Silicate glass-ceramics properties

Chemical Properties. The chemical durability is a function of the durability of the crystals and the residual glass. Generally, highly siliceous glass-ceramics with low alkali residual glasses, such as glass-ceramics based on quartz and (3-spodumene, have excellent chemical durability and corrosion resistance similar to that obtained in borosilicate glasses. 

W. Donald, B. L. Metcalfe, D. J. Wood, J. R. Copley, The preparation and properties of some lithium zinc silicate glass-ceramic, J. Mater. Sci., 24, 3892-903(1989)... 

M. R. DeSocio, in Effect of Potassium Content on the Properties of Lithium Silicate Glass-Ceramics, M. S. Thesis, Alfred University, 1985. 

Since phosphate glass-ceramics exhibit some properties that are more favorable and others that are less favorable than those of silicate glass-ceramics, their special characteristics should be addressed at the beginning of this section. Most of these features are attributed to the different nucleation and crystallization conditions of phosphate glass-ceramics compared with those of silicate glass-ceramics. The glass structure provides the basis for these characteristics. Therefore, the structures of the two glass-ceramics fiunilies must be addressed. 

McMillan P.W, Philips S.V., and Partridge G., "The Structure and Properties of a Lithium Zinc Silicate Glass-Ceramic,"/. Mater. Sci., 1, 269-79 (1966). 

Silicon shows a rich variety of chemical properties and it lies at the heart of much modern technology/ Indeed, it ranges from such bulk commodities as concrete, clays and ceramics, through more chemically modified systems such as soluble silicates, glasses and glazes to the recent industries based on silicone polymers and solid-state electronics devices. The refined technology of ultrapure silicon itself is perhaps the most elegant example of the close relation between chemistry and solid-state physics and has led to numerous developments such as the transistor, printed circuits and microelectronics (p. 332). 

Fluorosilicates. Compared to the simple silicates, these crystals have more complex chain and sheet structures. Examples from nature include hydrous micas and amphiboles, including hornblende and nephrite jade. In glass-ceramics, fluorine replaces the hydroxyl ion fluorine is much easier to incorporate in glass and also makes the crystals more refractory. Four commercial fluorosilicate glass-ceramic compositions and their properties are listed in Table 2. 

Oxides. Although not widespread commercially, glass-ceramics consisting of various oxide crystals in a matrix of siliceous residual glass offer properties not available with more common silicate crystals. In particular, glass-ceramics based on spinels and perovskites can be quite refractory and can yield useful optical and electrical properties. 

Rather conventional means for the manufacturing of hollow microspheres with diameters between 1 and 1000 pm have been developed [11.9]. Methods include spray drying and dripping as well as emulsion or suspension techniques. The microspheres feature low effective and bulk densities coupled with high specific surfaces. Typical wall thicknesses are in the range 1-10% of the diameter. Potential wall materials include glass, ceramic and mixed oxides, silicates and aluminosilicates, polymers and polycondensates, and metals. Surface phenomena, which may be modified by chemical reactions, additives, and/or post-treatments, play an important role for microsphere formation, properties, and stability. Fig. 11.12 is the photomicrograph of a calcined hollow microsphere [11.9]. 

A great variety of silicate matrices has been considered for the fabrication of fibre-reinforced glass and glass-ceramic matrix composites [4-6,29-31]. Typical matrices investigated are listed in Table 1. Table 2 gives an overview of different composite systems developed and some of the most remarkable properties achieved. 

E. Bernardo, G. Scarinci, S. Hreglich, Mechanical Properties of Metal-Particulate Lead-Silicate Glass Matrix Composites Obtained by Means of Powder Technology, J. Europ. Ceram. Soc. 23, 1819-1827 (2003). 

In all cases, the composite systems to be considered contain a silicate glass or glass-ceramic phase. Aspects of their processing, microstmctural characterisation, properties andappKca-tions are discussed and relevant references are provided. 

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