A Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl₂O₄)

Abstract

A review of the densification mechanisms and the microstructural development for transparent spinel made by free sintering and by hot pressing is given. The paper is divided into two main parts. The first part considers spinel without any sintering additives because there still is some controversy concerning the role of cation stoichiometry on sintering and grain growth. The second part discusses the role of the classic sintering aid, LiF, in processing transparent spinel. LiF is shown to have multiple behaviors: (1) it initially wets spinel and forms a liquid phase at relatively low temperatures, which affects early‐stage densification and also grain growth; (2) upon cooling from intermediate temperatures, or even from higher temperatures if microstructure evolution (e.g., formation of closed porosity) prevents volatization, the LiF‐containing liquid dewets and resides in isolated pockets; (3) LiF alters the cation stoichiometry, thereby enhancing diffusion via an increase in the concentration of oxygen vacancies; this affects both the densification rate and grain growth; and (4) it reacts with impurities in the system, thereby acting as a cleanser. For the production of transparent spinel, it is critical that LiF or associated reaction products not be retained as a secondary phase.

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

“…Some chemical reaction of LiF with carbon and gas formation may also be present. 18 Important densification behavior was noted in pure nc-Y 2 O 3 heated to 1500 • C (Fig. 3c).…”

“…[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. Nevertheless, excess LiF additive was found to degrade the chemical stability of the spinel.…”

Spectroscopic study of the discoloration of transparent MgAl2O4 spinel fabricated by spark-plasma-sintering (SPS) processing

“…The LiF doped into spinel acts also as carbon cleanser by reacting with C (LiF + C ! Li + (CF) gas ") and can remove the CF phases as gas [18,59,70]. Second, Bonnefont et al [21] used higher purity starting spinel powder (S30CR, Baikowski Chimie, France) and utilized a very slow heating rate of a = 1°C min À1 .…”

Microstructural development of interface layers between co-sintered alumina and spinel compacts

“…In 31 lattice O 2− ion diffusion could be the limiting diffusion mechanism of growth of the spinel columnar grains because of its low diffusion coefficient, limiting the diffusion process. As Mg 2+ diffuse much faster than O 2− ion, the solid state chemical reaction (1) at the end of spinel columnar grains on the alumina side appears to be limited by the O 2− ion bulk diffusion.…”

“…This apparent activation energy and corresponding diffusion coefficients correspond well with the parameters of oxygen diffusion in spinel lattice (volume diffusion) reported in the literature. 31 On the other hand, there are many mechanisms which may interact and it is difficult to separate one controlling mechanism. Quantification of apparent activation energy in this study has no direct connection with one mechanism.…”