Elastic-Electric-Magnetic Coupling Mechanism And Properties Of Multiferroic Composites

Multiferroic materials, which simultaneously exhibit two or more coupled ferroelectricity, ferromagnetism and ferroelaticity, an external electric field can induce magnetization and an external magnetic field can induce polarization, there exits the productive magnetoelectric coupling effect. The peculiar properties give more freedoms for design of materials and multifunctional devices, which lead to the potential applications in multi-field data-read-and-write device, wireless microwave deviced, wireless sensor net-work and varies multifunctional devices and more attention has been payed on the multiferroic materials.Single phase materials are rare exist and exhibit only small magnetoelectric effect at low temperatures. An alternative route is to combine the electrostriction of piezoelectric materials and magnetostriction of magnetic materials though the elastic coupling in the interface of the two portions. The coupling effects in such composites are believed to be significantly with respect to the elastic interaction through the interfaces of the two portions and film/substrate. It is thus highly desirable to study the stress state in the composites and the mechanics on the ME coupling effect to guide the design of such materials with better ME coupling effect. The purpose of this thesis is to study the coupling mechanics of the multiferroic composites and the size effect, surface effect, volume fractions, thickness and external and internal strain and stress effect on the properties of the materials. Moreover, works of the thesis also pay attention to controlling the properties of some multifunctional structures using multi-fields with more freedom and the fracture toughness of the multiferroic composite. The main ideas, approaches and results are listed as follows:1) Based on the Landau theory, 1-3 type multiferroic thin films were investigated considering electrostrictive and magnetostrictive effects, misfit strains between the interfaces of FE/FM and film/substrate, and the relaxation of the stress because of the dislocation in the interfaces. Thermodynamic energy density was constructed considering the elastic coupling energy. By using variation method, the time-dependent Ginzburg-Landau equations with respect to spontaneous polarization and magnetization were derived. According to linear analysis theory, the dynamic stabilities of the stationary states were probed. The stable polarization, magnetization and stress state were calculated numerically. Butterfly loops of the polarization and magnetization were simulated based on the Landau-Khalatnikor.2) Based on Landau theory, magnetostrictive/dielectric laminate material (terfenol-D/Ba0.6Sr0.4TiO3) was studied and designed. Dielectric properties of Ba0.6Sr0.4TiO3 layer can be adjusted by the magnetostrictive strain of terfenol-D layers under applied magnetic fields. Results show that the small signal dielectric constant of Ba0.6Sr0.4TiO3 can be tuned in a certan range by the magnetic field. By adjusting the Ba/Sr ratio and relative layer thickness ratio, more broad adjustable dielectric constant and tunability of BST layer can be designed.3) Based on the magnetoelectric coupling effect, a bilayer BaTiO3/terfenol-D bending system was design. Using the Hsueh's method, bending response was investigated, which can be significantly adjusted by the magnetostrictive strain of terfenol-D layers under applied magnetic fields. The vertical displacement induced by an asymmetry stress in the bilayer system can be controlled by the thickness ratio, external electric field, and magnetic field. A high relative static displacement of 55% can be obtained under the magnetic field.4) Fracture toughness of unidirectional ferromagnetic fiber reinforced ferroelectric matrix composite was studied based on the energy approach in a view of large scale. The released energy density was derived explicitly considering the magnetoelectric coupling under combined mechanical, electric and magnetic loading. Because of the magnetoelectric coupling through the interface, the fracture toughness is highly dependent on the polarization properties of the ferroelectric and ferromagnetic portions besides the volume fraction and the elastic properties of each composite.