Congruent evaporation of borosilicate glass strengthening silica fiber

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A. Eronyan, A. Yu. Kulesh, M. K. Tsibinogina, D. V. Glita, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4029039/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract The work aims to study the evaporation process of borosilicate glass used for strengthening of silica fiber light guides. The composition of glass after high-temperature treatment was determined based on refractive index measurement. It was found that at temperature 2100 o C two-component SiO 2 -B 2 O 3 glass vaporizes without changing its composition at a boron oxide content of ≈ 4 mol% in both oxygen and nitrogen atmospheres. The IR absorption spectra of borosilicate glass indicate the absence of boroxol rings of B 2 O 3 , which can explain the low partial pressure of its vapors compared to the ideal melt state. It is experimentally shown that the content of only 1 mol % B 2 O 3 in the outer cladding of silica fiber increases its strength by 10% when the drawing temperature is reduced to 1900°C. Such a small addition of boron oxide to silica glass decreases its viscosity, ensuring the occurrence of compressive stresses in the borosilicate cladding. borosilicate glass temperature pressure congruent evaporation silica fiber strength Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1 Introduction Optical fibers (OF) based on silica glass feature unique transparency, high mechanical strength and radiation-optical stability. As known, lowering the drawing temperature of optical fibers helps to reduce their optical losses [ 1 , 2 ] and increase their radiation-optical stability [ 3 ]. However, at the same time, the drawing force of fiber increases, reducing its strength [ 1 ]. The surface layer of borosilicate glass on silica glass fiber, creating compressive stresses, contributes to its strength [ 4 ]. It increases with lowering the drawing temperature [ 5 , 6 ]; this is due to the occurrence of compressive stresses in an easy-melting borosilicate cladding when the fiber drawing force increases. However, the application of this effective technological technique has not been widely used, probably, because of two main reasons. First, the thermal expansion coefficient (TEC) of the surface cladding doped with B 2 O 3 is greater than that of the silica glass preform [ 7 ]. Therefore, cracks may occur in the borosilicate layer after its high-temperature sintering as the preform cools down. Second, the equilibrium vapor pressure over B 2 O 3 melt is much higher than over SiO 2 . Therefore, the boron oxide vaporization rate of borosilicate glass is also higher than that of silicon oxide. This decreases boron in the cladding and increases its viscosity during high-temperature fiber drawing and, consequently, to weakens of the strengthening effect. Nevertheless, the results of experimental studies [ 5 ] confirm the effectiveness of such a technical solution. As for the high evaporation rate of boron oxide, it appears to not completely leave the silica glass during high-temperature treatment. Thus, when heating the core containing B 2 O 3 and GeO 2 in the modified method of chemical vapor deposition (MCVD), the tube compression process at a temperature of more than 2000°C leads to the evaporation of germanium dioxide, but boron oxide does not completely evaporate [ 8 ]. This behavior of B 2 O 3 may indicate a significant decrease in the rate of evaporation of boron oxide compared to the ideal state of the silicon and boron oxides melt. This work aims to determine the composition of borosilicate cladding glass after high-temperature treatment, as well as the dependence of the OF strength with such a cladding on the temperature of its drawing. 2 Materials and methods Experimental studies of borosilicate glass evaporation were carried out on an automated complex OFC-12-729 (Nextrom). For the experiments a silica glass tube of F 300 grade was used with an outer diameter of 25 mm and a wall thickness of 3 mm. Its heating temperature was measured with an optical IR pyrometer. Layers of silica glass doped with B 2 O 3 in an amount of approximately 10 mol% were applied to the inner surface of the tube by the MCVD method. In the process of high-temperature compression, the inner channel of the tube was purged with nitrogen or oxygen in three passes of the burner at a temperature of 2100 ºC (Fig. 1 ). High-purity nitrogen contained no more than 1·10 − 4 vol% of impurity oxygen. Then, the tube with deposited glass layers was collapsed into a rod to eliminate the inner channel. In the sample thus obtained, the radial refractive index (RI) profile was measured on a P-101 refractometer to analyze the change in the composition of borosilicate glass during its high-temperature evaporation. The concentration of B 2 O 3 in silica glass (C B2O3 , mol%) was determined on the basis of the known dependence of the RI change (Δ n) on its content [ 8 ]: Δn = (− 0,51C B2O3 + 0,0076 C 2 B2O3 )·10 − 3 , (1) It should be noted here that due to the small mass of boron atom, the measurement of its content in glass by X-ray microanalyzer is impossible [ 9 ]. To investigate the strengthening effect of OF due to borosilicate cladding, a preform with a germanosilicate core was fabricated by the MCVD method [ 5 ]. Then, the outer chemical vapor deposition method was applied to deposit a porous silicon dioxide layer on the preform, then the porous layer was impregnated with saturated aqueous B 2 O 3 solution [ 10 ]. The B 2 O 3 content in the dry porous layer of the cladding was evaluated by IR spectrometry on Bruker Tensor 37 spectrometer in the emission range of 4000 cm -1 − 600 cm -1 with a resolution of 2 cm -1 . For this purpose, IR absorption spectra of pure oxide powders were obtained (Fig. 2 ), indicating the possibility of quantitative analysis of boron oxide by the ratio of intensity of absorption bands for B 2 O 3 (3200 cm -1 ) and SiO 2 (1100 cm -1 ). IR spectra of powders with different B 2 O 3 content were taken to determine the porous borosilicate cladding composition. At the same time, absorption spectra of borosilicate glass of different compositions were obtained to determine the specificity of boron incorporation into the matrix of silica glass. Glass samples with different boron oxide content were prepared by mixing powdered oxides (SiO 2 and B 2 O 3 ) and heating them at a temperature of 1000 ºC for 1 hour or melting them in an oxygen-hydrogen flame. The outer porous layer of the preform containing boron oxide was melted at a temperature of ≈ 1800 ºC. Then, the fiber with a diameter of 125 µm was drawn out of it at a rate of 30 m/min with simultaneous application of a single-layer UV-curing epoxyacrylate coating with a thickness of 45 µm. The preform was heated in a furnace with a graphite heater, which was shielded from the glass by a stream of high-purity argon containing no more than 10 − 4 vol% of impurity oxygen and moisture. The OF sections were drawn at heater temperatures of 1900, 2000 and 2100 ºC and the drawing force of 3.5, 1.2 and 0.5 N, respectively. Under the same conditions, fiber was drawn from a similar preform without borosilicate claddingto evaluate its strengthening effect. Fiber strength (σ) was measured by two-point bending at room temperature and calculated according to formula [ 11 ]: σ = Е 0 [1 + 6.9/2(1.219 d / D -1.137( d / D ) 2 )](1.219 d / D -1.137( d / D ) 2 ), where E 0 - elastic modulus of silica glass, equal to 73.5 GPa; d - diameter of glass fiber; D - diameter of the neutral axis of the bent fiber at its destruction. When evaluating the strength of silica fiber with borosilicate cladding, it was assumed that a small addition of boron oxide to silica glass could insignificantly affect its elastic modulus. To increase the statistical significance of the results, 20 measurements were made for each fiber drawing temperature. The error of the measured strength value equal to 2 standard deviations of its arithmetic mean (0.08 GPa) corresponds to 97% of its probability level. To evaluate the thickness of the borosilicate layer on the fiber, its diameter was measured during etching in 10% hydrofluoric acid. 3 Thermodynamic analysis of vapor composition during evaporation of borosilicate glass According to the Hertz-Knudsen equation, the vaporization rate of a substance is proportional to its pressure. If the pressure of borosilicate glass components is different, the glass composition changes during its high-temperature processing. Therefore, it is reasonable to evaluate the temperature dependence of the equilibrium pressure over silicon and boron oxides. Boron-doped silica glass evaporates forming the following major gaseous components: SiO 2 , B 2 O 3 , SiO and B 2 O 2 . The pressure of these substances above the pure oxide melts is determined by the corresponding equilibrium processes: SiO 2 (l) = SiO 2 (g), (2) B 2 O 3 (l) = B 2 O 3 (g), (3) SiO 2 (l) = SiO(g) + 0,5 O 2 (g), (4) B 2 O 3 (l) = B 2 O 2 (g) + 0,5 O 2 (g), (5) where (l) and (g) denote the liquid and gaseous states of matter, respectively. According to the Gibbs phase rule, the first two processes have one degree of freedom. Therefore, the equilibrium pressure of gases depends only on temperature. The equilibrium (4, 5) have two degrees of freedom and are determined by the temperature and equilibrium pressure of oxygen. The temperature dependence of the pressure of gaseous components SiO 2 and B 2 O 3 (Pi) over pure oxides (Fig. 3 ) is borrowed from [ 12 , 13 ]. Using the data from the reference book for the free energy of formation of substances [ 14 ], we calculated the temperature dependence of the equilibrium constant for reactions (4, 5) and the pressure of gaseous components containing silicon or boron at two values of the equilibrium oxygen pressure: 0.1 and 2∙10 4 Pa (Fig. 4 ). The high oxygen pressure (2∙10 4 Pa) corresponds to the conditions of flame treatment of the preform in air atmosphere, and its low pressure (0.1 Pa) is determined by the conditions of its heating during high-temperature fiber drawing in atmosphere of high-purity argon or nitrogen. 4 Experimental results The radial profile of RI preform with borosilicate glass after its heating at 2100 o C indicates that boron oxide does not completely evaporate in both nitrogen (Fig. 5 a) and oxygen (Fig. 5 b) atmospheres. The glass evaporation occurs without changing its composition. The content of B 2 O 3 in such glass leading to a change in the silica glass RI by about 0.002 is 4 mol% according to Eq. (1). The RI radial profile of a preform with a germanosilicate core and borosilicate cladding (Fig. 6 , 7 ) revealed a peripheral region with reduced refraction. The RI difference between this cladding and the bordering pure silica glass cladding is ≈ 0.0004, which corresponds to the content of 0.8 +/- 0.2 mol% B 2 O 3 according to Eq. (1). The IR spectrum of the porous borosilicate cladding before sintering by estimating the absorbance in the 3200 cm − 1 region gave a close value of 1.0 +/- 0.2 mol%. The slight decrease in the boron oxide content may be due to its evaporation while sintering the porous cladding layer with B 2 O 3 . The preform radial profile of RI (Fig. 6 , 7 ) indicates that on a 125 µm diameter fiber the thickness of borosilicate cladding equals ≈ 5 µm. The discontinuous change of RI at the cladding outer boundary (Fig. 7 ) indicates that there is no change in its composition during high-temperature sintering of the porous SiO 2 layer containing boron oxide. At the same time, a transition zone with a RI gradient is observed at the boundary of the cladding with silica glass, which indicates boron diffusion. The silica fiber strength measurement (Fig. 8 ) showed that the borosilicate cladding provided a 10% increase in silica fiber strength (from 5.9 to 6.55 GPa) when its drawing temperature was reduced to 1900°C. Fiber strengthening due to the occurrence of compressive stresses in the cladding does not affect the statistical nature of the scatter of strength measurement data. As known, this depends on the distribution of microinhomogeneity of impurity nature [ 11 ]. The source of defects can be dust particles in the atmosphere of the graphite furnace used for fiber drawing [ 15 ]. When such defects are completely eliminated, the experimentally measured strength of silica fiber in the conditions of natural humidity reaches 26 GPa [ 16 ]. At complete removal of moisture from the surrounding gas atmosphere, fiber strength can be higher than 40 GPa, which corresponds to theoretical estimates. Soaking the fibers in hydrofluoric acid solution (Fig. 9 ) revealed an increased etching rate of the borosilicate cladding compared to the silica glass fiber. From the results of this experiment, we can estimate the thickness of the borosilicate cladding to be ≈ 3 µm. This value within the measurement error corresponds to the estimate of the borosilicate cladding thickness (5 µm) obtained from the radial profile of the preform RI (Fig. 6 , 7 ). This slight difference in the estimated cladding thickness may also be due to its evaporation during the fiber drawing process. The IR absorption spectrum of the silicon and boron oxide mixture after its high temperature treatment (Fig. 10 ) showed the presence of two bands at 1400 and 950 cm − 1 . The absorption band at 1400 cm − 1 indicates a three-coordinated boron atom, and band at 950 cm − 1 indicates their four-coordinated state in the silica glass melts [ 15 ]. The ratio of boron coordination [BO 3 ] and [BO 4 ] does not depend on temperature. The absorption band in the region of 3200 cm − 1 , which is the characteristic of pure boron oxide (Fig. 2 ), is absent in the IR spectrum of borosilicate glass. 5 Discussions The comparison of temperature dependence of pressure of gases over pure silicon and boron oxides (Figs. 3 and 4 ) indicates that regardless of the equilibrium oxygen pressure, SiO 2 and B 2 O 3 molecules are the main vapor components. The pressure of the boron-containing gases is much higher than that of the silicon-containing components. In the case of an ideal state of boron oxide solution in silica glass this should lead to complete vaporization of the volatile oxide. However, the experimental results (Fig. 5 ) demonstrate the phenomenon of congruent evaporation at a B 2 O 3 content of ≈ 4 mol%. Such a phenomenon was observed earlier in similar experiments during high-temperature treatment of borosilicate glass [ 16 ], but this fact was not given due attention. The absorption bands at 1400 and 950 cm -1 in borosilicate glass (Fig. 10 ) are caused by vibrations of trigonally [BO 3 ] and tetrahedrally [BO 4 ] coordinated boron groups included in the silica glass matrix. Boron oxide is fully incorporated into the structure of silica even at such a low temperature as 1000 o C. The absorption bands by boroxol rings are observed in the IR absorption spectra of pure boron oxide (Fig. 1 ), but they are absent in borosilicate glass. This structural rearrangement of boron may serve as an explanation for the decrease in its vaporization rate when dissolved in silica glass. The high equilibrium vapor pressure over pure B 2 O 3 is due to the specific structure of boroxyl rings. These rings form flat networks linked by weak van der Waals forces. There is no chemical interaction of oxides in borosilicate glass [ 17 ]. Therefore, the deviation of vapor pressure of boron-containing gases over borosilicate glass from the ideal solution state is caused only by the specificity of the structural transformation of boron in the glass matrix. In addition, it should be noted that the energy of the B-O bond (496 kJ/mol) is greater than that of the Si-O bond (444 kJ/mol). The structural transformation of boron with the decrease of its content in borosilicate glass leads to a significant decrease in its TEC compared to the linear dependence [ 7 ]. The effect of compressive stress formation in the borosilicate cladding of silica fiber in our experiments (0.6 GPa) is the same as in similar studies [ 18 ] (85 ksi). However, the strenthening cladding of our fibers contained significantly less B 2 O 3 (1 instead of 10 mol%), and the fiber drawing force in our studies was almost two times less (3.5 instead of 6.8 N). Thus, the hardening efficiency of the borosilicate layer increases with decreasing boron oxide content. This may be due to the competition of two mechanisms of stress generation in borosilicate cladding. According to one mechanism, compressive stresses in the cladding grow proportionally to the fiber drawing force [ 4 ], and according to another mechanism, tensile stresses arise due to the increased TEC of borosilicate glass compared to the TEC of silica glass fiber. In the preform fabrication, the first mechanism of forming compressive stresses does not work, and the second mechanism can lead to the formation of cracks in the borosilicate layer. Therefore, to strengthen silica fiber with borosilicate cladding, the content of B 2 O 3 in it should not exceed 1 mol%. This guarantees to exclude the formation of cracks in the preform borosilicate layer if the porous cladding is sintered at the stage of OF drawing [ 10 ]. A more significant advantage of the silica optical fiber technology with borosilicate cladding, along with increasing its strength, is the possibility to reduce the temperature of its drawing. This can significantly minimize the optical losses of light guides, which are due to the fundamental mechanism of Rayleigh scattering [ 21 ]. Moreover, changing the temperature of silica fiber drawing from 2100 to 1900°C leads to a tenfold decrease in SiO 2 vapor pressure (Fig. 3 ), therefore, reducing the chemical corrosion of the graphite heater. 6 Conclusions The work results have shown that during high-temperature treatment of borosilicate glass, boron oxide does not completely evaporate from the glass, in both oxidizing and neutral gas atmosphere. At 2100°C, this glass vaporizes congruently withB 2 O 3 content of ≈ 4mol%. The strength of silica optical fibers increases by 10% in the presence of an outer cladding doped with 1mol% B 2 O 3 with decreasing their drawing temperature to 1900°C. The efficiency of compressive stresses formation in such a borosilicate cladding of silica fiber is higher than that in similar fibers containing 10% boron oxide in the cladding. The results of studies have shown the possibility of drawing silica optical fiber at low temperatures, which is of particular importance for obtaining promising light guides for long-distance communication lines with particularly low optical losses. It should be noted that the effect of strengthening of silica fiber with low-viscosity cladding can be provided by its doping with fluorine [ 22 ]. However, for stability of fluorosilicate glass composition during fiber drawing it is necessary to create an atmosphere of chemically aggressive fluorine-containing gas. Declarations Acknowledgements The authors are thankful to Academician of the Russian Academy of Sciences, Professor V. G. Peshekhonov for supporting this work and A.V. Khokhlov for the scientific and technical helps. Author contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by A.Yu. Kulesh, M.K. Tsibinogina, D.V. Glita, A.A. Untilov, V. E. Sitnikova, K.V. Dukelskiy, P.A. Zlobin I.K. Meshkovskiy. The first draft of the manuscript was written by M.A. Eronyan. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Availability of Data and Materials The authors of the manuscript declare the availability of the data and materials of the manuscript. Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. Competing Interests The authors declare no competing interests. Consent to Participate All authors agreed to participate in the authorship of the manuscript. Consent for Publication All authors gave their consent to publish the manuscript. Ethics Approval The authors assure that: the described work has not been previously published elsewhere; it is not being considered for publication elsewhere; the publication has been approved by all co-authors and responsible persons at the institutions where the work was done; all sources used are properly disclosed; all authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content. Conflict of Interest The authors declare that they have no financial or non-financial conflict of interest. References Oh PS, Mcalarney JJ, Nath DK (1983) Effect of Fiber Drawing Tension on Optical and Mechanical Properties of Optical Fiber Waveguides. J Am Ceram Soc 66:C84-C85. https://doi.o?rg/10.1111/j.1151-2916.1983.tb10061.x Bisyarin MA, Eronyan MA, Kulesh AY, Meshkovskiy IK, Reutsky AA, Scheglov AA, Ustinov SV (2017) Light-emitting optical fibers with controllable anomalous small-angle scattering. 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Dukel'skii KV, Eronyan MA, Komarov AV, Kondrat'ev YuN, Levit LG, Romashova EI, Serkov MM, Khokhlov AV, and Shevandin VS (2002) MCVD technology for single-mode low-damping fiber lightguides stable against microbends. J Opt Technol 69: 849 https://opg.optica.org/jot/abstract.cfm?URI=jot-69-11-849 Eronyan MA, Tsibinogina, MK Zlobin PA (2004) Method of high-temperature chemical treatment of glass surface, Patent RU 2272003. Additional Declarations No competing interests reported. Supplementary Files Graphicalabstract.png Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 08 Apr, 2024 Reviews received at journal 27 Mar, 2024 Reviewers agreed at journal 25 Mar, 2024 Reviewers invited by journal 24 Mar, 2024 Submission checks completed at journal 19 Mar, 2024 Editor assigned by journal 19 Mar, 2024 First submitted to journal 07 Mar, 2024 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. 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-4029039","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":281602698,"identity":"e472083e-414f-4b37-b60a-ea405f320ca7","order_by":0,"name":"M. A. 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Sitnikova","email":"","orcid":"","institution":"St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University","correspondingAuthor":false,"prefix":"","firstName":"V.","middleName":"E.","lastName":"Sitnikova","suffix":""},{"id":281602704,"identity":"9bf264e1-d8c7-4aca-8fd8-ed36e605d628","order_by":6,"name":"K. V. Dukelskiy","email":"","orcid":"","institution":"JSC “Research and Production Corporation S.I. Vavilov”","correspondingAuthor":false,"prefix":"","firstName":"K.","middleName":"V.","lastName":"Dukelskiy","suffix":""},{"id":281602705,"identity":"cded441b-8465-4cf4-8aac-6b250745c037","order_by":7,"name":"P. A. Zlobin","email":"","orcid":"","institution":"JSC “Research and Production Corporation S.I. Vavilov”","correspondingAuthor":false,"prefix":"","firstName":"P.","middleName":"A.","lastName":"Zlobin","suffix":""},{"id":281602709,"identity":"baba9680-618a-4fee-82f3-03faebf84b77","order_by":8,"name":"I. K. Meshkovskiy","email":"","orcid":"","institution":"St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University","correspondingAuthor":false,"prefix":"","firstName":"I.","middleName":"K.","lastName":"Meshkovskiy","suffix":""}],"badges":[],"createdAt":"2024-03-07 17:06:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4029039/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4029039/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":53229811,"identity":"15cc02bc-4671-4e4a-b91b-3a7d42c1ef3c","added_by":"auto","created_at":"2024-03-22 07:12:38","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":12564,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the experiment for vaporization of a borosilicate glass layer inside a tube heated to 2100 ºC\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/1c44490c7985abeaccab1003.png"},{"id":53229257,"identity":"2bf6ab59-9b1c-4b05-ad5b-b09768624644","added_by":"auto","created_at":"2024-03-22 07:04:37","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":11581,"visible":true,"origin":"","legend":"\u003cp\u003eIR absorption spectra of porous samples of pure oxides: SiO\u003csub\u003e2\u003c/sub\u003e (red line) and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (green line)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/d7b93b31f528e27b167110a5.png"},{"id":53229813,"identity":"c28143ab-adc9-4ed4-9d9c-34d64a66fc4d","added_by":"auto","created_at":"2024-03-22 07:12:38","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":6364,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature dependence of the pressure of gaseous oxides SiO\u003csub\u003e2\u003c/sub\u003e (red line) and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (green line) over pure SiO\u003csub\u003e2 \u003c/sub\u003eand B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3 \u003c/sub\u003emelts\u0026nbsp;\u0026nbsp;\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/625ccab3737857a4ff1a4dbc.png"},{"id":53229262,"identity":"dd0da0b0-cbb6-4b69-bc8e-2cef12319509","added_by":"auto","created_at":"2024-03-22 07:04:38","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":7878,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature dependence of the pressure of SiO (red line) and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (green line) gases over pure SiO\u003csub\u003e2\u003c/sub\u003e and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3 \u003c/sub\u003emelts at equilibrium oxygen pressure of 2·10\u003csup\u003e4\u003c/sup\u003e Pa (solid line) and 0.1 Pa (dashed line)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/24fa698c31abfbef2ddc7310.png"},{"id":53229265,"identity":"32bd3bff-5685-4067-b8a1-2a29f3078208","added_by":"auto","created_at":"2024-03-22 07:04:38","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":11627,"visible":true,"origin":"","legend":"\u003cp\u003eRI radial profile of preform compressed by nitrogen (a) and oxygen (b) purging of the inner channel\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/0bf032cbcba2d8336c7cb328.png"},{"id":53229263,"identity":"2f8a1678-aba1-440b-a934-7cef819cc5b5","added_by":"auto","created_at":"2024-03-22 07:04:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":7001,"visible":true,"origin":"","legend":"\u003cp\u003eRadial RI profile of perform\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/0db500987b00e6f8fdaae24e.png"},{"id":53229260,"identity":"e8a228dc-e8d5-4ea5-a494-26cb9d7c422b","added_by":"auto","created_at":"2024-03-22 07:04:37","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":7929,"visible":true,"origin":"","legend":"\u003cp\u003eRadial profile of borosilicate cladding RI\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/4790e8b99bbf94da1cfe6b9b.png"},{"id":53229810,"identity":"ed446ea8-3d36-4e0e-b6cf-5032133f8b60","added_by":"auto","created_at":"2024-03-22 07:12:37","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":11028,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the effect of drawing temperature on the OF strength with borosilicate cladding (green line) and without cladding (red line)\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/30b6fea4051babee47d93043.png"},{"id":53229264,"identity":"663ea9e1-0347-4f43-b05e-5c5b8698393c","added_by":"auto","created_at":"2024-03-22 07:04:38","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":11848,"visible":true,"origin":"","legend":"\u003cp\u003eTime dependence (t) of the layer thickness (δ) removed by etching from the surface of pure silica glass (red line) and borosilicate cladding fiber (green line). The dot size corresponds to the accuracy of the fiber diameter measurement\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/9badc0f3e80751fcc3830d38.png"},{"id":53229267,"identity":"64b3f738-e9b2-4c52-a691-a07315ec2307","added_by":"auto","created_at":"2024-03-22 07:04:38","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":10780,"visible":true,"origin":"","legend":"\u003cp\u003eIR spectra of SiO\u003csub\u003e2\u003c/sub\u003e + B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3 \u003c/sub\u003epowder mixture: boron oxide content of 2 mol% (dashed line) and 10 mol% (solid line) after heating at 1000\u003csup\u003eo\u003c/sup\u003eC (blue line) and melting in a burner flame (red line)\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/c5abf876d3f9692d49308019.png"},{"id":53230428,"identity":"cafc3062-2579-447a-a659-325a9dc5f171","added_by":"auto","created_at":"2024-03-22 07:20:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":340994,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/b7532754-de02-45f9-a210-f20d534d5125.pdf"},{"id":53229256,"identity":"482b8f6f-b47e-4a45-8880-f5fcd3cc865c","added_by":"auto","created_at":"2024-03-22 07:04:37","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11945,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.png","url":"https://assets-eu.researchsquare.com/files/rs-4029039/v1/1e46290a3455dde412919a94.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Congruent evaporation of borosilicate glass strengthening silica fiber","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eOptical fibers (OF) based on silica glass feature unique transparency, high mechanical strength and radiation-optical stability. As known, lowering the drawing temperature of optical fibers helps to reduce their optical losses [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] and increase their radiation-optical stability [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, at the same time, the drawing force of fiber increases, reducing its strength [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe surface layer of borosilicate glass on silica glass fiber, creating compressive stresses, contributes to its strength [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. It increases with lowering the drawing temperature [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]; this is due to the occurrence of compressive stresses in an easy-melting borosilicate cladding when the fiber drawing force increases. However, the application of this effective technological technique has not been widely used, probably, because of two main reasons.\u003c/p\u003e \u003cp\u003eFirst, the thermal expansion coefficient (TEC) of the surface cladding doped with B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is greater than that of the silica glass preform [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Therefore, cracks may occur in the borosilicate layer after its high-temperature sintering as the preform cools down.\u003c/p\u003e \u003cp\u003eSecond, the equilibrium vapor pressure over B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e melt is much higher than over SiO\u003csub\u003e2\u003c/sub\u003e. Therefore, the boron oxide vaporization rate of borosilicate glass is also higher than that of silicon oxide. This decreases boron in the cladding and increases its viscosity during high-temperature fiber drawing and, consequently, to weakens of the strengthening effect.\u003c/p\u003e \u003cp\u003eNevertheless, the results of experimental studies [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] confirm the effectiveness of such a technical solution. As for the high evaporation rate of boron oxide, it appears to not completely leave the silica glass during high-temperature treatment. Thus, when heating the core containing B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and GeO\u003csub\u003e2\u003c/sub\u003e in the modified method of chemical vapor deposition (MCVD), the tube compression process at a temperature of more than 2000\u0026deg;C leads to the evaporation of germanium dioxide, but boron oxide does not completely evaporate [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. This behavior of B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e may indicate a significant decrease in the rate of evaporation of boron oxide compared to the ideal state of the silicon and boron oxides melt.\u003c/p\u003e \u003cp\u003eThis work aims to determine the composition of borosilicate cladding glass after high-temperature treatment, as well as the dependence of the OF strength with such a cladding on the temperature of its drawing.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cp\u003eExperimental studies of borosilicate glass evaporation were carried out on an automated complex OFC-12-729 (Nextrom). For the experiments a silica glass tube of F 300 grade was used with an outer diameter of 25 mm and a wall thickness of 3 mm. Its heating temperature was measured with an optical IR pyrometer.\u003c/p\u003e \u003cp\u003eLayers of silica glass doped with B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in an amount of approximately 10 mol% were applied to the inner surface of the tube by the MCVD method. In the process of high-temperature compression, the inner channel of the tube was purged with nitrogen or oxygen in three passes of the burner at a temperature of 2100 \u0026ordm;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). High-purity nitrogen contained no more than 1\u0026middot;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e vol% of impurity oxygen.\u003c/p\u003e \u003cp\u003eThen, the tube with deposited glass layers was collapsed into a rod to eliminate the inner channel. In the sample thus obtained, the radial refractive index (RI) profile was measured on a P-101 refractometer to analyze the change in the composition of borosilicate glass during its high-temperature evaporation.\u003c/p\u003e \u003cp\u003eThe concentration of B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in silica glass (C\u003csub\u003eB2O3\u003c/sub\u003e, mol%) was determined on the basis of the known dependence of the RI change (Δ n) on its content [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003eΔn = (\u0026minus;\u0026thinsp;0,51C\u003csub\u003eB2O3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;0,0076 C\u003csup\u003e2\u003c/sup\u003e\u003csub\u003eB2O3\u003c/sub\u003e)\u0026middot;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, (1)\u003c/p\u003e \u003cp\u003eIt should be noted here that due to the small mass of boron atom, the measurement of its content in glass by X-ray microanalyzer is impossible [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo investigate the strengthening effect of OF due to borosilicate cladding, a preform with a germanosilicate core was fabricated by the MCVD method [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Then, the outer chemical vapor deposition method was applied to deposit a porous silicon dioxide layer on the preform, then the porous layer was impregnated with saturated aqueous B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e solution [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content in the dry porous layer of the cladding was evaluated by IR spectrometry on Bruker Tensor 37 spectrometer in the emission range of 4000 cm\u003csup\u003e-1\u003c/sup\u003e \u0026minus;\u0026thinsp;600 cm\u003csup\u003e-1\u003c/sup\u003e with a resolution of 2 cm\u003csup\u003e-1\u003c/sup\u003e. For this purpose, IR absorption spectra of pure oxide powders were obtained (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), indicating the possibility of quantitative analysis of boron oxide by the ratio of intensity of absorption bands for B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (3200 cm\u003csup\u003e-1\u003c/sup\u003e) and SiO\u003csub\u003e2\u003c/sub\u003e (1100 cm\u003csup\u003e-1\u003c/sup\u003e). IR spectra of powders with different B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content were taken to determine the porous borosilicate cladding composition.\u003c/p\u003e \u003cp\u003eAt the same time, absorption spectra of borosilicate glass of different compositions were obtained to determine the specificity of boron incorporation into the matrix of silica glass. Glass samples with different boron oxide content were prepared by mixing powdered oxides (SiO\u003csub\u003e2\u003c/sub\u003e and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) and heating them at a temperature of 1000 \u0026ordm;C for 1 hour or melting them in an oxygen-hydrogen flame.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe outer porous layer of the preform containing boron oxide was melted at a temperature of \u0026asymp;\u0026thinsp;1800 \u0026ordm;C. Then, the fiber with a diameter of 125 \u0026micro;m was drawn out of it at a rate of 30 m/min with simultaneous application of a single-layer UV-curing epoxyacrylate coating with a thickness of 45 \u0026micro;m. The preform was heated in a furnace with a graphite heater, which was shielded from the glass by a stream of high-purity argon containing no more than 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e vol% of impurity oxygen and moisture. The OF sections were drawn at heater temperatures of 1900, 2000 and 2100 \u0026ordm;C and the drawing force of 3.5, 1.2 and 0.5 N, respectively. Under the same conditions, fiber was drawn from a similar preform without borosilicate claddingto evaluate its strengthening effect. Fiber strength (σ) was measured by two-point bending at room temperature and calculated according to formula [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003eσ = \u003cem\u003eЕ\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e [1\u0026thinsp;+\u0026thinsp;6.9/2(1.219\u003cem\u003ed\u003c/em\u003e/\u003cem\u003eD\u003c/em\u003e-1.137(\u003cem\u003ed\u003c/em\u003e/\u003cem\u003eD\u003c/em\u003e)\u003csup\u003e2\u003c/sup\u003e)](1.219\u003cem\u003ed\u003c/em\u003e/\u003cem\u003eD\u003c/em\u003e-1.137(\u003cem\u003ed\u003c/em\u003e/\u003cem\u003eD\u003c/em\u003e)\u003csup\u003e2\u003c/sup\u003e),\u003c/p\u003e \u003cp\u003ewhere E\u003csub\u003e0\u003c/sub\u003e - elastic modulus of silica glass, equal to 73.5 GPa; \u003cem\u003ed\u003c/em\u003e - diameter of glass fiber; \u003cem\u003eD\u003c/em\u003e - diameter of the neutral axis of the bent fiber at its destruction.\u003c/p\u003e \u003cp\u003eWhen evaluating the strength of silica fiber with borosilicate cladding, it was assumed that a small addition of boron oxide to silica glass could insignificantly affect its elastic modulus.\u003c/p\u003e \u003cp\u003eTo increase the statistical significance of the results, 20 measurements were made for each fiber drawing temperature. The error of the measured strength value equal to 2 standard deviations of its arithmetic mean (0.08 GPa) corresponds to 97% of its probability level.\u003c/p\u003e \u003cp\u003eTo evaluate the thickness of the borosilicate layer on the fiber, its diameter was measured during etching in 10% hydrofluoric acid.\u003c/p\u003e"},{"header":"3 Thermodynamic analysis of vapor composition during evaporation of borosilicate glass","content":"\u003cp\u003eAccording to the Hertz-Knudsen equation, the vaporization rate of a substance is proportional to its pressure. If the pressure of borosilicate glass components is different, the glass composition changes during its high-temperature processing. Therefore, it is reasonable to evaluate the temperature dependence of the equilibrium pressure over silicon and boron oxides.\u003c/p\u003e \u003cp\u003eBoron-doped silica glass evaporates forming the following major gaseous components: SiO\u003csub\u003e2\u003c/sub\u003e, B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, SiO and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. The pressure of these substances above the pure oxide melts is determined by the corresponding equilibrium processes:\u003c/p\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e(l)\u0026thinsp;=\u0026thinsp;SiO\u003csub\u003e2\u003c/sub\u003e(g), (2)\u003c/p\u003e \u003cp\u003eB\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(l)\u0026thinsp;=\u0026thinsp;B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(g), (3)\u003c/p\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e(l)\u0026thinsp;=\u0026thinsp;SiO(g)\u0026thinsp;+\u0026thinsp;0,5 O\u003csub\u003e2\u003c/sub\u003e(g), (4)\u003c/p\u003e \u003cp\u003eB\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(l)\u0026thinsp;=\u0026thinsp;B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e(g)\u0026thinsp;+\u0026thinsp;0,5 O\u003csub\u003e2\u003c/sub\u003e(g), (5)\u003c/p\u003e \u003cp\u003ewhere (l) and (g) denote the liquid and gaseous states of matter, respectively.\u003c/p\u003e \u003cp\u003eAccording to the Gibbs phase rule, the first two processes have one degree of freedom. Therefore, the equilibrium pressure of gases depends only on temperature. The equilibrium (4, 5) have two degrees of freedom and are determined by the temperature and equilibrium pressure of oxygen.\u003c/p\u003e \u003cp\u003eThe temperature dependence of the pressure of gaseous components SiO\u003csub\u003e2\u003c/sub\u003e and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (Pi) over pure oxides (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) is borrowed from [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUsing the data from the reference book for the free energy of formation of substances [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], we calculated the temperature dependence of the equilibrium constant for reactions (4, 5) and the pressure of gaseous components containing silicon or boron at two values of the equilibrium oxygen pressure: 0.1 and 2∙10\u003csup\u003e4\u003c/sup\u003e Pa (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe high oxygen pressure (2∙10\u003csup\u003e4\u003c/sup\u003e Pa) corresponds to the conditions of flame treatment of the preform in air atmosphere, and its low pressure (0.1 Pa) is determined by the conditions of its heating during high-temperature fiber drawing in atmosphere of high-purity argon or nitrogen.\u003c/p\u003e"},{"header":"4 Experimental results","content":"\u003cp\u003eThe radial profile of RI preform with borosilicate glass after its heating at 2100\u003csup\u003eo\u003c/sup\u003eC indicates that boron oxide does not completely evaporate in both nitrogen (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea) and oxygen (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) atmospheres. The glass evaporation occurs without changing its composition. The content of B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in such glass leading to a change in the silica glass RI by about 0.002 is 4 mol% according to Eq.\u0026nbsp;(1).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe RI radial profile of a preform with a germanosilicate core and borosilicate cladding (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) revealed a peripheral region with reduced refraction. The RI difference between this cladding and the bordering pure silica glass cladding is \u0026asymp;\u0026thinsp;0.0004, which corresponds to the content of 0.8 +/- 0.2 mol% B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e according to Eq.\u0026nbsp;(1). The IR spectrum of the porous borosilicate cladding before sintering by estimating the absorbance in the 3200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e region gave a close value of 1.0 +/- 0.2 mol%. The slight decrease in the boron oxide content may be due to its evaporation while sintering the porous cladding layer with B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eThe preform radial profile of RI (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) indicates that on a 125 \u0026micro;m diameter fiber the thickness of borosilicate cladding equals\u0026thinsp;\u0026asymp;\u0026thinsp;5 \u0026micro;m.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe discontinuous change of RI at the cladding outer boundary (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) indicates that there is no change in its composition during high-temperature sintering of the porous SiO\u003csub\u003e2\u003c/sub\u003e layer containing boron oxide. At the same time, a transition zone with a RI gradient is observed at the boundary of the cladding with silica glass, which indicates boron diffusion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe silica fiber strength measurement (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) showed that the borosilicate cladding provided a 10% increase in silica fiber strength (from 5.9 to 6.55 GPa) when its drawing temperature was reduced to 1900\u0026deg;C.\u003c/p\u003e \u003cp\u003eFiber strengthening due to the occurrence of compressive stresses in the cladding does not affect the statistical nature of the scatter of strength measurement data. As known, this depends on the distribution of microinhomogeneity of impurity nature [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The source of defects can be dust particles in the atmosphere of the graphite furnace used for fiber drawing [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. When such defects are completely eliminated, the experimentally measured strength of silica fiber in the conditions of natural humidity reaches 26 GPa [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. At complete removal of moisture from the surrounding gas atmosphere, fiber strength can be higher than 40 GPa, which corresponds to theoretical estimates.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSoaking the fibers in hydrofluoric acid solution (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e) revealed an increased etching rate of the borosilicate cladding compared to the silica glass fiber. From the results of this experiment, we can estimate the thickness of the borosilicate cladding to be \u0026asymp;\u0026thinsp;3 \u0026micro;m. This value within the measurement error corresponds to the estimate of the borosilicate cladding thickness (5 \u0026micro;m) obtained from the radial profile of the preform RI (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e,\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis slight difference in the estimated cladding thickness may also be due to its evaporation during the fiber drawing process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe IR absorption spectrum of the silicon and boron oxide mixture after its high temperature treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e) showed the presence of two bands at 1400 and 950 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The absorption band at 1400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates a three-coordinated boron atom, and band at 950 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates their four-coordinated state in the silica glass melts [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The ratio of boron coordination [BO\u003csub\u003e3\u003c/sub\u003e] and [BO\u003csub\u003e4\u003c/sub\u003e] does not depend on temperature.\u003c/p\u003e \u003cp\u003eThe absorption band in the region of 3200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is the characteristic of pure boron oxide (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), is absent in the IR spectrum of borosilicate glass.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5 Discussions","content":"\u003cp\u003eThe comparison of temperature dependence of pressure of gases over pure silicon and boron oxides (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) indicates that regardless of the equilibrium oxygen pressure, SiO\u003csub\u003e2\u003c/sub\u003e and B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e molecules are the main vapor components. The pressure of the boron-containing gases is much higher than that of the silicon-containing components. In the case of an ideal state of boron oxide solution in silica glass this should lead to complete vaporization of the volatile oxide. However, the experimental results (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) demonstrate the phenomenon of congruent evaporation at a B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content of \u0026asymp;\u0026thinsp;4 mol%. Such a phenomenon was observed earlier in similar experiments during high-temperature treatment of borosilicate glass [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], but this fact was not given due attention.\u003c/p\u003e \u003cp\u003eThe absorption bands at 1400 and 950 cm\u003csup\u003e-1\u003c/sup\u003e in borosilicate glass (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e) are caused by vibrations of trigonally [BO\u003csub\u003e3\u003c/sub\u003e] and tetrahedrally [BO\u003csub\u003e4\u003c/sub\u003e] coordinated boron groups included in the silica glass matrix. Boron oxide is fully incorporated into the structure of silica even at such a low temperature as 1000 \u003csup\u003eo\u003c/sup\u003eC. The absorption bands by boroxol rings are observed in the IR absorption spectra of pure boron oxide (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), but they are absent in borosilicate glass. This structural rearrangement of boron may serve as an explanation for the decrease in its vaporization rate when dissolved in silica glass.\u003c/p\u003e \u003cp\u003eThe high equilibrium vapor pressure over pure B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is due to the specific structure of boroxyl rings. These rings form flat networks linked by weak van der Waals forces.\u003c/p\u003e \u003cp\u003eThere is no chemical interaction of oxides in borosilicate glass [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Therefore, the deviation of vapor pressure of boron-containing gases over borosilicate glass from the ideal solution state is caused only by the specificity of the structural transformation of boron in the glass matrix. In addition, it should be noted that the energy of the B-O bond (496 kJ/mol) is greater than that of the Si-O bond (444 kJ/mol). The structural transformation of boron with the decrease of its content in borosilicate glass leads to a significant decrease in its TEC compared to the linear dependence [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe effect of compressive stress formation in the borosilicate cladding of silica fiber in our experiments (0.6 GPa) is the same as in similar studies [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] (85 ksi). However, the strenthening cladding of our fibers contained significantly less B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (1 instead of 10 mol%), and the fiber drawing force in our studies was almost two times less (3.5 instead of 6.8 N). Thus, the hardening efficiency of the borosilicate layer increases with decreasing boron oxide content. This may be due to the competition of two mechanisms of stress generation in borosilicate cladding.\u003c/p\u003e \u003cp\u003eAccording to one mechanism, compressive stresses in the cladding grow proportionally to the fiber drawing force [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], and according to another mechanism, tensile stresses arise due to the increased TEC of borosilicate glass compared to the TEC of silica glass fiber.\u003c/p\u003e \u003cp\u003eIn the preform fabrication, the first mechanism of forming compressive stresses does not work, and the second mechanism can lead to the formation of cracks in the borosilicate layer. Therefore, to strengthen silica fiber with borosilicate cladding, the content of B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in it should not exceed 1 mol%. This guarantees to exclude the formation of cracks in the preform borosilicate layer if the porous cladding is sintered at the stage of OF drawing [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA more significant advantage of the silica optical fiber technology with borosilicate cladding, along with increasing its strength, is the possibility to reduce the temperature of its drawing. This can significantly minimize the optical losses of light guides, which are due to the fundamental mechanism of Rayleigh scattering [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Moreover, changing the temperature of silica fiber drawing from 2100 to 1900\u0026deg;C leads to a tenfold decrease in SiO\u003csub\u003e2\u003c/sub\u003e vapor pressure (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), therefore, reducing the chemical corrosion of the graphite heater.\u003c/p\u003e"},{"header":"6 Conclusions","content":"\u003cp\u003eThe work results have shown that during high-temperature treatment of borosilicate glass, boron oxide does not completely evaporate from the glass, in both oxidizing and neutral gas atmosphere. At 2100\u0026deg;C, this glass vaporizes congruently withB\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e content of \u0026asymp;\u0026thinsp;4mol%.\u003c/p\u003e \u003cp\u003eThe strength of silica optical fibers increases by 10% in the presence of an outer cladding doped with 1mol% B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e with decreasing their drawing temperature to 1900\u0026deg;C. The efficiency of compressive stresses formation in such a borosilicate cladding of silica fiber is higher than that in similar fibers containing 10% boron oxide in the cladding.\u003c/p\u003e \u003cp\u003eThe results of studies have shown the possibility of drawing silica optical fiber at low temperatures, which is of particular importance for obtaining promising light guides for long-distance communication lines with particularly low optical losses.\u003c/p\u003e \u003cp\u003eIt should be noted that the effect of strengthening of silica fiber with low-viscosity cladding can be provided by its doping with fluorine [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, for stability of fluorosilicate glass composition during fiber drawing it is necessary to create an atmosphere of chemically aggressive fluorine-containing gas.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003eThe authors are thankful to Academician of the Russian Academy of Sciences, Professor V. G. Peshekhonov for supporting this work and A.V. Khokhlov for the scientific and technical helps.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u0026nbsp;\u003c/strong\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by\u0026nbsp;A.Yu. Kulesh, M.K. Tsibinogina, D.V. Glita, A.A. Untilov, V. E. Sitnikova, K.V. Dukelskiy, P.A. Zlobin I.K. Meshkovskiy.\u0026nbsp;The first draft of the manuscript was written by M.A. Eronyan. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of Data and Materials\u003c/strong\u003e The authors of the manuscript declare the availability of the data and materials of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e \u003cem\u003eThe authors declare that no funds, grants, or other support were received during the preparation of this manuscript.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate\u003c/strong\u003e All authors agreed to participate in the authorship of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e All authors gave their consent to publish the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Approval\u003c/strong\u003e The authors assure that: the described work has not been previously published elsewhere; it is not being considered for publication elsewhere; the publication has been approved by all co-authors and responsible persons at the institutions where the work was done; all sources used are properly disclosed; all authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e The authors declare that they have no financial or non-financial conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eOh PS, Mcalarney JJ, Nath DK (1983) Effect of Fiber Drawing Tension on Optical and Mechanical Properties of Optical Fiber Waveguides. 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V. 3, Book 2: The elements B, Al, Ga, In, Tl, Be, Mg, Ca, Sr and Ba and their compounds.\u003c/li\u003e\n\u003cli\u003eJANAF (1985) Thermochemical Tables, 3rd Ed. J. Phys. Chem. \u003c/li\u003e\n\u003cli\u003eSakaguchi S, Nakahara M, Tajima Y (1984) Drawing of high-strength long-length optical fiber. J Non Cryst Solids 64:173\u0026ndash;183. https://doi.org/10.1021/acs.cgd.5b00253 \u003c/li\u003e\n\u003cli\u003eBrambilla G, Payne DN (2009) The ultimate strength of glass silica nanowires. Nano Lett 9:831\u0026ndash;835. https:// doi. org/ 10. 1021/ nl803 581r\u003c/li\u003e\n\u003cli\u003eKonijnendijk WL (1975) The structure of borosilicate glasses https://doi.org/10.6100/IR146141 \u003c/li\u003e\n\u003cli\u003eKirchhof J, Unger S, Dellith J, Scheffel A, Teichmann C (2012) Diffusion coefficients of boron in vitreous silica at high temperatures. Opt Mater Express 5: 534-547.\u003c/li\u003e\n\u003cli\u003eRockett TJ, Foster WR (1965) Phase relation in the system boron oxide \u0026ndash; silica 48: 75-80.\u003c/li\u003e\n\u003cli\u003eRongved L, Kurjian CR, Geyling FT (1980) Mechanical tempering of optical fibers. J Non-Cryst Solids 42: 579\u0026ndash;584. \u003c/li\u003e\n\u003cli\u003eDukel\u0026apos;skii KV, Eronyan MA, Komarov AV, Kondrat\u0026apos;ev YuN, Levit LG, Romashova EI, Serkov MM, Khokhlov AV, and Shevandin VS (2002) MCVD technology for single-mode low-damping fiber lightguides stable against microbends. J Opt Technol 69: 849 https://opg.optica.org/jot/abstract.cfm?URI=jot-69-11-849\u003c/li\u003e\n\u003cli\u003eEronyan MA, Tsibinogina, MK Zlobin PA (2004) Method of high-temperature chemical treatment of glass surface, Patent RU 2272003.\u003c/li\u003e\n\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":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"borosilicate glass, temperature, pressure, congruent evaporation, silica fiber, strength","lastPublishedDoi":"10.21203/rs.3.rs-4029039/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4029039/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe work aims to study the evaporation process of borosilicate glass used for strengthening of silica fiber light guides. The composition of glass after high-temperature treatment was determined based on refractive index measurement. It was found that at temperature 2100 \u003csup\u003eo\u003c/sup\u003eC two-component SiO\u003csub\u003e2\u003c/sub\u003e-B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e glass vaporizes without changing its composition at a boron oxide content of \u0026asymp;\u0026thinsp;4 mol% in both oxygen and nitrogen atmospheres. The IR absorption spectra of borosilicate glass indicate the absence of boroxol rings of B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, which can explain the low partial pressure of its vapors compared to the ideal melt state. It is experimentally shown that the content of only 1 mol % B\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in the outer cladding of silica fiber increases its strength by 10% when the drawing temperature is reduced to 1900\u0026deg;C. Such a small addition of boron oxide to silica glass decreases its viscosity, ensuring the occurrence of compressive stresses in the borosilicate cladding.\u003c/p\u003e","manuscriptTitle":"Congruent evaporation of borosilicate glass strengthening silica fiber","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-22 07:04:32","doi":"10.21203/rs.3.rs-4029039/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-04-08T15:51:43+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-03-27T21:41:58+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"1b734442-df86-483f-ab46-cec16c1098a7","date":"2024-03-25T10:07:05+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-03-25T01:47:30+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-03-20T03:02:40+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-03-20T03:02:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Silicon","date":"2024-03-07T16:32:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"84727615-d234-47f5-abc2-bfb0c6d4e8b6","owner":[],"postedDate":"March 22nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-04-28T00:41:25+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-22 07:04:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4029039","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4029039","identity":"rs-4029039","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}