Ceramics, advanced powder processing

Table 20.1 lists the desirable powder characteristics for advanced ceramics. For most processing methods we want a small particle size. The small size helps shape the product and during densi-fication (sintering) at high temperature, allows higher density bodies at lower firing temperatures. 

Polymer pyrolysis to form advanced ceramics allows the production of highly covalent refractory components (fibers, films, membranes, foams, joints, monolithic bodies, ceramic matrix composites) that are difficult to fabricate via the traditional powder processing route [1-4]. Yajima was the first to demonstrate the feasibility of producing high-strength SiC-based fibers from pyrolysis of polycarbosilane [5]. In this process, a thermoplastic pre-ceramic polymer is first shaped into the desired form, cross-linked into a pre-ceramic network and finally converted into a ceramic material by a pyrolysis process in a controlled atmosphere (Fig. 1). A common feature of the polymer route is the formation of intermediates called amorphous covalent ceramics (ACC) [6]. These are formed after removal of the organic components and before crystallization that occurs at higher temperatures. 

Sigmund WM, Bell NS, Bergstrom L (2000) Novel powder-processing methods for advanced ceramics. J Am Ceram Soc 83 1557-1574... 

In the case of advanced ceramics and powders of nanometric size, the use of the classical selected area diffraction Patterns (SADP) by using different apertures in the electron path in the TEM is very short. In these cases and similarly for Zr02/mullite materials, the very small size of intragranular zirconia formed in the reaction sintered process inside the matrix, reaching 50-300 nm sizes, makes it fully necessary to use higher resolution in the electron diffraction analysis. 

R. Nitzsche, H. Friedrich, G. Boden, W. Hermel, in Elektrokinetic Surface Investigations An Important Technique for Ceramic Powder Processing, ed by R.A. Williams, N.C. De Jaeger. Advances in Measurement and Control of Colloidal Processes. (Butterworth-Heinemann, 1991), pp. 280-291... 

Ceramic Powder Processing Science series Ceramic Powder Science, Advances in Ceramics, Vol. 21, (G. L. Messing, K. S. Mazdiyasni, J. W. McCauley, and R. A. Haber, Eds.) The American Ceramic Society, Columbus, OH, 1987 Ceramic Powder Science II, Ceramic Transactions, Vol. 1, Parts A ... 

R. J. Pugh, Dispersion and Stability of Ceramic Powders, in Surface and Colloid Chemistry in Advanced Ceramics Processing, Marcel Dekker, New York, 1994, Chapter 4. 

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

The preparation of ceramic powders by CVD is a promising and growing field. These powders are used as precursors for the processing of advanced ceramics, or directly as powder for such applications as abrasives. 

M. Heule, S. VuUlemin, and L.J. Gauckler. Powder-based ceramic meso- and microscale fabrication processes , Advanced Materials 15 (2003), 1237-1245. 

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