Direct Temperature Change Measurements Research Articles

Dynamical response of Gadolinium in alternating magnetic fields up to 9Hz

In magnetic refrigerator, the magnetic induction and its alternating frequency have great influence on the magnetocaloric effects of materials. In order to relationship between the magnetocaloric effect and alternating frequency of magnetic field, the direct measurements of temperature change for Gadolinium near working conditions in alternating magnetic fields up to 9 Hz are carried out. In this device, the magnetic field is alternately changed by rotating the magnet while sample holder keeps stationary to defend the measurement signal interference. Results shows that the temperature change of Gd decreases from 1.5 K to 0.7 K with increasing field alternating frequency from 2.2 Hz to 9 Hz under magnetic field of 0.72T. Furthermore, based on the heat transfer process between sample, thermocouple and environment, we have established a theoretical model to investigate the effects of different influencing factors on the temperature change of Gadolinium. It reveals that near work conditions, there are lots of influencing factors to affect the temperature change, such as sample mass, applied magnetic field mode, adiabatic conditions, cycle frequency, and so on. The experimental and simulation results provide the guidance for screening the best candidate of magnetocaloric materials and developing the magnetic refrigerator.

Direct measurement of adiabatic temperature change in (La0.7Sm0.3)0.67Ba0.33MnO3

In this work, the magnetic and magnetocaloric properties of (La0.7Sm0.3)0.67Ba0.33MnO3 manganite produced by sol–gel method were investigated by magnetization and direct-temperature change measurements. As the temperature decreases in M(T) measurements, the sample shows the transition from paramagnetic to ferromagnetic state at 160 K, which is the magnetic phase transition temperature where magnetization suddenly drops. The maximum magnetic entropy change value from M(H) measurements in the transition region is determined as 2.12 Jkg−1 K−1 under 4.8 T magnetic field. It was found that the material had a second-order transition due to the positive slope of the Arrott plots, and this behavior was confirmed by the universal curve obtained by the normalized magnetic entropy change curves versus the rescaled temperature axis. ΔTad of the sample was measured with the adiabatic magneto-calorimetry system directly and the maximum value of ΔTad was measured as 0.72 K under 3 T magnetic field change.

Giant room-temperature barocaloric effects in PDMS rubber at low pressures

The barocaloric effect is still an incipient scientific topic, but it has been attracting an increasing attention in the last years due to the promising perspectives for its application in alternative cooling devices. Here, we present giant values of barocaloric entropy change and temperature change induced by low pressures in PDMS elastomer around room temperature. Adiabatic temperature changes of 12.0 K and 28.5 K were directly measured for pressure changes of 173 MPa and 390 MPa, respectively, associated with large normalized temperature changes (∼70 K GPa−1). From adiabatic temperature change data, we obtained entropy change values larger than 140 J kg−1 K−1. We found barocaloric effect values that exceed those previously reported for any promising barocaloric materials from direct measurements of temperature change around room temperature. Our results stimulate the study of the barocaloric effect in elastomeric polymers and broaden the pathway to use this effect in solid-state cooling technologies.

Tuneable Giant Magnetocaloric Effect in (Mn,Fe)₂(P,Si) Materials by Co-B and Ni-B Co-Doping.

The influence of Co (Ni) and B co-doping on the structural, magnetic and magnetocaloric properties of (Mn,Fe)(P,Si) compounds is investigated by X-ray diffraction (XRD), differential scanning calorimetry, magnetic and direct temperature change measurements. It is found that Co (Ni) and B co-doping is an effective approach to tune both the Curie temperature and the thermal hysteresis of (Mn,Fe)(P,Si) materials without losing either the giant magnetocaloric effect or the positive effect of the B substitution on the mechanical stability. An increase in B concentration leads to a rapid decrease in thermal hysteresis, while an increase in the Co or Ni concentration hardly changes the thermal hysteresis of the (Mn,Fe)(P,Si) compounds. However, the Curie temperature decreases slowly as a function of the Co or Ni content, while it increases dramatically for increasing B concentration. Hence, the co-substitution of Fe and P by Co (Ni) and B, respectively, offers a new control parameter to adjust the Curie temperature and reduce the thermal hysteresis of the (Mn,Fe)(P,Si) materials.

Magnetocaloric effect in (La1−xSmx)0.67Pb0.33MnO3 (0 ≤ x ≤ 0.3) manganites near room temperature

We present an investigation on structure, magnetic and magnetocaloric properties of the perovskite manganites (La1−xSmx)0.67Pb0.33MnO3 (x = 0, 0.1, 0.2, 0.3) synthesized by sol–gel technique. The XRD patterns show that all synthesized samples have reflections typical of the perovskite structure with orthorhombic symmetry. Thermomagnetic measurements showed that all the samples display a paramagnetic–ferromagnetic transition with decreasing temperature. The Curie temperature decreases with increasing Sm content from 358 K for x = 0–286 K for x = 0.3. We determined the isothermal magnetic entropy changes of all the samples from the magnetization measurements and furthermore measured the adiabatic temperature change of the sample for x = 0.3 directly. The results showed that the adiabatic temperature change, determined using together the entropy change and the specific heat and the value obtained by direct temperature change measurements both give 1.3 K for a magnetic field change of 3 T at about 280 K. We also measured the cyclic adiabatic temperature-change of the sample and the results indicate that the sample undergoes a reversible temperature-change on field-cycling which is essential for magnetic refrigeration.

Adiabatic temperature change around coinciding first and second order magnetic transitions in Mn3Ga(C0.85N0.15)

The inverse magnetocaloric material in Mn3GaC relies on the presence of a first order antiferromagnetic–ferromagnetic transition at around 160K. Since Mn3GaN is antiferromagnetic, the partial substitution of carbon by nitrogen in Mn3GaC enhances the antiferromagnetic exchange and shifts this transition to higher temperatures. At the Mn3Ga(C0.85N0.15) stoichiometry, the transition takes place at 180K. The hysteresis at the transition reduces so that the inverse magnetocaloric effect is practically reversible. We study the magnetocaloric effect and the reversibility of the adiabatic temperature-change by magnetization and direct temperature-change measurements up to field-changes of 5T.