Formation of Glass and Amorphous Oxide Fibers from Solution
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FORMATION OF GLASS AND AMORPHOUS OXIDE FIBERS FROM SOLUTION
SUMIO SAKKA Institute for Chemical Research, Uji, Kyoto-Fu 611, Japan
Kyoto University,
ABSTRACT A review has been made of our works on two types of sol-gel methods for obtaining oxide fibers through low temperature process: (1) the alkoxide method and (2) the In the alkoxide method, metal freezing-of-gel method. alkoxides in an alcoholic solution are hydrolyzed and polycondensed into chain-like polymers, drawn into gel fibers near room temperature and heated to several hundred 0C. Various problems encountered in the process are discussed In the freezing-of-gel method, a with SiO 2-based fibers. hydrogel is unidirectionally frozen by lowering the gel Thawing of unidirectionally cylinder into a cold bath. grown ice crystals leaves a bundle of hydroxide fibers. Factors affecting the formation of fibers are discussed with titanium and zirconium hydroxide fibers as examples.
INTRODUCTION Conventionally, glass and ceramic fibers have been prepared by drawing This from the high temperature melts of corresponding compositions. This makes method needs conversion of raw materials to a homogeneous melt. it difficult to prepare fibers of the compositions which are high melting, immiscible in the liquid state or easily crystallizable during cooling. The sol-gel technique of fiber preparation in which fibrous gels drawn from the solution near room temperature are converted to glass or ceramic fibers at several hundred degrees makes it possible to avoid these difficulties. Recently, various kinds of glass and ceramic fibers such as SiOgiAI20 3 TiO2 , ZrO2 and SiC fibers have been prepared by the sol-gel tectnique. 't can be said that the chemistry at low temperatures is important in this technique in contrast with the conventional method involving melting. There are several variations in fiber preparation based on the sol-gel In this paper the preparation of fibers using method, as shown in Table I. TABLE I.
Variations of fiber preparations through sol-gel processes
Method
Fiber formation
Starting materials
Forming temperature
Fibers
1.
Low temp., Drawing during hydrolysis. Low temp., Drawing by adding viscosity increasing reagent. High temp., Drawing from glass rods Unidirectional freezing of gel
Metal alkoxide
Near room temp. Near room temp.
SiO,2Si 2 -ZrO2 , SiO2 -TiO 2
Higher than 12500 C2
SiO
near O0C
SiO2 , TiO2 ,
2.
3.
4.
Metal alkoxide, Inorganic compounds Metal alkoxide, Colloidal silica Inorganic compounds
Mat. Res. Soc. Syrp. Proc. Vol. 32 (1984) CEleevier science Publishing Co., Tnc.
SiO, A12 0 3 TiO2 , ZrU2
ZrO2
92
metal alkoxides as starting materials (called as the alkoxide method in this paper) and the preparation of fibers by unidirectional freezing of gels (called as the freezing-of-gel method) will be reviewed mainly on the basis of experiences in our laboratories of Mie and Kyoto Universities.
PREPARATION OF FIBERS BY THE ALKOXIDE METHOD Hydrolysis and accompanying polycondensation of metal alkoxides in solution produce
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