Metal powders, reactions

Condensation of metal vapors followed by deposition on cooler surfaces yields metal powders as does decomposition of metal hydrides. Vacuum treatment of metal hydrides gives powders of fine particle size. Reaction of a metal haHde and molten magnesium, known as the KroU process, is used for titanium and zirconium. This results in a sponge-like product. 

Use of excess sodium drives the reaction, usually done under an argon or helium blanket, to completion. After cooling, the excess sodium is leached with alcohol and the sodium and potassium fluorides are extracted with water, leaving a mass of metal powder. The metal powder is leached with hydrochloric acid to remove iron contamination from the cmcible. 

Titanium carbide may be prepared by a thermochemical reaction between finely divided carbon and titanium metal powder. The reaction proceeds exothermically. 

T oxides, carbides via high iatensity arc metallic powders via vacuum or catalytic reactions... 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

No binary hydrides have been characterized, but reactions of the metal powders with alkali metal hydrides in a hydrogen atmosphere lead to Li3RhH4 (planar RhH4 ) and M3MH6 (M = Li, Na M = Rh, Ir) with octahedral MHj [34],... 

Praseodymium tri-iodide, Prl3, as the starting material for reduction reactions, might be easily produced by the oxidation of praseodymium metal with elemental iodine [17]. With catalytic amounts of hydrogen dissolved in praseodymium metal powder, the reaction temperature can be as low as 230 °C [18]. Sublimation in high vacuum in tantalum tubes yields pure Prl3. 

As already discussed, low-valent titanium can be generated in situ from catalytic amounts of TiCls, by use of excess Zn powder and TCS 14 in acetonitrile, to cy-clize reductively 2-benzoylaminoacetophenone 2091 to the indole 2092, in 80% yield, in an elegant version of the McMurry reaction [23, 64]. Replacement of the Zn metal powder by Ti powder and TCS 14 is very effective - 2092 is obtained in 97% yield [64]. In these reactions the intermediate Ti(0)Cl is apparently recycled by MesSiCl 14 into TiCh [64]. In these Fiirstner versions of the McMurry reaction... 

T. Baker Chemicals were used. The arsenic metal powder was Imbedded in indium foil for examination. An arsenic mirror formed on a reaction flask was also examined. 

Since the utility of these materials is improved by the incorporation of these reactive functionalities without severely decreasing other favorable properties such as thermooxidative stability and solvent resistance the chemistry of the isoimide isomerization and acetylene crosslinking reactions is of considerable interest. Previous work in our laboratory has shown that these materials, when loaded with metal powders, provide a convenient and effective method of optimizing the electrical conductance and thermal stability of aluminum conductor joints. 

The nitrides can be prepared by heating a metal powder in an N2 or NH3 atmosphere to temperatures above 1100°C. The carbides form upon heating mixtures of the metal powders with carbon to temperatures of about 2200 °C. Both the nitrides and carbides can also be made by chemical transport reactions by the van Arkel-de Boer method if the metal deposition takes place in an atmosphere of N2 or a hydrocarbon. Their remarkable properties are ... 

Limiting currents measured for a deposition reaction may be excessively high due to surface roughness formation near the limiting current. Rough deposits in the case of copper deposition have been mentioned several times in previous sections, since this reaction is one commonly used in limiting-current measurements. However, many other metals form dendritic or powdery deposits under limiting-current conditions, for example, zinc (N lb) and silver. Processes of electrolytic metal powder formation have been reviewed by Ibl (12). 

Potassium perchlorate Metal powders See other thermite reactions... 

During investigation of pyrotechnic flare formulations, it was found that mixtures of the metal powder and oxidant underwent a low-temperature exothermic reaction at 70-135°C in presence of moisture. 

The activity of metals other than platinum for skeletal reactions of larger molecules is not well documented, particularly in a mechanistic sense. Carter, Cusumano, and Sinfelt (157) have recently studied the reaction of n-heptane on a series of group VIII metals in the form of hydrogen-reduced (300°C) metal powders. The nature of the reaction pathways is summarized in Table IX. Although many metals have been... 

Reaction of n-Heptane over Reduced Metal Powders ... 

Halides of the less electropositive metals are quickly reduced to highly dispersed and very active metal powders if they are exposed to ultrasonic waves in the presence of lithium and other group I metals(20). Ultrasound not only accelerates the reduction of the halides but also increases the rate of subsequent reactions of these less active metals. These reactions are covered in the chapter by K. Suslick. 

In 1972, we reported a general procedure for the preparation of highly reactive metal powders. The basic procedure involved the reduction of a metal salt in a hydrocarbon or ethereal solvent. The reductions are most generally carried out with alkali metals such as potassium, sodium, or lithium. A wide range of methods have been developed to carry out the reductions. The reactivities of these resulting black powders exceed other reports in the literature for metal powders. This high reactivity has resulted in the development of several new synthetic techniques and vast improvements in many older, well established reactions. This review concentrates on the metals Mg, Ni, Zn, Cd, Co, Cu, Fe, and U. 

Several basic approaches are possible and each has its own particular advantages. For some metals, all approaches lead to metal powders of identical reactivity. However, for some metals one method will lead to far superior reactivity. High reactivity, for the most part, refers to oxidative addition reactions. 

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