Aggregates, metallic powders

These highly disperse metal powders are very active chemically, and hence unstable they readily aggregate to coarser particles, and readily oxidize when in contact with air. Their stability rises significantly when they have been applied to a suitable substrate. 

ASTM D75-97 (1997), Standard practice for sampling aggregate, 12 ASTM B215-96 (1996), Standard practice for sampling finished lots of metal powders, 12... 

Nevertheless the use of dielectric materials obtained by conductive filler dispersion (carbon black, graphite fibres, metallic powders) is limited. As a matter of a fact material performances are dependent on the filler content as well as particle aggregation phenomena. These composites require a high level of reproducibility and their behaviour is linked to the control of electronic inter-particular transfer. The measured parameter (complex permittivity) depends on the texture of the percolation aggregates and consequently on the processing conditions. The percolation threshold (the particle concentration, after which particles are in contact and the electrical current exists) depends on the particle shape (sphere, plates or fibres). 

The reaction products consist of loosely aggregated cakes of thorium metal powder and calcium oxide. Each cake is individually quenched and immersed in water for 4 hr. They disintegrate to a fine powder as the excess calcium is destroyed and the calcium oxide is converted to the hydroxide. The hydroxide is then dissolved by the addition of nitric acid to the solution until an excess of about 0 3N remains. The time of exposure of the thorium metal powder to the acidic conditions is limited, to prevent its dissolution. It is fairly rapidly filtered by vacuum, rinsed with water, and dried in a vacuum oven. The dry metal powder is about 300 mesh in particle size and has the following typical analysis ... 

In addition to the decreased polarizability of the heavier metals, their larger radii require higher metal coordination numbers to achieve steric saturation. As a result, extensive aggregation, frequently coinciding with rather limited solubility in non-donor solvents, and occasionally even in donor solvents, complicates the characterization of these species in solution and the solid state. In fact, several structural characterizations of organoalkali species have relied on recent advances in powder diffraction techniques using synchrotron radiation.1 ... 

Spray pyrolysis routes have been extensively investigated to prepare Pt-based catalysts. Typically, a liquid feed of metal precursor and carbon is atomized into an aerosol and fed into a continuous furnace to evaporate and heat-treat to form a collectable powder. The method has good control over final aggregate particle size and metal particle size distributions, as well as producing powder without further isolation or separation. Hampton-Smith et al. have reviewed efforts of Superior MicroPowder (now Cabot Fuel Cells) in this area. ... 

Tetrameric [MeLi]4 (1), [EtLi]4 (2) and [t-BuLi]4 (3) are white pyrophoric powders. While methyllithium is soluble only in polar solvents like diethyl ether, the two others are soluble even in non-polar hydrocarbons like hexane. In non-donating solvent the tetrameric aggregation is retained. Each of the four U3 triangles is /ra-capped by a Ca atom above the center of the equilateral metal triangle. Even in the solid-state none of the three tetramers adopts ideal symmetry (Figure 6). 

As mentioned in the preceding section, to stabilize a particle, metallic particles must be produced in an inert gas atmosphere and then trapped in appropriate liquids to finely disperse them. Unless liquid is used, particles tend to coalesce, forming a larger particle or aggregate, the size of which exceeds several tens of nanometers as a powdered sample. First we introduce the normal gas evaporation technique to show the principle of aerosol method. Then several modifications are described to get dispersed metallic systems as a colloid. 

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