M. Straßheim et al. Journal of Applied Physics 137 (2025) 045106/1-9

Cryogenic magnetic refrigeration becomes more and more important nowadays, especially for the liquefaction of gases such as hydrogen. In this study, we have synthesized La_1-xCe_x(Fe_0.88-yMn_ySi_0.12⁠)₁₃ samples and investigated their magnetic and magnetocaloric properties in order to assess their potential for cryogenic applications. By adjusting the Mn and Ce content and adding excess rare-earth elements, the first-order ferromagnetic transition was lowered from 200 to 40 K and the adiabatic temperature change of the samples was measured directly using pulsed magnetic fields. The sample with the lowest transition temperature still showed a significant adiabatic temperature change in magnetic fields up to 10 T, with an increasingly stronger first-order transition observed in samples with higher Ce substitution. In addition, we synthesized spherical powder with diameters between 20 and 120 μm using ultrasonic atomization while maintaining the magnetic transition, which is a promising starting material for future additive manufacturing of magnetocaloric materials.

Q. Badosa et al. Journal of Applied Physics 134 (2023) 113902/1-6

We have studied the impact of demagnetizing fields on the magnetocaloric effect of commercial-grade gadolinium plates. Adiabatic temperature changes (⁠dT) were measured for magnetic fields applied along the parallel and perpendicular directions of the plates. The differences in the obtained dT values were accounted for by differences in the internal field due to demagnetizing effects. A combination of calorimetric measurements under a magnetic field and thermometric measurements has enabled us to obtain Brayton cycles for the two different magnetic field orientations. It has been found that the refrigerant capacity for a Brayton cycle working at 1.6 T around room temperature reduces from RC=9.4 to RC=5.5 J/kg when the demagnetizing factor changes from ND=0.035 to ND=0.928 for the parallel and perpendicular configurations, respectively. It has been shown that it is possible to obtain significant demagnetizing field-induced magnetocaloric effects by rotating the sample in a region of a constant applied magnetic field. The refrigerant capacity of a Brayton cycle around room temperature for a 1.6T constant applied magnetic field is  RC=0.6 J/kg ⁠. The feasibility of these demagnetizing field-induced effects has been confirmed by direct thermometric measurements, which reveal adiabatic temperature changes of 1 K when the sample is rotated between the perpendicular and parallel configurations.

L. Beyer et al. Journal of Magnetism and Magnetic Materials 536 (2021) 168115/1-6

Gd cladded in a seamless 316L austenitic steel tube has been swaged into wires by the powder-in-tube (PIT) technology, resulting in an outer diameter of 1 mm, a wall thickness of approx. 100 µm and a filling factor of around 62 vol%. Such wires provide an advantageous geometry for heat exchangers and have the benefit to protect the Gadolinium, i.e. from corrosion when being in contact with a heat transfer fluid. The magnetocaloric composite has been studied by static and pulsed magnetic-field measurements in order to evaluate the performance of Gd as a core material. By the analysis of magnetization and heat capacity data, the influences of deformation-induced defects on Gadolinium are presented. The subsequent heat treatment at 773 K for 1 h in Ar atmosphere allowed restoring the magnetic properties of the wire after deformation.

Data of the pulsed magnetic-field measurements on the Gd-filled PIT-wires and a Gd–core separated from the jacket are presented, with an achievable temperature change of 1.2 K for the wire and 5.2 K for the Gd in 2 T, respectively. A comparison to previously studied La(Fe, Co, Si)₁₃-filled composite wires is included. It indicates that performance losses due to the passive matrix material cannot be overcome only by an increased adiabatic temperature change of the core material, but instead the wire components need to be chosen regarding an optimized heat capacity ratio, as well.

T. Samanta et al. Journal of Applied Physics 129 (2021) 023901/1-6

The magnetostructural transition (MST) can be tuned close to room temperature for an isostructurally alloyed (MnNiGe)_1−x(Fe₂Ge)_x (x = 0.1) compound by partially substituting a small amount of Si for Ge (7 at. %). In this study, the effect of hydrostatic pressure (p) on MST is investigated. In comparison to purely magnetically induced phase transition, pressure initiates structural transition more abruptly, which results in an increase in the isothermal entropy change by a factor of 2 from −Δs = 25.6 (p = 0) to 45.6 J/kg K (p = 190 MPa) for a magnetic field change of 2 T. Since the direct assessment of the adiabatic temperature change, ΔT_ad, is difficult due to the large volume change and subsequent structural breakdown at MST, an indirect method has been employed to estimate ΔT_ad.

M. Krautz et al. Materials & Design 193 (2020) 108832/1-7

Magnetocaloric composite wires have been studied by pulsed-field measurements up to μ₀ΔH = 10 T with a typical rise time of 13 ms in order to evaluate the evolution of the adiabatic temperature change of the core, ΔT_ad, and to determine the effective temperature change at the surrounding steel jacket, ΔT_eff, during the field pulse. An inverse thermal hysteresis is observed for ΔT_ad due to the delayed thermal transfer. By numerical simulations of application-relevant sinusoidal magnetic field profiles, it can be stated that for field-frequencies of up to two field cycles per second heat can be efficiently transferred from the core to the outside of the jacket. In addition, intense numerical simulations of the temperature change of the core and jacket were performed by varying different parameters, such as frequency, heat capacity, thermal conductivity and interface resistance in order to shed light on their impact on ΔT_eff at the outside of the jacket in comparison to ΔT_ad provided by the core.

A. Funk et al. Materials Today Energy 9 (2018) 223-228

The powder-in-tube (PIT) technology was applied to La(Fe, Co, Si)13 powder cladded by a thin seamless austenitic steel jacket. Wires appear to be promising in the search for alternative regenerator geometries, since they offer various possibilities of arrangements allowing to optimise heat transfer and pressure loss within the boundaries set by parallel plate and sphere beds. Here, pre-alloyed La(Fe, Co, Si)13 powder was  filled in a AISI 316L austenitic steel tube and swaged to wires with an outer diameter of 1 mm. By mechanical deformation, the steel jacket thickness was reduced to about 100 mm surrounding the magnetocaloric core. A post-annealing of only 10 min at 1050  C is sufficient to form the magnetocaloric NaZn13-type phase resulting in an entropy change close to the value of a pure reference sample. The presented technology is not limited to La(Fe, Co, Si)13/steel combination but can be extended to material pairs involving wire core materials with a first order transition, such as Fe2P-type or Heusler alloys.
 

M. Krautz et al. Materials and Design 193 (2020) 108832

Magnetocaloric composite wires have been studied by pulsed-field measurements up to μ0ΔH = 10 T with a typical rise time of 13 ms in order to evaluate the evolution of the adiabatic temperature change of the core, ΔTad, and to determine the effective temperature change at the surrounding steel jacket, ΔTeff, during the field pulse. An inverse thermal hysteresis is observed for ΔTad due to the delayed thermal transfer. By numerical simulations of application-relevant sinusoidal magnetic field profiles, it can be stated that for field-frequencies of up to two field cycles per second heat can be efficiently transferred from the core to the outside of the jacket. In addition, intense numerical simulations of the temperature change of the core and jacket were performed by varying different parameters, such as frequency, heat capacity, thermal conductivity and interface resistance in order to hed light on their impact on ΔTeff at the outside of the jacket in comparison to ΔTad provided by the core.

L. Beyer et al. Journal of Magnetism and Magnetic Materials 536 (2021) 168115/1-6

Gd cladded in a seamless 316L austenitic steel tube has been swaged into wires by the powder-in-tube (PIT) technology, resulting in an outer diameter of 1 mm, a wall thickness of approx. 100 µm and a filling factor of around 62 vol%. Such wires provide an advantageous geometry for heat exchangers and have the benefit to protect the Gadolinium, i.e. from corrosion when being in contact with a heat transfer fluid. The magnetocaloric composite has been studied by static and pulsed magnetic-field measurements in order to evaluate the performance of Gd as a core material. By the analysis of magnetization and heat capacity data, the influences of deformation-induced defects on Gadolinium are presented. The subsequent heat treatment at 773 K for 1 h in Ar atmosphere allowed restoring the magnetic properties of the wire after deformation.
Data of the pulsed magnetic-field measurements on the Gd-filled PIT-wires and a Gd–core separated from the jacket are presented, with an achievable temperature change of 1.2 K for the wire and 5.2 K for the Gd in 2 T, respectively. A comparison to previously studied La(Fe, Co, Si)₁₃-filled composite wires is included. It indicates that performance losses due to the passive matrix material cannot be overcome only by an increased adiabatic temperature change of the core material, but instead the wire components need to be chosen regarding an optimized heat capacity ratio, as well.