Berthold Krevet

SMA microgripper system

A microgripper system is presented, which consists of a monolithic shape memory alloy (SMA) device of 2 mm x 5.8 mm x 0.23 mm size and an integrated optical position sensor. Gripper closing and opening is performed by two integrated actuators, which form an antagonistic pair. Investigations of temperature profiles by coupled finite-element simulations and infrared microscopy demonstrate a sufficient thermal insulation of the actuators for their selective control. The motion of gripping jaws is transmitted by an integrated gearing mechanism into a linear motion of an integrated optical slit, which is detected by change of optical transmission. The maximum stroke and force of the gripping jaws are 300 μm and 35 mN, respectively. In the range between 10 and 90% of the maximum stroke, positioning is achieved within 140 ms with an accuracy of about 2 μm.

Recent Progress in FSMA Microactuator Developments

The giant magneto-strain effect is particularly attractive for actuator applications in micro- and nanometer dimensions as it enables contact-less control of large deformations, which can hardly be achieved by other actuation principles in small space. Two different approaches are being pursued to develop ferromagnetic shape memory (FSMA) microactuators based on the magnetically induced reorientation of martensite variants: (1) the fabrication of free-standing epitaxial Ni-Mn-Ga thin film actuators in a bottom-up manner by magnetron sputtering, substrate release and integration technologies and (2) the top-down approach of thickness reduction of bulk Ni-Mn-Ga single crystals to foil specimens of decreasing thicknesses (200 – 40 μm) and subsequent integration. This review describes the fabrication technologies, procedures for thermo-mechanical training adapted to the quasi-two-dimensional geometries of film and foil specimens as well as the performance characteristics of state-of-the art actuators after processing and training.

Magnetic Shape Memory Microactuators

By introducing smart materials in micro systems technologies, novel smart microactuators and sensors are currently being developed, e.g., for mobile, wearable, and implantable MEMS (Micro-electro-mechanical-system) devices. Magnetic shape memory alloys (MSMAs) are a promising material system as they show multiple coupling effects as well as large, abrupt changes in their physical properties, e.g., of strain and magnetization, due to a first order phase transformation. For the development of MSMA microactuators, considerable efforts are undertaken to fabricate MSMA foils and films showing similar and just as strong effects compared to their bulk counterparts. Novel MEMS-compatible technologies are being developed to enable their micromachining and integration. This review gives an overview of material properties, engineering issues and fabrication technologies. Selected demonstrators are presented illustrating the wide application potential.

A magnetic shape memory foil actuator loaded by a spring

This paper presents experimental and simulation results on the performance of a novel linear actuator that uses the magnetic shape memory (MSM) effect in a Ni?Mn?Ga foil device loaded by a mechanical spring. The linear MSM actuator shows reversible actuation cycles with maximum magnetic field induced strain change of 5.6% for optimized spring constant and prestress. The experimental results are compared with simulations based on a thermodynamics-based Gibbs free energy model. The model has been implemented in a finite element program, which uses beam elements and an integral magnetic solver. The simulations qualitatively describe the observed tensile stress dependence of the magnetostrain of the MSM foil actuator. We demonstrate that the effects of material inhomogeneity need to be taken into account to further improve the agreement with the experiment.

Ni-Mn-Ga shape memory nanoactuation

To probe finite size effects in ferromagnetic shape memory nanoactuators, double-beam structures with minimum dimensions down to 100 nm are designed, fabricated, and characterized in-situ in a scanning electron microscope with respect to their coupled thermo-elastic and electro-thermal properties. Electrical resistance and mechanical beam bending tests demonstrate a reversible thermal shape memory effect down to 100 nm. Electro-thermal actuation involves large temperature gradients along the nanobeam in the order of 100 K/μm. We discuss the influence of surface and twin boundary energies and explain why free-standing nanoactuators behave differently compared to constrained geometries like films and nanocrystalline shape memory alloys.

Thermodynamic Modelling of Ferromagnetic Shape Memory Actuators

We present a thermodynamic Gibbs free energy model for the finite element simulation of the coupled thermo-magneto-mechanical behavior of ferromagnetic shape memory alloys (FSMAs). Starting from a free energy model for the conventional shape memory effect, additional terms are included to take into account the magnetic anisotropy and the geometry-dependent magnetostatic energy. Different functions are considered for the strain dependence of the anisotropy energy in order to describe the experimentally found strong dependence of the anisotropy energy on the ratio of short and long crystallographic axis c/a. The resulting energy landscape is used to calculate the transition probabilities between three martensite variants and the austenite state under applied stress and external magnetic field. The magnetic shape memory effect is simulated for different loading conditions and sample geometries. We demonstrate the influence of the c/a dependence of the anisotropy energy as well as the influence of twinning strain and elastic modulus on the transition between martensite variants. The model calculations are compared with experimental results on Ni-Mn-Ga single crystals.

Modeling and FEM Simulation of Shape Memory Microactuators

This article reports on two models for the shape memory effect and explains, how they are implemented in a finite element method program. The first model uses a phenomenological approach. For the example of a microgripper, the performance prediction of real actuators made of polycrystalline materials is demonstrated. In the second model, the martensite-austenite phase transition is treated as a thermodynamically activated process. Thermodynamic laws, like e.g. the minimization of the Gibbs free energy, are used for the formulation. To simplify the model, it is primarily intended to describe the behavior of single crystals. By comparing the simulated bending characteristic of a cantilever beam with experimental data, the applicability to polycrystalline material is tested. Due to the physics based formulation, this model gives more insight into the structural processes involved. This is very useful, e.g., for physical extensions needed for the simulation of the magnetic shape memory effect. It is shown, how the model can be extended to predict the behavior of actuators made of ferromagnetic Ni-Mn-Ga single crystals in a magnetic field.

Evolution of Temperature Profiles during Stress-Induced Transformation in NiTi Thin Films

This paper presents a time-resolved investigation of tensile loading induced temperature profiles in pseudoelastic NiTi thin film specimens. A finite element model for coupled mechanical and time dependent thermal analysis is presented that accounts for the different effects of local generation of latent heat and heat transfer. For strain rates larger than 0.24/s, the maximum temperature increases from room temperature to above 50 °C. Subsequent stress relaxation after temperature compensation results in a temperature decrease from room temperature down to -20 °C. The observed evolution of temperature profiles is in qualitative agreement with infrared thermography experiments on NiTi films fabricated by magnetron sputtering.

Development of a meso-scale thermo-magneto-mechanical free energy model for NiMnGa

This paper motivates a one-dimensional thermo-magneto-mechanical free energy model for NiMnGa. Following a discussion of material behavior and modeling purpose, we present what might be referred to as a meso-scale model, incorporating micro-scale physics while striving for macro-scale simplicity. Development of the model begins with the construction of a free energy landscape for the material, with strain and magnetization as its order parameters. This landscape includes paraboloidal energy wells - isolated from each other by energy barriers - to represent stable states of the material. The energy well positions and barrier heights are allowed to vary as functions of stress, magnetic field, and temperature. The resulting equations are employed within the theory of thermally activated processes to find the phase-fraction evolution of a sample. Previous results demonstrating the potential of the modeling approach are included.

Shape memory micromechanisms for microvalve applications

Two micromechanisms including a microactuator of a shape memory alloy (SMA) and a retaining system are presented, which are implemented in a microvalve to maintain a closed condition while no power is supplied. In one design, the retaining system is realized by a pseudoelastic SMA microspring coupled to the SMA microactuator. Alternatively, a pressure compensation mechanism is developed based on two mechanically coupled membranes, which are located above and below the SMA microactuator. The mechanical, electrical and thermal behaviors of the SMA microactuator are simulated by a coupled finite element program. Based on force-displacement characteristics of microspring and microactuator, a design of the two micromechanisms is developed. The investigation reveals several advantages of the pressure-compensation mechanism. In particular, pressure compensation allows a maximum controllable pressure difference of more than 500 kPa compared to 100 kPa for the microspring mechanism. Furthermore, a larger actuation stroke close to the maximum possible design value is achieved. Dynamic flow measurements reveal similar time constants for both mechanisms of 15 and 55 ms for opening and closing, respectively.