Preparation And Properties Of Magnetoelectric Composite Materials

There exist some problems on the study of magnetoelectric （ME） composites at present. Firstly,the magnetic-mechanical-electric transform of layered ME composites is achieved by interfacecoupling between magnetostrictive and piezoelectric layers. Therefore, the interface coupling is a keyfactor that determines the ME coupling of layered composites. However, for most of layered MEmaterials, the magnetostrictive and the piezoelectric layers are separated by an epoxy bonding layer,or the surfaces of the piezoelectric layer must be metallized with nonmagnetostrictive layers whichreduce the ME coupling. Secondly, there are two resonance frequencies in1-150kHz for ME bilayers.However, only one of the ME voltage coefficients is large enough. Finally, non-interface bonding typeME composites have recently stimulated tremendous fundamental and practical interests as their MEcoupling is not achieved by interface bonding. However, the structure of the non-interface bondingtype ME composites is too complex to be used for devices. The goal of this thesis is to obtain thecomposites with giant ME coupling. To solve the problem mentioned above, the electroless depositionis used to prepare layered ME composites with desired thickness with neither electrodes nor bondinglayers. The magnetostrictive layers are in direct contact with the piezoelectric layers. TheNi/PZT/TbFe₂trilayers are also prepared with the negative-magnetostrictive/piezoelectric/positive-magnetostrictive layered structure and two remarkable resonance peaks have been observed.The bi-rectangular structure and the ME structure with end-bonding have been prepared, the simplestructures of which benefit their application in devices.The thermodynamics and reaction mechanism of electroless Ni-deposition have been studied.The influence of the thermodynamics and kinetics of electroless deposition processes on themicrostructure, magnetic and magnetostrictive properties of the deposited Ni layer and the MEcoupling of Ni/PZT/Ni trilayers has also been investigated. The results indicate that, Ni can besuccessfully deposited on the surface of PZT layers without any impurities. With the increase of pHand/or temperature, the nucleation rate of Ni and the diffusivity of Ni²⁺increase, which leads to anincrease in the deposition rate and a decrease of the grain size in the deposited Ni layers. Therefore,the soft magnetic and magnetostrictive properties of the deposited Ni layers are improved and thecomposites with strong ME coupling are obtained.To study the infuence of the structure factors such as the size, the interface condition, thecurvature and shape on the ME effect of layered composites prepared by electroless deposition, theME coupling of flake-like trilayered, arc shaped trilayered and cylindrical trilayered ME composites have been investigated. The results indicate that, with the increase of length or cylinder diameter, theME voltage coefficient increases, while the resonance frequency decreases. The ME effect of MEcomposites increases as the interface roughness increases. With arc radius increasing （curvaturedecreasing）, the resonance frequency of layered arc Ni/PZT/Ni composites gradually decreases, whilethe maximum of the ME voltage coefficient of the composites increases monotonously. The MEvoltage coefficient increases linearly with high static magnetic field because of the piezoinductiveeffect of Ni/PZT/Ni cylindrical layered composites, which results from the combination of theelectromagnetic induction in the Ni cylindrical layers and the piezoelectric effect in the PZT cylinder.The negative-magnetostrictive/piezoelectric/positive-magnetostrictive layered ME structuresmade up of Ni, PZT and TbFe₂layers have been prepared. Compared to the ME coupling of bilayersand trilayers, the ME effect of Ni/PZT/TbFe₂trilayers has been studied. There appear two remarkableresonance peaks of αE,31in Ni/PZT/TbFe₂trilayers. The first bending resonance frequency iscontrolled by TbFe₂layer, while the first planar acoustic resonance frequency is controlled by Ni layer.The working principle of Ni/PZT/TbFe₂trilayers is in a bending vibration mode. With the increase ofthe thickness of Ni layer, the ME voltage coefficient at the first bending resonance frequency changesvery fewly, while the ME voltage coefficient at the first planar acoustic resonance frequency increasesremarkably. The first bending resonance frequency and the first planar acoustic resonance frequencyincrease as the thickness of Ni layer increases.The ME effect of the bi-rectangular structure and the ME structure with end-bonding have beenstudied. The ME coupling of these structures are achieved without interface coupling and onlyinfluenced by properties of composite materials, the size and structure of the composites. The MEeffect of the bi-rectangular structure is affected by negative and positive magnetostrictive flakes. Theresonance frequency and the maximum ME voltage coefficient of the bi-rectangular ME compositestructure are influenced by the length of PZT flakes. The maximum ME voltage coefficient is alsoinfluenced by the length of TbFe₂flakes, but the resonance frequency is not affected by the length ofTbFe₂flakes. There are several resonance peaks in1-150kHz for the ME structure with end-bonding.In the PZT/TbFe₂/PZT composite structure a large ME voltage coefficient αE,31as high as10.5V·cm^-1·Oe^-1can be obtained at the first planar acoustic resonance frequency of f=85.5kHz and themaximum value of αE,31=9.9V·cm^-1·Oe^-1at69.5kHz for the third bending resonance was alsoobserved.