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Effects and elimination of nanoporosity in transparent sintered spinel (MgAl2O4)

: Krell, A.; Waetzig, K.; Klimke, J.


Tustison, R.W. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Window and dome technologies and materials XII : 27 - 28 April 2011, Orlando, Florida, United States; Part of SPIE defense, security + sensing
Bellingham, WA: SPIE, 2011 (Proceedings of SPIE 8016)
ISBN: 978-0-8194-8590-8
Paper 801602
International Conference on Window and Dome Technologies and Materials <12, 2011, OrlandoFla.>
Defense, Security and Sensing Symposium <2011, Orlando/Fla.>
Fraunhofer IKTS ()
ceramic windows; spinel (sintered); MgAl2O4; transmittance; nanopores

MgAl2O4 is a candidate for sintered windows, domes and lenses for UV, visible, and IR applications. However, exact Mie calculation shows that for imaging uses with a window thickness of e.g. 5 mm even IR transmission will not tolerate smallest amounts of 0.01% of 50-100 nm small pores, and the impact of such pores is even worse at shorter wave lengths. Principles of solid state sintering suggest that smallest pores should be eliminated more easily than larger ones. It is, however, observed that a significant population of 50-100 nm small nanopores exists in undoped transparent spinel ceramics after hot-isostatic pressing with the higher concentration the finer the particles of the raw spinel powder are. On the other hand, it is demonstrated that sintering densification is governed not only by the size of the ceramic powder particles and the homogeneity of their mutual coordination but also by the state of the crystal lattice. Taking advantage of this latter effect, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al2O3 mixtures resulting in an in-line transmittance which fits comparable spinel single crystals from 200nm wave length up to the IR range.