Pre-ceramic powders

Advanced Methods Towards Making Pre-Ceramic Powders... 

The use of sintering for the induration of ceramic products (bricks, pots, vases, etc., see also Chapters 2 and 3) is quite old but through the centuries, even though empirically improved, was exclusively carried out in batch kilns. Continuous heat treatment of ceramic, powder metal, and other pre-agglomerated parts is less than 150 years old. 

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. 

PoLLMANN D., Leis F., Tolg G., Tschopel P. and Broekaert (. A. C. (1994) MiJtielement trace determinations in AI2O3 ceramic powders by inductively coupled plasma mass spectrometry with special reference to on-line pre-concentration, Spectrochim. Acta, Part B 49 1251-1258. 

Smith and coworkers (6-10) have recently conducted a series of Investigations on the extraction and precipitation of a variety of organic polymers and inorganic materials including ceramic and preceramic powders. Silica powders were precipitated by rapid expansion following their extraction with supercritical water at 580-590 bar and pre-expansion temperatures higher than 723 K. The particle size and agglomeration characteristics of these powders were found to be sensitive to the concentration of silica before the expansion as... 

Selection of the most perfect and reliable electrolyte-anode adjoining method based on CIP techniques was performed as follows. Anode and electrolyte layers in various conditions (press-powder, green compact, partially sintered compact, or sintered ceramics) were jointly isostatically pressed at pressure values from 0.05 to 0.8 GPa. Since the electrolyte layer should be much thinner than the anode one, such joint isostatical pressing, as a rule, had a nature of adpressing the electrolyte powder onto a pre-formed anode compact. The two-layered plate produced by such a way was sintered at 1500°C. Thereafter a cathode layer was build up in a similar manner. Alternatively the cathode layer was applied by molding a slip prepared from a mixture of the powder and alcohol. A green cathode layer on the plate was sintered at 1200°C. 

Satisfactory results were obtained by adpressing an electrolyte layer onto a pre-compacted anode layer or at slip painting the powder cathode onto a partially sintered electrolyte. A flowchart of the developed process for manufacturing ceramic membranes using the CIP procedure is presented in Fig. 1. 

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