| Because of its great mechanical properties and impact resistance under high temperature as well as its good dielectrical property, the mullite ceramics have been widely used in different areas. Traditional ways of sintering mullite ceramics has a low cost and widely application. However, they are not solutions good enough to make full use of the great properties of mullite. In this paper, through chemical synthesis, we can get mullite precursor which has mean crystal grain and high purity. By using those high quality mullite we can explore the best sintering technique to synthesis mullite powder with ultra-fine grain, which is of great importance. In this paper we use Sol-Gel to synthesis precursor mullite powder which is evengranules and highly purified. Through SPS, we use those powder to synthesis block materials which is totally densified and equiaxed in grain shape. Those ceramics have good mechanical properties. At the foundation of the SPS, we explored the influence of two-step sintering on grain growth, micro structure and densification of sintering.Precursor mullite powder was prepared by sol-gel process using tetrathoxysilance(TEOS) and Al(NO3) 3 as raw materials. The mullite precursor was refined by ball-milling process. And the precursor was synthesized by SPS. By our study, we find that when the temperature is 1400℃, the heating rate is 100℃/min, the holding time is 3min, the axial pressure is 80MPa, we can get mullite ceramic with ultra-fine grain and high purity. When exerting the axial pressure at the temperature of 1150℃, the sintering temperature would be decreased and densification enhanced, meanwhile, the grain size of mullite can be decreased. At the temperature of 1380℃, we can get the mullite ceramics with density of 99.37%, grain size of 239nm. The axial pressure played a very important role in the synthesis of the mullite ceramics. We also use CIP to treat precursor. After cold isostatic pressing(200MPa) of 15min, when the sintering temperature by SPS is 1340℃, the relative density of sample is 96.4%, the grain size is 160nm. The precursor has an uniform force in the CIP, so it's better to process the mullite ceramic with higher density and finer grain size. We designed the sharper which can bear high pressure. The result showed that the sintering temperature of mullite by high pressure sharper was much lower than the sample using common graphite sharper. In this paper we design a mold which can add the pressure to be more than 300MPa. The results shows that increasment the pressure can decrease the sintering temperature. In this paper, we also discuss the two-step sintering. There are two different experimental plans. The first method was heating the samples to T1 and then cooling down to T2 and holding in SPS. When the first sintering temperature to be 1250℃, the holding temperature is 1150℃and kept for 40min, the relative density increased and the distribution of grain size became more uniform. The other method was that using SPS to get the top sintering temperature, and to cool down to room temperature, then to held it for a long time in a muffle furnace. When the first sintering temperature to be 1250℃, the holding temperature is 1180℃and kept it for 15 hours, the density of sinteing samples have more significant enhancement, the grain size do not have great increasement. When the first step sintering method adopts SPS sintering, we can have the samples reach a higher density at the low temperature and in a short time. By using of two-step sintering, the first step sintering temperature is lower than the SPS sintering temperature of dense mullite ceramics. During the sintering, the mullite achieve rapid post-sintering to get a certain density after long period of holding at low temperature. Relying on grain boundary diffusion and suppressing crystal boundary migration, we can restrain growing of the grain size while achieve densification.
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