Ceramic materials pyrolytic ceramization

One potential solution to these problems, suggested some 20 years ago by Chantrell and Popper (1), involves the use of inorganic or organo-metallic polymers as precursors to the desired ceramic material. The concept (2) centers on the use of a tractable (soluble, meltable or malleable) inorganic precursor polymer that can be shaped at low temperature (as one shapes organic polymers) into a coating, a fiber or as a matrix (binder) for a ceramic powder. Once the final shape is obtained, the precursor polymer can be pyrolytically transformed into the desired ceramic material. With careful control of the pyrolysis conditions, the final piece will have the appropriate physical and/or electronic properties. 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

This criterion, which is product rather than precursor-property driven, is critical to the design and synthesis of new precursors. The need for high ceramic yields arises because of the excessive volume changes associated with pyrolytic conversion to ceramic materials. Scheme 1 illustrates these changes for a SiC precursor with an 80% ceramic yield of phase pure SiC (3.2 gml-1). Most precursors densities are close to 1 gml-1, whereas most Si ceramic densities range from 2.5 to 3.5 gml-1. 

Figure 9.1 Definitions of the various species used as starting materials and condensation products during pyrolytic conversion to ceramics.

Scheme 31 Upper panel-. Complex formation between acetylenes and cobalt carbonyls. Lower panel Formation of polymer complexes 81 and 82 via metal complexation and transformation of the complexes into soft ferromagnetic materials 83 and 84 by pyrolytic ceramization...

This chapter gives an introduction to the preceramic polymer route to ceramic materials and focuses on the reasons why this new approach was needed and on the chemical considerations important in its implementation, with examples from research on organosilicon polymers. Novel polysilazanes have been prepared by the dehydro-cyclodimerization reaction, a new method for polymerizing suitably substituted cyclooligosilazanes. The living polymer intermediate in this reaction has been used to convert Si-H-containing organosilicon polymers that are not suitable for pyrolytic conversion to ceramics into useful preceramic polymers. 

To confer electroconductivity upon porous ceramics these materials were coated with a layer of pyrolytic carbon. The deposition of carbon was carried out in the gas phase by pyrolysis of natural gas. The ceramic materials were placed in a quartz reactor under 50 mL/min flow of natural gas. The pyrolysis took place at 900°C for 30 min. 

A development program is underway at the ICPP to examine methods of treating WCF calcine some methods retain the granular nature of the WCF product while others convert it to a more massive form. Methods of treatment include (1) conversion to a cermet by the incorporation of the calcine in a metal matrix such as aluminum or lead-tin alloy (2) conversion to a ceramic by the addition of a ceramic material such as clay and sintering (3) incorpoartion into a concrete by the addition of binders or cement and (4) coating of the calcine particles with pyrolytic carbon, glazes, metals, etc. 

Advanced waste form work is also being carried out in the Ceramics and Graphite Section at PNL, where high temperature gas-cooled reactor fuel technology is applied to waste solidification. Waste particles are coated with pyrolytic carbon followed by a cover coat of silicon carbide. These coated particles would then be placed in a matrix of inert material contained in a canister of yet another material. 

The Yajima et al. process [3] possesses general applicability to the preparation of ceramic materials from polymeric and oligomeric precursors via pyrolysis. In some cases even monomeric units can be used as precursors. Thus, the invention of the Yajima et al. process [3] has generated tremendous research activities in the synthesis of precursors and their pyrolytic conversion to ceramic powders and/or fibers, leading to the fields of what generally are known now as preceramic polymer chemistry and polymer pyrolysis technology [1,6]. 

PYROLYTIC CARBON is a CARBON MATERIAL deposited from gaseous hydrocarbon compounds on suitable underlying substrates (CARBON MATERIALS, metals, ceramics) at temperatures ranging 1000-2500K (chemical vapor deposition). 

Intermediate layers of metals or ceramics have been used to enhance resin bonding for veneers and pontics by a combination of micro-mechanical and mechanical bonding. In one commercial system, the metal surface is cleaned by sandblasting and then a thin layer of silica is pyrolytically deposited on the surface. This bonds to the metal and the layer is then coated with a vinyl silane to enable bonding to the resin. As an alternative, the metal surface is blasted with a combination of alumina and silica, which bonds and fuses to the surface under the kinetic energy of impact. The impacted material layer becomes coloured to allow accurate control. Again this is coated with a vinyl silane. 

Carbon is a commonly used solid electrode material as a substrate for biosensors, particularly in the form of glassy carbon, due to its wide positive potential window, mechanical stability, and low porosity. Carbon film, carbon composite and graphite are other substrates which have been investigated. Carbon film electrodes made from carbon film electrical resistors have been extensively employed by us as electrode substrate [43], obtained by pyrolytically coating a ceramic cylindrical substrate with a thin carbon layer. 

Low-Z ceramics for high heat-flux component materials for fusion reactors have been tested by out-of-pile electron beams and by in-pile TEXTOR limiter tests. While pyrolytic a-BN shows high thermal shock resistivity (and its use on the limiter ion side rules out arcing), the main erosion occurs by sublimation decomposition. There is evidence that superthermal electrons play a significant role in the limiter erosion [143 to 150]. 

Electrodes for electric discharge machining are partially coated with pyrolytic boron nitride in order to prevent undesirable electric discharge between the electrode and a workpiece [330]. A boron nitride layer is used to prevent bonding of a conductive material, applied for indicating wear, to hard tool ceramics [331]. Equipment for applying pyrolytic boron nitride layers has been described [332], and also for applying boron nitride as protective sheets for immersion pyrometers for molten metals [333]. 

Polymer pyrolysis refers to the pyrolytic decomposition of metal-organic polymeric compounds to produce ceramics. The polymers used in this way are commonly referred to as preceramic polymers in that they form the precursors to ceramics. Unlike conventional organic polymers (e.g., polyethylene), which contain a chain of carbon atoms, the chain backbone in preceramic polymers contains elements other than carbon (e.g., Si, B, and N ) or in addition to carbon. The pyrolysis of the polymer produces a ceramic containing some of the elements present in the chain. Polymer pyrolysis is an extension of the well-known route for the production of carbon materials (e.g., fibers from pitch or polyacrylonitrile) by the pyrolysis of carbon-based polymers (54). It is also related to the solution sol-gel process described in the previous section where a metal-organic polymeric gel is synthesized and converted to an oxide. 

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