling of the oxide layer, the loose and porous parts at the interfaces are all peeled off from the substrate, while the external oxide layer tightly connect with matix may form nodular oxides. Combining the XRD and EMPA results, it can be seen that the main phase of the nodular oxide is CuO, which is accompanied by a small amount of Cu 2 O around it. 4. Discussion 4.1 Thermodynamic calculations of copper oxides The high-temperature oxidation of Cu mainly involves the following four reactions Eqs. 1–4 It can be seen from their Gibbs free-energy-variation curves (Fig. 10), the largest tendency is the formation of Cu 2 O from the reaction of Cu with O 2 . Although the most thermodynamically stable product of copper is Cu 2 O, the Cu + in Cu 2 O is in an intermediate valence state in terms of chemical activity and would be continuously oxidized by oxygen to a stable highest valence state. These two steady states are distinct and can be affected by temperature and oxygen partial pressure [ 15 ] . As shown in Fig. 11, Cu can react with an oxidizing composition to form Cu 2 O when the partial pressure of ambient oxygen is greater than the oxygen partial pressure of Cu 2 O formation at some temperature. The oxygen partial pressures required for the formation of CuO within the temperature range of 0-1000 ℃ as shown in Fig. 12 and Fig. 13 . The equivalent partial pressures of oxygen are all in the range of 2.4×10 − 9 -1.4×10 − 6 atm when the oxidizing composition is CO 2 , H 2 O and CO 2 + H 2 O (Table 1 ).The maximum value of them is only larger than that of the partial pressure of oxygen formed by the reaction of Cu and O 2 to generate Cu 2 O. Therefore, only Cu 2 O can be formed but CuO cannot under the above three atmospheres, which is consistent with the macroscopic (Figs. 3) and microscopic (Figs. 7) morphology of residual oxides on copper surface under these conditions. The equivalent partial pressure of oxygen are all about 4.41×10 − 3 atm when the oxidizing composition is O 2 , O 2 + CO 2 , O 2 + H 2 O and O 2 + CO 2 + H 2 O (Table 1 ), which is greater than the partial pressure of oxygen formed by the reaction of Cu and O 2 to generate Cu 2 O and CuO. Therefore, both Cu 2 O and CuO are generated on copper surface, which is consistent with the results in Fig. 3. However, when the temperature is greater than 850 ℃, the reaction of Cu 2 O with O 2 to form CuO will not occur because its partial pressure of oxygen formed for this reaction (Fig. 13) has exceeded the equivalent partial pressure of oxygen for the heated atmosphere (4.41×10 − 3 atm). As can be seen from Fig. 2 , the heating stage from room temperature to 850 ℃ only accounts for about 20% in the whole heating stage. Therefore, the large-scale external oxide (CuO) on copper surface is mainly generated by the reaction between Cu and O 2 when the heating temperature is greater than 850 ℃. In addition to the above oxidation reactions, when the oxidizing composition is H 2 O, O 2 + H 2 O and O 2 + CO 2 + H 2 O,, H 2 decomposed from H 2 O is also able to undergo a reduction reaction with copper oxides at temperature exceeding 784°C because it’s higher than the critical temperature for water vapor decomposition [ 16 ] . From the Gibbs free energy changes for each reaction (Table 2 ), it is clear that the reduction of CuO to Cu 2 O is the easiest and the reduction of Cu 2 O to Cu is the most difficult. When the oxidizing composition is CO 2 , O 2 + CO 2 and O 2 + CO 2 + H 2 O, the decomposition of CO 2 in these atmospheres is more difficult than the decomposition of H 2 O mentioned above. It has been documented that CO 2 decomposes by only 1.8% at 2000°C [ 17 ] . It can also be obtained from Table 1 that the equivalent partial pressure of oxygen in the oxidizing composition of H 2 O at 940°C is slightly larger than that in CO 2 . While the grain sizes of the oxides on copper surface formed in H 2 O are much larger than those formed in CO 2 [Fig. 7(b), (c)]. This further indicates that the amount of CO 2 decomposition is extremely small than that of H 2 O at same temperature. Although CO 2 is difficult to decompose, it can undergo adsorption-desorption reactions with copper oxides (Table 3 ). The thermodynamic values show that CO 2 adsorbs on the CuO surface and reacts with it to form CuCO 3 , which in turn decomposes rapidly into CuO and CO 2 . However,CO 2 is very difficult to react with Cu 2 O. Table 2 Gibbs free energy of the reduction reaction between H 2 and copper oxide at 940℃ Reaction ΔG/(J/mol) (5) 2CuO(s)+H 2 (g)=Cu 2 O(s)+H 2 O(g) -1.64×10 5 (6) CuO(s)+H 2 (g)=Cu(s)+H 2 O(g) -1.32×10 5 (7) Cu 2 O(s)+H 2 (g)=2Cu(s)+H 2 O(g) -1.01×10 5 Table 3 Thermodynamics of adsorption-desorption reactions involving CO 2 at various temperature Reaction ΔH/(kJ mol -1 ) ΔS/ (J mol -1 K -1 ) ΔG/(kJ mol -1 ) T/(K) Reaction possibility (8) Cu 2 O(s)+CO 2 (g)→2CuO(s)+CO(g) (adsorption) +141.0 -23.0 +147.9 298.0 Not favourable (9) CuO(s)+CO 2 (g)→CuCO 3 (s) (adsorption) -45.5 -169 +4.9 298.0 Favourable (10) CuCO 3 (s) →CuO(s)+CO 2 (g)(desorption) +45.5 +169 -4.9 298.0 Favourable (very low temperature) (11) 2CuCO 3 (s)→Cu 2 O(s) + 2CO 2 (g) + 1/2O 2 (g) (desorption) +233.0 +447.6 -4.2 530.0 Favourable (high temperature) 4.2 Formation process of exfoliated and residual oxides When the oxidizing composition is O 2 + CO 2 + H 2 O, O 2 , O 2 + CO 2 and O 2 + H 2 O, it can be seen from the previous analysis that the main physical phases of the exfoliated oxide layer and the residual nodular oxide are all CuO. In the macroscopic morphology, there is dark-red Cu 2 O on the copper billet surface after the external oxide layer is peeled off (Fig. 3), while there is no Cu 2 O at the interface between the residual nodular oxide and the substrate (Fig. 4). In terms of microscopic morphology, the external oxide layer is more prone to spalling when there is a greater amount of Cu 2 O at the interface with the substrate (Fig. 7). EPMA results in Fig. 6 confirm that the position where nodular oxide partially peels off is copper substrate rather than the Cu 2 O. The exfoliated oxide layer is characterized by the presence of continuous large-sized pores at the exfoliation interface (Fig. 8), whereas small discontinuous pores are present at the nodular oxide-matrix interface (Fig. 9). Based on the above phenomena, it is speculated that the cause of the nodular oxide is: (1) CuO is formed directly on the copper surface without Cu 2 O between them. (2) Small discontinuous pores are present at the CuO-matrix interface. On the contrary, an easily exfoliated external oxide layer is formed when loose and porous Cu 2 O exists at the CuO-matrix interface; meanwhile, continuous pores are formed at the exfoliated interface when the Cu 2 O particles are small in size and large in number. Synthesizing the previous analyses and experimental results, and combining them with the current studies on the kinetics of copper oxidation [3–5,18−19] , the high-temperature oxidation of copper in the oxidizing compositions of O 2 , O 2 + H 2 O and O 2 + CO 2 could be crudely speculated, which is schematically shown in Fig. 14. The formation of oxide layer on copper surface is shown in Fig. 14(a) When the oxidizing composition is O 2 . Firstly, O 2 is adsorbed on the copper substrate (stage ①). Then Cu react with O 2 to form Cu 2 O and CuO respectively. Since the partial pressure of oxygen for the Cu 2 O formation is much lower, Cu 2 O is generated in most of the zone on the copper surface and CuO in the remaining zone (stage ②). Because both Cu 2 O and CuO are P-type semiconductor oxides, copper ions have to migrate continuously to the oxide/heating atmosphere interface, which leads to the formation of metal vacancies at the oxide-matrix interface, resulting in a large number of pores. It takes two copper ions to produce one Cu 2 O, while it takes only one copper ion to produce a CuO. Therefore, the metal vacancies underneath the Cu 2 O are more than those underneath the CuO. This is also observed by Fig. 7, the CuO grain size is larger and the oxide layer in CuO zone is dense, meanwhile the Cu 2 O grain size is smaller and the number of pores in this region is significantly higher. As the heating temperature increases (< 850 ℃), Cu 2 O would react with O 2 to form CuO resulting in the thickness of oxide layer increases (stage ③). It takes one Cu 2 O to produce two CuO, therefore the growth pores also appear at the Cu 2 O-CuO interface. When the heating temperature is greater than 850 ℃ to 940 ℃ insulation stage, Cu 2 O no longer reacts with O 2 to generate CuO. Then the copper ions that continue to migrate to the oxide layer-heating atmosphere interface react with the adsorbed oxygen to form CuO, resulting in the thickness of oxide layer continues to increase (stage ④). The rapidly growing CuO also grows laterally to form a "mushroom-shaped" oxide layer, which in turn forms pores inside the CuO layer parallel to the oxide-copper interface [ 20 ] . In addition, the oxide layer releases growth stresses through plastic deformation as it thickens, resulting in the pores appear within it as well. During the cooling of copper sample (stage ⑤), since the more porosity at the Cu 2 O-Cu interface and the greater difference in the thermal expansion coefficients of Cu 2 O-Cu (> CuO-Cu 2 O and CuO-Cu), the Cu 2 O-Cu interface is more susceptible to cracking [ 21 ] , which cause the external oxide layer to spall off. Compared with the above, the fewer pores exists at CuO-Cu interface, so that the binding force is stronger. This makes it difficult for external oxide layer to peel off and form " nodular" oxides on the copper substrate. Due to the formation of the above mentioned "parallel pores" during the growth of CuO, part of the "nodular" oxide may also fracture at the "parallel" pore position. Ultimately only part of the nodular oxides remains on the substrate. When the oxidizing composition is O 2 + H 2 O [Fig. 14(b)], O 2 and H 2 O are first adsorbed on the substrate (Stage ①). Then Cu reacts with O 2 to form Cu 2 O and CuO, respectively (Stage ②), which likewise forms pores at the oxide-substrate interface and inside the oxide layer. As the heating temperature increases (< 850 ℃), Cu 2 O reacts with O 2 to form CuO. Meanwhile, the CuO is reduced to Cu 2 O by a small amount of H 2 that is decomposed by H 2 O (stage ③). At this point, the small-sized H 2 which is incorporated into the oxide layer increases the mobility of dislocations and the plasticity of oxide layer. Therefore, the sufficient plastic flow effectively eliminates the vacancies generated by the outward migration of metal ions [ 22 ] . When the heating temperature is greater than 850 ℃ to 940 ℃ insulation stage, the thickness of oxide layer continues to increase. Meanwhile H 2 O decomposes more H 2 at the same time to reduce the CuO at CuO- Cu 2 O interface to Cu 2 O (stage ④). At this point, the pores in the oxide layer are also annihilated due to strong plasticity. During the cooling of copper (stage ⑤), almost all zones at the CuO-matrix interface are interspersed with loose and porous Cu 2 O, which results in a complete exfoliation of the external oxide layer, with virtually no "nodular" oxides remaining. When the oxidizing composition is O 2 + CO 2 [Fig. 14(c)], less Cu 2 O and CuO may generate on the substrate due to the large-sized CO 2 occupies part of O 2 (Stage ①). After CuO is generated, CO 2 is adsorbed on its surface and reacts to form CuCO 3 . Then CuCO 3 is rapidly decomposed into CuO and CO 2 , and a small portion of CO 2 is left inside the oxide layer which causes pores (Stage ②,③,④).Thus the oxidation process when the oxidizing composition is O 2 + CO 2 is generally similar to that in the oxidizing composition O 2 . It is just that the size of CO 2 decomposed by CuCO 3 is so large, which cannot be fully incorporated into the oxide layer to increase the plasticity and eliminate the pores, but instead exacerbates the pores in the oxide layer [ 23 ] . 5. Conclusions The morphology observation and phase analysis of the copper samples after oxidation experiment were carried out by various material characterization methods, so as to find out some effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer. The following results have been obtained: The main phase of the exfoliated oxides and the residual nodular oxides on the copper surface are CuO. The reason for the difference in their morphology is the formation steps of CuO, which results in the difference in the bonding strength between external oxide layer and copper matrix. When CuO is generated by the reaction between Cu 2 O and O 2 , the loose and porous Cu 2 O forms between external oxide layer and copper matrix. Then the external oxide layer is easy to peel off. When CuO is generated by the direct reaction between Cu and O 2 , the bonding is strong, which may lead to part of the CuO finally remain on copper matrix to form nodular oxides. Water vapour(H 2 O) may attenuate the formation of nodular oxides on the copper surface. The reason for this may that H 2 O promote the reaction of CuO at oxide layer-matrix interface to generate loose Cu 2 O. When the number and size of Cu 2 O particles at the connection interface are larger, the bonding strength between the external oxide layer and the substrate is weaker. Water vapour(H 2 O) and its decomposition products may effectively eliminate the pores within the oxide layer and at the oxide-matrix interface, while CO 2 may increase the pore size inside the oxide layer. 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Surface morphology, composition and wettability Cu 2 O/CuO composite thin films prepared by a facile hydrothermal method[J]. Applied Physics A,2015,118(3). Haugsrud R, Kofstad PK. On the Oxygen Pressure Dependence of High Temperature Oxidation of Copper[J]. Materials Science Forum,1997,343(251–254). McCrea WH. The specific heat of water vapour and the theory of the dissociation of water vapour at high temperatures[J]. Mathematical Proceedings of the Cambridge Philosophical Society,1927,Vol.23(8): 942–950 Isahak WNRW, Ramli ZAC, Ismail MW, et al. Adsorption-desorption of CO 2 on different type of copper oxides surfaces: Physical and chemical attractions studies[J]. Journal of CO 2 Utilization,2013,Vol.2: 8–15 Zhu YF, Mimura K, Isshiki M. Oxidation mechanism of Cu 2 O to CuO at 600–1050 ℃[J]. Oxidation of Metals,2004,Vol.62: 207–222 Haugsrud R. On the Influence of Non-Protective CuO on High-Temperature Oxidation of Cu-Rich Cu-Ni Based Alloys[J]. Oxidation of Metals,1999,52(5–6). Chen YR, Yuen DYW. Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon-Low Silicon Steel in 1%O 2 –N 2 at 1173 and 1273 K[J]. Oxidation of Metals,2013,79(5–6). Lee SJ, Jang YB, Kim SJ, et al. Comparison Study of Oxidation Behavior of Copper at Elevated Temperatures[J]. Advanced Materials Research,2014,3482(1025–1026). Haugsrud R. The Influence of Water Vapor on the Oxidation of Copper at Intermediate Temperatures[J]. Journal of The Electrochemical Society,2002,149(1). Koitaya T, Yamamoto S, Shiozawa Y, et al. Real-Time Observation of Reaction Processes of CO 2 on Cu(997) by Ambient-Pressure X-ray Photoelectron Spectroscopy[J]. Topics in Catalysis,2016,59(5–7). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 13 Mar, 2024 Read the published version in High Temperature Corrosion of Materials → Version 1 posted Editorial decision: Revision requested 24 Jan, 2024 Reviews received at journal 22 Sep, 2023 Reviewers agreed at journal 20 Sep, 2023 Reviewers invited by journal 20 Sep, 2023 Editor assigned by journal 08 Sep, 2023 Submission checks completed at journal 08 Sep, 2023 First submitted to journal 05 Sep, 2023 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3326982","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":231772040,"identity":"be239e4a-f6c1-4987-917e-17d14ec690c9","order_by":0,"name":"Chufeng Lv","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Chufeng","middleName":"","lastName":"Lv","suffix":""},{"id":231772041,"identity":"011cad77-6477-44e0-bd21-ca6a915c9a37","order_by":1,"name":"Yue Guo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYPACGyjNRryWNCBmJk3LYRK0yEckH5PmqTifuOH8+QMMH8oOM/DPbsCvxfBGWpo0z5nbiRtuJDMwzjh3mEHizgECWmbkmEnztt3O3XCDmYGZt+0wg4FEAjFa/p3L3XD+MAPzX2K0yEuAtDQcyN1wIJmBmZEYLQY8z5It5xxLrp95I9ngYM+5dB6JG4RsaU8+eONNjZ0x3/mDDx/8KLOW459ByJYDDCwSMM4BIObBrx5kSwMD8weCqkbBKBgFo2BkAwB9K0NzSdd+EAAAAABJRU5ErkJggg==","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Yue","middleName":"","lastName":"Guo","suffix":""},{"id":231772042,"identity":"efb823ed-ad55-4d33-b5d2-2bf0e57512c2","order_by":2,"name":"Jian Zhao","email":"","orcid":"","institution":"CHINA COPPER HUAZHONG COPPER CO.,LTD","correspondingAuthor":false,"prefix":"","firstName":"Jian","middleName":"","lastName":"Zhao","suffix":""},{"id":231772043,"identity":"ce687d2f-b630-4767-83c2-c438381ee265","order_by":3,"name":"Fangqin Dai","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Fangqin","middleName":"","lastName":"Dai","suffix":""},{"id":231772044,"identity":"632c5e22-9a1a-4060-8f33-311a70fa706a","order_by":4,"name":"Weidong Zeng","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Weidong","middleName":"","lastName":"Zeng","suffix":""},{"id":231772045,"identity":"3c566f82-fd5a-4346-ad7b-01092ed211c8","order_by":5,"name":"Ming Liu","email":"","orcid":"","institution":"CHINA COPPER HUAZHONG COPPER CO.,LTD","correspondingAuthor":false,"prefix":"","firstName":"Ming","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2023-09-05 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8","display":"","copyAsset":false,"role":"figure","size":101217,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/56fb686e49257dde7dd5c7a4.jpg"},{"id":43029993,"identity":"9139af37-6e29-4efd-a791-45f9467e6ef8","added_by":"auto","created_at":"2023-09-12 20:50:28","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":179018,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/011894a1f578d7ca61c279bd.jpg"},{"id":43030215,"identity":"33d0001c-55a1-42e8-ab35-727d9da3faa8","added_by":"auto","created_at":"2023-09-12 21:06:28","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":50906,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/20bcbae48bac702d23706b83.jpg"},{"id":43030135,"identity":"0fc5eed6-57f9-447b-bd42-a0a0645dc77e","added_by":"auto","created_at":"2023-09-12 20:58:28","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":46209,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/47598a28cc99fad9aac7cb47.jpg"},{"id":43030133,"identity":"af8e8480-edc1-4d7d-8265-a8a7abcb4e3b","added_by":"auto","created_at":"2023-09-12 20:58:28","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":45156,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/c29c4dc0a577706b82a21022.jpg"},{"id":43030137,"identity":"23ee4230-9f40-42c6-92f7-886721ad25f1","added_by":"auto","created_at":"2023-09-12 20:58:28","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":46398,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/1f861f0f466592d186f7afb1.jpg"},{"id":43029995,"identity":"970b2cbf-f621-43fe-af74-2e9aaa9c64ab","added_by":"auto","created_at":"2023-09-12 20:50:28","extension":"jpg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":209157,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/5da6f3a0f3a42eea3f8d5789.jpg"},{"id":52823880,"identity":"38176351-a0bd-4e6e-a757-880f43666a77","added_by":"auto","created_at":"2024-03-16 20:07:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1528513,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3326982/v1/c5e9ae2f-50b8-4075-b297-2d5b4172852c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eCopper is one of the most widely used non-ferrous metals, which is an indispensable basic material for economic development and technological progress. Due to the rapid development of electronic information, the consumption of high-precision copper strip is increasing in recent years\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. However, the nodular oxides may form on the copper surface in the heating atmosphere during rolling reheating furnace, resulting in quality decline on the product surface\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eCopper oxidation at high temperature is generally summarized as oxygen first adsorbed on copper substrate to generate Cu\u003csub\u003e2\u003c/sub\u003eO, and then Cu\u003csub\u003e2\u003c/sub\u003eO continues to react with oxygen to form CuO\u003csup\u003e[\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. From the perspective of oxidation thermodynamics and kinetics, the oxidation essence of copper is mutual diffusion of Cu and O. The thickness growth of oxide layer mainly relies on the diffusion of Cu ions to the outside\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. However, the actual oxidation is quite complicated due to many influencing factors. Among them, one important factor is metallic properties such as microstructure and purity \u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. Moreover, the oxides properties play an important role in its further oxidation. For example, the stress of oxide layer may increase due to the different thermal expansion coefficients of CuO and Cu\u003csub\u003e2\u003c/sub\u003eO. This may cause the oxide layer to crack or flake easily when its stress exceeds the bonding strength between oxides and substrate. Then, the direct contact of reaction atmosphere with the copper matrix may accelerate oxidation rate, sometimes resulting in residual nodular oxides on the copper surface \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. The fundamental factor affecting copper oxidation is the atmosphere environment which it is exposed. Feng\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e investigated the oxidation kinetics of copper in pure oxygen and air using thermogravimetric analysis (TGA), who suggested that the oxidation rate of copper in moist air is higher than that in dry air, but the rate in pure oxygen is almost the same as that in dry air. Bridges\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e et al. proposed that the weight gain of copper is parabolic with respect to time when oxidized in oxygen (0.026\u0026ndash;20.4 atm) at 600\u0026ndash;1000 ℃. Yongfu Zhu\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e et al. further investigated the mechanism of copper oxidation in oxygen (0.1 MPa) at 350\u0026ndash;1050 ℃, who found that the restrictive step of oxidation rate is the outward diffusion of Cu\u003csup\u003e+\u003c/sup\u003e in Cu\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e \u003cp\u003eAbove studies mainly focused on the copper oxidation in oxygen or air, in which the growth mechanism of oxide layer is mostly investigated. However, the heating atmosphere is a multi-component mixture [N\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e and water vapour(H\u003csub\u003e2\u003c/sub\u003eO)] in rolling reheating furnace. Each atmosphere composition would play an effect on the copper oxidation, which make part of the oxide layer peel off. Some of the oxides remain on the copper surface and form nodules, which are significantly thicker than the substrate around it. In the subsequent rolling process, the nodules would be rolled into the copper matrix, which increases the milling thickness and thus improves the production cost. This study investigated the micro-morphology and phase evolution of the exfoliated and residual oxides and analyzed the formation mechanism of nodular oxides by controlling the composition and content of atmosphere in the reheating furnace (N\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e and water vapor characterized in terms of dew-point temperature).\u003c/p\u003e"},{"header":"2. Experimental Procedures","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eThe experimental samples were copper billet (Cu content of 99.999%) before entering the rolling reheating furnace, which were cut into block with the size 50 mm\u0026times;30 mm\u0026times;5 mm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Experimental Methodology\u003c/h2\u003e \u003cp\u003eThe oxidation experiment of copper was carried out in a vacuum tube atmosphere furnace ( Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). According to the laboratory equipment capacity and combined with the actual production process (average heat temperature is 940 ℃ and holding time is 3.5 h), the copper heating process curve (shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) was designed. In addition, the corresponding experimental atmosphere was designed based on the composition of the atmosphere in the rolling reheating furnace of copper billets, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In this experiment, the total flow rate of the atmosphere was 2 L/min, in which the contents of N\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e were controlled by the corresponding mass flow meters, and the water vapor was regulated by controlling the dew point temperature\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eExperiment atmosphere composition for high-temperature oxidation (volume fraction, %)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSerial number\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDew point/℃\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u003csub\u003eO2\u003c/sub\u003e(\u003cem\u003eT\u003c/em\u003e\u0026thinsp;=\u0026thinsp;25 ℃)/atm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u003csub\u003eO2\u003c/sub\u003e(\u003cem\u003eT\u003c/em\u003e\u0026thinsp;=\u0026thinsp;940 ℃)/atm\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e69.724\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.441\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.411\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20.424\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e4.411\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e4.411\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e90.589\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.411\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\\\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e2.452\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e4.722\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e79.576\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20.424\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e3.523\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e8.476\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e70.165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.411\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20.424\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e3.768\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e1.320\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e99.559\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.441\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\\\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e4.410\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e4.410\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e90.148\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.441\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.411\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\\\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e4.410\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e4.410\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e79.135\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.441\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20.424\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c7\"\u003e \u003cp\u003e4.410\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c8\"\u003e \u003cp\u003e4.411\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe exfoliated oxide layer and residual oxides of oxidized samples were detected by a small angle diffraction with the X-ray diffractometer (XRD). The surface morphology of residual oxides (SE), the cross-sectional morphology of exfoliated oxide layer (OM), and the cross-sectional morphology of the residual oxide (BSE) were observed using optical microscopy and field emission scanning electron microscopy (Nova 400 Nano SEM), respectively. In addition, the elemental composition and distribution of the residual oxides on the copper surface were observed by an electron probe microanalyzer (EPMA 8050G).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Macroscopic morphology of residual oxides on copper surface with different heating atmosphere\u003c/h2\u003e\n \u003cp\u003eThe macroscopic morphology of the residual oxides on the copper surface with different heating atmospheres is shown in Fig.\u0026nbsp;3. There is no external oxide layer is formed on the copper surface when the oxidizing composition in heating atmosphere is CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO mixture. However, many small oxides liked white \u0026quot;spots\u0026quot; are generated in above reaction condition, meanwhile the order of its number is CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO༞CO\u003csub\u003e2\u003c/sub\u003e༞H\u003csub\u003e2\u003c/sub\u003eO [Fig.\u0026nbsp;3(d), (b), (c)]. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e mixture and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO mixture, the external oxide layer is formed on the copper surface, and part of them peels off. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, an external oxide layer is formed on the copper surface and it completely peels off [Fig. 3(g)]. The macroscopic morphology of exfoliated oxides is shown in Fig. 3(h), in which the black oxides formed at the atmosphere-copper interface, and the dark-red oxides formed at the interface between substrate and external oxide layer, as shown in Fig. 4(a).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Phase composition and microscopic morphology of residual oxides on copper surface with different heating atmosphere\u003c/h2\u003e\n \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\n \u003ch2\u003e3.2.1 Phase and element distribution of residual oxides\u003c/h2\u003e\n \u003cp\u003eThe phase composition of oxides on copper surface with different heating atmospheres is shown in Fig.\u0026nbsp;4. When the oxidizing composition is CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO mixture, the residual oxide on copper surface is only Cu\u003csub\u003e2\u003c/sub\u003eO. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e, the residual nodular oxides are the mixture of CuO and Cu\u003csub\u003e2\u003c/sub\u003eO, in which CuO accounts for a very high proportion. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO mixture, both the exfoliated and residual oxides on the copper surface are mixture of CuO and Cu\u003csub\u003e2\u003c/sub\u003eO, in which the exfoliated oxide is almost entirely CuO, and the residual oxide is mostly Cu\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e\n \u003cp\u003eWhen the heating atmosphere is N\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e, the microscopic morphology of residual oxides at the oxide-matrix interface is shown in Fig. 5. The black nodule oxide [Fig. 5(a) 1 position] is dense which macroscopic morphology is showed in Fig. 5(b) (1 position). The dark-red oxide [Fig. 5(a) 2 position] distributed near the nodular oxide is a few small particles, which macroscopic morphology is showed in Fig. 5(b) (2 position).\u003c/p\u003e\n \u003cp\u003eThis experiment analyzes the elements concentration of O and Cu at oxide-matrix interface by EPMA in order to determine the physical phase composition of the nodular oxides and its their nearby small particles. Figure\u0026nbsp;6(a) shows the EPMA mapping of the interface position, in which region A and B are not exactly in the same plane due to the presence of nodular oxide. For this reason, the focusing position selected in this experiment is in region B. Based on this, the oxygen concentration in region A could be observed [Fig.\u0026nbsp;6(a)], which value is extremely low because the Popper peak of O is broader. However, the copper concentration in region A could be not observed [Fig.\u0026nbsp;6(a)] because the Popper peak of Cu is narrower. Therefore, the mapping of O and Cu elements in region A and region B at position 1,2,3 are locally enlarged. The nodular oxides in region A are dense with large grain size, which phase is mainly CuO as shown in Fig.\u0026nbsp;6(b). It is in agreement with the results of Sneha Samal\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;6(c) shows that the phase of oxides are exfoliated and the residual is mainly Cu at position 1 in region B. Figure\u0026nbsp;6(d) shows that the phase exposed after oxides peeling off in the left zone is Cu, and the phase of residual small particles in the right zone is mainly Cu\u003csub\u003e2\u003c/sub\u003eO at position 2 in region B \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;6(e) shows that at position 3 in region B, there are small particles of Cu\u003csub\u003e2\u003c/sub\u003eO on the left side and a bare copper matrix on the right side. The above results show that the residual nodular oxides on the copper billet surface are mainly dense CuO with large grain size; the dark red area near the residual nodular oxides is composed of a mixture of Cu\u003csub\u003e2\u003c/sub\u003eO and Cu, in which the grain size of Cu\u003csub\u003e2\u003c/sub\u003eO is small and the distribution is uneven.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\n \u003ch2\u003e3.2.2 Surface microscopic morphology of residual oxides\u003c/h2\u003e\n \u003cp\u003eThe microscopic morphology of the residual oxides on copper surface with different heating atmospheres is shown in Fig.\u0026nbsp;7. When the oxidizing composition is CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO mixture, there is no large-sized oxide grains observed on the surface. Meanwhile combined with the XRD results under the condition of CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO (Fig.\u0026nbsp;4), it can be seen that the surface oxides are all Cu\u003csub\u003e2\u003c/sub\u003eO, and their grain sizes are in the order of H\u003csub\u003e2\u003c/sub\u003eO༞CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO༞CO\u003csub\u003e2\u003c/sub\u003e [Fig. 7(c), (d), (b)]. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, the residual oxide on the surface is known to be a mixture of CuO and Cu\u003csub\u003e2\u003c/sub\u003eO from its XRD results. But the large-sized grains of CuO are not observed on the sample surface, which is replaced by small particles of Cu\u003csub\u003e2\u003c/sub\u003eO that are uniformly distributed [Fig.\u0026nbsp;7(g)]. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, it can be seen from its XRD and EPMA results that the residual oxide on the surface is a mixture of CuO and Cu\u003csub\u003e2\u003c/sub\u003eO. At this time, there are both large-sized grains of CuO and small particles of Cu\u003csub\u003e2\u003c/sub\u003eO on sample surface. As seen from Fig.\u0026nbsp;3, the oxide surface is dark-red when nodular oxide is thin [Fig.\u0026nbsp;3(a)]. At this time the amount of CuO remaining on the substrate is small, which is cross-distributed with Cu\u003csub\u003e2\u003c/sub\u003eO [Fig. 7(a)]. The oxide surface is black when nodular oxide is thicker [Fig. 3(e), (f)], in which a large area of CuO remains on the sample surface [Fig. 7(e), (f)].\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Microscopic morphology of oxide layer on copper surface with different heating atmosphere\u003c/h2\u003e\n \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.1 Microscopic morphology of peeled oxide layer\u003c/h2\u003e\n \u003cp\u003eFigure 8 shows the cross-sectional microcosmic morphology of exfoliated oxide layer when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO. As seen from the figure, the thickness of exfoliated oxide layers are all around 200 \u0026micro;m, which main phase is CuO combined with the XRD results.\u003c/p\u003e\n \u003cp\u003eFigure\u0026nbsp;8(b) shows the exfoliated oxide layer is loose when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e, in which there is a row of pores parallel to the oxide-matrix interface. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e, the \u0026quot;parallel pores\u0026quot; move down to the external oxide-matrix interface [Fig.\u0026nbsp;8(c)]. Compared with that in Fig.\u0026nbsp;8(b), the number and size of pores at this interface are reduced. When the oxidizing atmosphere is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, the external oxide-matrix interface is almost poreless. When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO [Fig.\u0026nbsp;8(a)], there are no \u0026quot;parallel pores\u0026quot; in the external oxide layer. Meanwhile the size and number of pores at oxide-matrix interface are in between compared with Fig.\u0026nbsp;8(b) and Fig.\u0026nbsp;8(d). In addition, the number of pores inside the exfoliated oxide layer with each atmosphere is in the order of O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;O\u003csub\u003e2\u003c/sub\u003e+CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, whereas there are almost no pores inside the external oxide layer under the atmosphere of O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\n \u003ch2\u003e3.3.2 Sectional microscopic morphology of residual oxides\u003c/h2\u003e\n \u003cp\u003eAs shown in Fig.\u0026nbsp;9, the thickness of residual oxides on copper surface is about 35 \u0026micro;m, 60 \u0026micro;m, 250 \u0026micro;m, and 30 \u0026micro;m, respectively when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO. When nodular oxides (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e) are formed on the copper surface, its oxide thickness is significantly greater than the residual oxide thickness when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO. Meanwhile, the nodular oxides are tightly connected to the matrix, with only small-sized pores remaining at the connection surface. Comparison with Fig.\u0026nbsp;8 shows that when the number and size of pores at the external oxide-matrix interface are more and larger [Fig.\u0026nbsp;8(a)-(c)], the nodular oxides are more likely to remain on the substrate [Fig.\u0026nbsp;9(a)-(c)]. It is speculated that compared with the oxidizing composition of O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, the pores formed in external oxide layer are unevenly distributed when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e. During the peeling of the oxide layer, the loose and porous parts at the interfaces are all peeled off from the substrate, while the external oxide layer tightly connect with matix may form nodular oxides. Combining the XRD and EMPA results, it can be seen that the main phase of the nodular oxide is CuO, which is accompanied by a small amount of Cu\u003csub\u003e2\u003c/sub\u003eO around it.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e4.1 Thermodynamic calculations of copper oxides\u003c/h2\u003e\n \u003cp\u003eThe high-temperature oxidation of Cu mainly involves the following four reactions Eqs.\u0026nbsp;1\u0026ndash;4 It can be seen from their Gibbs free-energy-variation curves (Fig.\u0026nbsp;10), the largest tendency is the formation of Cu\u003csub\u003e2\u003c/sub\u003eO from the reaction of Cu with O\u003csub\u003e2\u003c/sub\u003e. Although the most thermodynamically stable product of copper is Cu\u003csub\u003e2\u003c/sub\u003eO, the Cu\u003csup\u003e+\u003c/sup\u003e in Cu\u003csub\u003e2\u003c/sub\u003eO is in an intermediate valence state in terms of chemical activity and would be continuously oxidized by oxygen to a stable highest valence state. These two steady states are distinct and can be affected by temperature and oxygen partial pressure \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. As shown in Fig.\u0026nbsp;11, Cu can react with an oxidizing composition to form Cu\u003csub\u003e2\u003c/sub\u003eO when the partial pressure of ambient oxygen is greater than the oxygen partial pressure of Cu\u003csub\u003e2\u003c/sub\u003eO formation at some temperature. The oxygen partial pressures required for the formation of CuO within the temperature range of 0-1000 ℃ as shown in Fig.\u0026nbsp;12 and Fig.\u0026nbsp;13 .\u003c/p\u003e\n \u003cp\u003eThe equivalent partial pressures of oxygen are all in the range of 2.4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e-1.4\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e atm when the oxidizing composition is CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).The maximum value of them is only larger than that of the partial pressure of oxygen formed by the reaction of Cu and O\u003csub\u003e2\u003c/sub\u003e to generate Cu\u003csub\u003e2\u003c/sub\u003eO. Therefore, only Cu\u003csub\u003e2\u003c/sub\u003eO can be formed but CuO cannot under the above three atmospheres, which is consistent with the macroscopic (Figs.\u0026nbsp;3) and microscopic (Figs.\u0026nbsp;7) morphology of residual oxides on copper surface under these conditions. The equivalent partial pressure of oxygen are all about 4.41\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e atm when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), which is greater than the partial pressure of oxygen formed by the reaction of Cu and O\u003csub\u003e2\u003c/sub\u003e to generate Cu\u003csub\u003e2\u003c/sub\u003eO and CuO. Therefore, both Cu\u003csub\u003e2\u003c/sub\u003eO and CuO are generated on copper surface, which is consistent with the results in Fig.\u0026nbsp;3. However, when the temperature is greater than 850 ℃, the reaction of Cu\u003csub\u003e2\u003c/sub\u003eO with O\u003csub\u003e2\u003c/sub\u003e to form CuO will not occur because its partial pressure of oxygen formed for this reaction (Fig. 13) has exceeded the equivalent partial pressure of oxygen for the heated atmosphere (4.41\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e atm). As can be seen from Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, the heating stage from room temperature to 850 ℃ only accounts for about 20% in the whole heating stage. Therefore, the large-scale external oxide (CuO) on copper surface is mainly generated by the reaction between Cu and O\u003csub\u003e2\u003c/sub\u003e when the heating temperature is greater than 850 ℃.\u003c/p\u003e\n \u003cp\u003eIn addition to the above oxidation reactions, when the oxidizing composition is H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO,, H\u003csub\u003e2\u003c/sub\u003e decomposed from H\u003csub\u003e2\u003c/sub\u003eO is also able to undergo a reduction reaction with copper oxides at temperature exceeding 784\u0026deg;C because it\u0026rsquo;s higher than the critical temperature for water vapor decomposition \u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. From the Gibbs free energy changes for each reaction (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), it is clear that the reduction of CuO to Cu\u003csub\u003e2\u003c/sub\u003eO is the easiest and the reduction of Cu\u003csub\u003e2\u003c/sub\u003eO to Cu is the most difficult. When the oxidizing composition is CO\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, the decomposition of CO\u003csub\u003e2\u003c/sub\u003e in these atmospheres is more difficult than the decomposition of H\u003csub\u003e2\u003c/sub\u003eO mentioned above. It has been documented that CO\u003csub\u003e2\u003c/sub\u003e decomposes by only 1.8% at 2000\u0026deg;C\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. It can also be obtained from Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e that the equivalent partial pressure of oxygen in the oxidizing composition of H\u003csub\u003e2\u003c/sub\u003eO at 940\u0026deg;C is slightly larger than that in CO\u003csub\u003e2\u003c/sub\u003e. While the grain sizes of the oxides on copper surface formed in H\u003csub\u003e2\u003c/sub\u003eO are much larger than those formed in CO\u003csub\u003e2\u003c/sub\u003e [Fig. 7(b), (c)]. This further indicates that the amount of CO\u003csub\u003e2\u003c/sub\u003e decomposition is extremely small than that of H\u003csub\u003e2\u003c/sub\u003eO at same temperature. Although CO\u003csub\u003e2\u003c/sub\u003e is difficult to decompose, it can undergo adsorption-desorption reactions with copper oxides (Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). The thermodynamic values show that CO\u003csub\u003e2\u003c/sub\u003e adsorbs on the CuO surface and reacts with it to form CuCO\u003csub\u003e3\u003c/sub\u003e, which in turn decomposes rapidly into CuO and CO\u003csub\u003e2\u003c/sub\u003e. However,CO\u003csub\u003e2\u003c/sub\u003e is very difficult to react with Cu\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eTable 2\u0026nbsp;\u003c/strong\u003eGibbs free energy of the reduction reaction between H\u003csub\u003e2\u003c/sub\u003e and copper oxide at 940℃\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"74.4186046511628%\" colspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003eReaction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.58139534883721%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026Delta;G/(J/mol)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"17.05426356589147%\" valign=\"top\"\u003e\n \u003cp\u003e(5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"57.36434108527132%\" valign=\"top\"\u003e\n \u003cp\u003e2CuO(s)+H\u003csub\u003e2\u003c/sub\u003e(g)=Cu\u003csub\u003e2\u003c/sub\u003eO(s)+H\u003csub\u003e2\u003c/sub\u003eO(g)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.58139534883721%\" valign=\"top\"\u003e\n \u003cp\u003e-1.64\u0026times;10\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"17.05426356589147%\" valign=\"top\"\u003e\n \u003cp\u003e(6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"57.36434108527132%\" valign=\"top\"\u003e\n \u003cp\u003eCuO(s)+H\u003csub\u003e2\u003c/sub\u003e(g)=Cu(s)+H\u003csub\u003e2\u003c/sub\u003eO(g)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.58139534883721%\" valign=\"top\"\u003e\n \u003cp\u003e-1.32\u0026times;10\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"17.05426356589147%\" valign=\"top\"\u003e\n \u003cp\u003e(7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"57.36434108527132%\" valign=\"top\"\u003e\n \u003cp\u003eCu\u003csub\u003e2\u003c/sub\u003eO(s)+H\u003csub\u003e2\u003c/sub\u003e(g)=2Cu(s)+H\u003csub\u003e2\u003c/sub\u003eO(g)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"25.58139534883721%\" valign=\"top\"\u003e\n \u003cp\u003e-1.01\u0026times;10\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cstrong\u003eTable 3\u0026nbsp;\u003c/strong\u003eThermodynamics of adsorption-desorption reactions involving CO\u003csub\u003e2\u003c/sub\u003e at various temperature\u003c/p\u003e\n \u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"38.488576449912124%\" colspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003eReaction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.490333919156415%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026Delta;H/(kJ mol\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026Delta;S/ (J mol\u003csup\u003e-1\u003c/sup\u003eK\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026Delta;G/(kJ mol\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003eT/(K)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003eReaction possibility\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"5.799648506151143%\" valign=\"top\"\u003e\n \u003cp\u003e(8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"32.68892794376099%\" valign=\"top\"\u003e\n \u003cp\u003eCu\u003csub\u003e2\u003c/sub\u003eO(s)+CO\u003csub\u003e2\u003c/sub\u003e(g)\u0026rarr;2CuO(s)+CO(g) (adsorption)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.490333919156415%\" valign=\"top\"\u003e\n \u003cp\u003e+141.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e-23.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e+147.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e298.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003eNot favourable\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"5.799648506151143%\" valign=\"top\"\u003e\n \u003cp\u003e(9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"32.68892794376099%\" valign=\"top\"\u003e\n \u003cp\u003eCuO(s)+CO\u003csub\u003e2\u003c/sub\u003e(g)\u0026rarr;CuCO\u003csub\u003e3\u003c/sub\u003e(s) (adsorption)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.490333919156415%\" valign=\"top\"\u003e\n \u003cp\u003e-45.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e-169\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e+4.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e298.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003eFavourable\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"5.799648506151143%\" valign=\"top\"\u003e\n \u003cp\u003e(10)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"32.68892794376099%\" valign=\"top\"\u003e\n \u003cp\u003eCuCO\u003csub\u003e3\u003c/sub\u003e(s)\u0026nbsp;\u0026rarr;CuO(s)+CO\u003csub\u003e2\u003c/sub\u003e(g)(desorption)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.490333919156415%\" valign=\"top\"\u003e\n \u003cp\u003e+45.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e+169\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e-4.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e298.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003eFavourable (very low temperature)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"5.799648506151143%\" valign=\"top\"\u003e\n \u003cp\u003e(11)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"32.68892794376099%\" valign=\"top\"\u003e\n \u003cp\u003e2CuCO\u003csub\u003e3\u003c/sub\u003e(s)\u0026rarr;Cu\u003csub\u003e2\u003c/sub\u003eO(s) + 2CO\u003csub\u003e2\u003c/sub\u003e(g) + 1/2O\u003csub\u003e2\u003c/sub\u003e(g) (desorption)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.490333919156415%\" valign=\"top\"\u003e\n \u003cp\u003e+233.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e+447.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e-4.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003e530.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.005272407732864%\" valign=\"top\"\u003e\n \u003cp\u003eFavourable (high temperature)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e4.2 Formation process of exfoliated and residual oxides\u003c/h2\u003e\n \u003cp\u003eWhen the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO, it can be seen from the previous analysis that the main physical phases of the exfoliated oxide layer and the residual nodular oxide are all CuO. In the macroscopic morphology, there is dark-red Cu\u003csub\u003e2\u003c/sub\u003eO on the copper billet surface after the external oxide layer is peeled off (Fig.\u0026nbsp;3), while there is no Cu\u003csub\u003e2\u003c/sub\u003eO at the interface between the residual nodular oxide and the substrate (Fig.\u0026nbsp;4). In terms of microscopic morphology, the external oxide layer is more prone to spalling when there is a greater amount of Cu\u003csub\u003e2\u003c/sub\u003eO at the interface with the substrate (Fig.\u0026nbsp;7). EPMA results in Fig.\u0026nbsp;6 confirm that the position where nodular oxide partially peels off is copper substrate rather than the Cu\u003csub\u003e2\u003c/sub\u003eO. The exfoliated oxide layer is characterized by the presence of continuous large-sized pores at the exfoliation interface (Fig.\u0026nbsp;8), whereas small discontinuous pores are present at the nodular oxide-matrix interface (Fig.\u0026nbsp;9). Based on the above phenomena, it is speculated that the cause of the nodular oxide is: (1) CuO is formed directly on the copper surface without Cu\u003csub\u003e2\u003c/sub\u003eO between them. (2) Small discontinuous pores are present at the CuO-matrix interface. On the contrary, an easily exfoliated external oxide layer is formed when loose and porous Cu\u003csub\u003e2\u003c/sub\u003eO exists at the CuO-matrix interface; meanwhile, continuous pores are formed at the exfoliated interface when the Cu\u003csub\u003e2\u003c/sub\u003eO particles are small in size and large in number.\u003c/p\u003e\n \u003cp\u003eSynthesizing the previous analyses and experimental results, and combining them with the current studies on the kinetics of copper oxidation\u003csup\u003e[3\u0026ndash;5,18\u0026minus;19]\u003c/sup\u003e, the high-temperature oxidation of copper in the oxidizing compositions of O\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO and O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e could be crudely speculated, which is schematically shown in Fig. 14.\u003c/p\u003e\n \u003cp\u003eThe formation of oxide layer on copper surface is shown in Fig.\u0026nbsp;14(a) When the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e. Firstly, O\u003csub\u003e2\u003c/sub\u003e is adsorbed on the copper substrate (stage ①). Then Cu react with O\u003csub\u003e2\u003c/sub\u003e to form Cu\u003csub\u003e2\u003c/sub\u003eO and CuO respectively. Since the partial pressure of oxygen for the Cu\u003csub\u003e2\u003c/sub\u003eO formation is much lower, Cu\u003csub\u003e2\u003c/sub\u003eO is generated in most of the zone on the copper surface and CuO in the remaining zone (stage ②). Because both Cu\u003csub\u003e2\u003c/sub\u003eO and CuO are P-type semiconductor oxides, copper ions have to migrate continuously to the oxide/heating atmosphere interface, which leads to the formation of metal vacancies at the oxide-matrix interface, resulting in a large number of pores. It takes two copper ions to produce one Cu\u003csub\u003e2\u003c/sub\u003eO, while it takes only one copper ion to produce a CuO. Therefore, the metal vacancies underneath the Cu\u003csub\u003e2\u003c/sub\u003eO are more than those underneath the CuO. This is also observed by Fig.\u0026nbsp;7, the CuO grain size is larger and the oxide layer in CuO zone is dense, meanwhile the Cu\u003csub\u003e2\u003c/sub\u003eO grain size is smaller and the number of pores in this region is significantly higher. As the heating temperature increases (\u0026lt;\u0026thinsp;850 ℃), Cu\u003csub\u003e2\u003c/sub\u003eO would react with O\u003csub\u003e2\u003c/sub\u003e to form CuO resulting in the thickness of oxide layer increases (stage ③). It takes one Cu\u003csub\u003e2\u003c/sub\u003eO to produce two CuO, therefore the growth pores also appear at the Cu\u003csub\u003e2\u003c/sub\u003eO-CuO interface. When the heating temperature is greater than 850 ℃ to 940 ℃ insulation stage, Cu\u003csub\u003e2\u003c/sub\u003eO no longer reacts with O\u003csub\u003e2\u003c/sub\u003e to generate CuO. Then the copper ions that continue to migrate to the oxide layer-heating atmosphere interface react with the adsorbed oxygen to form CuO, resulting in the thickness of oxide layer continues to increase (stage ④). The rapidly growing CuO also grows laterally to form a \u0026quot;mushroom-shaped\u0026quot; oxide layer, which in turn forms pores inside the CuO layer parallel to the oxide-copper interface\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. In addition, the oxide layer releases growth stresses through plastic deformation as it thickens, resulting in the pores appear within it as well. During the cooling of copper sample (stage ⑤), since the more porosity at the Cu\u003csub\u003e2\u003c/sub\u003eO-Cu interface and the greater difference in the thermal expansion coefficients of Cu\u003csub\u003e2\u003c/sub\u003eO-Cu (\u0026gt;\u0026thinsp;CuO-Cu\u003csub\u003e2\u003c/sub\u003eO and CuO-Cu), the Cu\u003csub\u003e2\u003c/sub\u003eO-Cu interface is more susceptible to cracking\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e, which cause the external oxide layer to spall off. Compared with the above, the fewer pores exists at CuO-Cu interface, so that the binding force is stronger. This makes it difficult for external oxide layer to peel off and form \u0026quot; nodular\u0026quot; oxides on the copper substrate. Due to the formation of the above mentioned \u0026quot;parallel pores\u0026quot; during the growth of CuO, part of the \u0026quot;nodular\u0026quot; oxide may also fracture at the \u0026quot;parallel\u0026quot; pore position. Ultimately only part of the nodular oxides remains on the substrate.\u003c/p\u003e\n \u003cp\u003eWhen the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO [Fig.\u0026nbsp;14(b)], O\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO are first adsorbed on the substrate (Stage ①). Then Cu reacts with O\u003csub\u003e2\u003c/sub\u003e to form Cu\u003csub\u003e2\u003c/sub\u003eO and CuO, respectively (Stage ②), which likewise forms pores at the oxide-substrate interface and inside the oxide layer. As the heating temperature increases (\u0026lt;\u0026thinsp;850 ℃), Cu\u003csub\u003e2\u003c/sub\u003eO reacts with O\u003csub\u003e2\u003c/sub\u003e to form CuO. Meanwhile, the CuO is reduced to Cu\u003csub\u003e2\u003c/sub\u003eO by a small amount of H\u003csub\u003e2\u003c/sub\u003e that is decomposed by H\u003csub\u003e2\u003c/sub\u003eO (stage ③). At this point, the small-sized H\u003csub\u003e2\u003c/sub\u003e which is incorporated into the oxide layer increases the mobility of dislocations and the plasticity of oxide layer. Therefore, the sufficient plastic flow effectively eliminates the vacancies generated by the outward migration of metal ions\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. When the heating temperature is greater than 850 ℃ to 940 ℃ insulation stage, the thickness of oxide layer continues to increase. Meanwhile H\u003csub\u003e2\u003c/sub\u003eO decomposes more H\u003csub\u003e2\u003c/sub\u003e at the same time to reduce the CuO at CuO- Cu\u003csub\u003e2\u003c/sub\u003eO interface to Cu\u003csub\u003e2\u003c/sub\u003eO (stage ④). At this point, the pores in the oxide layer are also annihilated due to strong plasticity. During the cooling of copper (stage ⑤), almost all zones at the CuO-matrix interface are interspersed with loose and porous Cu\u003csub\u003e2\u003c/sub\u003eO, which results in a complete exfoliation of the external oxide layer, with virtually no \u0026quot;nodular\u0026quot; oxides remaining.\u003c/p\u003e\n \u003cp\u003eWhen the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e [Fig. 14(c)], less Cu\u003csub\u003e2\u003c/sub\u003eO and CuO may generate on the substrate due to the large-sized CO\u003csub\u003e2\u003c/sub\u003e occupies part of O\u003csub\u003e2\u003c/sub\u003e (Stage ①). After CuO is generated, CO\u003csub\u003e2\u003c/sub\u003e is adsorbed on its surface and reacts to form CuCO\u003csub\u003e3\u003c/sub\u003e. Then CuCO\u003csub\u003e3\u003c/sub\u003e is rapidly decomposed into CuO and CO\u003csub\u003e2\u003c/sub\u003e, and a small portion of CO\u003csub\u003e2\u003c/sub\u003e is left inside the oxide layer which causes pores (Stage ②,③,④).Thus the oxidation process when the oxidizing composition is O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e is generally similar to that in the oxidizing composition O\u003csub\u003e2\u003c/sub\u003e. It is just that the size of CO\u003csub\u003e2\u003c/sub\u003e decomposed by CuCO\u003csub\u003e3\u003c/sub\u003e is so large, which cannot be fully incorporated into the oxide layer to increase the plasticity and eliminate the pores, but instead exacerbates the pores in the oxide layer\u003csup\u003e[\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e .\u003c/p\u003e\n\u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThe morphology observation and phase analysis of the copper samples after oxidation experiment were carried out by various material characterization methods, so as to find out some effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer. The\u0026nbsp;following results have been obtained:\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003eThe main phase of the exfoliated oxides and the residual nodular oxides on the copper surface are CuO. The reason for the difference in their morphology is the formation steps of CuO, which results in the difference in the bonding strength between external oxide layer and copper matrix. When CuO is generated by the reaction between Cu\u003csub\u003e2\u003c/sub\u003eO and O\u003csub\u003e2\u003c/sub\u003e, the loose and porous Cu\u003csub\u003e2\u003c/sub\u003eO forms between external oxide layer and copper matrix. Then the external oxide layer is easy to peel off. When CuO is generated by the direct reaction between Cu and O\u003csub\u003e2\u003c/sub\u003e, the bonding is strong, which may lead to part of the CuO finally remain on copper matrix to form nodular oxides.\u003c/li\u003e\n \u003cli\u003eWater\u0026nbsp;vapour(H\u003csub\u003e2\u003c/sub\u003eO) may attenuate the formation of nodular oxides on the copper surface. The reason for this may that H\u003csub\u003e2\u003c/sub\u003eO promote the reaction of CuO at oxide layer-matrix interface to generate loose Cu\u003csub\u003e2\u003c/sub\u003eO. When the number and size of Cu\u003csub\u003e2\u003c/sub\u003eO particles at the connection interface are larger, the bonding strength between the external oxide layer and the substrate is weaker.\u003c/li\u003e\n \u003cli\u003eWater\u0026nbsp;vapour(H\u003csub\u003e2\u003c/sub\u003eO) and its decomposition products may effectively eliminate the pores\u0026nbsp;within the oxide layer and at the oxide-matrix interface, while CO\u003csub\u003e2\u003c/sub\u003e may increase the pore size inside the oxide layer.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eInvestigation, Conducting experiments, Formal analysis, Writing-original draft: \u003cstrong\u003eChufeng Lv\u003c/strong\u003e;\u0026nbsp;Conceptualisation, Data curation, Validation, Supervision, Writing-review and editing: \u003cstrong\u003eYue Guo\u003c/strong\u003e; Methodology, Supervision: \u003cstrong\u003eJian Zhao;\u003c/strong\u003e Review, Resources:\u003cstrong\u003e\u0026nbsp;Fangqin Dai\u003c/strong\u003e; Review:\u003cstrong\u003e\u0026nbsp;Weidong Zeng\u003c/strong\u003e; Supervision: \u003cstrong\u003eMing Liu\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.\u003c/p\u003e\n\u003cp\u003eConflict of Interest: The authors declare that they have no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003ePevzner MZ, Sergeev DG. Power Consumption, Control of Properties and of Continuous Annealing Process of Copper and Brass Rolled Products in Transverse Magnetic Field[J]. Metal Science and Heat Treatment, 2022,63(9\u0026ndash;10).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMinich WR,Cazamias UJ,Kumar M, et al. Effect of Microstructural Length Scales on Spall Behavior of Copper[J]. Metallurgical and Materials Transactions A,2004,35(9).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eValerii V Belousov, A A Klimashin. High-temperature oxidation of copper[J]. 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Topics in Catalysis,2016,59(5\u0026ndash;7).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"copper billet, high-temperature oxidation, water vapour, carbon dioxide, cupric oxide, cuprous oxide","lastPublishedDoi":"10.21203/rs.3.rs-3326982/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3326982/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWhen copper billet is heated in the rolling reheating furnace, some oxides which affect the surface quality may remain on the substrate. This study investigates the effects of different heating atmosphere compositions and contents (N\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO) on the micro-morphology and phase evolution of the exfoliated and residual oxides on copper billet surface by high-temperature oxidation experiments. The results show that the main phase of the exfoliated oxides is CuO attached to loose Cu\u003csub\u003e2\u003c/sub\u003eO, and the residual nodular oxides are also CuO which directly adhere to copper matrix; water vapour can increase the number and size of Cu\u003csub\u003e2\u003c/sub\u003eO particles on the interface between exfoliated oxide and copper matrix, and effectively reduces the number of residual nodular oxides; the vapour and its decomposition products may effectively eliminate the pores within the oxide layer and at the oxide-matrix interface, while CO\u003csub\u003e2\u003c/sub\u003e may increase the pore size inside the oxide layer.\u003c/p\u003e","manuscriptTitle":"Effect of heating atmosphere composition and content on phase and morphology distribution of copper oxide layer","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-09-12 20:50:22","doi":"10.21203/rs.3.rs-3326982/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-01-25T02:38:15+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2023-09-22T16:17:56+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"1be86efe-1cb4-4a18-8351-68a665b7d94f","date":"2023-09-21T01:21:02+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-09-21T00:21:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-09-08T12:52:55+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2023-09-08T12:01:02+00:00","index":"","fulltext":""},{"type":"submitted","content":"High Temperature Corrosion of Materials","date":"2023-09-05T09:06:08+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"72cbbfa2-4e5a-4b1f-b7cd-f260e9b46ab3","owner":[],"postedDate":"September 12th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-03-16T20:07:07+00:00","versionOfRecord":{"articleIdentity":"rs-3326982","link":"https://doi.org/10.1007/s11085-024-10237-y","journal":{"identity":"high-temperature-corrosion-of-materials","isVorOnly":false,"title":"High Temperature Corrosion of Materials"},"publishedOn":"2024-03-13 20:07:07","publishedOnDateReadable":"March 13th, 2024"},"versionCreatedAt":"2023-09-12 20:50:22","video":"","vorDoi":"10.1007/s11085-024-10237-y","vorDoiUrl":"https://doi.org/10.1007/s11085-024-10237-y","workflowStages":[]},"version":"v1","identity":"rs-3326982","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3326982","identity":"rs-3326982","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}