Ceramic hydration with expansion: The structure and reaction of water layers on magnesium oxide. A cyclic cluster study.

Bettina Heidberg 1,2Thomas Bredow 1Karl Jug 1Klaus Littmann 2

1. Universität Hannover, Fachgebiet Theoretische Chemie, Am Kleinen Felde 30, Hannover 30167, Germany
2. Universität Hannover, Fachgebiet für Beschichtungstechnik und Werkstoffchemie, Herrenhäuser Straße 8, Hannover 30419, Germany

Abstract

The search for novel materials, the reduction of dimensions and the need for optimization are challenges which frequently have to consider the surfaces or interfaces of ceramics. Among these the alkaline earth oxides MgO and CaO are model substrates. They find widespread applications as insulators, cements, catalysts, sensors, supports for metals or semiconductors. An important reaction is their hydration, proceeding with large expansion. Its first step is the adsorption of water.
The adsorption of water on the magnesium oxide surface has been studied using the semiempirical SCF MO method MSINDO. The cyclic cluster model including long range Madelung potentials is implemented. Both molecular adsorption and dissociative adsorption was found to occur. The latter was stabilized by hydrogen bonding from neighboring water molecules and surface oxygen. Whereas a single water molecule does not dissociate on MgO (100) surface, several energy minima exist for the experimentally observed low temperature (3 x 2) monolayer consisting of a mixture of dissociated and molecular water. The energy and structure of the fully hydroxylated (1 x 1) MgO (100) surface experimentally observed at 300 K, has been studied theoretically for the first time. It is shown that this structure is favorable for the naturally occuring reaction of MgO with water, which leads to the thermodynamically stable brucite Mg(OH)2. The reaction proceeds with appreciable volume increase, which, in a rigid matrix can generate large pressure inducing phase transitions and harmful cracking.

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Presentation: poster at E-MRS Fall Meeting 2004, Symposium H, by Bettina Heidberg
See On-line Journal of E-MRS Fall Meeting 2004

Submitted: 2004-05-25 10:49
Revised:   2009-06-08 12:55
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