The /spl Delta/e Effect and Magnetomechanical Coupling in Epoxy-Bonded Terfenol Composites

Abstract

It has been reported that the resonance and antiresonance frequencies of composite Terfenol materials decrease with increasing bias field. Such behaviour implies a negative DE effect and is opposite to that of bulk Terfenol. In this paper we present measurements of EH and EB (and the magnetomechanical coupling coefficient k) for a range of epoxy bonded composites with different particle sizes and volume fractions. The results indicate that the elastic modulii have a variation with bias field which is complex and dependent on both of these compositional variables. The compression bonded samples were produced using Terfenol powder particles in the size rnages (106-150, 150-210, 210-300, 300-500 and 500-710 mm) each in three concentrations (of nominal volume fractions 55, 62 and 70%). EH and EB were determined from the resonance and antiresonance frequencies, respectively, which were obtained from the free-frequency dependence of the electrical impedance of a 50 turn coil around the sample. The measurements show that EH and EB increase with increasing volume fraction in agreement with model predictions. The bias field dependence of the moduli is, however, complex and shows an initial decrease followed by an increase, as the field is increased, with the minima occurring at lower bias fields for increasing volume fractions. The optimum coupling coefficient k is about 0.25 for all samples and shows only a weak dependence on particle size and volume fraction. In contrast, the bias field dependence of k differs widely for samples with different composition parameters. The low value of k in these materials is discussed and is attributed principally to the low elastic modulus.