Water-atomized metal particles

Most water-atomized metal particles (powders) have been observed to follow the log-normal size distribution pattern. Relatively narrow size distributions of both fine and coarse particles may be generated by water atomization. A review of published data for droplet size distributions generated by gas and water atomization of a variety of liquid metals and alloys has been made by Lawley,[4] along with presentations of micrographs of surface morphology and internal microstructure of solidified particles. 

Further studies are required to develop more comprehensive and general correlations for water-atomized metal droplets/particles. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Amorphous carbon is characterized by a highly imperfect structure and high reactivity. This shows by a considerable amount of mobile carbon atoms at a surprisingly low temperature. Besides, a vast number of defects and small sizes of graphene sheets make the carbon matrix very labile. As a result, it may be deformed under the action of adsorbates. For example, granules of amorphous carbon swell [88,89] in water with concomitant changes in the carbon substructure and porosity [90,91]. These properties of the support weaken rapidly as its crystal structure becomes more perfect. The labile structure of amorphous carbon is responsible for at least two mechanisms of blocking of the surface of supported metal particles. 

A metal atom reactor is often used in a variety of "dirty" and "clean" operations. Accordingly, it should be remembered that radiation, adventitious water, oxygen, hydrocarbons, metallic particles and so on, can affect the properties of the isolated products. In the case of minute magnetic structures it is important to determine clearly the role of such agents in affecting volume and surface magnetic properties. Incorporation of a high vacuum Schlenk manifold such as the one described by Wayda in this book, should be considered an important supplement to the VS equipment. 

A very common objection to the role of Cu in methanol synthesis was based on the general experience of producers of Cu catalysts for methanol synthesis and water-gas shift, namely, that the activity is proportional to the metallic surface area. However, this problem is only apparent. Cu may have two functions to be an active centre when accessible to the gas phase and to be an anchor for a metallic particle. The role of ions in stabilizing highly dispersed metal is, indeed, a known fact. Cu may also supply H atoms. It is known to be a poor adsorbent for H2 but it does adsorb some at high temperatures and pressures. ... 

Unless the metal is introduced as such, e.g. as a colloid or by metal-atom-vapour deposition (see later), the final and critical step is inevitably a reduction, performed either ex situ or in situ (or both). Molecular hydrogen is most often used, although carbon monoxide has a thermodynamic advantage, which is useful for less easily reducible species because the carbon dioxide produced is less effective than water in reversing the process. Reduction of a base metal oxide can be effected by hydrogen atoms spilling over (see Section 3.34) from reduced noble metal particles." More exotic reductants (e.g. Cr ions," oxirane" and... 

The field of nanotechnology is typically defined as dealing with particles approximately 1 to 100 nm in size. Because of the tiny size of nanoscale particles, they behave quite differently from normal-sized particles, even though the two types of particles may contain the same types of atoms. An important reason for these different behaviors is the incredible surface area present in a sample of nanosized particles. For example, about one gram of nanobeads used in a device to filter water contains an amazing 1000 square meters of surface area. This extremely high surface area of nanoparticles is very useful for catalyzing the decomposition of soil contaminants. For instance, several studies have shown that nanosized metallic particles are very efficient catalysts for the breakdown of the chlorinated solvents often found in contaminated soil. ... 

Air pollutant particles range in size from fly ash particles, which are big enough to see, down to individual molecules, ions, or atoms. Many pollutants are attracted into the water droplets of fog. Solids and liquid droplets suspended in the atmosphere are collectively known as particulates. The solids may be metal oxides, soil particles, sea salt, fly ash from electric generating plants and incinerators, elemental carbon, or even small metal particles. Aerosol particles range upward from a diameter of 1 nanometer (nm) to about 10,000 nm and may contain as many as a trillion atoms, ions, or small molecules. Particles in the 2000-nm range are largely responsible for the deterioration of visibility. 

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