Crystals metallic

Feibelman P J 1987 Force and total-energy calculations for a spatially compact adsorbate on an extended, metallic crystal surface Phys. Rev. B 35 2626... 

The atom radius of an element is the shortest distance between like atoms. It is the distance of the centers of the atoms from one another in metallic crystals and for these materials the atom radius is often called the metal radius. Except for the lanthanides (CN = 6), CN = 12 for the elements. The atom radii listed in Table 4.6 are taken mostly from A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Addison-Wesley, Reading, Mass., 1970. 

Metal Crystal 22° C stmeture 1000° c Melting point, °C Density, g/cm Thermal expansion coefficient at RT, ioV°c Thermal conductivity at RT, W/(m-K)" Young s modulus, GPa "... 

An electrolytic process for purifying cmde vanadium has been developed at the U.S. Bureau of Mines (16). It involves the cathodic deposition of vanadium from an electrolyte consisting of a solution of VCI2 in a fused KCl—LiCl eutectic. The vanadium content of the mixture is 2—5 wt % and the operating temperature of the cell is 650—675°C. Metal crystals or flakes of up to 99.995% purity have been obtained by this method. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

The small precious metal crystals can exist as metal crystallites or as metal oxides, both of which are catalytic (31). Rodium oxide has a tendency to react with alumina to form a soHd solution (35). To minimize this reaction, zirconia is used with the alumina (36). PubHcations regarding the TWC function of precious metals abound (37—42). 

The things that we have been talking about so far - metal crystals, amorphous metals, solid solutions, and solid compounds - are all phases. A phase is a region of material that has uniform physical and chemical properties. Water is a phase - any one drop of water is the same as the next. Ice is another phase - one splinter of ice is the same as any other. But the mixture of ice and water in your glass at dinner is not a single phase because its properties vary as you move from water to ice. Ice + water is a two-phase mixture. 

We saw in Chapter 6 that diffusive transformations (like the growth of metal crystals from the liquid during solidification, or the growth of one solid phase at the expense of another during a polymorphic change) involve a mechanism in which atoms are attached to the surfaces of the growing crystals. This means that diffusive transformations can only take place if crystals of the new phase are already present. But how do these crystals - or nuclei - form in the first place ... 

In this chapter we review, briefly, the essential features of polymer structures. They are more complicated than those of metal crystals, and there is no formal framework (like that of crystallography) in which to describe them exactly. But a looser, less precise description is possible, and is of enormous value in understanding the properties that polymers exhibit. 

For the alkali metal doped Cgo compounds, charge transfer of one electron per M atom to the Cgo molecule occurs, resulting in M+ ions at the tetrahedral and/or octahedral symmetry interstices of the cubic Cgo host structure. For the composition MaCgg, the resulting metallic crystal has basically the fee structure (see Fig. 2). Within this structure the alkali metal ions can sit on either tetragonal symmetry (1/4,1/4,1/4) sites, which are twice as numerous as the octahedral (l/2,0,0) sites (referenced to a simple cubic coordinate system). The electron-poor alkali metal ions tend to lie adjacent to a C=C double... 

Elam, C.F. (1935) Distortion of Metal Crystals (Clarendon Press, Oxford). 

After 1934, research on dislocations moved very slowly, and little had been done by the time the War came. After the War, again, research at first moved slowly. In my view, it was not coincidence that theoretical work on dislocations accelerated at about the same time that the first experimental demonstrations of the actual existence of dislocations were published and turned invention into discovery . In accord with my remarks in Section 3.1.3, it was a case of seeing is believing all the numerous experimental demonstrations involved the use of a microscope. The first demonstration was my own observation, first published in 1947, of the process of polygonization, stimulated and christened by Orowan (my thesis adviser). When a metal crystal is plastically bent, it is geometrically necessary that it contains an excess of positive over negative dislocations when the crystal is then heated, most of the dislocations of... 

Probably the first to take up this technique for purposes of scientific research was Michael Polanyi (1891-1976) who in 1922-1923, with the metallurgist Erich Schmid (1896-1983) and the polymer scientist-to-be Hermann Mark (1895-1992), studied the plastic deformation of metal crystals, at the Institute of Fibre Chemistry in Berlin-Dahlem in those days, good scientists often earned striking freedom to follow their instincts where they led, irrespective of their nominal specialisms or the stated objective of their place of work. In a splendid autobiographical account of those... 

In addition to the above oxides M2O, M2O2, M4O6, MO2 and MO3 in which the alkali metal has the constant oxidation state 4-1, rubidium and caesium also form suboxides in which the formal oxidation state of the metal is considerably lower. Some of these intriguing compounds have been known since the turn of the century but only recently have their structures been elucidated by single crystal X-ray analysis. Partial oxidation of Rb at low temperatures gives RbeO which decomposes above —7.3°C to give copper-coloured metallic crystals of Rb902 ... 

An intriguing 3-coordinate organoantimony compound, which is the first example of trigonal-planar Sb, has been characterized. " The stibinidene complex [PhSb Mn(CO)2(rj -CsH5))2] has been isolated as shiny golden metallic crystals (mp 128°) from the crown-ether catalysed reaction ... 

Polonium is unique in being the only element known to crystallize in the simple cubic form (6 nearest neighbours at 335 pm). This a-form distorts at about 36° to a simple rhombohedral modification in which each Po also has 6 nearest neighbours at 335 pm. The precise temperature of the phase change is difficult to determine because of the self-heating of crystalline Po (p. 751) and it appears that both modifications can coexist from about 18° to 54°. Both are silvery-white metallic crystals with substantially higher electrical conductivity than Te. 

Pseudomorphism received methodical study from about 1905. A micro-section taken across the interface between a substrate and an electrodeposit shows the grain boundaries of the former continue across the interface into the deposit (Fig. 12.5). As grain boundaries are internal faces of metal crystals, when they continue into the deposit the latter is displaying the form of the substrate. Hothersall s 1935 paper contains numerous excellent illustrations with substrates and deposits chosen from six different metals, crystallising in different lattice systems and with different equilibrium spacing. Grain boundary continuation and hence pseudomorphism is evident despite the differences. 

A crystal may be defined as an orderly three-dimensional array of atoms, and all metals are aggregates of more or less imperfect crystals. In considering the structure of metals, therefore, it is convenient to start with the arrangement of atoms in a perfect metal crystal and then to proceed to the imperfections which are always present in the crystal structure. 

The atoms in a metal crystal can often be satisfactorily regarded as hard spheres of radius about 10" m. A sheet of such hard balls can be... 

The smallest imperfections in metal crystals are point defects, in particular vacant lattice sites (vacancies) and interstitial atoms. As illustrated in Fig. 20.21a, a vacancy occurs where an atom is missing from the crystal structure... 

Figure 9.1 Id illustrates a simple model of bonding in metals known as the electron-sea model. The metallic crystal is pictured as an array of positive ions, for example, Na+, Mg2+. These are anchored in position, like buoys in a mobile sea of electrons. These electrons are not attached to any particular positive ion but rather can wander through the crystal. The electron-sea model explains many of the characteristic properties of metals ...

Ductility and malleability. Most metals are ductile (capable of being drawn out into a wire) and malleable (capable of being hammered into thin sheets). In a metal, the electrons act like a flexible glue holding the atomic nuclei together. As a result, metal crystals can be deformed without shattering. 

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