Search

A virgin phase transition is observed in spherical particles of the industrially relevant magnetocaloric material La(Fe,Mn,Si)13Hy. Upon initial cooling, the phase transition is observed 2–3 K below the heating transition on all subsequent cooling and heating transitions. This virgin transition has been studied using differential scanning calorimetry and vibrating sample magnetometry. Incremental measurements show not only how the phase transition can be carefully approached but also that the initial full transformation requires cooling of about 6 K below the observed phase transformation. No signs of structural damage due to the thermal cycling were observed, neither macroscopically or by scanning electron microscopy.

This roadmap aims to define the guidelines to maximise the impact of the 3D printing revolution on the next generation of devices for the energy transition. It also outlines the current status, challenges and required advances in Science and Technology for a series of power generation technologies (fuel cells, solar cells, thermoelectric generators and turbomachinery) and energy storage technologies (electrolysers, batteries and supercapacitors). Finally, the roadmap discusses the role of 3D printing in improving the mass and heat transfer to improve the energy efficiency of chemical reactors (CO2 conversion) and novel cooling systems. With this document, the authors intend to provide a valuable tool for researchers, technology developers, and policymakers when defining their strategies for the energy sector's future.

A novel, environmentally friendly, fast, and flexible polishing process for Nitinol parts is presented in this study. Nitinol samples with both superelastic and shape memory properties at room temperature were investigated. The chemical contamination and surface roughness of superelastic Nitinol plates were examined before and after plasma electrolytic polishing. The shift in phase transformation temperature and tensile strength before and after the polishing process were analysed using Nitinol wire with shape memory properties. The obtained experimental results were compared to the data obtained on reference samples examined in the as-received condition. It was found that plasma electrolytic polishing, when the right process parameters are applied, is capable of delivering Nitinol parts with extremely high surface quality. Moreover, it was experimentally proven that plasma electrolytic polishing does not have a negative impact on functionality or mechanical properties of polished parts.

A promising solid‐state refrigeration technology, known as magnetic refrigeration, has reached a groundbreaking result. The potential for environmentally friendly cooling using the magnetocaloric effect is disadvantaged by small temperature windows of effective cooling and the requirement of expensive, high magnetic field producing Nd‐Fe‐B permanent magnets. Researchers from Imperial College London, Ames Laboratory (USA) and DTU Energy, Technical University of Denmark (see article no. 1700143) show through utilising a larger portion of the phase diagram of so‐called soft first order magnetic materials that they can reduce the maximum field required to within that attainable with cheap ferrite‐based permanent magnets (around 0.5 T) and significantly broaden the working temperature range. Traditionally, magnetocaloric materials are controlled through temperature and applied magnetic field, but with the addition of applied hydrostatic pressure it is possible to move around in the phase diagram of the La(Fe,Mn,Si)13Hz material series, taking advantage of its so‐called multicaloric properties. By careful choice of magnetic field and pressure controlled cooling cycles this work shows that a significant bottleneck towards commercially competitive refrigeration devices based on the magnetocaloric effect can be overcome.

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 σT, in order to model the chemical inhomogeneity. Good agreement is obtained between measurements and model with values of η ∼ 1.8 and σ(T) = 1.0 K.

We use powder X-ray diffraction to evaluate the temperature dependence of the crystalline properties during the magnetic phase transition of La(Fe,Mn,Si)13Hz as a function of the Fe/Mn/Si ratio. Both the paramagnetic and ferromagnetic phases were observed as peak overlaps in the patterns around the Curie temperature (TC) occurring continuously in a temperature range of about 5 K around TC. Using the Williamson-Hall method, we evaluate the strain developing in the crystallites during the transition and find that it is associated with the growth of the paramagnetic phase as the transition occurs. Based on our measurements and microstructure analyses, we propose that cracking during the phase transition is due to or aggravated by the small content of a La-rich phase.

We report a thorough study of the thermal hysteretic behaviour of a single phase sample of the magnetocaloric material La0.67Ca0.33MnO3. Previous reports in the literature have variously found hysteretic and non-hysteretic behaviour. We show the importance of measuring under carefully defined heating and cooling procedures. Careful analysis of the specific heat, measured at five different temperature ramp rates, and the magnetic entropy change indicates that there is no observable hysteresis, even though the behaviour of both quantities is consistent with a first-order phase transition. We discuss the reasons for this and for the differing results previously found.

The magnetocaloric effect of a magnetic material is characterized by two quantities, the isothermal entropy change and the adiabatic temperature change, both of which are functions of temperature and applied magnetic field. We discuss the scaling properties of these quantities close to a second-order phase transition within the context of the theory of critical phenomena. Sufficiently close to the critical temperature of a second-order material, the scaling of the isothermal entropy change will be determined by the critical exponents and will be the same as that of the singular part of the entropy itself. However, this is only true in the critical region near Tc and for small fields; for finite fields, scaling with constant exponents, in general, break down, even at Tc. The field dependence can then be described by field-dependent scaling exponents. We show that the scaling exponents at finite fields are not universal, showing significant variation for models in the same universality class. As regards the adiabatic temperature change, it is not determined exclusively by the singular part of the free energy and its derivatives. We show that the field dependence of the adiabatic temperature change at the critical temperature depends on the nonsingular part of the specific heat. The field dependence can still be fitted to a power-law expression but with nonuniversal exponents, as we show explicitly both within mean-field theory and using the so-called Arrott-Noakes equation of state. Within the framework of the Bean-Rodbell model, we briefly consider the scaling properties of the magnetocaloric effect in first-order materials. Finally, we discuss the implications of our findings for a widely used phenomenological scaling procedure for magnetocaloric quantities.

We have applied mixed valance manganite perovskites as magnetocaloric materials in a magnetic refrigeration device. Relying on exact control of the composition and a technique to process the materials into single adjoined pieces, we have observed temperature spans above 9 K with two materials. Reasonable correspondence is found between experiments and a 2D numerical model, using the measured magnetocaloric properties of the two materials as input.