Preparation method of rare earth silicate ceramic

Abstract

The invention discloses a preparation method of rare earth silicate ceramic, and belongs to the technical field of preparation of inorganic ceramic materials. Compared with a traditional sintering process, the process integrating in-situ synthesis of nano powder, gel casting and microwave-assisted hot pressing sintering has the advantages that the sintering time is greatly shortened, the energy consumption is reduced, the density and uniformity of the ceramic are remarkably improved, the defects such as air holes are effectively reduced, and the production efficiency is improved. The prepared rare earth silicate ceramic has excellent mechanical properties, thermal properties and chemical stability, and meets the strict requirements of aerospace, energy and other high-end fields on high performance of materials.

Description

Technical Field

[0001] The present invention relates to the field of inorganic ceramic material preparation, and particularly to a preparation method of rare earth silicate ceramics. Background Art

[0002] As an advanced functional material with excellent properties such as high temperature resistance, oxidation resistance, and low thermal conductivity, rare earth silicate ceramics are widely used in high-end fields such as aerospace, energy, and electronics. However, there are still many deficiencies in the current preparation methods of rare earth silicate ceramics. Usually, high-temperature solid-state sintering is also adopted, which has problems such as high energy consumption, large grain size (>5μm), and poor densification. Similarly, in the process of preparing rare earth silicate powder, the high-temperature solid-state method is used, which requires long-time high-temperature calcination, resulting in high energy consumption and serious particle agglomeration. Although the sol-gel method can reduce the temperature, the process is complex and impurities are easily introduced. Moreover, this method does not solve the process connection problem of ceramic forming and densification, and there are problems such as low ceramic density, uncontrollable grain growth, uneven distribution of rare earth elements, and disconnection between the forming process and the sintering process.

[0003] Based on this, a method for efficiently, controllably preparing high-performance rare earth silicate ceramics is proposed. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of rare earth silicate ceramics to solve the problems in the background art.

[0005] To achieve the above purpose, the present invention provides a preparation method of rare earth silicate ceramics, including the following steps: S1. Co-dissolve rare earth nitrate and citric acid in deionized water, adjust the pH to form a rare earth-citric acid complex solution; add a silicon source, fully stir, then add a double template agent, and perform ultrasonic dispersion and magnetic stirring to form a homogeneous sol; S2. Transfer the homogeneous sol to a reaction kettle lined with polytetrafluoroethylene for hydrothermal reaction to generate a gel precursor; perform microwave calcination on the gel precursor in a dynamic atmosphere in three stages to obtain rare earth silicate powder; S3. Mix the rare earth silicate powder with 1-3 wt% of a sintering aid and 0.5-2 wt% of acrylamide monomer, add deionized water for ball milling. After the ball milling is completed, inject the obtained slurry into a mold, add 0.1 wt% of ammonium persulfate initiator, and cure at 65-90°C. After demolding, obtain a green body; S4. Place the green body in a microwave sintering furnace, heat it to 950-1050°C in a CO2 atmosphere, keep it warm for 1 h, then transfer it to a hot pressing furnace, apply a pressure of 10-30 MPa, and keep it warm at 1300-1500°C for 2-4 h to obtain a dense rare earth silicate ceramic.

[0006] Preferably, in the step S1, the molar ratio of rare earth nitrate to citric acid is 1:2-3, and the rare earth nitrate is one or a combination of more than one of Eu(NO3)3, Ce(NO3)3, La(NO3)3, Gd(NO3)3, and Tb(NO3)3; When the rare earth nitrate is a combination of multiple types, the rare earth nitrates are added in a predetermined order. After the first rare earth nitrate forms a complex solution with citric acid, the subsequent rare earth nitrates are added after an appropriate time; The pH is adjusted to 3-4 by dilute nitric acid, sodium hydroxide, or ammonia water.

[0007] Preferably, in the step S1, the silicon source is tetraethyl orthosilicate or silica sol, and the molar ratio of rare earth in the rare earth nitrate to silicon in the silicon source is 1:1-4; the double template agent is 0.5-1 wt% polyethylene glycol and 5-10 vol% mesoporous SiO2 microspheres, the ultrasonic dispersion time is 30-60 min, and magnetic stirring is carried out for 2-4 h.

[0008] Preferably, in the step S2, the temperature of the hydrothermal reaction is 160-200 °C, and the time is 3-5 h.

[0009] Preferably, in the step S2, after the gel precursor is freeze-dried, it is placed in a microwave oven equipped with an infrared real-time temperature measurement module for three-stage calcination. The specific steps are as follows: 1) Stage 1: CO2 is introduced, the flow rate is controlled at 30-80 ml / min, the temperature is raised to 400-500 °C, and it is kept warm for 30 min; 2) Stage 2: Switch to an N2 atmosphere, raise the temperature to 700-800 °C, and keep warm for 15 min; 3) Stage 3: Switch to a 5% H2 / N2 mixed gas, raise the temperature to 900-1000 °C, keep warm for 2 h, and the microwave power is 600-800 W.

[0010] Preferably, in the step S3, the sintering aid is one of Y2O3, AIN, or graphene nanosheets; the ball milling time is 4-8 h, the solid content of the obtained slurry is 50-60 vol%, and the viscosity is ; In the step S3, 0.1-0.3 wt% polyacrylamide dispersant is also added during the ball milling process; N,N'-methylenebisacrylamide is also added during the curing process.

[0011] Preferably, in the step S4, an Ar / 5% H2 mixed gas is used in the hot press furnace, and the heating rate is 5-10 °C / min.

[0012] Preferably, the above rare earth silicate ceramics are applied to the preparation of nuclear radiation shielding ceramics, fluorescence-structure integrated ceramics, or conductive ceramics to realize the preparation of functional ceramics; Among them, the specific preparation steps of the nuclear radiation shielding ceramic are as follows: The rare earth silicate ceramic is treated by plasma electrolytic oxidation technology. Under a DC voltage of 300 - 500V and a current density of 0.1 - 0.5A / cm 2 2 , it is placed in a silicate electrolyte and treated for 5 - 10 minutes to form a rare earth silicate / Al2O3 composite coating with a thickness of 5 - 10μm on the ceramic surface, thus obtaining the nuclear radiation shielding ceramic.

[0013] Therefore, the preparation method of a rare earth silicate ceramic of the present invention has the following beneficial effects: (1) During the preparation of the rare earth silicate powder, first in the hydrothermal reaction, the high - pressure environment promotes the hydrolysis and polycondensation of silicate, and cooperates with the template agent to generate a gel precursor with a hierarchical pore structure, improving the subsequent sintering activity; at the same time, the hierarchical pore structure constructed by the double template agent significantly increases the exposure rate of the reaction active sites of the catalytic carrier ceramic.

[0014] (2) During the preparation of the rare earth silicate powder in the present invention, three - stage microwave calcination is adopted. By precisely controlling the atmosphere, temperature, and time in different stages, not only the organic components in the precursor are effectively removed, but also the grain size and morphology of the ceramic powder can be regulated. In the first stage, the organic components in the precursor are removed by the action of the high - frequency magnetic field of the microwave, and at the same time, the carbon dioxide atmosphere can inhibit the oxidation of rare earth elements, avoiding the out - of - control of the valence state of rare earth elements and improving the stability of the material; in the second stage, the preliminary polymerization of the silicon - oxygen network is promoted, and in the third stage, under the atmosphere of H2 / N2 mixed gas, the oxygen vacancies are repaired and the grain size is controlled, and high - purity and high - quality rare earth silicate powder is obtained by calcination, providing high - quality raw materials for the preparation of high - performance ceramics.

[0015] (3) In the ceramic preparation stage of the present invention, the method of combining microwave sintering and hot pressing is adopted. Compared with the traditional sintering process, the sintering time is significantly shortened and the energy consumption is reduced. At the same time, under specific atmosphere and pressure conditions, the density and uniformity of the ceramic can be significantly improved, effectively reducing defects such as pores, making the prepared rare earth silicate ceramic have excellent mechanical properties, thermal properties, and chemical stability, meeting the strict requirements for high - performance materials in high - end fields such as aerospace and energy.

[0016] Next, through examples, the technical solutions of the present invention will be further described in detail. Specific Embodiments

[0017] The following further illustrates the technical solutions of the present invention through examples.

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention.

[0019] Example 1 Prepare a highly dense gadolinium silicate nuclear radiation shielding ceramic as follows: S1. Synthesize Gd2Si2O7 nanometer powder by double template agent guidance: First, dissolve 0.2 mol, , 0.5 mol, and 96.0 g of citric acid in 200 mL of deionized water, and stir magnetically for 30 min to form a colorless transparent solution; adjust the pH to 3.5 with 0.1 mol / L dilute nitric acid to obtain a uniformly dispersed Gd-citric acid complex solution, avoiding the precipitation of Gd 3+ hydrolysis to form hydroxide precipitation, laying a foundation for the subsequent dispersion of the silicon source.

[0020] Slowly add 0.3 mol, 50.43 g of tetraethyl orthosilicate dropwise at a dropping rate of 1 mL / min. After the dropping is completed, continue to stir for 1 h. Add 2.0 g, 1 wt% PEG-20000, and 16 mL of 8 vol% mesoporous SiO2 microspheres with a diameter of 15 nm, and ultrasonically disperse for 45 min, and then stir magnetically for 3 h to form a translucent homogeneous sol with a solid content of 25 vol%.

[0021] In this process, PEG inhibits particle agglomeration through steric hindrance effects, and the mesoporous SiO2 microspheres act as hard templates to guide the formation of hierarchical pores, and the sol stability reaches 72 hours without precipitation.

[0022] S2. Transfer the sol to a 500 mL polytetrafluoroethylene-lined reaction kettle, seal it and put it into an oven, heat it to 180 °C at a rate of 5 °C / min, and keep the temperature for 4 h. After the reaction is completed, naturally cool it to room temperature, pour out the gel-like substance to obtain a white porous gel precursor, which forms a "micropore (2 - 5 nm) - mesopore (15 - 30 nm)" hierarchical structure inside, providing a uniform microstructure for subsequent calcination; Freeze-dry the gel precursor at -50 °C under a vacuum of <10 Pa for 24 h. After crushing, place it in a microwave calcination furnace and process it in three stages: Stage 1. Organic matter removal stage: Introduce CO2 (purity 99.99%, flow rate 80 mL / min), heat it to 450 °C at a rate of 10 °C / min, keep the temperature for 30 min, the microwave power is 500 W, and the decomposition rate of organic matters such as citric acid >99%. After detection, the oxidation rate of Gd 3+ <1%, avoiding the formation of high-valent Gd 4+ which affects the neutron absorption performance.

[0023] Stage 2, the silicon-oxygen bond crosslinking stage: Switch to N2 atmosphere (flow rate 100 mL / min), heat up to 700 °C, hold for 15 min, and the microwave power is 600 W. During this process, the silicon source is initially condensed to form an amorphous SiO2 framework, and the weight loss rate of the precursor reaches 15%, providing a skeleton for subsequent crystallization.

[0024] Stage 3, the grain oriented growth stage: Switch to a 5% H2 / Ar mixed gas (flow rate 120 mL / min), heat up to 900 °C, hold for 2 h, and the microwave power is 700 W to obtain monodisperse Gd2Si2O7 nanometer powder. This process can reduce the oxygen vacancy concentration and crystal defects.

[0025] S3. Mix 100 g of the above powder with 2.0 g of 2 wt% Y2O3, 1.5 g of 1.5 wt% acrylamide monomer, and 0.2 g of 0.2 wt% ammonium polyacrylate dispersant, add 80 mL of deionized water, and ball mill at 300 rpm for 6 h to form a stable slurry with a viscosity ; the solid content of the slurry is 55 vol%, the particle dispersibility is excellent, and it is found by laser scattering instrument that there is no agglomeration phenomenon.

[0026] Then inject the slurry into a Φ50 mm×5 mm cylindrical mold, vacuum degas for 5 min, add 0.1 g of 0.1 wt% ammonium persulfate, seal it and put it into an 80 °C constant temperature oven, cure for 2 h, and demold to obtain a green body with a dimensional accuracy of ±0.05 mm.

[0027] The green body has a three-dimensional network polymer support structure, a compressive strength of 5 MPa, and can withstand the mechanical stress during the subsequent sintering process.

[0028] S4. Put the green body into a microwave sintering furnace, introduce CO2 with a flow rate of 100 mL / min, heat up to 1000 °C at a rate of 15 °C / min, and hold for 1 h; Real-time monitor the temperature of the green body, and the temperature uniformity is ±5 °C. During this process, the internal micropores of the green body are quickly eliminated, the porosity is reduced, and the density is increased, providing a basic dense structure for hot pressing sintering.

[0029] Then transfer the green body to a hot press furnace, apply an axial pressure of 20 MPa, introduce a 5% H2 / Ar mixed gas with a flow rate of 150 mL / min, heat up to 1400 °C at a rate of 8 °C / min, hold for 3 h, and real-time monitor the displacement change during the sintering process, and control the shrinkage rate within 10±1%.

[0030] Measure the relative density of the product of this step by the Archimedes method to be 99.2%, achieving full densification, and the average grain size is 1.2 μm.

[0031] S5. Place the prepared rare earth silicate ceramic into a silicate electrolyte solution containing 0.5 mol / L Na2SiO3 and 0.1 mol / L NaOH. Use the ceramic as the anode and a stainless-steel plate as the cathode. Connect a DC power supply and apply a voltage of 350 V with a current density of 0.3 A / cm 2 , and process for 8 minutes.

[0032] Detect the final product and find that a GdSiO4 / Al2O3 composite coating with a thickness of 8 μm is formed on the surface, the microhardness is 22 GPa, and the high-temperature oxidation resistance is improved by 3 times.

[0033] Detect the properties of the obtained ceramic. Its thermal neutron absorption cross-section is 1.15×10 4 barns / cm 2 , which is better than the requirement of 8×10 3 barns / cm 2 specified in the ASTM C1237 standard; Conduct a three-point bending test on it with a span of 30 mm. The measured flexural strength is 398 MPa, and the strength retention rate after 10 thermal shock cycles from 1300 °C to room temperature is 92%; After SEM observation, the retention rate of its hierarchical pores is >80%, and there are no obvious pores at the grain boundaries.

[0034] Comparative Example 1 Prepare gadolinium silicate ceramic by the traditional high-temperature solid-state method as follows: 1) Mix 0.2 mol (42.8 g) of Gd2O3 with 0.3 mol (18.0 g) of SiO2, add 5 wt% anhydrous ethanol as a dispersant, and ball-mill in a ball mill for 12 h to obtain a mixed powder with an average particle size of 5 μm.

[0035] 2) Dry-press the mixed powder into a green body with a size of Φ50 mm×5 mm under a pressure of 200 MPa; 3) Heat the green body in a muffle furnace under an air atmosphere at a rate of 5 °C / min to 1600 °C, hold for 6 h, and then cool with the furnace.

[0036] 4) Grind and polish the sintered body without surface modification.

[0037] The relative density of the product in Comparative Example 1 is 94.5%, the grain size is 8.5 μm, the flexural strength is 210 MPa, and the thermal neutron absorption cross-section is 8.2×10 3 barns / cm 2 . Compared with Example 1, the traditional method results in coarser grains, more grain boundary defects, and significantly lower density and strength due to high-temperature sintering; without using a template agent and microwave assistance, hierarchical pores cannot be formed, and the neutron absorption performance is limited.

[0038] Example 2 Prepare Eu-Y gradient doped fluorescence-structural integrated ceramics as follows: S1. Preparation of Eu 3+ / Y 3+ gradient doped nanopowders: Take 0.05 mol, 19.82 g dissolve it in 100 mL of deionized water, add 0.125 mol, 24.02 g of citric acid, stir magnetically for 45 min, use 0.1 mol / L NaOH solution to adjust the pH to 3.2 to form a light red Eu-citric acid complex solution; After 30 min, add 0.1 mol, and continue stirring for 1.5 h to coordinate Y 3+ with the remaining citric acid, and the solution color remains transparent and colorless.

[0039] Slowly dropwise add 0.3 mol, 50.43 g of tetraethyl orthosilicate at a dropping rate of 1.5 mL / min, and continuously stir during the dropping process. After the dropping is completed, add 1.5 g, 1 wt% PEG-20000 and 10 mL, 5 vol% mesoporous SiO2 microspheres with a particle size of 10 nm, ultrasonically disperse for 60 min, and then stir magnetically for 4 h to form a pale yellow sol with a solid content of 22 vol%.

[0040] S2. Transfer the sol to a 250 mL polytetrafluoroethylene reaction kettle, seal it, and heat it to 190 °C at a rate of 5 °C / min, and keep it warm for 5 h. After the reaction is completed, cool it to room temperature with water, pour out the white gel precursor, and wash it 3 times with deionized water to remove the unreacted citric acid.

[0041] Freeze-dry the gel precursor at -40 °C for 48 h, crush it, and place it in a microwave calcination furnace for three-stage treatment: Stage 1. Removal of organic matter: Pass in a CO2 atmosphere with a flow rate of 60 mL / min, keep it warm at 450 °C for 30 min, and the microwave power is 600 W; Stage 2. Intermediate crystallization: Switch to an N2 atmosphere with a flow rate of 150 mL / min, keep it warm at 750 °C for 20 min to form an amorphous RE-Si-O precursor; Stage 3. Grain oriented growth: Switch to a 5% H2 / Ar mixed gas with a flow rate of 180 mL / min, keep it warm at 950 °C for 2.5 h, and the microwave power is 750 W to obtain nanopowders.

[0042] S3. Mix 100 g of the above powder with 1.0 g (1 wt%) of XC-72 type graphene nanosheets with a thickness of 5 - 10 nm and 1.5 g (1.5 wt%) of acrylamide monomer. Add 85 mL of deionized water and 5 mL of absolute ethanol, and ball mill for 8 h. The graphene is dispersed uniformly in the form of single sheets in the slurry through the synergistic dispersion of ultrasonic and ball milling.

[0043] Then inject the slurry into a custom-made arc-shaped mold with a radius of curvature of 50 mm, degas it under vacuum for 10 min, add 0.15 g (0.15 wt%) of ammonium persulfate and 0.08 wt% of N,N'-methylenebisacrylamide, and cure it at a constant temperature of 80 °C for 3 h. After demolding, an arc-shaped green body is obtained.

[0044] S4. Place the green body into a microwave sintering furnace, introduce CO2 with a flow rate of 90 mL / min, heat it to 1050 °C at a rate of 12 °C / min, and hold for 1.5 h. Then transfer it to a hot pressing furnace, apply a pressure of 15 MPa, introduce a 5% H2 / Ar atmosphere with a flow rate of 200 mL / min, heat it to 1350 °C at a rate of 6 °C / min, and hold for 2.5 h.

[0045] After sintering, the ceramic is annealed in an air atmosphere at 500 °C for 2 h to eliminate residual stress and enhance the stability of fluorescence emission.

[0046] The final product is detected by an absolute quantum efficiency meter, and its fluorescence quantum yield is increased from 80% to 82%, which is attributed to the reduction of the influence of lattice distortion on the luminescence center by the annealing treatment.

[0047] The obtained ceramic is subjected to performance testing. Its fluorescence concentration quenching threshold is 1.2 mol%; the room temperature volume resistivity , and the resistivity at 1000 °C (change rate < 25%); the flexural strength is 325 MPa, and the fracture toughness , which is significantly improved compared with traditional fluorescent ceramics.

[0048] Comparative Example 2 Prepare a single-template agent microwave-sintered fluorescent ceramic as follows: 1) Use the Eu-Y co-doping formula of Example 2, but only add 1 wt% of PEG-20000. The hydrothermal reaction temperature is 160 °C, and a single N2 atmosphere is used for microwave calcination to obtain nano-powders without hierarchical pores, and its specific surface area is 20 m 2 / g.

[0049] 2) Mix the powder with a 3 wt% polyvinyl alcohol binder, dry press it into shape, and conduct conventional microwave sintering. The sintering temperature is 1350 °C, in an N2 atmosphere, and hold for 2 h.

[0050] The fluorescence quantum yield of the product in Comparative Example 2 was 65%, and the volume resistivity , and the fluorescence concentration quenching threshold was 0.8 mol%. The single template agent in Comparative Example 2 could not form hierarchical pores, and the low specific surface area led to a decrease in fluorescence efficiency; moreover, gradient doping and atmosphere regulation were not carried out for the rare earth silicate powder, and the Eu 3+ concentration quenching occurred in advance, and the conductivity decreased due to poor pore connectivity.

[0051] Therefore, in the preparation method of a rare earth silicate ceramic of the present invention, through the integrated process of in-situ synthesis of nano-powders - gel-casting molding - microwave-assisted hot-pressing sintering, compared with the traditional sintering process, the sintering time is greatly shortened, the energy consumption is reduced, the density and uniformity of the ceramic are significantly improved, and defects such as pores are effectively reduced, so that the prepared rare earth silicate ceramic has excellent mechanical properties, thermal properties and chemical stability, meeting the strict requirements for high-performance materials in high-end fields such as aerospace and energy.

[0052] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a rare earth silicate ceramic, characterized in that, It includes the following steps: S1. Co-dissolve rare earth nitrate and citric acid in deionized water, adjust the pH to form a rare earth-citric acid complex solution; Add a silicon source, fully stir, then add a double template agent, ultrasonically disperse, and then perform magnetic stirring to form a homogeneous sol; S2. Transfer the homogeneous sol to a reaction kettle lined with polytetrafluoroethylene for hydrothermal reaction to generate a gel precursor; microwave calcine the gel precursor in a dynamic atmosphere in three stages to obtain rare earth silicate powder; S3. Mix the rare earth silicate powder with 1-3 wt% sintering aid and 0.5-2 wt% acrylamide monomer, add deionized water for ball milling. After ball milling is completed, inject the obtained slurry into a mold, add 0.1 wt% ammonium persulfate initiator, and cure at 65-90 °C. After demolding, obtain a green body; S4. Place the green body in a microwave sintering furnace, heat it to 950-1050 °C in a CO2 atmosphere, keep it warm for 1 h, then transfer it to a hot press furnace, apply a pressure of 10-30 MPa, and keep it warm at 1300-1500 °C for 2-4 h to obtain a dense rare earth silicate ceramic.

2. The preparation method of a rare earth silicate ceramic according to claim 1, characterized in that: In the above S1, the molar ratio of rare earth nitrate to citric acid is 1:2-3, and the rare earth nitrate is one or a combination of Eu(NO3)3, Ce(NO3)3, La(NO3)3, Gd(NO3)3, Tb(NO3)3.

3. The preparation method of a rare earth silicate ceramic according to claim 1, characterized in that: In the above S1, the silicon source is tetraethyl orthosilicate or silica sol, and the molar ratio of rare earth in rare earth nitrate to silicon in the silicon source is 1:1-4; the double template agent is 0.5-1 wt% polyethylene glycol and 5-10 vol% mesoporous SiO2 microspheres, the ultrasonic dispersion time is 30-60 min, and the magnetic stirring is 2-4 h.

4. The preparation method of a rare earth silicate ceramic according to claim 1, characterized in that: In the above S2, the temperature of the hydrothermal reaction is 160-200 °C, and the time is 3-5 h.

5. The preparation method of a rare earth silicate ceramic according to claim 1, characterized in that: In the above S2, after freeze-drying the gel precursor, place it in a microwave oven equipped with an infrared real-time temperature measurement module for three-stage microwave calcination. The specific steps are as follows: 1) Stage 1: Pass in CO2, control the flow rate to be 30-80 ml / min, heat up to 400-500 °C, and keep it warm for 30 min; 2) Stage 2: Switch to N2 atmosphere, heat up to 700-800 °C, and keep it warm for 15 min; 3) Stage 3: Switch to 5% H2 / N2 mixed gas, heat up to 900-1000 °C, keep it warm for 2 h, and the microwave power is 600-800 W.

6. The preparation method of a rare earth silicate ceramic according to claim 1, characterized in that: In S3, the sintering aid is one of Y2O3, AIN or graphene nanosheets; the ball milling time is 4 to 8 h, the solid content of the obtained slurry is 50 to 60 vol%, and the viscosity is .

7. The preparation method of a rare earth silicate ceramic according to claim 1, characterized in that: In the above S4, an Ar / 5% H2 mixed gas is used in the hot press furnace, and the heating rate is 5-10 °C / min.