Getting the most out of magnetocaloric materials for high efficiency refrigeration

For getting the most out of magnetocaloric materials (MCMs), the influence of their polycrystalline nature on the magnetic phase transition (MPT) needs to be understood. The MPT from the ferromagnetic to the paramagnetic state, if conducted adiabatically, results in the change of the material’s temperature. This is called the magnetocaloric effect (MCE). In order to investigate the grain behaviour in the micrometre range during this transition, an infrared (IR) thermography setup was developed and selected sample materials were characterised. The investigated samples, i.e., pure Gd, La_0.67Ca_0.33−xSr_xMn_1.05O₃(LCSMO), and La_0.85Ce_0.15Fe_11.25Mn_0.25Si_1.5H_y (LaCeFe), have their MPT around room temperature, to mimic those employed in magnetocaloric refrigeration (MR) devices, but this fact also allows the accurate temperature calibration of the IR thermography (IRT) setup and, therefore, the determination of the sample’s emissivity.

In particular, the ceramic perovskite LCSMO is a promising candidate for high efficiency MR for various reasons. While its caloric properties, such as the aforementioned adiabatic temperature change and the isothermal entropy change are relatively low compared to benchmark material Gd, LCSMO shows excellent non-caloric properties, as good chemical and mechanical stability, cost-effective production, easy shaping, and suitability for chemical doping. The latter of which results in the possibility to tune the temperature of the MPT. This feature is an important requirement in order to improve the efficiency of MCMs in prototypes that employ them as active magnetic regenerator (AMR) material. Furthermore, by means of high resolution differential scanning calorimetry (DSC), La(Fe_11.47 Si_1.28 Mn_0.25)H_1.65 (LaFeSi) was investigated in presence of various magnetic fields ranging from 0 T to 1.5 T. This material is similarly suitable as AMR material, but shows a volume-change during its MPT that puts additional constraint on its mechanical stability. It was found that in some samples the phase transition occurs distributed at different temperatures within a 0.4 K range. This finding suggests that, for this small temperature range around the material’s transition temperature, the Bean-Rodbell model parameter eta, which characterises the order of the MPT, is decoupled from the transition temperature. The developed IRT setup was intended to support this hypothesis due to the high spatial (μm) and temperature (mK) resolution of the IR camera. Because, if these distributed phase transitions could be attributed to the independent magnetocaloric activity of materials grains, the LaFeSi-type and possibly most MCMs that show a first order phase transition would turn to become too expensive for commercial application due to the generally higher production costs for materials with optimised grain structure. However, sample vibrations upon magnetic field change and too small thermal equilibration time between material grains (consistent with observations in LaCeFe), prevented further investigation in that direction.

Therefore, the ability of the IRT setup to measure the emissivity of magnetocaloric samples as a function of temperature and external magnetic field was investigated with the result that La_0.67Ca_0.33−xSr_xMn_1.05O₃ shows decreased emissivities in the presence of magnetic field, below its Curie temperature. This was found in a side-by-side tape cast sample, i.e., a sample where the two stoichiometries of x = 0.0375 and x = 0.075 are present locally close to each other. Hence, it was concluded that ordered electron spins do prevent the emittance of IR light from the surface of LCSMO plates in the waveband of the IR camera, i.e., 2.5 μm to 5.1 μm. This conclusion is based on the reproducibility of the effect in LCSMO samples of unique stoichiometry. Thus, this phenomenon, which can be deduced from the correlation between magnetoresistance and emissivity, additionally accounts for the difficulty to determine the adiabatic temperature change of application relevant MCMs by means of IR thermography.