Silicon nitride powder is the key raw material for the preparation of silicon nitride ceramics. Its performance is a key factor affecting the formation and sintering of silicon nitride green bodies, and it has an important impact on the density and mechanical properties of the final silicon nitride ceramic products.
1.Powder requirements for silicon nitride ceramics
(1) High particle dispersibility and good homogeneity
For most processes, sub-micron-sized powders are required, that is, the surface area is 10-25 cm2 / g, which can produce high-density particulate structure materials.
(2) α-phase content
Because the sintering and structure formation of silicon nitride ceramics are related to the accompanying α-Si3N4 → β-Si3N4 phase transition, and this change occurs based on the mechanism of liquid crystal recrystallization. Although this requirement is recognized, the successfully used silicon nitride powders are still amorphous or contain some crystalline phases.
(3) Controllable oxygen amount
Oxygen, as a basic impurity, participates in the silicon nitride powder in an adsorbed form, and also participates in the form of SiO2 and Si2N2O covering the surface of the Si3N4 particles. The oxygen content determines the amount of liquid phase during sintering and affects the phase composition, structure and properties of the material.
(4) Very little metal impurities and carbon
The presence of iron, calcium, and magnesium in the silicon nitride powder will reduce the viscosity of the liquid phase during sintering and increase the final density by 1% -5%, but it can promote the development of Si3N4 grains and the formation of coarser ceramic structures, reducing mechanical properties. High temperature strength under stress and accelerated deformation speed. The presence of carbon reduces the amount of liquid phase and changes its composition, inhibits sintering and promotes structural cohesion.
Regarding the effects of silicon nitride powder types on the properties of sintering and thermal diffusivity of silicon nitride ceramics, Peng Mengmeng, etc. of the State Key Laboratory of New Ceramics and Fine Technology of Tsinghua University, etc. synthesized β-nitride in a self-propagating high temperature in experiments Silicon powder and α-silicon nitride powder are used as raw materials, and rare earth compounds and MgO composite sintering aids are added. The silicon nitride ceramics are prepared by high temperature heat treatment after discharge plasma sintering. Impact on performance such as conductivity. Studies have shown that the thermal diffusivity of silicon nitride ceramics prepared using β-silicon nitride powder is nearly 20% higher than that of α-silicon nitride powder silicon nitride ceramics prepared using the same process, and the bending strength is higher than that of α- Silicon nitride powder silicon nitride ceramic is nearly 50% lower.
Silicon nitride powder has an important influence on the performance of the final ceramic product, and the basic properties of silicon nitride powder depend on its synthesis method and the quality of the initial reactant. The following table is the basic characteristics of silicon nitride powders prepared by different synthesis methods.
2. Granulation method of silicon nitride powder
In addition to the influence of the synthetic method, in the preparation of engineering ceramics, the raw material powder generally needs to be processed, that is, by "granulation", the raw material powder has an ideal shape, size, and reasonable particle size distribution. The granulation of powder is particularly critical in the preparation of silicon nitride ceramics: in order to ensure the sintering activity of the silicon nitride green body, a raw material powder with a small particle size is required, and a too fine raw material powder has the problem of poor fluidity and sintering. The important problems such as the need to uniformly disperse the additives in the silicon nitride powder need to be solved by the granulation treatment of the raw material powder. Studying the granulation treatment method of silicon nitride powder, analyzing the characteristics, advantages and disadvantages of each method, and selecting the best powder treatment solution according to the actual production conditions are of great significance for the preparation of high-performance silicon nitride ceramics.
(1) Dry pressing granulation
Dry pressing granulation refers to the process of forming powder through a mold, then crushing and spheroidizing. The specific steps of dry pressing granulation are: pre-pressure conveying → roll forming → crushing granulation. Dry pressing granulation has the advantages of high granulation efficiency and low production cost. In particular, compared with some granulation methods, such as spray granulation, the binder content required for dry-press granulation is very low, which can reduce the problems of low sintering density and many pores caused by the binder.
(2) Cold isostatic pressing granulation
Cold isostatic pressing granulation is similar to dry pressing granulation. It is also a granulation method in which the powder is shaped under a certain pressure and then crushed and spheroidized. The difference is that the ceramic powder is placed in a specific mold and then placed in a cold isostatic press. Cold isostatic pressure takes advantage of the incompressibility of the liquid medium and the characteristic of uniformly transmitting pressure, which can achieve uniform pressure on the sample from all directions, ensuring that the pressure on the powder in all directions is uniform and the same size. The powder is pressed into a green body by a cold isostatic pressing process, and then crushed by a crusher and sieved to complete the granulation process.
The above dry pressing granulation and cold isostatic pressing granulation belong to the pressure granulation method. The advantages of pressure granulation are the large bulk density of the formed pellets, which is conducive to improving the compactness of the green body; the content of organic substances such as plasticizers is small, and the subsequent processing is simple; the equipment and technology are simple; The crusher is broken, the shape of the pellet is difficult to control, and it is generally difficult to be ideal spherical, and there are problems of noise and dust, which adversely affects the health of the operator and the environment of the production line.
(3) Spray granulation method
Spray granulation refers to a granulation method in which the mixed slurry is directly sprayed into hot air, and quickly dried to obtain spherical powder particles with a regular shape. The spray granulation process can avoid re-agglomeration and sedimentation separation of the components in the slurry, can maintain the original uniformity of the slurry, and obtain a uniform particle size distribution and good fluidity. Spray granulation process can realize continuous automatic production, which can improve production efficiency and reduce dust pollution. The spray granulation process has a regular particle shape, which is conducive to improving the density and uniformity of the green body, and then improving the sintering performance of the green body.
The spray granulation method usually uses centrifugal or pressure spray granulation equipment to granulate the mixed silicon nitride slurry while stirring, so that the silicon nitride powder is evenly distributed, thereby improving the sphericity of the silicon nitride particles. And powder flowability, and improve the particle size distribution of the powder. Factors such as temperature, pressure, feeding speed, stirring time, type of binder, etc. during spray granulation have important effects on the particle size and dispersibility of silicon nitride powder after granulation.
However, the shape of spray granulated particles is not easy to control, and defects such as hollowness and surface pits are prone to occur. In addition, the powder obtained by spray granulation usually needs to burn out the organic matter to ensure that the organic matter is completely eliminated so as not to affect the sintering performance of the silicon nitride ceramic. Therefore, it is important to formulate a reasonable debinding system and accurately control the debinding process.