Crystallisation of Glasses

Lasocka parameter, activation energy of crystallisation, frequency factor, and Avrami parameter can be evaluated for different systems. Along with these parameters, crystallisation constants as a function of different temperatures as well as different heating rates (p) should be studied. Tg and its dependence on heating rates can be given by the following original empirical Lasocka equation. 

This may be due to the fact that thermodynamic factors such as (Tc - 2/)/2/ and - Tg)ITai are involved in the evaluation of ky(T) and ky(T) and parameter Hr is used to determine kh(T). Hence, kj,(T) cannot be regarded as stability criteria in the present glasses. Hence, these parameters play a substantial role in determining the nature of glasses. In addition to this, the concept of fluctuation free volume, the transition of an amorphous substance from a liquid to solid state, is accompanied with a decrease in the fraction of fluctuation free volume, /g. 

According to the Frenkel equation, the abrupt change in coefficient of thermal expansion Of which is associated with an increase in number of microvoids (free volume) upon heating of glass can be determined merely by the fraction fgi 

Example 1 Crystallisation kinetics of ML, CL, SL, and BL glass The non-isothermal method is widely used to obtain various characteristic parameters from DTA and the glass sample is heated up at a fixed heating rate. The 

Equations (5.41)-(5.43) yield the crystallisation constants as shown in Table 5.8. It is quite observable from Table 5.8 that ky T) and k T) are quite independent of 

On the other hand, the annealed glass, having been cooled to ambient, can be reheated above Tg to reaccess the supercooled liquid, again for shaping purposes, during which time again imwanted devitrification may occur. 

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As explained earlier in the chapter on Electroceramics, glass ceramics are made by the controlled crystallisation of glass. Since 1967 research has been carried out into special glass formulas for bioceramic applications. This has resulted in a material which forms a bond on the line between the implant and the host tissue, the so-called bio-glass with the following composition, expressed in mass percentages ... 

Crystallisation of glasses in the Nd-Mg-Si-O-N system to form glass-ceramic composites has been investigated. Morrissey et al. (1990) showed that heat treatment at a single temperature resulted in only a small increase in hardness for a 12 24 64 Nd Mg Si composition, but two-stage heat treatments resulted in a much higher increase. They found that the optimum nucleation temperature was related to the glass transition temperature of the materials (usually 7g... 

Keir discovered the crystalline hydrate of sulphuric acid H2SO4, HgO, the m.p. of which he found to be 45° F. He also published on native soda from Bengal, nitric acid, the decomposition of common salt and sal ammoniac by copper, the crystallisation of glass, and the production of brandy in Chatra (Ramgur) from the flowers of a tree mdhwah)J... 

Controlled inducing of the phase separation of a glass is useful for the determined partial crystallisation of glass components and will be described in Sect. 2.3. 

Partial crystallisation of glasses has been described by many authors, see for instance Vogel [538], Scholze [449] and Hinz [225], The process comprises both steps the nucleation and the crystal growth. 

Lead formate separates from aqueous solution without water of crystallisation. It can therefore be used for the preparation of anhydrous formic acid. For this purpose, the powdered lead formate is placed in the inner tube of an ordinary jacketed cond ser, and there held loosely in position by plugs of glass-wool. The condenser is then clamped in an oblique position and the lower end fitted into a receiver closed with a calcium chloride tube. A current of dry hydrogen sulphide is passed down the inner tube of the condenser, whilst steam is passed through the jacket. The formic acid which is liberated... 

If difficulty is experienced in inducing the first crude crop of the a-glucoside to ciystallise, place a few drops of the solution on a watch glass and expose freely to the air, with occasional scratching meanwhile keep the main volume of the solution securely corked in the conical flask. After an interval of varying length (possibly several days), partial crystallisation occurs in the material on the watch-glass. Then seed the solution with this material crystallisation of the first main crop will rapidly follow. 

Using present ideas of glass structure, frits can be formulated to have maximum durability. Zirconia enamels have a lower than optimum acid resistance, but titania enamels have superior durability at low pH values. Encouraging crystallisation in the enamel enhances mechanical strength and therefore chemical resistance by reducing the incidence of damage sites which act as foci for attack. The addition of an extra (clear) coat on to the top most coat of enamel gives superior chemical resistance". 

Romero, M., Rawlings, R. D. Rincon, J. Ma. 1999. Development of a new glass ceramic by means of controlled vitrification and crystallisation of inorganic wastes from urban incineration. Journal of the European Ceramic Society, 19, 2049-2058. 

At the onset of biomineralization the mechanism of phosphate and silica deposition is essentially the same. Both start with a highly hydrated amorphous phase having glass-like physical-chemical properties. The kinetics of crystallisation of the two differs. ACP will rapidly alter in the direction of apatite in hours or days, whereas amorphous silica requires thousands of years or higher temperatures to yield quartz. 

The explosion of the glassy material at—183°C was attributed to crystallisation of the glass. 

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