A method for preparing a sapphire optical fiber double clad resistant to high temperature and long service in air

Abstract

The application provides a kind of sapphire optical fiber double cladding preparation method of high temperature resistant long time service in air, dense and continuous BN / SiC double cladding is prepared on the surface of sapphire optical fiber by chemical vapor deposition method.For sapphire optical fiber / BN / SiC, the refractive index of BN is slightly lower than that of sapphire optical fiber, as the total reflection layer of optical fiber, and the slow oxidation rate of SiC at high temperature can provide more durable protection for BN inner cladding, by preparing BN on the surface of sapphire optical fiber, the defects on the surface of sapphire optical fiber can be repaired, on the other hand, the transmission loss can be reduced;Secondly, the BN / SiC double cladding prepared on the surface of sapphire optical fiber can improve the mechanical properties of sapphire optical fiber at high temperature, and enhance the linearization of sapphire optical fiber stress-strain curve;In addition, the slow oxidation rate of SiC outer cladding in 1500 DEG C-air environment enhances the long time service ability of sapphire optical fiber / BN / SiC in this environment, and the service ability of sapphire optical fiber / BN / SiC in 1500 DEG C-air environment can reach more than 150h with suitable thickness of SiC.

Description

Technical Field

[0001] The invention belongs to the field of low-loss and high-temperature-resistant cladding preparation of sapphire optical fibers, and mainly relates to a method for preparing a multilayer ceramic cladding that is high-temperature-resistant and can serve for a long time. Background Art

[0002] In fields such as aviation and aerospace, some structural components will creep during long-term service. Once the creep of a component reaches its limit, the entire device may have catastrophic consequences. Before the component reaches its expected lifespan, timely detection of problems and replacement of damaged components are of positive significance for reducing maintenance costs and improving mission efficiency. Sapphire fiber optic sensors have the advantages of small size, light weight, high temperature resistance, anti-electromagnetic interference, safety and reliability, and have great potential for component health detection. However, current sapphire fiber Bragg sensors lack suitable cladding at high temperatures and have high transmission loss at high temperatures (>1200°C). In addition, impurities in the environment adsorbed on the surface of the sapphire fiber easily cause light scattering. In addition, the insufficient mechanical properties of sapphire fiber at high temperatures restrict its wider application. Therefore, it is necessary to improve its mechanical and optical properties at high temperatures by preparing a cladding on the surface of the sapphire fiber.

[0003] Currently, sapphire fiber outer claddings include high-melting-point metals and ceramic claddings. Metal claddings include elements such as platinum, niobium, and iridium, while non-metallic claddings include BN, SiBCN, SiO2, TiO2, ZrO2, and MgAl2O4. Metal coatings deposited on the fiber surface through processes such as magnetron sputtering and sol-gel deposition often face issues such as oxidation, agglomeration, and evaporation at high temperatures, making them difficult to use for extended periods in harsh, high-temperature environments. While ZrO2 claddings are stable in oxidizing and moderately reducing atmospheres, the refractive index of zirconium oxide cladding is higher than that of sapphire fiber, making it unsuitable for use as a total reflection layer. Porous Al2O3 claddings can be used up to 1600°C, but because the cladding and substrate share the same chemical composition, diffusion occurs at the interface at high temperatures, allowing polycrystalline alumina to fuse with the sapphire, resulting in the disappearance of the core interface between the cladding and the sapphire fiber. Spinel thin films (MgAl2O4) have a good thermal expansion coefficient match with sapphire fiber, making the cladding used to fabricate sapphire fiber interfacially compatible. Spinel films maintain stoichiometry at around 900°C, but as the temperature continues to rise, spinel will gradually dissolve Al2O3 to form a solid solution; when the temperature rises to 1400°C, the increase in solubility will promote the diffusion of aluminum from the sapphire fiber to the MgAl2O4 cladding; the long-term stability of spinel films at high temperatures restricts their widespread application in sapphire optical fiber cladding.

[0004] In summary, the application temperature conditions for sapphire fiber cladding reported in current research are limited. Many cladding materials cannot be reliably used in air environments at 1500°C, resulting in insufficient long-term service capability. Therefore, the present invention aims to achieve long-term service of sapphire fiber at high temperatures through a unique double-cladding design. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, improve the stability in an air environment of 1500°C, and enhance the long-term service capability, the present invention provides a method for preparing a double-clad sapphire optical fiber that is resistant to high temperatures and long-term service in air.

[0006] This invention aims to enhance the long-term service life of sapphire optical fibers at high temperatures through a unique double-cladding structure design, material selection, and fabrication methods. In this double-cladding design, the inner cladding heals surface defects and improves mechanical properties, while also acting as a total reflection layer to reduce optical transmission loss. The outer cladding, due to its high-temperature resistance and chemical stability, protects the inner cladding from oxidation. Calculations of sapphire fiber / BN / SiC transmission loss and interface residual stress using COMSOL software provide a basis for the design of the sapphire fiber double-cladding structure. A dense and continuous BN / SiC double cladding is then deposited on the sapphire fiber surface using chemical vapor deposition (CVD).

[0007] A method for preparing a double-clad sapphire optical fiber capable of withstanding high temperatures and long-term service in air comprises the following steps:

[0008] Preparation of BN inner cladding:

[0009] Step 1: Clean the sapphire optical fiber and dry it;

[0010] Step 2: Place the dried sapphire fiber into the constant temperature zone of the BN deposition furnace;

[0011] Step 3: Reduce the pressure in the BN deposition furnace, introduce argon gas and increase the temperature;

[0012] The pressure in the BN deposition furnace is reduced to less than 3 kPa, and argon gas is introduced at the same time, and then the BN deposition furnace is heated to 600-1000° C. The argon gas flow rate is 100 ml / min;

[0013] Step 4: BN deposition furnace is introduced with gaseous precursor;

[0014] After the temperature in the BN deposition furnace stabilizes, gaseous precursors are introduced into the BN deposition furnace. The gaseous precursors introduced into the BN deposition furnace include H2, argon, BCl3, and NH3 gases. The gaseous precursors react in the reaction chamber of the BN deposition furnace to deposit BN on the surface of the sapphire optical fiber. The BN deposition furnace maintains the temperature for 1-20 hours to obtain a BN inner cladding with a target thickness of 1 to 10 μm.

[0015] Step 5: After the BN inner cladding deposition is completed, cool it to room temperature and place the reacted sapphire fiber into the SiC deposition furnace;

[0016] Preparation of SiC outer cladding:

[0017] Step 6: After the temperature in the SiC deposition furnace is raised, the SiC cladding gaseous precursor is introduced;

[0018] The temperature of the SiC deposition furnace is raised to 900-1100°C. After the temperature in the SiC deposition furnace is stabilized, a SiC cladding gaseous precursor is introduced into the SiC deposition furnace. The gaseous precursors in the SiC deposition furnace include H2, argon, and MTS. The temperature of the methyltrichlorosilane MTS oil bath is raised to 30-35°C. H2 is introduced into the MTS oil bath by bubbling. After the H2 is introduced, gaseous trichloromethylsilane is carried out into the reaction chamber. Argon is simultaneously introduced into the reaction chamber for dilution. The gaseous precursors in the SiC deposition furnace are decomposed to generate SiC on the surface of the sapphire fiber / BN, thereby obtaining a SiC outer cladding with a target thickness of 1 to 10 μm.

[0019] Step 7: After the deposition is completed, the introduction of the reaction gas source is stopped and the deposition furnace is cooled to room temperature to obtain sapphire fiber / BN / SiC.

[0020] Furthermore, in step 4, the H2 flow rate is 50-200 ml / min; the BCl3 flow rate is 10-100 ml / min; the NH3 flow rate is 30-200 ml / min; and the argon flow rate is 50-200 ml / min.

[0021] Furthermore, in step 6, the H2 flow rate is 20-2000 ml / min; the argon flow rate is 10-2000 ml / min.

[0022] Furthermore, in step 6, the temperature is maintained for 1-100 hours.

[0023] The present invention proposes a design and preparation method for a BN / SiC double-clad sapphire optical fiber capable of long-term service in an air environment at 1500°C. This method has a simple design, a wide range of operability, strong substitutability, and good repeatability. Compared with bare sapphire optical fiber, the present invention has the following advantages:

[0024] (1) The BN / SiC double cladding is prepared on the surface of sapphire optical fiber by chemical vapor deposition. The surface is continuous and dense, the interface is well bonded, and it has good structural stability. In the double cladding design, the inner cladding heals the surface defects of the optical fiber and improves its mechanical properties. On the other hand, it acts as a total reflection layer to reduce light transmission loss. The outer cladding is high temperature resistant and chemically stable to protect the inner cladding from oxidation.

[0025] (2) The strength of sapphire fiber / BN / SiC in 1500℃-air environment is significantly improved while the modulus decreases, and the mechanical properties are significantly improved;

[0026] (3) The BN inner cladding acts as a total reflection layer, which can significantly reduce the transmission loss of sapphire optical fiber;

[0027] (4) The double cladding designed and prepared by the present invention can improve the service capability of sapphire fiber at high temperatures. The oxides generated by the slow oxidation of the SiC outer cladding at high temperatures reduce the air permeation rate, thereby improving the long-term service capability of the sapphire fiber / BN / SiC. The appropriate thickness of SiC can enable the sapphire fiber / BN / SiC to serve for at least 150 hours in a 1500°C-air environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 : Some propagation modes of sapphire fiber / BN-5μm / SiC: (a) Lp01; (b) Lp02; (c) Lp21; (d) Lp31; (e) Lp41; (f) Lp22;

[0029] Figure 2 : Changes in transmission loss of some propagation modes of sapphire fiber / BN / SiC under different BN thicknesses;

[0030] Figure 3 :(a) Stress-strain curves of sapphire fiber / BN / SiC at 1500℃ in air; (b) Simulation results of radial residual stress of sapphire fiber / BN / SiC at room temperature and 1500℃;

[0031] Figure 4 : Fracture morphologies of sapphire fiber / BN-4μm / SiC-6μm at 1500℃: (a) and (b) fracture morphologies; (c) and (d) surface morphologies;

[0032] Figure 5 : Morphological changes of sapphire fiber / BN-3μm / SiC-20μm before and after oxidation at 1500℃ in air for 8h: (a) before oxidation; (b) after oxidation; DETAILED DESCRIPTION

[0033] A technical solution for preparing a double-clad sapphire optical fiber that is resistant to high temperatures and long-term service in air is as follows:

[0034] Step 1: Clean the sapphire optical fiber and dry it;

[0035] The sapphire optical fiber was cleaned in an ultrasonic bath using anhydrous ethanol and deionized water in sequence, and the cleaned sapphire optical fiber was placed in an oven to dry;

[0036] Step 2: Place the dried sapphire fiber into the constant temperature zone of the BN deposition furnace;

[0037] Place the dried sapphire fiber into a graphite box, fix one end of the sapphire fiber and leave the other end free, and place the graphite box into the constant temperature zone of the BN deposition furnace;

[0038] Step 3: Reduce the pressure in the BN deposition furnace while introducing argon and increasing the temperature.

[0039] The pressure in the BN deposition furnace was pumped down to below 3 kPa, and argon gas was introduced at the same time, and then the temperature of the BN deposition furnace was raised to 600-1000° C. The argon gas flow rate was 100 ml / min;

[0040] Step 4: BN deposition furnace is introduced with gaseous precursor;

[0041] After the temperature in the BN deposition furnace stabilizes, gaseous precursors are introduced into the BN deposition furnace. The gaseous precursors in the BN deposition furnace include H2, argon, BCl3, and NH3 gases. The gaseous precursors react in the reaction chamber of the BN deposition furnace to deposit BN on the surface of the sapphire optical fiber. The BN deposition furnace maintains the temperature for 1-20 hours to obtain a BN inner cladding with a target thickness of 1 to 10 μm.

[0042] The H2 flow rate is 50-200 ml / min; the BCl3 flow rate is 10-100 ml / min; the NH3 flow rate is 30-200 ml / min; the argon flow rate is 50-200 ml / min;

[0043] Step 5: After the BN inner cladding deposition is completed, cool it to room temperature and place the reacted sapphire fiber into the SiC deposition furnace;

[0044] After the BN inner cladding deposition is completed, the reaction gas source is stopped, and the deposition furnace is cooled to room temperature, and the graphite box is transferred to the SiC deposition furnace;

[0045] Preparation of SiC outer cladding:

[0046] Step 6: The SiC cladding gaseous precursor is introduced into the SiC deposition furnace;

[0047] The temperature of the SiC deposition furnace is raised to 900-1100° C. After the temperature in the SiC deposition furnace is stabilized, a SiC cladding gaseous precursor is introduced into the SiC deposition furnace; the SiC deposition furnace gaseous precursor includes H2, argon and MTS; the temperature of the methyltrichlorosilane MTS oil bath is raised to 30-35° C., H2 is introduced into the MTS oil bath by bubbling, and the gaseous trichloromethylsilane is carried out into the reaction chamber after the H2 is introduced, and argon is simultaneously introduced into the reaction chamber for dilution, so that the gaseous precursor in the SiC deposition furnace decomposes and SiC is generated on the surface of the sapphire fiber / BN. The temperature is maintained for 1-100 hours to obtain a SiC outer cladding with a target thickness of 1-10 μm; the H2 flow rate is 20-2000 ml / min; the argon flow rate is 10-2000 ml / min;

[0048] Step 7: After the deposition is completed, the introduction of the reaction gas source is stopped and the deposition furnace is cooled to room temperature to obtain sapphire fiber / BN / SiC.

[0049] Example process parameters:

[0050]

[0051]

[0052] The present invention will be further described below with reference to the accompanying drawings and examples.

[0053] Preparation of BN inner cladding:

[0054] (1) Clean the sapphire optical fiber in an ultrasonic bath with anhydrous ethanol and deionized water, and then dry it in an oven;

[0055] (2) Place the dried sapphire fiber into a graphite box and fix one end to ensure that the other end of the sapphire fiber is free, and then place it in the constant temperature zone of the BN deposition furnace;

[0056] (3) Pumping the pressure in the BN deposition furnace to below 3 kPa, while introducing argon gas, and then heating the deposition furnace to 600-1000° C.; the argon gas flow rate is 100 ml / min;

[0057] (4) After the temperature stabilizes, introduce H2, Ar, BCl3, and NH3 gases. The gaseous precursors react within the reaction chamber of the deposition furnace, thereby depositing BN on the surface of the sapphire fiber. Maintain this temperature for 1-20 hours to obtain the target BN thickness. The H2 flow rate is 50-200 ml / min; the BCl3 flow rate is 10-100 ml / min; the NH3 flow rate is 30-200 ml / min; and the Ar flow rate is 50-200 ml / min.

[0058] (5) After the deposition is completed, stop introducing the reaction gas source, wait for the deposition furnace to cool to room temperature, and transfer the graphite box to the SiC deposition furnace.

[0059] Preparation of SiC outer cladding:

[0060] (6) The SiC deposition furnace is heated to 900-1100°C, and the methyltrichlorosilane (MTS) oil bath temperature is simultaneously raised to 30-35°C. H2 is then introduced into the MTS oil bath by bubbling to remove some gaseous trichloromethylsilane into the reaction chamber. Ar is then introduced to dilute the gaseous precursor, causing it to decompose and generate SiC on the sapphire fiber / BN surface. This temperature is maintained for 1-100 hours to obtain SiC of the target thickness. The H2 flow rate is 20-2000 ml / min; the Ar flow rate is 10-2000 ml / min.

[0061] (7) After the deposition is completed, the introduction of the reaction gas source is stopped and the deposition furnace is cooled to room temperature to obtain sapphire fiber / BN / SiC.

[0062] Preparation of SiC outer cladding:

[0063] The gas flow rate and deposition temperature required for the BN and SiC deposition processes are not limited to the above ranges, and are dynamically related to the size specifications of the deposition furnace.

[0064] The gas sources for BN deposition process include but are not limited to: H2, Ar, BCl3, NH3; the gas sources for SiC deposition process include but are not limited to: H2, Ar, MTS.

[0065] In order to better illustrate the advantages of the present invention, the present invention is further described below with reference to examples and drawings.

[0066] The sapphire fiber / BN / SiC transmission model was simulated by COMSOL, and the results are as follows Figure 1 As shown in Figure 2, the transmitted light is almost entirely confined to the core of the sapphire fiber / BN / SiC. In addition, the propagation loss (leakage loss) corresponding to each propagation mode varies with the BN thickness as shown in Figure 2. Figure 2 As shown in the figure, it can be seen that with the increase of BN thickness, the transmission loss corresponding to each mode continues to decrease. When the thickness is higher than 3μm, the loss is negligible.

[0067] The mechanical properties test results of sapphire fiber / BN / SiC in 1500℃-air environment are shown in Table 1. The strength of the sample is 648MPa, which is 28% higher than that of the original sapphire fiber; while the elastic modulus is 245GPa, which is 23% lower than that of the original sapphire fiber. The strength of sapphire fiber / BN / SiC is increased and the flexibility is enhanced. The stress-strain curve of the sample at high temperature is shown in Figure 3As shown in (a), compared with the original sapphire fiber, the stress-strain curve of sapphire fiber / BN / SiC has little fluctuation and remains highly linear. The residual stress in the radial direction of the sapphire fiber / BN / SiC sample at room temperature and high temperature is shown in Figure 3 As shown in (b), since the thermal expansion coefficient and Poisson's ratio of sapphire fiber and SiC are very close, the radial residual stress at the sample interface is limited at both room temperature and high temperature, and the cracking tendency between the cladding interfaces is limited, and the sapphire fiber / BN / SiC has structural stability.

[0068] The fracture morphology of the sapphire fiber / BN-4μm / SiC-6μm high temperature tensile specimen is as follows Figure 4 As shown in the figure, the fracture surface of the sample is divided into a smooth fracture mirror and a relatively rough, saw-toothed, diamond-shaped dissociation surface. Figure 4 (a) and (b) show that the BN in the inner layer of the sample has been oxidized and lost. This is mainly caused by the BN on the fracture surface being exposed to a high-temperature oxidizing environment after the sample is broken. Figure 4 (a), (b) and (c) show that the SiC in the outer layer of the sample is continuously distributed along the circumference and axial directions of the optical fiber, with no obvious signs of cracking. Secondly, large hemispherical cauliflower-like morphologies exist in local areas of the sample surface. These large agglomerates are formed by the fusion of a large number of micron-sized polycrystalline SiC. Overall, the SiC cladding is continuous, with no gaps between the unit cells. Figure 4 (d) is the morphology of the tensile specimen after the cladding is detached at a local location. It can be seen that many wrinkles appear on the surface of the sapphire fiber after stretching.

[0069] The morphology of sapphire fiber / BN-3μm / SiC-20μm after being tested in 1500℃-air environment for 8h is as follows Figure 5 As shown in the figure, it can be found that 1μm oxide is generated on the surface of the sample after 8 hours of testing, while the BN in the innermost layer of the sample does not undergo any oxidation. It can be inferred that the cladding of the sapphire fiber / BN / SiC prepared by chemical vapor deposition is continuous and dense and will not crack at room temperature and high temperature, and has the expected structural stability, which is consistent with the Figure 3 (d) The residual stress calculation results are consistent. In addition, according to Figure 5 It can be seen that it takes 8 hours for the SiC outer cladding to oxidize 1μm. At the same time, the oxides formed on the surface will reduce the air permeation rate. Therefore, based on this oxidation rate, it can be estimated that the sample can be used in a 1500℃ air environment for at least 150 hours.

[0070] High temperature mechanical properties of sapphire fiber / BN / SiC:

[0071] In this paper, sapphire fiber / BN / SiC was prepared and its mechanical properties at high temperature (1500℃) were tested. The results are shown in Table 1. The strength of the sample is 648MPa, which is 28% higher than that of the original sapphire fiber; while the elastic modulus is 245GPa, which is 23% lower than that of the original sapphire fiber. The stress-strain curve of the sample at high temperature is shown in Table 1. Figure 3 As shown in (a), it can be seen that the stress-strain curve fluctuates very little and remains highly linear. The residual stress between the radial direction of the sapphire fiber / BN / SiC sample at room temperature and high temperature is shown in Figure 3 As shown in (b), since the thermal expansion coefficient and Poisson's ratio of sapphire fiber and SiC are very close, the radial residual stress at the sample interface is relatively limited regardless of room temperature or high temperature.

[0072] Table 1 Tensile test results of sapphire fiber / BN-4μm / SiC-6μm at 1500℃

[0073]

[0074] The fracture morphology of the sapphire fiber / BN-4μm / SiC-6μm high temperature sample is as follows Figure 4 As shown in the figure, similar to the fracture of the aforementioned specimen, the specimen's fracture surface is divided into a smooth fracture mirror and a rough, jagged, diamond-shaped dissociation surface. The fiber surface on the side of the fracture mirror is the fracture source. During stretching, the crack that initiates at the fracture source initially expands slowly. When the crack front is large enough and reaches a high expansion rate, the crack begins to branch, passing through the remaining portion of the fiber cross section, forming a jagged, diamond-shaped dissociation surface.

[0075] from Figure 4 (a) and (b) show that the BN in the inner layer of the sample has been oxidized and lost, while the SiC in the outer layer of the sample is continuously distributed along the circumference of the fiber with no obvious cracking. Figure 4 As shown in (c), there are large hemispherical cauliflower-like morphologies in local areas of the sample surface. These large agglomerates are formed by the fusion of a large number of micron-sized polycrystalline SiC. Overall, the SiC coating is continuous and there are no gaps between the unit cells. Figure 4 (d) is the morphology of the tensile specimen after the coating falls off at a local location. It can be seen that many wrinkles appear on the surface of the sapphire fiber after stretching. The reason for these wrinkles is the same as the above situation.

Claims

1. A method for preparing a double-clad sapphire optical fiber capable of withstanding high temperatures and long-term service in air, characterized in that: The steps include: Preparation of BN inner cladding: Step 1: Clean the sapphire optical fiber and dry it; Step 2: Place the dried sapphire fiber into the constant temperature zone of the BN deposition furnace; Step 3: Reduce the pressure in the BN deposition furnace, introduce argon gas and increase the temperature; The pressure in the BN deposition furnace is less than 3 kPa, and argon gas is introduced at the same time, and then the BN deposition furnace is heated to 600-1000° C.; the argon gas flow rate is 100 ml / min; Step 4: BN deposition furnace is introduced with gaseous precursor; After the temperature in the BN deposition furnace stabilizes, gaseous precursors are introduced into the BN deposition furnace. The gaseous precursors introduced into the BN deposition furnace include H2, argon, BCl3, and NH3 gases. The gaseous precursors react in the reaction chamber of the BN deposition furnace to deposit BN on the surface of the sapphire optical fiber. The BN deposition furnace maintains the temperature for 1-20 hours to obtain a BN inner cladding with a target thickness of 1 to 10 μm. Step 5: After the BN inner cladding deposition is completed, cool it to room temperature and place the reacted sapphire fiber into the SiC deposition furnace; Preparation of SiC outer cladding: Step 6: After the temperature in the SiC deposition furnace is raised, the SiC cladding gaseous precursor is introduced; The temperature of the SiC deposition furnace is raised to 900-1100°C. After the temperature in the SiC deposition furnace is stabilized, a SiC cladding gaseous precursor is introduced into the SiC deposition furnace. The gaseous precursors in the SiC deposition furnace include H2, argon, and MTS. The temperature of the methyltrichlorosilane MTS oil bath is raised to 30-35°C. H2 is introduced into the MTS oil bath by bubbling. After the H2 is introduced, gaseous trichloromethylsilane is carried out into the reaction chamber. Argon is simultaneously introduced into the reaction chamber for dilution. The gaseous precursors in the SiC deposition furnace are decomposed to generate SiC on the surface of the sapphire fiber / BN, thereby obtaining a SiC outer cladding with a target thickness of 1 to 10 μm. Step 7: After the deposition is completed, stop introducing the reaction gas source and wait for the deposition furnace to cool to room temperature to obtain sapphire fiber / BN / SiC.

2. The method for preparing a double-clad sapphire optical fiber capable of high temperature resistance and long-term service in air according to claim 1, characterized in that: In step 4, the H2 flow rate is 50-200 ml / min; the BCl3 flow rate is 10-100 ml / min; the NH3 flow rate is 30-200 ml / min; and the argon flow rate is 50-200 ml / min.

3. The method for preparing a double-clad sapphire optical fiber capable of high temperature resistance and long-term service in air according to claim 1, characterized in that: In step 6, the H2 flow rate is 20-2000 ml / min; the argon flow rate is 10-2000 ml / min.

4. The method for preparing a double-clad sapphire optical fiber capable of high temperature resistance and long-term service in air according to claim 1, characterized in that: In step 6, the temperature is maintained for 1-100 hours.

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