Powder technology

Nanoscaled powders are the main materials used to prepare ceramics and thick fihns for a variety of applications during gas sensor design and fabrication. It was fonnd that the varions synthesis techniqnes nsed for producing nanoscaled powders can generally be divided into three major types assembly of clnsters/nano-particles produced by (1) wet-chemical routes, (2) gas-phase synthesis, and (3) electrolytic deposition. 

Metal oxide Precursor, temperature of hydrolysis Temperature and time of calcination Grain size 

A range of collection devices has been used to separate nanoparticles from the gas (Choy 2003 Vahlas et al. 2006). Traditionally, a rotating cylindrical device cooled with liquid nitrogen has been employed for particle collection. The nanoparticles are subsequently removed from the surface of the cylinder with a scraper, usually in the form of a metallic plate. However, the simplest method of collecting nanopowders is to use a mechanical filter with a small pore size. Nanoparticles ranging from 2 to 50 mn in size may be extracted from the gas flow by thermophoretic forces from an applied permanent temperature gradient and then deposited loosely on the surface of the collection device as a powder of low density with no agglomeration. 

For some applications, especially solid-state gas sensor design, it is necessary to use nanoscaled powders with sizes smaller than 5-10 mn (Nayral et al. 2000 Leite et al. 2001). In many cases, however. 

If Vp is the volume of particles and Vy, the volume of the powder bed, the degree of packing is often characterized by the packing fraction (Vp/Vb). Another useful parameter is the fractional voidage or porosity defined by (1 - V /Vy). An idealized model made up of equally sized cubes would have a packing fraction of one and thus a porosity of zero. The theoretical porosity for closest packed, uniform, spherical particles is 0.26. The porosity of other simple models may be calculated however, in practice, the heterogeneity of size and shape of particles and the interactions between them usually make theoretical predictions of porosity 

It might be expected that inclusion of small particles in a powder could decrease porosity if they fit into voids between larger particles. In practice, the opposite effect is usually observed. The explanation for this follows from consideration of the main factors that determine closeness of packing and flow properties of powders the size, shape, and surface properties of the particles. It is often found that the effects of surface properties outweigh the others because they govern the friction and adhesion between particles. As the size of particles decreases, the ratio of surface to volume increases, thus magnifying frichonal resistance. Other factors that may contribute to increased friction or stickiness are the presence of liquid films and electrical charge effects. 

Butcher, J., and N. L. Stenvert. 1973. Conditioning studies on Australian wheats. III. The role of the rate of water penetration into the wheat grain. Journal of the Science of Food and Agriculture 24 1077-1084. 

Interfacial effects in particulate, fibrous and layered composite materials. Kep Engineering Materials 116-117 307-330. 

Kelfkens, and G. McMaster. 1998. Ash determination—A useful standard or a flash in the pan Satake Corporation, UK division. Presented at ICC Technical Conference, Valencia, Spain (http //www.satake.co.uk/labo-ratory / Ash%20Tony %20Evers.htm). 

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COATINGPROCESSES - POWDER TECHNOLOGY] (Vol 6) -melting ternperamre PLASTIC PROCESSING] (Vol 19)... 

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