Ceramic fibre

At the start of this Chapter, an essay by Peter Day was quoted in which he lauds the use of soft chemistry , exemplifying this by citing the use of organometallic precursors for making thin films of various materials used in microelectronics. The same approach, but without the softness, is increasingly used to make ceramic fibres here, ceramic includes carbon (sometimes regarded as almost an independent state of matter because it is found in so many forms). 

Turning now to other types of ceramic fibre, the most important material made by pyrolysis of organic polymer precursors is silicon carbide fibre. This is commonly made from a poly(diorgano)silane precursor, as described in detail by Riedel (1996) and more concisely by Chawla (1998). Silicon nitride fibres are also made by this sort of approach. Much of this work originates in Japan, where Yajima (1976) was a notable pioneer. 

Refactory ceramic fibres or special purpose fibres, with the exception of those specified elsewhere in the Approved Supply List (man-made vitreous (silicate) fibres with random orientation with alkaline oxide and alkali earth oxide (Na20 + K2O + CaO + MgO + BaO) content less than or equal to 18% by weight)... 

We will confine ourselves to those applications concerned with chemical analysis, although the Raman microprobe also enables the stress and strain imposed in a sample to be examined. Externally applied stress-induced changes in intramolecular distances of the lattice structures are reflected in changes in the Raman spectrum, so that the technique may be used, for example, to study the local stresses and strains in polymer fibre and ceramic fibre composite materials. 

The Geramn air threshold value (technical reference concentration TRK) for biopersistent mineral wools and ceramic fibres 250,000 t/m (certain areas 500,000 f/m (German TRGS 900/901). 

Technical advantage/fimction Ceramic fibres are used in automotive catalytic converters as bearing and adjustment materials for the catalytic converter (monolith), where the chemical reactions for exhaust cleaning take place. They are also used for thermal and acoustic insulation. Series-tested ceramic fibre substitutes for converter-specific usage conditions are not yet available... 

Market and actors In Germany there is one manufacturer of suitable ceramic fibres (three in Europe and two in Japan). Workers in vacuum forming, modular stmcture and punching plants as well as other plants, where ceramic fibre products are installed and used, also handle ceramic fibres. 

Innovation process Unlike silencers, a substitute for ceramic fibres in catalytic converters is not yet available because very stringent technical requirements have to be met. The test cycles and clarification of the technical and other properties (occupational health and safety and environmental protection) are also correspondingly extensive. Solutions are being developed in cooperation with manufacturers, users and official bodies. 

Innovation drivers An important impulse was the classification of ceramic fibres as carcinogenic category K2 (Directive 97/69/EC), which has been applicable since January 1998. Since July 2001 there has been a ban on the marketing of ceramic fibres for the general public . An impending ban also for the industrial sector caused alarm in the automotive industry. A scientific study by Wuppertal University" also directed attention on the subject of ceramic fibres in catalytic converter recycling . 

Direction of innovation (from the standpoint of market actors - 6al For automotive and catalytic converter manufacturers the direction was clearly away from ceramic fibres classified as K2 as per the 1998 EU classification. Official bodies (especially FIOSH) were consulted for assessing the hazardousness and were thus included in developing substitutes. A development was thus accelerated with the highest possible security of direction. 

Direction of imovation (from the standpoint of the SubChem research group - 6b ) The substitution of ceramic fibres in the constmction of catalytic converters would avoid exposure (manufacture and recycling) to potentially carcinogenic fibres. Inclusion of specialised official bodies in the evaluation of alternative materials is one step towards greater security of direction. 

Chemical vapour deposition (CVD) is employed to prepare adherent films of controlled composition and thickness. Protective coatings, micro- and opto-electronics, ceramic fibres and ceramic-matrix composites production represent the usual applications of this technique, which allows surfaces of complex geometry to be uniformly coated. 

Lawrence, C. W. (1990). Acoustic microscopy of ceramic fibre composites. D. Phil. 

Reinforcements in the form of continuous fibres, short fibres, whiskers or particles are available commercially. Continuous ceramic fibres are very attractive as reinforcements in high-temperature structural materials. They provide high strength and elastic modulus with high temperature-resistant capability and are free from environmental attack. Ceramic reinforcement materials are divided into oxide and non-oxide categories, listed in Table 3.1. The chemical compositions of some commercially available oxide and non-oxide reinforcements are given in Table 3.2 and Table 3.3. 

Ceramic oxide fibres, both continuous and discontinuous, have been commercially available since the 1970s, and processing and microstructure control are very important in obtaining the desired properties. Among the desirable characteristics in any ceramic fibre for structural applications are ... 

Commercially available non-oxide ceramic reinforcements are in three categories continuous, discontinuous, and whiskers. The great breakthrough in the ceramic fibre area has been the concept of pyrolysing polymers under controlled conditions, containing the desired species to produce high-temperature ceramic fibres. Silicon carbide fibre is a major development in the field of ceramic reinforcements. 

There are other promising ceramic fibres, e.g. boron carbide and boron nitride. Boron nitride fibre has the same density (2.2 g cm-3) as carbon fibre, but has a greater oxidation resistance and excellent dielectric properties. Boron carbide fibre is a very light and strong material. 

Laffon, C., Flank, A.M. and Lagarde, P., et al., Study of Nicalon-based ceramic fibres and powders by EXAFS spectrometry, X-ray diffractometry and some additional methods , J. Mat. Sci, 24, 1503 (1989). 

SiC fibres (SiCf) are some of the more refractory ceramic fibres, and perform very satisfactorily in oxidising environments. A key problem in the development of SiC fibre composites is thermal degradation of the SiC fibre (Johnson et al 1987). The properties of SiC fibre start degrading above 600°C because... 

Caputo, A. J., Lackey, W.J. and Stinton, D.P., (1985), Development of new, faster process for the fabrication of ceramic fibre reinforced ceramic composites by chemical vapour infiltraton , Ceram. Eng. Sci. Proc., 6(7-8), 694-706. 

Janes, Neumann and Sethna ° reviewed the general subject of solid lubricant composites in polymers and metals. They pointed out that the reduction in mechanical properties with higher concentrations of solid lubricant can be offset by the use of fibre reinforcement. Glass fibre is probably the most commonly used reinforcing fibre, with carbon fibre as a second choice. Metal and ceramic fibres have been used experimentally to reinforce polymers, but have not apparently been used commercially. To some extent powders such as bronze, lead, silica, alumina, titanium oxide or calcium carbonate can be used to improve compressive modulus, hardness and wear rate. 

Not burning, even in pure Glass and ceramic fibres, inorganic... 

G. A. Kriegsmann and B. A. Wagner, Microwave heating of carbon-coated ceramic fibres a mathematical model, IMA Journal of Applied Mathematics, 55, 243-255 (1995). 

The reactor is constructed of Inconel 600 alloy and has an international diameter of 15 cm at the bottom and 30 cm in the free-board area. Fluidizing gas (air and steam mixture) is introduced through a specially designed Inconel distributor plate. The reactor is insulated with Kaowool ceramic fibre. The bed materials consist of a graded sand fraction of 60—80 mesh. 

In high-temperature insulations (up to 1250—1450 °C), use is made of fibres based on the system AI2O3—Si02 containing 50—75 wt. % Al203. The material is also known as refractory or ceramic fibre. 

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