Giant magnetostrictive Terfenol-D can be employed as actuator or transducer element under AC magnetic field up to approximately ~10 kHz, but at higher frequencies above 10 kHz, eddy current losses are known to restrict its applications. Magnetostrictive particulate composites have been developed to minimize the restrictions imposed by monolithic Terfenol-D, such as eddy current loss and brittleness since the 1970s. The disadvantages of these magnetostrictive composites are random crystal orientation and low magnetization and thus much lower magnetostriction. The first objective of this thesis is to develop the fabrication process of Terfenol-D/Epoxy composite, which can achieve the enhancement of magnetostriction as well as mechanical properties. It is necessary to know how the initial composition and annealing temperature can be affected on microstructure of Terfenol-D/Epoxy composite to obtain the optimum magnetostriction of Terfenol-D/Epoxy composite.
The effect of the Tb/Dy, the RE/Fe ratio and annealing temperature on microstructure is thus investigated because these factors can significantly influenced on magnetostriction properties of Terfenol-D/Epoxy composite. The second objective is to analyze the effect of eutectic phase, residual stress caused by epoxy infiltration on magnetostriction and magnetic properties of Terfenol-D/Epoxy composite. Finally, modelling of magnetostriction of Terfenol-D/Epoxy composite is proposed based on a modified uniform strain assumption that considers the elastic modulus coupled in the preferred direction.
The fabrication process of Terfenol-D/Epoxy composite consists of fabrication of as-grown Terfenol-D based on unidirectional solidification technique, reaction heat treatment and infiltration process. A directionally aligned cell structure of REFe2 phase in as-grown Terfenol-D is obtained by zone melting among unidirectional solidification method. During the reaction heat treatment of as-grown Terfenol-D, the eutectic phase is extracted from as-grown Terfenol-D because it is reacted with the quartz granules, which are mainly composed of $SiO_2$, at the contact surface. The extracted fraction of eutectic phase significantly depends on the initial composition as well as the reaction heat treatment temperature. The Terfenol-D pre-form prepared by reaction heat treatment is well infiltrated by a liquid epoxy phase at 70℃ with lowest viscosity. In Terfenol-D/Epoxy composite, the volume fraction of the $REFe_2$ phase is easily controlled by the initial composition and reaction heat treatment temperature.
Effects of fabrication processing parameter (heat treatment, reaction heat treatment) and each constituent (the eutectic phase, residual stress caused by epoxy infiltration) on magnetostriction have been investigated. The magnetostriction of heat treated Terfenol-D improved about 10 % compared to as-grown Terfenol-D. Furthermore, the maximum magnetostriction coefficient d33 increased from 5.4 to 46.3 nm/A as the RE/Fe ratio increased from 1.00 to 1.85. It is explained that it reduced the pinning of domain walls since the heat treatment changed the residual stress distribution of the as-grown Terefol-D due to the localized melting around the eutectic phase. The magnetostriction of Terfenol-D pre-forms with $REFe_2$ fraction of 0.62, 74 and 0.91 showed 971, 1041, and 1199 ppm, respectively. The magnetostriction of Terfenol-D pre-form was enlarged by about 2~3 times compared to as-grown Terfenol-D. This indicates that the increased amount of eutectic phase inhibits the domain wall motion (or domain rotation). The deviation of optimum prestress, at which a peak magnetostriction occurs in both Terfenol-D pre-form and Terfenol-D/Epoxy composite, was regarded as a residual stress caused by epoxy infiltration and the cure. As volume fraction of epoxy decreased from 0.38 to 0.09, a residual stress in Terfenol-D/Epoxy composite decreased from 2.4 to 0.8 MPa. Furthermore, it was investigated the magnetomechanical coupling coefficient $k_{33}$ of Terfenol-D/Epoxy composite since the coupling coefficient, which is equal to the ratio of the energy converted to the total energy stored, is used as a measure of energy conversion efficiency for applications such as a transducer or actuator. When the $REFe_2$ fraction was 0.91, the maximum magneto-mechanical coupling coefficient $k_{33}$ of Terfenol-D/Epoxy composite was 4.5 at 47.2 kA/m.
The normalized texture fraction and elastic modulus with each preferred direction were investigated to get the magnetostriction model of Terfenol-D/Epoxy composite maintaining the texture of a unidirectionally solidified crystal. Based on these results, a new model is proposed for predicting the effective magnetostriction of Terfenol-D/Epoxy composite, utilizing a modified uniform strain assumption that considers the elastic modulus coupled in the preferred direction. Since the proposed model considers the anisotropic properties of REFe2 phase contrary to other magnetostrictive pariticulate composites, the predicted magnetostriction of Terfenol-D/Epoxy composite using the modified uniform strain assumption showed good agreement with the experimental results. The proposed model can be used to assess the influences of different contributions on the effective magnetostriction since its methodology accounts for the preferred direction of the unidirectionally grown crystal and the elastic modulus change related to each crystal direction. Therefore, the proposed model can provide a criterion for choosing the optimum composition of constituent materials for high-frequency magnetostrictive transducers or sensors.
기존 고분자기지 Terfenl-D 분말 복합재료는 Terfenol-D 분말을 기반으로 하기 때문에 벌크 Terfenl-D에 비해 결정배향성(crystal orientation) 이 떨어지고, 자장인가 시 자장 역방향으로 반자장(demagnetization)을 형성하기 때문에 자기변형특성이 낮은 단점을 갖고 있다.
본 연구의 목적은 벌크 Terfenol-D의 결정배향성을 이용함으로써 자장 인가 시 반자장 효과를 최소화 하고, 강도 향상을 위해 일방향응고기술에 기반한 새로운 개념의 Terfenol-D/Epoxy 복합재료의 제조공정을 제안하고, 복합재료의 자기변형율에 미치는 제조공정 인자의 영향을 분석하는 것이다. Terfenol-D/Epoxy 복합재료는 3단계 과정으로 제조하였다. 첫번째 단계는 일방향응고 기술을 이용하여 합금의 RE/Fe비의 제어를 통해 일방향 응고재 내 공정상의 분율을 조절할 수 있었다. 둘째 단계는 일방향 응고재 내 공정상을 공정상의 액상온도(~880℃) 이상에서 $SiO_2$ 와 반응시켜서 공정상을 제거하는 반응열처리(reaction heat treatment)공정을 개발하였다. 기공분율은 Terfenol-D의 초기 화학성분비(RE/Fe, Tb/Dy)를 조절함으로써 0.34~0.91의 $REFe_2$ 분율로 제어할 수 있었다. 셋째 단계는 Terfenol-D preform을 고분자 함침법(polymer infiltration)으로 에폭시를 함침하는 기술을 개발하였다.
Terfenol-D/Epoxy 복합재료의 자기변형 특성을 분석하기 위해 자기변형특성에 미치는 열처리 효과, 공정상 및 에폭시 함침시 나타나는 잔류응력 효과에 대한 분석을 실시하였다. 열처리에 의한 자기변형율 효과는 RE/Fe 비에 관계없이 일방향 응고재에 비해 약 10% 정도의 자기변형율 향상을 나타내었다. 이는 일방향 응고 시 발생한 잔류응력을 열처리공정을 통해 완화시켜 자장 인가 시 자구이동(domain wall movement or domain rotation)을 용이하게 하여 주기 때문이다. 일방향 Pre-form Terfenol-D의 자기변형율은 자화용이축으로 자구이동을 방해하는 공정상의 제거로 크게 향상시켰다($REFe_2$ 분율이 0.62에서 0.91로 증가함에 따라 971에서 1199 ppm으로 증가). 에폭시 함침율이 0.38에서 0.09로 감소함에 따라 최대자기변형율이 나타나는 압축 응력의 차이는 2.4 MPa에서 0.09 MPa로 감소하였다. 이는 에폭시 함침 및 curing 처리 시 발생하는 축방향의 압축 잔류응력에 기인한 것으로 복합재료의 응력이방성이 작아지는 원인이 되었다. Terfenol-D /Epoxy 복합재료의 최대 자기결합계수값은 4.9로 나타났으며, $REFe_2$ 분율 증가에 따라 에폭시에 의한 압축잔류응력 감소로 비 180° 자구가 용이하게 이동할 수 있기 때문에 자기결합계수값은 증가하였다. 또한 복합재의 굽힘강도는 $REFe_2$ 분율 증가에 따라 증가하였으며 최대 굽힘강도는 약 100 MPa로 나타났다. Terfenol-D/Epoxy 복합재료의 자기변형을 예측하기 위해 자기변형율 변화에 영향을 미치는 $REFe_2$ 분율, 탄성계수 및 $REFe_2$ 상의 집합조직을 기반으로 하여 자기변형율 모델식을 유도하였다. Terfenol-D 합금의 XRD법을 이용하여 각 방향별 ODF값을 구해서 방향별 탄성계수와 $REFe_2$ 분율을 계산하여 자기변형율을 계산할 수 있었다. 자기변형 모델식을 이용한 최대자기변형율은 실험적으로 계산한 최대자기변형율과 잘 일치하는 결과를 얻을 수 있었으며, 자기 변형율 고려 시 Terfenol-D/Epoxy 복합재료의 집합조직의 영향을 고려하는 필요하다는 것을 알 수 있다. 본 연구결과는 향후 다양한 응용분야를 가질 Terfenol-D/Epoxy 복합재료의 제조 공정 최적화 및 자기변형율 예측에 크게 기여할 것으로 판단된다.
