3.091_Notes_7 - LN7 3.091 Introduction to Solid State...

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LN–7 1 3.091 – Introduction to Solid State Chemistry Lecture Notes No. 7 GLASSES * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Sources for Further Reading: 1. Kingery, W.D., Bowen, H.C., and Uhlmann, D.R., Introduction to Ceramics , 2nd Edition, John Wiley and Sons, New York, 1976. 2. Guy, A.J., Essentials of Materials Science , McGraw-Hill, 1976. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 1. INTRODUCTION When a liquid phase is cooled to below its freezing temperature, it usually transforms into a crystalline solid, i.e. it crystallizes. Some liquids, because of complex molecular configuration or slow molecular transport, do not “crystallize” (assume an ordered configuration) upon being cooled to low temperatures, but instead form a rigid disordered network, known as glass, which is very similar in structure to that of the liquid. Most metals and alkali halides crystallize easily upon cooling through the freezing temperature because the structural rearrangement from the liquid to the crystalline state is simple and bonding is nondirectional. At temperatures just above the freezing temperature, most metals and molten salts have fluidities approximately equivalent to that of water at room temperature. Thus, the required structural rearrangements can take place relatively easily. In contrast, many inorganic silicates form glasses upon cooling because the fluidity at and even above the freezing temperature is very low. This glass formation is related to the high silicon-oxygen single-bond energies and the directional bonding requirements imposed by sp 3 hybridization of silicon. The disordered liquid cannot flow easily and thus cannot undergo the rearrangements required for crystallization. Moreover, the
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LN–7 2 fluidity decreases very rapidly as the temperature is lowered. [For large organic and polymeric liquids, crystallization is difficult because of their chain lengths. In polymer systems the intermolecular bonding (Van der Waals) is weak and expected to permit individual molecules to readily slide past one another. Thus, it is the “difficult” structural rearrangement required for crystallization that induces glass formation in these systems.] The fluidity of liquids (the reciprocal of the viscosity) is a measure of their ability to flow. In turn, the viscosity ( η ) (fig. 1) is a measure of their resistance to flow. Viscosity is A v x F τ = F / A [N/m 2 ] (shear stress) τ z v x / z [sec -1 ] (shear strain rate) η = τ v x /z [ N.s m 2 ] ( ) viscosity Figure 1 Dimensional analysis of viscosity formulated as the ratio of an applied shear stress to the resultant shear-strain rate - its dimensions are Newton seconds per meter 2 . Liquid flow, involving the motion of molecules past one another, requires the breaking and making of new bonds. Thus the fluidity, like chemical reactions and diffusion in solids (to be discussed later) is expected to exhibit an exponential temperature dependence and may be modelled as: 1 h T e * ǒ Bond Energy kT Ǔ It is noteworthy that inorganic glasses occur in nature as do organic glasses (such as the sap from trees) whereas metallic glasses do not occur naturally and have been manufactured only within the last two decades. The major obstacle to achieving metallic
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3.091_Notes_7 - LN7 3.091 Introduction to Solid State...

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