Introduction and application of silicon carbide ceramics

1. Introduction to silicon carbide ceramics

 

Silicon carbide has two main crystal structures, namely cubic β-SiC and hexagonal α-SiC. α-SiC is stable at high temperature, and β-SiC is stable at low temperature.

 

green silicon carbide/black silicon carbide

 

2. Manufacturing process of silicon carbide ceramics

(1) Preparation of silicon carbide raw materials

There are many preparation methods for silicon carbide raw materials, which are briefly described as follows:

①Carbothermal reduction method

In this way, metal oxides or nonmetal oxides react with carbon to form carbides.

②Vapor deposition

In this method, metal halides, hydrocarbons and hydrogen react with each other to produce SiC when decomposition occurs. This method can be used to prepare highly purified SiC powders or to produce membranes.

③Self-propagating high-temperature synthesis method (SHS method)

It is a method of preparing refractory compounds developed in recent years. The SHS method is a kind of transformation method, but the generalization method relies on the external heat source to maintain the reaction, while the SHS method relies on the heat emitted during the reaction to maintain the reaction. SHS method has the advantages of energy saving, simple process and high purity.

(2) Molding

Conventional forming method can be used to mold silicon carbide ceramics, but mud pouring method and injection molding method can be used for the complex shape of the billet.

(3) Sintering

Silicon carbide ceramics are difficult to be sintered. The following methods are usually adopted:

①Atmospheric sintering method

The actual situation shows that, if using high purity ultrafine powder, selecting reasonable process, the composition phase, appropriate additives at atmospheric sintering, then high density  silicon carbide products are obtained. For example, submicron β-sic powders are used, which contain oxygen content < 0.2 %, and add 0.5 % boron and 1.0% carbon, in 1950~2100℃ temperature in inert atmosphere or vacuum are used.

The carborundum products which are almost completely densified can also be obtained by compression sintering.

②Reaction sintering method

This method is also known as self - binding or silicon - cementing. It is made of -SIC and graphite powder mixed in a certain proportion and pressed into a billet, which is heated at about 1650℃, Si was infiltrated into the human body through liquid or gas phase to react with graphite to produce -SIC, and at the same time, the original -SIC particles were combined to achieve densification. This sintering does not have any size change, but the sintered body contains 8%~10% free silicon, therefore, the use temperature is limited.Reaction sintering is usually accomplished by heating graphite crucible by induction under vacuum. The equipment USES reaction sintering furnace.

③Hot pressing sintering method

Adding SiC powder to additive, put it into graphite mold, and sintering under pressure of 1950℃ and above 200MPa can be done. Then SiC products with theoretical density are obtained. Commonly used additives are Al2O3, AIN, BN, B4C, B, B C, among which Boron is the most effective additive. The practice shows that the fineness of raw material, phase, carbon content, pressure, temperature, type and content of additives have great influence on sintering.

④mmersion method

In this method, the raw material of SiC fiber is polycarbosilane, which is used as the binder, added to SiC powder and sintered into porous silicon carbide products, and then impregnated in polycarbosilane, and then sintered at 1000℃, so as to increase the density. Repeated for many times, the volume density may reach 80%~95% of the theoretical density. The main characteristic of this method is that it can obtain high purity sum at low temperature. High strength silicon carbide material, can be manufactured in a variety of complex shapes.

⑤Recrystallization method

This method is also called post-sintering, which is a newly developed sintering method. This is the reaction sintered and atmospheric sintering of SiC at high temperature
Then recrystallization sintering (>2000℃) was carried out to obtain the dense sintered surface or object.

 

3. Properties and applications of silicon carbide ceramics.

 

(1) Performance

Silicon carbide ceramics have the best oxidation resistance among carbides. However, the oxidation rate of SiC in air is relatively high between 1000~1140℃. It can be broken down by molten base.Silicon carbide ceramics have good chemical stability, high mechanical strength and thermal shock resistance.

The volume resistivity of silicon carbide varies little in the range of 1000~1500℃. This characteristic can be used as the material of resistance heating element. Silicon carbide heating resistance itself can also be called thermistor semiconductor resistance. The resistivity of different types of silicon carbide thermistors varies with temperature.

(2) Application

Silicon carbide ceramics are widely used in various industrial fields. Their uses are as follows:

 

Silicon carbide ceramic parts