CN111099919B - Silicon nitride ceramic with hierarchical pore structure and preparation method thereof - Google Patents
Silicon nitride ceramic with hierarchical pore structure and preparation method thereof Download PDFInfo
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- 239000000919 ceramic Substances 0.000 title claims abstract description 109
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 title claims abstract description 108
- 229910052581 Si3N4 Inorganic materials 0.000 title claims abstract description 82
- 239000002149 hierarchical pore Substances 0.000 title claims abstract description 23
- 238000002360 preparation method Methods 0.000 title claims abstract description 21
- 238000005245 sintering Methods 0.000 claims abstract description 34
- LIVNPJMFVYWSIS-UHFFFAOYSA-N silicon monoxide Chemical compound [Si-]#[O+] LIVNPJMFVYWSIS-UHFFFAOYSA-N 0.000 claims abstract description 30
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- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims abstract description 13
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- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 5
- 239000007864 aqueous solution Substances 0.000 claims description 5
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- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 description 1
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Abstract
The invention discloses a preparation method of silicon nitride ceramics with a hierarchical pore structure, which comprises the following steps: uniformly mixing silicon nitride, silicon monoxide, a sintering aid, a pore-forming agent and a gel aid, adjusting the pH value to 8.5-11.5, and performing ball milling to obtain mixed slurry; the mixed slurry is subjected to injection molding, and is dried, demoulded and discharged after being cured to obtain a ceramic blank; placing the ceramic blank in a chemical vapor infiltration furnace, and carrying out deposition sintering in the atmosphere of a gaseous precursor to obtain a ceramic sample; and (3) paving silicon sources below the ceramic sample at intervals, and carrying out non-pressure discharge plasma sintering on the ceramic sample and the silicon sources under a protective atmosphere to obtain the silicon nitride ceramic with the hierarchical pore structure. The silicon nitride ceramic prepared by the invention has good air permeability, is suitable for filtering smoke dust, and has simple preparation process and adjustable microstructure.
Description
Technical Field
The invention relates to the field of preparation of porous ceramics, in particular to silicon nitride ceramics with a hierarchical pore structure and a preparation method thereof.
Background
In recent years, the industrial pollution problem in China is increasingly serious, and the illegal discharge of industrial dust is particularly prominent. Industrial dust is mainly derived from mechanical crushing and grinding of solid materials, mixing, screening, packaging and transporting of powdery materials, smoke and dust generated by combustion of materials and the like. The particle size distribution of such dusts is often several hundred microns to several hundred nanometers, wherein the emissions from particles with a particle size below 10 μm are a serious health hazard, especially toxic metal and non-metal dusts (chromium, manganese, cadmium, lead, mercury, arsenic, etc.) can cause poisoning or death. In addition, the random emission of industrial dust also contributes to the pollution of PM10 and PM2.5 in the atmosphere. Therefore, it is important to develop an effective removal technique of industrial dust.
Common industrial dust removal methods include mechanical dust removal (including gravity settling, inertial dust removal, cyclone dust removal and the like), filtration dust removal, wet gas scrubbing, electrostatic dust removal, and new dust removal methods developed in recent years, such as composite dust removal, magnetic dust removal, electrocoagulation dust removal technology and the like. For the smoke dust (PM below 2.5) with small particle size emitted by primary stages of smelting plants, power plants and the like, the temperature is very high, the requirement on equipment is strict by adopting a conventional dust removal method, and the method adopting filtration is more effective. This puts harsh requirements on the filter material, such as good chemical stability and thermal stability, strong acid and corrosion resistance, and no destructive chemical reaction with the powder at room temperature and high temperature; light weight and high strength; the porosity is high, the permeability is good, and the pore size gradient distribution is reasonable; low cost, easy processing and forming, etc.
The porous silicon nitride ceramic has the advantages of high temperature resistance, high oxidation resistance temperature, good mechanical property, thermal shock resistance, wear resistance, acid and alkali resistance and chemical corrosion resistance, and excellent comprehensive performance, and is one of ideal materials of a high-temperature smoke dust filtering device. Porous silicon nitride ceramics are structurally classified into single-stage homogeneous and gradient porous. The single-stage homogeneous porous structure has a certain effect on filtering smoke dust, but the effective filtering of the smoke dust with wide particle size distribution is difficult to realize. The porosity or aperture or pore structure of the gradient porous structure is regularly changed along with the size of a sample, the asymmetric structure easily realizes small aperture and large ventilation, has the characteristics of high filtering precision, large ventilation coefficient and good backwashing effect, can greatly improve the filtering precision and the filtering efficiency when being used in the field of filtering separation, is particularly suitable for the separation of mixed fluid containing fine particles, such as high temperature, corrosivity and the like, high-temperature flue gas dust removal, fine filtration and the like, and is more and more emphasized by people.
Patent CN102417366A discloses a pore gradient porous SiC ceramic and a preparation method thereof, wherein a pore-forming agent and fine SiC particles are mixed and stirred uniformly according to different proportions, then mixed powder with different proportions is spread layer by layer on a metal mold, and the mixed powder is sintered after compression molding to obtain the gradient porous SiC ceramic. Patent CN101323528A discloses a method for preparing a gradient SiC ceramic tube with continuous pores, which comprises injecting uniformly dispersed gel-casting suspension into a mold for centrifugation, heating the suspension slurry under the action of centrifugal force to perform gelation reaction, curing and molding, and demolding, drying and sintering the wet blank to obtain the gradient SiC ceramic tube with continuous pores. Patent CN104311114A discloses a vacuum foaming and freeze drying combined technology for preparing gradient porous silicon nitride ceramics, which is prepared by mixing silicon nitride powder, sintering aid, binder and water, ball milling, vacuum foaming, freeze drying and high temperature sintering. The first two processes mainly introduce preparation methods of silicon carbide gradient porous materials, while the third process requires preparation under low temperature conditions, the production cost is high, and the pore structure of the obtained material has no obvious gradient change along with the size of a sample. How to simply and inexpensively construct the gradient porous structure with adjustable microstructure is a difficult problem.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide the silicon nitride ceramic with the hierarchical pore structure, which has good air permeability, is suitable for filtering smoke dust, has simple preparation process and adjustable microstructure and the preparation method thereof.
In order to solve the technical problems, the invention adopts the following technical scheme:
a preparation method of silicon nitride ceramics with a hierarchical pore structure comprises the following steps:
s1, uniformly mixing silicon nitride, silicon monoxide, a sintering aid, a pore-forming agent and a gel aid to obtain mixed powder, dividing the mixed powder into two parts, marking the two parts as mixed powder I and mixed powder II, mixing the mixed powder I with deionized water, and then carrying out pre-ball milling to obtain pre-mixed slurry;
s2, dividing the mixed powder II in the step S1 into n parts, adding the n parts into the premixed slurry in batches, ball-milling the mixed powder II after each time of adding the mixed powder II, adding a pH regulator to adjust the pH value to 8.5-11.5 after the nth part of the mixed powder II is added, and uniformly ball-milling to obtain mixed slurry; wherein n is more than or equal to 4;
s3, performing injection molding on the mixed slurry obtained in the step S2, performing heat preservation and solidification, and drying, demolding and removing glue to obtain a ceramic blank;
s4, placing the ceramic body in the step S3 in a chemical vapor infiltration furnace, and performing deposition sintering in the atmosphere of a gaseous precursor until the porosity reaches the expected target to obtain a ceramic sample;
s5, placing silicon sources below the ceramic sample in the step S4 at intervals, and carrying out non-pressure discharge plasma sintering on the ceramic sample and the silicon sources in a protective atmosphere to obtain the silicon nitride ceramic with the hierarchical pore structure.
As a further improvement to the above technical solution:
in the step S1, the sintering aid is Y2O3And Al2O3Silicon nitride, silicon monoxide, Y2O3And Al2O3The mass ratio of (A) to (B) is 100: 1-20: 0.1-3.
In the step S1, the pore-forming agent and the gel aid are both egg white powder, and the mass fraction of the egg white powder in the mixed powder is 5% to 25%.
In the step S5, the protective atmosphere is nitrogen, and the pressureless discharge plasma sintering includes the specific steps of: heating to 1300-1700 ℃ at the heating rate of 50-300 ℃/min, preserving the heat for 2-20 min, and cooling to room temperature. Is composed of
In the step S4, the gaseous precursors are silicon tetrachloride and ammonia gas, and the deposition and sintering specifically includes: raising the temperature to 800-1200 ℃ at the temperature raising rate of 2-20 ℃/min, and keeping the temperature for 50-600 h, wherein the total air pressure is 0.5-5.0 kPa.
In the step S2, n =5, the mass percentages of the first part to the fifth part of the mixed powder II are respectively 35-25%, 25-20%, 20-15% and 15-10%, and the adding time interval between the adjacent batches of the mixed powder II is 20-60 min; the ball milling time after the nth powder mixed powder II is added is 6 to 12 hours.
The pH regulator is tetramethylammonium hydroxide aqueous solution with the mass fraction of 10-30%.
The volume fraction of the solid phase in the premixed slurry is 20-60%.
In the step S3, the injection molding temperature is 50-100 ℃, and the heat preservation and curing time is 10-300 min; the drying method comprises the following specific steps: drying for 12-36 h at room temperature, and then drying for 36-90 h at 30-60 ℃; the rubber discharging method comprises the following specific steps: firstly, the temperature is kept at 60-100 ℃ for 0.5-6 h, then the mixture is heated to 500-800 ℃ at the heating rate of 0.5-10 ℃/min, and the temperature is kept for 0.5-6 h.
In the step S5, the mass ratio of the ceramic sample to the silicon source is 10: 1-2: 1.
The invention also provides a silicon nitride ceramic with a hierarchical pore structure, which is prepared by the preparation method and comprises primary porous silicon nitride ceramic, secondary porous silicon nitride ceramic and silicon nitride nanowires, wherein the secondary porous silicon nitride ceramic is deposited on the pore walls of the primary porous silicon nitride ceramic and forms a secondary pore structure, the silicon nitride nanowires are grown on the pore walls of the primary porous silicon nitride ceramic and/or the secondary porous silicon nitride ceramic, and the silicon nitride nanowires are intersected and stacked to form a tertiary pore structure.
The principle of the invention is as follows:
the invention can realize the near net shape of the complex component based on Chemical Vapor Infiltration (CVI) and Gel-casting process, and is easy to realize industrialization. The Gel-casting process can be used for preparing porous silicon nitride ceramics to form a primary pore structure, and then the pore diameter is regulated and controlled by a CVI (chemical vapor infiltration) process to form a secondary pore structure. Meanwhile, one-dimensional nanowires grow in the porous material, and the nanowires are crossed at intervals to form a three-level pore structure, so that the nasal hair in the nasal cavity of a higher animal can effectively resist dust in the air. The gradient porous silicon nitride ceramic material which is composed of a three-level pore structure, has the pore diameter covering the range of 1 to more than 100 mu m and has a nasal cavity-like structure is expected to realize effective filtration of high-temperature smoke dust.
Compared with the prior art, the invention has the advantages that:
1. the invention relates to a silicon nitride ceramic material with a hierarchical pore structure and a preparation method thereof, and a primary pore structure (of the silicon nitride ceramic material)>100 mu m) is formed by a gel casting process, a secondary pore structure (10-50 mu m) is prepared by depositing silicon nitride ceramics on the wall of a primary pore by adopting a CVI process, and a tertiary pore structure is formed by intersecting and spacing silicon nitride nanowires which grow in a gas-solid mode. Similar to nasal cavity of higher animals, good air permeability (air permeability of 5.16 cm)3·cm/(cm2s.kPa)) is expected to be used for filtering particles with different particle sizes, including PM2.5, in industrial high-temperature smoke dust.
2. According to the invention, the porosity, the pore size and the distribution of the silicon nitride ceramic material can be controlled by controlling the solid content of silicon nitride in the mixed slurry, the using amount of the pore-forming agent and the sintering system (sintering rate and sintering temperature); and the control of the number of windows on the inner wall of the pore and the deposition thickness, namely the number of secondary pore structures, can be realized by controlling the deposition time of CVI. Meanwhile, the silicon nitride nanowires grow on the surface of the wall of the primary pore of the porous silicon nitride ceramic and cross-grow to the inside of the cavity of the secondary pore structure, so that the multistage through hole structure formed at the early stage is favorable for the growth of the silicon nitride nanowires at the later stage.
3. The gradient porous ceramic material is prepared by utilizing the pressureless discharge plasma sintering technology, the heat transfer efficiency is high, the heating rate is high, the sintering time is short, on one hand, local pre-melting can be realized at a crystal grain contact point, necking is generated, and the strength of a blank body is rapidly improved; on the other hand, one-dimensional nanowires can be rapidly grown on the walls of the secondary pores and stacked to form a tertiary pore structure.
4. According to the invention, the one-dimensional nano material is grown by using a gas-solid mechanism, the silicon source (silicon nitride and silicon monoxide) is added into the raw material, and the silicon source is paved outside, so that the silicon vapor concentration under a high-temperature condition can be improved, the silicon nitride nanowire can be grown in a short time, the silicon source is silicon monoxide (SiO), the price is low, the corresponding nanowire can be directly obtained under the condition of not adding a catalyst, the process is greatly simplified, the cost is reduced, and the industrial production can be realized.
Drawings
FIG. 1 is a schematic structural view of a silicon nitride ceramic according to the present invention.
FIG. 2 is an optical photograph of a silicon nitride ceramic sample in example 1 of the present invention.
FIG. 3 shows a sample of silicon nitride ceramic prepared in example 1 of the present invention and its XRD spectrum.
FIG. 4 is an SEM photograph of a silicon nitride ceramic obtained in example 1 of the present invention.
FIG. 5 is a graph showing the pore size distribution of the silicon nitride ceramic obtained in example 1 of the present invention.
Detailed Description
The invention will be described in further detail below with reference to the drawings and specific examples. Unless otherwise specified, the following materials or apparatus are commercially available.
The invention relates to a preparation method of silicon nitride ceramics with a hierarchical pore structure, which comprises the following steps:
s1, uniformly mixing silicon nitride, silicon monoxide, a sintering aid, a pore-forming agent and a gel aid to obtain mixed powder, dividing the mixed powder into two parts, marking the two parts as mixed powder I and mixed powder II, mixing the mixed powder I with deionized water, and then carrying out pre-ball milling to obtain pre-mixed slurry;
s2, dividing the mixed powder II in the step S1 into n parts, adding the n parts into the premixed slurry in batches, ball-milling the mixed powder II after each time of adding the mixed powder II, adding a pH regulator to adjust the pH value to 8.5-11.5 after the nth part of the mixed powder II is added, and uniformly ball-milling to obtain mixed slurry; wherein n is more than or equal to 4;
s3, performing injection molding on the mixed slurry obtained in the step S2, performing heat preservation and solidification, and drying, demolding and removing glue to obtain a ceramic blank;
s4, placing the ceramic body in the step S3 in a chemical vapor infiltration furnace, and performing deposition sintering in the atmosphere of a gaseous precursor until the porosity reaches the expected target to obtain a ceramic sample;
s5, placing silicon sources below the ceramic sample in the step S4 at intervals, and carrying out non-pressure discharge plasma sintering on the ceramic sample and the silicon sources in a protective atmosphere to obtain the silicon nitride ceramic with the hierarchical pore structure.
The silicon nitride ceramic with the hierarchical pore structure comprises primary porous silicon nitride ceramic, secondary porous silicon nitride ceramic and silicon nitride nanowires, wherein the secondary porous silicon nitride ceramic is deposited on the pore wall of the primary porous silicon nitride ceramic to form the secondary pore structure, the silicon nitride nanowires are grown on the pore wall of the primary porous silicon nitride ceramic and/or the secondary porous silicon nitride ceramic, and the silicon nitride nanowires are intersected and stacked to form a tertiary pore structure. As shown in FIG. 1, the silicon nitride ceramic with the hierarchical pore structure of the invention has a primary pore structure (> 100 μm) formed by a pore-forming agent, a secondary pore structure (10-50 μm) prepared by depositing the silicon nitride ceramic on the wall of the primary pore by a CVI process, and a tertiary pore structure formed by intersecting and spacing gas-solid grown silicon nitride nanowires at the bottom of a sample.
Example 1:
the preparation method of the silicon nitride ceramic with the hierarchical pore structure comprises the following steps:
(1) 100g of silicon nitride ceramic powder, 3g of silicon monoxide and 1.8g of 1.8gY2O3、1.2gAl2O3And mixing the mixed powder with 10g of egg white powder uniformly, weighing the mixed powder with the mass fraction of 30% as mixed powder I, and the balance as mixed powder II, and adding 66.7g of deionized water into the mixed powder I for pre-ball milling to obtain pre-mixed slurry.
(2) Dividing the mixed powder II into five parts, namely 30%, 20%, 20%, 15% and 15% by mass of the mixed powder II, adding the premixed slurry obtained in the step (1) in batches, performing ball milling for 30min after each addition, namely, the time interval of each addition is 30min, after the last part of the mixed powder II is added, adjusting the pH value of the slurry to 10.0 by using a pH regulator (tetramethylammonium hydroxide aqueous solution in the embodiment) with the mass fraction of 25%, and continuing ball milling for 6h to obtain the mixed slurry.
(3) Performing injection molding on the mixed slurry obtained in the step (2), preserving heat and solidifying for 30min under the condition of a 80 ℃ water bath steamer, cooling the sample to room temperature along with the water bath, standing for 36h at the room temperature, transferring to a 60 ℃ oven for drying for 72h, then demolding, preserving heat for 1h at 100 ℃, then increasing the temperature to 600 ℃ at the rate of 1 ℃/min, preserving heat for 2h, and obtaining a ceramic blank with the density of 0.46g/cm3The opening ratio was 82.1%.
(4) Putting the ceramic blank obtained in the step (3) into a CVI furnace for deposition sintering, heating to 950 ℃ at the heating rate of 8 ℃/min, introducing precursor silicon tetrachloride and ammonia gas, keeping the air pressure at 1.0kPa, preserving the heat for 160h, and naturally cooling to room temperature along with the furnace to obtain a gradient porous silicon nitride ceramic sample with the density of 0.67g/cm3The porosity was 77.6%, and the optical photograph thereof is shown in FIG. 2, where a distinct dark and light color boundary line was observed, indicating that the deposition direction was from the outside to the inside.
(5) And (3) erecting 4.8g of the silicon nitride ceramic sample obtained in the step (4) on a crucible, flatly paving 2.0g of SiO powder in the crucible, enabling the silicon nitride ceramic sample not to contact the powder, then integrally placing the crucible and the silicon nitride ceramic sample into a graphite mould for spark plasma sintering, sintering in a nitrogen atmosphere, heating to 1500 ℃ at a heating rate of 100 ℃/min, keeping the temperature for 5min, and naturally cooling to room temperature along with the furnace to obtain the silicon nitride ceramic with the hierarchical pore structure, wherein the phase composition and the microscopic morphology of the silicon nitride ceramic are respectively shown in fig. 3 and 4.
FIG. 3 shows a sample of silicon nitride ceramic prepared in example 1 of the present invention and its XRD spectrum. Fig. 3a shows the XRD pattern after CVI and fig. 3b shows the XRD pattern after SPS sintering, and it can be seen that after CVI and SPS sintering, the phase is mainly α silicon nitride and a small amount of β silicon nitride is present. FIG. 4 is an SEM photograph of a silicon nitride ceramic obtained in example 1 of the present invention. Wherein FIG. 4a is a microstructure of a cross-section of a silicon nitride ceramic, and FIGS. 4b and 4c are enlarged partial views of inner walls of pores in light and dark regions of FIG. 4 a; FIG. 4c is an inset showing the thickness of the silicon nitride deposition on the inner wall surface of the dark area hole in FIG. 4 a; figure 4d shows the growth state of the silicon nitride nanowires on the bottom pore walls. As can be seen from FIG. 4, a plurality of windows appear on the porous silicon nitride gas pore wall obtained after CVI to form a through hole structure, the size of the aperture of the window is between 10 and 50 μm, the inner wall pore surface of the light color region in FIG. 4a is mostly short rod-shaped α -silicon nitride, while the inner wall pore surface of the dark color region is completely covered by amorphous silicon nitride, and the CVI deposition thickness is obviously observed to be 6 to 7 μm from the inset of FIG. 4 c. And finally, the nanowires grown at the bottom of the sample are in a loose porous structure, not only cover the surface of the primary pore wall, but also extend into the secondary pore cavity to form a mesh structure, a dense layer is formed on the outer surface of the bottom of the sample, and the mesh gaps of the mesh structure formed by the nanowires are smaller than 2 microns.
The silicon nitride ceramic prepared by the embodiment has the density of 0.68g/cm through detection3The open porosity was 76.5%, and the pore size distribution curve is shown in fig. 5, which is consistent with the SEM characterization result (where the pore size around 0.25 μm in the mercury intrusion test is not the third-order pore in the SEM photograph, but the pores between silicon nitride particles). The compressive strength was 20.6 MPa. The porosity of the porous ceramic obtained by testing according to GB/T1968-1980 is5.16 cm3·cm/(cm2S.kpa) which is excellent in air permeability and can be used for effective filtration of smoke.
Example 2:
the preparation method of the silicon nitride ceramic with the hierarchical pore structure comprises the following steps:
(1) 100g of silicon nitride ceramic powder, 3g of silicon monoxide and 2.0g of 2.0g Y2O3、1.5g Al2O3And 7.5g of egg white powder, weighing the mixed powder with the mass fraction of 35% as mixed powder I, and the balance as mixed powder II, and adding 66.7g of deionized water into the mixed powder I for pre-ball milling to obtain pre-mixed slurry.
(2) Adding the mixed powder II into the premixed slurry obtained in the step (1) in batches when the mixed powder II is divided into five parts with the mass percentage of 35%, 20%, 20%, 15% and 10%, ball-milling for 20min after each addition, namely, the time interval of each addition is 20min, after the last part of the mixed powder II is added, adjusting the pH value of the slurry to 10.6 by using a pH regulator (tetramethylammonium hydroxide aqueous solution in the embodiment) with the mass percentage of 25%, and continuing ball-milling for 8h to obtain the mixed slurry.
(3) And (3) performing injection molding on the mixed slurry obtained in the step (2), preserving heat and solidifying for 40min under the condition of a 80 ℃ water bath steamer, cooling the sample to room temperature along with the water bath, standing for 36h at the room temperature, transferring to a 60 ℃ oven for drying for 72h, then demolding, and performing gel discharge in a muffle furnace: keeping the temperature at 100 ℃ for 1h, then heating to 600 ℃ at the heating rate of 1 ℃/min, and keeping the temperature for 2h to obtain a ceramic blank with the density of 0.42g/cm3The open porosity was 85.4%.
(4) Putting the ceramic blank obtained in the step (3) into a CVI furnace for deposition sintering, raising the temperature to 1000 ℃ at the heating rate of 8 ℃/min, introducing precursor silicon tetrachloride and ammonia gas, keeping the air pressure at 1.2kPa, preserving the heat for 250 hours, and naturally cooling to room temperature along with the furnace to obtain a gradient porous silicon nitride ceramic sample with the density of 0.96g/cm3The porosity was 51.4%.
(5) And (3) erecting 6.5g of the silicon nitride ceramic sample obtained in the step (4) on a crucible, flatly paving 3.0g of SiO powder in the crucible, enabling the silicon nitride ceramic sample not to contact the powder, then integrally placing the crucible and the silicon nitride ceramic sample into a graphite die for spark plasma sintering, sintering in a nitrogen atmosphere, heating to 1500 ℃ at a heating rate of 200 ℃/min, keeping the temperature for 3min, and naturally cooling to room temperature along with the furnace to obtain the silicon nitride ceramic with the hierarchical pore structure.
The silicon nitride ceramic prepared in the example has a density of 1.0 g/cm3The porosity is 50.2%, the pore size distribution is mainly 0.2 μm, 10-30 μm, 100-200 μm in three regions, and the compressive strength is 28.9 MPa.
Example 3:
the preparation method of the silicon nitride ceramic with the hierarchical pore structure comprises the following steps:
(1) 100g of silicon nitride ceramic powder, 10g of silicon monoxide and 1.8g of 1.8gY2O3、1.2gAl2O3And 15.0g of egg white powder are uniformly mixed, the mixed powder with the mass fraction of 30% is weighed and recorded as mixed powder I, the balance is recorded as mixed powder II, and 72.3g of deionized water is added into the mixed powder I for pre-ball milling to obtain pre-mixed slurry.
(2) Adding the premixed slurry obtained in the step (1) into the mixed powder II in five parts by mass, wherein the five parts by mass are 30%, 25%, 20%, 15% and 10%, performing ball milling for 30min after each addition, namely, the time interval of each addition is 30min, adjusting the pH value of the slurry to 10.3 by using a pH regulator (tetramethylammonium hydroxide aqueous solution in the embodiment) with the mass fraction of 25% after the last part of mixed powder is added, and continuing ball milling for 9h to obtain the mixed slurry.
(3) And (3) performing injection molding on the mixed slurry obtained in the step (2), preserving heat and solidifying for 50min under the condition of a 70 ℃ water bath steamer, cooling the sample to room temperature along with the water bath, standing for 36h at the room temperature, transferring to a 60 ℃ oven for drying for 72h, then demolding, and performing gel discharge in a muffle furnace: keeping the temperature at 100 ℃ for 1h, then heating to 600 ℃ at the heating rate of 1 ℃/min, and keeping the temperature for 2h to obtain a ceramic blank with the density of 0.50g/cm3The void ratio was 83.8%.
(4) The step (A) is3) Putting the obtained ceramic blank into a CVI furnace for deposition sintering, raising the temperature to 1000 ℃ at the heating rate of 8 ℃/min, introducing precursor silicon tetrachloride and ammonia gas, keeping the air pressure at 0.8kPa, preserving the heat for 90 hours, naturally cooling to room temperature along with the furnace to obtain a gradient porous silicon nitride ceramic sample with the density of 0.57g/cm3The porosity was 80.0%.
(5) And (3) erecting 7.7g of the silicon nitride ceramic sample obtained in the step (4) on a crucible, flatly paving 3.0g of SiO powder in the crucible, enabling the silicon nitride ceramic sample not to contact the powder, then integrally placing the crucible and the silicon nitride ceramic sample into a graphite die for spark plasma sintering, sintering in a nitrogen atmosphere, heating to 1400 ℃ at a heating rate of 100 ℃/min, keeping the temperature for 5min, and naturally cooling to room temperature along with the furnace to obtain the silicon nitride ceramic with the hierarchical pore structure.
The silicon nitride ceramic prepared in the example has a density of 0.61 g/cm3The porosity is 78.9%, the pore size distribution is mainly 1 μm, 10-50 μm and 100-300 μm in three regions, and the compressive strength is 16.5 MPa.
Although the present invention has been described with reference to the preferred embodiments, it is not intended to be limited thereto. Those skilled in the art can make numerous possible variations and modifications to the present invention, or modify equivalent embodiments to equivalent variations, without departing from the scope of the invention, using the teachings disclosed above. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical spirit of the present invention should fall within the protection scope of the technical scheme of the present invention, unless the technical spirit of the present invention departs from the content of the technical scheme of the present invention.
Claims (6)
1. A preparation method of silicon nitride ceramics with a hierarchical pore structure is characterized by comprising the following steps: the method comprises the following steps:
s1, uniformly mixing silicon nitride, silicon monoxide, a sintering aid, a pore-forming agent and a gel aid to obtain mixed powder, dividing the mixed powder into two parts, marking as mixed powder I and mixed powder II, mixing the mixed powder I with deionized water, and then performing pre-ball milling to obtain pre-mixed powderSizing agent; the sintering aid is Y2O3And Al2O3Silicon nitride, silicon monoxide, Y2O3And Al2O3The mass ratio of (A) to (B) is 100: 1-20: 0.1-3; the pore-forming agent and the gel auxiliary agent are egg white powder, and the mass fraction of the egg white powder in the mixed powder is 5-25%; the volume fraction of the solid phase in the premixed slurry is 20-60%;
s2, dividing the mixed powder II in the step S1 into n parts, adding the n parts into the premixed slurry in batches, ball-milling the mixed powder II after each time of adding the mixed powder II, adding a pH regulator to adjust the pH value to 8.5-11.5 after the nth part of the mixed powder II is added, and uniformly ball-milling to obtain mixed slurry; wherein n is more than or equal to 4;
s3, performing injection molding on the mixed slurry obtained in the step S2, performing heat preservation and solidification, and drying, demolding and removing glue to obtain a ceramic blank;
s4, placing the ceramic body in the step S3 in a chemical vapor infiltration furnace, and performing deposition sintering in the atmosphere of a gaseous precursor until the porosity reaches the expected target to obtain a ceramic sample; the gaseous precursor is silicon tetrachloride and ammonia; the deposition sintering comprises the following specific steps: raising the temperature to 800-1200 ℃ at the temperature rise rate of 2-20 ℃/min, keeping the temperature for 50-600 h, and keeping the total air pressure at 0.5-5.0 kPa;
s5, paving silicon sources below the ceramic sample in the step S4 at intervals, and performing non-pressure discharge plasma sintering on the ceramic sample and the silicon sources in a protective atmosphere to obtain silicon nitride ceramic with a hierarchical pore structure; the pressureless discharge plasma sintering method comprises the following specific steps: heating to 1300-1700 ℃ at the heating rate of 50-300 ℃/min, preserving the heat for 2-20 min, and cooling to room temperature.
2. The method of claim 1, wherein: in step S5, the protective atmosphere is nitrogen.
3. The preparation method according to claim 1 or 2, wherein in the step S2, n =5, the mass percentages of the first part to the fifth part of the mixed powder II are respectively 35-25%, 25-20%, 20-15% and 15-10%, and the adding time interval between the adjacent batches of the mixed powder II is 20-60 min; the ball milling time after the nth powder mixed powder II is added is 6 to 12 hours.
4. The production method according to claim 3, characterized in that: in the step S2, the pH adjusting agent is a tetramethylammonium hydroxide aqueous solution with a mass fraction of 10% to 30%.
5. The production method according to claim 1 or 2, characterized in that: in the step S3, the injection molding temperature is 50-100 ℃, and the heat preservation and curing time is 10-300 min; the drying method comprises the following specific steps: drying for 12-36 h at room temperature, and then drying for 36-90 h at 30-60 ℃; the rubber discharging method comprises the following specific steps: firstly, the temperature is kept at 60-100 ℃ for 0.5-6 h, then the mixture is heated to 500-800 ℃ at the heating rate of 0.5-10 ℃/min, and the temperature is kept for 0.5-6 h.
6. A silicon nitride ceramic having a hierarchical pore structure, characterized in that: the silicon nitride ceramic is prepared by the preparation method of any one of claims 1 to 5, and comprises a primary porous silicon nitride ceramic, a secondary porous silicon nitride ceramic and silicon nitride nanowires, wherein the secondary porous silicon nitride ceramic is deposited on the pore walls of the primary porous silicon nitride ceramic and forms a secondary pore structure, the silicon nitride nanowires are grown on the pore walls of the primary porous silicon nitride ceramic and/or the secondary porous silicon nitride ceramic, and the silicon nitride nanowires are intersected and stacked to form a tertiary pore structure.
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