MgAl2O4 is considered a promising material for optical applications and hence object to research for more than 40 years worldwide [1]. The densification mechanism of MgAl2O4 spinel doped with lithium fluoride as a polycrystalline transparent ceramic has been intensively studied.
LiF greatly reduces the sintering temperature and facilitates enhanced densification at low temperatures. However, the basic mechanisms behind the sintering process are still not fully understood, as neither LiF nor an additional secondary phase is detectable in the final product.
Based on individual studies Reimanis, Kleebe and Rozenburg [2-5] postulated three major processes during sintering of spinel with LiF including (i) Enhanced volume diffusion by incorporation of O-vacancies (ii) Dissolution – Reprecipitation and (iii) Wetting – Dewetting: At this early stage of sintering, the densification mechanism can be described by a classical liquid phase sintering process facilitating particle rearrangement. At temperatures above 1000°C, no secondary phase is detectable along grain boundaries. In a recent work it was shown that these mechanisms occur simultaneously interacting with each other [5], however, the verification was made by indirect methods as for example the double fringe technique for the wetting-dewetting mechanism.
The overall aims of the present study are (i) to verify the postulated mechanisms and (ii) to transfer this knowledge to a pressure less sinter process. Based on dedicated model experiments and a characterisation using e.g. state-of-the-art electron microscopy it was shown for the first time that (a) a dissolution-reprecipation process occurs at significantly lower temperatures by the formation of a variety of transient phases, (b) a vapour transport mechanism leads to a notable mass transport involving the magnesium and (c) an exaggerated grain growth of a second generation of spinel hinders the densification process.
1. M. Rubat du Merac, H.-J. Kleebe, M.M. Müller and I.E. Reimanis, “Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel” J. Am. Ceram. Soc., 96 [11] (2013) 3341-3365.
2. I.E. Reimanis and H.-J. Kleebe, “A Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl2O4)”, J. Am. Ceram. Soc. 98 [12] (2007) pp. 1273-78.
3. K. Rozenburg, I.E. Reimanis, H.-J. Kleebe, and R. L. Cook, “Chemical interaction between LiF and MgAl2O4 spinel during sintering” J. Am. Ceram. Soc. 90 [7] (2007) pp. 2038-2042.
4. M. Rubat du Merac, I.E. Reimanis, C. Smith, H.-J. Kleebe and M.M. Mueller “Effect of Impurities and LiF Additive in Hot-Pressed Transparent Magnesium Aluminate Spinel”, Int. J. App. Ceram. Tech. 10 [1] (2013) pp. E33-E48.
5. M.M. Müller and H.-J. Kleebe “Sintering Mechanisms of LiF-Doped Mg-Al-Spinel” J. Am. Ceram. Soc., 95 [10] (2013) pp. 3022-3024.