Electrochemically Produced Metal Powders

Powders are finely divided solids, smaller than 1000 in its maximum dimension. A particle is defined as the smallest unit of a powder. The particles of powder may assume various forms and sizes, whereas the powders, as an association of such particles, exhibit, more or less, the same characteristics as if they were formed under identical conditions and if the manipulation of the deposits after removal from the electrode was the same [1,2]. The size of particles of many metal powders can vary in a quite wide range from a few nanometers to several hundreds of micrometers. The most important properties of a metal powder are the specific surface, the apparent density, the flowability, and the particle grain size distribution. These properties, called decisive properties, characterize the behavior of a metal powder. 

Dendrites are the most common shape of electrochemically produced powder particles [5]. Aside from dendrites, powder particles can be also obtained in the form of flakes, fibrous, spongy, wires, cauliflower-like, and the many other irregular forms. The powder particles can spontaneously fall off or can be removed from the 

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Electrochemically and chemically produced metal powders from aqueous solutions are of high purity. These powders find applications in metallurgy, automotive, aerospace, energy device, electronics, and biomedical industries. Disperse deposits and electrochemically produced metal powders are also very suitable for use as catalytic surfaces in chemical industry. 

In the second part of the 20th century, the tantalum capacitor industry became a major consumer of tantalum powder. Electrochemically produced tantalum powder, which is characterized by an inconsistent dendrite structure, does not meet the requirements of the tantalum capacitor industry and thus has never been used for this purpose. This is the reason that current production of tantalum powder is performed by sodium reduction of potassium fluorotantalate from molten systems that also contain alkali metal halides. The development of electronics that require smaller sizes and higher capacitances drove the tantalum powder industry to the production of purer and finer powder providing a higher specific charge — CV per gram. This trend initiated the vigorous and rapid development of a sodium reduction process. 

Nikolic ND, Zivkovic PM, Jokic B, Pavlovic MG, Stevanovic JS (2014) Comparative analysis of the polarisation and morphological characteristics of electrochemically produced powder forms of the intermediate metals. Maced J Chem Chem Eng 33 169-180... 

Powders of both academically and technologically important metals, such as silver, lead, cadmium, cobalt, nickel, and iron, were produced by the electrolytic processes. The constant regimes of electrolysis and the regime of pulsating overpotential (PO) were used for the electrochemical synthesis of powders of these metals. Morphologies of such obtained powders were characterized by the scanning electron microscopy and optical microscopy, as well as by X-ray diffraction techniques. 

The production of metallic powders without an external current source has many advantages compared to electrochemical (electrolytic) methods. Generally speaking, all the powders that can be produced electrolyticaUy from aqueous solutions can also be obtained using chemical methods, i.e. without an external current source. Classical examples of metallic powders produced from aqueous solutions by chemical methods on an industrial scale via the so-called hydrometallurgical processes include nickel and copper. 

Several other methods for the synthesis of PPVs have been described in the literature. The McMurry reaction, which consists of the deoxygenative coupling of aromatic dialdehydes in the presence of titanium compounds, has been employed to obtain 2,5-dihexyl-PPVs [69], para- and meta-FFV (20) [70], Scheme 11, the latter very unlikely to be prepared by the Wessling-Zinmerman or electrochemical routes, since there is no possibility of forming the required quinodimethane intermediate. Nevertheless, a drawback of this method is the somewhat tedious workup needed for the isolation of insoluble PPVs from the elementary metal powder (Ti and/or Zn), which is also produced. 

Anodic Oxidation. The abiUty of tantalum to support a stable, insulating anodic oxide film accounts for the majority of tantalum powder usage (see Thin films). The film is produced or formed by making the metal, usually as a sintered porous pellet, the anode in an electrochemical cell. The electrolyte is most often a dilute aqueous solution of phosphoric acid, although high voltage appHcations often require substitution of some of the water with more aprotic solvents like ethylene glycol or Carbowax (49). The electrolyte temperature is between 60 and 90°C. 

The electrochemical generation of Cr(II) produces yields comparable to the conventional zinc powder route. Advantages of the electrochemical method include reduced revenue costs and a more environmentally acceptable process with respect to the heavy metal effluents. 

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