Peltier element driven ac calorimetry

J. Leys1, C. Glorieux1 and J. Thoen1

1Laboratorium voor Akoestiek en Termische Fysica, Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Leuven, Belgium

Keywords: AC calorimetry
property: heat capacity
material: liquids, solids

Heat capacity measurements are an important tool to observe the changes in energy content of condensed matter systems. Commonly used calorimetric techniques are differential scanning calorimetry (DSC), adiabatic heat pulse calorimetry, adiabatic scanning calorimetry (ASC), and ac calorimetry (AC). In particular the last two high-resolution techniques have contributed substantially to our present understanding of many phase transitions by revealing subtle thermal features and fluctuation effects. In this contribution, we describe a novel approach to ac calorimetry using miniature commercial Peltier elements for the modulation of the heat input to a sample.

In AC a very small oscillatory heat input at circle frequency ω of the form Qac = Q0(1+cosωt)/2 is supplied to the sample, usually loosely coupled to a heat bath at a given temperature T0, and the amplitude ΔTac of the resulting temperature oscillations is measured for a given average temperature Ta. Under suitable circumstances, the following simple relation is obtained:

Cp = Q0/(2ω ΔΤac).

One of the side effects with resistive heating as well as with photothermal excitation is the fact that it also results in a net heating, the positive dc component Ta-T0. However, the Peltier effect offers an opportunity to generate an ac temperature modulation without a dc contribution, by changing the direction of the current in the Peltier element, both heating and cooling are possible. In this paper we give a full account of the implementation of commercial miniature Peltier elements as ac power sources for AC measurements on solid and liquid samples. As detectors of the temperature modulation in the samples, miniature thermistors are used as in traditional AC.

For the solid samples results will be discussed for the ferromagnetic to paramagnet phase transition in gadolinium and for the antiferromagnetic to paramagnetic phase transition in Cr2O3. Different types of liquid crystalline phase transitions in compounds of the alkylcyanobiphenyl (nCB) homologous series will be reported.

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