Metals and atmosphere

In the first scheme the metal boiling point is less than the oxide boiling point and the model consists of a vaporising droplet of metal surrounded by a detached reaction zone where condensed oxides appear as fine droplets. The reaction rate is said to be controlled by the vapour phase diffusion of metal and atmospheric oxygen into the reaction zone as in Figure 5.6. 

The scope of the present paper is to emphasize that the interactions between support, metal and atmosphere are responsible for both the physical (size distribution, shape of the crystallites, wettability of the substrate by the crystallites and vice versa), the chemical and the catalytic (suppression of chemisorption, increased activity for methanation, etc.) manifestations of the supported metal catalysts. In the next section of the paper, a few experimental results concerning the behaviour of iron crystallites on alumina are presented to illustrate the role of the strong chemical interactions between the substrate and the compounds of the metal formed in the chemical atmosphere. Surface energetic considerations, similar to those already employed by the author (7,8), are then used to explain some of the observed phenomena. Subsequently, the Tauster effect is explained as a result of the migration, driven by strong interactions,... 

Active metals are always covered by native surface films formed by reactions between the metal and atmospheric components. The native surface films on active metals usually have a bilayer stmcture. The inner layer comprises... 

For their wide applications, the investigation on the wettability of nitrides by liquid metals and alloys keeps going. The active metal and atmosphere affect strongly the wettability in nitrides. 

The above rate law has been observed for many metals and alloys either anodically oxidized or exposed to oxidizing atmospheres at low to moderate temperatures—see e.g. [60]. It should be noted that a variety of different mechanisms of growth have been proposed (see e.g. [61, 62]) but they have in common that they result in either the inverse logaritlnnic or the direct logarithmic growth law. For many systems, the experimental data obtained up to now fit both growth laws equally well, and, hence, it is difficult to distinguish between them. 

It is a shiny, white, soft, and ductile metal, and takes on a bluish cast when exposed to air at room temperatures for a long time. The metal starts to oxidize in air at 200oC, and when processed at even moderate temperatures must be placed in a protective atmosphere. 

In arc welding, the coalescence of metals is achieved through the intense heat of an electric arc, which is estabUshed between the base metal and an electrode. The processes Hsted in Table 1 are differentiated by various means of shielding the arc from the atmosphere (1 3). 

Polymerization and GycliZation. Acetylene polymerizes at elevated temperatures and pressures which do not exceed the explosive decomposition point. Beyond this point, acetylene explosively decomposes to carbon and hydrogen. At 600—700°C and atmospheric pressure, benzene and other aromatics are formed from acetylene on heavy-metal catalysts. 

Alternatively, tows of fibers can be passed through a Hquid metal bath, where the individual fibers are wet by the molten metal, wiped of excess metal, and a composite wine is produced. A bundle of such wines can be consoHdated by extmsion to make a composite. Another pressureless Hquid metal infiltration process of making MMCs is the Prim ex process (Lanxide), which can be used with certain reactive metal alloys such as Al—Mg to iafiltrate ceramic preforms. For an Al—Mg alloy, the process takes place between 750—1000°C ia a nitrogen-rich atmosphere (2). Typical infiltration rates are less than 25 cm/h. 

Ca.rbonylProcess. Cmde nickel also can be refined to very pure nickel by the carbonyl process. The cmde nickel and carbon monoxide (qv) react at ca 100°C to form nickel carbonyl [13463-39-3] Ni(CO)4, which upon further heating to ca 200—300°C, decomposes to nickel metal and carbon monoxide. The process is highly selective because, under the operating conditions of temperature and atmospheric pressure, carbonyls of other elements that are present, eg, iron and cobalt, are not readily formed. 

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

Uranium hexafluoride [7783-81-5], UF, is an extremely corrosive, colorless, crystalline soHd, which sublimes with ease at room temperature and atmospheric pressure. The complex can be obtained by multiple routes, ie, fluorination of UF [10049-14-6] with F2, oxidation of UF with O2, or fluorination of UO [1344-58-7] by F2. The hexafluoride is monomeric in nature having an octahedral geometry. UF is soluble in H2O, CCl and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The importance of UF in isotopic enrichment and the subsequent apphcations of uranium metal cannot be overstated. The U.S. government has approximately 500,000 t of UF stockpiled for enrichment or quick conversion into nuclear weapons had the need arisen (57). With the change in pohtical tides and the downsizing of the nation s nuclear arsenal, debates over releasing the stockpiles for use in the production of fuel for civiUan nuclear reactors continue. 

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