Effect of Impurities and LiF Additive in Hot‐Pressed Transparent Magnesium Aluminate Spinel

Abstract

The effect of impurities and LiF sintering additive on the microstructure and optical properties of hot‐pressed transparent MgAl2O4 spinel was investigated. A lower and a higher purity powder were hot pressed with and without LiF and process parameters varied. Microstructure was examined using optical and electron microscopy, optical properties using spectrophotometry, and chemistry using various spectroscopic techniques. Impurities present in parts‐per million were found to segregate at grain boundaries and form an amorphous phase, restricting grain growth and causing scatter and opacity. It was found that LiF reacts with impurities to form volatile species that can be removed with proper processing, resulting in larger grain size and increased transmittance. LiF also counteracts absorption caused by reduction of spinel, but results in MgO loss, grain boundary embrittlement, and if trapped in higher concentration, restricts grain growth and causes scatter and opacity.

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

“…in diameter made of transparent spinel. 11 These exciting results implicitly suggest that a solution has been found for the still debated questions of how transparency can be improved, 12,13 and cast some doubts on the real utility of pursuing investigations on the subject. On the other hand, the huge investments needed for high-capacity equipment mean that small or medium enterprises and research institutions operating outside the defense sector cannot afford to manufacture large transparent components.…”

“…[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).…”

“…18,20 Even below 10 −4 , powder impurities can segregate at the grain boundaries, causing scatter and opacity. 13 A simplified thermodynamic model describing the main densification phases was developed to elucidate the optimal processing conditions, in terms of LiF content and the most appropriate hot pressing cycle. This model clarifies the interaction between LiF and the other constituents, the impact of the pressure applied and the influence of the graphite tooling and heating elements generally used in hot pressing hardware.…”

Luminescence of photoactivated pristine and Cr-doped MgAl2O4 spinel

“…Experimental errors caused by the non-reproducibility of the original state of the sample surfaces is unlikely to exceed ca. ±10 % [16].…”

Aqueous gelcasting of the transparent MgAl2O4 spinel ceramics

“…Generally, transparent MgAl 2 O 4 spinel ceramics were fabricated using lithium fluoride (LiF) as sintering aids by traditional hot pressing (HP) directly or HP combined with hot isostatic pressing (HIP) technology [5][6][7][8], and both steps are performed at relatively high temperatures. However, the size and shape of sample was limited due to the size and shape of processing equipments.…”