Yaniv Ganor

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.

Photorefractive solitons and light-induced resonance control in semiconductor CdZnTe.

We demonstrate the formation of (1+1) - and (2+1) -dimensional solitons in photorefractive CdZnTe:V, exploiting the intensity-resonant behavior of the space-charge field. We control the resonance optically, facilitating a 10-mus soliton formation times with very low optical power.

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)

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.

Testing system for ferromagnetic shape memory microactuators

Ferromagnetic shape memory alloys are a class of smart materials that exhibit a unique combination of large strains and fast response when exposed to magnetic field. Accordingly, these materials have significant potential in motion generation applications such as microactuators and sensors. This article presents a novel experimental system that measures the dynamic magnetomechanical behavior of microscale ferromagnetic shape memory specimens. The system is comprised of an alternating magnetic field generator (AMFG) and a mechanical loading and sensing system. The AMFG generates a dynamic magnetic field that periodically alternates between two orthogonal directions to facilitate martensitic variant switching and to remotely achieve a full magnetic actuation cycle, without the need of mechanical resetting mechanisms. Moreover, the AMFG is designed to produce a magnetic field that inhibits 180 degrees magnetization domain switching, which causes energy loss without strain generation. The mechanical loading and sensing system maintains a constant mechanical load on the measured specimen by means of a cantilever beam, while the displacement is optically monitored with a resolution of approximately 0.1 microm. Preliminary measurements using Ni(2)MnGa single crystal specimens, with a cross section of 100x100 microm(2), verified their large actuation strains and established their potential to become a material of great importance in microactuation technology.

High sensitivity nanoscale mapping of elastic moduli

Recently, a new technique has been developed, which allows quantitative nanoscale mapping of elastic moduli by means of a hybrid nanoindentation and force modulation instrument. We introduce a procedure for finding the experimental parameters that provide an optimal modulus contrast. An application of the procedure on a BaTiO3 single crystal reveals a clear contrast between domains that have different orientations of the tetragonal unit cell. The obtained results are in good agreement with reported bulk elastic moduli and show that the elastic modulus sensitivity is 5%. Thus, the improved modulus mapping procedure can be applied not only to composite materials but also to many multiphase and multidomain material systems.

Zig-zag twins and helical phase transformations.

We demonstrate the large bending deformation induced by an array of permanent magnets (applied field ∼0.02 T) designed to minimize poles in the bent state of the crystal. Planar cantilevers of NiMnGa (5M modulated martensite) ferromagnetic shape memory alloy deform into an arched shape according to theory, with a zig-zag microstructure that complies with the kinematic and magnetic compatibility between adjacent twin variants. A general theory of bent and twisted states is given, applicable to both twinning and austenite/martensite transformations. Some of these configurations achieve order-of-magnitude amplification of rotation and axial strain. We investigate also atomistic analogues of these bent and twisted configurations with perfect interfaces between phases. These mechanisms of large deformation, induced by small magnetic fields or temperature changes, have potential application to the development of new actuation technologies for micro-robotic systems.

Work Output Enhancement of Ferromagnetic Shape Memory Micro Actuators

Ferromagnetic shape memory (FSM) alloys are a class of materials which are both ferromagnetic and capable of undergoing a structural phase transformation. FSM alloys have significant advantage over conventional shape-memory temperature-based actuators because they can be remotly actuated by fast alternating magnetic fields. Therefore, FSM alloys attract keen attention as promising candidates for a variety of MEMS applications, as they can provide large strokes using small components. The most commonly used FSM alloy is Ni2 MnGa and its off-stoichiometric alloys, which are used in commercial cm-scale FSM actuator. However, at the current stage, no experiments of the magneto-mechnical behavior of micro-scale actuators were conducted. Overall, the behavior of FSM alloys involves motion of twin boundaries and is significantly influenced by its microstructure. Based on a theoretical model, we have shown that down-scale specimens have finer twin boundary microstructure that consequently may increase the blocking stress characteristic such that it will enhance the output work for actuation. In light of this, a novel experimental method was realized to establish this conjecture and to provide comprehensive information on the behavior of small actuators. A series of tests demonstrated no actuation strain reduction up to extraordinary loads of 10MPa, and thus paves the route for engineering FSM high-power micro actuators by controlling their microstructure.Copyright

Estimation of ambient pressure changes using nonlinear acoustic properties of ultrasound contrast agents

A reliable tool for noninvasive blood pressure measurement is of interest since it will provide painless and infectionless measurements of ambient pressure changes within the body. We suggest to use clinically available ultrasound contrast agents (UCA) as a blood pressure sensor. The current investigation focused on the effects of hydrostatic pressures on the harmonic and sub-harmonic response of a clinically available UCA, Optison™. Samples of UCA suspension in saline, at commonly used in-vivo concentration of 50000 microspheres/cm^3, were subjected to hydrostatic pressure variations between 0 and 20 kPa (mimicking in-vivo pressures) and to insonations at 4MHz, 150-250 kPa, 9 cycles, PRF of 8 Hz bursts. The total duration of each experiment was 300s. The sub-harmonic-to-basic-frequency (SHBF) intensity ratio of the harmonic microbubble response was found to be the most accurate and robust measure of ambient pressure variations.