Direct magnetocaloric characterization and simulation of thermomagnetic cycles

Simple practical system for directly measuring magnetocaloric effects under large magnetic fields

Direct measurements of the adiabatic temperature change (ΔT_ad) in Gd and Mn_1.15Fe_0.8P_0.5Si_0.5C_0.05 are made using a homemade adiabatic magnetocalorimeter at 260-360 K and 0-7 T. The system uses a servo motor to drive the samples into and out of the magnetic field under a vacuum environment provided by the Physical Property Measurement System (PPMS). The peak values of ΔT_ad for Gd and Mn_1.15Fe_0.8P_0.5Si_0.5C_0.05 at 7 T are 8.71 K and 6.41 K at ambient temperatures of 303 K and 317 K, respectively. Based on the theory model, it is found that ΔT_ad of Gd depends on the 2/3 exponential function of magnetic field H (ΔT_ad ∝ H^2/3), whereas the Mn_1.15Fe_0.8P_0.5Si_0.5C_0.05 compound follows the power law of ΔT_ad ∝ H^0.66-1.04 due to the first order magnetic transitions. Furthermore, using the constructed experimental instrument, the adiabatic temperature change in different magnetic materials, including materials with first/second order magnetic transition and blocks, flakes, or powders, can be directly measured under large magnetic fields and wide temperature spans.

Elastocaloric effect and superelastic stability in Ni-Mn-In-Co polycrystalline Heusler alloys: hysteresis and strain-rate effects

Controlling material hysteresis and working frequency variability are fundamentally important for refrigeration cycle efficiency and power density in solid-state cooling systems. For elastocaloric cooling, understanding the relationship between the width of the stress hysteresis and elastocaloric behaviour during superelastic cycles under varied strain rates is important. In this work, we report the effects of strain rate effects on the superelastic and elastocaloric responses in Ni₄₅Mn_50−xIn_xCo₅ (x = 13.6–14.0 in at%) polycrystalline alloys. We observed a strong correlation between stress hysteresis and superelastic stability during mechanical cycling under increasing strain rates. Excellent transformation reversibility and stable superelastic responses are observed for x = 13.6 with a narrow hysteresis (49 MPa), whereas transformation irreversibility and dramatically deteriorated superelastic stability occur for x = 13.8, with a wide hysteresis (138 MPa). Furthermore, isothermal loading–unloading cycles under increasing and constant maximum applied stress were performed for the x = 13.6 samples, with a combination of low transformation stress and small transformation hysteresis. We suggest that a balance between transformation strain and hysteresis energy loss is fundamental to achieving a high coefficient of performance for elastocaloric materials.

Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys

We report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy versus temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Non-contact direct measurement of the magnetocaloric effect in thin samples

An experimental setup, based on a non-contact temperature sensor, is proposed to directly measure the magnetocaloric effect of samples few micrometers thick. The measurement of the adiabatic temperature change of foils and ribbons is fundamental to design innovative devices based on magnetocaloric thin materials or micro-structuring bulk samples. The reliability of the proposed setup is demonstrated by comparing the measurements performed on a bulk gadolinium sample with the results obtained by an experimental setup based on a Cernox bare chip thermoresistance and by in-field differential scanning calorimetry. We show that this technique can measure the adiabatic temperature variation on gadolinium sheets as thin as 27 μm. Heat transfer simulations are added to describe the capability of the presented technique.

Taming the first-order transition in giant magnetocaloric materials

Large magnetically driven temperature changes are observed in MnFe(P,Si,B) materials simultaneously with large entropy changes, limited (thermal or magnetic) hysteresis, and good mechanical stability. The partial substitution of B for P in MnFe(P,Si) compounds is found to be an ideal parameter to control the latent heat observed at the Curie point without deteriorating the magnetic properties, which results in promising magnetocaloric properties suitable for magnetic refrigeration.