Magnesium oxide surface defects

Extensive study has been devoted to paramagnetic defects that are formed on high-surface area alkaline earth oxides, particularly magnesium oxide. The work carried out by Wertz el al. 187, 188) and Henderson and Wertz 139) on bulk defects formed in MgO single crystals has been quite valuable in the identification of the surface defects. Both the bulk and surface defects may be divided into two classes those in which an electron... 

Similar results on the kinetics and temperature dependence of the recombination of electron and hole centres have been obtained [68-71] in studying highly dispersed samples of magnesium oxide MgO. As distinct from CaO, however, in MgO the hole centres are mainly stabilized on the surface (so-called Vs -centres) while the electron centres are stabilized both on the surface (Fs+ -centres) and in the volume (F1 -centres). After irradiation is over a slow recombination of radiation defects is observed... 

Abstract. An embedded-cluster approximation is adopted for simulating the heterolytic dissociation of hydrogen at two intrinsic defects on the (001) surface of magnesium oxide the isolated anion vacancy, and the tub divacancy. The dissociation process is shown to be critically dependent on the structure of the electrostatic field at the surface both as concerns energetics and final configuration. 

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. ... 

Nanocrystalline magnesium oxide (NAP-MgO) exhibits unusual surfaee morphologies and possesses more reactive surfaces than the commercial MgO due to the presence of high concentrations of edge/corner sites and other defects, imparting it with unique surface chemistry. 

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. 

Similar to hydrogen ion implantation, spinel magnesium indium oxide films were irradiated with 1.5 MeV Li+ ions for various fluences and their optical absorption spectra are shown in Figure 9.14a. It is inferred that the absorption spectra have three features the slope from 300 to 350 nm, strong absorption at 365 nm, and broadband around 540 nm. The hump at 2.3 eV (540 nm) is ascribed to surface plasmon resonance of Li nanoparticles and the wide slope is due to the transitions from the lithium sublevels including widely spread defect bands. In comparison with the low fluence irradiated and as-deposited Aims, Mglu204 Alms implanted with LF ions at high fluence have... 

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