Elastocaloric cooling is a low-global-warming-potential alternative to vapor-compression refrigeration; however, the high cost of shape memory alloys limits its large-scale deployment. Polymer-based materials such as natural rubber (NR) and thermoplastic polyurethanes (TPU) provide cost-effective alternatives but undergo large operational strains (up to 600\%), which challenge conventional regenerator designs and numerical modeling approaches. In this study, a transient one-dimensional numerical model with a dynamic mesh formulation is developed to capture coordinate transformations and coupled thermo-hydraulic behavior in large-deformation elastocaloric regenerators. Four microchannel geometries (parallel plate, square, triangular, and circular) are evaluated. Using NR as the working material, the triangular configuration achieves the highest specific cooling power (SCP) due to enhanced heat transfer and reduced viscous dissipation. Parametric optimization identifies an optimal pore size of $\sim$500 $\mu$m at 50\% porosity. Replacing NR with high elastocaloric response TPU (HM-PTMEG) further improves cooling performance. At an operating frequency of 1 Hz, the optimized triangular TPU regenerator achieves an SCP of 20.1 Wg$^{-1}$, which is three times higher than that of NR. The systems exhibit the cost per unit cooling power of $3.7-4.6 \times 10^{-4}$ \$/W, demonstrating strong economic competitiveness. This work provides quantitative design guidelines for scalable polymer-based regenerator elastocaloric cooling systems.

This paper presents the experimental and model-based investigation of a compact miniature-size generator for conversion of low-grade waste heat at temperatures near 100 °C to electricity leveraging the combined transducer effects of a thermomagnetic (TM) and a piezoelectric (P) layer. TM energy conversion is achieved via the recently developed effect of resonant self-actuation of a cantilever using the large temperature-induced change of magnetization at the ferromagnetic transition of a Ni-Mn-Ga film. The resulting kinetic energy is converted into electrical energy via the piezoelectric effect of a thin lead-zirconium-titanate (PZT) layer, which enables rectification of the harvested energy using standard electrical circuitry. The performance of the generator is characterized with respect to the effects of tip mass and length of the piezoelectric layer as well as heat transfer and damping. A lumped element model (LEM) of the coupled system shows that heat intake and heat dissipation have to be balanced to achieve resonant self-actuation, while damping limits the oscillation frequency of the cantilever. A single P-TM generator generates an average power output of 0.8 µW at 105 °C. After rectification, this corresponds to 1.25 µW/cm² of DC power at a voltage of 2 V DC.

The influence of tungsten (W)-doping on the structural, magnetic, and magnetocaloric properties of Ni47Mn40Sn13-xWx (x = 0, 0.5, 1, 1.25) metamagnetic shape memory alloys was investigated using a combination of experimental and first-principles methods. W-doping in Ni-Mn-Sn shifts the martensitic transformation (MT) towards room temperature and enhances the inverse magnetocaloric effect (MCE), with a maximum magnetic-field-induced entropy change of 13.2 J/kg·K at 5 T for the x = 1 alloy. Direct cyclic measurements at 1.96 T show stable adiabatic temperature changes (ΔTad ≈ 0.5 K at the MT temperature and ΔTad ≈ 1.4 K at the Curie temperature) over hundreds of cycles, confirming the cyclic stability. Density functional theory calculations reveal that the W-dopant increases the energy gap between the cubic and tetragonal crystal structures (from 15.8 to 43.6 meV), which correlates with the higher MT temperature observed in W-doped samples. The analysis of the total and local density of states identified W-induced enhancement of hybridization and a change in the density of states near the Fermi level, resulting in a shift of MT to room temperature and an improvement in MCE. W-doping improves the MCE properties of Ni–Mn–Sn alloys, providing a path towards energy-efficient magnetic cooling.

Ti-Ni-Cu shape memory alloy (SMA) thin films with thicknesses ranging from 150 nm to 1000 nm were deposited by magnetron sputter deposition on planar Si/SiO2 substrates and prestructured silicon-on-insulator nano-hinges with the aim to provide optimal films for (bidirectional) microelectromechanical systems (MEMS) actuators. SMA films exhibited excellent compositional homogeneity with a minimal deviation of approximately 2 at.% across a 100 mm diameter wafer. The films transitioned from compressive residual stress in the as-deposited state to tensile stress after annealing, with tensile stress values decreasing from 294 to 71 MPa with increasing film thickness. This correlated with a monotonic increase of the transformation temperatures. In situ transmission electron microscopy between room temperature and 100 degrees C confirmed the B19 -> B2 phase transformation, consistent with X-ray diffraction results, which also revealed minor B19 ' and Ti2Ni precipitates; atom probe tomography further verified Ti2Ni formation. An ion beam etching sidewall removal was implemented to eliminate unwanted deposition on nano-hinges, supporting reliable fabrication of bidirectional MEMS actuators.

Efficient thermal management is critical for next-generation energy systems. Elastocaloric cooling with shape memory alloy (SMA) films offers rapid heat transfer, high specific cooling capacity and simple solid–solid heat exchange, but current devices rely on bulky, electrically powered actuators that limit scalability and efficiency. Here we demonstrate a thermally powered shape memory actuator that drives elastocaloric cooling in tailored SMA films, establishing a heat-driven cooling system that can use waste heat or solar thermal energy with reduced electricity demand. Our prototype achieves a temperature span of 12.9 K at the refrigerant film level and 4.0 K at the device level under Joule-heated actuation at 86 °C. When driven by an external heat source, the system maintains a device-level temperature span of 2.2 K, confirming the feasibility of heat-driven elastocaloric cooling. These results mark a step towards electricity-minimized elastocaloric cooling technologies that transform heat into useful cooling. Elastocaloric cooling in shape memory alloy films is efficient but limited by electrically powered actuators. Here the authors demonstrate a heat-driven system that achieves a temperature span of up to 12.9 K (4.0 K at device level), showing that waste heat can be used for electricity-minimized cooling.

In micro cantilever fracture, a bridge notch geometry with material ligaments at the notch ends helps to reduce focused ion beam artefacts near the notch root by arresting initial cracks and promoting fracture from sharp, natural cracks. Thus, it significantly reduces the statistical scatter in fracture toughness, a common but undesirable feature in micro fracture testing. Although this concept has been validated in simulations and experiments, systematic investigations into the optimal geometry remain lacking. In this study, we experimentally examine the influence of bridge width and notch depth on the fracture toughness of micro cantilevers, using single crystalline silicon as a model material. We found that samples with thinner material bridges and deeper notches exhibit crack arrest before failure, while those with thicker bridges do not show crack arrest instead exhibit apparent toughening. Cantilevers with an optimized bridge notch geometry for crack arrest exhibit a KIC of 1.09 +/- 0.02 MPa m0.5, which agrees with previously reported fracture toughness for the Si (111) surface. Additionally, discrepancies between the bridge geometry in the experiment and the ideal structure resulted in a mismatch between the predicted and observed notch requirements for crack arrest. Our findings offer practical guidelines for designing bridge notch geometries to promote bridge failure, thus improving statistical analysis in micro fracture.

This paper introduces a novel concept of in-plane bistable switching of a freely suspended Si platform using a co-integrated Ni-Ti-Cu/Si micro-actuator with large stroke. The microactuator consists of two coupled prestrained Ni-Ti-Cu/Si beams with nano-scale hinge dimensions, which are designed to exhibit buckling instability. By selective Joule heating, the one-way shape memory effect in the Ni-Ti-Cu layer results in a large shape recovery force allowing to switch between the stable states. First, we show the model-based design of the bistability range and switching force. Then, we present a process flow for co-integration onto a Si photonic chip for optical waveguide switching and demonstrate mechanical switching performance by in-situ experiments in a SEM.

A bending microactuator with large angular range is presented consisting of a novel antagonistic design of cascaded shape memory alloy (SMA) hinges. The antagonistic design comprises two counteracting hinges with double-bridge design to enable bidirectional folding of a pair of rigid tiles serving as a building block of an origami-inspired microsystem. Additional connection of the hinges in series results in a cascaded design to enhance the bidirectional folding angle. A microfabrication process is developed using a combination of magnetron sputtering of TiNiCu films and UV photolithography. The strongly coupled thermo-mechanical properties of the SMA hinges are taken into account in finite element simulations to determine optimized hinge geometries, which allow for selective shape setting at either maximum or minimum bending angle to functionalize their performance as protagonists or antagonists. Thus, localized Joule heating enables folding and unfolding to discrete angles ensuring reversible and programmable folding motions. The bidirectional folding angles of the bending microactuators cover the range of about +/- 105 degrees, which is more than a factor of two compared to microactuators with single hinge design.

We present the design, simulation and characterization of a compact microactuator with overall dimensions of 2.5 x 2.5 x 4 mm(3) allowing for bistable switching a magnetic pin of 1.5 mm diameter with a stroke of 0.4 mm. This performance is achieved through the synergy of shape-memory (SM) and thermomagnetic (TM) microactuation leveraging the one-way SM effect and large magnetization change at the Curie temperature, respectively. A superelastic (SE) spring is designed to balance forces and ensure bistability. This approach enhances mechanical performance, reduces power consumption, and minimizes size compared to previous concepts. Integrating SM-TM microactuators in matrix arrays enables new options for ultra-compact digital micro-mechanical systems, such as haptic devices and Braille displays.

A reprogrammable microactuator system is presented, consisting of antagonistic shape memory alloy (SMA) microactuators for bidirectional folding of miniature-scale tiles following the concept of origami. Additional integrated heatable soft-magnetic pads with low ferromagnetic transition temperature allow for control of magnetic latching forces. The strongly coupled thermo-mechanical and thermo-magnetic properties of the microactuator and magnetic subsystems are taken into account in a model-based design to enable their selective control by Joule heating. A procedure for local shape setting of the SMA microactuators is presented to adjust their memory shape at either maximum or minimum bending angle and, thus, to functionalize their performance as protagonists or antagonists. A microfabrication process is developed that takes the specific requirements for processing the various materials and structures into account. A demonstrator system consisting of four triangular tiles with an edge length of 500 µm and an angular range of about ±100° is introduced that is programmed to adopt the shape of a pyramid and later on reprogrammed to self-unlatch, self-unfold, and subsequently to adopt the shape of a table.

Micro‐grained high surface‐to‐volume ratio thin ribbons of magnetic shape memory Ni 42 Co 8 Mn 39 Sn 11‐ x Ge x ( x = 1, 2, 3) alloys are prepared, and their martensitic transformation (MT) behavior, magnetic and magnetocaloric properties are investigated. X‐ray diffraction reveals that the incorporation of Ge consistently decreases the lattice parameters and the transformation volume change, thereby improving geometric compatibility between martensitic and austenitic crystal lattices. This improvement facilitated a reduction of the thermal hysteresis of MT to a minimum of ≈12.1 K at the Ge concentration of x = 2. Direct measurements of adiabatic temperature changes show that the x = 2 alloy (SnGe2 ribbon) exhibited the highest peak value (≈2 K) at a moderate magnetic field change of 1.96 T. In addition, SnGe2 ribbon demonstrates exceptional isothermal entropy changes of ΔS iso = 35.5 J kg −1 K −1 K at 7 T and 22.8 J kg −1 K −1 K at 2 T, which are competitive with those of bulk alloys and surpassing previously reported melt‐spun ribbons of the Heusler‐type magnetocaloric materials. The significance of intricate microstructure in boosting the magnetocaloric effect is emphasized. These results highlight the substantial potential of Ni–Co–Mn–Sn–Ge thin ribbons as highly effective, micro‐sized magnetocaloric materials for cutting‐edge solid‐state refrigeration systems.

Elastocaloric cooling is an emerging solid-state technology that leverages the reversible latent heat of phase transformation in superelastic shape memory alloys to achieve efficient and environmentally friendly refrigeration. The performance of the regenerator, where elastocaloric effect and heat transfer occur, critically depends on the elastocaloric material and structural design. This study employs a 1-D numerical active elastocaloric regenerator model to evaluate the cooling performance of different woven-structure regenerators. Three typical woven structures-plain weave, twilled weave, and dutch weave-are investigated across varying porosities. The results indicate that a plain weave regenerator with 30 % porosity achieves the highest performance, delivering a maximum cooling power of 485 W, a specific cooling power of 11.3 W/g, and a COP of 1.48 under a 20 K temperature span. Twilled weave regenerators exhibit comparable performance, whereas dutch weave regenerators show significantly lower cooling capabilities. Both plain and twilled weave regenerators outperform the parallel plate regenerator, increasing cooling power by a factor of 3.3, due to enhanced heat transfer and specific heat transfer area. The study highlights the potential of woven-structure regenerators for high-performance elastocaloric cooling, offering insights into optimizing regenerator design and operational parameters while emphasizing their promise for efficient and sustainable solid-state cooling systems.

The smart coupling of microactuators to cooperative microactuator systems enables new functionalities like active bi- and multistability requiring no external force for switching between stable states. This review explores different concepts of cooperative microactuator systems combining microactuation based on either the same or different transducer principles. The transducer principles comprise electrostatic, magnetic, dielectric elastomer and shape memory effects as well as combinations thereof. Thereby, active bi-/multistable switching is achieved via selective control of the microactuators using diverse control signals based on thermal, electrical or magnetic stimuli. The combination of the microactuators in confined space gives rise to various coupling effects and cross-sensitivities that need to be considered. In the following, the engineering aspects of material properties, microsystems design and fabrication, as well as experimental and numerical characterization of system performances and dependencies on design parameters will be discussed. The presented microactuator systems will be assessed with respect to their energy characteristics and critical forces for switching. Their application potential will be highlighted.

We report on the design and characterization of a demonstrator device for miniature-scale elastocaloric (eC) cooling using a series of natural rubber (NR) foil specimens of 9 × 26.5 mm 2 lateral size and thicknesses in the range of 290–900 μ m. NR has the potential to meet the various challenges associated with eC cooling, as it exhibits a large adiabatic temperature change in the order of 20 K and high fatigue resistance under dynamic load, while loading forces are low. Owing to the large surface-to-volume ratio of rubber-based foils, heat transfer to heat sink and source elements is accomplished by mechanical contact enabling compact designs. Two actuators are implemented to control the performance in loading direction independent from the performance of mechanical contacting. The study of operation parameters is complemented by lumped-element modeling to understand the cycle frequency-dependent dynamics of heat transfer and resulting cooling capacity. The single-stage device operates in the strain range of 300%–700% and exhibits a temperature span up to 4.1 K, while the specific cooling power reaches 1.1 Wg −1 and the absolute cooling power 123 mW. The performance metrics show a pronounced dependence on foil thickness and heat transfer coefficient indicating a path toward future device optimization.

A novel solid-state elastocaloric cooling device is presented, making use of a bistable actuation mechanism for loading of a natural rubber (NR) foil refrigerant. The thicknesses of the foil refrigerants are 290 and 650 mu m in an initial undeformed state, while their lateral size is 9 x 26.5 mm(2). Owing to the large surface-to-volume ratio of the NR foils, heat transfer to the heat sink and source is accomplished by a solid-solid mechanical contact. The loading mechanism consists of a rotating lever arm providing for stable positions at contact to the heat sink and source, which allows for significant power saving during elastocaloric cycling. In addition, the negative biasing associated with bistability favors good thermal contact at the end positions, which improves heat transfer resulting in a maximum temperature span Delta T-device of 4.2 K in the strain range of 300%-700% under adiabatic conditions. The coefficient of performance of the device COPdevice reaches values up to 5.7 for foil refrigerants of 290 mu m thickness. The maximum cooling power is 214 mW corresponding to a specific cooling power of 3.4 Wg(-1).

A finite-element (FE) analysis of the active bistability of an antagonistic shape memory alloy (SMA) beam actuator of TiNiCu is presented. The actuator comprises two coupled SMA beams that are clamped at both ends and coupled in their center by a spacer having different memory shapes being deflected in opposite out-of-plane directions. The actuator is characterized by two equilibrium positions. To determine bistable behavior as a function of geometrical parameters, a force criterion is defined by the coupling force of the beams in austenitic and martensitic states. Bistable behavior is achieved, if the coupling force does not change sign in the entire displacement range. This implies that the austenitic beam dominates the opposing martensitic beam. Thus, selective heating of the SMA beams results in a snap-through motion of the coupled SMA beams. Depending on which of the two beams is in austenitic state, either of the two equilibrium positions is reached without the need for an external force. It is demonstrated that geometrical parameters like initial predeflection and spacer length have a crucial effect on the bistable performance. Bistable regions as well as critical limits characterized by geometry-dependent stability ratios, beyond which the actuator's performance becomes monostable, are identified. Bistable shape memory alloy (SMA) beam actuators are designed and optimized using a validated finite-element model. The actuators consist of two antagonistically coupled SMA beams with clamped ends and different memory shapes deflected in opposite out-of-plane directions. Force- and energy-displacement characteristics of the coupled SMA beams are analyzed and monostable and bistable regions depending on design parameters are identified.image (c) 2024 WILEY-VCH GmbH

The ongoing trend of miniaturization and increasing power density in miniaturized systems demand for active temperature control and cooling. The cooling technologies utilized today depend on environmentally harmful substances or are bound to low efficiencies. This leads to an urgent need for innovative cooling technologies that are both environmentally friendly and efficient. This report focuses on shape memory alloy (SMA) film-based elastocaloric (eC) cooling, as SMA films exhibit a large eC effect and enable efficient heat transfer through solid-to-solid contact due to their large surface-to-volume ratio. Among the different material candidates, TiNiCuCo films are of special interest due to their ultra-low fatigue properties and small hysteresis. Single-stage SMA film-based eC devices reach a device temperature span up to 14 K combined with a high specific cooling capacity of up to 19 W g−1. However, absolute cooling capacities in the small-scale device are limited to 220 mW and the device temperature span already reaches the adiabatic limit of the used SMA film. To overcome the limitations in cooling capacity and device temperature span, novel advanced device architectures are explored. A parallelized cooling device combining the eC effect of five SMA films is engineered to increase the absolute cooling capacity up to 900 mW, while it preserves the high specific cooling capacity unique to SMA film-based cooling devices. A cascaded eC cooling device is developed, which consists of a serial arrangement of three SMA films. In this case, the device temperature span is increased beyond the adiabatic limit of a single film to 27.3 K. These results provide a basis for next-generation SMA film-based eC cooling technology.

Abstract The ancient art of origami enables to create 3D shapes from a flat sheet of paper. This concept can be applied to a planar matrix composed of interconnected tiles, which can be self-folded and unfolded using actuated SMA bridges. Origami-inspired self-folding structures offer several advantages, including rapid in-plane fabrication, reduced assembly time, and lightweight designs with high rigidity. Previous research has primarily concentrated on macro-sized demonstrators that have proven advanced functionality in a variety of fields, including robotics and aerospace. In this work, we extend this concept to the microscale by creating a network of self-folding microactuators that can be programmed and controlled through local Joule heating. Our previous concepts faced limitations for reversible latching of tiles in their position after actuation. This paper presents the design, fabrication, and characterization of an origami-inspired bistable microdevices that utilize antagonistic SMA microactuators for bi-directional self- folding and heatable magnetic SMA films for reversible magnetic micro-latching.

We present the design and characterization of solid-state elastocaloric cooling devices based on a natural rubber (NR) foil. Two different loading mechanisms are explored using either two linear actuators or a bistable actuation mechanism using a rotating lever arm, which allows for reduced loading forces by a factor of three. The thicknesses of the investigated NR foil refrigerants are 290 and 650 micrometer in initial undeformed state while their lateral size is 9 x 26.5 mm(exp 2). Owing to the small thickness, heat transfer is accomplished by solid-solid contact between the NR foil and planar heat sink and source elements. The optimal operation frequency depends on the NR foil thickness increasing from 140 to 540 mHz for a thickness of 650 and 290 micrometer, respectively. The maximum temperature span DeltaTDevice is 4.1 K while the coefficient of performance COPDevice reaches values up to 4.7. A maximum cooling power of 144 mW is achieved for bistable actuation, which is enhanced by 17% compared to linear actuation. The specific cooling power increases with decreasing foil thickness reaching 1.2 Wg-1 at 290 micrometer for bistable actuation.