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The Coming of Materials Science Episode 11

Tham khảo tài liệu 'the coming of materials science episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 380 The Coming of Materials Science between experiment and theory it may well be a prototype of ceramic research programmes of the future. There is no room here to give an account of the many adventures in processing which are associated with modern high-tech ceramics. The most interesting aspect perhaps is the use of polymeric precursors which are converted to ceramic fibres by pyrolysis Section 11.2.5 another material made by this approach is glassy carbon an inert material used for medical implants. The standard methods of making high-strength graphite fibres from poly acrylonitrile and of silicon carbide from a poly carbosilane precursor both developed more than 25 years ago are examples of this approach. These important methods are treated in Chapters 6 and 8 of Chawla s 1998 book and are discussed again here in Chapter 11. Another striking innovation is the creation in Japan of ceramic composite materials made by unidirectional solidification in ultra-high-temperature furnaces Waku et al. 1997 . This builds on the metallurgical practice developed in the 1960s of freezing a microstructure of aligned tantalum carbide needles in a nickelchromium matrix. An eutectic microstructure in Al2O3 GdAlO3 mixtures involves two continuous interpenetrating phases this microstructure proves to be far tougher more fracture-resistant than the same mixture processed by sintering. The unidirectionally frozen structure is still strong at temperatures as high as 1600 C. 9.6. GLASS-CERAMICS In Chapter 7 1 gave a summary account of optical glasses in general and also of the specific kind that is used to make optical waveguides or fibres for long-distance communication. Oxide glasses of course are used for many other applications as well Boyd and Thompson 1980 and the world glass industry has kept itself on its toes by many innovations with respect to processing and to applications such as coated glasses for keeping rooms cool by reflecting part of the solar spectrum. Another .