The outstanding strain capabilities of Terfenol-D make it useful for transducer and sensor applications. But due to highly conductive and brittle nature, its uses are restricted to low frequency application such as underwater sound transducers. Doping with light elements, such as silicon, increases the resistivity of Terfenol-D, but the operating range is still small and the magnetomechanical properties are seriously degraded. Over a decade, much effort has been devoted to the studies on the Terfenol-D particulate composite that can achieve both of the improvements of mechanical properties and reduction of the eddy current effects at once. However, the manufacture of these composites requires relatively high cost due to the particle manufacturing processes, and the properties such as magnetomechanical coupling coefficient, k, still needs to be improved. Recently we have developed a new Terfenol/epoxy composite which is called Polymer Infiltrated Grain Aligned Composite (PIGAC) and shown that it has good static magnetostrictive properties (e.g., d ~20nm/A) higher than that of monolithic Terfenol-D. This study focuses on the dynamic magnetomechanical properties of composite, in particular the magnetomechanical coupling and verifying the effect of eddy current on it.
The composites were made through three steps of fabrication, which are vertical zone melting, annealing and polymer infiltration. The raw metals with different iron content $(Tb_{0.30}Dy_{0.70}Fe_y, y = 0.74, 0.89, 1.05, 1.21, 1.36, 1.51, 1.67)$ were processed in to the composites. The volume fraction of $RFe_2$ phase was determined to be from 30% to 94% by analysing the mass after each step of fabrication and investigating the microstructure with image analysis program. Considering the solubility of rare earth in $RFe_2$ phase, it was concluded that the removal of eutectic phase was successful and the most of the remnant phase is $RFe_2$.
The dynamic magnetomechanical properteis of PIGACs were investigated using both of resonance method and plane wave modelling (PWM) technique as a function of magnetic bias field and $RFe_2$ volume fraction of composite. It was found that the magnetomechanical coupling (k) of composite reaches 0.49 at the R$RFe_2$ volume fraction of 75% and bias field of 29kA/m, which is the largest value reported for Terfenol-D composite. Also the values of as-grown samples that are preforms of the composites were compared with those of PIGAC. It is thought that the remnant eutectics decrease k in the composite with a higher $RFe_2$ volume fraction. In addition, the non 180˚domain wall motion was used to explain the elastic modulus of PIGC as a function of bias field, which coincide with the bias field dependence of the coupling coefficient in consequence of the 180˚and non 180˚domain wall motions. Observing the variation of the bias field at which maximum k realized, it seems important to decrease impurity contents in $RFe_2$ phase of the composite with a low $RFe_2$ volume fraction. Also the uniform strain and uniform stress models were used to expect the modulus of composite showing that the uniform strain model is proper to explain the microstructure of PIGAC in which the epoxy and $RFe_2$ phases are combined parallel to the growth axis.
From the PWM results various dynamic magnetomechanical properties were obtained in example piezomagnetic constant g, permeability μ, which can not be obtained with classical resonance method. And the effect of eddy currents was quantitified by obtaining conductivities of the PIGCs assuming that the classical skin effect dominates anomalous losses originating from the motions of magnetic domains. From the results it is concluded that the microstructure of PIGAC reduces the conductivity successfully(up to 95%) and extends the operational frequency range to ten's of kHz frequencies. This implies that it is possible to improve the brittleness and resistivity at once, reducing the damage of magnetomechanical properties of Terfenol to a minimum. Moreover, the frequency dependent coupling was calculated using PWM method, which verifies that the low conductivity of the composite decreases the penetration of magnetic field.
자기변형이란 인가자장하에서 물리적 변형을 일으키는 재료를 뜻한다. Terfenol-D는 거대자기변형 재료로서 변형률이 1500ppm이 넘지만 금속간화합물의 특성상 취성이 있고 전도도가 높아 고주파에서의 응용이 제한되므로 지난 10여년간 Terfenol-D 분말상 복합재에 대한 많은 연구가 이루어져 왔다. 최근 이러한 Terfenol-D 분말상 복합재료에 구분되는 새로운 복합재료가 개발되었는데 수지함침고분자 복합재료(Polymer Infiltrated Grain Aligned Composite, PIGAC)라 명명된 이 재료는 결정성장기술을 적용하여 기존의 monolithic Terfenol-D에서 자기변형 상인 RFe2상은 그대로 유지하면서 비 자성 상인 공정상을 제거하고 그 자리에 고분자 수지를 채워 넣는 방식을 택하였다. 이 재료에 대한 기존의 연구에서는 monolithic Terfenol-D에 비교될만한 정적 특성(static properties)을 가지고 있음이 확인되었으며, 분말상 Terfenol-D 복합재료와 비교할 때 저비용의 제조가 가능하다는 점에서 상당한 발전 가능성을 가지고 있다고 할 수 있다.
본 연구는 이 새로운 복합재료의 동적 특성을 정량화 하는데 그 중점을 두었으며 특히 자기변형 복합재료에서 가장 중요한 변수인 자기기계결합계수와 고주파에서의 와전류의 영향을 규명하였다. 복합재료의 전기 전도도는 전도상과 비전도상이 복잡하게 얽혀 있는 미세구조로 인해 정확한 값을 얻기가 실제로 불가능하며 와전류의 영향을 직접 파악하는 것은 매우 어려운 일이다. 따라서 본 연구에서는 복합재료의 특성 평가를 위해 기존에 널리 사용되던 고전적인 공진법(resonance method)에 더하여 최근 새로 제시된 Plane Wave Model(PWM)법을 사용하여 특성을 평가하였다.
