Transparent Solid Solution Magnesium Aluminate Spinel Polycrystalline Ceramic with the Alumina‐Rich Composition MgO·1.2 Al₂O₃

Abstract

Solid solution magnesium aluminate spinel with the alumina‐rich composition MgO·1.2 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 84.8 ± 2.7% and >82% throughout the visible spectrum. Starting powders are prepared from mixtures of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry. Water is removed by rotary evaporation. The solids are collected, dried, calcined, mixed with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa Ar. Final grain sizes ranged between 300 and 1000 μm. Samples exhibited flexural strength of 176.8 ± 46.2 MPa; hardness of 12.3 ± 0.2 GPa; and elastic modulus of 292.9 ± 7.5 GPa. Control samples of stoichiometric magnesium aluminate spinel (MgO·Al2O3) were prepared with the same procedure and exhibited comparable values for transmission and physical properties.

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

“…6. In the mixture with no added LiF, the onset of the reaction between Al 2 O 3 and MgO to form spinel was located at 1000-1100 • C, confirming previous reports from Esposito et al and Sutorik et al [17,24]. In the presence of LiF, the spinel peaks are seen at a considerably lower temperature (800 • C), suggesting that LiF plays an active part by markedly reducing the activation energy of the reactions leading to the formation of spinel phase.…”

“…[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 Effect of Grain Size on the Mechanical and Optical Properties of Spark Plasma Sintering‐Processed Magnesium Aluminate Spinel MgAl2O4

“…The conditions for PMAS fabrication by conventional sintering procedures (pressureless sintering, hot pressing [HP], and hot isostatic pressing [HIP]) were investigated and have been reported in several publications . Krell et al .…”

Diffusion limited precipitation of alumina in magnesium aluminate spinel

“…MgO·nAl 2 O 3 powders of compositions n =1.00 and 2.00, henceforth referred to as n =1 and 2, were made by mixing Mg(OH) 2 and γ‐Al 2 O 3 powders in appropriate amounts and calcined at 1273 K to form material containing spinel and Al 2 O 3 phases. Further details are provided elsewhere . In this study, n =2 material was chosen because both Sutorik et al .…”