A Peltier cell calorimeter for the direct measurement of the isothermal entropy change in magnetic materials

Abstract

We developed a calorimetric technique to measure the isothermal magnetocaloric entropy change. The method consists in the use of Peltier cells as heat flow sensor and heat pump at the same time. In this paper, we describe the setup, the constitutive equations of the Peltier cell as sensor and actuator, and the calibration procedure. The Peltier heat is used to keep the sample isothermal when magnetic field is changed. The temperature difference between the sample and the thermal reservoir is kept by a digital control within 5 mK for a magnetic field rate of 20 mT s(-1). The heat flux sensitivity around 1 microW. With this method, it is possible to measure the magnetocaloric effect in magnetic materials by tracing the curves of the exchanged entropy Delta(e)s as a function of the magnetic field H. The method proves to be, in particular, suitable to reveal the role of the entropy production Delta(i)s, which is connected with hysteresis. Measurement examples are shown for Gd, BaFe(12)O(19) ferrite, and Gd-Si-Ge.

Materials Challenges for High Performance Magnetocaloric Refrigeration Devices

“…This is done by using the Peltier cells actively to keep the sample isothermal when the external fi eld is changed. [164][165][166] The heat required to do this is then equal to the isothermal entropy change times the sample temperature. Sasso et al measured the history dependence in single crystals of Ni 55 Mn 20 Ga 25 , which exhibit signifi cant hysteresis, using this method.…”

Direct magnetocaloric characterization and simulation of thermomagnetic cycles

“…[10][11][12][13][14][15] Therefore, the need for novel thermal devices has been continuously pushing scientists to carefully analyze the thermal behavior of magnetic critical processes. During the last years the realization of several in-field calorimetric techniques [16][17][18][19][20][21][22][23][24][25][26][27][28][29] contributed both to a better understanding of the thermomagnetic behavior of the most promising compounds showing first order phase transitions and to encourage the development of new materials. 15,30 Thus, the study of the time and frequency dependence of the MCE constitutes the next step to be taken.…”

Recent Developments in Magnetic Measurements: from Technical Method to Physical Knowledge

“…i) Magnetic measurements M(H a , T), where the isothermal entropy change Δs iso is computed integrating the Maxwell relation ds/dH a = μ 0 dM/dT [40]. This method applies well to systems with second order phase transitions, but it is not appropriate to transitions with temperature hysteresis [41]. ii) Adiabatic methods, where the temperature change ΔT ad due to magnetic field is directly accessed [42].…”

“…It is the magnetocaloric effect. Magnetic refrigeration is therefore envisaged and, with this prospective goal in mind, a number of special measuring techniques have been developed to study the magnetocaloric phenomena [6][7][8].…”