Range of metastability of tetragonal zirconia in some rare earth doped zirconia fibers

Abstract

Studies on the thermal treatment of amorphous zirconia (gel, precipitate, etc.) have shown that its conversion to crystalline forms usually starts around 400±500 8C [1±4], and that the precise temperature of conversion may vary with the composition (e.g. the type and amount of cationic dopants, hydroxyl content). It has been reported by various workers [2± 4] that the product of this initial crystallization is a metastable tetragonal (t) phase, not the thermodynamically stable monoclinic (m) phase. Further, it has been shown that this metastable t-phase persists, even at higher temperatures within the stability range of the m-phase before ®nally converting to the latter. Thus, for an undoped zirconia gel, crystallization mainly to the t-phase was observed at 400 8C, and t! m conversion at 1000 8C [4]. Generally speaking, this follows Ostwald's step rule [5]. In addition, the apparent stabilization of t-ZrO2 at low temperatures has been explained to be a function of structural similarities of an amorphous phase with the t-phase [3], crystallite size [6], effect of strain [5] etc., apart from the presence of ionic impurities (as mentioned previously). Naskar and Ganguli [7] recently studied the crystallization behavior of rare earth (RE) doped zirconia gel ®bers that were obtained by spinning doped zirconium acetate sols. They showed (Fig. 1) that the metastable t-phase obtained from ZrO2RE2O3 gel ®bers yields partially or fully to the mphase at higher temperatures as a function of the size of the cationic dopant (2 mol % rare earth oxide in each case). Fig. 1 indicates that although all the dopant cations yielding t-ZrO2 were larger in size than Zr4‡ (La . Pr . Nd . Sm . Gd . Dy . Zr), the larger cations among them yielded the phase with progressively shorter thermal ` stability'' ranges. On