Brief Description of Three Types of Silicon Nitride Ceramic Sintering Auxiliaries

Release time：

2022-11-23 14:24

As a structural ceramic material with the best comprehensive performance, silicon nitride has special application value in extreme environments such as high temperature, high speed, and strong corrosive media. It is considered to be one of the most promising structural ceramic materials for development and application. It has been widely used in mechanical engineering, metallurgy, chemical industry, aerospace, biomedicine, semiconductor and other industries.

Silicon nitride will undergo α → β phase transition during sintering, which belongs to structural reconstruction type, and chemical bonds must be broken and formed. For silicon nitride materials, high-energy covalent bonds are an unfavorable factor in the sintering process, and the existence of Si-N covalent bonds leads to relatively low atomic diffusion coefficients. Therefore, the sintering of silicon nitride usually adopts the method of liquid phase sintering, which is easier to densify and optimize performance in all aspects. In order to improve the performance of silicon nitride, it is very important to add suitable sintering aids to adjust the composition and content of the liquid phase.

Oxide sintering aid

Oxide sintering aids are the most studied type of sintering aids in Si3N4 ceramic sintering. In the earliest research on sintering aids, only a single metal oxide sintering aid was added for sintering, which played a certain role in promoting the sintering of Si3N4, but often the compactness was not enough, or the formed glass phase had a low melting point and poor mechanical properties at high temperature, which could not meet the application. Subsequently, the researchers used rare earth oxides instead of metal oxides as sintering aids, and found the crystal phase formed by rare earth elements and silicon oxide and nitrogen in the grain boundaries, and prepared Si3N4 ceramic materials with better properties.

At present, commonly used metal oxides and rare earth oxides are Al2O3, MgO, ZrO2, SiO2, RE2O3 (RE = La, Nd, Gd, Y, Yb, Sc) and so on.

In addition, the research on sintering aids has developed from a single sintering aid to two or more composite sintering aids. The study found that the use of a variety of composite sintering aids can significantly improve the liquid phase viscosity and improve the high temperature and thermal properties of Si3N4 ceramics. The usual method is to use metal oxides and rare earth oxides multi-component compounding, such as Y2O3/Al2O3, Y2O3/MgO, Y2O3/Al2O3/MgO, Y2O3/Al2O3/La2O3, Y2O3/La2O3, etc.

Non-oxide sintering aid

With the deepening of research on Si3N4 ceramics, it is found that reducing the content of lattice oxygen during sintering can effectively improve the thermal conductivity and other properties of Si3N4 ceramics.

Using non-oxide sintering aids such as boride (LaB6, YB2C2), silicide (ZrSi2), fluoride (YF3, YbF3, MgF2, LiF), nitride (MgSiN2, Y2Si4N6C), hydride (YH2, GdH2, ZrH2) to replace the corresponding oxide sintering aids to improve the thermal conductivity of silicon nitride proved to be an economical and effective way. The use of non-oxide sintering aids effectively reduces the oxygen content in the liquid phase, thereby hindering the formation of lattice oxygen, and after the sintering body cools, the amorphous glass phase content is reduced, and the thermal conductivity is improved.

Oxide-non-oxide composite sintering aid

The results show:

(1) Using the Li + Y sintering aid system, the addition of LiF and Li2O can achieve good low-temperature densification effect, especially the final relative density of silicon nitride ceramics with LiF-Y2O3 as an aid can reach more than 99%;

(2) Compared with the addition of Li2O, the F-containing liquid phase promotes the transition of α → β phase, so that Si and O are continuously removed from the liquid phase to increase the O content in the liquid phase and reduce the O content of the system. The hot-pressed silicon nitride ceramics obtained by the composite additive have higher thermal conductivity. After hot-pressing and sintering at 1750 ° C for 8 hours, the thermal conductivity of silicon nitride ceramics can reach 59W/(m · K).

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