chapter  2
16 Pages

Thermodynamic Properties

Writing a chapter on thermodynamics of glass melts raises the question of how to relate it to other chapters in this book. If irreversible thermodynamics is included, this alone should be more than enough for one chapter. If the treatment were restricted to reversible thermodynamics, it would still include acid-base and redox properties, solubility of gases, transport phenomena, phase diagrams etc., to which, however, separate chapters are already devoted. Thus, what primarily remains is to try to compile an overview of thermodynamic constants like the heat of a reaction or enthalpy,

H

; the degree of disorder in the system; the available part of the enthalpy, i.e., the free enthalpy or Gibbs free energy,

G

; the temperature derivative of the enthalpy or heat capacity,

C

; and the deviation from ideality due to mixing or activity. As an alternative to activity, the corresponding mixing functions

H

,

S

,

and

G

could be treated. Again,

H

,

S

,

G

and

C

values for pure compounds can be found in thermodynamic tables. Thus, in general, deviations from ideality due to mixing remain to be considered. If the treatment is further restricted to areas of interest to the glass industry, only the effect of deviation from ideality on the liquidus temperature of multicomponent silicate glasses remains; binary and ternary phase

The heat capacity, that is, the change in enthalpy for a given change in temperature, is of limited concern to the industry, but is used to transform accessible enthalpy data (usually given at NTP) to a temperature of interest. Some attention should be paid to how a published value is defined; the mean heat capacity, , is a linearization of the molar heat capacity,

C

, over a temperature range

T

.