State, metallic

The electrical conductivity is mainly determined by the carrier density, n, relaxation time, T, and effective mass, m, of the carrier (electrical conductivity, r = ne r/m, where e is the electron charge). According to the loffe-Regel criterion, the interatomic distance is considered as the lower limit for the mean free path, A, in a metallic system. Hence, for a metallic system kpX is greater than 1, where kpA. = [h(37f) ] / kp is the Fermi 

Polymer Abbreviation Repeat Unit Orientability CrystaUinity Ccmductivity  

Reproduced by permission from M. Ahlskog, R. Menon, Journal of Physics Condensed Matter, lOP Publishing, 1998, 10, 32, 31-33, 7171.  

Iodine-doped polyacetylene is one of the most extensively studied systems among 

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The principal ha2ards of plutonium ate those posed by its radioactivity, nuclear critical potential, and chemical reactivity ia the metallic state. Pu is primarily an a-emitter. Thus, protection of a worker from its radiation is simple and usually no shielding is requited unless very large (kilogram) quantities are handled or unless other isotopes are present. 

Because no process has been developed for selectively removing impurities in vanadium and vanadium alloys in the metallic state, it is essential that all starting materials, in aggregate, be pure enough to meet final product purity requirements. In addition, the consoHdation method must be one that prevents contamination through reaction with air or with the mold or container material. 

The capability of zinc to reduce the ions of many metals to theh metallic state is the basis of important appHcations. However, metals are removed from zinc solutions by displacement with finely divided zinc before winning by electrolysis. Gold and silver are displaced from cyanide leach solutions with zinc and the following metals are similarly recovered from various solutions platinum group, cadmium, indium, thallium, and sometimes copper. 

Discharging to this lower cell voltage usually results ia shorter cycle life. Enough excess iron should be provided ia the cell design to avoid this problem. Active iron ia the metallic state is slowly attacked by the alkaline electrolyte according to... 

Cesium [7440-46-2] Cs, is a member of the Group 1 alkali metals. It resembles potassium and mbidium ia the metallic state, and the chemistry of cesium is more like that of these two elements than like that of the lighter alkaU metals. 

Cesium was first produced ia the metallic state by electrolysis of a molten mixture of cesium and barium cyanides (2). Subsequentiy the more common thermochemical—reduction techniques were developed (3,4). There were essentially no iadustrial uses for cesium until 1926, when it was used for a few years as a getter and as an effective agent ia reduciag the electron work function on coated tungsten filaments ia radio tubes. Development of photoelectric cells a few years later resulted ia a small but steady consumption of cesium and other appHcations for cesium ia photosensing elements followed. 

Copper [7440-50-8] Cu, critically important to the development of civilization, is the only common metal found naturally in the metallic state. It was thus suitable for the production of tools, and ancient people made use of its easy workabiUty and beauty. Furthermore, the ease with which the oxide can be reduced to the metal, together with the tendency of copper to alloy with other metals naturally present in the ores, promoted broad usage. 

The higher ionisation energy and smaller ionic radius of copper contribute to its forming oxides much less polar, less stable, and less basic than those of the alkah metals (13). Because of the relative instabiUty of its oxides, copper joins silver in occurring in nature in the metallic state. 

A guide to tire stabilities of inter-metallic compounds can be obtained from the semi-empirical model of Miedema et al. (loc. cit.), in which the heat of interaction between two elements is determined by a contribution arising from the difference in work functions, A0, of tire elements, which leads to an exothermic contribution, and tire difference in the electron concentration at tire periphery of the atoms, A w, which leads to an endothermic contribution. The latter term is referred to in metal physics as the concentration of electrons at the periphery of the Wigner-Seitz cell which contains the nucleus and elecUonic structure of each metal atom within the atomic volume in the metallic state. This term is also closely related to tire bulk modulus of each element. The work function difference is very similar to the electronegativity difference. The equation which is used in tire Miedema treatment to... 

Metallic materials consist of one or more metallic phases, depending on their composition, and very small amounts of nonmetallic inclusions. In the metallic state, atoms donate some of their outer electrons to the electron gas that permeates the entire volume of the metal and is responsible for good electrical conductivity (10 S cm ). Pure elements do not react electrochemically as a single component. A mesomeric state can be approximately assumed... 

The optimised interlayer distance of a concentric bilayered CNT by density-functional theory treatment was calculated to be 3.39 A [23] compared with the experimental value of 3.4 A [24]. Modification of the electronic structure (especially metallic state) due to the inner tube has been examined for two kinds of models of concentric bilayered CNT, (5, 5)-(10, 10) and (9, 0)-(18, 0), in the framework of the Huckel-type treatment [25]. The stacked layer patterns considered are illustrated in Fig. 8. It has been predicted that metallic property would not change within this stacking mode due to symmetry reason, which is almost similar to the case in the interlayer interaction of two graphene sheets [26]. Moreover, in the three-dimensional graphite, the interlayer distance of which is 3.35 A [27], there is only a slight overlapping (0.03-0.04 eV) of the HO and the LU bands at the Fermi level of a sheet of graphite plane [28,29],... 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

Ruthenium and osmium are generally found in the metallic state along with the other platinum metals and the coinage metals. The major source of the platinum metals are the nickel-copper sulfide ores found in South Africa and Sudbury (Canada), and in the river sands of the Urals in Russia. They are rare elements, ruthenium particularly so, their estimated abundances in the earth s crustal rocks being but O.OOOl (Ru) and 0.005 (Os) ppm. However, as in Group 7, there is a marked contrast between the abundances of the two heavier elements and that of the first. 

In the concentrated ores most metals are in chemical compounds, as oxides or sulfides. Reducing these compounds to the metallic state in the final stage in producing metal can be accomplished by chemical processes or electrolysis. Two examples of chemical reduction are... 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

Table 21-IV. properties of the alkaline earths in the metallic state...

X-ray diffraction analysis of the spent catalyst (Table VI) revealed that the nickel was present only in the metallic state. Chemical analyses demonstrated very little difference in catalyst composition at the gas inlet and outlet. 

Here, for nearly all of the elements, the number of 4f electrons in the metallic state and in the trichloride is the same, so we expect a largely smooth energy... 

Fig. 8.7 Superposition of density-of-states for B-N bonding BN>. states with corresponding metal states, reflected by their valence states from alkaline-earth (as Ca), lanthanide (as La), and 3d-metal (as Ni), and corresponding block schemes...

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

Platinum-rhenium catalysts have been reduced in one atmosphere of flowing hydrogen and then examined, without exposure to the atmosphere, by ESCA. The spectra indicate that the Group VIII metal is present in a metallic state in the reduced catalyst and that the majority of the rhenium is present in a valence state higher than Re(0). 

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