Crystalline structure of metals

The selection of materials for high-temperature applications is discussed by Day (1979). At low temperatures, less than 10°C, metals that are normally ductile can fail in a brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with the crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more liable to brittle failure than those with a face-centred-cubic (fee) or hexagonal lattice. For low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fee) or aluminium alloys (hex) should be specified see Wigley (1978). 

Concurrent stream of the development of nanomaterials for solid-state hydrogen storage comes from century-old studies of porous materials for absorption of gasses, among them porous carbon phases, better known as activated carbon. Absorption of gases in those materials follows different principles from just discussed absorption in metals. Instead of chemisorption of gas into the crystalline structure of metals, it undergoes physisorption on crystalline surfaces and in the porous structure formed by crystals. The gases have also been known to be phy-sisorbed on fine carbon fibers. 

The crystalline structure of metals is often disturbed when metals are worked into pieces of jewelry. For example, the artisan might pound, pull, twist, bend, and cut a piece of gold to produce a single, unique bracelet or earring (students should examine jewelry that they are wearing to find evidence of metal manipulation). Metals can be annealed, heated, and then carefully cooled, to make them soft. Metals do not shatter when they are... 

Metals are malleable (can be flattened) and ductile (can be drawn into a wire). Especially significant is the fact that the crystalline structure of metals is preserved under moderate deformations. 

X-ray diffraction (XRD) can be used to do a quick and easy determination of the crystalline structure of metal oxides by comparing the diffraction angle (2 ) with that of metal oxides of known composition. However, this is a qualitative and bulk method. 

When corrosion products such as hydroxides are deposited on a metal surface, a reduction in oxygen supply occurs, since the oxygen has to diffuse through deposits. Since the rate of metal dissolution is equal to the rate of oxygen reduction, a Hmited supply and limited reduction rate of oxygen will also reduce the corrosion rate. In this case the corrosion is said to be under cathodic control. In other cases corrosion products form a dense and continuous surface film of oxide closely related to the crystalline structure of metal. Films of this type prevent primarily the conduction of metal ions from metal-oxide interface to the oxide-liquid interface, resulting in a corrosion reaction that is under anodic control. When this happens, passivation occurs and metal is referred as a passivated metal. Passivation is typical for stainless steels and aluminum. 

Applicability Most hazardous waste slurried in water can be mixed directly with cement, and the suspended solids will be incorporated into the rigid matrices of the hardened concrete. This process is especially effective for waste with high levels of toxic metals since at the pH of the cement mixture, most multivalent cations are converted into insoluble hydroxides or carbonates. Metal ions also may be incorporated into the crystalline structure of the cement minerals that form. Materials in the waste (such as sulfides, asbestos, latex and solid plastic wastes) may actually increase the strength and stability of the waste concrete. It is also effective for high-volume, low-toxic, radioactive wastes. 

The atomic and crystalline structure of the two non-metallic carbides, boron and silicon carbides, is less complex than that of the... 

Different ways of the structural classification of deposits exist. In one system, the following structures are distinguished arbitrarily (1) fine-crystalline deposits lacking orientation, (2) coarse-crystalline deposits poorly oriented, (3) compact textured deposits oriented in field direction (prismatic deposits), and (4) isolated crystals with a predominant orientation in the field direction (friable deposits, dendrites). The structure of metal deposits depends on a large number of factors solution composition, the impurities present in the solntion, the current density, surface pretreatment, and so on. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Analogous calculations were made for dozens of crystalline structures of penetration -metal carbides and hydrocarbides (only some of them are given in table 4). In all these cases the relative difference of values of P-parameters of interacting systems can be considered the stability criterion - (coefficient a) based on the following equation ... 

The crystalline structure of the metal is also affected by the metal-support interaction. Metal particles supported on CNFs have a highly crystalline structure due to strong metal-support interaction [155], whereas Pt particles supported on Vulcan and OMCs have a more dense globular morphology due to weak metal-support... 

Palladium shows a great capacity for hydrogen absorption. This takes place with changes in the crystalline structure of the metal with the formation of Pd2H or Pd4H2 hydrides. 

The most obvious future data needs concern the missing, uncertain, and conflicting data identified above. Additional experimental investigations are needed in the case of Fe(III) and Zr(IV) carbonate complexation, and in the case of the Sn(IV)/Sn(II) and the Se(0)/Se(-II) redox couples. The molecular structure of metal silicate complexes needs clarification in order to remove ambiguities in the speciation scheme of these complexes. A rather challenging topic concerns the supposed transformation of crystalline tetra-valent actinide oxides, AnOz(cr), to solids with an amorphous surface layer as soon as the An4+ ion hydrolyses. The consequences of such... 

All substances, except helium, if cooled sufficiently form a solid phase the vast majority form one or more crystalline phases, where the atoms, molecules, or ions pack together to form a regular repeating array. This book is concerned mostly with the structures of metals, ionic solids, and extended covalent structures structures which do not contain discrete molecules as such, but which comprise extended arrays of atoms or ions. We look at the structure and bonding in these solids, how the properties of a solid depend on its structure, and how the properties can be modified by changes to the structure. 

Metallic Glasses. Under highly specialized conditions, the crystalline structure of some metals and alloys can be suppressed and they form glasses. These amorphous metals can be made from transition-metal alloys, eg, nickel—zirconium, or transition or noble metals in combination with metalloid elements, eg, alloys of palladium and silicon or alloys of iron, phosphoms, and carbon. 

In order to show that the origin of this difference is not a function of the particular substrate analogue used, similar NMR relaxation studies have been performed with dimethyl sulfoxide (DMSO)1401 since the crystal structure of the enzyme-NADH-DMSO ternary complex is well resolved.1366 From the relaxation data, the distance between the methyl protons of DMSO and Co11 was calculated to be 8.9 0.9 A, again too great for direct coordination of the sulfoxide group to the metal ion. Since the cobalt enzyme appears to be functionally similar to the native enzyme, the difference is unlikely to be a direct result of substitution. One possibility is that there may actually be a difference between the solution and crystalline structure of the enzyme ternary complex, particularly since it is well established that the crystalline enzyme is 1000 times less active than in solution.1402... 

Iron usually substitutes for some nickel and cobalt in skutterudite (Klein, 2002), 369. The arsenic in the crystalline structure of skutterudite occurs as AS4 rings (Cotton et al., 1999), 387. The rings are planar and rectangular with bond lengths of 2.464 0.002 A and 2.572 0.002 A at 22 °C (Mandel and Donohue, 1971). In skutterudite, each atom of cobalt or another divalent metal is surrounded by six arsenic atoms in a roughly octahedral formation (Mandel and Donohue, 1971). Chloanthite and smaltite are arsenic-deficient forms of nickel and cobalt skutterudite, respectively (Table 2.5). 

When we determined the crystalline structure of solids in Chapter 4, we noted that most transitional metals form crystals with atoms in a close-packed hexagonal structure, face-centered cubic structure, or body-centered cubic arrangement. In the body-centered cubic structure, the spheres take up almost as much space as in the close-packed hexagonal structure. Many of the metals used to make alloys used for jewelry, such as nickel, copper, zinc, silver, gold, platinum, and lead, have face-centered cubic crystalline structures. Perhaps their similar crystalline structures promote an ease in forming alloys. In sterling silver, an atom of copper can fit nicely beside an atom of silver in the crystalline structure. 

How does the crystalline structure of a metal affect the physical properties of the metal ... 

Chromophore identification Identification of species and functional groups via group absorbance peaks Determination of molecular structure of crystalline compounds Shows structure of metal complexes, environment around central element Structure determination, shows bonding conditions... 

It must be recognized that, in general, the physical properties of any metal are dependent on (1) the crystalline structure of the metal, (2) the presence of impurities, and (3) the mode of production and the mechanical treatment to which the metal may have been subjected. 

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