No abstract

Recent developments on Heusler alloys including Ni–Mn–X and Ni–Co–Mn–X (X = Ga, In, Sn,…) demonstrate multiferroic phase transformations with large abrupt changes in lattice parameters of several percent and corresponding abrupt changes in ferromagnetic ordering near the transition temperatures. These materials enable a new generation of thermomagnetic generators that convert heat to electricity within a small temperature difference below 5 K. While thermodynamic calculations on this energy conversion method predict a power density normalized to material volume of up to 300 mW cm−3, experimental results have been in the range of µW cm−3. Challenges are related to the development of materials with bulk‐like single‐crystal properties as well as geometries with large surface‐to‐volume ratio for rapid heat exchange. This study demonstrates efficient thermomagnetic generation via resonant actuation of freely movable thin‐film devices of the Heusler alloy Ni–Mn–Ga with unprecedented power density of 118 mW cm−3 that compares favorably with the best thermoelectric generators. Due to the large temperature‐dependent change of magnetization of the films, a periodic temperature change of only 3 K is required for operation. The duration of thermomagnetic duty cycle is only about 12 ms, which matches with the period of oscillatory motion.

Cooling and thermal management comprise a major part of global energy consumption. The by far most widespread cooling technology today is vapor compression, reaching rather high efficiencies, but promoting global warming due to the use of environmentally harmful refrigerants. For widespread emerging applications using microelectronics and micro-electromechanical systems, thermoelectrics is the most advanced technology, which however hardly reaches coefficients of performance (COP) above 2.0. Here, we introduce a new approach for energy-efficient heat pumping using the elastocaloric effect in shape memory alloys. This development is mainly targeted at applications on miniature scales, while larger scales are envisioned by massive parallelization. Base materials are cold-rolled textured Ti 49.1 Ni 50.5 Fe 0.4 foils of 30 μm thickness showing an adiabatic temperature change of +20/−16 K upon superelastic loading/unloading. Different demonstrator layouts consisting of mechanically coupled bridge structures with large surface-to-volume ratios are developed allowing for control by a single actuator as well as work recovery. Heat transfer times are in the order of 1 s, being orders of magnitude faster than for bulk geometries. Thus, first demonstrators achieve values of specific heating and cooling power of 4.5 and 2.9 W g −1 , respectively. A maximum temperature difference of 9.4 K between heat source and sink is reached within 2 min. Corresponding COP on the device level are 4.9 (heating) and 3.1 (cooling).

Strain and temperature profiles of magnetronsputtered ferroelastic TiNi-based films of 20 lm thickness are investigated during tensile load cycling with respect to strain, strain rate, and cycle number in order to assess their potential for elastocaloric cooling. Two different ferroelastic film specimens are considered, binary TiNi and quaternary TiNiCuCo films, which strongly differ regarding their phase transformation hysteresis and fatigue behavior. In situ digital image correlation and infrared thermography measurements reveal a correlated response of strain and temperature bands that is determined by mesoscale stress and temperature fields on the kinetics of phase transformation. In the case of binary TiNi films, this response is also strongly affected by cycling-induced fatigue causing vanishing band formation and decreasing elastocaloric effect size. In contrast, TiNiCuCo films show negligible fatigue and retain the local characteristics of the elastocaloric effect. Compared to TiNi films, they exhibit not only a reduced temperature change, but also a reduced work input for pseudoelastic cycling resulting in an improved material's coefficient of performance of 15.

optical, medical and lab-on-chip systems including mobile, wearable and implantable devices. [ 5 ] Various concepts for energy harvesting on a miniature scale have been developed in the past based, e.g., on piezoelectric, [ 1,6 ] electromagnetic induction, [ 7 ] electrostatic, [ 8 ] and thermoelectric principles. [ 9 ] In the fi rst three cases, kinetic energy is generated by using vibration in the environment. These systems are commonly based on electromechanically coupled spring-damper systems.Thermoelectric principles use temperature gradients in the environment to produce electrical power (Seebeck effect). In contrast to the aforementioned principles, no moving parts are required. While being versatile for various applications, they face a number of diffi culties when it comes to miniaturization. The effi ciency of thermoelectric devices is determined by the fi gure of merit ZT , which is in the order of 1 in best cases. [ 10 ] In order to obtain a reasonable output, relatively large temperature differences Δ T and means of heat sinking beyond natural convection are required. [ 11 ] In small dimensions, however, Δ T is reduced and, thus, energy conversion becomes ineffi cient.For the use of energy resources stored at small Δ T below 20 K, smart materials showing a fi rst order phase transformation without diffusion are highly attractive. Recent developments on MSMAs demonstrate abrupt changes in lattice parameters beyond 10% and corresponding large changes in their magnetic properties (magnetization, magnetic anisotropy) at small Δ T . [12][13][14][15][16][17] Owing to their multifunctional properties, these materials may perform different tasks while keeping the design simple, which is important for downscaling. Due to these reasons, magnetic SMAs are predestined for thermal microenergy harvesting.Recently, a new series of magnetic SMA systems, Ni-Mn-X-Y (X: In, Sn, Sb, Y: Co, Fe) has been found showing a fi rst order phase transformation with a large change of magnetization Δ M . [14][15][16][17] Ni-Mn-In and Ni-Co-Mn-In alloys show a particularly drastic Δ M effect due to a martensitic transition from a ferromagnetic austenite phase to a nonferromagnetic martensite phase. [ 18,19 ] An important prerequisite for energy conversion in a cyclic process are highly reversible phase transformations. The stress associated with the change of lattice parameters during phase transformation can cause pronounced microstructural changes including the formation of dislocations and other A new method for thermal energy harvesting at small temperature difference and high cycling frequency is presented that exploits the unique magnetic properties and actuation capability of magnetic shape memory alloy (MSMA) fi lms. Polycrystalline fi lms of the Ni 50.4 Co 3.7 Mn 32.8 In 13.1 alloy are tailored, showing a large abrupt change of magnetization and low thermal hysteresis well above room temperature. Based on this material, a free-standing fi lm device is designed that exhibits thermomagnetically induced actuation between a hea...