The Formation of Metal Powders

Figure 7.15 Schematic illustration of compaction and fracture processes in the formation of metallic powders via ball milling. ASM Handbook, Vol. 7, Powder Metallurgy (1984), ASM International, Materials Park, OH 44073-0002. p. 2.

Hydrometallurgical processes, i.e. production of Ni, Co, Cu, or similar metals, are well established in practice. Similarly, the electrodeposition of metallic powders has been extensively studied over the last century. A significant portion of this book is devoted to electrodeposition of metallic powders. The study of electroless deposition of metal powders has to a certain extent been neglected. The formation of metallic powders occurring during electroless deposition of metallic powders is considerably less investigated than its electrolytic counterpart. 

The production of metallic powders through hydrometallurgical or electrodeposition routes has extensively been studied over the last century. The formation of metallic powders during electroless deposition of films of metals or alloys is observed when the so-called bath instability takes place [1, 16, 17]. As such, the electroless deposition was far less investigated than its electrolytic counterpart. The bath instability is usually seen in the experimental conditions at elevated temperatures or when the concentration of the reducing agent is too high. 

It is necessary to avoid heating of solutions of heavy late transition and heavy main group elements (especially important for Bi(III) ) to avoid the formation of metal powders via reduction through the )8-hydride transfer mechanism (Parola, 1997). 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

Silicon and lead oxide Mixtures of powdered silicon and lead oxide/lead dioxide/red lead burn fiercely and rapidly with the formation of metallic lead and fusible lead silicate. The reactions may be shown in Equation 5.9 ... 

In relation to elemental metals, o-quinones are extremely reactive ligands [11,12,14,133,192-208], especially those containing t-butyl substituents. Complexes of o-benzoquinones, o-semiquinones, and catechols of this type have been obtained by direct interaction between metal powders and the corresponding ligands and reviewed [14]. It was established that thermal decomposition of such complexes of copper in solution leads to the formation of metallic copper and the initial o-quinone. 

Therefore, local dissolution and recrystallization seem to play an important role in the gas uptake mechanism in these type of sensor materials. The coordination of SO2 to the platinum center (and the reverse reaction) is therefore likely to take place in temporarily and very locally formed solutes in the crystalline material, whereas the overall material remains crystalline. The full reversibility of the solid-state reaction was, furthermore, demonstrated with time-resolved solid-state infrared spectroscopy (observation at the metal-bound SO2 vibration, vs= 1072 cm-1), even after several repeated cycles. Exposure of crystalline samples of 26 alternat-ingly to an atmosphere of SO2 and air did show no loss in signal intensities, e.g. due to the formation of amorphous powder. The release of SO2 from a crystal of 27 was also observed using optical cross-polarization microscopy. A colourless zone (indicative of 26) is growing from the periphery of the crystal whereas the orange colour (indicative for 27) in the core of the crystal diminishes (see Figure 9). 

Chang, D. K., Won, C. W Chun, B. S and Shim, G. C., Purifying effects and product microstructure in the formation of TIC powder by the self-propagating high-temperature synthesis. Metall. Mater. Trans. B, 26B, 176 (1995). 

It is obvious that an increase in the concentration of the reducing agent leads to an increase in the concentration of OH- ions, as shown by the reaction (20). This increase in the concentration of OH- ions will lead to the formation of hydroxy complexes in bulk solution and the reduction of these complexes or hydroxides may take place into the bulk solution. This scenario could consequently lead to the precipitation of metallic powders in the bulk solution.8... 

The addition of metal powders with a high thermal value, such as aluminum, magnesium, titanium, boron, etc. Because the formation heats of their explosion products are relatively large and their thermal capacities are not increased very much, their addition is beneficial to the increase of explosion temperature. 

Finally, hychogen evolution becomes crucial factor determining the shape of powder particles of the group of the inert metals, and the concept of effective overpotential is applicable to explain the formation of these powder particles. Due to vigorous hychogen evolution, dendritic growth is almost completely inhibited. Analysis of the polarization curves for Co and Ni [12, 26, 30] showed that their... 

Finally, in Chapter 7 Djokic discusses the deposition of metallic powders from aqueous solutions without an external current source. Metallic powders can be successfully produced via galvanic displacement reaction or by electroless deposition from homogenous aqueous solutions or slurries. The formation of various metallic powders without an external current source e.g., Cu, Ni, Co, Ag, Pd, and Au, using appropriate reducing agents is presented. The mechanistic aspects of electroless deposition of powders are also discussed. It is shown that the hydrolysis of metallic ions is the most important factor leading to the deposition of metal powders from aqueous solutions. 

The formation of metal carbonyls by the direct reaction of the metal with CO at elevated temperatures and pressures is a well-established procedure for a wide variety of metals. However, for certain metals this procedure fails due to the low reactivity of the metal. Chromium is such a metal, and all attempts to react the metal with CO have failed to yield any Cr(CO)6. We would like to report the preparation of chromium powder by the Rieke process and the direct reaction of the metal with CO to yield Cr(CO)6 [1]. 

Most metals will precipitate as the hydroxide in the presence of concentrated NaOH. Metals forming amphoteric hydroxides, however, remain soluble in concentrated NaOH due to the formation of higher-order hydroxo-complexes. For example, Zn and AP will not precipitate in concentrated NaOH due to the formation of Zn(OH)3 and Al(OH)4. The solubility of AP in concentrated NaOH is used to isolate aluminum from impure bauxite, an ore of AI2O3. The ore is powdered and placed in a solution of concentrated NaOH where the AI2O3 dissolves to form A1(0H)4T Other oxides that may be present in the ore, such as Fe203 and Si02, remain insoluble. After filtering, the filtrate is acidified to recover the aluminum as a precipitate of Al(OH)3. 

Rhenium hexafluoride is readily prepared by the direct interaction of purified elemental fluorine over hydrogen-reduced, 300 mesh (ca 48 pm) rhenium powder at 120°C. The reaction is exothermic and temperature rises rapidly. Failure to control the temperature may result in the formation of rhenium heptafluoride. The latter could be reduced to rhenium hexafluoride by heating with rhenium metal at 400°C. 

Heterogeneous vapor-phase fluorination of a chlorocarbon or chlorohydrocarbon with HP over a supported metal catalyst is an alternative to the hquid phase process. Salts of chromium, nickel, cobalt or iron on an A1P. support are considered viable catalysts in pellet or fluidized powder form. This process can be used to manufacture CPC-11 and CPC-12, but is hampered by the formation of over-fluorinated by-products with Httle to no commercial value. The most effective appHcation for vapor-phase fluorination is where all the halogens are to be replaced by fluorine, as in manufacture of 3,3,3-trifluoropropene [677-21 ] (14) for use in polyfluorosiHcones. 

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