Multi-field Coupling Finite Element Analysis Of Magnetoelectric Composites And Device Design

The magnetoelectric composite consists of two functional materials,it can convert the magnetic energy into electrical energy by coupling effect,or vice versa.Among various connections,the laminated structure is the simplest one,and it's easy to fabricate.Thus the laminated composite attracts a lot of attention from the researchers in physics and mechanics.And the magnetoelectric composite can be applied widely in the manufacturing of multi-functional devices,such as magnetic sensors,magnetoelectric energy harvesters and magnetoelectric memories.The main focus of the magnetoelectric composite is in the following two aspects: Improvement of magnetoelectric coefficient and the broadband of the magnetoelectric response.Both of them have made great strides in recent years.In previous studies,there are many researches focus on the influence of stress,magnetic field and temperature on magnetoelectric effect.Partly of them reported the effect of geometric size on magnetoelectric coefficient,but most of them are relevant to thickness direction,i.e.,the volume fraction of two components.And the influences of other two dimensions are seldom reported,because most of the theoretical models are limited to two dimensional.When the lengths of two components are different,the expression of magnetoelectric coefficient can't be obtained by theoretical method.That is the defect of various theoretical models.Besides,most of the theoretical models only consider the steady state case.Actually,the magnetoelectric composite is surrounded by an instantaneous condition,and the geometric size is three dimensional(3D).Moreover,how to broaden the magnetoelectric response is another important issue.In view of this,firstly,a 3D finite element coupling model is established to study the dynamic response of the laminated magnetoelectric composite;secondly,a novel magnetoelectric structure is proposed to broad the magnetoelectric response by structural design.With the development of multi-functional nano-scale devices,some researchers proposed a new noncontact way to tune the carrier transportation in piezoelectric semiconductor by magnetoelectric effect.The brief review of this academic dissertation are as following:Firstly,based on the nonlinear constitutive relations of magnetostrictive material and the linear relations of piezoelectric material,a 3D multi-field coupling finite element model is established to investigate the dynamic response of the laminated magnetoelectric composite.The numerical results agree well with the experimental data,and it indicates that the finite element model can be used to predict the magnetoelectric response of the magnetoelectric composite effectively.By selecting a cross section of the composite structure,the contour distributions of magnetic flux,displacement,voltage and stress inside the magnetoelectric composite are plotted,and it can be seen that the distribution is inhomogeneous,because the demagnetization and edge effect always exist in the composite structure.Furthermore,when the length of the composite structure keeps invariant,it's can be found that the width of the composite structure show a significant effect on magnetoelectric response,and the demagnetization become more obvious when the length-to-width ratio is small.The influences of the length-to-width ratio in low,moderate,high magnetic field are different.Under the action of transient magnetic field,the magnetoelectric coefficient increases gradually,and then become stable after a period of time.Based on the Fast Fourier Transformation(FFT)of magnetoelectric response and harmonic magnetic field,it can be found that the frequency of magnetoelectric response is twice of that of the input harmonic magnetic field,i.e.,the multiplying frequency phenomenon.The strains of the two functional materials are coupled through the interface,thus the stress inside the composite structure can influence the voltage distribution directly,and when the magnetoelectric composite is under resonance,the stress at the interface can be enhanced many times,it may lead to de-bonding at the interface.Secondly,for the purpose of broading the bandwidth of the magnetoelectric response,a novel magnetoelectric structure is proposed.Compared with the traditional magnetoelectric structure,there are many resonant modes inside the new magnetoelectric structure,because it's a multiple degree of freedom structure.From the experiment,the multi-peaks phenomenon can be observed in a wide range inside the novel magnetoelectric structure,and the resonant frequency is low,thus it can avoid the eddy current loss at high frequency.Because the resonant frequencies are close to each other or overlapped,which makes the magnetoelectric composite can stay good performance in a wide range.Besides,a finite element model is established to analyze the resonant mode of the magnetoelectric structure,the numerical results agree well with the experimental measurements.From the finite element analysis and experimental measurement,it indicates that the novel magnetoelectric composite can realize the broadband of the magnetoelectric response at low frequency,which may be of great potential for designing broadband magnetoelectric devices.Finally,due to the development of the multi-functional devices based on piezoelectric semiconductors,and the relationship of mechanics,electricity and carrier transportation are coupled.Besides,the magnetoelectric response is a multi-field coupling problem as well,thus,a new multi-functional nano-scale devices based on piezoelectric semiconductor and magnetoelectric composite is a complicated multi-field coupling problem.At present,some experiments are trying to combine the magnetoelectric characteristics with piezoelectric semiconductor devices.Based on the creative idea,a new noncontact way is proposed to tune the piezopotential in piezoelectric semiconductor,but the theoretical research on this issue is rare,thus,it's an urgent need to establish an appropriate model to predict the influences of different factors on piezoelectric semiconductor.In the following,the p-n junction is the elementary building block of many semiconductor devices,so the influence of the p-n junction should be considered.Furthermore,the size of the structure is nano scale,the surface stress plays an important role in the performance of the piezoelectric semiconductor multi-functional devices.Based on the Gurtin-Muedoch theory,the influence of the surface stress is investigated at the end of the academic dissertation.In brief,a 3D multi-field coupling finite element model is established to study the dynamic magnetoelectric response,which perfect the theoretical research of the magnetoelectric composite.In order to broaden the magnetoelectric response,a novel magnetoelectric structure is proposed.Based on the magnetoelectric effect and piezoelectric semiconductor properties,the theoretical model predict the influence of different factors on nano-scale devices,which may be useful to tune carrier transportation indirectly.