Glasses ceramic

Glass Type Si02 NU20 CaO Al20j B2O2 Other Characteristics and Applications 

Fused silica 99.5 High melting temperature, very low coefficient of expansion (thermally shock resistant) 

96% Silica (Vycor) 96 4 Thermally shock and chemically resistant—laboratory ware 

Borosilicate (Pyrex) 81 3.5 2.5 13 Thermally shock and chemically resistant—ovenware 

Container (soda-lime) 74 16 5 1 4MgO Low melting temperature, easily worked, also durable 

Glass-ceramics can also be made starting from a powdered glass followed by a single step combined sintering and controlled crystal nucleation and growth steps. This route is the one most relevant to the electroceramics context. 

Glass-ceramics based on the LijO-AljC -SiC can be tailored, principally through varying the alumina content, to have linear thermal expansivities in the range from close to zero to approximately 18 MK-1. The low expansion materials have excellent resistance to thermal shock whilst those with the higher expansivities can be successfully joined to a range of metals. 

The stages in the manufacture of a controlled atmosphere housing for a high current electrical switch are illustrated in Fig. 3.7. 

Glass-ceramics are an important class of materials that have been commercially quite successful. They are polycrystalline materials produced by the controlled crystallization of glass and are composed of randomly oriented crystals with some residual glass, typically between 2 and 5 percent, with no voids or porosity. 

A typical temperature versus time cycle for the processing of a glass-ceramic is shown in Fig. 9.15, and it entails four steps. 

Mixing and melting. Raw materials such as quartz, feldspar, dolomite, and spodumene are mixed with the nucleating agents, usually TiOi or Zr02, and melted. 

Growth. Following nucleation, the temperature is raised to a point where growth of the crystallites occurs readily. Once the desired microstructure is achieved, the parts are cooled. During this stage the body usually shrinks slightly — by about 1 to 5 percent. 

Glass-ceramics offer several advantages over both the glassy and crystalline phases, including these  

Glass-ceramics are distinguished from glasses and from ceramics by the characteristics of their manufacturing processes (see introduction to this chapter 3.4) as well as by their physico-chemical properties. 

Such a = 0 glass-ceramics can be subjected to virtually any thermal shock or temperature variation below 

700 °C. Wall thickness, wall thickness differences, and complicated shapes are of no significance. 

Another technical advantage is the exceptionally high dimensional and shape stability of objects made from these materials, even when the objects are subjected to considerable temperature variations. 

The Zerodur glass-ceramic, whose coefficient of linear thermal expansion at room temperature can be kept at 0.05 x 10 /K (Table 3.4-19), was especially developed for the production of large mirror blanks for astronomical telescopes. Zerodur has further applications in optomechanical precision components such as length standards, and mirror spacers in lasers. With 

Refractory stone. (From W. D. Kingery et al.. Introduction to Ceramics, 2nd Edn., John Wiley, New York, 1976, pp. 783-812.) 

Submicrostmcture of Li20-Al203-Si02 glass—ceramic nucleated with TiOj—approximately 80% crystalline. Small rutile crystals shown in larger crystals of P-spodumene. 

A glass ceramic is a solid that is largely crystalline, made by the crystallisation or devitrification of a glass object of the desired shape. Glass ceramics are therefore composite materials that consist of crystals and some glass. They combine the ease of production of glass with much enhanced thermal and mechanical properties. In this section, the microstmctures of some glass ceramics are described, and the way in which the superior properties are achieved is outlined. 

The two most important factors in glass ceramic production are the composition of the melt and the microstmcture of the final product. These are interrelated, of course. The composition controls the ability of the substance to form a glass with the correct viscosity and workability, as the starting solid is completely glassy in nature. Composition also controls what nuclei can form in the glass and the types of crystal that can grow. Most crystals have a definite crystal habit, and this factor greatly influences the microstructure of the final solid. 

There are two main divisions of polymeric materials thermoplastic and thermosetting. Thermoplastic materials can be formed repeatedly that is, they can be melted and reformed a number of times. Thermosetting materials can be formed only once they cannot be remelted. They are usually strong. 

Although polymers are associated with electrically insulating behaviour, the increasing ability to control both the fabrication and the constitution of polymers has led to the development of polymers that show metallic conductivity superior to that of copper (see Section 13.2.8) and to polymers that can conduct ions well enough to serve as polymer electrolytes in batteries and fuel cells (see Sections 9.2.5 and 9.3.7). 

In this section, polymers will be discussed largely from a structural and microstructural point of view. Several typical and differing polymers are used as examples polyethylene (polythene), nylon and epoxy resins. In addition, elastomers are described, as they differ in a fundamental way from other materials. 

The key innovations in turning optical waveguides (fibres) into a successful commercial product were made by R.D. Maurer in the research laboratories of the Corning Glass Company in New York State. This company was also responsible for introducing another family of products, crystalline ceramics made from glass precursors - glass-ceramics. The story of this development carries many lessons for 

The factors that favour successful industrial innovation have been memorably analysed by a team at the Science Policy Research Unit at Sussex University, in England (Rothwell et al. 1974). In this project (named SAPPHO) 43 pairs of attempted similar innovations one successful in each pair, one a commercial failure - were critically compared, in order to derive valid generalisations. One conclusion was The responsible individuals (i.e., technical innovator, business innovator, chief executive, and - especially - product champion) in the successful attempts are usually more senior and have greater authority than their counterparts who fail . 

Stookey s reflection on a lifetime s industrial research is An industrial researcher must bring together the many strings of a complex problem to bring it to a conclusion, to my mind a more difficult and rewarding task than that of the academic researcher who studies one variable of an artificial system . 

(1994) in Ultra High Purity Base Metals (UHPM-94), ed. Abiko, K. et al. (Japan Institute of Metals, Sendai) p. 1. 

Arunachalam, V.S. and Sundaresan, R. (1991) Powder metallurgy, in Processing of Metals and Alloys, ed. Cahn, R.W. Materials Science and Technology, vol 15, eds. Cahn, R.W., Haasen, P. and Kramer, E.J. (VCH, Weinheim) p. 137. 

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Borax is used in the production of pyrex glass, ceramics, as a flux in soldering and welding, and in laundering to impart a glaze to linen. 

Fluorine is the most electronegative and reactive of all elements. It is a pale yellow, corrosive gas, which reacts with most organic and inorganic substances. Finely divided metals, glass, ceramics, carbon, and even water burn in fluorine with a bright flame. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

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