A method for producing a rare-earth silicate ceramic

Abstract

The application discloses a preparation method of a rare earth silicate ceramic and belongs to the technical field of inorganic ceramic material preparation.The in-situ synthesis-gel injection molding-microwave-assisted hot-pressing sintering integrated process of the application greatly shortens sintering time, reduces energy consumption, significantly improves the density and uniformity of the ceramic, effectively reduces defects such as pores, and makes the prepared rare earth silicate ceramic have excellent mechanical properties, thermal properties and chemical stability, thereby meeting the strict requirements of high-end fields such as aerospace and energy on high performance of materials.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of inorganic ceramic material preparation, and particularly to a preparation method of rare earth silicate ceramic. BACKGROUND

[0002] As an advanced functional material with excellent high-temperature resistance, oxidation resistance, low thermal conductivity and other properties, rare earth silicate ceramic is widely used in high-end fields such as aerospace, energy, electronics, etc. However, the current preparation method of rare earth silicate ceramic still has many deficiencies. It usually adopts high-temperature solid-phase sintering, which has problems such as high energy consumption, coarse grain (> 5 μm), poor density, etc. Similarly, in the process of preparing rare earth silicate powder, high-temperature solid-phase 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, the method does not solve the process connection problem of ceramic forming and densification, and has problems such as low ceramic density, uncontrollable grain growth, uneven distribution of rare earth elements, and disconnection between forming process and sintering process.

[0003] Therefore, a method for efficiently, controllably and high-performance preparation of rare earth silicate ceramic is proposed. SUMMARY

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

[0005] To achieve the above-mentioned purpose, the present application provides a preparation method of rare earth silicate ceramic, comprising the following steps:

[0006] S1, dissolve rare earth nitrate and citric acid in deionized water to form a rare earth-citric acid complex solution; add a silicon source, stir thoroughly, then add a bimodal template agent, ultrasonic dispersion, and then magnetic stirring to form a homogeneous sol;

[0007] S2, transfer the homogeneous sol to a reaction kettle lined with polytetrafluoroethylene and perform hydrothermal reaction to generate a gel precursor; perform three-stage microwave calcination on the gel precursor under a dynamic atmosphere to obtain a rare earth silicate powder;

[0008] 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, then inject the obtained slurry into a mold, add 0.1 wt% ammonium persulfate initiator, and solidify at 65-90℃ to obtain a green body after demolding;

[0009] S4, placing the embryo body in a microwave sintering furnace, heating to 950-1050℃ under CO2 atmosphere, keeping warm for 1h, then transferring to a hot press furnace, applying a pressure of 10-30MPa, keeping warm at 1300-1500℃ for 2-4h, obtaining a dense rare earth silicate ceramic.

[0010] Preferably, in S1, the molar ratio of the rare earth nitrate to citric acid is 1:2-3, and the rare earth nitrate is one or more of Eu(NO3)3, Ce(NO3)3, La(NO3)3, Gd(NO3)3, and Tb(NO3)3.

[0011] When the rare earth nitrate is a combination of multiple, the rare earth nitrate is added in a predetermined order, and after the first rare earth nitrate forms a complex solution with citric acid, the rare earth nitrate of subsequent order is added after a suitable time;

[0012] The pH is adjusted to 3-4 by dilute nitric acid, sodium hydroxide or ammonia water.

[0013] Preferably, in S1, the silicon source is tetraethyl orthosilicate or silica sol, 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 0.5-1wt% polyethylene glycol and 5-10vol% mesoporous SiO2 microspheres, the ultrasonic dispersion time is 30-60min, and the magnetic stirring time is 2-4h.

[0014] Preferably, in S2, the temperature of the hydrothermal reaction is 160-200℃, and the time is 3-5h.

[0015] Preferably, in S2, after freeze-drying the gel precursor, it is placed in a microwave oven equipped with an infrared real-time temperature measurement module for three-stage calcination, and the specific steps are as follows:

[0016] 1) Stage one: pass in CO2, control the flow rate to be 30-80ml / min, heat to 400-500℃, keep warm for 30min;

[0017] 2) Stage two: switch to N2 atmosphere, heat to 700-800℃, keep warm for 15min;

[0018] 3) Stage three: switch to 5%H2 / N2 mixed gas, heat to 900-1000℃, keep warm for 2h, and the microwave power is 600-800W.

[0019] Preferably, in S3, the sintering aid is one of Y2O3, AIN or graphene nanosheet; the ball milling time is 4-8h, and the obtained slurry has a solid content of 50-60vol% and a viscosity of ;

[0020] In the S3, 0.1-0.3wt% polyacrylamide dispersant is added in the ball milling process; and N,N'-methylene bisacrylamide is added in the solidification process.

[0021] Preferably, in the S4, Ar / 5%H2 mixed gas is used in the hot-pressing furnace, and the heating rate is 5-10℃ / min.

[0022] Preferably, the rare earth silicate ceramic is applied to preparation of nuclear radiation shielding ceramic, fluorescent-structure integrated ceramic or conductive ceramic to realize functional ceramic preparation.

[0023] 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 300-500V direct current voltage, 0.1-0.5A / cm 2 current density for 5-10min in a silicate electrolyte to generate a 5-10μm-thick rare earth silicate / Al2O3 composite coating on the ceramic surface, thus obtaining the nuclear radiation shielding ceramic.

[0024] Therefore, the preparation method of the rare earth silicate ceramic has the following beneficial effects:

[0025] (1) In the preparation of the rare earth silicate powder, firstly, in the hydrothermal reaction, the high-pressure environment promotes the hydrolysis and polycondensation of the silicate, and cooperates with the template agent to generate a gel precursor with hierarchical pores, thus improving the subsequent sintering activity; meanwhile, the hierarchical pore structure constructed by the double template agent significantly improves the exposure rate of the reaction active sites of the catalytic carrier ceramic.

[0026] (2) In the preparation of the rare earth silicate powder, three-stage microwave calcination is adopted, the atmosphere, temperature and time of different stages are accurately controlled, not only the organic components in the precursor are effectively removed, but also the grain size and morphology of the ceramic powder are controlled; in the first stage, the high-frequency magnetic field of the microwave is used to remove the organic components in the precursor, and the carbon dioxide atmosphere can inhibit the oxidation of the rare earth elements, avoid the out-of-control of the valence state of the rare earth elements, and improve the stability of the material; in the second stage, the silicon-oxygen network is preliminarily polymerized; in the third stage, under the H2 / N2 mixed gas atmosphere, the oxygen vacancies are repaired and the grain size is controlled, thus obtaining the rare earth silicate powder with high purity and high quality, which provides high-quality raw materials for the preparation of high-performance ceramics.

[0027] (3) The application adopts the combination of microwave sintering and hot pressing in the ceramic preparation stage, compared with the traditional sintering process, the sintering time is greatly shortened, and the energy consumption is reduced. At the same time, under the conditions of specific atmosphere and pressure, the density and uniformity of the ceramic can be significantly improved, the defects such as pores are effectively reduced, the prepared rare earth silicate ceramic has excellent mechanical properties, thermal properties and chemical stability, and meets the strict requirements of high-end fields such as aerospace and energy on high performance materials.

[0028] The technical solutions of the present application will be further described in detail through examples. DETAILED DESCRIPTION

[0029] The technical solutions of the present application will be further described in detail through examples.

[0030] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are part of the embodiments of the present application, not all.

[0031] Example 1

[0032] The preparation of high-density gadolinium silicate nuclear radiation shielding ceramic is as follows:

[0033] S1, double-template agent guided synthesis of Gd2Si2O7 nano powder: first, 0.2mol、 , 0.5mol, 96.0g citric acid, dissolved in 200mL deionized water, magnetic stirring for 30min, forming a colorless transparent solution; using 0.1mol / L dilute nitric acid to adjust pH to 3.5, obtaining uniformly dispersed Gd-citric acid complex solution, avoiding Gd 3+ hydrolysis to form hydroxide precipitate, laying foundation for subsequent silicon source dispersion.

[0034] Slowly add 0.3mol, 50.43g tetraethyl orthosilicate, dropwise addition rate 1mL / min, continue to stir for 1h after dropwise addition is completed, add 2.0g, 1wt% PEG-20000 and 8vol% mesoporous SiO2 microspheres with diameter of 15nm, 16mL, ultrasonic dispersion for 45min, then magnetic stirring for 3h, forming a semi-transparent homogeneous sol with solid content of 25vol%.

[0035] In this process, PEG inhibits particle agglomeration through steric hindrance effect, mesoporous SiO2 microspheres act as hard template to guide hierarchical pore formation, and the stability of the sol reaches 72 hours without precipitation.

[0036] S2, transfer the sol to a 500 mL PTFE-lined autoclave, seal it and put it in an oven, heat it to 180℃ at a rate of 5℃ / min, keep it at this temperature for 4 h, after the reaction is completed, cool it to room temperature naturally, pour out the gel-like substance, and obtain a white porous gel precursor with a "micropore (2-5 nm)-mesopore (15-30 nm)" hierarchical structure, which provides a uniform microstructure for subsequent calcination;

[0037] freeze dry the gel precursor at -50℃ under a vacuum of <10 Pa for 24 h, crush it and place it in a microwave calcination furnace, and process it in three stages:

[0038] Stage 1: organic matter removal stage: introduce CO2 (purity 99.99%, flow rate 80 mL / min), heat it to 450℃ at a rate of 10℃ / min, keep it at this temperature for 30 min, microwave power 500 W, the decomposition rate of citric acid and other organic matters is >99%, and the detection shows that the Gd 3+ oxidation rate is <1%, avoiding high-valence Gd 4+ which affects the neutron absorption performance.

[0039] Stage 2: silicon-oxygen bond crosslinking stage: switch to N2 atmosphere (flow rate 100 mL / min), heat it to 700℃, keep it at this temperature for 15 min, microwave power 600 W. In this process, the silicon source is preliminarily condensed to form an amorphous SiO2 framework, and the weight loss rate of the precursor reaches 15%, providing a skeleton for subsequent crystallization.

[0040] Stage 3: crystal grain directional growth stage: switch to 5% H2 / Ar mixed gas (flow rate 120 mL / min), heat it to 900℃, keep it at this temperature for 2 h, microwave power 700 W, and obtain monodisperse Gd2Si2O7 nanopowder. This process can reduce the oxygen vacancy concentration and reduce crystal defects.

[0041] S3, mix 100 g of the above-mentioned powder with 2.0 g, 2 wt% Y2O3, 1.5 g, 1.5 wt% acrylamide monomer, and 0.2 g, 0.2 wt% polyacrylamide dispersant, add 80 mL of deionized water, and ball mill at 300 rpm for 6 h to form a stable slurry with a viscosity of 100 mPa·s. The slurry has a solid content of 55 vol% and excellent particle dispersibility, and no agglomeration is found by laser scattering instrument detection.

[0042] Then inject the slurry into a Φ50 mm×5 mm cylindrical mold, vacuum degas for 5 min, add 0.1 g, 0.1 wt% ammonium persulfate, seal it and put it in an 80℃ constant temperature box, and solidify it for 2 h. After demolding, a green body with a size accuracy of ±0.05 mm is obtained.

[0043] The green body has a three-dimensional network polymer support structure and a compressive strength of 5 MPa, which can withstand mechanical stress in the subsequent sintering process.

[0044] S4, the green body is placed into a microwave sintering furnace, CO2 is introduced at a flow rate of 100 mL / min, and the temperature is raised to 1000℃ at a rate of 15℃ / min, and the temperature is kept for 1 h;

[0045] The temperature of the green body is monitored in real time, and the temperature uniformity is ±5℃. During this process, the micro-pores in the green body are quickly eliminated, the porosity is reduced, and the density is improved, thereby providing a basic dense structure for hot-pressing sintering.

[0046] Then the green body is transferred to a hot-pressing furnace, an axial pressure of 20 MPa is applied, 5% H2 / Ar mixed gas is introduced at a flow rate of 150 mL / min, the temperature is raised to 1400℃ at a rate of 8℃ / min, and the temperature is kept for 3 h. The displacement change is monitored in real time during the sintering process, and the shrinkage rate is controlled to be 10±1%.

[0047] The relative density of the product of this step is measured by the Archimedes method to be 99.2%, full densification is achieved, and the average grain size is 1.2 μm.

[0048] S5, the prepared rare earth silicate ceramic is placed into a silicate electrolyte containing 0.5 mol / L Na2SiO3 and 0.1 mol / L NaOH, the anode is a ceramic made of a stainless steel plate, a direct current power supply is connected, a voltage of 350 V is applied, and the current density is 0.3 A / cm 2 for 8 min.

[0049] The final product is detected, and it is found that a GdSiO4 / Al2O3 composite coating with a thickness of 8 μm is generated on the surface, the microhardness is 22 GPa, and the high-temperature oxidation resistance is improved by 3 times.

[0050] The performance of the obtained ceramic is detected, and the thermal neutron absorption cross section is 1.15×10 4 barns / cm 2 , which is better than the standard requirement of 8×10 3 barns / cm 2 of ASTM C1237;

[0051] Three-point bending test is performed on it with a span of 30 mm, and the bending strength is detected to be 398 MPa, and the strength retention rate is 92% after 10 times of thermal shock cycling at 1300℃-room temperature;

[0052] After SEM observation, the retention rate of the hierarchical pores is >80%, and there are no obvious pores at the grain boundaries.

[0053] Comparative Example 1

[0054] Gadolinium silicate ceramic is prepared by a traditional high-temperature solid-phase method as follows:

[0055] 1) Mix 0.2 mol, 42.8 g Gd2O3 with 0.3 mol, 18.0 g 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.

[0056] 2) The mixed powder is dry-pressed into a Φ50mm×5mm blank under a pressure of 200MPa;

[0057] 3) The billet is heated to 1600℃ in an air atmosphere in a muffle furnace at 5℃ / min, held for 6 hours, and then cooled with the furnace.

[0058] 4) The sintered body was polished but not surface modified.

[0059] The product in Comparative Example 1 had a relative density of 94.5%, a grain size of 8.5 μm, a flexural strength of 210 MPa, and a thermal neutron absorption cross-section of 8.210. 3 barns / cm 2 Compared with Example 1, the traditional method resulted in coarse grains and numerous grain boundary defects due to high-temperature sintering, and the density and strength were significantly lower than those of Example 1. Without the use of template agents and microwave assistance, hierarchical channels could not be formed, thus limiting the neutron absorption performance.

[0060] Example 2

[0061] The preparation of Eu-Y gradient-doped fluorescent-structure integrated ceramics is as follows:

[0062] S1, Eu 3+ / Y 3+ Preparation of gradient-doped nanopowders: Take 0.05 mol, 19.82 g Dissolve in 100 mL of deionized water, add 0.125 mol of 24.02 g of citric acid, stir magnetically for 45 min, and adjust the pH to 3.2 with 0.1 mol / L NaOH solution to form a light red Eu-citric acid complex solution;

[0063] Add 0.1 mol after 30 minutes. Continue stirring for 1.5 hours to allow Y to... 3+ When it coordinates with the remaining citric acid, the solution remains transparent and colorless.

[0064] 0.3 mol of 50.43 g of tetraethyl orthosilicate was slowly added dropwise at a rate of 1.5 mL / min, with continuous stirring during the addition. After the addition was complete, 1.5 g of 1 wt% PEG-20000 and 10 mL of 5 vol% mesoporous SiO2 microspheres with a particle size of 10 nm were added. The mixture was ultrasonically dispersed for 60 min and then magnetically stirred for 4 h to form a pale yellow sol with a solid content of 22 vol%.

[0065] S2, transfer the sol to a 250 mL Teflon reactor, after sealing, increase the temperature to 190 °C at a rate of 5 °C / min, and keep for 5 h. After the reaction is completed, cool to room temperature with water, pour out the white gel precursor, and wash with deionized water for 3 times to remove unreacted citric acid.

[0066] Freeze-dry the gel precursor at -40 °C for 48 h, crush, and then place in a microwave calcination furnace for three-stage treatment:

[0067] Stage 1, remove organic matter: pass CO2 atmosphere at a flow rate of 60 mL / min, keep at 450 °C for 30 min, and microwave power is 600 W;

[0068] Stage 2, intermediate crystallization: switch to N2 atmosphere at a flow rate of 150 mL / min, keep at 750 °C for 20 min, and form amorphous RE-Si-O precursor;

[0069] Stage 3, crystal grain directional growth: switch to 5% H2 / Ar mixed gas at a flow rate of 180 mL / min, keep at 950 °C for 2.5 h, and microwave power is 750 W, to obtain nanopowder.

[0070] S3, mix 100 g of the above powder with 1.0 g, 1 wt% of graphene nanosheet XC-72 with a thickness of 5-10 nm, 1.5 g, 1.5 wt% of acrylamide monomer, add 85 mL of deionized water and 5 mL of anhydrous ethanol, and ball mill for 8 h; the graphene is dispersed by ultrasonic-ball milling, and uniformly distributed in the form of single sheet in the slurry.

[0071] Then, the slurry is injected into a custom-made arc-shaped mold with a radius of curvature of 50 mm, vacuum degassed for 10 min, 0.15 g, 0.15 wt% of ammonium persulfate and 0.08 wt% of N,N'-methylenebisacrylamide are added, and the temperature is kept at 80 °C for 3 h. After demolding, an arc-shaped body is obtained.

[0072] S4, place the body into a microwave sintering furnace, pass CO2 at a flow rate of 90 mL / min, increase the temperature to 1050 °C at a rate of 12 °C / min, and keep for 1.5 h;

[0073] Then, it is transferred to a hot-pressing furnace, a pressure of 15 MPa is applied, 5% H2 / Ar atmosphere is passed at a flow rate of 200 mL / min, the temperature is increased to 1350 °C at a rate of 6 °C / min, and kept for 2.5 h.

[0074] After sintering, the ceramic is annealed in air atmosphere at 500 °C for 2 h to eliminate residual stress and enhance the stability of fluorescence emission.

[0075] The final product was detected by absolute quantum efficiency instrument, and the fluorescence quantum yield was increased from 80% to 82%, which was attributed to the annealing process reducing the influence of lattice distortion on the light emitting center.

[0076] The obtained ceramic was detected for performance, and the fluorescence concentration quenching threshold was 1.2 mol%; the room temperature volume resistivity , the resistivity at high temperature of 1000 DEG C (change rate < 25%); the bending strength was 325 MPa, and the fracture toughness was significantly improved compared with traditional fluorescent ceramics.

[0077] Comparative example 2

[0078] The single-template microwave sintering fluorescent ceramic was prepared as follows:

[0079] 1) The Eu-Y co-doped formula of example 2 was used, but only 1 wt% PEG-20000 was added, the hydrothermal reaction temperature was 160 DEG C, the single N2 atmosphere was used for microwave calcination, the nano-powder without hierarchical pores was obtained, and the specific surface area was 20 m 2 / g.

[0080] 2) The powder was mixed with 3 wt% polyvinyl alcohol binder, dry-pressed, and conventionally microwave sintered, the sintering temperature was 1350 DEG C, the N2 atmosphere, and the holding time was 2 h.

[0081] The fluorescence quantum yield of the product in comparative example 2 was 65%, the volume resistivity was 1.2*10-3 ohm*cm, and the fluorescence concentration quenching threshold was 0.8 mol%. In comparative example 2, the single template cannot form hierarchical pores, the low specific surface area leads to the decrease of fluorescence efficiency, and the Eu 3+ concentration quenching is advanced, and the conductivity is reduced due to the poor connectivity of the pores.

[0082] Therefore, the preparation method of the rare earth silicate ceramic integrates the nano-powder in-situ synthesis-gel injection molding-microwave assisted hot-pressing sintering process, 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, the defects such as pores are effectively reduced, the prepared rare earth silicate ceramic has excellent mechanical properties, thermal properties and chemical stability, and meets the strict requirements of high-end fields such as aerospace and energy on high performance materials.

[0083] It should be pointed out finally that the above examples are only used to illustrate the technical solutions of the present application but not to limit it, and although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can still be modified or replaced equivalently, and these modifications or equivalent replacements should not make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for producing a rare-earth silicate ceramic, characterized by, The method comprises the following steps: S1, dissolving rare earth nitrate and citric acid in deionized water to form a rare earth-citric acid complex solution by adjusting pH; After adding a silicon source and fully stirring, a double template agent is added, the double template agent is 0.5-1 wt% polyethylene glycol and 5-10 vol% mesoporous SiO2 microspheres, ultrasonic dispersion is performed, and then magnetic stirring is performed to form a homogeneous sol; S2, transferring the homogeneous sol to a reaction kettle lined with polytetrafluoroethylene to perform hydrothermal reaction to generate a gel precursor; the hydrothermal reaction is performed at a temperature of 160-200 DEG C for 3-5 h, the gel precursor is calcined in a dynamic atmosphere in three stages to obtain a rare earth silicate powder; specifically: After freeze-drying the gel precursor, the gel precursor is placed in a microwave oven equipped with an infrared real-time temperature measurement module to perform three-stage microwave calcination, and the specific steps are as follows: 1) stage one: CO2 is introduced, the flow rate is controlled to be 30-80 ml / min, the temperature is raised to 400-500 DEG C, and the temperature is kept for 30 min; 2) stage two: switch to N2 atmosphere, raise the temperature to 700-800 DEG C, and keep the temperature for 15 min; 3) stage three: switch to 5% H2 / N2 mixed gas, raise the temperature to 900-1000 DEG C, and keep the temperature for 2 h, the microwave power is 600-800 W; S3, mixing the rare earth silicate powder with 1-3 wt% sintering aid and 0.5-2 wt% acrylamide monomer, adding deionized water for ball milling, after ball milling, the obtained slurry is injected into a mold, 0.1 wt% ammonium persulfate initiator is added, and the mold is cured at 65-90 DEG C, and the green body is obtained after demolding; S4, placing the green body in a microwave sintering furnace, heating to 950-1050 DEG C under CO2 atmosphere, keeping the temperature for 1 h, then transferring to a hot pressing furnace, applying a pressure of 10-30 MPa, keeping the temperature at 1300-1500 DEG C for 2-4 h, using Ar / 5% H2 mixed gas in the hot pressing furnace, and raising the temperature at a rate of 5-10 DEG C / min to obtain a dense rare earth silicate ceramic; The prepared rare earth silicate ceramic is applied to prepare a nuclear radiation shielding ceramic, a fluorescent-structure integrated ceramic or a conductive ceramic to realize functional ceramic preparation.

2. The method of claim 1, wherein the method further comprises: In the S1, the molar ratio of the rare earth nitrate to the citric acid is 1:2-3, and the rare earth nitrate is one or more combinations thereof. ​ 3. The method of claim 1, wherein the method further comprises: In the S1, the silicon source is tetraethyl orthosilicate or silica sol, the molar ratio of rare earth in the rare earth nitrate to silicon in the silicon source is 1:1-4; the ultrasonic dispersion time is 30-60 min, and the magnetic stirring time is 2-4 h. ​ 4. The method of claim 1, wherein the method further comprises: The sintering aid in the S3 is one of Y2O3, AIN or graphene nanosheet; the ball milling time is 4-8h, the solid content of the obtained slurry is 50-60vol%, and the viscosity is . ​