Magnesium oxide defect reactions

Surface Superbasic Sites of One-electron Donor Character. - The reaction of alkali metal with anionic vacancies on the oxide surfaces (equation 1) leads to the creation of colour centres of F type. The transfer of one electron from the alkali metal atom to an anionic vacancy is the reason for the formation of these defects. The largest quantities of this type of active centre are obtained by evaporation of the alkali metal onto an oxide surface calcined at about 1023 K, at which temperature the largest quantity of anionic vacancies is formed. Oxide surfaces calcined at such high temperatures contain only a small quantity of OH groups ca. 0.5 OH per 100 for MgO and 0.8 OH per 100 for AI2O3), so their role in the reaction is small and the action of alkali metal leads selectively to the creation of defects of the electron in anionic vacancy type. The evidence for such a reaction mechanism is the occurrence of specific colours in the oxide. Magnesium oxide after deposition by evaporation of sodium, potassium, or a caesium turns blue, alumina after sodium evaporation becomes a navy blue in colour, and silica after sodium evaporation becomes violet-brown in colour. ... 

We open the Sect. 1.5.1 by illustrating the chemistry of magnesium oxide and their defects. This is also the support material of choice of the cluster deposition experiments presented here. These comprise three catal3ffic reactions, the oxidation of CO, the reduction of NO by CO, and the polymerization of acetylene. These studies are reviewed in Sects. 1.5.2-1.5.5. 

However, when low profile additives were applied to these new chemically thickened formulations rather than the BMC and preform unthickened systems, problems were observed. The reaction of the unsaturated polyesters carboxyl groups with the magnesium oxide changed the compatibility between the resin and low profile additive. This caused a separation of the components which resulted in an exudate on the surface of the SMC. Not only did this complicate the compound handling but caused molding defects such as scumming, sticking and porosity. 

At high temperatures the spinel MgAl204 can take in excess alumina to a composition of approximately 70 mol% A1203 (Fig. 4.5). (a) What are the possible formulas that fit the composition of this spinel Write the defect formation equation for the reaction if the excess A1 is (b) distributed over both magnesium and aluminum sites and (c) only over aluminum sites. Assume that there is no electronic compensation in the insulating oxide. 

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