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Transparent MgAl2O4/LiF ceramics by hot-pressing: Host-additive interaction mechanisms issue revisited

 
: Goldstein, Adrian; Räthel, Jan; Katz, Michael; Berlin, Mila; Galun, Ehud

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Journal of the European Ceramic Society 36 (2016), No.7, pp.1731-1742
ISSN: 0955-2219
English
Journal Article
Fraunhofer IKTS ()
MgAl2O4; magnesium aluminium spinel; hot pressing; dopants; lithium fluorite; LiF; Pulvermetallurgie; Keramik; Glaskeramik; Feuerfest-Werkstoffe; Heißpressen; Spinell; Sintertechnik; optische Eigenschaft; Graphit; Lösungsmittel; Lichtabsorption; Schmiermittel; Additiv

Abstract
Pure MgAl2O4 powder can be hot-pressed to transparency in the presence of LiF (1-3.5 %; best ~1 %), at relatively low pressure(50 MPa or less), if peak temperatures around 1650 °C are used. At the concentration levels and conditions present during hot-pressing, LiF does not react with spinel; it is inert also toward graphite. LiF additions have positive and negative effects on spinel densification. HP schedules able to ensure a proper balance between these effects allow fabrication of highly transparent parts. The lubricant and solvent abilities of liquid LiF make possible achievement of densification levels >90 %TD under 1200 °C. LiF-containing specimens also exhibit somewhat lower average pores size than undoped ones; this facilitates densification, especially in its final high temperature stage. LiF vapor pressure and its work as sintering aid at low temperatures, keep carbon atom clusters out of the specimen, thus facilitating its full densification and avoiding coloration. Liquid LiF ability to close porosity at temperatures lower than those (>1350 °C) which allow impede? significant carbon penetration into spinel, is essential in achievement of transparency. Excessive LiF gas pressure leads to microcracking, generating defects which act as light scatterers. In the absence of LiF or similar, transparent spinel results only if HPing is done at temperatures low enough to prevent carbon penetration; this is feasible if pressuresin the 150-400 MPa are used.

 

Hot-pressing schedules – able to ensure a proper balance between positive and negative effects of LiF – allow fabrication of highly transparent MgAl2O4 parts under low-pressure (≤50 MPa); peak temperatures in excess of 1550 °C are needed, with best results obtained at ∼1650 °C. At the concentration-levels and conditions present during hot-pressing, LiF does not react with spinel; it is inert also toward graphite. The lubricant and solvent abilities of liquid LiF make possible achievement of densification levels >90%TD under 1200 °C. All LiF leaves the specimens as such at temperatures ≤1550 °C. LiF's ability to close porosity at temperatures lower than those (>1350 °C) - which allow significant carbon penetration into spinel – is essential in achievement of transparency. Light absorption – occurring when carbon is present – is thus prevented. The main light-scattering defect produced by HPing, in the presence of LiF, is micro and macro-cracking; opaque spots, due to un-complete densification, are rare.

: http://publica.fraunhofer.de/documents/N-379593.html