Hybrid active FSMA/polymer micro-composites and electrostrictors:
Engineering FSMA-based actuators for Navy applications

Challenges Single crystals of ferromagnetic shape memory alloys (FSMAs, e.g. Ni-Mn-Ga) show D.C. field-induced strains of order 6%. In their present state of development, they appear to be suited for a few applications (e.g. "set and forget" actuators, energy absorption and conversion). However, for broader applications, there are three performance challenges that must be addressed:

	a)	the use of centimeter-sized single crystals poses problems of cost, brittleness, poor formability, and, in AC applications, large eddy-current loss;
	b)	present data indicate that even under quasi-static actuation, significant DC hysteresis is present;
	c)	the relatively large threshold field needed for actuation should be reduced in order to lower size of the drive coil/core, if light-weight active systems are to be realized.

Proposal The primary objective of this ONR-sponsored MURI program is to develop high-authority FSMA micro-composite actuators that will show large stroke and broad bandwidth suitable for vibration isolation and sonar applications, respectively. This means we must enhance the attractive features of FSMA materials and overcome limitations a) to c) (above). To achieve this, we combine into a coherent program, four major innovative thrusts. Thrust 1) Development, characterization and optimization of FSMA powder/polymer micro-composites (challenge a) [Figs. 1 and 2] for reduced DC hysteresis (b), lower eddy-current loss (a) and lower actuation field (c). FSMA particles will be made with a layer of soft magnetic material (e.g. Fe-Co) to enhance response to magnetic fields by exchange coupling. Micromagnetic and micro-mechanical modeling will guide the development of optimized particle size, exchange coupling, and twin boundary mobility, and in the later stages of the program, device optimization. Figure 3 shows how a 90-degree domain wall is associated with a twin boundary. These two planar entities have different energy densities and thicknesses which must be better understood. The position of the atoms across a twin boundary can be modeled as a soliton as suggested in Fig. 3, right. Figure 3. The magnetization rotates relatively gradually on crossing the boundary between one twin variant and another. The illustration at right shows that the twin boundary also has non-zero thickness. Thrust 2) Development of two new classes of FSMA materials. One class will be based on the known shape-memory, high-magnetization Fe-Ni-Co alloys. If successful, these Fe-based FSMAs would represent a less expensive, broader temperature range, and higher authority (larger saturation magnetization implies stronger response to applied magnetic fields) alternative to Ni-Mn-Ga alloys (challenge c). [Fig. 4] The other new class of FSMAa would operate as a magnetic analogue of the low-hysteresis, high-strain electrostrictive materials, showing reduced DC and AC loss as well as reduced threshold field (challenges a to c). In addition, new classes of electrostrictive materials would be developed for use with the FSMA/polymer micro-composite with an objective of developing a hybrid transducer. Figure 4. Strain versus electric field in BNBZT single crystals. Thrust 3) A thorough dynamic characterization effort focused on how FSMAs and FSMA/polymer composites function under complex operational conditions of multiple stresses, bias fields, frequency, and temperature (challenges a), b), c). This thrust will exploit the unique performance characteristics (e.g. positive slope of the strain-versus-bias-stress) as well as magnetic feedback from the FSMA, to integrate sensing and self-regulation for smart, active systems. Thrust 4) Thrusts 1) and 2) merge with the new active magnetic material(s) of Thrust 2) for the development of high-authority, low-cost linear actuators for ship-board vibration isolation. Thrusts 1) and 2) merge with the new material(s) of thrust 2) in development of hybrid FSME/polymer-electroactive (i.e. piezoelectric or electrostrictive) hybrid transducer expected to lead to high-actuation-authority and broad bandwidth, hybrid sonar transducers [Fig. 6]. Power and control systems will be designed and fabricated for each application.

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