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This paper presents the results of a comprehensive study of the pull-in phenomenon in the hybrid levitation microactuator (HLMA), in which square-shaped proof masses (PMs) of different sizes, namely, length sides of 2.8 and 3.2 mm and thicknesses of 25 and 10 μm were electromagnetically levitated. The pull-in actuation of the square-shaped PMs was performed by the electrostatic force generated by the set of energized electrodes and acting on the bottom surface of the PMs along the vertical direction. The pull-in parameters, such as pull-in displacements and the corresponding applied pull-in voltages, were measured with the developed setup. The experimental measurements showed that the pull-in actuation is nonlinearly dependent on the size and mass of the PMs and a levitation height. In particular, it was found that PMs levitated within a height range from 140 to 170 μm can be stably displaced within a range of 30 μm. The results of measurements were extensively simulated with the developed analytical model by means of the quasi-FEM method. The direct comparison of the results of simulation and measurements showed a very good agreement between the theory and experiments.

Refrigerants in vapor-compression systems have a global warming potential thousands of times that of carbon dioxide, yet their spread on the market is unrivalled. Elastocaloric systems, based on solid state cooling, feature among the most promising alternatives. In this paper, an elastocaloric device for air ventilation (ECV) composed by parallel and serial connection of multiple shape memory alloy (SMA) films, is investigated via volume-based finite difference simulation in MATLAB and dynamic building simulation in TRNSYS considering eight cities across the globe. The models assume experimentally demonstrated thermal parameters for the elastocaloric phase transformation around room temperature and a single-storey reference building. The ECV operates according to an optimized, energy-saving logic that includes load partialization and recirculation. Parametric analyses suggest that moderate terminal velocities (∼2 m/s) and a climate-specific design aimed at maximizing the use of the ECV device at nominal cooling capacity are key to reach building cooling needs reductions up to 70% in the considered scenarios. Partialization results in enhanced energy flexibility and conservation, whereas recirculation extends the ECV usability to extreme heat conditions. In absolute terms, the ECV works best under hot climates (e.g. Cairo, Dubai, Brisbane), with monthly cooling load reductions about 2/3-fold compared to cold locations (e.g. Milan, Hobart). The performance is extremely sensitive to the ventilation rate. Thermal zones requiring 1 to 2 air changes per hour are best suited. These findings provide initial insight into design criteria, opportunities and limitations on the use of elastocaloric devices for building ventilation to guide future experimental verification.

The exploitation of the elastocaloric effect in superelastic shape memory alloys (SMA) for cooling applications shows a promising energy efficiency potential but requires a better understanding of the non-homogeneous martensitic phase transformation. Temperature profiles on sputter-deposited superelastic $${\mathrm {Ti_{55.2}Ni_{29.3}Cu_{12.7}Co_{2.8}}}$$ shape memory alloy thin films show localized release and absorption of heat during phase transformation induced by tensile deformation with a strong rate dependence. In this paper, a model for the simulation of the thermo-mechanically coupled transformation behavior of superelastic SMA is proposed and its capability to reproduce the mechanical and thermal responses observed during experiments is shown. The procedure for experiment and simulation is designed such that a significant temperature change from the initial temperature is obtained to allow potential cooling applications. The simulation of non-local effects is enabled by the use of a model based on the one-dimensional Muller–Achenbach–Seelecke model, extended by 3D mechanisms such as lateral contraction and by non-local interaction, leading to localization effects. It is implemented into the finite element software COMSOL Multiphysics, and comparisons of numerical and experimental results show that the model is capable of reproducing the localized transformation behavior with the same strain rate dependency. Additionally to the thermal and the mechanical behavior, the quantitative prediction of cooling performance with the presented model is shown.

Summary Thermomagnetic (TM) generators based on Heusler alloy films have the potential to recover waste heat below 200°C at small temperature differences . Progress in the development of materials that exhibit large abrupt changes in ferromagnetic ordering and in film engineering enable efficient thermomagnetic generation via resonant self-actuation of freely movable film-based devices. Yet, power levels of individual devices are low, and upscaling becomes a key issue of material development and engineering. Here, we address the key question of how film thickness and device footprint affect power and efficiency. We investigate the scaling performance of heat intake, heat dissipation, and resulting local temperature changes. Based on this understanding, the electrical power per footprint could be increased by a factor of 3.4. Maximum values of electrical power per footprint are 50 μW/cm2 at a temperature change of only 3°C, which marks an important milestone in the upscaling of Heusler alloy film-based TM generators.

The thermal induced martensitic phase transition in TiNiHf was exploited for bi-directional actuation with TiNiHf/SiO2/Si composites. When compared to free-standing films of similar thickness, films on a substrate exhibit a reduced fatigue effect upon thermal cycling and a smaller hysteresis width. Differential scanning calorimetry (DSC) and cantilever deflection measurements (CDM) results showed that the transition temperatures of fabricated TiNiHf films and TiNiHf/SiO2/Si bimorph composites decrease with thermal cycling. The change in transition temperatures after 40 thermal cycles is significantly reduced for TiNiHf films bound to a SiO2/Si substrate compared to the functional fatigue DSC results reported for freestanding films. The thermal hysteresis width is also reduced for TiNiHf films constrained by SiO2/Si and Si substrates compared to freestanding films of similar thicknesses. With proper composition selection and microstructural control, TiNiHf films can be promising SMA films for bistable actuators with PMMA/TiNiHf/Si composites.

In this article, we report on a novel technological approach for miniaturization inductive levitating micro-suspension based on the nested 3D micro-coil structures. In the developed approach, each 3D micro-coil is fabricated separately, beginning with the innermost and thus smallest coil diameter of the nested microstructure. This helps to overcome fabrication restrictions due to the wire-bonding process and primarily caused by the size of the bond-head, and provides the opportunity to fabricate nested 3D micro-coil structures of a smaller size. We fabricated a nested two-micro-coil structure, the inner coil having a diameter of \SI{1000}{\micro\m} and 14 windings, the outer coil with a diameter of 1900m and 8 windings, and demonstrate its application as an inductive levitating micro-suspension. In particular, a fabricated 3D inductive levitating micro-suspension was able to levitate a 1100m diameter disc-shaped proof mass to a height up to 45m.

Due to their high-energy density, shape memory alloys (SMAs) are investigated as material for bending microactuators in applications of self-folding structures, realizing the concept of programmable matter. Here, for the numerical prediction of the electro-thermo-mechanical performance, the quantification of the time-dependent coupling effects in SMA materials during phase transformation is of crucial interest. Isothermal SMA material models cannot treat the time-dependent interaction between deformation, temperature and electric potential in thermally controlled actuation. In this paper, we extend an isothermal SMA model using standard thermodynamics (Coleman–Noll procedure) to treat the time-dependent behavior of polycrystalline SMAs. The model is implemented as a user material subroutine (UMAT) in a standard finite element (FE) code (Abaqus standard). The time-dependent loading of a tensile sample and a bending microactuator made from 20 $$\mu \hbox {m}$$ μ m thick SMA foil are simulated. A comparative study between experimental and simulation results on the thermoelastic and caloric effects during stress-induced phase transformation is presented. Joule heating simulations for shape recovery during both tensile and bending loading are conducted. Time-resolved temperature variations accompanying the loading and Joule heating processes are reported. The coupled SMA material model is found to be capable of approximating the time-dependent field quantities of a polycrystalline SMA microactuator subjected to electro-thermo-mechanical loading.

We investigate the cross coupling between parallel operating thermo-magnetic generators (TMGs) on their dynamic performance and power output. TMGs are an emerging technology for waste heat recovery. Here, TMG operation relies on the abrupt drop of magnetization at the Curie temperature of a magnetic shape memory (MSMA) film of Ni-Mn-Ga mounted at the front of a cantilever and on rapid heat transfer due to the film's large surface-to-volume ratio. Parallel designs are highly demanded to increase power output. When exposed to heat source temperatures Ts of 110-170 °C, resonant self-oscillation of each cantilever is excited in the range of 50-70 Hz resulting in a power output up to $30\ \text{mW}/\text{cm}^{3}$ for each TMG. At low Ts, oscillations remain stable even for a small distance $D$ of 0.4 mm between cantilevers, while at $\text{Ts}\ \geq\ 150^{\circ}\mathrm{C}$ , instabilities occur below $D=1$ mm affecting power output.

This paper describes an innovative miniature multi-port valve with a thin foil of shape memory alloy (SMA) as actuator for switching and dosing gaseous and liquid media. The normally closed (NC) microvalve has two structured SMA actuators that are switched independently of each other and either two inputs and one output or one input and two outputs. In addition to switching the media in the 3/4-way arrangement, it can also be used with a flow sensor in a closed loop control for dosing. Furthermore, the valve design is layer-based so that individual components can be manufactured according to given requirements or using different manufacturing technologies depending on the batch size. The SMA multi-port microvalve showed flow rates of about 2300 ml/min (nitrogen gas) and about 45 ml/min (water) for an applied pressure difference of 200 kPa and a heating current of about 400 mA. For flow regulation a closed loop control was realized and evaluated for a pressure difference of 100 kPa and a setpoint value of 900 ml/min.

International audience; As an alternative to thermoelectric generators, heat engines show great interest thanks to their ability to convert temperature spatial gradient into time-domain temperature variations or vibrations. To this end, MultiPhysic Memory Alloys (MPMAs), combining shape memory characteristics with ferromagnetic properties, provide significant attractive characteristics such as sharp transition with reduced hysteresis as well as magnetic properties enabled by heating, thus allowing easier device development and implementation. In this study, we report the development of a heat engine for small-scale energy harvesting where the MPMA transfers its heat to a pyroelectric element that provides thermal to electrical energy conversion, yielding a more direct energy conversion path compared to conventional electromechanical heat engines. Furthermore, thermally decoupling the pyroelectric element from the MPMA allows a faster cooling of the latter, accounting for higher variation frequency. Compared to the use of electromagnetic transduction through a coil attached to the moving MPMA, this approach is shown to provide 3 to 9 times more power density (according to considered volume), with theoretical potential gains from 8 to 25 with the use of nonlinear electrical interfaces.

This paper presents a lumped element model (LEM) to describe the coupled dynamic properties of thermomagnetic generators (TMGs) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a freely movable cantilever, caused by a large abrupt temperature-dependent change of magnetization and rapid heat transfer inherent to the MSMA films. The LEM is validated for the case of a Ni-Mn-Ga film with Curie temperature TC of 375 K. For a heat source temperature of 443 K, the maximum power generated is 3.1 µW corresponding to a power density with respect to the active material’s volume of 80 mW/cm3. Corresponding LEM simulations allow for a detailed study of the time-resolved temperature change of the MSMA film, the change of magnetic field at the position of the film and of the corresponding film magnetization. Resonant self-actuation is observed at 114 Hz, while rapid temperature changes of about 10 K occur within 1 ms during mechanical contact between heat source and Ni-Mn-Ga film. The LEM is used to estimate the effect of decreasing TC on the lower limit of heat source temperature in order to predict possible routes towards waste heat recovery near room temperature.

In recent years, considerable efforts have been undertaken to fabricate and investigate the low-dimensionally shaped Heusler-type magnetic shape memory alloys (MSMAs). As a result, new light-weight, multifunctional materials, such as MSMA/polymer composites, foams, thin films, micropillars, thin wires and ribbons, are developed for small-scale applications such as actuators, sensors, and energy conversion devices. These materials have been shown to exhibit a diversity of physical phenomena caused by size effects. Particularly, various thermo-magneto-mechanical coupling effects have been found in these materials at small scales opening up novel routes for microactuation and related micro-electro-mechanical-systems (MEMS) applications. This chapter highlights the last developments in the MSMA research field providing a brief overview of the properties, engineering issues, and fabrication technologies of such materials. The recent achievements in micro- and nanotechnology of MSMAs and first demonstrators for nanoactuation are represented.

This paper presents the design, fabrication and performance of origami-based folding microactuators based on a cold-rolled NiTi foil of 20 µm thickness showing the one-way shape memory effect. Origami refers to a variety of techniques of transforming planar sheets into three-dimensional (3D) structures by folding, which has been introduced in science and engineering for, e.g., assembly and robotics. Here, NiTi microactuators are interconnected to rigid sections (tiles) forming an initial planar system that self-folds into a set of predetermined 3D shapes upon heating. While this concept has been demonstrated at the macro scale, we intend to transfer this concept into microtechnology by combining state-of-the art methods of micromachining. NiTi foils are micromachined by laser cutting or photolithography to achieve double-beam structures allowing for direct Joule heating with an electrical current. A thermo-mechanical treatment is used for shape setting of as-received specimens to reach a maximum folding angle of 180°. The bending moments, bending radii and load-dependent folding angles upon Joule heating are evaluated. The shape setting process is particularly effective for small bending radii, which, however generates residual plastic strain. After shape setting, unloaded beam structures show recoverable bending deflection between 0° and 140° for a maximum heating power of 900 mW. By introducing additional loads to account for the effect of the tiles, the smooth folding characteristic evolves into a sharp transition, whereby full deflection up to 180° is reached. The achieved results are an important step towards the development of cooperative multistable microactuator systems for 3D self-assembly.

The control circuit for inductive levitation micro-actuators is developed in this research. The circuit performance and its electrical parameters are discussed. The developed control circuit was fabricated on a 4-layer PCB board having a size of 60 mm by 60 mm. It consists of a generator based on high-speed Flip-Flop components and a current amplifier build on a bridge configuration. The circuit is able to generate ac current with squared shape in a range of frequency from 8 to 43 MHz and with peak-to-peak amplitude up to 420 mA. To demonstrate the efficiency of the developed circuit and its compatibility with the micro-actuation system, an inductive levitation micro-actuator was fabricated by using 3D micro-coil technology. The device was composed of two solenoidal coil design, which consists of levitation and stabilization coil, having 2 mm and 3.8 mm in diameters, respectively. The levitation coil has 20 turns of a gold wire of a 25 µm diameter, while the stabilization one has 12 turns similar to the micro-structure presented previously by our group. Using the developed control circuit, the micro-actuator was successfully excited and it demonstrated the actuation of aluminum disc-shaped micro-objects having a diameter of 2.7 and 3.2 mm and, for the first time, an aluminum square shaped micro-object having a side-length of 3.4 mm at a frequency of 10 MHz. To characterize the actuation, the levitation height and the rms amplitude of current were measured. In particular, we showed that the square shaped micro-object can lift up on a height of 100 µm with rms current of 160 mA. The characterization is supported by the simulation using a 3D model based on the quasi-FEM approach.

The high work density and beneficial downscaling of shape memory alloy (SMA) actuation performance provide a basis for the development of actuators and systems at microscales. Here, we report a novel monolithic fabrication approach for the co-integration of SMA and Si microstructures to enable SMA-Si bimorph microactuation. Double-beam cantilevers are chosen for the actuator layout to enable electrothermal actuation by Joule heating. The SMA materials under investigation are NiMnGa and NiTi(Hf) films with tunable phase transformation temperatures. We show that Joule heating of the cantilevers generates increasing temperature gradients for decreasing cantilever size, which hampers actuation performance. In order to cope with this problem, a new method for design optimization is presented based on finite element modeling (FEM) simulations. We demonstrate that temperature homogenization can be achieved by the design of additional folded beams in the perpendicular direction to the active beam cantilevers. Thereby, power consumption can be reduced by more than 35 % and maximum deflection can be increased up to a factor of 2 depending on the cantilever geometry.

This paper presents a simulation model for the coupled dynamic properties of thermomagnetic generators based on magnetic shape memory alloy (MSMA) films. MSMA thermomagnetic generators exploit the large abrupt temperature-induced change of magnetization at the first- or second-order magnetic transition as well as the short heat transfer times due to the large surface-to-volume ratio of films. These properties allow for resonant self-actuation of freely movable MSMA cantilever devices showing thermomagnetic duty cycles in the order of 10 ms duration, which matches with the period of oscillatory motion. We present a numerical analysis of the energy conversion processes to understand the effect of design parameters on efficiency and power output. A lumped element model is chosen to describe the time dependence of MSMA cantilever deflection and of temperature profiles as well as the magnitude and phase dependency of magnetization change. The simulation model quantitatively describes experimentally observed oscillatory motion and resulting power output in the order of 100 mW cm−3. Furthermore, it predicts a power output of 490 mW cm−3 for advanced film materials with temperature-dependent change of magnetization ∆M/∆T of 4 A m2 (kg K)−1, which challenges state-of-the-art thermoelectric devices.

In the early nineties, microelectromechanical systems (MEMS) technology has been still in its infancy. As silicon (Si) is not a transducer material, it was clear at the very beginning that mechanically active materials had to be introduced to MEMS in order to enable functional microdevices with actuation capability beyond electrostatics. At that time, shape memory alloys (SMAs) have been available in bulk form, mainly as SMA wires and SMA plates. On the macro scale, these materials show highest work densities compared to other actuation principles in the order of 107 J/m3, which stimulated research on the integration of SMA to MEMS. Subsequently, two approaches for producing planar materials have been initiated (1) magnetron sputtering of SMA thin films and (2) the integration of rolled SMA foils, which both turned out to be very successful creating a paradigm change in microactuation technology. The following review covers important milestones of the research and development of SMA foil-based microactuators including materials characterization, design engineering, technology, and demonstrator development as well as first commercial products.

A model for the evolution of the mechanical and thermal properties of shape memory alloy (SMA) films during elastocaloric cycling is developed and compared with experiments. The focus is on Ti-Ni-Cu-Co films of 20 μm thickness showing ultra-low fatigue properties. The films undergo a highly localized pseudoelastic transformation under tensile load cycling featuring strain and temperature band patterns that depend on the loading conditions. The corresponding temperature change is of special interest for film-based elastocaloric cooling applications. Starting from a thermodynamics-based Gibbs free energy model comprising mechanical and chemical contributions, we include a martensite-austenite interface free energy term, for which formulations from a phase-field model are adapted. A 3D continuum mechanics description is modified to treat plane stress conditions appropriate for polycrystalline thin films. The nucleation mechanism of strain bands under dynamic loading is described by introducing a spatial random distribution of the transformation stress barriers reflecting the degree of material inhomogeneity. Heat transfer due to conduction and convection is taken into account. The simulations predict the correlated mechanical and thermal local response of the films including band formation and evolution, tilt angle as well as strain-rate dependence. Macroscopic stress-strain characteristics and thermal evolution curves well represent the experimental results.

This paper presents a novel approach to simultaneously fabricate a Si micro test platform and a nanostructured test specimen of a multiferroic material with critical dimension of 200 nm and lateral aspect ratio of 33 for in-situ characterization in a scanning electron microscope (SEM). The design is adjusted to allow for electrical resistance measurements and tensile tests. We report on a fabrication process comprising e-beam lithography and dry etching technology and, as a proof of concept, on the measurement of the force-displacement characteristics of Ni nanobeams.

This study investigated macroscopic inhomogeneous deformation occurring in single-crystal Ni–Mn–Ga foils under uniaxial tensile loading. Two types of single-crystal Ni–Mn–Ga foil samples were examined as-received and after thermo-mechanical training. Local strain and the strain field were measured under tensile loading using laser speckle and digital image correlation. The as-received sample showed a strongly inhomogeneous strain field with intermittence under progressive deformation, but the trained sample result showed strain field homogeneity throughout the specimen surface. The as-received sample is a mainly polycrystalline-like state composed of the domain structure. The sample contains many domain boundaries and large domain structures in the body. Its structure would cause large local strain band nucleation with intermittence. However, the trained one is an ideal single-crystalline state with a transformation preferential orientation of variants after almost all domain boundary and large domain structures vanish during thermo-mechanical training. As a result, macroscopic homogeneous deformation occurs on the trained sample surface during deformation.

Novel damping devices are presented and characterized that make use of energy dissipation due to the one-way shape memory effect in micromachined shape memory alloy (SMA) foil actuators of 30 µm thickness and lateral dimensions a few mm. The design of SMA foil actuators consists of free-standing planar suspensions with bridge, spiral and folded beam design for load control in-plane and out-of-plane. In contrast to SMA wire-based solutions, this new approach is compatible to MEMS batch fabrication technology. Under pulsed loading, a damping capacity of up to 0.92 is achieved, which compares favorably with the best viscoelastic damping concepts.

This paper explores, develops and demonstrates a novel miniature cooling device using the combined elastocaloric effect of three cascaded Ti-Ni-Cu-Co films allowing to double the temperature span of a single film device being 7.6 K to 15 K. Ultra-low fatigue shape memory alloy (SMA) films are arranged in series to investigate in detail the effects of operation conditions and number of cascaded units on the cooling performance. By the new concept, the temperature span of the elastocaloric cooling device can overcome the adiabatic limit of the used elastocaloric material. The specific cooling power of a first-of-its kind demonstrator reaches 17 Wg-1 which is in line with performance predictions.

In the pursuit of ever more efficient built environments, able to resiliently respond to the many implications of climate change, near room-temperature caloric cooling could be a game changer from multiple standpoints. In this paper, perspectives and challenges of successful implementation of elastocaloric devices in the built environment are explored by contrasting the current readiness level with the envisaged potentiality. Material-level and device-level criticalities are identified and potential solutions are discussed. The roadmap towards an informed and efficient use of this environmentally friendly technology is eventually proposed aiming at an increase of building’s energy efficiency, but also at counteracting the urban heat island effect.