Polycrystalline transition metals

Single-crystal and polycrystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and slip systems. The fee carbides show slip upon mechanical load within the (111) plane in the 110 direction. The ductile-to-brittle transformation temperature of TiC is about 800 °C and is dependent on the grain size. The yield stress of TiC obeys a Hall- Petch type relation, that is, the yield stress is inversely proportional to the square root of the grain size. TiC and ZrC show plastic deformation at surprisingly low temperatures around 1000 °C. 

Figure 3.21. (a) Heats of adsorption of CO on polycrystalline transition metal surfaces, (b) Heats of adsorption of CO on various single crystal surfaces of transition metals. 

Stradella, L., Heats of adsorption of different gases on polycrystalline transition metals. Adsorpt. Sci. Technol., 9(3), 190-198(1993). 

All transition metals from column 3 to 10, plus Cu, are exothermic H adsorbers (AHnads < 0 0H decreases when the temperature increases). Experimental heats of H adsorption (per mole of H2g) obtained on polycrystalline transition metal surfaces are shown in Figure 2.2a [69]. The M-H dj, bond energy may be obtained from the relation ... 

We ivill discuss the reaction of hydrogen and oxygen on transition metals first. This reaction has been extensively studied in our laboratory 18-32) using evaporated metal films as a catalyst. From our previous considerations it follows that as a consequence of the choice of this particular system we must restrict ourselves to certain problems only. We cannot identify the surface species (we can indirectly indicate only some of them) nor understand completely their role in the reaction. Because of the polycrystalline character of the film, all the experimental results are averaged over all the surface. Several new problems thus arise, such as grain boundaries, and, consequently, the exact physical interpretation of these results is almost impossible it is more or less a speculative one. However, we can still get some valuable information concerning the chemical nature of the active chemisorption complex. The experimental method and the considerations will be shown in full detail for nickel only. For other metals studied in our laboratory, only the general conclusions will be presented here. 

Furthermore, the method of orientation selection can only be applied to systems with an electron spin-spin cross relaxation time Tx much larger than the electron spin-lattice relaxation time Tle77. In this case, energy exchange between the spin packets of the polycrystalline EPR spectrum by spin-spin interaction cannot take place. If on the other hand Tx < Tle, the spin packets are coupled by cross relaxation, and a powder-like ENDOR signal will be observed77. Since T 1 is normally the dominant relaxation rate in transition metal complexes, the orientation selection technique could widely be applied in polycrystalline and frozen solution samples of such systems (Sect. 6). 

In this monograph it has been demonstrated that ENDOR is a very powerful tool to study transition metal ions in organic, inorganic and bioinorganic single crystals, polycrystalline samples and frozen solutions. Due to the high resolution of this double resonance technique, many problems in magnetic resonance, which cannot be solved with ordinary EPR, become accessible. 

There has been considerable work in the literature on the structure of very small particles and clusters. Interest in this field has been primarily due to Ino s (1966) early experimental studies of normally fee metals prepared by vapour condensation which showed that a sizable portion of the particles exhibited non-crystallographic structures. These non-crystallographic atomic clusters or polycrystalline nuclei have been observed to consist of pentagonal bi-pyramid or icosahedra form of twinned structures and are known as multiply twinned particles (MTPs). EM studies of supported transition metal catalyst systems have indicated that MTPs sinter faster in catalytic reactions leading to the loss of surface area and are not beneficial to catalysis (Gai 1992). We describe the structure and the role of MTPs in catalysis in the following sections. 

Studies of semiconductor films have shown many facets. The properties of epitaxial films have mainly been investigated on Ge and Si, and to a lesser degree on III—V compounds. Much work, lias been done on polycrystalline II-VI films, particularly with regard to the stoichiometry of the deposits, doping and post-deposition treatments, conductivity and carrier mobility, photo-conductance, fluorescence,electroluminescence, and metal-semiconductor junction properties. Among other semiconductors, selenium, tellunum. and a few transition metal oxides have found some interest. 

All the metallic transition-metal hydrides can be prepared by direct reaction at elevated temperatures. The method has been used commonly to prepare polycrystalline material according to the exothermic reaction described by the above equation. 

The general level of catalytic activity of some metals has in the last few years been explained in terms of the electronic interaction between the metal surface and the reacting molecules. This subject has been well discussed elsewhere (16). Studies of electronic interactions, however, have been concerned with explaining the general level of catalytic activity of polycrystalline materials and not the nature of the active regions in any one catalytic material. At this point it should be emphasized that the differences in activity between different faces on one catalyst with a particular d character can be much greater than the differences in measured activity between two catalysts with appreciably different d characters. It is of interest that the greatest differences in rate with face have been found with the transition metals, the activity of which in the polycrystalline form has been interpreted in terms of the d character. 

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