Study Of Fine-grain Mullite Ceramics

Mullite is a more versatile material, it has good chemical stability. Under the environment of high temperature, mullite has good creep resistance and excellent mechanical properties. At the same time, the oxidation resistance and corrosion resistance of mullite is also good. So mullite is ideal for high-grade refractory materials, has been widely used in the traditional high temperature structural ceramics. In addition, the dielectric constant of mullite is low and under the high temperature mullite also has good infrared-transparent coefficient. Therefore, in the field of optical and electronic, mullite has received widespread attention. Ordinary mullite powder as the raw material to sinter mullite ceramics in traditional methods has a low cost and widely application. However, the excellent properties of mullite ceramics can not be fully displayed. We can get high purity and mean crystal grain mullite precursor powder through chemical synthesis. We can also use advanced sintering technique to sinter mullite ceramics with fine grain. In this paper, we use Sol-Gel method to synthesis mullite precursor powder which has high purity, ultra-fine grain and high sintering activity. We can use those powder to synthesis mullite ceramics which has high density and fine grain through spark plasma sintering(SPS) technology. Because of the sintering activity of mullite precursor powder, we can add it to the TiB2 powder to prepare mullite/TiB2 composite. In this way, we can not only improve the oxidation resistance in high temperature of TiB2, but also can not affect the good thermal conductivity and electrical conductivity of TiB2.In this paper, TEOS(tetraethoxysilance) and Al(NO3)3·9H2O(aluminum nitrate nonahydrate) were used to prepare mullite precursor powder by sol-gel method. And the precursor powder was synthesized to mullite ceramics by SPS technology. Research shows that, reducing the heating rate in the middle and later periods of the sintering not only can decrease the defects which was induced by the direct current, but also can reduce the driving force of grain growth and inhibit the grain growth, and by increasing the holding time to increase the density of the sample. In this way, we can prepare the ceramics with high density and fine grain. In this article, through the experiment research to determine, the turning point of heating rate is 1300 ?, after this temperature we will reduce the heating rate of sintering. In the sintering process of SPS, the optimized sintering process is as follows: the heating temperature is 1400?, the heating rate before the turning point of heating rate is 100?/min, and after the turning point of heating rate is 20?/min, the holding time is 20 min, the axial pressure is 80 MPa, and add the pressure when the temperature is 1150?. By this sintering process, we can get the mullite ceramics with high density of 99.55% and grain size of 98 nm.In this article, we also explore the preparing methods of mullite/titanium diboride composite materials. The mullite precursor powder and TiB2 powder were mixed in a volume ratio of 3:7. The powder after mixing was sintering in the tube furnace at 1400 in Ar, and we find that there was no reaction between the two powders. ? After that we sintered the mixed powder, when the sintering temperature is 1600?, the heating rate is 100?/min, the holding time is 3min, the axial pressure is 30 MPa, the density of the sintering sample is 99.03%. This illustrates that the mullite precursor powder can improve the sintering activity of TiB2. The mullite/titanium diboride sample which sintering temperature is 1500?, the density of 94.17% was chose to do the static oxidation experiments. The oxidation temperature is 500?, 800 and ?1100? respectively, the holding time is 4h. By comparing the quality change of the samples before and after oxidation(accurate to 10-4g), and compared with the oxidation experiments of pure TiB2 ceramic. It can be found that after adding mullite, the oxidation resistance of the mullite/titanium diboride composite is improve obviously, especially in the high temperature.