Study On Preparation Of Superfine Mullite Powders And Their Application In Tialite Composite Ceramics

Mullite?3Al₂O₃·2SiO₂?ceramics exhibit series of outstanding properties,such as excellent high-temperature stability and resistance to thermal shock,as well as low thermal expansion,thermal conductivity and dielectric constant,which makethem widely applied in refractory,high-temperature structural,electronic and optical materials.Therefore,the preparation of superfine mullite powders has become a research hotspot.Many studies on methods for mullite powders preparation have been reported in recent years,most of which focus on the traditional hydrolytic sol-gel route.However,hydrolytic sol-gel route always leads to precursor which shows significant agglomeration and requires high crystallization temperature.Other methods were less studied,which also have flaws such as agglomeration,high equipment requirements or emission of gas pollution.This paper aims to explore new technologies for the preparation of superfine mullite powders,providing a theoretical basis for the synthesis and application of mullite powders.Fine mullite powders with narrow particle size distribution were prepared via reverse microemulsion method.The influence of the raw materials fomula and the process parameters on the stability of reverse microemulsion system and the performance of powders were studied in detail.The research results showed that the particle size of the precursors prepared were less than 100 nm in size,and after being calcined at 1200?for 2 hours,they turned into powders of which the average particle size was 0.29?m,with single mullite phase and loss structure,as the concentration of Al³⁺in Al/Si solution was 0.6 mol/L,the pH value of ammonia was 12,the terminal pH value was 7,the molar ratio of water phase to surfactant was 20:1 and the titration method was the reverse one.In addition,mullite powders were prepared at low temperature by non-hydrolytic sol-gel route employing aluminium powders as one of the raw materials.The mechanism of crystallization at low temperature was studied,and the evolution processes for the composition and the phase of these two preparation methods were compared and analyzed in detail.The research results showed that the precursors prepared contained single-phase gel,of which the initial crystallization temperature was about 600?,and they completely turned to mullite powders with single phase at 990?,the average particle size of calcined powders was1.33?m,as the temperature and the time of gelation were 70?and 24 hours respectively,the ratio of catalyst to Al was 0.100:1,and the mixing order of raw materials was that aluminium powders,ethanol and catalyst were mixed firstly and then tetraethyl orthosilicate was added into the system.The mechanism of crystallization at low temperature was that within the route of non-hydrolytic sol-gel,a single phase gel containing Al-O-Si bonding was synthesized which was very close to the one formed in the crystal mullite.Both of the precursors prepared via non-hydrolytic sol-gel route and reverse microemulsion one contained Al-O-Si bonding,while the bonding of the former one was closer to the bonding in crystal mullite,besides,the initial crystallization temperature of non-hydrolytic sol-gel route was 400?lower than reverse microemulsion one.After being calcined at 1200?for 2 hours,the precursors prepared via non-hydrolytic sol-gel route turned into tetragonal mullite,while the one prepared via reverse microemulsion route turned into orthorhombic mullite.On this basis,tialite-mullite composite ceramics were respectively prepared with the mullite powders synthesized via the two methods mentioned above.The influence of the raw materials on the ceramic performance was analyzed.The research results showed that mullite had a strong effect on limiting the abnormal growth and preferred orientation of the crystal grains.Tialite composite ceramics prepared with the mullite powders synthesized via reverse microemulsion method enjoyed better performance,of which the relative density was 96.04%,the open porosity was 7.35%,the bending strength was 32.57 MPa,the crystal grain size was fine and the cracks were few in number,as the sintering temperature was 1500?and the holding time was 3 hours.