Formation of zirconia fibres on unidirectional freezing of a gel

A zirconia hydrogel prepared by dialysing ZrOCI₂ aqueous solution against water buffer was unidirectionally frozen in a cylindrical polyethylene cell which was immersed in a -78° C cold bath at various rates. Freezing rate,R, and temperature gradient,G, at the frozen gel front were continuously measured during freezing. It was found that zirconia fibres longer than 20 mm are formed whenG/R = 2 to 8° C h cm^–2. Either lower or higherG/R values gave only fragmental zirconia. The diameter,d, of the long zirconia fibres was changed from 25 to 55 m with the temperature gradient and the freezing rate according to the equationd ^– A/(GR) +B, whereA andB are constants. It was concluded that long fibres of the desired diameter are obtained by controlling both the freezing rate and the temperature gradient. These results were interpreted in terms of cellular growth of ice crystals.