Zirconia monoclinic crystal

N. Bonanos and E. P. Butler [1985] Ionic Conductivity of Monoclinic and Tetragonal Yttiia-Zirconia Single Crystals, /. Mat. Sci. Lett. 4, 561-564. 

On heating to 1500 °C/6 h/Ar, the Zr material crystallizes to a mixture of monoclinic and tetragonal zirconia and crystobalite with loss of considerable original surface area (36 m2 g 1). The Hf material behaves similarly, although it partially crystallizes at 1000 °C to produce cubic or tetragonal hafnia. Cristobalite is only observed in materials heated to 1400 °C. Finally, thin films of the Zr and Hf derivatives could be cast from hydrocarbon solutions on quartz and then converted to thin films of the corresponding amorphous or ceramic materials. 

The crystal structure of zirconia and the catalytic properties of SZ generally depend on the synthesis method and thermal treatment adopted. In particular zirconia crystallises in three different polymorphs characterised by monoclinic, tetragonal and cubic symmetry. Among them only the tetragonal SZ phase displays significant catalytic properties [5-7]. Unfortunately, the synthesis of the pure tetragonal polymorph is difiBcult and, in the absence of promoted oxides [8], it could be stabilised only through an accurate control of the synthesis parameters, with particular attention to the thermal treatments. 

SZl sample, reference materials of this work, is constituted by pure tetragonal ZrOz, with an average crystal size (determined with the classical Scherrer equation) of 85 A (Table 1). The stabilisation of the tetragonal ZrOz depends on the presence of TPAOH in the reaction mixture. In fact, variable amounts of monoclinic phase are detected in the products synthesised in the absence of organic base (SZ2 sample. Table 1). This is probably related to the pH of the sol, but this hypothesis should be confirmed. Basic medium usually enhances the condensation reactions [15], but at the same time the ability of sulphates to form complexes with Zr precursors up to pH = 11 [16] and the low water content in the sol prevent the growth of zirconia particles. 

The peak 5 at around 470°C, observed in DSC curve, corresponds to an exothermic phenomenon without any loss of weight. It can thus be attributed to the crystallization of the amorphous zirconia. This is confirmed by the X-ray diffraction pattern obtained after the thermogravimetric analysis (Fig. 9). According to del Monte et al. [17] the peaks located at 20 = 28 and 31.5° are characteristic of the monoclinic zirconia whereas those situated at 20 = 30, 34.5 and 50° belong to the tetragonal structure.This study showed that nanostructured porous zirconia has a low thermal stability. In the preparation of the stable and efficient catalysts for the complete oxidation of aromatics, this low thermal stability will be taken into account. 

A synthesis protocol of porous zirconia catalyst support, through a neutral Ci3(EO)6-Zr(OC3H7)4 assembly pathway has been developed. Our studies evidenced the role played by the surfactant. It has also been observed that the increase of hydrothermal treatment time and temperature have a benefical effect on tailoring the pore sizes. The synthesized materials will be used in preparation of Au / ZrOz, Au / VO / ZrOz catalysts, which will be tested in the benzene oxidation reaction. Thermogravimetric analysis shows that the recovered zirconia present a relatively low thermal stability. Then the structure collapses due to the crystallization to more stable tetragonal and monoclinic phase. 

Fig. 5 Crystal structure of monoclinic zirconia as seen from the [131] direction...

In addition, II NMR results show that, even after crystallization, it is not correct to describe the sample composition as Zr02. Data show that, after being heated to 300°C, the sample s composition is Zr()l 42(OH), 6. At 500°C, well above the crystallization temperature, it is still ZrO176(OH)048. The OH content found below this temperature is not related to the usual surface hydroxylation upon exposure to the atmosphere, instead the hydroxyls are structural units within the sample. Above 700°C, the hydroxide content is no longer measurable and with subsequent heating the sample changes to the monoclinic structure. Hence, the reaction for the formation of zirconia by precipitation can be described as ... 

Zirconia nanotubes were also obtained using a similar method with a zirconium propoxide precursor [75]. After oxidizing the carbon, zirconia tubes with a diameter of 40 nm, 6 nm wall thickness, and several micrometers long were obtained. The Zr02 was composed of mixed crystal phases (monoclinic and tetragonal). Increased temperature treatment led to collapse of the nanotubes. The addition of yttria in a slightly modified procedure gave a more stable nanotube structure with similar wall thicknesses. The yttria-stabilized zirconia had a cubic structure. 

Zirconium oxide, or zirconia, occurs as the mineral baddeleyite, but zirconium oxide is obtained commercially mainly via its recovery from zircon. Zircon is treated with molten sodium hydroxide to dissolve the silica. Zirconia is used as a ceramic, but it must be doped with about 10 percent CaO or Y2O3 to stabilize it in its face-centered cubic form. Zirconia is monoclinic, meaning that it has one oblique intersection of crystallographic axes, but it undergoes a phase change at about 1,100°C (2,012°F), its crystal structure becoming tetragonal, and above 2,300°C (4,172°F) it becomes cubic. To... 

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