Dynamic magneto-mechanical properties of epoxy-bonded Terfenol-D composites

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Abstract

The dependence of the dynamic magneto-mechanical properties of epoxy-bonded Terfenol on the composition parameters of particle size (Ps) and volume fraction of Terfenol (Vf) is investigated as functions of bias field (H) and high frequency (f). Samples were prepared with powder in five size ranges between 106 and 710 μm using three volume fractions. The dynamic strain coefficient (d) and magneto-mechanical coupling coefficient (k) were found to be independent of Ps and Vf while the dynamic relative permeability (μr) was proportional to Vf. d showed less variation with H up to 100 kA m−1, than for bulk Terfenol with d also exhibiting a flat response with frequency, up to 20 kHz. The frequency dependence of μr, up to 200 kHz, was also flat apart from the occurrence, at ∼15 kHz, of a fundamental shape resonance which was also experienced by d. Classical eddy current theory was used to model the behaviour of μr with f and it was found that experiment and theory were qualitatively similar.

Introduction

The highly magnetostrictive material TbxDy1−xFe1.95 (where 0.27<x<0.3) known as Terfenol-D has been increasingly used as a transducer material particularly in sonar and actuator devices [1]. However, the wider application of this material at frequencies above a few kilohertz is limited by the effects of eddy current losses. The frequency bandwidth of Terfenol can be considerably increased by incorporating powdered Terfenol into an insulating matrix to form a composite [2]. To date, little work has been published on the relationships between the dynamic magnetomechanical properties and the composition parameters of volume fraction (Vf) and particle size (Ps). In this paper, we report the results of a systematic study of the composition dependence of the dynamic strain coefficient (d) and dynamic relative permeability (μr) along with their variation with bias field (H) and frequency (f). The results of measurements of the dynamic magneto-mechanical coupling coefficient (k) are also presented using the `three-parameter method'. In addition, the variation of μr with f is compared with classical eddy current theory.

Section snippets

Manufacture

The epoxy bonded samples were prepared from randomly-oriented Terfenol powder in five particle size ranges (106–150, 150–212, 212–300, 300–500, 500–710 μm) each in three volume fractions (nominally 0.55, 0.62 and 0.70), using a compression moulding technique. The method of manufacture initially involved preparing Terfenol powder by grinding arc-cast Terfenol in an inert atmosphere and sieving to produce the correct powder size range. Pre-determined quantities of the powder and a two-part, low

Results and discussion

The dynamic strain coefficient and relative permeability were measured, for all the samples, as functions of H up to 100 kA m−1 with Hrms=2 kA m−1 at a frequency of 1 kHz. In Fig. 1 the results of the d and μr measurements are shown, for a sample with a powder size range of 212–300 μm and a 0.61 Vf. The bias field dependence of d is typical of all the samples studied and shows a much flatter variation with H, but a lower peak value, dmax, than that for bulk Terfenol. The flat variation of d

Conclusion

The d-coefficient, relative permeability and coupling coefficient of the composites have been measured as a function of bias field, and frequency. d and k are independent of Vf and Ps, and μr is proportional to Vf. The effects of eddy currents are found to be small so that the effective frequency range is extended to greater than 200 kHz.

Acknowledgements

This work has been carried out with the support of B&W Loudspeakers (Steyning, UK).

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