Alumina intermetallics

Gas Atomization 50-300 Standard deviation 1.9-2.5 <10-50 at high gas pressures with close-coupled atomizer Solder materials. Precious metals, Cu, Fe, Al, Mg, Co, Ti, Zn, Al-6Cr-3Fe-2Zr alloy. Low-alloy steels. High speed steels. Stainless steels, Specialty alloys, Ni-base superalloys, Alumina, Intermetallics io3-.o5 1-70 Spherical smooth particles. Cleanliness, Rapidly-solidified structures, Acceptable production rates High cost, Low 1 volume, Low energy efficiency (EE), Gas-filled porosity in particles H... 

The best catalyst for the synthesis of methanol from CO + H2 mixtures is copper/zinc oxide/alumina. Intermetallic compounds of rare earth and copper can be used as precursors for low-temperature methanol synthesis as first reported by Wallace et al. (1982) for RCu2 compounds (R = La, Ce, Pr, Ho and Th). The catalytic reaction was performed under 50 bar of CO + H2 at 300°C, and XRD analyses revealed the decomposition of the intermetallic into lanthanide oxide, 20-30 nm copper particles and copper oxide. Owen et al. (1987) compared the catalytic activity of RCux compounds, where R stands mainly for cerium in various amounts, but La, Pr, Nd, Gd, Dy and even Ti and Zr were also studied (table 4). The intermetallic compounds were inactive and activation involved oxidation of the alloys using the synthesis gas itself. It started at low pressures (a few bars) and low temperatures (from 353 K upwards). Methane was first produced, then methanol was formed and it is believed that the activation on, for example, CeCu2, involved the following reaction, as already proposed for ThCu2 (Baglin et al. 1981) ... 

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

Af-dipropyl-p- toluidine, 2 550t a, a -dinitroanthraquinones, 9 315—316 a-alumina, 2 406t 14 103. See also Corundum transition to, 2 403 a-aluminum-iron—silicon alloys, 2 317 intermetallic phases, 2 316t a-aluminum oxide-hydroxide. 

Several fiber types have been mentioned so far, and several other types have been neglected that have been worked on over the past few years. Some of those not discussed may become important fibers for reinforcement in the years ahead. To date though, they have not been available in sufficient quantity for thorough evaluation in composite specimens. Included in this group are boron carbide, spinel, polycrystalline alumina and silica, titanium diboride, and miscellaneous silicides and intermetallics. Ten years from now as we look back on the 70s we no doubt will have an entirely different view of some of these materials. 

The important ceramic matrix materials are glass, silicon carbide, silicon nitride, alumina, glass-ceramics, sialons, intermetallics and some elemental materials. A list of some ceramic matrix materials is given in Table 3.5. 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

This unusual reaction has not been reported previously, however, a number of studies have demonstrated the formation of analogous structures when platinum/alumina specimens were heated to temperatures in excess of 8009C. Baker and co-workers [23] using the CAEM technique to study the sintering characteristics of platinum on alumina in oxygen observed spectacular transitions in the appearance of the specimens at temperatures in excess of 800°C. The metal particles initially spread on the alumina to from diffuse islands and then quite suddenly reconstructed to produce well defined dense shapes. Sprys and Mencik [24] found the same effect when platinum/alumina specimens were subjected to intense electron beams within the electron microscope and characterized the structures as the intermetallic compound PtaAl from electron diffraction analysis. 

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Since the significant majority of the published literature on high temperature crack growth under static and cyclic loads is predicated upon experiments conducted on alumina and alumina matrix composites, the examples cited in the present review have centered around oxide ceramics and their composites. However, the implications of the results to other classes of ceramics, intermetallics, and brittle matrix composites are also described, wherever feasible, along with any available information in an attempt to illustrate the generality of the concepts developed here. 

Since metallic Sn has a high capacity for reversible Li insertion, pure Sn as well as its intermetallic compounds have been considered as promising anode materials in lithium ion batteries. In intermetallic compounds of Sn, the second metal is normally electrochemically inactive and cannot be alloyed with Li. Such an inactive metal performs as a buffer to accommodate volume variations during Li insertion/deinsertion in Sn.224 Among various Sn intermetallic compounds, Ni3Sn4232-235 and CU6S115236 237 are the most commonly studied materials. Intermetallic compounds of Sn with Ni and Cu can be electrochemically deposited. Templates are conventionally used to improve the morphological properties and, thus, the electrochemical behavior of the electrodeposited Sn and Sn intermetallic compounds. Copper nanopillars can be electrodeposited within alumina templates on a Cu foil to provide a unique template for the subsequent electrodeposition of Sn or its intermetallic compounds. 

R. A. Perkins, K. T. Chiang, G. H. Meier and R. Miller, Formation of alumina on niobium and titanium alloys, Oxidation of High-temperature Intermetallics, eds. T. Grobstein and J. Doychak (TMS, Warrendale, 1988) 157-169. 

Other experiments on nitrogen fixation have used transition metai compiexes of nitrogen.74 One used a tungsten nitrogen compiex with a ruthenium hydrogen complex at 55°C to produce a 55% yield (based on tungsten) of ammonia.75 Intermetallic compounds of iron and titanium have been used with ruthenium on alumina to make some... 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

A number of intermetallic compounds, which form protective alumina or silica scales at high temperature, undergo accelerated degradation at intermediate temperatures. This subject has been recently reviewed [29]. The observations actually involve several different, but related, phenomena which may be subdivided into accelerated oxidation , internal oxidation , intergranular oxidation and disintegration . The following definitions will be used throughout this paper. 

Accelereated Oxidation - the alumina or silica is not continuous and significant amounts of the other component(s) of the intermetallic are present in the surface film. The overall oxidation rate is substantially faster than that for the growth of alumina or silica. 

Aluminides based on the intermetallic phases Ni3Al and Fe3Al are considered both as structural materials and as coatings for high temperature applications [1-6]. Their excellent corrosion resistance is due to their forming a dense, protective alumina scale. Alumina, especially ot-Al203, shows low rate constants even at temperatures above 1000°C [7]. Unlike chromia, which is formed on conventional stainless steels and nickel base alloys, alumina does not evaporate above 1000°C [8] and it is even stable in oxygen deficient atmospheres. 

Since classical Cu/ZnO catalysts exhibited a poor stability while the addition of alumina resulted in much better systems, it was tempting to add alumina to Cu-Ce intermetallic compounds. Jennings et al. (1992a), prepared ternary Cu-Ce-Al alloys of various compositions and also tried a variety of other metals (Ca, Cr, Mn, Pd, Zn). Among these ternary alloys aluminum-containing catalysts were the best. In spite of lower initial activities as compared to binary alloys, they exhibited a much better long-term stability. It is believed that the role of aluminum is to stabilize the disperse copper-ceria phases responsible for methanol synthesis activity, although the mechanism for such a process remains unclear. 

Hydrogenolysis of triphenylarsine (AsPh ) on alumina supported nickel (Ni/Al Oj) has been studied as model reaction for metallic catalyst poisoning. The hydrogenolysis of AsPh on Ni/Al Oj occurs at temperature ranging from 303 to 443 K under 12 bars of hydrogen and in n-heptane solution. It has been followed by kinetics analysis of the AsPh consumption and Benzene and cylohexane evolution as well as XRD measurements of the metallic and intermetallic phase(s). 

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