CN101407420A - Method for preparing non-grain boundary phase porous silicon nitride ceramic based on carbon thermal reduction - Google Patents

Abstract

The invention discloses a method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction. Firstly, the following components are mixed according to weight percentage: 93-95% of silicon nitride and 5-7% of sintering aid According to the conventional porous silicon nitride preparation process, a porous silicon nitride sample with a porosity of 35-55% is obtained. Use hydrofluoric acid, nitric acid, and sulfuric acid to remove the glass phase in silicon nitride and the compound formed by the sintering aid; prepare a sol of phenolic resin and silicon dioxide in a certain proportion, and take the porous silicon nitride matrix sample after pickling Immerse in the sol for many times, put it into the atmosphere furnace after drying, heat to 1200°C, pass nitrogen gas, raise the temperature to 1750-1800°C, keep it under the nitrogen pressure of 2-6 atmospheres for 1-2 hours, and obtain amorphous Boundary phase porous silicon nitride ceramics. The porous ceramics can be widely used as base materials for gas separation filters in high-temperature atmospheres and corrosive atmospheres, as high-temperature heat-resistant materials for gas turbines for power generation, engines, space shuttles, radomes, etc.

Description

A method for preparing porous silicon nitride ceramics without grain boundary phase based on carbothermal reduction

technical field

The invention relates to a method for preparing porous silicon nitride ceramics, in particular to a method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction.

Background technique

Porous silicon nitride has excellent mechanical properties such as high wear resistance, high strain resistance and damage resistance, and can be used in various fields such as filters, catalyst supports and bioreactors under high and low temperature, as well as reinforcement of composite materials. Porous silicon nitride also has good thermal stability, low dielectric constant, low dielectric loss, and high erosion resistance, and is regarded as the most promising new generation of wave-transmitting materials. Silicon nitride is a strong covalent bond compound, its self-diffusion coefficient is very small, and the volume diffusion and grain boundary diffusion speed necessary for densification are very small. At the same time, the ratio of its grain boundary energy V _gb to powder surface energy V _sv , ( V _gb /V _sv ) is much larger than ionic compounds and metals, making the sintering driving force ΔV smaller, so it is difficult to sinter pure silicon nitride by solid phase, and sintering aids (usually metal oxides) and nitrogen must be added The silicon dioxide oxide layer or silicon nitride on the surface of silicon nitride powder forms a eutectic liquid with a low melting point, which promotes the sintering and densification of silicon nitride ceramics through the liquid phase sintering mechanism. So far, porous silicon nitride ceramics have been prepared by adding oxide sintering aids such as MgO, Al ₂ O ₃ , Y ₂ O ₃ , Yb ₂ O ₃ to produce liquid phase sintering, and carbothermal reduction. For example, in Japanese Patent Laid-Open No. 2000-225985, the molded body formed by the mixed powder containing silicon nitride particles and oxide sintering aids is heated in nitrogen, and the sintering temperature and the amount of sintering aids are controlled to obtain silicon nitride porous ceramic materials. . In Japanese Patent Laid-Open No. 2001-206775, it is proposed to prepare porous silicon nitride ceramics by heat-treating a molded body containing silicon dioxide particles, carbon particles and metal silicon particles with an average particle size of 5 μm or less in nitrogen.

In the above preparation method, due to the addition of sintering aids, the porous silicon nitride ceramics containing the glass phase are formed through liquid phase sintering. Because it contains more glass phase, the strength is low, and the glass phase is easy to soften at high temperature, so its high temperature resistance, insulation and dielectric properties have certain influences.

Contents of the invention

The purpose of the present invention is to provide a method for preparing porous silicon nitride ceramics without a grain boundary phase, so as to improve the high-temperature performance and application of the existing porous silicon nitride ceramics.

To achieve the above object, the present invention is achieved by taking the following technical solutions:

A method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction, comprising the steps of:

(1) By weight percentage, the following components are mixed: 93-95% of silicon nitride and 5-7% of sintering aid, and are sintered at 1750°C for 2h in _N2 atmosphere according to the conventional preparation process of porous silicon nitride to obtain a porous silicon nitride sample with a porosity of 35-55%;

(2) Pickling with one or more of hydrofluoric acid, nitric acid, and sulfuric acid to remove the compound formed by the glass phase and sintering aid in the porous silicon nitride sample, and then washing with alkali to remove the acid to prepare an amorphous Silicon nitride substrate samples in the world;

(3) Prepare the sol of phenolic resin and silicon dioxide, the molar ratio of the carbon and silicon dioxide cracked by the phenolic resin is 1～3, the porous silicon nitride substrate sample after pickling is immersed in the sol, dipped several times Seep until saturated;

(4) Dry the impregnated and saturated porous silicon nitride substrate sample, place it in an atmosphere furnace, heat it to 1200°C at a heating rate of 5-30°C/min, feed nitrogen gas, and rise to 1400°C after 1-2 hours. ~1600°C, heat up to 1750~1800°C after 1~4 hours, and keep warm for 1~2 hours under nitrogen pressure of 2~6 atmospheres to obtain porous silicon nitride ceramics without grain boundary phase.

In the above method, the sintering aid is at least one of alumina or trivalent rare earth metal oxides. The acid precipitation is to remove the glass phase with 96% HF acid+98% _HNO3 acid in _a 60°C water bath for 2 to 4 hours; to remove the glass phase with 98% _H2SO4 acid in a 60°C water bath for 1 hour Yttrium compounds. The alkali washing is to use ammonia water with a mass concentration of 28% at 69° C. for 1 hour to remove acid.

The preparation method of the phenolic resin and silica sol: take the phenolic resin and dissolve it in ethanol, magnetically stir in a constant temperature water bath, mix evenly, and mix ethyl orthosilicate, absolute ethanol, and water in a molar ratio=1:2:3 Mix, stir evenly in a constant temperature water bath with magnetic force, add hydrochloric acid with a mass concentration of 38% to adjust the pH value, hydrolyze to obtain a silica sol, and mix the two liquids evenly to obtain a sol of phenolic resin and silica.

The present invention uses normal pressure sintering to prepare a porous silicon nitride ceramic sample, which ensures a good lap joint of the silicon nitride rod-like structure, and then uses a carbothermal reduction method to prepare a new silicon nitride lap joint without grain boundary phase porous nitrogen Silicon ceramics.

The beneficial effect of the present invention is that, according to the method of the present invention, the uniformity of impregnation is ensured by preparing the sol of phenolic resin and silicon dioxide. By adjusting nitrogen pressure and sintering temperature, porous silicon nitride ceramics with high porosity and good mechanical properties can be obtained without grain boundary phase. The grain boundary phase-free porous silicon nitride ceramics of the present invention, compared with the existing porous silicon nitride ceramics, has excellent high-temperature performance due to the lack of grain boundary glass phases, and can be widely used in gases under high-temperature atmospheres and corrosive atmospheres. Base materials for separation filters, reinforcement materials for heat-resistant materials used in gas turbines for power generation, engines, space shuttles, radomes, etc., reinforcement materials for metal matrix composite materials, and various heat insulation, sound absorption, substrates, etc.

Description of drawings

Fig. 1 is the XRD pattern of the sample after normal pressure sintering in Example 1.

Fig. 2 is the XRD pattern of the sample after carbothermal reduction in Example 1.

The SEM figure of the sample after Fig. 3 embodiment 1 carbothermal reduction

Detailed ways

The present invention will be further described in detail below in conjunction with specific examples.

Porous silicon nitride ceramics, the composition of which is shown in Table 1. In Examples 1 to 9 shown in Table 1, the silicon nitride powder content is generally 93 to 95%, and the amount of sintering aid added is 5 to 7 wt%. , the added amount exceeds 7wt%, the shrinkage rate of porous ceramics increases, resulting in a decrease in porosity and a large number of intercrystalline glass phases. Sintering aids refer to metal oxides that change into glass phases in the high temperature range of sintering, which can be expressed as M ₂ O ₃ or MO (M is metal), and also include oxides of one or several components that can change into glass phases through reactions. A mixture of glass phases. Such a metal oxide is at least one of Y ₂ O ₃ , Al ₂ O ₃ , Eu ₂ O ₃ or Lu ₂ O ₃ used in the examples in Table 1, and a mixture of several oxides may also be added according to circumstances.

The preparation method of porous silicon nitride: mix silicon nitride and sintering aid according to the ratio in Table 1 and put them into a ball milling tank, use absolute ethanol as the ball milling medium, and use silicon nitride balls to mill on a rolling ball mill for 24 hours. The mixed slurry was dried in a 60°C drying oven and passed through a 200-mesh sieve. The prepared powder is uniaxially pressed into a strip sample in a pressure forming machine, and then the pressed green body is placed in a graphite crucible coated with BN, and the graphite crucible is placed in a Japanese-made HIGH-MULTI-5000 In a multifunctional furnace, sinter at 1750° C. for ₂ h in N atmosphere to obtain a porous silicon nitride sample with a porosity of 35-55% (Example 1-Example 9).

Preparation method of the silicon nitride substrate sample without grain boundary: use 96% concentrated HF acid+98% concentrated _HNO3 to prepare a porous silicon nitride sample with a porosity of 35-55% in Table 1, Example 1-Example 9 Acid, temperature 60°C, water bath 2-4h, pickling to remove glass phase; 98% concentrated _H2SO4 acid, 60°C 1h, pickling to remove yttrium compound; 28% _concentrated ammonia, temperature 69°C, time 1h , Alkaline washing to remove the acid, and prepare a silicon nitride matrix sample without grain boundaries.

The preparation method of the sol: take the phenolic resin (70%C) and dissolve it in ethanol, stir it magnetically in a constant temperature water bath, and mix evenly. Mix tetraethyl orthosilicate (TEOS, density: 0.933g·mL ^-1 ), absolute ethanol (density: 0.79g·mL ^-1 ), water in a certain ratio, (the molar ratio is 1:2:3 ), stir evenly in a constant temperature water bath, add hydrochloric acid (density is 1.19g·mL ⁻¹ , mass percentage concentration is about 38%), adjust the pH value, the amount of hydrochloric acid is HCl/TEOS=0.01mol (molar ratio), carry out hydrolysis, The silicon dioxide sol is obtained, and the two mixtures are uniformly mixed to prepare a sol of phenolic resin and silicon dioxide.

The molar ratio of the carbon cracked by the phenolic resin to the silicon dioxide hydrolyzed by the ethyl orthosilicate is 1:3-3:1. In the carbothermal reduction reaction, the optimum molar ratio of carbon to silicon dioxide is 2. When it is less than 1, the composite ceramics of silicon nitride and silicon dioxide can be produced due to the low carbon content. When it is greater than 3, the carbon content The increase of carbon dioxide will increase the weight loss on ignition, which will greatly affect the strength of porous silicon nitride ceramics without grain boundary phase. Carbon and silicon dioxide are easy to form silicon carbide, so the molar ratio is selected at 1-3.

The sample examples 1 to 9 of the grain boundary-free silicon nitride matrix after pickling were immersed in sols with different molar ratios of carbon to silicon dioxide (Table 1), and soaked several times until saturated. After drying, put it into a multi-functional atmosphere furnace, heat at 1200°C at a heating rate of 5-30°C/min, pass in nitrogen, rise to 1400-1600°C after 1-2 hours, and rise to 1750-1800°C after 1-4 hours , keep the heat for 1-2 hours under the nitrogen pressure of 2-6 atmospheres to obtain porous silicon nitride ceramics without grain boundary phase.

The sintering temperature of the molded body is 1750-1800° C. under nitrogen atmosphere. If the temperature is less than 1750 degrees, the silicon nitride phase transformation cannot be completely completed. If the temperature exceeds 1800°C, the silicon nitride grains may grow abnormally, resulting in uneven grains and a decrease in mechanical properties. In the carbothermal reduction reaction, carbon and silicon dioxide can react with silicon carbide at high temperature, so the formation of silicon carbide can be prevented by increasing the nitrogen pressure. If the holding time is less than 1 hour, the phase transition of silicon nitride is not sufficient. According to XRD, the α-β phase transformation has been completely completed after 2 hours of holding time. The prolonged time will cause waste of energy. Therefore, the holding time is preferably 1-2 hours.

The quality of the sample was measured with an electronic balance, and the bending strength was measured by the three-point bending method. The Archimedes drainage method was used to determine the open porosity. X-ray diffraction (XRD) analysis of the phase. The microstructure of the samples was observed with a scanning electron microscope (SEM). Table 1 Composition and sintering process conditions of Examples 1-9 of the present invention. Table 2 Test performance of porous silicon nitride ceramics of the present invention. Example 1 The XRD pattern of the sample after normal pressure sintering is shown in FIG. 1 , the XRD pattern of the sample after carbothermal reduction is shown in FIG. 2 , and the SEM pattern is shown in FIG. 3 .

Table 1 Composition and sintering process conditions of Examples 1-9 of the present invention

example Proportion Porosity (%) Bending strength (MPa) carbon to silica molar ratio Temperature (°C) Insulation (h) nitrogen pressure Example 1 95% Si ₃ N ₄ +5% Y ₂ O ₃ 53.1 139.2 2 1750 2 6 Example 2 94% Si ₃ N ₄ +5% Y ₂ O ₃ +1% Al ₂ O ₃ 42.4 197.2 1 1750 1 2 Example 3 93%Si ₃ N ₄ 4+5%Y ₂ O ₃ +2%Al ₂ O ₃ 35.1 216.8 1 1750 1 2 Example 4 95% Si ₃ N ₄ +5% Lu ₂ O ₃ 54.1 148.3 2 1800 1.5 6 Example 5 94% Si ₃ N ₄ +5% Lu ₂ O ₃ +1% Al ₂ O ₃ 44.4 206.3 2 1800 1.5 6 Example 6 93% Si ₃ N ₄ +5% Lu ₂ O ₃ +2% Al ₂ O ₃ 37.1 234.2 2 1800 1.5 6 Example 7 95% Si ₃ N ₄ +5% Eu ₂ O ₃ 53.6 143.4 3 1750 2 4 Example 8 94% Si ₃ N ₄ +5% Eu ₂ O ₃ +1% Al ₂ O ₃ 43.1 217.5 3 1750 2 4 Example 9 93% Si ₃ N ₄ +5% Eu ₂ O ₃ +2% Al ₂ O ₃ 36.7 229.4 3 1750 2 4

Table 2 Test performance of porous silicon nitride ceramics of the present invention

It can be seen from Table 2 that the mass of the matrix decreases by 5-10% after pickling, the weight gain of the sample after impregnation is 9-14%, and the mass decreases by 4-7% after carbothermal reduction, so the corroded mass is basically replenished , The porosity of the sample increased by 2-3% after pickling. In the sintering temperature range of 1750-1800°C, the material exhibits high bending strength at a porosity of 35-55%, up to 220.67MPa. The mass loss increases as the molar ratio of cracked carbon to silica in the phenolic resin increases. After the sample was corroded, the glass phase connecting the silicon nitride rod-like structures was corroded, leaving only the rod-like structures, and there was no overlap between them, so the strength was reduced, almost to half of the original strength. When the matrix sample is infiltrated with phenolic resin and silicon dioxide, the carbon decomposed by the phenolic resin reacts with silicon dioxide and nitrogen carbothermal reduction to form silicon nitride, and the rod-shaped silicon nitride is rejoined, so the strength increases, which is similar to the original The sintered samples have almost the same level of strength as the grain boundary phase-free porous silicon nitride ceramics.

It can be seen from Figure 1 that the main crystal phase of the atmospheric pressure sintered sample is β-Si ₃ N ₄ , and there is a small amount of grain boundary crystal phase Y ₂ Si ₃ O ₃ N ₄ . This shows that α-Si ₃ N ₄ has been completely transformed into β-Si ₃ N ₄ , the phase transition has been completed, and a small amount of Y ₂ Si ₃ O ₃ N ₄ is formed by the reaction of Y ₂ O ₃ with Si ₃ N ₄ and SiO ₂ of. In addition, since the sintering aid participated in the formation of the liquid phase, part of the amorphous glass grain boundary phase existed in the sintered sample after final sintering.

It can be seen from Figure 2 that there is only the diffraction peak of β-Si ₃ N ₄ in the carbothermal reduction sintering XRD, indicating that the composition of the sample is β-Si ₃ N ₄ , and no diffraction peak of SiC appears, indicating that under nitrogen atmosphere, Carbon decomposed by phenolic resin reacts with silicon dioxide and nitrogen carbothermal reduction to generate β-Si ₃ N ₄ . High nitrogen pressure can promote the formation of silicon nitride and inhibit the formation of silicon carbide.

It can be seen from Figure 3 that the newly formed rod-shaped grains overlap the original silicon nitride rod-shaped grains, which improves the strength of the sample, so porous silicon nitride ceramics without grain boundary phase can be prepared.

Claims (5)

1. A method for preparing grain boundary phase porous silicon nitride ceramics based on carbothermal reduction, is characterized in that, comprises the following steps: (1) By weight percentage, the following components are mixed: 93-95% of silicon nitride and 5-7% of sintering aid, and are sintered at 1750°C for 2h in _N2 atmosphere according to the conventional preparation process of porous silicon nitride to obtain a porous silicon nitride sample with a porosity of 35-55%; (2) Pickling with one or more of hydrofluoric acid, nitric acid, and sulfuric acid to remove the compound formed by the glass phase and sintering aid in the porous silicon nitride sample, and then washing with alkali to remove the acid to prepare an amorphous Silicon nitride substrate samples in the world; (3) Prepare the sol of phenolic resin and silicon dioxide, the molar ratio of the carbon and silicon dioxide cracked by the phenolic resin is 1～3, the porous silicon nitride substrate sample after pickling is immersed in the sol, dipped several times Seep until saturated; (4) Dry the impregnated and saturated porous silicon nitride substrate sample, place it in an atmosphere furnace, heat it to 1200°C at a heating rate of 5-30°C/min, feed nitrogen gas, and rise to 1400°C after 1-2 hours. ~1600°C, heat up to 1750~1800°C after 1~4 hours, and keep warm for 1~2 hours under nitrogen pressure of 2~6 atmospheres to obtain porous silicon nitride ceramics without grain boundary phase. 2. The method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction according to claim 1, characterized in that the sintering aid is at least one of alumina or trivalent rare earth metal oxides. 3. The method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction as claimed in claim 1, characterized in that the pickling is with 96% HF acid+98% _HNO3 acid , 60°C water bath for 2 to 4 hours to remove the glass phase; use H ₂ SO ₄ acid with a mass concentration of 98% to remove the yttrium compound in a 60°C water bath for 1 hour. 4. The method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction according to claim 1, characterized in that the alkali washing is to use ammonia water with a mass concentration of 28% at 69°C for 1 hour to remove the acid. 5. The method for preparing porous silicon nitride ceramics without a grain boundary phase based on carbothermal reduction as claimed in claim 1, characterized in that, the preparation method of the phenolic resin and silica sol: take the phenolic resin and dissolve it in ethanol , magnetic stirring in a constant temperature water bath, mixing evenly, mixing ethyl orthosilicate, absolute ethanol, and water in a molar ratio = 1:2:3, stirring evenly in a constant temperature water bath, adding hydrochloric acid with a mass concentration of 38% to adjust the pH value, and hydrolyzing The silica sol is obtained, and the two liquids are uniformly mixed to obtain a sol of phenolic resin and silica.

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