Kinetic Study of Amorphous Yttria‐Alumina Fibers Using Differential Thermal Analysis

Abstract

Differential thermal analysis (DTA) was used to study the crystallization phenomena in Y2O3‐Al2O3 fibers formed via a melt extraction process. Such fibers are X‐ray amorphous and optically transparent regardless of composition in the as‐extracted state but can be crystallized to produce various equilibrium phases that depend on the original composition of the ceramic rods used in the extraction process. The activation energy values were calculated using Kissinger and Augis‐Bennett equations by measuring the variation of the peak temperature in the differential thermal patterns with heating rate. X‐ray diffractometry analysis was used to determine the phase formation in the fibers crystallized after the DTA scan.

Low‐Temperature High‐Pressure Consolidation of Amorphous Al2O3–15 mol% Y2O3

“…The true densities of the decomposed powders prepared by co‐precipitation and spray pyrolysis were found to be similar at 2.87 and 2.89 g/cm 3 , respectively. The powder densities are significantly less than the value of 4.247 g/cm 3 for the equilibrium mixture of alumina and YAG and are also much smaller than the values of 3.8 and 3.4 g/cm 3 reported for amorphous samples of the eutectic composition of 21 mol% yttria in RF‐sputtered films and melt‐extracted fibers, respectively 14,15 . The density of this amorphous A15Y phase is only ∼67% of the density of the equilibrium microstructure.…”

Effects of micro‐sized mullite on cristobalite crystallization and properties of silica‐based ceramic cores

“…As shown in Figure 2B, the height of cristobalite peaks increased sharply as the temperature increased to 1530℃ and Table 3 shows the cristobalite contents in secondary sintered samples are much higher than sintered samples. This can be ascribed to crystal evolution kinetics Equation (1) of fused silica 25 (where k is the crystallization rate, E is the activation energy of crystallization, T is the temperature in Kelvin and R is the universal gas constant), k increases with T , thereby accelerating cristobalite crystallization.As shown in Table 3, the cristobalite content of sintered sample was decreased from 11.47% to 4.20%, and the cristobalite content of secondary sintered sample was decreased from 40.62% to 19.09%, as the micro‐sized mullite content increased from 0 to 2.3 wt%, suggesting that micro‐sized mullite could inhibit cristobalite crystallization at high temperatures. This can be attributed to the formation of compressive stresses on the surface regions of fused silica particles.…”

“…As shown in Figure 2B, the height of cristobalite peaks increased sharply as the temperature increased to 1530℃ and Table 3 shows the cristobalite contents in secondary sintered samples are much higher than sintered samples. This can be ascribed to crystal evolution kinetics Equation (1) of fused silica 25 (where k is the crystallization rate, E is the activation energy of crystallization, T is the temperature in Kelvin and R is the universal gas constant), k increases with T, thereby accelerating cristobalite crystallization.…”

Kinetic Studies on Crystallization and Grain Growth of Amorphous Al₂O₃-ZrO₂ Powder

“…At 1100°C, the appearance of characteristic peaks for α-Al 2 O 3 (PDF#74-1081) indicated that crystallization of α-Al 2 O 3 occurred at 1100°C; all XRD peaks narrowed at higher temperatures in Fig. 1 According to Kissinger equation [8], activation energy for crystallization, which is interpreted as the energy barrier opposing the transformation from amorphous phase to crystalline, can be expressed by…”