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Ch6 Viscosity

Ch6 Viscosity - CHAPTER 6 Viscosity of Glass Forming Melts...

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CHAPTER 6 Viscosity of Glass Forming Melts 1 INTRODUCTION The kinetic model of glass formation indicates that the temperature dependence of the viscosity plays a major role in determining the ease of glass formation for any melt. Glasses are most easily formed if either (a) the viscosity is very high at the melting temperature of the crystalline phase which would form from the melt, or (b) if the viscosity increases very rapidly with decreasing temperature. In either case, crystallization is impeded by the kinetic barrier to atomic rearrangement which results from a high viscosity. In addition to controlling the ease of glass formation, viscosity is also very important in determining the melting conditions necessary to form a bubble-free, homogeneous melt, the temperature of annealing to remove internal stresses, and the temperature range used to form commercial products. The viscosity also determines the upper use temperature of any glass object and the conditions under which devitri- fication (crystallization) may occur. The very high viscosity encountered in the glass transformation range leads to viscoelastic behavior, and to time dependence in many of the properties of the melt. 2 VISCOSITY DEFINITIONS AND TERMINOLOGY Viscosity is a measure of the resistance of a liquid to shear deformation, i.e., a measure of the ratio between the applied shearing force and the rate of flow of the liquid. If a tangential force difference, F, is applied to two parallel planes of area, A, which are separated by a distance, d, the viscosity, q, is given by the expression: Fd v=- Av 111
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112 Chapter 6 where v is the relative velocity of the two planes. If the velocity varies directly with the applied shear force, the viscosity is independent of force and the liquid is said to behave as a Newtonian liquid. At high shear stresses, many glass forming melts exhibit an apparent decrease in viscosity with increasing shear stress. This form of non-Newtonian behavior is known as pseudoplastic flow, or shear thinning, and is important in high shear rate forming processes. The original unit for viscosity was based on the cgs system, where the viscosity is given in dyne s cm-2. This unit, which is termed a Poise and given the symbol P, is used in virtually all literature prior to 1970 and is still used extensively throughout the glass industry. In SI units, which have replaced cgs units in much of the recent literature, viscosity is given in N s m-2, or, since a Pascal is a N m-2, the viscosity is reported in Pa s. Since 1 Pa s = 10 P, the conversion of viscosity data from one unit to the other is very straightforward, The viscosity of water at room temperature is = 0.01 P, or 0.001 Pa s. Fluidity is the reciprocal of the viscosity. A melt with a large fluidity will flow readily, whereas a melt with a large viscosity has a large resistance to flow. While fluidity is often used in dealing with ordinary liquids, virtually all literature dealing with glass forming melts discusses flow behavior in terms of the viscosity.
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Ch6 Viscosity - CHAPTER 6 Viscosity of Glass Forming Melts...

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