Ultrafine products, preparation

This method is one of the dry methods in which no chemical reaction is involved. Preparation of ultrafine particles by physical vapor deposition (PVD) dose not require washing and calcination, which are indispensable for chemical preparation such as in CP and DP methods. As waste water and waste gases are not by-produced, the arc plasma (AP) method is expected to grow in popularity as one of the industrial production methods for gold catalysts and as a clean preparation method. 

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. 

As mentioned in Chapter 13, ultrafine materials have a number of excellent characteristics therefore it is of interest to prepare HAP sized at the nano level. Some of the methods mentioned above can yield ultrafine particles of HAP, and progress has been made in the control of particle size and coalescence of the products however, problems relating to particle appearance, dispersion and size distribution etc. have not yet been completely solved. 

Zhao, Bin, Liu, Zhijie, Cai, Mengjun and Hu, Liming (1997). A study on the preparation of ultrafine copper powder by hydrazine hydrate reducing and stability of the product. J. Eastern China University of Technology, 23(3) 372-376 (in Chinese). 

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). 

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. 

Vaporisation of materials in plasmas has been extensively studied and is becoming of increasing interest from the commercial point of view, esp aally for the preparation of ultrafine powders characterised by enhanced chemical activity, and also as a means of parating valuable products from otherwise intractable materials From the ceramics point of view, plasma processing offers the possibility of preparing powders for subsequent sintering which either cannot be prej ied in any other way, or have superior properties to powders prepared by conventional means. The process... 

Clay and mineral fillers have been used for reducing production costs and improving the comprehensive water absorbing properties of superabsorbent materials For example, a poly(acrylic acid)/mica superabsorbent has been synthesized with water absorbency higher than 1100 g H20/g In a typical method of preparation, acrybc acid monomer is neutralized at ambient temperature with an amount of aqueous sodium hydroxide solution to achieve 65% neutralization (optimum) Dry ultrafine (<0.2 tm) mica powder (10 wt%) is added, followed by cross-linker N,N-methylene-bisacrylamide (0.10 wt%) and radical initiator, potassium persulfate The mixture is heated to 60-70°C in a water bath for 4 h. The product is washed, dried under vacuum at 50°C, and screened. 

One of the most important developments in recent years has been the technology to extrude extremely fine filaments of less than 1.0 denier while maintaining all of the strength, uniformity, and processing characteristics expected by textile manufacturers and consumers. This product development began with the preparation of conjugated bicomponent filaments that were post-processed to split into ultrafine fibers. The process technology later matured and has been applied to polyesters, polyamides, polypropylene, and polyethylene and polyphenylene sulfide. 

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