Structural and morphological features of MgO powders. The key role of the preparative starting compound
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Structural and morphological features of MgO powders. The key role of the preparative starting compound S. Ardizzone, C. L. Bianchi, and B. Vercelli Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, 20133 Milan, Italy (Received 22 April 1997; accepted 9 October 1997)
The present paper reports data concerning magnesia samples obtained by calcination of different precursor salts at different increasing temperatures (873–1253 K). The oxides are characterized by x-ray diffraction, scanning electron microscopy, and N2 adsorption at subcritical temperatures. The samples appear to be composed, at any temperature, of pure periclase with a degree of crystallinity which increases with the temperature of calcination. Morphologically, the products have the shape either of lamellas or of cubes of variable dimensions, depending on the nature and route of preparation of the precursor salts. The variation of the specific surface area and the degree of porosity with the nature of the precursors and the temperature is discussed.
I. INTRODUCTION
Magnesia powders have several technological applications. To oxide, either pure or mixed with other phases, is used as a precursor for a large class of ceramic materials with precise electrical, optical, and chemical properties.1 These materials are also employed as catalysts in organic reactions.2 Further, MgO together with ZrO2 and other compounds is used for the preparation of unit cells for solid electrolyte fuel cells.3,4 The oxide powders, both pure and promoted with either alkali metals5–9 or rare earth metals,10–12 (Pb,13,14 Zn,15 Mn,16 metal chlorides,17 etc.) play a key role as catalysts in the oxidative coupling of methane.18,19 While the literature is rich in data concerning these applications of the material, there is no complete and systematic information on the features of MgO, such as bulk and surface properties of the oxide. Often, some isolated data concerning surface area (Brunauer-Emmett-Teller, BET)20 transmission and scanning electron microscopy (TEM, SEM)18–20 and x-ray diffraction (XRD)21 are randomly presented together with the description of the catalytic performance of the oxides. In light of the dispersed nature of the information on these intrinsic physico-chemical aspects of the materials, the present work is aimed at obtaining a general collection of structural and morphological features of the powders as a function of both the nature of the precursor salts and the temperature of calcination. In fact, while MgO for practical applications is mainly prepared by direct calcination of the carbonate, in the literature, the characteristics of the oxide are reported to vary not only with the nature of the starting salt but also, for the same precursor, with its preparative route.22 The results reported in this paper are based on the characterization by XRD, SEM, and N2 adsorption at 2218
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 8, Aug 1998
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