We have evaluated the thermal stability of BaAl2Si2O8:Eu2+ (BAS:Eu2+) phosphor using its polymorph property and hexagonal and monoclinic crystal structure, depending upon firing temperature for plasma display panel application. When BAS:Eu2+ samples having each characteristic crystal structure were baked at 500 °C in air for 30 min, the baked monoclinic BAS:Eu2+ showed the same photoluminescence (PL) intensity as the fresh one, whereas the baked hexagonal one lost its PL intensity significantly, corresponding to about 34%. From analyses of electron spin resonance on Eu2+ and Rietveld refinement, the difference of thermal stability between hexagonal and monoclinic BAS:Eu2+ could be ascribed to both the crystal structure of host materials and the average interatomic distances between the Eu2+ ion and oxygen which plays the key role of shield for Eu2+ ions against an oxidation atmosphere.

Thermal Stability Study of BaAl2Si2O8:Eu2+ Phosphor Using Its Polymorphism for Plasma Display Panel Application

Effect of aluminizing of Cr-containing ferritic alloys on the seal strength of a novel high-temperature solid oxide fuel cell sealing glass

The mechanism of additive-enhanced kinetics of the hexacelsian–celsian transformation in BAS was assessed by doping various mineralizers having different cationic radii and valence. The semi-quantitative data of XRD analysis indicate that an ionic valence of +3 is less efficient and ionic radii ranging from 0.68 to 1.12 A are more effective in promoting celsian formation. Insights about the sites in which the doped cations reside are obtained by examining the possible defect structures developed in the mineralizer-containing hexacelsian. Oxygen- and barium-vacancies defects induced by the addition of mineralizers in BAS could assist the breaking of BaO or (Al, Si)O bonds and promote the polymorphic transformation.

Role of mineralizers on the hexacelsian to celsian transformation in the barium aluminosilicate (BAS) system

In this study room temperature X-ray diffraction patterns and FTIR spectra of zeolite A in its Na form, Ba-exchanged zeolite A, hexacelsian, monoclinic celsian and Ba-exchanged zeolite A samples thermally treated at temperatures ranging from 200 to 1000 °C are reported. The combined inspection of room temperature X-ray diffraction patterns and FTIR spectra reveals the following points. 1) The thermal collapse of the microporous zeolite structure goes to completion upon thermal treatment at 200–300 °C; in the resulting amorphous phase the presence of the secondary building units of the zeolite A structure may be detected. 2) Thermal treatment in the temperature range 200–400 °C results in a middle-range order which favours the crystallisation of small crystallites of monoclinic celsian at 500 °C. 3) The further evolution of the amorphous phase in the 500–800 °C temperature range of thermal treatment instead of promoting the crystalline growth of monoclinic celsian, creates a middle-range order favourable to the crystallisation of hexacelsian which occurs at 1000 °C.

FTIR study of the thermal transformation of barium-exchanged zeolite A to celsian

Kokchetavite, a new polymorph of K-feldspar (KAlSi3O8), has been identified as micrometer-size inclusions in clinopyroxene and garnet in a garnet-pyroxene rock from the Kokchetav ultrahigh-pressure terrane, Kazakhstan. Kokchetavite has a hexagonal structure with a =5.27(1) A, c=7.82(1) A, V=188.09 A3, Z=1, and is found to be associated with phengite + α/β-cristobalite (or quartz) + siliceous glass ± phlogopite/titanite/calcite/zircon, occurring as multi-phase inclusions in clinopyroxene and garnet. It is concluded that kokchetavite could not be an exsolution phase in host minerals. Instead, it might be metastably precipitated from an infiltrated K-rich melt during rock exhumation. Alternatively, although less likely, kokchetavite might be derived from dehydration of K-cymrite, which, in turn, was formed at high pressures. In either case, kokchetavite is a metastable polymorph of K-feldspar.

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H. C. Lin

Papers

Studies in the system BaO-Al2O3-SiO2 I. The polymorphism of celsian