| The composites consist of magnetostrictive and piezoelectric materials are with the ability of magneto-mechanical-electric conversion.The magnetoelectric(ME)effect is dramatically enhanced when the magnetostrictive/piezoelectric composites are operated around the resonance frequency.The resonant ME effect has become a focused hot research topic due to its potential promising for many applications in the field of ME sensor and transducer.In this thesis,a low-damping resonant ME sensitive unit is developed by employing a magnetostrictive material and a quartz tuning fork resonator.The performance of the ME sensitive unit has been investigated by the means of theoretical analysis,finite element simulation and experiment.The main work is as follows:(1)The properties of the dielectric,elasticity and piezoelectricity as well as the first type of piezoelectric equation of the quartz crystal are introduced summarily.Several vibration modes of the quartz tuning fork resonator are illustrated with examples,especially for the detailed structural design,electrode configuration and equivalent circuit for the optimized vibration mode.The vibration modes of the quartz tuning fork resonator are simulated and analyzed by using finite element software.In addition,the typical principles and methods for atomic force microscope,magnetic force microscope with the quartz tuning fork resonator are summarized and analyzed,which is significant to design the resonant ME unit employing the tuning fork resonator.(2)The performances of the commonly used magnetostrictive materials are compared.The physics characteristics,fabrication technology,the constitutive equation and the factors affecting performances of the selected Fe-Ga alloys are introduced.Finite element software is used to simulate the magnetostrictive characteristics of the Fe-Ga alloys under different boundary conditions such as the middle fixed,the single-ended fixed,the double-ended fixed and the fully fixed conditions.The results show that the piezomagnetic coefficient achieves a maximum value of 10.2nm/A under the middle fixed condition.(3)The structure the resonant magnetostrictive/quartz tuning fork ME sensitive unit is designed.The two tines of the quartz tuning fork is operated in the bending vibration mode,in which the direction of stress/strain and bending moments are opposite in the coupling area and canceled out with each other.The coupling area is actually the decoupling area of stress/strain.Thus,the magnetomechanical damping of the magnetostrictive material is isolated from the fixed region when the bottom of the quartz tuning fork resonator is bonded to the magnetostrictive material,resulting in high quality factor(Q-factor)in the resonant ME sensitive unit.Under the action of dynamic magnetic field,the magneto-mechanical-electric conversion process can be regarded as a forced vibration process.When the frequency of the excitation magnetic field is close to the resonance frequency of the tuning fork,the output electrical signal is dramatically enhanced.The resonant ME response of the ME sensitive unit is analyzed by taking into account of the forced vibration process,the mode distribution function and the piezoelectric equation.(4)The resonant ME sensitive unit is fabricated by using FeGa alloy and commercial tuning fork resonator.The resonant ME response characteristics under different boundary conditions are tested by using a self-built ME effect measurement system.The experimental results show that the ME sensitive units show excellent performances under the middle fixed boundary condition.The Q-factor of the ME sensitive units achieve 5843.As compared with the value in sub-resonance region,the increment of the resonant ME current achieves 63.2nA under ac drive field of 0.1Oe superposed with dc bias field 200 Oe.The sensitivity of the output ME current coefficient with respect to dc magnetic field is 7.9(nA/Oe)/Oe when the ME sensitive unit is operated at the selected frequency around the resonance frequency of 32.769 kHz. |
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