Polycrystalline Oxides

We consider problems related to electrophysical properties of sintered polycrystalline oxides as well as their adsorption changes. We also analyze the difference in adsorption induced changes of electrophysical characteristics of stoichiometric and non-stoichiometric partially reduced oxide adsorbents. 

Adsorption response of electrophysical characteristics of semicondnctor sensors made of barrier-disordered polycrystalline oxides... 

Summing up the above said we examined in our experiments the adsorption-caused change in ohmic electric conductivity and the inclination angle of pre-relaxation VAC of polycrystalline oxide adsorbents. 

Fig. 1.22. The kinetics of change in electric conductivity of polycrystalline oxide during adsorption of donor particles 1 - CO on Sn02, 0.11 vol.-% CO in atmosphere, T = 300 C 2 - H atoms on ZnO, = 510-2 Torr, Tp = 1400 C,...

Thus, the whole complex of existing experimental data indicates that the major part of polycrystalline contacts in vacuum sintered polycrystalline oxides are provided by bridges of open type. Moreover, the vacuum sintering at moderate temperatures 300 - 350°C leads to formation of a unified pattern (see Fig. 2.4, b) which cannot be disjoint into specific microcrystals and connecting bridges [37, 40]. The structure of adsorbents obtained presents a complex intertwining of branches of various thickness. 

UPS studies of supported catalysts are rare. Griinert and coworkers [45] recently explored the feasibility of characterizing polycrystalline oxides by He-II UPS. A nice touch of their work is that they employed the difference in mean free path of photoelectrons in UPS, V 2p XPS and valence band XPS (below 1 nm, around 1.5 nm, and above 2 nm, respectively) to obtain depth profiles of the different states of vanadium ions in reduced V205 particles [45]. However, the vast majority of UPS studies concern single crystals, for probing the band structure and investigating the molecular orbitals of chemisorbed gases. We discuss examples of each of these applications. 

Polycrystalline oxide materials, both undoped and doped, have been extensively examined for use as photoanodes. Ti02 electrodes have been prepared by thermal oxidation of a Ti plate in an electric furnace in air at 300-800°C (15-60 min) and in a flame at 1300°C (20 min) [27-30]. XRD analysis of thermally oxidized samples indicates the formation of metallic sub-oxide interstitial compounds, i.e. TiOo+x (x < 0.33) or Ti20i y (0 < y < 0.33) and Ti30 together with rutile Ti02 [27]. The characteristic reflection of metallic titanium decreases in intensity after prolonged oxidation (60 min) at 800° C indicating the presence of a fairly thick oxide layer (10-15 pm). Oxidation at 900°C leads to poor adhesion of the oxide film... 

Figure 5.50 Low-stress creep rates of several polycrystalline oxides. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

UPS studies of supported catalysts are rare. However, Heber and Grunert [63, 64] recently explored the feasibility of characterizing polycrystalline oxides by Hell UPS. One attractive point of their investigations was that they employed the difference in mean free path of photoelectrons in UPS, V 2p XPS and valence... 

D. J. Pysher and R. E. Tressler, Tensile Creep Rupture Behavior of Alumina-Based Polycrystalline Oxide Fibers, Cer. Eng. Sci. Proc., 13[7-8], 218-226 (1992). 

The presence of an oriented oxide and the type of orientation are determined to a marked extent by the nature of the metal surface. Two factors in particular, the presence or absence of contaminating materials and the topography of the metal surface, have a very strong influence on the oxide formation. The presence of a contaminant in many cases leads to the formation of a randomly oriented polycrystalline oxide rather than an oriented one. A faceted or terraced surface can lead to the formation of orientations which are different from those found on a macroscopically smooth surface. It is obviously important to have well prepared and characterized surfaces if epitaxial studies are to have any real meaning. Despite this, there has been a notable lack of attention to this point and some authors have made no indication at all of the nature of the metal surface. 

EPR (Electron Paramagnetic Resonance) Spectroscopy of Polycrystalline Oxide Systems... 

I 7 EPR (Electron Paramagnetic Resonance) Spectroscopy of Polycrystalline Oxide Systems Table 1.2 Coefficients for the binominal expansion (1 +x)". 

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