Ultrafine powder, preparation

Ultrafine powders can be prepared in high-temperature plasmas. Particles below 1 [Lm and larger particles with unusual surface structures are formed according to WaJdie [Trans. Inst. Chem. Eng., 48(3), T90 (1970)]. Energy costs are discussed. 

Sonochemical Preparation of Ultrafine Powders and Nanostructured Materials... 

Ultrafine powders are usually considered to be particles with diameters between 1 -100 run [141]. A focus for recent work in material chemistry has been the preparation of such particles [142] because of their special properties optical, photocatalytic, electrochemical, magnetic, thermal resistance, high melting point and sintering abilities [143,144]. A variety of chemical and physical methods have been used for the synthesis of such materials including the decomposition of organometallics [145] and the controlled reduction of salts [146-148]. 

PREPARATION OF ULTRAFINE POWDERS BY REACTION-PRECIPITATION IN IMPINGING STREAMS I ULTRAFINE WHITE CARBON BLACK... 

Scientifically, the term ultrafine powder or ultrafine particles is used to describe solid products in which the particle sizes are no greater than 100 nm. The ultrafine white carbon black to be discussed in this chapter is the product of particles of a smaller size than those in common products, i.e., ultrafine here is not a scientific but a commercial term. The ultrafine powders in the scientific sense, e.g., nano copper, nano TiO and nano hydroxyapatite, and related topics will be discussed in later chapters. Nevertheless, the principles involved in the preparation of ultrafine white carbon black by impinging stream reaction-precipitation are very similar to those involved in the preparation of the nano powders mentioned above. Therefore this topic is discussed here under the overall title Preparation of ultrafine powders . 

Because of their important application value, much research and development on the preparation technologies of ultrafine powders has been carried out in the last twenty years and more, and hundreds of preparation methods have been proposed. Since they are not the major topic of this book, neither a description of the classification of the methods nor an introduction to the details of the various methods will be covered here. On the other hand, reaction-precipitation methods generally have a number of advantages such as lower cost, moderate operating conditions, lower equipment requirements, convenience of operation, and normally yield good-performing products etc. thus they occupy an important position among the various methods. 

The liquid-continuous impinging stream (LFS) device has the features of efficient micromixing and strong pressure fluctuation and, being the major equipment in the preparation of ultrafine powders by reaction-precipitation, it has exhibited excellent performances in a number of application cases. However, of the various preparation... 

ADAPTABILITY OF LIQUID-CONTINUOUS IMPINGING STREAMS FOR PREPARATION OF ULTRAFINE POWDERS... 

Essentially, the key process in the preparation of ultrafine powders by reaction-precipitation is crystallization from a solution. As mentioned in the previous chapter, crystallization from a solution includes two steps nucleation and crystal-growth. Both can occur only in a supersaturated solution and spontaneous nucleation can occur only when the concentration of the solute in the solution is over the super solubility of the substance involved. The rate equation for nucleation derived from the principles of thermodynamics is represented by [ 182]... 

Summarizing the discussions above, the necessary conditions for the preparation of ultrafine powders by reaction-precipitation may be concluded to be the following (1) Very fast reaction that can yield instantaneously a quantity of the substance to be precipitated (2) Very low solubility of the substance to be prepared in the form of ultrafine particles (3) Extremely high supersaturation and (4) Very uniform supersaturation. 

Because of their important application values, ultrafine powders have been the subject of a number of investigations and developments in the last two or three decades, and many kinds of preparation methods have been proposed. By examining the status of the research and development and the various methods proposed, it is not... 

In addition to the areas of LIS application introduced above, such as chemical reactions, crystallization, and preparation of ultrafine powders, the following may be fields where LIS could be successfully applied and so are worthy of note ... 

A soft chemical route, known as the sol-gel method, has also been employed for the preparation of nano-oxides with uniform size and shape. This is a multistep process, usually consisting of hydrolysis of a metal alkoxide in an alcoholic solution to yield a metal hydroxide, followed by polymerization by elimination of water (gel-formation), drying off the solvent, and densification of the product to yield an ultrafine powder (Rao and Raveau, 1998 Khaleel and Richards, 2001). 

Qian Y.-T., Chen Q.-W., Chen Z.-Y, Fan C.-G., Zhou G.-E., Preparation of ultrafine powders of Ti02 by hydrothermal H2O2 oxidation starting from metallic Ti. J. Mater. Chem., 3 (1993) pp. 203-205. 

Waldie 56, 57), in an attempt to prepare ultrafine powders from coarser materials, obtained spheroids of oxide powders in low-power RF torches. When silica powder (50-72 p.m) was injected into a 2.5-kW, 34-MHz argon plasma at 15 gm/hr, a 15% conversion to ultrafine particles (0.015-0.15 / m) and coarse spheroids were obtained. Ultrafine powders of barium oxide (50% <0.1 ju.m) and alumina spheroids were also prepared by this technique. When alumina was injected cocur-rently into a 3.5-kW, 10-MHz argon plasma 57), 48% spheroidization of a 180-250 /xm powder was obtained at a feed rate of 36 gm/hr. Waldie obtained better results by use of countercurrent particle flow similar to the technique used by Haiti 26). Up to 26% spheroidization of a 300-500 /xm powder was measured for an alumina feed rate of up to 140 gm/hr. It is evident from this work that countercurrent spheroidization can achieve not only higher yields of spheroids but also spheroidization of a larger size range of solid. 

Tsuzuki and McCormick have shown that suitably designed reduction reactions can also be used for the synthesis of ultraline oxide powders. In their study, ultrafine powder particles of Cr203 were prepared by the reduction of Na2Cr207 with sulfur. 

Particles of microcrystalline size can also be obtained by spray-drying procedures, resulting in a porous, free-flowing, easily wetted, essentially monodispersed powder. With proper control of process variables, spherical particles are obtained that may be coated with agents to aid suspension and promote stability. However, the process is not normally considered for the preparation of ultrafine powders. 

Betametasone EP PC Ultrafine powder to be used for preparation of solid dosage forms to achieve a satisfactory rate of solution... 

Boron. - Raman spectroscopy was used to characterise ultrafine CaB6 powder, prepared from CaCl2 and NaBH4 at 500°C.36 Ab initio and DFT calculations of vibrational wavenumbers were reported for M+B7, where M = Li, Na, K, Rb, Cs.37... 

Several fluid bed processes are under development for production and encapsulation of nanoparticles, for example, WC-Co composites, bioceramics (i.e., calcium phosphate hydroxyapapite), carbon encapsulation of iron magnetic nanoparticles, and carbon nanotubes. These nano- or ultrafine powders have broad industrial and pharmaceutical applications. Production processes usually include solution preparation (sol-gel), drying, calcination, and sintering. The last three steps may be realized in a fluidized bed, but fluidization of nano- and ultrafine powders is very difficult because of strong interparticle forces. 

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