Caloric materials offer a promising and novel route to providing high efficiency and environmentally-friendly solid-state refrigeration solutions for air conditioning and other heat-pumping applications.

What are caloric materials?

Caloric materials are characterised by a temperature change due to their entropy variation whilst an external field (electric, magnetic, pressure or stress) is applied/withdrawn, under adiabatic conditions.

The principal caloric effects are:

• Electrocaloric effect (Electric field, E)
• Magnetocaloric effect (Magnetic field, M)
• Piezocaloric (Mechanical stress)

Schematic diagram of a caloric refrigeration cycle: The basic operating principle of caloric refrigeration is analogous to that of gas compression cooling. This is constituted by the transportation of heat from a heat load to the surrounding environment through a caloric material that is subjected to a thermodynamic cycle. Refrigeration cycles are driven by the entropy variation of the working body, while an external field (electric, magnetic or mechanical) varies.

Direct measurements of caloric effects

Most measurements of caloric effects in materials are derived from indirect measurements using thermodynamic constitutive relations which can result in unquantified errors through the assumptions made, and as such, do not form a good basis for a traceable measurement of the caloric effect. Also, direct measurement of adiabatic temperature change in thin films is challenging, as generated heat leaks away through the substrate.

NPL is focused on the development of advanced traceable measurement systems and methods for caloric properties under different intensive quantities (magnetic field, electric field, mechanical stress).

Current NPL research on caloric materials measurements includes:

Schematic illustration of multi-caloric coupling effects

• Metrology for spatial- and time-resolved temperature change in caloric materials
• On-chip metrology for direct measurement in caloric thin films
• Metrology for Figure-of-Merit for caloric materials

Multi-caloric coupling effects

The Functional Materials Group is also dedicated to developing new models to describe the coupling between different caloric effects and metrology to measure multi-caloric coupling properties in materials that are simultaneously activated by different external fields, i.e., electric fields, magnetic fields or mechanical stress.

People

Tatiana Correia

Publications

• PST thin films for electrocaloric coolers
  T. M. Correia, S. Kar-Narayan, J. S. Young, J. F. Scott, N. D. Mathur, R. W. Whatmore, Q. Zhang
  J. Phys. D: Appl. Phys. 44, 165407 (2011)
• Electrocaloric effect: a metrologic perspective
  T. M. Correia
  NPL Report MAT 4894 (2011)