Solid-state chemistry of zirconium dioxide
A systematic literature search on the system Na2O-ZrO2-SiO2 is presented which reveals inconsistencies on the number and identity of the ternary phases. New X-ray diffraction data are presented for Na2ZrSiO5, Na2ZrSi2O7 and Na2ZrSi4O11; these data are substantiated by good agreement with data calculated from single-crystal work. The unit-cell parameters were redetermined. The compounds Na2ZrSi4O11 and Na4Zr2Si10O31 have now been successfully synthesised by solid state reaction, although the latter could not be prepared phase-pure; these phases had previously only been synthesised hydrothermally. The subsolidus compatibility relations at 1000oC have been reassessed, taking into account the two phases mentioned above, which have not featured in any previously published phase diagram. The addition of dopants, such as calcia, magnesia or yttria, as the oxide, carbonate etc., into solid solution with zirconia are known to stabilise the cubic phase to room temperature. A comparative trial is reported of the physical properties of these powders produced using either organic or inorganic gel-processing routes; the organic route consistently provided powders with greater thermal stability and surface area, which should lead to a more sinterable product, therefore this method was extended to include lanthanide oxide dopants. The incorporation of these dopants has been previously reported, but long sintering times at elevated temperatures were necessary; the organic precursor route described above successfully produced cubic zirconia more rapidly and at much reduced temperatures. The gelation tendencies of the lanthanide dopants are discussed. The binary system ZrO2-SnO2 was investigated at various temperatures; at 1000oC one metastable phase, zirconium stannate, which was observed previously but was poorly characterised, has been studied by X-ray diffraction techniques; new unit-cell parameters and X-ray diffraction data are presented; these data have been substantiated by agreement with calculated data. This phase has now been shown, by various techniques, to exsolve to two distinct phases with average compositions 90ZrO210SnO2 and 82SnO218ZrO2. The monoclinic zirconia solid-solution has been proven by the influence of this inclusion of tin oxide on the physical properties of pure monoclinic zirconia. A discussion of the crystallography of zirconium titanate, which is isostructural with zirconium stannate, is also reported. The extent of solid solubility of tin oxide in zirconium titanate has been determined at 1000, 1350, 1500 and 1550oC; the compositions in this single-phase region are known as zirconium tin titanates and are used as microwave dielectric resonators. A large metastable region of ZrO2-TiO2 solid solution was noted at 1000oC which is similar to data previously published. At 1350oC, the extent of solid solution in the ternary region was also as previously published, but a vast increase in solid-solubility was noted at higher temperatures. A comparison of the dielectric properties of pellets produced by mixed oxide sintering and gel-processing is reported.