TITLE:
On Thermodynamic Analysis of Substances with Negative Coefficient of Thermal Expansion
AUTHORS:
Kal Renganathan Sharma
KEYWORDS:
Negative Thermal Expansion; NTE Materials; Helmholz Free Energy; Second Law of Thermodynamics; Speed of Sound; Volume Expansivity
JOURNAL NAME:
Engineering,
Vol.5 No.11,
November
6,
2013
ABSTRACT:
There are reports in the
literature on the discovery of novel materials that were observed to shrink
upon heating. Treatment of these materials in the same manner as the materials with positive
coefficient of thermal expansion can lead to the misinterpretation of the laws
of thermodynamics. This is because volume expansivity is usually defined at constant
pressure. Negative values for volume expansivity can be shown using Maxwell’s
reciprocity relations to lead to negative values for absolute temperature for ideal gas. For real systems,
using Helmholz free energy analysis at equilibrium an expression for the volume
expansivity was derived. It can be seen that this expression would be always
positive for real physical changes, either heating or cooling. Isentropic
volume expansivity is proposed as better suited for analysis of materials with
negative thermal expansion, NTE and composites used in space such as Hubble
telescope and Chandra telescope with zero coefficient of thermal expansion.
This kind of a switch from isobaric to isentropic has precedence in the history
of development of speed of sound.