Doron Shilo

Implications of twinning kinetics on the frequency response in NiMnGa actuators

The explicit kinetic relation for twin wall motion in NiMnGa is used to correlate basic material properties to magneto-mechanical actuation rates in these crystals. In particular, we identify two parameters: the Peierls energy barrier and the twin wall mobility, which directly determine the dynamic response of NiMnGa actuators at frequencies above 10 Hz. Comparison between the kinetics of type I and type II twin walls reveals a correlation between the Peierls energy barrier and the commonly used twinning stress property. However, it is shown that twinning stress dictates twin wall dynamics only at very slow frequencies, typically below 1 Hz.

Breaching the work output limitation of ferromagnetic shape memory alloys

One important parameter that quantifies the performance of ferromagnetic shape memory alloys is the blocking stress. To date, the low blocking levels (<5 MPa) impede the utilization of these alloys in applications where high work output is required. In this paper, we demonstrate an increase in the blocking stress by more than 100% by reducing the actuator size. A new theoretical model shows that smaller specimens have increased values of the blocking stress due to an enhancement in the energy barrier to magnetization rotation and indicates on a fundamental relationship among the specimen size, its microstructure, and its physical properties.

Visualization of 10 μm surface acoustic waves by stroboscopic x-ray topography

X-ray topographs under acoustic wave excitation were taken from LiNbO3-based surface acoustic wave devices with He-implanted waveguide layers, using monochromatized synchrotron radiation from the European Synchrotron Radiation Facility (ESRF), Grenoble. Measurements were performed in the stroboscopic mode, i.e., by synchronizing the electron bunch frequency with the resonant frequency of the acoustic wave excitation in the 300 MHz range. These x-ray diffraction images showed plane acoustic wave propagation through the LiNbO3 crystals as well as a weak wavefront distortion due to the scattering on dislocation deformation fields. In some images secondary spherical waves were observed as a result of the strong acoustic wave interaction with the submicron size density perturbations.

Investigation of twin boundary thickness and energy in CuAlNi shape memory alloy

The nanometer-scale thickness and energy of twin boundaries govern the martensitic twin structure and its dynamics, which are responsible for the actuation and superelasticity of shape memory alloys. In this letter the authors apply a method, which has been recently developed for investigating twin wall structures in ferroelectric crystals, on a shape memory alloy crystal. Fittings of simulated displacement fields to atomic force microscopy measurements allow them to extract the thickness of type-I twin boundaries in CuAlNi. Further, a relation between the twin boundary thickness and energy is developed and used for evaluating the twin boundary energy.

Ferromagnetic shape memory flapper for remotely actuated propulsion systems

Generating propulsion with small-scale devices is a major challenge due to both the domination of viscous forces at low Reynolds numbers as well as the small relative stroke length of traditional actuators. Ferromagnetic shape memory materials are good candidates for such devices as they exhibit a unique combination of large strains and fast responses, and can be remotely activated by magnetic fields. This paper presents the design, analysis, and realization of a novel NiMnGa shear actuation method, which is especially suitable for small-scale fluid propulsion. A fluid mechanics analysis shows that the two key parameters for powerful propulsion are the engineering shear strain and twin boundary velocity. Using high-speed photography, we directly measure both parameters under an alternating magnetic field. Reynolds numbers in the inertial flow regime (>700) are evaluated. Measurements of the transient thrust show values up to 40 mN, significantly higher than biological equivalents. This work paves the way for new remotely activated and controlled propulsion for untethered micro-scale robots. (Some figures may appear in colour only in the online journal)

Application of a bi-stable chain model for the analysis of jerky twin boundary motion in NiMnGa

The “jerky” motion of a twin boundary in the ferromagnetic shape memory alloy NiMnGa is studied experimentally and theoretically. We employ a bi-stable chain model in order to interpret macroscopic stress-strain experiments and extract important micro-level properties. The analysis reveals the existence of a periodic barrier for type I twin boundary motion with an average distance of 19 μm and amplitude of 0.16 J/m2. Further, we show that the macroscopic mechanical response depends on the length of the crystal and predict a significant decrease of the hysteresis in sub-mm length specimens.

Stroboscopic x-ray topography in crystals under 10-μm-surface acoustic wave excitation

High-frequency stroboscopic x-ray topography is developed with the goal to visualize surface acoustic wave propagation. For this purpose the resonant frequencies of LiNbO3-based surface acoustic wave devices (290 and 355 MHz) are synchronized with the frequency of x-ray flashes (5.68 MHz) delivered by the European Synchrotron Radiation Facility (ESRF, Grenoble). X-ray topographs, taken in such a manner, revealed periodic contrast caused by a “frozen” acoustic deformation field. This technique allows for visualization of individual wavefronts of traveling acoustic waves having a wavelength in a 10 μm range. X-ray stroboscopic topographs showed a weak wavefront distortion due to the scattering of acoustic waves on linear dislocations. Secondary spherical waves were observed as a result of the strong acoustic wave interaction with submicron size density perturbations.

In situ elastic modulus measurements of ultrathin protein-rich organic layers in biosilica: towards deeper understanding of superior resistance to fracture of biocomposites

Biogenic ceramics are known to exhibit superior toughness due to a laminated architecture with ultrathin organic layers separating the ceramic blocks. Theoretical analyses relate the toughness increase to the modulus contrast, Ec/Eo between the stiff, Ec, and the compliant, Eo, components. However, experimental data on this contrast are extremely difficult to obtain by any known technique due to the very small thickness and low modulus values of the organic layers. Here we adapt a recently developed nanoscale modulus mapping technique combined with reverse finite element analysis in order to map the elastic modulus across a 35 nm thick organic layer within biosilica in a giant anchor spicule of the glass sponge Monorhaphis chuni. We find a modulus of 0.7 GPa in the organic layer as compared to 37 GPa in the bioglass. Furthermore, a modulus gradient extends 50 nm into the glass layer, probably due to the spatial distribution of small organic inclusions. With this new methodology it becomes possible to determine the elastic moduli of nanometric inclusions even when embedded in a matrix which is 50 times stiffer.

Modulus mapping of nanoscale closure variants in Ni–Mn–Ga

The twinned magnetic microstructure of Ni2MnGa ferromagnetic shape-memory alloy is investigated by high resolution nanoscale modulus mapping. A surprisingly fine near-surface nanoscale substructure of closure magnetic twin variants was observed. The lateral distance between adjacent closure variants was found to be 100nm. The small size of twin variant prisms provides a unique opportunity for evaluating the twin boundary energy by considering the competition between the magnetic field and interface energies. Our estimate shows a relatively small twin boundary energy of 3ergs∕cm2, which suggests the ability of Ni2MnGa to form nanoscale structures of magnetic twin variants.

Mechanical Characterization of Released Thin Films by Contact Loading

The design of reliable micro electro-mechanical systems (MEMS) requires understanding of material properties of devices, especially for free-standing thin structures such as membranes, bridges, and cantilevers. The desired characterization system for obtaining mechanical properties of active materials often requires load control. However, there is no such device among the currently available tools for mechanical characterization of thin films. In this paper, a new technique, which is load-controlled and especially suitable for testing highly fragile free-standing structures, is presented. The instrument developed for this purpose has the capability of measuring both the static and dynamic mechanical response and can be used for electro/magneto/thermo mechanical characterization of actuators or active materials. The capabilities of the technique are demonstrated by studying the behavior of 75 nm thick amorphous silicon nitride (Si_3N_4) membranes. Loading up to very large deflections shows excellent repeatability and complete elastic behavior without significant cracking or mechanical damage. These results indicate the stability of the developed instrument and its ability to avoid local or temporal stress concentration during the entire experimental process. Finite element simulations are used to extract the material properties such as Youngs modulus and residual stress of the membranes. These values for Si_3N_4 are in close agreement with values obtained using a different technique, as well as those found in the literature. Potential applications of this technique in studying functional thin film materials, such as shape memory alloys, are also discussed.