Best practices in evaluation of the magnetocaloric effect from bulk magnetization measurements

Conventional magnetometry is irreplaceable in evaluating bulk magnetization of materials over broad temperature and field ranges. The technique is also effective in quantifying hysteresis that may be associated with magnetic and structural phase transitions that occur during the magnetizing/demagnetizing cycling, and the derived magnetic field-induced isothermal entropy change – one of the most important properties in the field of magnetocalorics. Both systematic and random errors present during the measurements of magnetization, however, may lead to erroneous conclusions. Using two well-known materials – elemental Gd and intermetallic Gd5Si2Ge2 as examples, we consider best practices in performing reliable and rapid magnetization measurements for proper characterization of magnetocaloric properties

From a magnet to a heat pump

The magnetocaloric effect (MCE) is the thermal response of a magnetic material to an applied magnetic field. Magnetic cooling is a promising alternative to conventional vapor compression technology in near room temperature applications and has experienced significant developments over the last five years. Although further improvements are necessary before the technology can be commercialized.Researchers were mainly focused on the development of materials and optimization of a flow system in order to increase the efficiency of magnetic heat pumps. The project, presented in this paper, is devoted to the improvement of heat pump and cooling technologies through simple tests of prospective regenerator designs. A brief literature review and expected results are presented in the paper. It is mainly focused on MCE technologies and provides a brief introduction to the magnetic cooling as an alternative for conventional vapor compression technology

Magneto-elastic coupling in La(Fe, Mn, Si)₁₃H<i>y</i> within the Bean-Rodbell model

First order magnetic phase transition materials present a large magnetocaloric effect around the transition temperature, where these materials usually undergo a large volume or structural change. This may lead to some challenges for applications, as the material may break apart during field change, due to high internal stresses. A promising magnetocaloric material is La(Fe, Mn, Si)13Hy, where the transition temperature can be controlled through the Mn amount. In this work we use XRD measurements to evaluate the temperature dependence of the unit cell volume with a varying Mn amount. The system is modelled using the Bean-Rodbell model, which is based on the assumption that the spin-lattice coupling depends linearly on the unit cell volume. This coupling is defined by the model parameter η, where for η > 1 the material undergoes a first order transition and for η  ≤ 1 a second order transition. We superimpose a Gaussian distribution of the transition temperature with a standard deviation σ T 0 , in order to model the chemical inhomogeneity. Good agreement is obtained between measurements and model with values of η  ∼ 1.8 and σ(T0) = 1.0 K

Influence of the starting temperature of calorimetric measurements on the accuracy of determined magnetocaloric effect

Availability of a restricted heat capacity data range has a clear influence on the accuracy of calculated magnetocaloric effect, as confirmed by both numerical simulations and experimental measurements. Simulations using the Bean-Rodbell model show that, in general, the approximated magnetocaloric effect curves calculated using a linear extrapolation of the data starting from a selected temperature point down to zero kelvin deviate in a non-monotonic way from those correctly calculated by fully integrating the data from near zero temperatures. However, we discovered that a particular temperature range exists where the approximated magnetocaloric calculation provides the same result as the fully integrated one. These specific truncated intervals exist for both first and second order phase transitions and are the same for the adiabatic temperature change and magnetic entropy change curves. The effect of this truncated integration in real samples was confirmed using heat capacity data of Gd metal and Gd5Si2Ge2 compound measured from near zero temperatures

Influence of the starting temperature of calorimetric measurements on the accuracy of determined magnetocaloric effect

Availability of a restricted heat capacity data range has a clear influence on the accuracy of calculated magnetocaloric effect, as confirmed by both numerical simulations and experimental measurements. Simulations using the Bean-Rodbell model show that, in general, the approximated magnetocaloric effect curves calculated using a linear extrapolation of the data starting from a selected temperature point down to zero kelvin deviate in a non-monotonic way from those correctly calculated by fully integrating the data from near zero temperatures. However, we discovered that a particular temperature range exists where the approximated magnetocaloric calculation provides the same result as the fully integrated one. These specific truncated intervals exist for both first and second order phase transitions and are the same for the adiabatic temperature change and magnetic entropy change curves. The effect of this truncated integration in real samples was confirmed using heat capacity data of Gd metal and GdSiGe compound measured from near zero temperatures.Work in Sevilla supported by the Spanish MINECO (project MAT2013-45165-P), AEI/FEDER-UE (project MAT-2016-77265-R) and the PAI of the Regional Government of Andalucía – Spain. L.M. Moreno-Ramírez acknowledges a FPU fellowship from the Spanish MECD. Work in Ames supported by the U.S. Department of Energy (DOE) Advanced Manufacturing Office of the Office of Energy Efficiency and Renewable Energy (CaloriCool®). Ames Laboratory is supported by the U.S. DOE Office of Science and operated by Iowa State University under Contract No. DE-AC02-07CH11358.Peer Reviewe

Novel design of a high efficiency multi-bed active magnetic regenerator heat pump

Supporting data for publication 'Novel design of a high efficiency multi-bed active magnetic regenerator heat pump" submitted to the International Journal of Refrigeration (authors are awaiting DOI)The videos show the operation of the novel magnetocaloric heat pump called the MagQueen, which has been developed by DTU Energy.The Excel sheet summarizes the experimental output parameters for the performance data presented in the publication. All data were measured continuously after reaching steady-state conditions, and the data were averaged over a time span of 600 s.</div