Study On Magneto-mechanical-electric Coupling Effect In Layered Magnetoelectric Composites And Novel Devices

Compared to single-phase magnetoelectric materials, 0-3-type particulate composites and 1-3 fiber/rod composites, 2-2 laminate composites can present remarkable magneto-electric coupling effect, and achieve effective transform between magnetic field and electric field, which are very useful in many fields such as magnetic sensors, electric current sensors, magnetoelectric energy harvesters, frequency multipliers, memorizers and so on. Thus, they attract more extensive attention. At present, experimental works on laminate composite are aimed at enhancing magnetoelectric coefficient and designing new types of devices with a high sensitivity. At the same time, theoretical studies focus mainly on the accurate mathematical models that may predict the coupling relationship between each two components and discuss the influence of some factors on magnetoelectric coefficient.Recently, experiments have shown that magnetostrictive materials will exhibit much complex nonlinear characteristic when they are conducted under applied magnetic field, which not only leads to mechanical-thermo-magnetic coupling property, but also causes frequency-multiplier signal or signal distortion etc. in magnetoelectric laminate composite. However, linear theoretical models, based on linear piezoelectric and piezomagnetic constitutive equations, are no longer available to reveal mechanism for such phenomena and study more influence factors. The existing nonlinear theoretical models pay more attention to the bulk with rectangular structure instead of the other complex configurations and thin films. In addition, previous theories were also not very successful in describing bending effect, surface effect and high-order voltage etc.. Therefore, it is very necessary to promote further theoretical study. As for the design of magnetoelectric devices, amount of effort is still required to improve conversion efficiency and broaden responsive frequency. Based on these issues, the magneto-mechanical-electric coupling properties of magnetoelectric composite and devices are studied deeply in this paper. Nonlinear theoretical models for complex configuration and thin film are established, and a new magnetic field sensor is designed using a novel coupling mechanism. Specific studies are as follows:Firstly, based on nonlinear constitutive equations of giant magnetostrictive material, nonlinear magneto-mechanical coupling models are developed for both layered magnetoelectric cylindrical shell and rectangle structures. For the cylindrical shell structure, the magnetoelectric coefficient under static and dynamic magnetic fields are derived respectively, and the cases containing four different boundaries and pre-stress are analyzed. To describe the imperfect interfacial condition, the spring model is introduced in this work, then the dependence of magnetoelectric coefficient on interfacial factor is studied. For the rectangle structure, considering the structural asymmetry, we explain how the bending effect can weaken magnetoelectric effect. At the same time, the mechanical-magneto coupling problem is solved by the proposed iterative method, so the true coupling stress in the composite is calculated. The difference between the magnetoelectric coefficients with and without coupling term is discussed finally.Secondly, nanostructures are important for increased functionality in magnetoelectric miniature devices, so it is very necessary to study their surface piezomagnetic effect and piezoelectric effect theoretically. In this paper, we further proposed a theoretical model for magnetoelectric effect in nanobilayer on a substrate, in which the special structure and size characteristic, and surface effect are considered. Gurtin-Murdoch theory is employed to describe surface piezoelectric and piezomagnetic effects of two materials. The influence of surface effect including deformation-independent residual surface stress, bending strain and the size and material constants of substrate material on magnetoelectric coefficient and resonant frequency is studied. Furthermore, the combined effect of pre-stress and bias magnetic field on magnetoelectric coupling behaviors in Terfenol-D/SrTiO3/PZTand Ni6/SrTiO3/PZT are investigated, respectively. At last, the dependence of magnetoelectric effect on the properties of piezoelectric material is studied.Then, the high-order magnetoelectric effect in layered composite is investigated theoretically. Based on the nonlinear mechanical-thermo-magneto coupling constitutive equations, both linear and nonlinear magnetostrictive coefficients are obtained. Combining governing equation with geometric equation and boundary conditions, fundamental signal and second harmonic signal are derived, and then the specific influence of applied magnetic field, temperature, interfacial factor and fraction ratio on two signals is studied. In addition, the competition relationship between linear and nonlinear magnetostrictive effect is discussed to reveal the physical mechanism of frequency doubling behavior. For investigating the influence factors of signal distortion, a distortion coefficient is defined as the linear signal to nonlinear signal ratio, by which some adjustable methods for signal distortion using ambient temperature and bias field are provided.Finally, we design a new functionally graded magnetoelectric sensor using the magneto-mechanical-electric coupling effect in layered piezoelectric/magnet composite. All materials properties of two components are assumed to be graded through the thickness and described by the given graded function. In this study, two types of sensors are proposed according to different distribution characteristics of piezoelectric material, and compare their performance to select the one with superiority after the calculations of output voltage, power, resonant frequency and stress at interface. The relevant experiment is conducted on the homogeneous sensor to verify the theoretical model and confirm the operability. Compared to the traditional ones, the proposed sensor can harvest both magnetic field and vibration energy with a lower resonant frequency and high power density. When the power of graded function is large enough, the stress concentration at interface between piezo and non-piezo materials can be successfully controlled, therefore the sensor can be applied widely in low-frequency condition with a longer life.In a word, this research will consummate the nonlinear theory of magnetoelectric effect in layered magnetoelectric composite, and may provide some useful guidance for the design of new type magnetoelectric sensors.