Sintering Kinetics of a MgAl₂O₄Spinel Doped with LiF

Abstract

The sintering kinetics of a pure magnesium aluminate spinel, MgAl2O4, and that doped with LiF were determined through the use of the master sintering curve technique developed by Su and Johnson.20 Powders with 0%, 0.5%, and 1.0% by mass LiF were densified in a vacuum hot press under a range of unaxial pressures. After the sintering mechanisms in each temperature and pressure regime were determined, an optimized vacuum hot‐pressing schedule was formulated for spinel powders doped with 1.0% by mass of LiF. In addition to forming a transient liquid phase, the presence of LiF leads to the formation of oxygen vacancies that promote late‐stage sintering in MgAl2O4.

A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing

“…[20][21][22][23] The role of the fugitive LiF additive is still being debated, however, and has yet to be fully understood. 13,19,24,25 MgAl 2 O 4 spinel formation from the initial oxide powders with LiF evaporation is thermodynamically possible at the temperature considered, but many other species can be formed, in quantities that also depend on pressure and temperature. A thermodynamic study was conducted to study the behavior of the system comprising Al 2 O 3 , MgO and LiF as a function of temperature and pressure, also taking any effects due to carbon contamination into account (carbon activity: a C = 1).…”

“…The previous study could not demonstrate that the ultimate spinel transmittance had been reached. Further improvements might hopefully derive from the use of ultrapure LiF powder and a better understanding of the role of LiF 18,19 and the reactions underway during densification, with a view to fine-tuning the HP cycle. 18,20 Even below 10 −4 , powder impurities can segregate at the grain boundaries, causing scatter and opacity.…”

Effect of 1wt% LiF additive on the densification of nanocrystalline Y2O3 ceramics by spark plasma sintering

“…LiF was used as a sintering aid for fabrication of transparent oxides. [14][15][16][17] Different functionalities were tailored to LiF additives such as grain boundary liquid phase formation for diffusion enhancement, increasing the oxygen vacancies and thus the volume diffusion, low-solubility such that it segregates into isolated pockets during the cooling, and finally reaction with the impurities. 18 These functionalities were also used for rapid densification of transparent spinel 19 and YAG 20 by the SPS process.…”

Point Defects in Magnesium Aluminates Spinel Ceramics Doped with Lithium Fluoride

“…Therefore, the mechanism of formation of this band could be related to solid chemical reaction between spinel MgAl 2 O 4 , and LiF. Analysis of reaction between MgAl 2 O 4 and LiF leads to the conclusion that oxygen vacancies are formed due to the incorporation of both Li and F ions into crystal lattice [8]: + , may serve as electron traps forming electron centers, which could be responsible for the absorption band at 5.65 eV.…”

“…Based on earlier electron microscopy studies, these microstructure variations correlate with variation of point defects and impurities in the lattice [7]. Investigations of sintering kinetics of spinel doped with LiF show that the presence of LiF lowers the effective activation energy for the densification of spinel [8]. Based on TEM observations it was proposed that oxygen vacancies are generated by a mechanism that originates via the presence of LiF.…”