Complex permittivity and remarkable microwave absorption of hydrothermally synthesized CoxP-loaded porous Al2O3 composites

Complex permittivity and remarkable microwave absorption of hydrothermally synthesized CoxP-loaded porous Al2O3 composites | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Complex permittivity and remarkable microwave absorption of hydrothermally synthesized CoxP-loaded porous Al2O3 composites Jiaojiao Yu, Liang Zhou, Taotao An, Hong bo Wang, Ming kang Zhang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7727233/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Materials with porous structure gradually become the research hotspot as microwave absorption materials (MAMs) due to their low density and high specific surface area. In this paper, the porous Al 2 O 3 ceramics decorated with Co x P were prepared by hydrothermal and low-temperature phosphating. The morphology, complex permittivity and reflection loss were explored to study their potential microwave absorption properties. The results show that the Co x P content can be regulated by adjusting the concentration of NH 4 F in the precursor solution. Meanwhile, the and microstructure and complex permittivity could be adjusted by the addition of NH 4 F. As the concentration of NH 4 F is 0.05 mol/L, Co x P@Al 2 O 3 composites exhibit excellent microwave absorption properties with reflection loss of -35.8 dB at 9.9 GHz and effective absorption bandwidth of 4 GHz in the X-band at the thickness of 3.2mm. This study provides a new way for the effective application of porous ceramic as functional materials, and the investigated Co x P@Al 2 O 3 composites can be qualified as favorable MAMs for broadband and efficient microwave absorption. CoxP@Al2O3 Hydrothermal method Low-temperature phosphating Complex permittivity Microwave absorption Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction With the development of modern science and technology, microwave has become a new pollution source with unexpected harmfulness besides water contamination, atmospheric pollution and noise pollution. 1 – 2 In recent years, microwave radiation is constantly being proved to have detrimental impacts on the health of organism and the normal operation of electronic equipment. 3 – 4 The microwave absorption materials could convert energy of electromagnetic wave into heat or other form energy to consume it, which make it a hot spot. 5 – 6 In order to meet the requirements of microwave absorption materials (strong absorption, wide bandwidth, thin thickness, low density), many efforts have been paid into the development of new structures and materials. 7 – 8 Wang et al. synthesized the Ni and S co-doped honeycomb-like C/Ni 3 S 2 composites using freeze drying and pyrolysis technique. 1 The composites with 10 wt.% filler loading exhibits wide effective absorption bandwidth (EAB, RL< -10 dB) of 7.0 GHz within 8–18.0 GHz and a strong absorption peak of -46.8 dB at 13.4 GHz, when the thickness is 2.5 mm. Recently, many composites with complicated structures such as core-shell structure, 9–10 sandwich structure, 11,7 porous structure, 9,12 hollow structure 13 – 14 have been prepared and proved to possess favorable microwave absorption properties. Among the complicated structures, porous structure has captured the attention of scientists because of the following advantages: ⅰ) The presence of pores can adjust the electromagnetic parameters of the composites to obtain superior impedance matching; 15–16 ⅱ) Interconnected pore channels increase the propagation path of incident electromagnetic waves and thus enhance the microwave absorption performance; 17–18 ⅲ) High specific surface area means more interfaces, which help to improve interfacial loss. 19 – 20 Chen et al. prepared porous Si-O-C ceramics by pyrolysis technology. 12 Compared with traditional paraffin-based Si-O-C, the porous Si-O-C ceramic has promising microwave absorption properties with a strong absorption peak of -39.13 dB at 11.76 GHz and wide effective absorption bandwidth of 4.64 GHz in 7.6–12.24 GHz with the thickness of 3.0 mm. Porous Al 2 O 3 ceramics has the advantages of high temperature resistance, good corrosion resistance, large elastic modulus, high specific surface area and light-weight, which has been widely used in filter, heat preservation, sound-absorbing and many other fields in recent years. 21 – 22 However, porous Al 2 O 3 ceramic is rarely used as microwave absorption materials. Our previous studies have shown that Al 2 O 3 ceramic is an optimum matrix. 23 – 24 Therefore, the present work uses porous Al 2 O 3 ceramic as the matrix to develop microwave absorption material with excellent performance. Co x P is widely used as hydrogen evolution catalyst due to its active sites and acid/basic inertness. 25 Green et al. have proved that Co 2 P has rosy microwave absorption performance with the reflection loss of -9.3 dB and the wide effective absorption bandwidth of 2.4 GHz. 26 Meanwhile, cobalt phosphides (including CoP, Co 2 P and CoP 2 ) are candidate cathode materials in sodium ion batteries to have uncommon electronic conductivity, which are also promising as new-type microwave absorbents. 27 Inspired by the above research, the Co x P-porous Al 2 O 3 was prepared and the effect of the absorbent content and morphology on the microwave absorption performance was studied. 2. Experimental 2.1 Preparation Porous Al 2 O 3 ceramic (Jiangsu Wuxi BaiRaid New Material Co., Ltd., China) with an average aperture size of 70 µm and density of 2.3 g/cm 3 were used as the matrix. Cobalt chloride hexahydrate (CoCl 2 ·6H 2 O) and ammonium fluoride (NH 4 F, morphological adjustment reagent) were purchased from Guangzhou Guanghua Technology Co. Ltd. Urea (CO(NH 2 ) 2 , precipitant) was supplied by Red Rock Reagent Factory (Hedong District, Tianjin) and sodium hypophosphite (NaH 2 PO 2 ·H 2 O) was produced by Tianjin Tianli Chemical Reagent Co., Ltd. The porous Al 2 O 3 ceramic was ultrasonicated respectively in acetone, ethanol and deionized water for 15 min to clean the impurity, and oven-dried at 80℃ for 6 h. To prepare the precursor solution, CoCl 2 ·6H 2 O, CO(NH 2 ) 2 and NH 4 F were dipped into a beaker with 15 mL deionized water. The concentration of CoCl 2 ·6H 2 O and CO(NH 2 ) 2 was 0.05 mol/L and 0.25 mol/L, respectively. Besides, the concentration of NH 4 F in the precursor solution of the three samples was 0 mol/L, 0.05 mol/L and 0.1 mol/L, respectively. After the precursor solution was magnetically stirred at room temperature for 20 min, the pretreated porous Al 2 O 3 ceramic was put into the precursor solution. In order to ensure the uniformity of deposition, the beaker was placed in vacuum drying oven (-0.1 MPa) at room temperature for 20 min. Subsequently, the above solution was transferred into 25 mL Teflon-lined stainless-steel autoclave and maintained at 120℃ for 5 h. When the reactor temperature drops to room temperature, the porous Al 2 O 3 composite ceramic and the powder in the reactor were removed and ultrasonically washed with deionized water for three times. After cleaning, the porous Al 2 O 3 composite ceramic and the powder were oven-dried at 80°C for 12 h under vacuum. Finally, NaH 2 PO 2 ·H 2 O (1 g) was put into tube furnace at upstream side and the porous Al 2 O 3 composite ceramic and the powders were placed at downstream side, annealing in the 50 mL/min Ar stream at 350°C for 2 h with heating rate of 5°C/min. The Co x P-porous Al 2 O 3 was obtained until the reactor was naturally cooled down to room temperature. 2.2 Characterization The microstructure of the composites was revealed by scanning electron microscope (SEM, Hitachi S-4800). The crystalline phase was identified by X-ray diffraction (XRD, AXS, D8 ADVANCE, BRUKER, Germany) with a Cu-Kα radiation at 40 kV and 40 mA operated in the 2θ range of 20–80°. The surface chemical composition samples were analyzed by an X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The electromagnetic parameters were measured in 8.2–12.4 GHz (X-band) by an E8362B vector network analyzer with the rectangle wave-guide method. The testing samples were cut into rectangular blocks (22.86 mm × 10.16 mm). Based on the transmission line theory, the formulas for calculating reflection loss (RL), and input impedance ( \(\:{\text{Z}}_{\text{i}\text{n}}\) ) of the composites are expressed as following: 28 $$\:\text{R}\text{L}（\text{d}\text{B}）=20\text{l}\text{g}\left|\left({\text{Z}}_{\text{i}\text{n}}-\left.{\text{Z}}_{0}\right)/\left.\left({\text{Z}}_{\text{i}\text{n}}+\left.{\text{Z}}_{0}\right)\right.\right|\right.\right.$$ 1 $$\:{\text{Z}}_{\text{i}\text{n}}={\text{Z}}_{0}\sqrt{{{\mu\:}}_{\text{r}}/{{\epsilon\:}}_{\text{r}}}\text{t}\text{a}\text{n}\text{h}\left[\text{j}\left(2{\pi\:}\text{f}\text{d}/\text{c}\right)\sqrt{{{\mu\:}}_{\text{r}}{{\epsilon\:}}_{\text{r}}}\right]$$ 2 where Z 0 is the impedance of the free space (377 Ω), µ r and ε r are the relative permeability and permittivity of the composites, respectively, f is the frequency of the incident electromagnetic wave, c is the speed of light in vacuum, and d is the sample thickness. 3. Results and discussion 3.1 Phase composition and microstructure From Fig. 2 a, it is obvious that the main components of the precursor powder are Co(CO 3 ) 0.5 (OH)·0.11H 2 O (PDF#48–0083). The peaks of the crystal shifts to the left due to N doping. Besides, two peaks at 43.931° and 61.579° corresponding to the C (PDF#26-1083) and Co(OH) 2 (PDF#30–0443), respectively. Due to the relatively small content of Co x P in the matrix, the XPS was measured to characterize the presence of Co x P and the spectra are shown in Fig. 2 (b-d). The survey spectrum proves the existence of Co, O, N, P, Al elements within the Co x P-porous Al 2 O 3 , in which the N element come from the added ammonium fluoride and urea. The XPS spectrum of Co 2p reveals three peaks at 779 eV, 782.4 eV and 785.5 eV assigned to Co 2p 3/2 and the peak at 788.79 eV, assigned to Co 2p 1/2 , respectively indicating the coexistence of Co 2+ and Co 3+ in the composites, which is the hierarchical nanostructures of Co x P. Among them, the peak at 785.5 eV is satellite peaks, which is produced by the vibration excitation of the high-spin Co 2+ ions. The high-resolution P 2p region exhibits two peaks at 129.5 eV and 133.7 eV corresponding to the P 2p 3/2 and P 2p 1/2 states. The peaks at 782.4 eV and 133.7 eV corresponding to the oxidized states of Co and P, which is due to the contact between the sample and air during phosphating. In addition, the peaks of Co-P are located at 779 eV of Co 2p spectrum and 129.5 eV of P 2p spectrum, which confirmed the presence of Co x P in the sample. As observed in Fig. 3 , the content of Co x P increases with the increase of NH 4 F concentration and the Co x P absorbent is uniformly distributed in the Al 2 O 3 matrix. The precursor solution without NH 4 F will lead to the formation of needle-like Co x P structures. However, when 0.05 mol/L NH 4 F was added during hydrothermal reaction, branch-like Co x P structures were obtained. With further increase the concentration of NH 4 F, the final structure of Co x P turned to flake with micropores. Combining with the XRD results of precursor powders, the possible reactions involved in the hydrothermal process can be described as follows: NH 4 F→NH 4 + +F ๣ (3) Co 2+ +xF ๣ →[CoF] (x−2)๣ (4) CO(NH 2 ) 2 +H 2 O→2NH 3 + CO 2 (5) CO 2 + H 2 O→CO 3 2๣ +2H ་ (6) [CoF] (x−2)๣ +0.5CO 3 2๣ +OH ๣ +0.11H 2 O→Co(CO 3 ) 0.5 (OH)·0.11H 2 O + xF ๣ (7) According to the above reactions, the morphology regulation of NH 4 F mainly depends on the effect of F ๣ ,which can be summarized as follows: ⅰ) The complexes produced by F ๣ and Co 2+ is beneficial to the formation of crystal nucleus on the Al 2 O 3 surface and the tight adhesion of nanostructures to Al 2 O 3 matrix; ⅱ) Due to the ionic hydration effect of F ๣ , multiple water molecules are linked around F ๣ , thus reducing the concentration of water molecules and inhibiting the movement of water molecules 29 . This effect can enhance the average activity coefficient of ions in the precursor solution, which increase the reaction rate and affect the morphology of the products. Besides, the addition amount of NH 4 F directly affect the pH value of the precursor solution, which affect the concentration of CO 3 2๣ and OH − in the precursor solution and then affect the growth rate of Co(CO 3 ) 0.5 (OH)·0.11H 2 O. When the NH 4 F concentration is 0 and 0.05 mol/L, Co(CO 3 ) 0.5 (OH)·0.11H 2 O can grow in order and finally produce needle-like and branch-like structures, respectively. As the concentration of NH 4 F increases, the excess F ๣ can be adsorbed onto the surface of the newborn Co(CO 3 ) 0.5 (OH)·0.11H 2 O crystal and prevent the orderly growth of Co(CO 3 ) 0.5 (OH)·0.11H 2 O crystal. Therefore, the final morphology of Co x P is flake with micropores. 3.2 Electromagnetic parameters Electromagnetic parameters are the key factors affecting the microwave absorption performance, which were measured in X-band. The complex permittivity of Co x P-porous Al 2 O 3 is shown in Fig. 4 (a-b). It could be found that the sample with the 0.1 mol/L NH 4 F had the highest complex permittivity with the average value of real and imaginary part are 6.5 and 3.1, respectively. As shown above, the ε′ and ε′′ values of Co x P-porous Al 2 O 3 increase with the increase of NH 4 F concentration. Recalling the SEM results, the NH 4 F concentration changes the content and morphology of Co x P, which indicates that the complex permittivity relies heavily on the absorbent content and morphology. Meanwhile, the ε′′ values decrease evidently with the increasing frequency, which is due to the charge transfer and polarization behind the changing electric field. 30 Based on the mixing law(lnε=∑λ i lnε i ), the complex permittivity values of composite materials are strongly related to their components 24 . Owning to the much higher complex permittivity of Co x P than porous Al 2 O 3 , the ε′ and the ε′′ values of the composites increases with increasing the content of Co x P. To the best of our knowledge, ε′ describes the electromagnetic energy storage capability of materials, which is mainly controlled by polarization. 4 For the investigated composites, it can be seen from the SEM images that the interfacial area will increase due to the increasing NH 4 F concentration. The increasing interface area is beneficial to the enhancement of the interface polarization, thus enhance the ε′ value. Besides, the increasing content of absorbent will increase the number of dipoles in the composites. Under the effect of electromagnetic field, the intrinsic dipole moment of the dipole will be shifted to the direction of the external electric field, thus producing orientation polarization. In brief, the increasing number of dipoles enhances the orientation polarization, thereby increasing the ε′ value. According to free electron theory ( \(\:{\epsilon\:}^{”}={\epsilon\:}_{relax}^{”}+\raisebox{1ex}{$\sigma\:$}\!\left/\:\!\raisebox{-1ex}{$2\pi\:f{\epsilon\:}_{0}$}\right.\) ), 30 the ε′′ values is mainly related to the electrical conductivity(σ) and relaxation loss ( \(\:{\epsilon\:}_{relax}^{”}\) ). For the composites studied, the interface polarization and orientation polarization contribute to the increasing ε′ value, and the related relaxation loss enhances the ε′′ values. Besides, it can be seen from the Fig. 3 that the increasing NH 4 F concentration will lead to the increase of absorbent content and the formation of conductive network. The above two points are beneficial to improve the electrical conductivity of the composites, thus further enhance the ε′′ values. To further confirm the analysis, the electrical conductivity of specimens was measured and the image is shown in Fig. 4 c. The electrical conductivities of the Co x P-porous Al 2 O 3 are 1.826×10 − 6 , 2.311×10 − 6 and 3.198×10 − 6 S/m, respectively, which is a strong support. The complex permeability of Co x P-porous Al 2 O 3 is illustrated in Fig. 4 (d-e). It is evidently found that the Co x P-porous Al 2 O 3 have the lowest complex permeability when there is no NH 4 F in the precursor solution, which is mainly due to the low content of Co x P in the samples. For the other two samples, the sample prepared by the precursor solution containing 0.05 mol/L NH 4 F has higher µ′, while the sample with 0.1 mol/L NH 4 F in the precursor solution has higher µ′′. Compared with the sample prepared by the precursor solution containing 0.05 mol/L NH 4 F, the sample with 0.1 mol/L NH 4 F in the precursor solution has higher electrical conductivity, which will produce strong eddy current loss, thus reducing the µ′ value and enhancing the µ′′ value. 32 – 33 Meanwhile, the µ′ decrease with the increasing frequency for the composites, which is ascribed to the hysteresis of domain motion in relation to rapidly changing electromagnetic fields. The µ″(µ′) −2 (f) −1 value is the parameter to determine whether eddy current loss is the main source of magnetic loss. 34 When the value is constant with different frequency, the eddy current loss is the main source of magnetic loss. According to Fig. 4 f, the main magnetic loss mechanism of the Co x P-porous Al 2 O 3 is exchange resonance and natural resonance. 3.3 Microwave absorption properties To measure the microwave absorption properties of the composites, the RL value is calculated and its curve with frequency is shown in the Fig. 5 a. It is evidently found that the minimum reflection loss (RL min ) of the samples decreases first and then increases with the increasing NH 4 F concentration in precursor solution. Meanwhile, the sample prepared by the precursor solution containing 0.05 mol/L NH 4 F have stronger RL of -25.35 dB at 11.25 GHz, while the sample with 0.1 mol/L NH 4 F in the precursor solution have wider effective bandwidth of 3.8 GHz in 8.6–12.4 GHz. The favorable impedance matching is a prerequisite for the material to have excellent microwave absorption properties, which ensures that the electromagnetic wave enters the absorbent as much as possible. Based on the transmission lines theory, the closer the impedance matching Z (Z=|Z in /Z 0 |) value is to 1, more electromagnetic waves can enter the absorbent for attenuation. 35 The impedance matching of Co x P-porous Al 2 O 3 with thickness of 2.8 mm was calculated and its diagram is shown in Fig. 5 b. It is easy to find that the impedance matching of Co x P-porous Al 2 O 3 increases in low frequency and decreases in high frequency with increasing frequency. Besides, when the NH 4 F concentration in precursor solution is 0.05 mol/L, the NH 4 F concentration in precursor solution has the best impedance matching. Altogether, this sample is most likely to have the best absorption properties. Except for impedance matching, attenuation coefficient α is also an important parameter to determine the microwave absorption properties of materials. The attenuation coefficient describes the ability of the microwave absorption material to absorb electromagnetic waves entering the material and is calculated by the following formula: 36–37 $$\:{\alpha\:}=\frac{\sqrt{2}{\pi\:}\text{f}}{\text{c}}\times\:\sqrt{\left({{\mu\:}}^{{\prime\:}{\prime\:}}{{\epsilon\:}}^{{\prime\:}{\prime\:}}-{{\mu\:}}^{{\prime\:}}{{\epsilon\:}}^{{\prime\:}}\right)+\sqrt{{\left({{\mu\:}}^{{\prime\:}{\prime\:}}{{\epsilon\:}}^{{\prime\:}{\prime\:}}-{{\mu\:}}^{{\prime\:}}{{\epsilon\:}}^{{\prime\:}}\right)}^{2}+{\left({{\mu\:}}^{{\prime\:}{\prime\:}}{{\epsilon\:}}^{{\prime\:}}+{{\mu\:}}^{{\prime\:}}{{\epsilon\:}}^{{\prime\:}{\prime\:}}\right)}^{2}}}$$ 8 Figure 5 c depicts attenuation coefficients of Co x P-porous Al 2 O 3 with the thickness of 2.8 mm. It is obviously found that the attenuation coefficient increases with the addition of NH 4 F. When the NH 4 F concentration in precursor solution is 0.1 mol/L, the Co x P-porous Al 2 O 3 have the best attenuation coefficient, but its poor impedance matching cause most electromagnetic waves reflect on the surface of the composites. Therefore, the Co x P-porous Al 2 O 3 prepared by the precursor solution containing 0.05 mol/L NH 4 F possess the promising microwave absorption properties due to its best impedance matching and better attenuation coefficient. The reflection loss and the typical effective absorption bandwidth of Co x P-porous Al 2 O 3 with different NH 4 F concentration in precursor solution are shown in Fig. 6 . According to Fig. 6 (a-c), when the concentration of NH 4 F in the precursor solution increases, the minimum reflection loss of the Co x P-porous Al 2 O 3 tend to appear in the region of thin thickness and high frequency. When the concentration of NH 4 F in the precursor solution is 0.1 mol/L, the minimum reflection loss value of the Co x P-porous Al 2 O 3 is -45.48 dB, and its effective absorption bandwidth is 2.1 GHz (Fig. 6 f). Compared with the other two samples, the Co x P-porous Al 2 O 3 with 0.05 mol/L NH 4 F in precursor solution has the best microwave absorption performance. When the thickness is 3.2 mm, its minimum reflection loss is -35.82 dB at 10.55 GHz, and its effective absorption bandwidth is 3.99 GHz (95% X-band) of 8.2–12.19 GHz (Fig. 6 e). As shown in Table 1 , the Co x P-porous Al 2 O 3 is comparable or even better than most reported absorbers. Table 1 Microwave Absorption Performance of Porous Ceramic Materials RLmin/dB Thickness/mm EAB/GHz Ref. Si-O-C -39.13 3 4.64 (7.6–12.24) [12] CNWs-SiO 2 /3Al 2 O 3 ·2SiO 2 -31 5 4.2 (8.2–12.4) [38] PDC-SiC/Si 3 N 4 -33 2.5 7 (11–18) [39] C-rich SiC NWs/Sc 2 Si 2 O 7 -35.5 2.75 4.2 (8.2–12.4) [40] Co x P-porous Al 2 O 3 -35.82 3.2 3.99 (8.2–12.19) this work Based on the analysis above, the microwave absorption mechanisms of Co x P-porous Al 2 O 3 can be simply depicted in Fig. 7 . The excellent microwave absorption performance of the Co x P-porous Al 2 O 3 mainly come from the following points. First, the air in the pore can effectively regulate the electromagnetic parameter of the Co x P-porous Al 2 O 3 to obtain better microwave absorption. In addition, the special structure of porous Al 2 O 3 and branch-like Co x P in the composite makes the incident electromagnetic wave have multiple reflection inside the material, which effectively prolongs the propagation path of electromagnetic wave, thus effectively attenuates the electromagnetic wave. In the end, the existence of branch-like Co x P and the porous Al 2 O 3 makes the composites have many heterogeneous interfaces, which leads to more interfacial loss to consume the electromagnetic wave. 4. Conclusions In the present work, the Co x P-porous Al 2 O 3 was prepared and its morphology, electromagnetic parameters and microwave absorption properties was investigated. According to the test results, the mole ratio of CoCl 2 .6H 2 O, NH 4 F and CO(NH 2 ) 2 in the precursor solution is 1:1:5 and the thickness of the sample is 3.2 mm, the Co x P-porous Al 2 O 3 has the positive microwave absorption properties with reflect loss of -35.8 dB at 9.9 GHz and effective absorption bandwidth of 4 GHz (95% of X-band) in 8.2–12.2 GHz. Meanwhile, the excellent absorption performance of the Co x P-porous Al 2 O 3 comes from the effective regulation of electromagnetic parameters by air, multiple reflections caused by the special structure, enhanced conduction loss and interfacial loss. Declarations Author Contribution CRediT authorship contribution statement Jiao-jiao YU: Conceptualization, Methodology, Data curation,Investigation,Writ-original draft; Liang ZHOU: Formal analysis, Methodology, Investigation, Writing – review & editing; Tao-tao AN: Methodology, Writing – Review & editing; Hong-bo WANG: Data curation, Validation, Supervision,Writing – Review & editing; Ming-kang ZHANG: Methodology, Writing – Review & editing. Acknowledgement This work is supported by Key Research and Development Program in Shaanxi Province of China (No. 2025CY-YBXM-128) and Fundamental Research Funds for the Central Universities of the Chang'an University (No. 300102313201). References L. Wang, X.Y. Bai, T. Zhao, Y. Lin, Facile synthesis of N, S-codoped honeycomb-like C/Ni 3 S 2 composites for broadband microwave absorption with low filler mass loading. J. Colloid Interface Sci. 580 , 126–134 (2020) Y.X. Zuo, Z.J. Yao, H.Y. Lin, J.T. Zhou, J. Lu, J. 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Int. 44 , 22784–22793 (2018) Additional Declarations No competing interests reported. Supplementary Files Graphicalabstract.jpg Cite Share Download PDF Status: Posted Version 1 posted 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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18:14:36","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1091157,"visible":true,"origin":"","legend":"\u003cp\u003ea) XRD patterns of precursor; XPS spectra of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with the NH\u003csub\u003e4\u003c/sub\u003eF concentration of 0.05 mol/L: (b) full spectra, (c) Co\u003csub\u003e2p\u003c/sub\u003e and (d) P\u003csub\u003e2p\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/3a64ebaab04f7f94ca3537fe.png"},{"id":94584548,"identity":"c70cbd36-7b83-439c-a443-de71e316972f","added_by":"auto","created_at":"2025-10-28 18:15:20","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4782971,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with different concentration of NH\u003csub\u003e4\u003c/sub\u003eF: (a-b) 0 mol/L; (c-d) 0.05 mol/L; (e-f) 0.1 mol/L.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/8f3c5966b890fadb40f71ed1.png"},{"id":94584550,"identity":"9d223367-bbf0-4cff-b538-98a9df16f725","added_by":"auto","created_at":"2025-10-28 18:15:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1427078,"visible":true,"origin":"","legend":"\u003cp\u003eThe (a) real (ε′) and (b) imaginary (ε′′) parts of complex permittivity, (c) the electrical conductivity, the (d) real (μ′) and (e) imaginary (μ′′) parts of complex permeability and (f) the μ″(μ′)−2(f)−1 values for CoxP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with different NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/8682d356f20bb7672e9f5079.png"},{"id":94584787,"identity":"94f98bb5-bcda-4d9d-9e35-ad9b9c9f5942","added_by":"auto","created_at":"2025-10-28 18:15:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":795814,"visible":true,"origin":"","legend":"\u003cp\u003eThe (a) reflection loss, (b) impedance matching and (c) attenuation coefficients of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with different NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution (d = 2.8 mm).\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/6e61e0aef841d9d71ec61761.png"},{"id":94584769,"identity":"d11bec37-af40-447e-aca6-521a0c38d4b0","added_by":"auto","created_at":"2025-10-28 18:15:35","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2274094,"visible":true,"origin":"","legend":"\u003cp\u003eThe reflection loss and the typical effective absorption bandwidth of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with different NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution: (a, d) 0 mol/L; (b, e) 0.05 mol/L; (c, f) 0.1 mol/L.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/f7d6770bf6c4a298fe5e3941.png"},{"id":94584220,"identity":"231a1ad2-f948-4249-a169-2a87adef6293","added_by":"auto","created_at":"2025-10-28 18:15:02","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1379755,"visible":true,"origin":"","legend":"\u003cp\u003eThe microwave absorption mechanisms of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/61020206e1296e542370a375.png"},{"id":94595608,"identity":"dc2adea5-ac55-484d-b85a-228454ef9114","added_by":"auto","created_at":"2025-10-28 18:35:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11644484,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/22dfcd81-2a31-4ba0-9078-1954aef4a6fc.pdf"},{"id":94584458,"identity":"f582de76-76ba-4b52-8fab-f5b29860533e","added_by":"auto","created_at":"2025-10-28 18:15:13","extension":"jpg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":142530,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7727233/v1/3652ee0431a6e4318997d8f3.jpg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Complex permittivity and remarkable microwave absorption of hydrothermally synthesized CoxP-loaded porous Al2O3 composites","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWith the development of modern science and technology, microwave has become a new pollution source with unexpected harmfulness besides water contamination, atmospheric pollution and noise pollution.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e In recent years, microwave radiation is constantly being proved to have detrimental impacts on the health of organism and the normal operation of electronic equipment.\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e The microwave absorption materials could convert energy of electromagnetic wave into heat or other form energy to consume it, which make it a hot spot.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e In order to meet the requirements of microwave absorption materials (strong absorption, wide bandwidth, thin thickness, low density), many efforts have been paid into the development of new structures and materials.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e Wang et al. synthesized the Ni and S co-doped honeycomb-like C/Ni\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e composites using freeze drying and pyrolysis technique.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e The composites with 10 wt.% filler loading exhibits wide effective absorption bandwidth (EAB, RL\u0026lt; -10 dB) of 7.0 GHz within 8\u0026ndash;18.0 GHz and a strong absorption peak of -46.8 dB at 13.4 GHz, when the thickness is 2.5 mm.\u003c/p\u003e\u003cp\u003eRecently, many composites with complicated structures such as core-shell structure,\u003csup\u003e9\u0026ndash;10\u003c/sup\u003e sandwich structure,\u003csup\u003e11,7\u003c/sup\u003e porous structure,\u003csup\u003e9,12\u003c/sup\u003e hollow structure\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e have been prepared and proved to possess favorable microwave absorption properties. Among the complicated structures, porous structure has captured the attention of scientists because of the following advantages: ⅰ) The presence of pores can adjust the electromagnetic parameters of the composites to obtain superior impedance matching;\u003csup\u003e15\u0026ndash;16\u003c/sup\u003e ⅱ) Interconnected pore channels increase the propagation path of incident electromagnetic waves and thus enhance the microwave absorption performance;\u003csup\u003e17\u0026ndash;18\u003c/sup\u003e ⅲ) High specific surface area means more interfaces, which help to improve interfacial loss.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e Chen et al. prepared porous Si-O-C ceramics by pyrolysis technology.\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e Compared with traditional paraffin-based Si-O-C, the porous Si-O-C ceramic has promising microwave absorption properties with a strong absorption peak of -39.13 dB at 11.76 GHz and wide effective absorption bandwidth of 4.64 GHz in 7.6\u0026ndash;12.24 GHz with the thickness of 3.0 mm. Porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics has the advantages of high temperature resistance, good corrosion resistance, large elastic modulus, high specific surface area and light-weight, which has been widely used in filter, heat preservation, sound-absorbing and many other fields in recent years.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e However, porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic is rarely used as microwave absorption materials. Our previous studies have shown that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic is an optimum matrix.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e Therefore, the present work uses porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic as the matrix to develop microwave absorption material with excellent performance.\u003c/p\u003e\u003cp\u003eCo\u003csub\u003ex\u003c/sub\u003eP is widely used as hydrogen evolution catalyst due to its active sites and acid/basic inertness.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e Green et al. have proved that Co\u003csub\u003e2\u003c/sub\u003eP has rosy microwave absorption performance with the reflection loss of -9.3 dB and the wide effective absorption bandwidth of 2.4 GHz.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e Meanwhile, cobalt phosphides (including CoP, Co\u003csub\u003e2\u003c/sub\u003eP and CoP\u003csub\u003e2\u003c/sub\u003e) are candidate cathode materials in sodium ion batteries to have uncommon electronic conductivity, which are also promising as new-type microwave absorbents.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e Inspired by the above research, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was prepared and the effect of the absorbent content and morphology on the microwave absorption performance was studied.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Preparation\u003c/h2\u003e\u003cp\u003ePorous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic (Jiangsu Wuxi BaiRaid New Material Co., Ltd., China) with an average aperture size of 70 \u0026micro;m and density of 2.3 g/cm\u003csup\u003e3\u003c/sup\u003e were used as the matrix. Cobalt chloride hexahydrate (CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) and ammonium fluoride (NH\u003csub\u003e4\u003c/sub\u003eF, morphological adjustment reagent) were purchased from Guangzhou Guanghua Technology Co. Ltd. Urea (CO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e, precipitant) was supplied by Red Rock Reagent Factory (Hedong District, Tianjin) and sodium hypophosphite (NaH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO) was produced by Tianjin Tianli Chemical Reagent Co., Ltd.\u003c/p\u003e\u003cp\u003eThe porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic was ultrasonicated respectively in acetone, ethanol and deionized water for 15 min to clean the impurity, and oven-dried at 80℃ for 6 h. To prepare the precursor solution, CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO, CO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and NH\u003csub\u003e4\u003c/sub\u003eF were dipped into a beaker with 15 mL deionized water. The concentration of CoCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and CO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e was 0.05 mol/L and 0.25 mol/L, respectively. Besides, the concentration of NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution of the three samples was 0 mol/L, 0.05 mol/L and 0.1 mol/L, respectively. After the precursor solution was magnetically stirred at room temperature for 20 min, the pretreated porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramic was put into the precursor solution. In order to ensure the uniformity of deposition, the beaker was placed in vacuum drying oven (-0.1 MPa) at room temperature for 20 min. Subsequently, the above solution was transferred into 25 mL Teflon-lined stainless-steel autoclave and maintained at 120℃ for 5 h. When the reactor temperature drops to room temperature, the porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composite ceramic and the powder in the reactor were removed and ultrasonically washed with deionized water for three times. After cleaning, the porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composite ceramic and the powder were oven-dried at 80\u0026deg;C for 12 h under vacuum. Finally, NaH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO (1 g) was put into tube furnace at upstream side and the porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composite ceramic and the powders were placed at downstream side, annealing in the 50 mL/min Ar stream at 350\u0026deg;C for 2 h with heating rate of 5\u0026deg;C/min. The Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was obtained until the reactor was naturally cooled down to room temperature.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Characterization\u003c/h2\u003e\u003cp\u003eThe microstructure of the composites was revealed by scanning electron microscope (SEM, Hitachi S-4800). The crystalline phase was identified by X-ray diffraction (XRD, AXS, D8 ADVANCE, BRUKER, Germany) with a Cu-Kα radiation at 40 kV and 40 mA operated in the 2θ range of 20\u0026ndash;80\u0026deg;. The surface chemical composition samples were analyzed by an X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The electromagnetic parameters were measured in 8.2\u0026ndash;12.4 GHz (X-band) by an E8362B vector network analyzer with the rectangle wave-guide method. The testing samples were cut into rectangular blocks (22.86 mm \u0026times; 10.16 mm). Based on the transmission line theory, the formulas for calculating reflection loss (RL), and input impedance (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{Z}}_{\\text{i}\\text{n}}\\)\u003c/span\u003e\u003c/span\u003e) of the composites are expressed as following:\u003csup\u003e28\u003c/sup\u003e\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\text{R}\\text{L}（\\text{d}\\text{B}）=20\\text{l}\\text{g}\\left|\\left({\\text{Z}}_{\\text{i}\\text{n}}-\\left.{\\text{Z}}_{0}\\right)/\\left.\\left({\\text{Z}}_{\\text{i}\\text{n}}+\\left.{\\text{Z}}_{0}\\right)\\right.\\right|\\right.\\right.$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{\\text{Z}}_{\\text{i}\\text{n}}={\\text{Z}}_{0}\\sqrt{{{\\mu\\:}}_{\\text{r}}/{{\\epsilon\\:}}_{\\text{r}}}\\text{t}\\text{a}\\text{n}\\text{h}\\left[\\text{j}\\left(2{\\pi\\:}\\text{f}\\text{d}/\\text{c}\\right)\\sqrt{{{\\mu\\:}}_{\\text{r}}{{\\epsilon\\:}}_{\\text{r}}}\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere Z\u003csub\u003e0\u003c/sub\u003e is the impedance of the free space (377 Ω), \u0026micro;\u003csub\u003er\u003c/sub\u003e and ε\u003csub\u003er\u003c/sub\u003e are the relative permeability and permittivity of the composites, respectively, f is the frequency of the incident electromagnetic wave, c is the speed of light in vacuum, and d is the sample thickness.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Phase composition and microstructure\u003c/h2\u003e\u003cp\u003eFrom Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, it is obvious that the main components of the precursor powder are Co(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e0.5\u003c/sub\u003e(OH)\u0026middot;0.11H\u003csub\u003e2\u003c/sub\u003eO (PDF#48\u0026ndash;0083). The peaks of the crystal shifts to the left due to N doping. Besides, two peaks at 43.931\u0026deg; and 61.579\u0026deg; corresponding to the C (PDF#26-1083) and Co(OH)\u003csub\u003e2\u003c/sub\u003e(PDF#30\u0026ndash;0443), respectively. Due to the relatively small content of Co\u003csub\u003ex\u003c/sub\u003eP in the matrix, the XPS was measured to characterize the presence of Co\u003csub\u003ex\u003c/sub\u003eP and the spectra are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b-d). The survey spectrum proves the existence of Co, O, N, P, Al elements within the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, in which the N element come from the added ammonium fluoride and urea. The XPS spectrum of Co 2p reveals three peaks at 779 eV, 782.4 eV and 785.5 eV assigned to Co 2p\u003csub\u003e3/2\u003c/sub\u003e and the peak at 788.79 eV, assigned to Co 2p\u003csub\u003e1/2\u003c/sub\u003e, respectively indicating the coexistence of Co\u003csup\u003e2+\u003c/sup\u003e and Co\u003csup\u003e3+\u003c/sup\u003e in the composites, which is the hierarchical nanostructures of Co\u003csub\u003ex\u003c/sub\u003eP. Among them, the peak at 785.5 eV is satellite peaks, which is produced by the vibration excitation of the high-spin Co\u003csup\u003e2+\u003c/sup\u003eions. The high-resolution P 2p region exhibits two peaks at 129.5 eV and 133.7 eV corresponding to the P 2p\u003csub\u003e3/2\u003c/sub\u003e and P 2p\u003csub\u003e1/2\u003c/sub\u003e states. The peaks at 782.4 eV and 133.7 eV corresponding to the oxidized states of Co and P, which is due to the contact between the sample and air during phosphating. In addition, the peaks of Co-P are located at 779 eV of Co 2p spectrum and 129.5 eV of P 2p spectrum, which confirmed the presence of Co\u003csub\u003ex\u003c/sub\u003eP in the sample.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the content of Co\u003csub\u003ex\u003c/sub\u003eP increases with the increase of NH\u003csub\u003e4\u003c/sub\u003eF concentration and the Co\u003csub\u003ex\u003c/sub\u003eP absorbent is uniformly distributed in the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix. The precursor solution without NH\u003csub\u003e4\u003c/sub\u003eF will lead to the formation of needle-like Co\u003csub\u003ex\u003c/sub\u003eP structures. However, when 0.05 mol/L NH\u003csub\u003e4\u003c/sub\u003eF was added during hydrothermal reaction, branch-like Co\u003csub\u003ex\u003c/sub\u003eP structures were obtained. With further increase the concentration of NH\u003csub\u003e4\u003c/sub\u003eF, the final structure of Co\u003csub\u003ex\u003c/sub\u003eP turned to flake with micropores. Combining with the XRD results of precursor powders, the possible reactions involved in the hydrothermal process can be described as follows:\u003c/p\u003e\u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003eF\u0026rarr;NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e+F\u003csup\u003e๣\u003c/sup\u003e (3)\u003c/p\u003e\u003cp\u003eCo\u003csup\u003e2+\u003c/sup\u003e+xF\u003csup\u003e๣\u003c/sup\u003e\u0026rarr;[CoF]\u003csup\u003e(x\u0026minus;2)๣\u003c/sup\u003e (4)\u003c/p\u003e\u003cp\u003eCO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e+H\u003csub\u003e2\u003c/sub\u003eO\u0026rarr;2NH\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e (5)\u003c/p\u003e\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO\u0026rarr;CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2๣\u003c/sup\u003e+2H\u003csup\u003e་\u003c/sup\u003e (6)\u003c/p\u003e\u003cp\u003e[CoF]\u003csup\u003e(x\u0026minus;2)๣\u003c/sup\u003e+0.5CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2๣\u003c/sup\u003e+OH\u003csup\u003e๣\u003c/sup\u003e+0.11H\u003csub\u003e2\u003c/sub\u003eO\u0026rarr;Co(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e0.5\u003c/sub\u003e(OH)\u0026middot;0.11H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;xF\u003csup\u003e๣\u003c/sup\u003e (7)\u003c/p\u003e\u003cp\u003eAccording to the above reactions, the morphology regulation of NH\u003csub\u003e4\u003c/sub\u003eF mainly depends on the effect of F\u003csup\u003e๣\u003c/sup\u003e,which can be summarized as follows: ⅰ) The complexes produced by F\u003csup\u003e๣\u003c/sup\u003e and Co\u003csup\u003e2+\u003c/sup\u003e is beneficial to the formation of crystal nucleus on the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e surface and the tight adhesion of nanostructures to Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e matrix; ⅱ) Due to the ionic hydration effect of F\u003csup\u003e๣\u003c/sup\u003e, multiple water molecules are linked around F\u003csup\u003e๣\u003c/sup\u003e, thus reducing the concentration of water molecules and inhibiting the movement of water molecules\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. This effect can enhance the average activity coefficient of ions in the precursor solution, which increase the reaction rate and affect the morphology of the products. Besides, the addition amount of NH\u003csub\u003e4\u003c/sub\u003eF directly affect the pH value of the precursor solution, which affect the concentration of CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2๣\u003c/sup\u003e and OH\u003csup\u003e\u0026minus;\u003c/sup\u003e in the precursor solution and then affect the growth rate of Co(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e0.5\u003c/sub\u003e(OH)\u0026middot;0.11H\u003csub\u003e2\u003c/sub\u003eO. When the NH\u003csub\u003e4\u003c/sub\u003eF concentration is 0 and 0.05 mol/L, Co(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e0.5\u003c/sub\u003e(OH)\u0026middot;0.11H\u003csub\u003e2\u003c/sub\u003eO can grow in order and finally produce needle-like and branch-like structures, respectively. As the concentration of NH\u003csub\u003e4\u003c/sub\u003eF increases, the excess F\u003csup\u003e๣\u003c/sup\u003e can be adsorbed onto the surface of the newborn Co(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e0.5\u003c/sub\u003e(OH)\u0026middot;0.11H\u003csub\u003e2\u003c/sub\u003eO crystal and prevent the orderly growth of Co(CO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e0.5\u003c/sub\u003e(OH)\u0026middot;0.11H\u003csub\u003e2\u003c/sub\u003eO crystal. Therefore, the final morphology of Co\u003csub\u003ex\u003c/sub\u003eP is flake with micropores.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Electromagnetic parameters\u003c/h2\u003e\u003cp\u003eElectromagnetic parameters are the key factors affecting the microwave absorption performance, which were measured in X-band. The complex permittivity of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a-b). It could be found that the sample with the 0.1 mol/L NH\u003csub\u003e4\u003c/sub\u003eF had the highest complex permittivity with the average value of real and imaginary part are 6.5 and 3.1, respectively. As shown above, the ε\u0026prime; and ε\u0026prime;\u0026prime; values of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e increase with the increase of NH\u003csub\u003e4\u003c/sub\u003eF concentration. Recalling the SEM results, the NH\u003csub\u003e4\u003c/sub\u003eF concentration changes the content and morphology of Co\u003csub\u003ex\u003c/sub\u003eP, which indicates that the complex permittivity relies heavily on the absorbent content and morphology. Meanwhile, the ε\u0026prime;\u0026prime; values decrease evidently with the increasing frequency, which is due to the charge transfer and polarization behind the changing electric field.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eBased on the mixing law(lnε=\u0026sum;λ\u003csub\u003ei\u003c/sub\u003elnε\u003csub\u003ei\u003c/sub\u003e), the complex permittivity values of composite materials are strongly related to their components\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Owning to the much higher complex permittivity of Co\u003csub\u003ex\u003c/sub\u003eP than porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, the ε\u0026prime; and the ε\u0026prime;\u0026prime; values of the composites increases with increasing the content of Co\u003csub\u003ex\u003c/sub\u003eP. To the best of our knowledge, ε\u0026prime; describes the electromagnetic energy storage capability of materials, which is mainly controlled by polarization.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e For the investigated composites, it can be seen from the SEM images that the interfacial area will increase due to the increasing NH\u003csub\u003e4\u003c/sub\u003eF concentration. The increasing interface area is beneficial to the enhancement of the interface polarization, thus enhance the ε\u0026prime; value. Besides, the increasing content of absorbent will increase the number of dipoles in the composites. Under the effect of electromagnetic field, the intrinsic dipole moment of the dipole will be shifted to the direction of the external electric field, thus producing orientation polarization. In brief, the increasing number of dipoles enhances the orientation polarization, thereby increasing the ε\u0026prime; value.\u003c/p\u003e\u003cp\u003eAccording to free electron theory (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\epsilon\\:}^{\u0026rdquo;}={\\epsilon\\:}_{relax}^{\u0026rdquo;}+\\raisebox{1ex}{$\\sigma\\:$}\\!\\left/\\:\\!\\raisebox{-1ex}{$2\\pi\\:f{\\epsilon\\:}_{0}$}\\right.\\)\u003c/span\u003e\u003c/span\u003e),\u003csup\u003e30\u003c/sup\u003e the ε\u0026prime;\u0026prime; values is mainly related to the electrical conductivity(σ) and relaxation loss (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\epsilon\\:}_{relax}^{\u0026rdquo;}\\)\u003c/span\u003e\u003c/span\u003e). For the composites studied, the interface polarization and orientation polarization contribute to the increasing ε\u0026prime; value, and the related relaxation loss enhances the ε\u0026prime;\u0026prime; values. Besides, it can be seen from the Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e that the increasing NH\u003csub\u003e4\u003c/sub\u003eF concentration will lead to the increase of absorbent content and the formation of conductive network. The above two points are beneficial to improve the electrical conductivity of the composites, thus further enhance the ε\u0026prime;\u0026prime; values. To further confirm the analysis, the electrical conductivity of specimens was measured and the image is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The electrical conductivities of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e are 1.826\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e, 2.311\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e and 3.198\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e S/m, respectively, which is a strong support.\u003c/p\u003e\u003cp\u003eThe complex permeability of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d-e). It is evidently found that the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e have the lowest complex permeability when there is no NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution, which is mainly due to the low content of Co\u003csub\u003ex\u003c/sub\u003eP in the samples. For the other two samples, the sample prepared by the precursor solution containing 0.05 mol/L NH\u003csub\u003e4\u003c/sub\u003eF has higher \u0026micro;\u0026prime;, while the sample with 0.1 mol/L NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution has higher \u0026micro;\u0026prime;\u0026prime;. Compared with the sample prepared by the precursor solution containing 0.05 mol/L NH\u003csub\u003e4\u003c/sub\u003eF, the sample with 0.1 mol/L NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution has higher electrical conductivity, which will produce strong eddy current loss, thus reducing the \u0026micro;\u0026prime; value and enhancing the \u0026micro;\u0026prime;\u0026prime; value.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Meanwhile, the \u0026micro;\u0026prime; decrease with the increasing frequency for the composites, which is ascribed to the hysteresis of domain motion in relation to rapidly changing electromagnetic fields. The \u0026micro;\u0026Prime;(\u0026micro;\u0026prime;)\u003csup\u003e\u0026minus;2\u003c/sup\u003e(f)\u003csup\u003e\u0026minus;1\u003c/sup\u003e value is the parameter to determine whether eddy current loss is the main source of magnetic loss.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e When the value is constant with different frequency, the eddy current loss is the main source of magnetic loss. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, the main magnetic loss mechanism of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is exchange resonance and natural resonance.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Microwave absorption properties\u003c/h2\u003e\u003cp\u003eTo measure the microwave absorption properties of the composites, the RL value is calculated and its curve with frequency is shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea. It is evidently found that the minimum reflection loss (RL\u003csub\u003emin\u003c/sub\u003e) of the samples decreases first and then increases with the increasing NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution. Meanwhile, the sample prepared by the precursor solution containing 0.05 mol/L NH\u003csub\u003e4\u003c/sub\u003eF have stronger RL of -25.35 dB at 11.25 GHz, while the sample with 0.1 mol/L NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution have wider effective bandwidth of 3.8 GHz in 8.6\u0026ndash;12.4 GHz.\u003c/p\u003e\u003cp\u003eThe favorable impedance matching is a prerequisite for the material to have excellent microwave absorption properties, which ensures that the electromagnetic wave enters the absorbent as much as possible. Based on the transmission lines theory, the closer the impedance matching Z (Z=|Z\u003csub\u003ein\u003c/sub\u003e/Z\u003csub\u003e0\u003c/sub\u003e|) value is to 1, more electromagnetic waves can enter the absorbent for attenuation.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e The impedance matching of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with thickness of 2.8 mm was calculated and its diagram is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. It is easy to find that the impedance matching of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e increases in low frequency and decreases in high frequency with increasing frequency. Besides, when the NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution is 0.05 mol/L, the NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution has the best impedance matching. Altogether, this sample is most likely to have the best absorption properties.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eExcept for impedance matching, attenuation coefficient α is also an important parameter to determine the microwave absorption properties of materials. The attenuation coefficient describes the ability of the microwave absorption material to absorb electromagnetic waves entering the material and is calculated by the following formula:\u003csup\u003e36\u0026ndash;37\u003c/sup\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{\\alpha\\:}=\\frac{\\sqrt{2}{\\pi\\:}\\text{f}}{\\text{c}}\\times\\:\\sqrt{\\left({{\\mu\\:}}^{{\\prime\\:}{\\prime\\:}}{{\\epsilon\\:}}^{{\\prime\\:}{\\prime\\:}}-{{\\mu\\:}}^{{\\prime\\:}}{{\\epsilon\\:}}^{{\\prime\\:}}\\right)+\\sqrt{{\\left({{\\mu\\:}}^{{\\prime\\:}{\\prime\\:}}{{\\epsilon\\:}}^{{\\prime\\:}{\\prime\\:}}-{{\\mu\\:}}^{{\\prime\\:}}{{\\epsilon\\:}}^{{\\prime\\:}}\\right)}^{2}+{\\left({{\\mu\\:}}^{{\\prime\\:}{\\prime\\:}}{{\\epsilon\\:}}^{{\\prime\\:}}+{{\\mu\\:}}^{{\\prime\\:}}{{\\epsilon\\:}}^{{\\prime\\:}{\\prime\\:}}\\right)}^{2}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec depicts attenuation coefficients of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with the thickness of 2.8 mm. It is obviously found that the attenuation coefficient increases with the addition of NH\u003csub\u003e4\u003c/sub\u003eF. When the NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution is 0.1 mol/L, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e have the best attenuation coefficient, but its poor impedance matching cause most electromagnetic waves reflect on the surface of the composites. Therefore, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e prepared by the precursor solution containing 0.05 mol/L NH\u003csub\u003e4\u003c/sub\u003eF possess the promising microwave absorption properties due to its best impedance matching and better attenuation coefficient.\u003c/p\u003e\u003cp\u003eThe reflection loss and the typical effective absorption bandwidth of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with different NH\u003csub\u003e4\u003c/sub\u003eF concentration in precursor solution are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a-c), when the concentration of NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution increases, the minimum reflection loss of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e tend to appear in the region of thin thickness and high frequency. When the concentration of NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution is 0.1 mol/L, the minimum reflection loss value of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is -45.48 dB, and its effective absorption bandwidth is 2.1 GHz (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). Compared with the other two samples, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with 0.05 mol/L NH\u003csub\u003e4\u003c/sub\u003eF in precursor solution has the best microwave absorption performance. When the thickness is 3.2 mm, its minimum reflection loss is -35.82 dB at 10.55 GHz, and its effective absorption bandwidth is 3.99 GHz (95% X-band) of 8.2\u0026ndash;12.19 GHz (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is comparable or even better than most reported absorbers.\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\u003eMicrowave Absorption Performance of Porous Ceramic\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" 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\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMaterials\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eRLmin/dB\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eThickness/mm\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eEAB/GHz\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRef.\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSi-O-C\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-39.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.64 (7.6\u0026ndash;12.24)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[12]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCNWs-SiO\u003csub\u003e2\u003c/sub\u003e/3Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026middot;2SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.2 (8.2\u0026ndash;12.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[38]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePDC-SiC/Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7 (11\u0026ndash;18)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[39]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC-rich SiC NWs/Sc\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-35.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.2 (8.2\u0026ndash;12.4)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[40]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCo\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-35.82\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e3.99 (8.2\u0026ndash;12.19)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003ethis work\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\u003eBased on the analysis above, the microwave absorption mechanisms of Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e can be simply depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The excellent microwave absorption performance of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e mainly come from the following points. First, the air in the pore can effectively regulate the electromagnetic parameter of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to obtain better microwave absorption. In addition, the special structure of porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and branch-like Co\u003csub\u003ex\u003c/sub\u003eP in the composite makes the incident electromagnetic wave have multiple reflection inside the material, which effectively prolongs the propagation path of electromagnetic wave, thus effectively attenuates the electromagnetic wave. In the end, the existence of branch-like Co\u003csub\u003ex\u003c/sub\u003eP and the porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e makes the composites have many heterogeneous interfaces, which leads to more interfacial loss to consume the electromagnetic wave.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn the present work, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was prepared and its morphology, electromagnetic parameters and microwave absorption properties was investigated. According to the test results, the mole ratio of CoCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO, NH\u003csub\u003e4\u003c/sub\u003eF and CO(NH\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e in the precursor solution is 1:1:5 and the thickness of the sample is 3.2 mm, the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e has the positive microwave absorption properties with reflect loss of -35.8 dB at 9.9 GHz and effective absorption bandwidth of 4 GHz (95% of X-band) in 8.2\u0026ndash;12.2 GHz. Meanwhile, the excellent absorption performance of the Co\u003csub\u003ex\u003c/sub\u003eP-porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e comes from the effective regulation of electromagnetic parameters by air, multiple reflections caused by the special structure, enhanced conduction loss and interfacial loss.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eCRediT authorship contribution statement Jiao-jiao YU: Conceptualization, Methodology, Data curation,Investigation,Writ-original draft; Liang ZHOU: Formal analysis, Methodology, Investigation, Writing \u0026ndash; review \u0026amp; editing; Tao-tao AN: Methodology, Writing \u0026ndash; Review \u0026amp; editing; Hong-bo WANG: Data curation, Validation, Supervision,Writing \u0026ndash; Review \u0026amp; editing; Ming-kang ZHANG: Methodology, Writing \u0026ndash; Review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work is supported by Key Research and Development Program in Shaanxi Province of China (No. 2025CY-YBXM-128) and Fundamental Research Funds for the Central Universities of the Chang'an University (No. 300102313201).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eL. 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Cheng, In situ growth of one-dimensional carbon-rich SiC nanowires in porous Sc\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e ceramics with excellent microwave absorption properties. Ceram. Int. \u003cb\u003e44\u003c/b\u003e, 22784\u0026ndash;22793 (2018)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"CoxP@Al2O3, Hydrothermal method, Low-temperature phosphating, Complex permittivity, Microwave absorption","lastPublishedDoi":"10.21203/rs.3.rs-7727233/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7727233/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMaterials with porous structure gradually become the research hotspot as microwave absorption materials (MAMs) due to their low density and high specific surface area. In this paper, the porous Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ceramics decorated with Co\u003csub\u003ex\u003c/sub\u003eP were prepared by hydrothermal and low-temperature phosphating. The morphology, complex permittivity and reflection loss were explored to study their potential microwave absorption properties. The results show that the Co\u003csub\u003ex\u003c/sub\u003eP content can be regulated by adjusting the concentration of NH\u003csub\u003e4\u003c/sub\u003eF in the precursor solution. Meanwhile, the and microstructure and complex permittivity could be adjusted by the addition of NH\u003csub\u003e4\u003c/sub\u003eF. As the concentration of NH\u003csub\u003e4\u003c/sub\u003eF is 0.05 mol/L, Co\u003csub\u003ex\u003c/sub\u003eP@Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites exhibit excellent microwave absorption properties with reflection loss of -35.8 dB at 9.9 GHz and effective absorption bandwidth of 4 GHz in the X-band at the thickness of 3.2mm. This study provides a new way for the effective application of porous ceramic as functional materials, and the investigated Co\u003csub\u003ex\u003c/sub\u003eP@Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composites can be qualified as favorable MAMs for broadband and efficient microwave absorption.\u003c/p\u003e","manuscriptTitle":"Complex permittivity and remarkable microwave absorption of hydrothermally synthesized CoxP-loaded porous Al2O3 composites","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-28 16:30:30","doi":"10.21203/rs.3.rs-7727233/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d0409d7c-194c-4871-8881-35f0c29f6ecd","owner":[],"postedDate":"October 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-06T02:38:10+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-28 16:30:30","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7727233","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7727233","identity":"rs-7727233","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}