Simplifying Analysis Framework For Multiferroic Structures And The Applications

Mulitiferroics (MF) represent an appealing class of multifunctional materials that simultaneously exhibit several ferroic (e.g. ferroelectric, ferromagnetic, or ferroelastic) orders. Based on the existence of cross coupling beween the magnetic and electric orders, termed the magnetoelectric (ME) effect, there will be a huge potential application in novel multifunctional devices, such as sensors, harvesters, memeories, and so on. The accuracy and reliable analysis of the ME effects in MF materials or structures can provide a valuable theoretical guidance in designs of device, especially when developing novel ones. Thus the main objective of this thesis is to develop systematically simplifying structural theories (SST) for MF structures.Based on the method of power series expansion, the displacements, electric potential and magnetic potential are expanded in a series of powers of appropriate coordinates, and one-and two-dimensional SST are established in a systematic way. We also present a more general form of equations of SST for MF plates and shells in an othorgenal curvilinear coordinate system. The ME effects in MF multilayers, bilayers, bilayer-cylindrical shell, bilayer-spherical shell, MF film deposited on an elastic substrate and MF fibers, are studied using the developed theories. The SST is very effective and has some prominent adventages that it is easy to obtain analytical solutions and can be applied to all kinds of actual device structures even with irregular geometric shapes.With the motivation of utilizing the ME effects in MF structures in order to scavenge the surrounding magnetic energy, the following three types of energy harvesters have been proposed:(1) magnetic energy harvester using extensional vibration of laminated MF plates, (2) low-frequency magnetic energy harvester using antisymmetric laminated MF plates, (3) wideband magnetic energy harvester using a cluster of MF fibers. The load impedence, deformation modes and the magnetic frequency play a key role and have an important effect on the performance of magnetic energy harvesters. One thing to be mentioned is that the wideband magnetic energy harvester, which can collect more energy in a wider frequency range, is a sound ideal harvester.The SST theory taking account of polarization gradient is also obtained on the basis of Mindlin's polarization gradent theory. This widens the research fields of the SST, and also lays a solid theoretical foundation for applications of nano-MF structures in novel devices.