What is Alumina ceramic sintering

1.Alumina ceramic sintering

High-density alumina ceramic sintering can be divided into two categories:
① through the diffusion process of solid-phase sintering to achieve densification, sintering process almost no liquid phase;

② with the liquid phase of the viscous flow to achieve dense sintering. Solid-phase sintered alumina ceramic materials are of high purity (> 99.9% by weight) and are mainly used in high temperature environments and require excellent corrosion resistance, such as transparent aluminum tubes for high pressure sodium lamps. Liquid sintering can reduce the sintering temperature and promote the densification, Al2O3 content range is generally 80% to 99%, the material contains grain boundary glass phase or other phase.

2.Solid phase sintering

Solid-phase sintered alumina is a single-phase ceramic made of high-purity ultra-fine alumina powder. A small amount of MgO or other additives can be introduced during the synthesis of the powder to inhibit the excessive growth of the Al2O3 grains and promote the densification , The densification sintering temperature depends on the particle size, specific surface area and sintering activity of the Al2O3 powder. For the high purity ultrafine alumina powder synthesized by chemical method, it can reach 99.5% relative density at lower sintering temperature (1350-1550 ℃), such as Japan’s Daming Chemical Co., Ltd., Japan Sumitomo Chemical Co., Ltd., SASOL produced by SASOL And WA series of Al2O3 powder.

3.MgO inhibits grain growth

High purity Al2O3 sintering process by adding a small amount of MgO (the amount of 0.05% to 0.25% (wt) can effectively inhibit the grain grow too much. United States GE’s ceramic scientist Goble in 1961 first found to add 0.25% WtO) MgO can reduce the pores produced during the sintering process of Al2O3, inhibit the grain growth and make the sintering tend to be completely densified.MgO can reduce the growth of discontinuous grains, eliminate the grain boundary separation of voids and reduce the density The average grain growth rate of aluminum.

The study on the mechanism of its mechanism suggests that when MgO is more than 0.1%, a second phase – spinel (MgAl2O3) is formed on the grain boundary surface, and the spinel is wrapped on the grain boundary outside the Al2O3 grains. Grain growth is achieved through the grain boundary movement, when the Al2O3grain grows, because the grain boundary spinel (pinning effect) barrier, grain boundary requires further movement must be over MgAl2O3phase, which It is obvious that Mg can inhibit the growth of Al2O3 crystals, especially in the later stages of sintering, which is significant .(Mg, 1994).

4.Solid solution activated sintering

By introducing some oxide additives that can form solid solution with Al2O3, solid phase sintering can be promoted. These additives mainly include TiO2, Cr2O3 and so on. TiO2 Ti4+ ion radius of 0.64nm, and Al2O3 in the Al3+ ion radius of 0.57nm is very close. (TiO +) is slightly larger than that ofAl3+, and the charge of Ti4+ is not balanced when it is replaced by Al3+. In order to achieve the neutralization of Ti4+, the cation is formed Absence, thus activating the lattice, is conducive to sintering. In addition, when Al2O3-TiO2 to high temperature, Ti4+ will be reduced to Ti3+, and Ti3 + ion radius of 0.69nm, more than 0.64nmTi4+. Leading to enhanced Al2O3character distortion, to promote sintering and reduce the sintering temperature.

It should be pointed out that TiO2promotes sintering and is related to the grain size of Al2O3 powder. If the original grain of Al2O3 is less than 1μm, the TiO2 into the Al2O3 lattice will form contact by the mechanism of vacancy volume diffusion, , Otherwise it is difficult.

Cr2O3promoted the solid-phase sintering mechanism of Al2O3 is also roughly the same, which is due to Cr3 + ion radius of 0.64nm, and Al3+ ion radius of 0.57nm difference of 14% can form a complete replacement type solid solution, making the Al2O3 lattice distortion, Of sintering. While the addition of Cr2O3 Al2O3 ceramic material, its hardness and strength can be improved.