Heat Capacity Measurement

High-precision calorimetry is a core area of expertise in Ames that is essential for discovery of new caloric materials.

Calorimetry provides critical information about thermodynamic properties of materials. Accurate heat capacity data are vital for evaluation of material suitability for caloric applications. Heat capacity, aka specific heat, is also needed to gauge first-principles theory and modelling results, thus making predictive design of new caloric materials reality. Moreover, calorimetry in the presence of magnetic fields is considered one of the best methods for the indirect measurement of the magnetocaloric effect. Two important magnetocaloric parameters: adiabatic temperature change and isothermal magnetic entropy change can be calculated from the heat capacity data.

When reliably measured, the magnetocaloric parameters obtained from heat capacity data are employed by the consortium researchers to make informed decisions about what material systems are promising for future material development. The knowledge about basic thermodynamics of materials from calorimetric study is also important for research on elastocaloric and electrocaloric materials.
Two different techniques are routinely employed by the Consortium researchers for heat capacity measurements: (1) semi-adiabatic heat pulse method, (2) relaxation method. The semi-adiabatic heat pulse method is the most accurate technique that achieves absolute errors that are generally lower than 1%, but it requires a relatively large (0.5 – 1.5 g) solid samples. Ames Laboratory has two highly accurate custom built adiabatic heat pulse calorimeters.

The relaxation technique is frequently utilized for calorimetric measurements of tiny samples weighing a few milligrams (typically 1-30 mg). Ames Laboratory employs two commercial Physical Property Measurement Systems (PPMS) manufactured by Quantum Design Inc., that rely on relaxation calorimetry. Using both methods, a wide range of materials including intermetallic compounds, metallic alloys, ceramics, and polymers can be examined.

Calorimetric experiments provide a wealth of important information about caloric solids including:
•    Thermodynamic nature of phase transitions
•    Transition temperatures
•    Heat capacity data as a function of temperature without and with applied magnetic fields
•    Standard enthalpies and enthalpies
•    Total entropies as functions of temperature without and with applied magnetic fields
•    Isothermal entropy changes as functions of magnetic field and temperature
•    Adiabatic temperature changes as functions of magnetic field and temperature