Synthesis of mullite fibre from an aluminosiloxane precursor

Toshinobu Yogo, Ilhan A. Aksay
1994 Journal of Materials Chemistry  
Mullite fibre was successfully synthesized by pyrolysis of aluminosiloxane formed from ethyl 3-oxobutanoatodiisopropoxyaluminium and di-(sec-butoxy)aluminoxytriethoxysilane. Aluminosiloxane increased in viscosity with increasing coordination number of aluminium, which was analysed by 27AI NMR spectroscopy. The viscosity of aluminosiloxane was controlled by the amount of added glacial acetic acid as well as the working temperature for the spinning of precursor fibre. Aluminosiloxane gave
more » ... s Si02-A120, at 500 "C, which began to crystallize to mullite at 930 "C. Single-phase mullite was produced on heating to 1000 "C for 1 h. The polymer fibre spun from aluminosiloxane was pyrolysed yielding crack-free mullite fibre at 1000 "C for 1 h. Mullite ( 3Al20,*2Si0,) has various attractive properties such as high-temperature strength, creep resistance and a low thermal expansion coefficient. Fibre-reinforced materials have been receiving great attention because of their applications in the field of composites Alumina, alumina-silica, and alumina-zirconia fibres are mainly used as temperatureresistant fibres., Refractory oxides have extremely high melting points and low viscosity of melts making melt spinning impractical, which leads to the development of chemical techniques including the sol-gel process for the synthesis of ceramic fibres. The synthesis of silica-stabilized alumina fibres by the sol process using aluminium oxychloride was demonstrated by Morton et aL4 Aluminosilicate and aluminium borosilicate fibres were made via similar processes employing aqueous aluminium acetate and colloidal ~i l i c a .~ Horikiri et al. reported the fabrication of alumina and silica-alumina fibres from the partially hydrolysed diethylaluminium isopropoxide and polysilicic acid esters.6 Usually, the spinning conditions are critical in the sol-gel process, since the viscosity of the sol changes with time, and increases very rapidly once gelation starts. The control of viscosity during rapid gelation, therefore, is a key processing factor, since the viscosity of the sol is not sufficiently high for spinning. Soluble organic polymers, such as polyvinyl alcohol and polyethylene oxide, were added not only to raise the viscosity of the sols but also to improve their spinning characteristic^.^ On the other hand, the time-independent viscosity of the starting polymer in the polymer route is one of the most distinct differences from the sol-gel process. Various spinning methods, such as melt-spinning and dryspinning result from the suitable solubility and viscoelastic properties of polymers. Andrianov synthesized various metalloxane polymers including aluminosiloxanes, which comprise A1 -0 -Si backbone^.^ Organoaluminosiloxane polymer obtained from diacetoxydimethylsilane and ethyl acetoacetatealuminium diisopropoxide was used for the fabrication of aluminosilicate films on metal substrate^.^'^ Oxyalkoxide polymer synthesized from aluminium butoxide and tetrachlorosilane was reported to form mullite when the initial A1 : Si ratio was appropriate." The formation process of mullite in the solution-precipitation processes in aqueous systems depends upon the mixing level of aluminium and silicon in the precipitated powders." The difference of the hydrolysis rates between aluminium alkoxide and silicon alkoxide results in the segregation of aluminium-and silicon-containing species in the precipitated powders.ll The degree of formation of silica and Al-Si spinel during mullitization is explained by the segregation level of starting materials.12 The slow hydrolysis/condensa tion of alkoxides under controlled conditions12 l4 and the spray pyrolysis of aluminium nitrate-silicon ethoxide ~y s t e m '~ are reported to be effective for chemical homogeneity in the powders. When the formation of mullite is accompanied by Al-Si spinel and silicious phase, the mechanical properties of the mullite degrade at high temperatures due to the formation of a grain-boundary liquid phase.15 The synthesis of' singlephase mullite is required to achieve the optimal hightemperature properties. This paper describes the synthesis of mullite fibre from an aluminosiloxane precursor synthesized by the reaction of ethyl 3-oxobutanoatodiisopropoxyaluminium and di-(see-butoxy) aluminoxytriethoxysilane in the presence of glacial ace tic acid. The synthesis conditions using glacial acetic acid were found to be useful for avoiding the segregation of aluminium-and silicon-containing species as well as avoiding gelation of the precursor. Aluminosiloxane with controlled viscosity was spun into precursor fibres, which were successfully crystallized in crack-free, single-phase mullite fibres.
doi:10.1039/jm9940400353 fatcat:3lfu6g6zmvf5pmef3pvmbkismi