Zirconia cubic-tetragonal crystal

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. 

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... 

Pure zirconia has a distorted fluorite (monoclinic) structure at room temperature, which transforms to a tetragonal structure at above 1200 C and finally to a cubic form at >2300°C. The cubic fluorite form has a crystal structure as shown in fig. la. The exact transformation temperature and behavior are probably very sensitive to any impurity present and also influenced by hysteresis. If the Zr is partially replaced by a divalent or trivalent cation with relatively large ionic radius, the fluorite structure can be stabilized at lower temperatures. This stabilized zirconia is often metastable at room temperature and does not decompose to the thermodynamically stable phases. 

Partially-stabilized zirconia(w th c. 8% MgO) has a cubic structure containing a fine dispersed precipitate of c. 0.1 pm tetragonal (and/or monoclinic) zirconia crystals. 

Figure 1.5 Crystal structure of tetragonal zirconia and relation between tetragonal (t) cell and pseudo-cubic fluorite (F) ceU. The closed and shaded open circles denote the cations and anions, respectively. An arrow in an open circle indicates the displacement of oxygen atoms along the c axis. The thick blue dashed lines indicate a primitive tetragonal cell and thin solid lines stand for two pseudo-fluorite cells. ...

However, after the heat treatment at 700 °C for 2 hours, the crystallization of zirconia was observed for all the solutions tested. There was no influence of the solvent used in the stabilization of the zirconia phase. The overlapping of the tetragonal and cubic zirconia p>eaks impedes the determination of the predominant stabilized phase (Wattnasiriwech et al., 2006). Given the importance of determining the stable phase, other techniques can be used to complement the analysis of YSZ X-ray diffraction. In this work Fourier Transform Infrared Sp>ectroscopy (FT-IR) was used to determinate the zirconia phase stabilized. 

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