Monoclinic ZrO

Figure 8.57 The transformation from tetragonal to monoclinic ZrO involves large strains (a to b), which creates elastic energy that will oppose the transformation. This allows the tetragonal phase to be retained to temperatures below its unconstrained transformation temperature. The transformation can be induced by the application of stress (a to c).

Prolonged rellux of solutions around pH 2.5 leads, after about 20h, to sols of monoclinic ZrO . (distorted fluorite structure) [100,101]. It does not appear that the tetramer [Zr4(OH)i6(OH2)g]° is involved in the reaction. Indeed, the geometry of the coordination polyhedron of zirconium is not the same in the tetramer (antiprismatic, 8-coordination) and in monoclinic zirconium (7-coordination, Figure 3.22). In addition, acidification of the medium by thermohydrolysis (the pH decreases from 2.5 to 0.4 after 40 h reflux) [100] probably destroys the tetramer, and it is likely that lire growth of the solid takes place via a mechanism of acidic oxolation between monomers (see Chapter 4). 

Transformation of tetragonal zirconia phase to monoclinic phase has been studied in [53], Calcination of zirconium hydroxide ZrO(OH)2 at various temperatures produced three types of paramagnetic centers assigned to trapped electrons located in oxygen vacancies of Zr02 (g = 2.0018), to adsorbed 02"... 

A range of Ni/ZrO catalysts with varying Ni loadings was prepared. Monoclinic zirconia used as support, (RC100), was supplied by Daichi Kigenso, Japan. It was calcined at 650°C for 15 hours in a stream of flowing air (30 ml/min). One catalyst was also prepared from support... 

Zr02 ZrO. monoclinic Norton from sulfate O.OOl-l mol dm KCl, NaC104 25... 

ZrO reagent grade, Johnsoii-Matthey monoclinic + tetragonal 0,01 mol dm NaNO ... 

ZrO synthetic, monoclinic, 3 different preparation routes O.OOI-0.1 mol dm " KNOj... 

ZrO(2 zl2)(OH)z tetragonal,crystalline monoclinic - like,disordered) —> ZrO, (monoclinic, crystalline)... 

At higher temperatures (i.e., 1,000°C) and excess partial pressure of oxygen (i.e., 10 6to latm.), monoclinic zirconia contains completely ionized zirconium vacancies [26], At 1,000°C, zirconia is stoichiometric at a pressure of 10 16 atm. At this point, the concentration of oxygen vacancies is equal to twice the concentration of zirconium vacancies. As the partial pressure of oxygen increases, the stoichiometry changes such that for ZrO,+, with the S value defined by ... 

Clearfield [64CLE] reviewed the structural aspects of zirconium chemistry. One of the important findings of this review was the substantial amount of evidence that was presented for the absence of the zirconyl (ZrO ) ionic structure in aqueous solution and in solids. Even in monoclinic zirconium oxide, Zr-O-Zr bonds are present rather than the Zr=0 double bond. Many compounds in both aqueous solution and the solid state contain hydroxo-zirconium bridges (see Section V.2.2). Clearfield presented evidence of the moiety [Zr4(0H)g(H20)i6] occurring in both aqueous solution and the solid state and argued that measurements by Zielen and Connick [56Z1E/CON] to determine the stability of polymeric zirconium hydrolysis species confirmed the existence of this species in aqueous solution. 

Figure 2. Bulk structural properties of glassy PdZrs metal alloy and Pd/Zr02 catalysts. XRD patterns (CuKa) of glassy PdZrs alloy (trace 1) PdZr-i (trace 2) PdZr-a (trace 2) PdZr-i after reduction in hydrogen (trace 4) and PdZr-c after reduction (trace 5). Denotation of reflections Pd, PdO, + ZrOs monoclinic, " Zr02 tetragonal, ° PdZr2 and Zr.

The amorphous ZrO 3, which is the first product obtained on heating the oi rchloride (300°C), converts at 500°Ctothe tetragonal modification, which then contains only traces of Cl. Above 600°C the material is monoclinic. 

Fig. 5.52 Plot of monoclinic fraction in partially stabilized ZrO,/ZnO composite [28]. With kind permission of John Wiley and Sons...

The high temperature electrolytes are mostly oxides of composition MO based upon the fluorite, structure. The best investigated is "calcia stabilized zirconia (CSZ) which consists of a solid solution of 12-15% CaO in ZrO. The addition of calcia transforms ZrO from the monoclinic to tne cubic (fluorite) structure and also introduces anion vacancies for charge compensation. Conduction is by 0 ion diffusion through anion vacancies and ZrO -CaO has a resistivity of 30 ohm-cm at 950 C. Trivalent cations may also be used to stabilise ZrO with resistivities at 950 C of 12 ohm-cm for ZrO -Y 0 and - 6 ohm-cm for ZrO -Yb O or ZrO -Sc O (Figure l). Staoilized zirconia is of interest as an electrolyte for fuel cells, but no battery applications have been proposed and the temperature of conduction is too high to be of real interest. 

Figure 7.13 Structure of monoclinic zirconia. (a) Idealized ZrO polyhedron, showing the two types of oxygen (0 and On) (b) Projection onto the (010) plane, showing the plane of twinning (dotted line).

When a crack starts in the ceramic the metastable ZrO grains absorb the crack energy and the energy is used to transform it to the stable monoclinic form. The propagation of the crack is interrupted and the material has been toughened (Figure 17.18). 

The IR spectra of Zr02 nanoparticles showed characteristic wavenumber shifts with average particle size. The Raman spectrum of monoclinic Zr02, including 160/I80 and Zr/ Zr shifts, showed bands at 334, 224, 192 and 180 cm due to ZrZr modes, 380, 349 and 308 cm due to ZrO modes. Several features were also seen due to 0-0 modes. Raman spectra were reported and assigned for monoclinic, tetragonal and cubic forms of Zr02- ... 

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