Poly crystalline Metals

Platinum electrodes are made usually from poly crystalline metal the crystal planes at the surface include both the (111) and (100) faces in approximately equal proportions. The electrochemical properties of Pt(lll) and Pt(100) faces are not identical. (Generally, the physical properties of individual metal crystal faces, such as work function, catalytic activity, etc., are different.)... 

Electronic Interaction on Poly crystalline Metal Surfaces... 

According to the standard electrochemical potentials (Table 1), with Cu/Cu2+ and Ru/Ru3+ couples, the amount of ruthenium deposited on metallic copper will be small, whereas the redox reaction carried out in presence of platinum or gold salts will occur to a large extent. On the other hand, for electrodes of first type (metal immersed in a solution of a salt of that metal), the standard electrochemical potentials as defined by thermodynamics are calculated with regard to a poly-crystalline metallic phase of infinite size. However, in the case of small metallic particles, characterized by metallic atoms of different coordination numbers, the notion of a local potential can be introduced. That no-... 

Other top-down methods are used for the production of ultrafine-grained (UFG) metals and alloys. These include the devitrification of metallic glass and severe plastic deformation, in which a coarse-grained poly crystalline metal or alloy is subjected to large shear strains under pressure, forcing the grains to subdivide into nanosized... 

It is evident that an increase in the repulsion between H atoms causes a flattening of both curves and a decrease in the maximum of the pseudocapacitance. It can be added that the Frumkin isotherm was fotmd to describe HUPD at Pt(lOO) in HCIO4, Pt(llO) in H2SO4, and Pt(lll) in both acids [241]. The value of the parameter g at Pt(lll) was approximately 12. Unfortunately, the isotherms at other surfaces or metals are much more complicated. The HUPD kinetics was studied on different poly crystalline metals. It was found that the kinetics at Pt [242] was about three orders of magnitude faster than that at Ru [243], Pd [244, 245], or Rh [246] electrodes, while that at Ir was intermediate between those groups [247] (on the... 

As a furtlier example for tire meaning of ex situ investigations of emersed electrodes witli surface analytical teclmiques, results obtained for tire double layer on poly crystalline silver in alkaline solutions are presented in figure C2.10.3. This system is of scientific interest, since tliin silver oxide overlayers (tliickness up to about 5 nm) are fonned for sufficiently anodic potentials, which implies tliat tire adsorjDtion of anions, cations and water can be studied on tire clean metal as well as on an oxide covered surface [55, 56]. For tire latter situation, a changed... 

Hing P. and Groves G.W. (1972). The strength and fracture toughness of poly-crystalline magnesium oxide containing metallic particles and fibers. J. Mater. Sci. 7, 427-434. 

As was the case for metals, poly crystalline ceramics show a dependence of mechanical properties on grain size, as given earlier by the Hall-Petch relation in Eq. (5.22), which in this case applies to fracture strength, ay ... 

Since the electron work functions and the specific surface energies of different planes of the same crystal may have different values, it would be interesting to study the electronic interaction during adsorption of foreign molecules on monocrystals. Investigations of monocrystals, however, encounter many difficulties therefore, one has to restrict oneself in general to poly crystalline surfaces, which also give remarkable results because the force of interaction essentially depends on the nature of the metal and differs for the same metal from one species of adsorbed molecules to the other. 

It is well known that contact between the bottom of the tip and the sample surface will not be between two smooth, regular surfaces. In particular, the bottom of the tip may contain many asperities, and one of these asperities will serve as the probe. In STM experiments the most common tip is made from a tungsten poly-crystalline wire, and other tip materials are commonly transition metals (platinum, iridium, alloys) [21]. It is generally agreed today that only a very sharp tip with a single atom at its pinnacle is suitable to obtain atomic resolution on close-packed surfaces. But, such a tip is highly unstable. Therefore, the fabrication and characterization of defined tips, e.g. by field ion microscopy, have not been achieved, nor can it... 

In the above analysis, we used the concept of space charge layer, to be more precise, a depletion layer that would form in a doped, wide-gap semiconductor contacting another phase (a metal, an electrolyte solution, or vacuum). The poly crystalline diamond/metal junctions (where metal is Au, Pt, Pd, etc.) often show rectifying properties [67, 68] and their capacitance characteristics resemble those of a diamond/electrolyte solution junction. 

There are at this time no IR-fibers which fulfill all of these demands. The mainly used fibers are made of poly-crystalline silver halide (refractive index 2.2 at 10.6 pm) with very good transmission over the whole MIR (2 to 16 pm, see Fig.6.5-5), and good mechanical properties. The disadvantages are that they react with many commonly used metals and that they are sensitive to visible radiation, especially to UV radiation (Ceram optec, 1991). 

Single-crystal and poly crystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and shp 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. 

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