Density and Thermal Expansion Behaviour

The density of material (p) is defined as the mass of substance per unit volume i.e.. 

From graph 5.18, we obtain three important parameters  

The expansion of glass is related to the non-harmonic vibration of particles. Particularly, for oxide glasses, the expansion can be determined by taking attraction between ions into account  

Usually, the potential well is not parabolic as Leonard Jones potential shows more resistance to the compression than the tension. Therefore, thermal vibrations tend to drive atoms apart causing thermal expansion. As the time of heat treatment increases, the thermal vibration amplitude within the glass network increases. The general increase in volume with temperature is mainly attributed to the increased amplitude of atomic vibrations about a mean position (Fig. 5.19) i.e.. 

19 Potential distance curve showing the cause of thermal expansion 

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