Nickolaus M. Bruno

Direct Measure of Giant Magnetocaloric Entropy Contributions in Ni-Mn-In

Off-stoichiometric alloys based on Ni2MnIn have drawn attention due to the coupled first order magnetic and structural transformations, and the large magnetocaloric entropy associated with the transformations. Here we describe calorimetric and magnetic studies of four compositions. The results provide a direct measure of entropy change contributions at low temperatures as well as at the first-order phase transitions. Thereby we determine the maximum possible field-induced entropy change corresponding to the giant magnetocaloric effect. We find a large excess entropy change above that of the magnetic moments, but only in compositions with no ferromagnetic order in the high-temperature austenite phase. Furthermore, a molecular field model corresponding to magnetic order in the low-temperature phases is in good agreement, giving an entropy contribution nearly independent of composition, despite significant differences in overall magnetic response of these materials.

Effect of niobium addition on the martensitic transformation and magnetocaloric effect in low hysteresis NiCoMnSn magnetic shape memory alloys

The effect of Nb substitution for Ni in Ni45Co5Mn40Sn10 magnetic shape memory alloys on their magnetic properties, martensitic transformation characteristics, transformation hysteresis, and magnetocaloric properties was studied using wavelength-dispersive X-ray spectroscopy, differential scanning calorimetry, and the temperature and field dependence of the magnetization. Ni45Co5Mn40Sn10 alloy has a very low transformation hysteresis; however, the martensitic transformation temperatures are notably above room temperature, which is not desirable for magnetic refrigeration applications. In this study, small quantities of Nb substitution were shown to drastically shift the transformation temperatures to lower temperatures, at a rate of 68 K/at. % Nb, which is needed for household refrigeration. The austenite Curie temperature also decreased with increasing Nb content. However, a decrease in the latent heat of the martensitic transition was observed, which negatively affects the magnetic field-induced adiabati...

Effect of grain constraint on the field requirements for magnetocaloric effect in Ni45Co5Mn40Sn10 melt-spun ribbons

The influence of grain constraint on the magnetic field levels required to complete the isothermal martensitic transformation in magnetic shape memory alloys has been demonstrated for a NiCoMnSn alloy, and the magnetocaloric performance of an optimally heat treated alloy was quantified. Ni45CoxMn45-xSn10 melt spun ribbons with x = 2, 4, 5, and 6 were characterized. The x = 5 sample was determined to exhibit the lowest transformation thermal hysteresis (7 K) and transformation temperature range during transformation from paramagnetic austenite to nonmagnetic martensite, as well as a large latent heat of transformation (45 J kg-1 K-1). For this composition, it was found that increasing the grain size to thickness ratio of the ribbons from 0.2 to 1.2, through select heat treatments, resulted in a decrease in the magnetic field required to induce the martensitic transformation by about 3 T due to the corresponding reduction in the martensitic transformation temperature range. This decrease in the field requirement ultimately led to a larger magnetocaloric entropy change achieved under relatively smaller magnetic field levels. The giant inverse magnetocaloric effect of the optimized alloy was measured and showed that up to 25 J kg-1 K-1 was generated by driving the martensitic transition with magnetic fields up to 7 T.

A theoretical and experimental investigation of power harvesting using the NiMnGa martensite reorientation mechanism

Magnetic shape memory alloys (MSMAs) can exhibit the shape memory effect when there is a magnetic field in the vicinity of a material point. The microstructure of the MSMAs is comprised of tetragonal martensite variants, each with their preferred internal magnetization orientation. Starting from a random variant orientation, the application of a large enough magnetic field will cause the variants to reorient so that the internal magnetization vectors align with the external field. Then, keeping the magnetic field constant and adding a variable compressive stress in a direction normal to that of the magnetic field, some or all of the martensitic variants may rotate into a stress preferred state. As the variants reorient, the internal magnetization vectors rotate, and the material’s magnetization changes. For power harvesting and sensing applications, the change in magnetization induces a current in a pickup coil placed around the MSMA specimen, resulting in an output voltage at its terminals according to Faraday’s law of inductance. This paper focuses on the evaluation of the voltage output, both experimentally and numerically, in an attempt to assess the ability of a MSMA thermodynamic based constitutive model, used in conjunction with Faraday’s law of induction, to predict the variant reorientation induced voltage output. Assessing the accuracy of the predicted voltage is beneficial for the design of both MSMA based power harvesting devices and MSMA based displacement sensors. (Some figures may appear in colour only in the online journal)

Calorimetric and magnetic study for Ni50Mn36In14 and relative cooling power in paramagnetic inverse magnetocaloric systems

The non-stoichiometric Heusler alloy Ni50Mn36In14 undergoes a martensitic phase transformation in the vicinity of 345 K, with the high temperature austenite phase exhibiting paramagnetic rather than ferromagnetic behavior, as shown in similar alloys with lower-temperature transformations. Suitably prepared samples are shown to exhibit a sharp transformation, a relatively small thermal hysteresis, and a large field-induced entropy change. We analyzed the magnetocaloric behavior both through magnetization and direct field-dependent calorimetry measurements. For measurements passing through the first-order transformation, an improved method for heat-pulse relaxation calorimetry was designed. The results provide a firm basis for the analytic evaluation of field-induced entropy changes in related materials. An analysis of the relative cooling power (RCP), based on the integrated field-induced entropy change and magnetizing behavior of the Mn spin system with ferromagnetic correlations, shows that a significant...

Reversible Martensitic Transformation under Low Magnetic Fields in Magnetic Shape Memory Alloys

Magnetic field-induced, reversible martensitic transformations in NiCoMnIn meta-magnetic shape memory alloys were studied under constant and varying mechanical loads to understand the role of coupled magneto-mechanical loading on the transformation characteristics and the magnetic field levels required for reversible phase transformations. The samples with two distinct microstructures were tested along the [001] austenite crystallographic direction using a custom designed magneto-thermo-mechanical characterization device while carefully controlling their thermodynamic states through isothermal constant stress and stress-varying magnetic field ramping. Measurements revealed that these meta-magnetic shape memory alloys were capable of generating entropy changes of 14 J kg−1 K−1 or 22 J kg −1 K−1, and corresponding magnetocaloric cooling with reversible shape changes as high as 5.6% under only 1.3 T, or 3 T applied magnetic fields, respectively. Thus, we demonstrate that this alloy is suitable as an active component in near room temperature devices, such as magnetocaloric regenerators, and that the field levels generated by permanent magnets can be sufficient to completely transform the alloy between its martensitic and austenitic states if the loading sequence developed, herein, is employed.

High-field magneto-thermo-mechanical testing system for characterizing multiferroic bulk alloys

Multiferroic meta-magnetic shape memory alloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of -100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memory alloy and a few selected results are presented here.

Improved thermodynamic model for magnetic shape memory alloys

Magnetic shape memory alloys (MSMAs) are a class of materials that can exhibit up to 10% recoverable strain as a result of the application of either magnetic field or compressive stress. This unique property makes MSMAs potentially suitable for commercial applications such as sensors, power harvesters, or actuators. Before any commercial applications are fully realized, effective models capable of accurately predicting the magneto-mechanical behavior of MSMAs need to be developed. This paper builds on an existing thermodynamic based constitutive model for MSMAs by accounting for the three-dimensional nature of the demagnetization phenomenon. In particular, the importance of using a demagnetization factor that comes from a solution to the three-dimensional magneto-static boundary value problem is highlighted. Also, the magnetic field present in directions other than that applied because of demagnetization is included in the model. Finally, this work proposes a more flexible means of calibrating thermodynamic based constitutive models for MSMAs.

Accessibility investigation of large magnetic entropy change in CoMn1−xFexGe

The influence of Fe doping for Mn in CoMn1−xFexGe alloys on their structural and magnetic phase transitions was investigated. CoMn1−xFexGe alloys display a first-order magneto-structural transition from a high temperature paramagnetic hexagonal Ni2In-type crystal structure to a low temperature ferromagnetic orthorhombic TiNiSi-type structure. An entropy change of ΔSM = −32.8 J/kg K was measured and adiabatic temperature change of −8.0 K was computed for the x = 0.085 alloy under a magnetic field of 7 T in the vicinity of the phase transition at 317 K. With increasing Fe content, transition temperatures and magnetic entropy changes decrease. In addition to investigating the effect of replacing Mn with Fe, the accessibility of the giant measured entropy change was quantified by means of the minimum magnetic field required to start and to complete the magneto-structural phase transition.

The Tunable Microstructure and Its Influence on the Giant Magnetocaloric Effect in Magnetic Shape Memory Alloys

The entropy and adiabatic temperature changes upon martensitic transformation induced by external magnetic fields were studied in various NiCoMn-X (X=Sn, In) magnetic shape memory alloys (SMAs). Their relative cooling power were determined using an extensive thermodynamic framework. Tunable materials parameters that lead to an improved caloric effect were identified, including magnetic-field sensitivities of martensitic transformation temperatures, transition ranges, and thermal hysteresis, and magnetization changes across the transition. These properties were determined experimentally from simple magneto-thermo-mechanical measurements before and after various annealing treatments and were identified to be related to microstructural features such as atomic ordering and grain size. The results indicated that large grain size and disordered austenite phases yield small transition ranges and hysteresis for the studied compositions. The improved microstructure after annealing also reduced the required driving force used to induce the temperature change, thereby revealing the possibility of using magnetic SMAs in room temperature solid state refrigeration.