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.
Highlights
The volume expansivity of pure substances is defined as [1]: 1 V V T P (1)It is a parameter that is used to measure the volume expansivity of pure substances and is defined at constant pressure, P
The negative thermal expansion coefficient of the glassy phase and the positive thermal expansion coefficient of the crystalline phase are expected to cancel out each other leading to a zero thermal coefficient material
The first two laws of thermodynamics was used and reversible changes were assumed in order to obtain Equation (10)
Summary
The volume expansivity of pure substances is defined as [1]:. It is a parameter that is used to measure the volume expansivity of pure substances and is defined at constant pressure, P. In the field of materials science, the property of linear coefficient of thermal expansion is an important consideration in materials selection and design of products This property is used to account for the change in volume when the temperature of the material is changed. In some cases it can be negative. ZerodourTM [3] is a glass-ceramic material that can be controlled to have zero or slightly negative thermal coefficient of expansion and was developed by Schott Glass Technologies. It consists of a 70 - 80 wt% crystalline phase with high-quartz structure. A dense, optically transparent and zero-thermal expansion material is necessary in these applications since any changes in dimensions as a result of the changes in the
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