Porous silicon nitride ceramics have the properties of both silicon nitride ceramics and porous materials. They not only have the advantages of high strength, good toughness, creep resistance, structural stability, rain corrosion resistance and thermal shock resistance of silicon nitride ceramics, but also have the characteristics of low density, low dielectric constant and dielectric loss of porous materials. The excellent performance of porous silicon nitride ceramics determines that it has broad application prospects. Due to its high temperature resistance, corrosion resistance, thermal shock resistance, high opening, internal pores and other characteristics, porous silicon nitride ceramics are widely used in the fields of smoke and gas filtration, catalyst carrier and so on. They can also be used in the field of aerospace and aviation as radome materials used in harsh environments.
The preparation of ceramic products with higher strength and higher porosity is the research subject of silicon nitride porous ceramic materials. Since the phase transition temperature of silicon nitride is carried out at high temperature, and the ceramic properties of silicon nitride also depend on the process and microstructure, reduce the sintering temperature of silicon nitride ceramics, Using advanced sintering technology is a hot issue in the research of silicon nitride porous ceramics.
The preparation methods of porous silicon nitride ceramics are briefly described below:
(1) Carbothermal reduction method: porous silicon nitride prepared by this method is formed in situ by carbothermal reduction reaction of cheap silicon dioxide and carbon in high temperature nitrogen atmosphere α- After Si3N4, under the action of sintering additives, the phase becomes β- Si3N4。 Porous silicon nitride was obtained due to 44% weight loss.
Because the silicon oxide and carbon powder particles are relatively small, the pore diameter of the sintered body is small, the specific surface area is large, and the spatial grid structure is formed, so it can be used as a filter device. Moreover, the wide range of carbon sources makes the finished products prepared by this method show diversity, which can meet the needs of different applications. For example, porous silicon nitride ceramics with similar wood structure can be prepared by using wood as the source of carbon.
(2) gel casting method: this method was first proposed by Oak Ridge National Laboratory, which means that the slurry can be solidified into a green body in situ by chemical reaction of slurry or a small amount of additives. In the process of gelcasting, porous silicon nitride was obtained due to the burning of organic compounds, partial densification of non pressure sintering and hydrolysis of silicon nitride under alkaline conditions.
Fig. 1 process flow chart of gelcasting for porous silicon nitride ceramics
Gelcasting is initially applied to the preparation of compact ceramics. It has the advantages of uniform structure, fine machining, and nearly net shape and complex shape parts. Porous silicon nitride ceramics were prepared by this method β- The porous structure of Si3N4 grain was prepared by using silica sol as monomer and sol-gel process. After molding, silicon oxide is wrapped on the surface of silicon nitride powder, which inhibits the oxidation and decomposition of silicon nitride at high temperature. Therefore, silicon nitride can be sintered in air atmosphere without adding protective atmosphere. Silicon oxide can also be used as sintering additive in the sintering process to improve the strength of the sintered body. Without adding a large amount of organic matter, the sintering process is simplified and the performance of the products is guaranteed.
(3) Partial hot pressing method: the porous silicon nitride ceramics prepared by this method with Y2O3, MgO and Cao as additives are composed of many columns β- Si3N4 grain and some residual α- Si3N4 is composed of long columnar pores β- Si3N4 is formed by overlapping, and its shape is irregular. It has the advantages of controlling the porosity of porous ceramics and no additives.
(4) Adding pore forming agent method: this method can arbitrarily change the type, amount and particle diameter of pore forming agent, and can make porous ceramics with different pore sizes and distributions. Compared with other methods, the cost of adding pore forming agent is the lowest. In this process, porous ceramics are prepared by adding pore forming agent into ceramic ingredients, using pore forming agent to occupy a certain space in the green body, and then sintering, and the pore forming agent leaves the matrix to form pores.
Fig. 2 process route of adding pore forming agent to prepare materials
(5) Starch Consolidation Method: by adding starch as pore forming agent and consolidation agent, this process can carry out partial oxidation sintering under air to prepare porous silicon nitride matrix composites with porosity up to 73.17%. In this method, silicon nitride, boron nitride and silicon dioxide are used as ceramic matrix materials, which are prepared by starch consolidation process and atmospheric pressure partial oxidation sintering. The porosity and pore structure are controlled by controlling the volume fraction, size and geometry of starch, so as to control the final properties of the material.
(6) Extrusion molding method: this method uses methylcellulose as binder to prepare silicon nitride slurry, and uses plunger extrusion die to prepare porous silicon nitride ceramics by extrusion molding method. Silicon nitride powder used α The phase content is more than 95%, and the average particle size is about 1 μ m。 Methylcellulose is used as binder and yttrium oxide as sintering additive.
(7) Coating porogen method: this method can prepare porous silicon nitride ceramics with porosity up to 70% and through porosity. The porosity of the sample increases with the increase of coating porogen. The use of coating method separates the pores from each other and avoids the macro fracture source caused by pore superposition. The bending strength of porous silicon nitride prepared by this method is much higher than that of uncoated silicon nitride, which shows the microstructure of high-density dislocations. Dislocations are in high-energy state, so the corrosion resistance is reduced.
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In foreign countries, the development and application of porous silicon nitride ceramics are ahead of that in China, especially in Japan. Taking the Ceramic Research Association in Nagoya, Japan as an example, they have made a major breakthrough in pore control technology and made anisotropic porous silicon nitride ceramic materials. Some researchers obtained porous silicon nitride ceramic products with porosity of 0 ~ 50% through the study of preparation methods. In the 1980s, based on the density of silicon nitride ceramics, Boeing successfully developed a preparation method that can control the density of ceramics. The density can be prepared from 0.5 ~ 1.8g/cm ³ Silicon nitride porous ceramics in the range, and a multiple broadband radome has been successfully developed.
The research progress of porous ceramics at home and abroad is analyzed. The preparation of porous silicon nitride ceramics with high strength and controllable pores is the development direction of silicon nitride. The domestic traditional method of preparing porous silicon nitride ceramics is mainly the method of adding pore forming agent. Although this method is simple, there are some problems to be solved, such as high sintering temperature and low porosity. Scholars have studied different preparation methods and improved preparation processes to obtain porous ceramic materials with good properties, which is still one of the research hotspots of silicon nitride ceramics.