M. Kohl

Shape memory microactuators

1 Introduction and Overview.- 2 Microactuators.- 3 Shape Memory Effects.- 4 Shape Memory Actuation.- 5 SMA Microvalves.- 6 SMA Linear Actuators.- 7 Summary.- References.

108 Gbit/s Plasmonic Mach–Zehnder Modulator with > 70-GHz Electrical Bandwidth

We report on high-extinction-ratio, ultrafast plasmonic Mach-Zehnder modulators. We demonstrate data modulation at line rates up to 72 Gbit/s (BPSK) and 108 Gbit/s (4-ASK). The driving voltages are Ud = 4 and 2.5 Vp for 12.5 and 25 μm short devices, respectively. The frequency response shows no bandwidth limitations up to 70 GHz. Static characterizations indicate extinction ratios > 25 dB.

Energy-efficient miniature-scale heat pumping based on shape memory alloys

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-electro-mechanical 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 Ti49.1Ni50.5Fe0.4 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).

High speed plasmonic modulator array enabling dense optical interconnect solutions

Plasmonic modulators might pave the way for a new generation of compact low-power high-speed optoelectronic devices. We introduce an extremely compact transmitter based on plasmonic Mach-Zehnder modulators offering a capacity of 4 × 36 Gbit/s on a footprint that is only limited by the size of the high-speed contact pads. The transmitter array is contacted through a multicore fiber with a channel spacing of 50 μm.

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.

Elastocaloric cooling using shape memory alloy films

The elastocaloric effect in magnetron-sputtered Ni50.4Ti49.6 films of 20 μm thickness is studied by means of uniaxial tensile tests and infrared thermography. For the investigated films, the usable quantity of latent heat is about 7.2 J/g. When relieving the stress after tensile loading and subsequent temperature equalization at strain rates larger than de/dt = 0.2 s−1, a maximum temperature change of ΔT = −16 K is observed as expected for adiabatic conditions. Compared to bulk specimens, the heat transfer times are reduced to about 850 ms due to the larger surface-to-volume ratio, which is attractive for rapid cooling.

Effect of rolling reduction on the deformation texture and anisotropy of transformation strain in Ti–50.2at.%Ni thin plates

Abstract Ti–Ni shape memory thin plates with a thickness of ∼0.1 mm can be used for making two-dimensional shape actuators. Such thin plates were made by a rolling process, which will induce a specific texture in the plates. The texture causes an anisotropy of the transformation strain to appear. The purpose of the present study is to investigate the effect of a final cold-rolling reduction in the range of 0–70% on the texture and anisotropy of the transformation strain in Ti–50.2at.%Ni thin plates which were annealed at 673 K for 3.6 ks after the cold-rolling, the initial thickness being 0.2 mm. Transformation strains along various directions in the rolling plane were calculated by using a crystallite orientation distribution function (ODF) which was measured by an X-ray diffraction (XRD) method. The transformation strain in specimens prepared by intermediate cold-rolling reduction was almost constant along directions between 0 and 40° from the rolling direction, and it decreased with further increasing the angles until 90°. Specimens prepared by 0 and 70% cold-rolling reduction showed a maximum strain at an angle of 30°. Transformation strains were also measured by thermal cycle tests under various constant stresses. Both the calculated and experimentally obtained strains showed qualitatively a similar orientation dependence.

TiNi-based films for elastocaloric microcooling— Fatigue life and device performance

The global trend of miniaturization and concomitant increase of functionality in microelectronics, microoptics, and various other fields in microtechnology leads to an emerging demand for temperature control at small scales. In this realm, elastocaloric cooling is an interesting alternative to thermoelectrics due to the large latent heat and good down-scaling behavior. Here, we investigate the elastocaloric effect due to a stress-induced phase transformation in binary TiNi and quaternary TiNiCuCo films of 20 μm thickness produced by DC magnetron sputtering. The mesoscale mechanical and thermal performance, as well as the fatigue behavior are studied by uniaxial tensile tests combined with infrared thermography and digital image correlation measurements. Binary films exhibit strong features of fatigue, involving a transition from Luders-like to homogeneous transformation behavior within three superelastic cycles. Quaternary films, in contrast, show stable Luders-like transformation without any signs of degradation. The elastocaloric temperature change under adiabatic conditions is −15 K and −12 K for TiNi and TiNiCuCo films, respectively. First-of-its-kind heat pump demonstrators are developed that make use of out-of-plane deflection of film bridges. Owing to their large surface-to-volume ratio, the demonstrators reveal rapid heat transfer. The TiNiCuCo-based devices, for instance, generate a temperature difference of 3.5 K within 13 s. The coefficients of performance of the demonstrators are about 3.