CN114804912A - Method for preparing high-toughness high-temperature-resistant directionally-arranged silicon nitride monolithic porous ceramic - Google Patents
Method for preparing high-toughness high-temperature-resistant directionally-arranged silicon nitride monolithic porous ceramic Download PDFInfo
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Abstract
Preparation method of high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic, alpha-Si 3 N 4 As a raw material, Y 2 O 3 As an auxiliary agent, the beta-Si with high length-diameter ratio is prepared by a normal pressure sintering method 3 N 4 Whiskers, beta-Si prepared 3 N 4 Preparing the crystal whisker with casting slurry, utilizing casting molding to directionally arrange the silicon nitride crystal whisker to prepare a film and carry out lamination sample preparation, obtaining silicon nitride by reaction of SiO high-temperature evaporation gas phase infiltration and residual carbon between fibers, and preparing to obtain the silicon nitride single crystalA stone porous ceramic material; the invention provides an organization design idea of fiber orientation arrangement and secondary silicon nitride bonding, realizes the effective preparation of porous ceramics constructed by pure silicon nitride fibers, exerts the advantage of anisotropy through orientation arrangement, thereby effectively improving the fracture toughness of the silicon nitride ceramics, greatly improving the high temperature resistance through the bonding of the pure silicon nitride, and greatly expanding the application range of the porous silicon nitride ceramic material.
Description
Technical Field
The invention belongs to the technical field of silicon nitride ceramic sintering, and particularly relates to a method for preparing high-toughness high-temperature-resistant directionally-arranged silicon nitride monolithic porous ceramic, which is suitable for various high-temperature filtering separators, catalyst carriers, sound absorption materials, wave transmission materials and the like.
Background
Along with the increase of the speed of the missile, the working environment of the missile radome is severe day by day, and when the missile flies in the atmosphere at hypersonic speed, pneumatic heating is very serious. In order to protect the normal operation of systems such as communication, remote measurement, guidance and detonation of an aircraft, the supersonic missile radome has the advantages of high temperature resistance, ablation resistance, excellent mechanical property and electrical property and the like. The silicon nitride material has the advantages of excellent mechanical property, good thermal shock resistance, high temperature resistance, corrosion resistance and the like, is a new-generation antenna housing material with great prospect, and the porous design of the material ensures the necessary wave-transmitting characteristic and bearing.
However, poor reliability due to poor brittleness of the ceramic material is a major obstacle affecting the application of the ceramic wave-transparent material, and particularly, the low toughness of the porous silicon nitride ceramic becomes an important index that the porous silicon nitride wave-transparent material must be improved.
The fracture toughness is a measure of the capability of the material for preventing the instability and the propagation of the macrocrack and is also a toughness parameter of the material for resisting the brittle failure, which determines the use reliability and the anti-damage capability of the material, and the low fracture toughness restricts the further development and the large-scale engineering application of the material. Patent No. 202110154424.3 discloses the preparation of intercrystalline-free glass phase beta-Si with high aspect ratio whiskers 3 N 4 Method for producing porous ceramics by compression molding process to obtain beta-Si 3 N 4 After compression molding, introducing a carbon source, and carrying out carbothermic reduction on beta-Si 3 N 4 alpha-Si is generated at the lap joint of the crystal whisker 3 N 4 Preparation of Si 3 N 4 Overlap of beta-Si 3 N 4 The whisker porous ceramic material has a porosity of 51.2% and a fracture toughness of only 3.1, so that the performance of the whisker porous ceramic material is not ideal.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a method for preparing high-toughness high-temperature-resistant oriented silicon nitride monolithic porous ceramics, and the method uses alpha-Si 3 N 4 As a raw material, Y 2 O 3 As an auxiliary agent, the beta-Si with high length-diameter ratio is prepared by a normal pressure sintering method 3 N 4 Whiskers, beta-Si prepared 3 N 4 Preparing casting slurry for the whiskers, carrying out directional arrangement on the silicon nitride whiskers by utilizing casting molding to prepare a film, laminating to prepare a sample, carrying out high-temperature evaporation gas-phase infiltration of SiO, reacting with residual carbon between fibers to obtain silicon nitride, and preparing to obtain the silicon nitride monolithic porous ceramic material, namely the beta-Si monolithic porous ceramic material with high aspect ratio 3 N 4 The crystal whisker has directional arrangement property by tape casting, and improves Si 3 N 4 Flexural strength and fracture toughness of ceramics, on the other hand by high temperature C-SiO-N 2 The carbon thermal reduction utilizes the bonding of pure silicon nitride, and the high temperature resistance is greatly improved.
In order to achieve the purpose, the invention adopts the technical scheme that:
a method for preparing high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic comprises the following steps:
step 1: weighing 90-95 wt% of alpha-Si according to mass percentage 3 N 4 And 5-10 wt% of rare earth oxide mixed powder, and after being uniformly mixed, the mixed powder is loosely placed into a graphite crucible;
step 2: placing the crucible filled with the mixed powder into a multifunctional sintering furnace, filling nitrogen as protective atmosphere, controlling the heating rate to be less than 10 ℃/min, preserving the heat at 1600-1750 ℃ for 2-3h, and then cooling along with the furnace;
and step 3: putting the reactant obtained in the step 2 into a polytetrafluoroethylene container filled with hydrofluoric acid, heating at 60-80 ℃ and carrying out magnetic stirring treatment until the block becomes powder, and steamingDistilled water is rinsed for a plurality of times until Ph is 7, and then the dried product is dried to obtain beta-Si 3 N 4 Whisker;
and 4, step 4: 20-40 wt% of beta-Si 3 N 4 Adding whiskers, 45.8-66.90% of an organic solvent, 22.9-33.45% of butanone and 1% of a dispersing agent into a beaker, adding a magnetic rotor, stirring for 6-8 h, adding 5-6% of a binding agent polyvinyl butyral ester, 3-3.6% of polyethylene glycol and 3-3.6% of dibutyl phthalate into the mixed slurry, and magnetically stirring for 6-8 h to obtain uniform casting slurry;
and 5: defoaming the casting slurry obtained in the step 4 through a vacuum defoaming machine for 30-60min, and taking out to prepare casting;
step 6: pouring the casting slurry into a container, inserting the casting substrate into the slurry, and vertically lifting and pulling out the slurry to uniformly cast the slurry on the substrate to form a raw material belt;
and 7: peeling off the raw material belt by using a pair of tweezers, and cutting the raw material belt to be overlapped to a thickness of 3-4 mm;
and 8: the overlapped samples are placed in a cold isostatic press for pressing under 180-250MPa after vacuum plastic packaging;
and step 9: drying the pressed and formed green body in an oven at the temperature of 40-60 ℃;
step 10: putting the green body into a tubular furnace for carbonization, and introducing flowing protective gas to crack the organic additives in the green body into carbon to obtain a porous silicon nitride ceramic green body;
step 11: vacuum impregnating the green body with 10-20 wt% phenolic resin alcohol solution, and curing in an oven at 150-180 ℃ for 6-8 h;
step 12: flowing protective gas is introduced, and the resin is cracked into carbon by heat preservation for 2-4h at the high temperature of 800-;
step 13: placing a graphite frame in the crucible, placing carbonized porous silicon nitride ceramic green bodies on the graphite frame, placing SiO powder under the graphite frame, and adding N 2 As a protective gas, the residual carbon in the green body is reacted with SiO and N at high temperature 2 Reaction to form alpha-Si 3 N 4 。
In the step 1, alpha-Si is selected 3 N 4 The model of the particle is UBE-E10, and the average particle size is 0.2-0.5 mu m; the rare earth oxide includes yttrium oxide, lutetium oxide, lanthanum oxide, and ytterbium oxide, preferably yttrium oxide.
In the step 3, the concentration of the hydrofluoric acid is changed between 1 and 2 mol/L.
The organic solvent in the step 4 is a binary azeotropic mixture, including ethanol/butanone, ethanol/trichloroethylene, trichloroethylene/methyl ethyl ketone, and the powder can be uniformly dispersed in the solvent, preferably 22.9-33.45% of absolute ethanol and 22.9-33.45% of butanone.
The dispersing agent in the step 4 comprises phosphate, an ethoxy compound and triolein, and triethyl phosphate is preferably used as the dispersing agent.
In the step 7, the raw material sheets are overlapped in a mutually parallel mode, a mutually vertical mode or a mode of overlapping at a certain angle, preferably in a parallel mode or a vertical mode.
In the step 10, inert gas which does not react with other systems is adopted as the flowing gas, including Ar, the room temperature is raised to 550-650 ℃, the temperature raising rate is 0.5-0.8 ℃/min, and the temperature is kept for 1-2h, so that no organic additive residue exists in the green body;
the protective gas flowing in the step 12 is inert gas which does not react with the rest of the system, including Ar.
In the step 11, in the vacuum impregnation process, the vacuum degree is not more than-0.08 MPa, so that the resin can uniformly enter the inside of the sample;
in the step 11, the phenolic resin can be replaced by other carbon-containing materials in the vacuum impregnation process, and the carbon-containing materials can be cracked into C only by ensuring that the liquidity of the solution meets the impregnation requirement.
In said step 13, N 2 The pressure is 4-6atm, the heating rate from room temperature to 1100 ℃ is 8-10 ℃/min, the heating rate above 1100 ℃ is 5-8 ℃/min, and the heat preservation temperature at high temperature can be changed between 1600-1700 ℃.
Compared with the prior art, the invention has the advantages that:
1) the beta-Si 3N4 crystal whisker with high length-diameter ratio prepared by the method is used as a raw material to prepare silicon nitride porous ceramics, and the beta-Si 3N4 crystal whisker with high length-diameter ratio in the material obviously improves the bending strength and the fracture toughness.
2) The method of vertical drawing casting is adopted, so that the beta-Si is obtained 3 N 4 The whiskers are directionally arranged in two-dimensional directions, so that the whiskers have high strength and fracture toughness in the directional arrangement direction, the strength can reach 300MPa while the porosity is kept at 40%, and the fracture toughness can reach 5.
3) The raw material sheet can adopt a vertical laminating mode, a parallel laminating mode and a mutual angle laminating mode, and the specific laminating mode can be determined according to the actual requirement so as to control the beta-Si 3 N 4 The directional distribution of the crystal whiskers is adapted to the specific requirements of different service conditions.
4) According to the invention, the silicon nitride ceramic is prepared by adopting a carbothermic reduction method, and no sintering aid is required to be added, so that the blank body after sintering realizes net-size sintering, the blank body does not shrink, and the subsequent processing of the material is facilitated.
5) The porosity of the material can be controlled by adjusting the concentration of the impregnation liquid and the impregnation times, so that the porosity can be changed between 40 and 60 percent, and the application requirements of different scenes can be met conveniently.
6) The porous silicon nitride material prepared by the method only has alpha-Si 3 N 4 And beta-Si 3 N 4 The two phases have no intercrystalline glass phase, the bending strength of the two phases does not obviously decrease at 1500 ℃, and the adverse effect of the intercrystalline glass phase on the high-temperature mechanical property of the liquid phase sintered porous silicon nitride ceramic is overcome.
Drawings
FIG. 1 shows β -Si obtained in example 2 3 N 4 Seed micrographs.
FIG. 2 is a micrograph of a cast sheet obtained in example 4.
FIG. 3 is the morphology of the fractured tissue of example 6.
Fig. 4 is an XRD pattern of the porous silicon nitride material obtained in example 9.
FIG. 5 is a graph showing the change of the bending strength of the porous silicon nitride material obtained in example 12 with temperature.
Detailed Description
The invention is further illustrated by the following examples:
the invention provides a method for preparing high-toughness high-temperature-resistant oriented silicon nitride monolithic porous ceramic, the compositions of the examples are shown in tables 1-3, and in the examples 1-10 shown in the table 1, a certain proportion of Y is added into silicon nitride 2 O 3 As sintering aid, prepare beta-Si with higher length-diameter ratio 3 N 4 The crystal whisker is prepared into the porous ceramic with the organization structure of silicon nitride and silicon nitride by tape casting, carbonization, phenolic resin impregnation, solidification, carbonization and carbothermic reduction, and silicon nitride is obtained by the reaction of high-temperature evaporation gas phase infiltration of SiO and residual carbon between fibers.
The preparation method of the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic described in the embodiments 1 to 10 includes the following specific steps:
step 1: weighing 95 wt% of alpha-Si according to mass percentage 3 N 4 And 5 wt% of Y 2 O 3 Mixing the powder, and loosely packing the powder into a graphite crucible after uniformly mixing;
step 2: placing the crucible filled with the mixed powder into a multifunctional sintering furnace, filling nitrogen as protective atmosphere, controlling the heating rate to be less than 10 ℃/min, preserving the heat at 1650-1750 ℃ for 2-3h, and then cooling along with the furnace;
in the step 2, N 2 The pressure is 5atm, the heating rate from room temperature to 1100 ℃ is 10 ℃/min, the heating rate above 1100 ℃ is 5 ℃/min, and the heat preservation temperature at high temperature is changed between 1650-1750 ℃;
and step 3: and (3) knocking the reactant obtained in the step (2) into small blocks, putting the small blocks into a polytetrafluoroethylene container filled with 1mol/L hydrofluoric acid, heating at 60 ℃, and magnetically stirring until the blocks become powder. Rinsing with distilled water for many times until Ph is 7, and drying to obtain beta-Si 3 N 4 Whisker;
and 4, step 4: reacting beta-Si 3 N 4 Adding the crystal whisker, triethyl phosphate serving as a dispersant, absolute ethyl alcohol and butanone into a beaker according to the mass ratio in the table 2, adding a magnetic rotor, stirring for 6 hours, uniformly mixing, adding a binder polyvinyl butyral ester and a plasticizer phthalic acid di-n-butyl ester into the mixed slurryMagnetically stirring butyl ester and polyethylene glycol for 6 hours to obtain uniform casting slurry;
and 5: defoaming the casting slurry obtained in the step 4 for 60min by using a vacuum defoaming machine, and taking out to prepare casting;
step 6: pouring the casting slurry into a container, inserting the casting substrate into the slurry, and vertically lifting and pulling out the slurry to uniformly cast the slurry on the substrate to form a raw material belt;
and 7: cutting the raw material sheet after being stripped from the casting substrate by using a pair of tweezers, and stacking the raw material sheet to a thickness of 4mm in a parallel stacking mode;
and 8: the overlapped samples are placed in a cold isostatic press for pressing under 200MPa after vacuum plastic packaging;
and step 9: the pressed and formed green body is placed in an oven to be dried at the low temperature of 50 ℃;
step 10: putting the green body into a tube furnace for carbonization, introducing flowing protective gas, heating to 215 ℃ at the speed of 0.5 ℃/min, preserving heat for 1h, heating to 550 ℃ at the speed of 0.5 ℃/min, preserving heat for 2h, and cracking organic additives in the green body into carbon to obtain a porous silicon nitride ceramic green body;
step 12: putting the sample into a three-neck flask, connecting a vacuum pump and a constant pressure funnel, and filling the funnel with alcohol solution of phenolic resin. Starting a vacuum pump, keeping the vacuum degree in the system for 20min after the vacuum degree reaches-0.09 MPa, unscrewing a constant-pressure funnel knob, enabling the phenolic resin solution in the funnel to flow into the three-neck flask, and keeping the negative pressure of the system for 20 min. Drying the sample at normal temperature, and then placing the dried sample in an oven for 6-8 h for curing at 150-180 ℃;
step 13: carbonizing a sample by a tubular furnace, preserving heat for 2 hours at 400 ℃ by using flowing Ar gas as protective gas, preserving heat for 2 hours at 800 ℃, and cracking the cured phenolic resin into carbon;
step 14: putting the sample and SiO powder into a graphite crucible, and preserving heat for 3-4h at 1600-1750 ℃ under the atmosphere of N2 to ensure that carbon thermal reduction reaction is carried out on carbon, SiO and N2 in the sample to generate alpha-Si 3N4, and preparing the beta-Si 3N4 porous ceramic lapped with Si3N4 through the combination effect of the alpha-Si 3N4 and the SiO 2.
The open porosity of the porous silicon nitride ceramic obtained by the method is measured by adopting an Archimedes drainage method; INSTRON-1195 TYPEThe bending strength of the samples was measured by a universal tester, and the average value of 5 measurements per sample was taken as the final result, with a sample size of 3mm × 4mm × 30mm (span 16mm), and a loading rate of 0.5 mm/min; the fracture toughness KIC of the sample is tested by using a unilateral notched beam (SEVNB) method, namely, firstly, a crack is cut in the middle of the sample, a three-point bending fracture test is measured by using an INSTRON-1195 type universal testing machine, and the fracture toughness of the material is calculated. The average of 5 tests per data was used as the final result, with specimen sizes: 2.5mm multiplied by 5mm multiplied by 30mm, the incision height is 2.5 mm; the prepared beta-Si was observed using a Gemini SEM 500 scanning electron microscope (accelerating voltage 15.0KV, secondary electron imaging mode) 3 N 4 Microscopic morphologies of whiskers, cast films, and sample fractures; the Bruker model D8 ADVANCE X-ray diffractometer analyzes the phase composition of the samples, test conditions: cu Kalpha radiation source, scanning speed of 12(°)/min, tube voltage of 40kV and tube current of 40 mA. The data obtained are shown in the following table:
table 1: the invention prepares the beta-Si with high length-diameter ratio 3 N 4 Whisker technological parameters (Steps 1-3)
Table 2: process parameters for preparing casting slurry (step 4)
Table 3: the sintering technological parameters and performance of the silicon nitride multi-ceramic
As can be seen from Table 3, example 1 used 95.0 wt% silicon nitride powder as a raw material and 5.0 wt% Y 2 O 3 Is taken as a sintering aid, and is subjected to heat preservation at 1650 ℃ for 2h and normal pressure sintering to obtain beta-Si with the average length-diameter ratio of 9.7 3 N 4 Powder according to the process parameters of Table 2Preparing a raw material sheet by tape casting after preparing slurry, superposing the raw material sheet to 4mm in parallel, carbonizing the raw material sheet after vacuum plastic packaging and isostatic pressing at 200MPa, preserving the heat of a carbonized sample at 1650 ℃ for 3h for carbothermic reduction sintering, and obtaining porous Si 3 N 4 The porosity of the material is 58.2%, the bending strength at normal temperature can reach 153.5MPa, the bending strength at 1500 ℃ and high temperature can reach 135.1MPa, and the fracture toughness can reach 1.9MPa m 1/2 。
As can be seen from Table 3, example 2 used 95.0 wt% silicon nitride powder as the raw material and 5.0 wt% Y 2 O 3 Is used as a sintering aid, and is sintered at 1750 ℃ for 2h under normal pressure to prepare beta-Si with the average length-diameter ratio of 20.1 3 N 4 Preparing powder, preparing slurry according to the process parameters of the table 2, carrying out tape casting to prepare a green sheet, parallelly overlapping to 4mm, carrying out vacuum plastic packaging, carrying out isostatic pressing at 200MPa, carbonizing, carrying out heat preservation at 1700 ℃ for 3.5h, carrying out carbothermic reduction sintering on the carbonized sample, and obtaining porous Si 3 N 4 The porosity of the material is 61.4%, the bending strength at normal temperature can reach 160.2MPa, the bending strength at 1500 ℃ can reach 134.6MPa, and the fracture toughness can reach 2.2MPa m 1/2 。
As can be seen from Table 3, example 6 used 95.0% by weight of silicon nitride powder as the raw material and 5.0% by weight of Y 2 O 3 Is used as a sintering aid, and is sintered at 1750 ℃ for 2h under normal pressure to prepare beta-Si with the average length-diameter ratio of 20.1 3 N 4 Preparing powder, preparing slurry according to the process parameters of the table 2, carrying out tape casting to prepare a raw material sheet, parallelly overlapping to 4mm, carrying out vacuum plastic packaging, carrying out isostatic pressing at 200MPa, carbonizing, carrying out single-cycle impregnation on a carbonized sample, carrying out heat preservation at 1650 ℃ for 3.5h, carrying out carbothermic reduction sintering, and thus obtaining the porous Si 3 N 4 The porosity of the material is 55.2%, the bending strength at normal temperature can reach 210.5MPa, the bending strength at 1500 ℃ can reach 181.0MPa, and the fracture toughness can reach 3.2MPa m 1/2 。
As can be seen from Table 3, example 8 used 95.0% by weight of silicon nitride powder as the raw material and 5.0% by weight of Y 2 O 3 Is used as a sintering aid, and is sintered at 1750 ℃ for 2h under normal pressure to prepare beta-Si with the average length-diameter ratio of 20.1 3 N 4 Powder bodyPreparing slurry according to the process parameters of the table 2, preparing a raw material sheet by tape casting, parallelly overlapping to 4mm, carbonizing after vacuum plastic packaging and isostatic pressing at 200MPa, carbonizing after twice circulating impregnation of a carbonized sample, preserving heat at 1750 ℃ for 3.5 hours, and carrying out carbothermic reduction sintering to obtain the porous Si 3 N 4 The porosity of the material is 48.5%, the bending strength at normal temperature can reach 267.1MPa, the bending strength at 1500 ℃ can reach 235MPa, and the fracture toughness can reach 4.3MPa m 1/2 . In the case of short commercially available beta-Si having an aspect ratio of 4.1 as in comparative example 1 3 N 4 In a comparison experiment of the porous silicon nitride material prepared by the crystal whisker, the bending strength is improved by 33.4%, the fracture toughness is improved by 34.4%, and the mechanical property is greatly improved.
As can be seen from Table 3, in example 11, 95.0% by weight of silicon nitride powder and 5.0% by weight of Y were used as raw materials 2 O 3 Is used as a sintering aid, and is sintered at 1750 ℃ for 2h under normal pressure to prepare beta-Si with the average length-diameter ratio of 20.1 3 N 4 Preparing powder, preparing slurry according to the process parameters of the table 2, carrying out tape casting to prepare a raw material sheet, parallelly overlapping to 4mm, carrying out vacuum plastic packaging, carrying out isostatic compaction at 200MPa, carbonizing, carrying out three-time circulating impregnation on a carbonized sample, carrying out carbonization at 1600 ℃ for 4 hours, carrying out carbothermic reduction sintering, and obtaining porous Si 3 N 4 The porosity of the material is 40.9%, the bending strength at normal temperature can reach 319.2MPa, the bending strength at 1500 ℃ can reach 277.7MPa, and the fracture toughness can reach 4.9MPa m 1/2 。
As can be seen from Table 3, in example 12, 95.0% by weight of silicon nitride powder and 5.0% by weight of Y were used as raw materials 2 O 3 Is taken as a sintering aid, and is subjected to heat preservation at 1650 ℃ for 2h and normal pressure sintering to obtain beta-Si with the average length-diameter ratio of 9.7 3 N 4 Preparing powder, preparing slurry according to the process parameters of the table 2, carrying out tape casting to prepare a raw material sheet, parallelly overlapping to 4mm, carrying out vacuum plastic packaging, carrying out isostatic compaction at 200MPa, carbonizing, carrying out cyclic impregnation twice on a carbonized sample, carrying out heat preservation at 1700 ℃ for 3 hours, carrying out carbothermic reduction sintering, and obtaining porous Si 3 N 4 The porosity of the material is 46.2%, the normal temperature bending strength can reach 248.2MPa, and the material has a high temperature of 1500 DEG CBending strength can reach 215.9MPa and fracture toughness can reach 3.8MPa m 1/2 。
As can be seen from Table 3, comparative example 2 used 95.0 wt% of silicon nitride powder as the raw material and 5.0 wt% of Y 2 O 3 The porous silicon nitride material is used as a sintering aid, the mixture is sieved and then directly subjected to compression molding under 200MPa, the heat preservation is carried out for 2 hours at 1750 ℃, and the normal-pressure sintering is carried out, so that the porosity of the obtained porous silicon nitride material is 48.1%, the bending strength at normal temperature reaches 181.2MPa, but the bending strength at 1500 ℃ is only 91.9MPa, and the bending strength is reduced by 49.3% compared with the bending strength at normal temperature. Compared with comparative example 2, the method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic can prepare the porous silicon nitride ceramic with room temperature strength similar to that of liquid-phase sintered silicon nitride with the same porosity, but the strength is not reduced up to 1500 ℃.
The microstructure of the cast green sheet prepared in example 2 is shown in fig. 2, and it can be seen that the high aspect ratio micron-sized silicon nitride rod crystals have strong alignment.
FIG. 1 shows β -Si obtained in example 2 3 N 4 Seed micrographs. As shown in the figure, the obtained silicon nitride powder has high aspect ratio.
FIG. 2 is a micrograph of a cast sheet obtained in example 4. As shown, beta-Si with high aspect ratio 3 N 4 The seed crystal has obvious directional arrangement.
Fig. 3 is the fracture morphology of example 6, and it can be seen that the fracture morphology is small particles with hexagonal cross section, which also proves that the casting process enables the silicon nitride rod-like fiber to be well oriented and arranged along the casting direction.
Fig. 4 is an XRD pattern of the porous silicon nitride material obtained in example 9. As shown in the figure, after carbothermal reduction sintering, the obtained phase is alpha-Si 3 N 4 With beta-Si 3 N 4 。
FIG. 5 is a graph showing the change of bending strength with temperature of the porous silicon nitride material obtained in example 12. As shown, the bending strength is substantially unchanged with increasing temperature. The high strength of 215.9MPa can be still maintained at the high temperature of 1500 ℃.
Claims (9)
1. A method for preparing high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic is characterized by comprising the following steps:
step 1: weighing 90-95 wt% of alpha-Si according to mass percentage 3 N 4 And 5-10 wt% of rare earth oxide mixed powder, and after being uniformly mixed, the mixed powder is loosely placed into a graphite crucible;
step 2: placing the crucible filled with the mixed powder into a multifunctional sintering furnace, filling nitrogen as protective atmosphere, controlling the heating rate to be less than 10 ℃/min, preserving the heat at 1600-1750 ℃ for 2-3h, and then cooling along with the furnace;
and step 3: putting the reactant obtained in the step 2 into a polytetrafluoroethylene container filled with hydrofluoric acid, heating at 60-80 ℃ and carrying out magnetic stirring treatment until the block becomes powder, rinsing with distilled water for multiple times, and drying until Ph is 7 to obtain beta-Si 3 N 4 Whisker;
and 4, step 4: 20-40 wt% of beta-Si 3 N 4 Adding whiskers, 45.8-66.90% of an organic solvent, 22.9-33.45% of butanone and 1% of a dispersing agent into a beaker, adding a magnetic rotor, stirring for 6-8 h, adding 5-6% of a binding agent polyvinyl butyral ester, 3-3.6% of polyethylene glycol and 3-3.6% of dibutyl phthalate into the mixed slurry, and magnetically stirring for 6-8 h to obtain uniform casting slurry;
and 5: defoaming the casting slurry obtained in the step 4 through a vacuum defoaming machine for 30-60min, and taking out to prepare casting;
and 6: pouring the casting slurry into a container, inserting the casting substrate into the slurry, and vertically lifting and pulling out the slurry to uniformly cast the slurry on the substrate to form a raw material belt;
and 7: peeling off the raw material belt by using a pair of tweezers, and cutting the raw material belt to be overlapped to a thickness of 3-4 mm;
and 8: the overlapped samples are placed in a cold isostatic press for pressing under the pressure of 180 ℃ and 250MPa after vacuum plastic packaging;
and step 9: drying the pressed and formed green body in an oven at the temperature of 40-60 ℃;
step 10: putting the green body into a tubular furnace for carbonization, and introducing flowing protective gas to crack the organic additives in the green body into carbon to obtain a porous silicon nitride ceramic green body;
step 11: vacuum-dipping the green body in 10-20 wt% phenolic resin alcohol solution, and curing after heat preservation for 6-8 h at 150-180 ℃ in a drying oven;
step 12: flowing protective gas is introduced, and the resin is cracked into carbon by heat preservation for 2-4h at the high temperature of 800-;
step 13: placing a graphite frame in the crucible, placing carbonized porous silicon nitride ceramic green bodies on the graphite frame, placing SiO powder under the graphite frame, and adding N 2 As a protective gas, the residual carbon in the green body is reacted with SiO and N at high temperature 2 Reaction to form alpha-Si 3 N 4 。
2. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps: in the step 1, alpha-Si is selected 3 N 4 The model of the particle is UBE-E10, and the average particle size is 0.2-0.5 mu m; the rare earth oxide includes yttrium oxide, lutetium oxide, lanthanum oxide, and ytterbium oxide, preferably yttrium oxide.
3. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps: in the step 3, the concentration of the hydrofluoric acid is changed between 1 and 2 mol/L.
4. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps: the organic solvent in the step 4 is a binary azeotropic mixture, including ethanol/butanone, ethanol/trichloroethylene, trichloroethylene/methyl ethyl ketone, and the powder can be uniformly dispersed in the solvent, preferably 22.9-33.45% of absolute ethanol and 22.9-33.45% of butanone.
5. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps: the dispersing agent in the step 4 comprises phosphate, an ethoxy compound and triolein, and triethyl phosphate is preferably used as the dispersing agent.
6. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps: in the step 7, the raw material sheets are overlapped in a mutually parallel mode, a mutually vertical mode or a mode of overlapping at a certain angle, preferably in a parallel mode or a vertical mode.
7. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps:
in the step 10, inert gas which does not react with other systems is adopted as the flowing gas, including Ar, the room temperature is raised to 550-650 ℃, the temperature raising rate is 0.5-0.8 ℃/min, and the temperature is kept for 1-2h, so that no organic additive residue exists in the green body;
the protective gas flowing in the step 12 is inert gas which does not react with the rest of the system, including Ar.
8. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps:
in the step 11, in the vacuum impregnation process, the vacuum degree is not more than-0.08 MPa, so that the resin can uniformly enter the inside of the sample;
in the step 11, the phenolic resin can be replaced by other carbon-containing materials in the vacuum impregnation process, and the carbon-containing materials can be cracked into C only by ensuring that the liquidity of the solution meets the impregnation requirement.
9. The method for preparing the high-toughness high-temperature-resistant silicon nitride monolithic porous ceramic according to claim 1, wherein the method comprises the following steps: in said step 13, N 2 The pressure is 4-6atm, the heating rate from room temperature to 1100 ℃ is 8-10 ℃/min, the heating rate above 1100 ℃ is 5-8 ℃/min, and the heat preservation temperature at high temperature can be changed between 1600-1700 ℃.
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