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Prof. Hans-Joachim Kleebe
the Technical University of Darmstadt, Germany
Micro- and Nanostructure of LiF-Doped Polycrystalline Transparent Mg-Al-Spinel; A TEM Study
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.
Prof. Hans-Joachim Kleebe is currently Director of the Institute of Applied Geosciences and holds the Chair in Applied Mineralogy at the Technical University of Darmstadt, Germany.
Prof. Kleebe studied Mineralogy at the University of Cologne, Germany, and received his Ph.D. degree in Crystallography at the University of Cologne in 1988, followed by his Habilitation in Applied Crystallography in 1996.
Previous positions include his postdoctoral stay at the University of California, Santa Barbara, followed by a position as research scientist at the Max-Planck Institute for Metal Research in Stuttgart, Germany. After his activities at the University of Bayreuth from 1993 through 2000, Prof. Kleebe accepted an offer from the Colorado School of Mines in Golden in 2001 as full professor, where he was in charge of the electron microscopy lab. In 2006, Prof. Kleebe took office at the Technical University of Darmstadt, Germany, where he is also responsible for the transmission electron microscopy laboratory at TU Darmstadt.
Prof. Kleebe is author/coauthor of more than 200 technical papers, holds 4 patents and published one book on Introduction to Material Science. 
His primary fields of research included polycrystalline Si3N4 ceramics with particular emphasis on amorphous grain-boundary films. In addition, Prof. Kleebe studied polymer-derived ceramics and is interested in the organic-inorganic transition. More recent activities span from synthesis and TEM characterization of transparent ceramic oxides via studies on the domain structure in ferroelectrics to defects in natural minerals as well as bio-mineralisation and biomaterials. 
Prof. Kleebe and acts as Associate Editor of the American Ceramic Society since January 2003. 
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