Research Papers on Microwave Sintering of Silicon Nitride

The needle-like beta; - Si3N4 samples can be formed by sintering at 1725 C in microwave equipment, while the needle-like beta; - Si3N4 samples can not be obtained by conventional sintering at 1800 C. When 2.45 GHz microwave source is used, mixed heating mode is generally used, while direct heating mode is used when 28 GHz or 30 GHz microwave source is used. Silicon nitride ceramics are a kind of important advanced high temperature structural ceramics. They have the characteristics of low thermal expansion coefficient, high hardness, high modulus of elasticity, good thermal temperature performance, chemical stability and insulation. In addition, they have the advantages of oxidation resistance, corrosion resistance and wear resistance, which make them have wide application prospects. Pure silicon nitride does not absorb microwave energy at room temperature, and is almost microwave transparent material. It is difficult to heat by its dielectric loss. How to make the sample temperature rise uniformly to a certain sintering temperature is the key to sintering. The dielectric loss of

SiN ceramics is not only very low, but also has little change with temperature. The thermal conductivity of SiN ceramics is not good. In the rapid heating process, the phenomenon of cracking and arcing or local overheating of samples will inevitably occur. Using high purity & alpha; - Si3N4 powder as raw material, Al2O3 and Y2O3 as sintering aids, adding 12% (Al2O3: Y2O3 = 1:3), mixing, ethanol as medium, milling with Al2O3 ball for 24 hours, drying and dispersing, the powder was pre-pressed with 30 MPa pressure, then pressed with 200 MPa isostatic pressure, and then pressed into 24-times; 12 (mm) samples. Using 2.45GHz and 6kW microwave ovens, the heating rate is 25 C/min. Conventional sintering adopts graphite furnace pressureless sintering, and the sintering system is the same as microwave sintering, and they are all sintered in nitrogen atmosphere.

Because of the small loss in microwave at low temperature ofγ-Si3N4, SiC is needed as a heater to absorb microwave at low temperature. When the sample temperature begins to pass the critical temperature of 1390 C, it will absorb microwave heating itself immediately. The results show that microwave can promote - alpha; - Si3N4 - alpha; - phase beta; the speed of phase transition and the increase of density.

Comparing the scanning electron microscopy (SEM) images of microwave sintered and conventional sintered samples at 1750 ~C, under the same formula and sintering system, the matrix of microwave sintered materials has larger long-shaped particles, the proportion and particle size of long-shaped particles have increased significantly after holding for 2 hours, while the conventional sintered samples have no such effect even after holding for 5 hours. Compared with conventional resistance heating, microwave sintering of Si3N4 can promote densification and growth of phase grains. As a result, the material maintains high flexural strength and achieves high fracture toughness.