On the Thermal Capacity of Solids.

Area of Science:

• Thermodynamics
• Solid-state Physics
• Materials Science

Background:

• The concept of thermal capacity is often misinterpreted as a measure of storable thermal quantity.
• Heat is a transient energy form, not a component of internal energy, making thermal capacity an inadequate descriptor for storage.
• Entropy, however, represents a storable quantity, necessitating a re-evaluation of thermal properties.

Purpose of the Study:

• To redefine and elucidate the concept of entropy capacity as the physically accurate measure of storable thermal quantities.
• To demonstrate the utility of entropy capacity in understanding the thermal behavior of solids and other materials.
• To highlight the advantages of explicitly using entropy capacity for fundamental insights into thermal processes.

Main Methods:

• Conceptual analysis of thermal capacity versus entropy capacity.
• Application of entropy capacity to analyze low-temperature contributions (phononic and electronic) in solids.
• Examination of entropy capacity in models like the Debye model, thermochemical modeling of carbon allotropes, and caloric materials.
• Analysis of an electrocaloric cycle in barium titanate using entropy capacity.

Main Results:

• Entropy capacity is identified as the correct physical coefficient for storable thermal quantities, functioning as a susceptibility.
• The inverse of entropy capacity relates temperature changes to entropy changes, analogous to fluid level responses.
• Implicit uses of entropy capacity are clarified, particularly in estimating solid entropy and in the thermoelectric figure of merit.
• Explicit application of entropy capacity provides fundamental understanding in diverse systems, including caloric materials and phase transitions.

Conclusions:

• Entropy capacity offers a more fundamental and intuitive understanding of thermal processes compared to thermal capacity.
• Its explicit use enhances the analysis of thermal behavior in solids, caloric materials, and phase transitions.
• Entropy capacity serves as a key concept for a deeper comprehension of thermodynamics in materials.