A Peltier cells differential calorimeter with kinetic correction for the measurement of cp(H,T) and Δs(H,T) of magnetocaloric materials

Abstract

In this paper we describe and test a setup for the characterization of the magnetocaloric effect around room temperature. The setup is a differential calorimeter able to measure both the specific heat c(p)(H,T) under constant magnetic field H and the isothermal entropy change induced by changing H, Δs(H,T), in the room temperature range. The setup uses miniaturized Peltier cells to measure the heat flux, with resolution of about 1 μW, and power Peltier cells to regulate the temperature in the range from 243 K (-30 °C) to 343 K (+70 °C). The kinetic effects due to the heat capacity of the measuring cells are taken into account by a simple model of the heat flux diffusion in the calorimetric cell. As measurement examples, we show the characterization of the magnetocaloric effect in magnetic materials with a second order transition [without latent heat and without hysteresis, as in the La(1)(Fe(1-x-y)Co(y)Si(x))(13) alloy with x=0.077 and y=0.079] and with a first order transitions (with latent heat and hysteresis as in Ni(50)Mn(36)Co(1)Sn(13)). As a result we compare the entropy change Δs(H,T) derived from (i) the integration of the specific heat c(p)(H,T) and (ii) the direct isothermal measurements, obtaining an excellent agreement.

Specific heat and entropy change at the first order phase transition of La(Fe-Mn-Si)13-H compounds

“…Characterization was done by commercial power compensation differential scanning calorimetry (DSC) and Peltier calorimetry under magnetic field 11 . Under zero applied field, the results of power compensation DSC and Peltier calorimetry agree within the measurement uncertainty.…”

“…We made an experimental investigation of the MCE by direct calorimetry in magnetic field in order to determine the effectiveness of the Mn to obtain optimized MCE properties [11][12][13] . We find that the transition, without a magnetic field applied, passes from first order (∆T hyst =1.5 K) at y = 0.06 (T t =339 K) to almost second order with no hysteresis at y = 0.46 (T t =270 K).…”

Evaluation of the reliability of the measurement of key magnetocaloric properties: A round robin study of La(Fe,Si,Mn)Hδ conducted by the SSEEC consortium of European laboratories

“…In the case of a first-order phase transition, these two temperatures can be significantly different (in principle the temperature of the sample should remain constant during the first order phase transition). In the INRIM calorimeter, the lag between the measured temperature (that of the thermal bath) and the sample temperature was properly taken into account and partially corrected, as explained elsewhere (Basso et al 2010) The difference in the general shape of the two PPMS results can be explained by a number of possible factors. Firstly there is the spread in sample properties that can result from the pressing process used.…”

“…The construction and operation details of the experimental setup, its calibration and many results have been presented recently (Basso et al 2010). As in a commercial DSC, reference materials are used either for the determination of the calibration constant, S p , (connecting the voltage measured at the ends of the Peltier cells to the heat flux traversing them), or the time constant,  p , governing the kinetics of the calorimeter.…”

Direct measurements of the magnetic entropy change

“…Our device resembles that of Ref. 2 with the addition that our device has a fully automated systematic resetting capability (both thermal and magnetic reset). Our magnetic field source is furthermore made of permanent magnets rather than an electromagnet and the field change from zero to max (1.5 T) field can be achieved in a continuous way.…”