Zirconium metastable

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). 

Attempts to prepare zirconium trifluoride, by a similar direct reduction of the tetrafluoride with zirconium (37), have been unsuccessful, with no new phase formed. It was suggested that the earlier report was based on material which had either oxide or hydride impurity or which was, possibly, a metastable phase. 

First attempts to check this hypothesis [23] revealed a superior catalytic activity of iron in amorphous iron-zirconium alloys in ammonia synthesis compared to the same iron surface exposed in crystalline conventional catalysts. A detailed analysis of the effect subsequently revealed that the alloy, under catalytic conditions, was not amorphous but crystallized into platelets of metastable epsilon-iron supported on Zr-oxide [24, 25]. 

Besides the dioxide MO2, suboxides of zirconium and hafnium of composition MeO, M3O, and metastable MO are known (see Oxides Solid-state Chemistry). The dioxides are high melting (mp Zr02 = 2850 °C, Hf02 = 2900 °C),... 

The chemical filing technique is very effective in modifying the redox property in the low temperature regions. The reduction temperatures of the chemically filed samples become lower than those of the non-filed ones without decreasing in the amount of the released oxygen. The redox activities of the chemically filed samples arc maintained even after several reduction and reoxidation aging at 1273 K. The reasons for these better redox activities have been attributed to the formation of trace amounts of Ce02 ultrafine particles with the evolution of zirconium and subsequent stabilization of the metastable k and phases. ... 

The tetragonal modification is stable at high temperatures, but may exist as a metastable one in the range 0 — 500 °C when it has been obtained by the decomposition of zirconium salts. At 700 — 800 C, it is converted irreversibly to a monoclinic stable form. The process takes place very slowly even at room temperature. Undercooling of pure Zr02 in the tetragonal form to room temperature has not been successful, but the structure may be stabilized by the introduction of Si02. 

Zirconium carbide is a hard, high melting compound. Carbon forms a solid solution with either a- or (3-Zr whereby a small fraction of interstitial sites are filled with carbon atoms [95FER]. At much higher contents of carbon, in non stoichiometrical ZrC where a considerable amount of the interstitial sites are filled, the compound is considered to be derived from a face centred cubic, metastable form of zirconium [95FER]. 

Al—Zr. This system (Fig. 19) has a peritectic reaction at 660.8 0 at which solubiHty is 0.28% zirconium [7440-67-7], Zr, soHd and 0.11% Zr Hquid. The equiHbrium phase on the aluminum-rich end of the phase diagram is tetragonal Al Zr [12004-83-0], p. Coarse primary particles of p-phase have a tendency to form during soHdification when the zirconium content is much above 0.12%. A metastable form of Al Zr having a cubic LI2 structure, p, is formed when supersaturated zirconium precipitates as a dispersoid Although this phase is nonequiHbrium, it is extremely resistant to transformation to the equiHbrium P-phase. 

A complete range of metastable cerium-zirconium mixed metal oxide powders (CexZr(i.x)Oy, 0 < X < 1) were prepared through a similar hydroxide precipitation technique reported by Hori, et al. [11]. Cerium (IV) ammonium nitrate and zirconium oxynitrate precursors are completely dissolved in de-ionized water with mild heat and precipitated through the addition of excess ammonium hydroxide (-100 vol%). The ceria-zirconia is thoroughly washed with excess distilled water and allowed to evaporate to dryness overnight. The ceria-zirconia system is calcined in atmosphere for 1 hour at 773 K and subsequently milled into a fine powder. The model ceria-zirconia catalysts are prepared from the ground cerium-zirconium oxide powders using a 13 mm diameter pellet die and hydraulic press. 

Oxidized, metastable clusters are obtained for both families via solution processes near room temperature and, in parallel, these contain 14-e niobium and tantalum members, but 12-e (and 13-e) zirconium chloride clusters. Rare-earth-metal clusters don t provide such persuasive evidence small main-group Z examples are rather scarce but similar (12-14-e for R7X-12Z), while iodides centered by large metals cover a sizable range (16-20), as was rationalized earlier (see 5). 

Furthermore, in sohd-phase reactions of diffusion amorphization (e.g., au-rum and lanthanum [8], nickel and zirconium [9]), a metastable amorphous phase, which is absent in an equilibrium state diagram, appears first. In multicomponent systems with two-phase zones, the problem of evolutionary path choice becomes even more complicated [10-16]. Therefore, the formulation of a heuristic principle of choice forecasting the evolutionary path based on a general thermodynamic bacl ound would be very important. 

ZrO has a monocHnic crystal structure (M) at room temperature but is tetragonal (T) at sintering temperature. The transformation temperature is about 1200°C. If zirconium oxide is mixed with stabiHzers such as yttrium oxide Y Oj (about 6%) before sintering, the tetragonal structure remains (metastable) to room temperature. 

Transformation toughening occurs when the zirconium oxide is in the metastable, tetragonal phase. This can be achieved by stabilising the tetragonal phase with another oxide. For example, by adding yttrium oxide (yttria, Y2O3) to zirconium oxide, the transformation temperature can be reduced to... 

One possible toughening mechanism in such systems could operate by phase transformation, which is well known for ceramic materials [200]. An example is represented by zirconium-containing ceramics [201]. The metastable tetragonal phase of zirconium is incorporated into the ceramic, and under the influence of the stress field ahead of a crack tip, this phase transforms to the stable monoclinic phase. Because the monoclinic phase is less dense than the tetragonal phase, compressive stresses are set up on one of the phases, which superposes on the tensile stress field ahead of the crack tip producing shear deformations, with the effect of increasing the critical fracture energy. 

Luk64] Luke, CA., Taggart, R., and Polonis, D.H., The Metastable Constitution of Quenched Titanivun and Zirconium-Base Binary Alloys, TYans. ASM, Vol 57,1964,p. 142-149... 

Ti-15Mo-5Zr is a metastable type alloy that exhibits good cold fonnability and age hardenabil-ity. It is P stabilized by molybdenum to enhance corrosion resistance to reducing atmospheres. Zirconium is added to (1) enhance corrosion resistance above the level achieved by molybdenum, (2) suppress (D transformation to prevent embrittlement, and (3) to improve thermal stability of the P phase. Zirconium additions of 5% Tniniminn are used to enhance thermal stability. 

Oxidized zirconium is a proprietary implant material of Smith Nephew Orthopedics (Memphis, Tennessee, USA) marketed under the trade name OXINIUM [114]. OXINIUM material is fabricated by heating the zirconium alloy device in the presence of air, which converts the surface to a black zirconium oxide ceramic ( 5pm thick) (Figure 6.13). In vivo phase transformation from metastable tetragonal to stable monoclinic is not an issue with oxidized zirconium because its phase content is over 95% stable monoclinic. Clinically introduced for the knee in December 1997 and for the hip in October 2002, oxidized zirconium is intended only for hard-on-soft bearings both in total hip... 

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