Ultrafine powders

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. 

This reduction can also be carried out with molecular hydrogen and as such is probably not of any commercial interest. However, it is suited for the study of the catalytic properties of the ultrafine powders and serves as a characterization and optimization technique for the titanium nitride nanoparticles in this study. 

Sonochemical reduction of aqueous solutions containing ammonium dichromate and potassium permanganate produced ultrafine powders of Cr203 and... 

Catalytic activity and electrochemical performance generally increase as the NiO and YSZ particle sizes are reduced. However, ultrafine powders are prone to agglomeration during the milling and mixing process the distributions of the phases (and hence the percolation threshold and many other important properties) are determined by the agglomeration size, not by the primary particle size. 

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]. 

Sonochemistry affords a technology which can be used to generate ultrafine powders and nanostructured materials of significant commercial interest which encompass both amorphous and crystalline properties [152]. 

Fig. 8.1.4 Schematic illustration of the evaporation apparatus with electron beam heating, DP and MBP mean diffusion pump and mechanical booster pump, respectively. (Reprinted from J Cryst Growth, 56 S Iwama et a].. Ultrafine powders of TiN and AIN produced by a reactive gas evaporation technique with electron beam heating, pp 265-269. Copyright 1982, with permission from Elsevier Science )...

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 ... 

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