The sol-gel preparation of magnesium oxide and nanocrystalline magnesium and barium fluoride ceramic thin films, and the crystal chemistry and thermal stability of sol-gel prepared fluoride-substituted talc.

摘要

Magnesium oxide thin-films are used as insulating layers in semiconductor devices and high voltage electrical components. Coatings on Si(100), deposited from sols made by treating methanolic magnesium methoxide with ten equivalents or one equivalent of hydrogen peroxide, decomposed on heating to give random or (111) oriented MgO films respectively. The xeogels isolated from these sols were composed of magnesium peroxide hydrate, and a mixture of magnesium peroxide hydrate and magnesium hydroxide methoxide respectively. The interaction of these compounds with carbon dioxide was studied.; Due to their extremely small size (10-100 A), nanocrystals may be used to prepare optically homogeneous materials in systems that would otherwise not form glasses. Two equivalents of HF, added to a sol with a Mg:{dollar}rm Hsb2Osb2{dollar} ratio of 1:1, gave a clear nanocrystalline MgF{dollar}sb2{dollar} sol, which was used to prepare MgF{dollar}sb2{dollar} coatings on Si(100). Barium fluoride thin-films were also prepared using this method. Magnesium fluoride sols, when combined with silicon alkoxide sols, gave xerogels composed of nanocrystalline MgF{dollar}sb2{dollar} dispersed in an amorphous silica matrix. Moderately abrasion resistant thin-films on Si(100) were prepared using these MgF{dollar}sb2{dollar}-silicon alkoxide sols.; The high temperature mechanical properties of fiber reinforced ceramics may be modified when a thin-layer of fluoridated sheet silicate exists at the fiber-ceramic interface. One possible fluoridated sheet silicate for high temperature fiber-ceramic interface modification is fluoride-substituted talc, {dollar}rm Mgsb3Sisb4Osb{lcub}10{rcub}(OHsb{lcub}1-x{rcub}, Fsb{lcub}x{rcub})sb2.{dollar} Talc with fluoride substitution as high as 75 mole percent, the highest ever reported, was prepared from the high pressure treatment of sol-gel prepared precursors. Changes in the crystal structure of talc caused by fluoride substitution were inferred from high resolution IR spectroscopic and x-ray diffraction analyses.; The thermal stability and decomposition products of fluoride-substituted talcs were studied. Synthetic talc without fluoride decomposed at 860{dollar}spcirc{dollar}C, whereas talc with 67.5% fluoride substitution was stable up to 1060{dollar}spcirc{dollar}C. The upper stability temperature of talc showed a non-linear dependence on fluoride substitution. Talc with 100% fluoride substitution is predicted to be stable up to {dollar}sim{dollar}1100{dollar}spcirc{dollar}C.