Zirconia monoclinic

In this study, the solubilities of crystalline zirconia (monoclinic form, m-Zr02 as well as Y-stabilized cubic form, c-Zr02) and of a fresh hydroxide doped with Zr have been investigated at pH 9. All measurements were carried out Ifom undersaturation starting from pure water, with or without addition of sodium bicarbonate (5 x 10, 0.05 and 0.5 M NaHCOs). Equilibration times ranged from 1 day up to 250 days. Because the only electrolyte added was NaHCOs in different concentrations, these solubility experiments do not comply with the usual requirement for a fixed ionic strength, which varied from almost zero to 0.5 M. 

Zirconia prepared by the thermal decomposition of zirconium salts is often metastable tetragonal, or metastable cubic, and reverts to the stable monoclinic form upon heating to 800°C. These metastable forms apparently occur because of the presence of other ions during the hydrolysis of the zirconium their stabiUty has been ascribed both to crystaUite size and surface energy (152—153) as well as strain energy and the formation of domains (154). 

The hydroxides as precipitated are amorphous, but if they are refluxed ia a neutral or slightly acidic solution they convert to a mixture of cubic and monoclinic hydrous zirconia crystaUites on continued refluxing, only the monoclinic form persists (196). If the refluxing is conducted in an alkaline solution, metastable cubic zirconia is formed (197). 

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

Fig. 7.12. The monoclinic to tetragonal conversion of shock-modified zirconia was studied with DTA by Hammetter and co-workers. The conversion temperature was found to be strongly changed and dependent on shock-modification conditions. The higher-pressure behavior was found to be strongly correlated with reduction in crystallite size [84H01],...

X-ray diffraction and surface area measurements suggest that these W-atom surface densities correspond to saturation coverages, which markedly inhibit zirconia sintering and tetragonal to monoclinic transformations at high temperatures. Zr02 surface areas after 1073 K calcination are 4 m g" and increase to an asymptotic value of 51 m g for W surface densities above 5-6 W-atoms nm (Figure 4). Similarly,... 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

Using the HRTEM image processing analysis, it was possible to deduce that the alloying of monoclinic zirconia and yttria and the formation of a new tetragonal Zr02 solid solution occur simultaneously. 

Zirconia exists in three solid phases-cubic, tetragonal and monoclinic. It undergoes the following transformations... 

As an attempt to solve this problem, zirconia is "stabilized" in the cubic phase by alloying it with an appropriate amount of di-or tri-valent oxide of cubic symmetry such as CaO, MgO or Y203. This results in a lowering of the temperature for the two lowest temperature transitions. These alloys are called partially stabilized zirconia, PSZ and they are a mixture of cubic and monoclinic or tetragonal phases and fully stabilized zirconia (all cubic phase) depending upon the concentration of the "dopant" or added metal oxide. 

The structure and nature of acidity of sulphated zirconia has been recently investigated (ref. 8-10). Untreated zirconium hydroxide which is amorphous, crystallises 1n the monoclinic form at 350°C. On the contrary, the sulphate treated zirconium hydroxide crystallises at higher temperature (500ooC) into tetragonal form, which has significantly greater surface area as compared to the monoclinic zirconia. The infra red spectra of absorbed pyridine indicate presence of only coordinatively bonded pyridine and not the protonated pyridine implying that the zirconia is predominantly of Lewis... 

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