Sintering principle and analysis of special ceramics

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The main preparation process of special ceramics includes three steps of blank preparation, forming and sintering. After the molding process is completed, sintering can control the growth of grains and has a crucial impact on the performance of the material. So far, ceramic sintering technology has been a field of continuous breakthroughs.

Special ceramic sintering principle

Sintering refers to the densification process of the formed green body under the action of high temperature, through the mutual bonding of particles and material transfer between the green bodies, the elimination of pores, the shrinkage of volume, the improvement of strength, and the gradual transformation into a sintered body with a certain geometric shape and a strong sintered body. . Observing the sintering phenomenon on the macro and micro level, it can be seen that the volume of the sintered product shrinks, the density increases, and the strength increases. Microscopically, the shape of the pores changes, the crystal grows, and the composition changes (doping elements). According to the changes in the sintering process, sintering is mainly divided into the following stages:

1 Pre-sintering stage

(1)Removal of binders, etc.: For example, paraffin wax is all vaporized and volatilized at 250~400℃.

(2)As the sintering temperature increases. The atomic diffusion is intensified, the voids are reduced, the point contact between the particles is transformed into surface contact, the voids are reduced, the connected voids become closed, and are distributed in isolation.

(3)Small particles are the first to appear grain boundaries, the grain boundaries move, and the grains become larger.

2 Late stage of sintering

(1) Elimination of pores: The material on the grain boundary continuously diffuses to the pores, so that the pores are gradually eliminated.

(2)Grain growth: The grain boundary moves and the grain grows.

Ceramic sintering can be mainly divided into solid-phase sintering and liquid-phase sintering, which correspond to different reaction mechanisms. The reaction mechanism of liquid phase sintering can be simply summarized as melting, rearrangement, dissolution-precipitation, and pore exclusion; according to the structural characteristics of the sintered body, the solid phase sintering mechanism is divided into three stages: initial sintering, middle sintering and late sintering.

Schematic diagram of solid phase sintering

Early stage of sintering: In the early stage of sintering, the particles are close to each other, and the contact between different particles forms a neck through material diffusion and body shrinkage. At this stage, the grains in the particles do not change, and the shape of the particles remains basically unchanged.

In the middle stage of sintering: the sintered neck begins to grow, the atoms migrate to the particle bonding surface, the distance between the particles shrinks, and a continuous pore network is formed. Both the density and the strength of the sintered body increase at this stage.

Late sintering stage: Generally, when the density of the sintered body reaches 90%, the sintering enters the late sintering stage. At this time, most of the pores are separated, and the substances on the grain boundaries continue to diffuse and fill the pores. As the densification continues, the grains continue to grow. At this stage, the sintered body shrinks mainly through the disappearance of small pores and the reduction of the number of pores, and the shrinkage is slow.

Special ceramic sintering method

People classify the sintering methods of ceramics according to different bases, and their characteristics and scope of application are as follows:

Brief introduction of ceramic sintering method

Factors Affecting Sintering

1 powder particle size

Fine particles increase the driving force of sintering, shorten the atomic diffusion distance, improve the solubility of particles in the liquid phase, and accelerate the sintering process, but too fine particles are easy to absorb a large amount of gas, hinder the contact between particles and hinder sintering, so it must be reasonable according to the sintering conditions. selection granularity.

2 The role of admixtures

In solid phase sintering, admixtures can promote sintering by increasing defects; in liquid phase sintering, admixtures can promote sintering by changing the properties of the liquid phase.

3 Sintering temperature and time

Increasing the sintering temperature is beneficial to mass transfer such as solid-phase diffusion, but an excessively high temperature will promote secondary crystallization and deteriorate the material properties. The low temperature stage of sintering is dominated by surface diffusion, and the high temperature stage is dominated by volume diffusion. Too long sintering time at low temperature is unfavorable for densification, and the performance of the material is deteriorated. Therefore, high temperature and short time sintering is usually used to improve the density of the material.

4 Sintering atmosphere

Sintering in air will generate vacancies and cause defects in the crystal, so the atmosphere should be selected for sintering different matrix materials. The influence of atmosphere on sintering is very complicated. General materials such as TiO2, BeO, Al2O3, etc., are sintered in a reducing atmosphere, and oxygen can directly escape from the crystal surface to form a defect structure, which is conducive to sintering; non-oxide ceramics, because they are easily oxidized at high temperatures, so in nitrogen and Sintering in an inert gas; PZT ceramics, in order to prevent the volatilization of Pb, require an atmosphere sheet or atmosphere powder for airtight sintering.

5 Forming pressure

The forming pressure of the green body is also critical to the properties of the material. The higher the molding pressure, the tighter the contact between the particles in the green body, and the smaller the diffusion resistance during sintering; too high molding pressure will cause brittle fracture of the powder, which is not conducive to sintering.

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