【作者】 路晓艳；

【导师】 王彪；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2009， 博士

【摘要】 多重铁性材料是指一类具有铁电、铁磁和铁弹两者或两者以上耦合性能的材料,这种材料在外电场下可以诱发磁极化,在外磁场下也可以诱发电极化,具有乘积效应的磁电耦合性能。这种奇特的性能为材料和器件的设计增加了自由度,在多场信息存贮与读写、无线微波器件、无线传感网络和多功能电子器件等方面有很大应用潜力,已成为功能材料领域的研究热点。具有多重铁性的单一物质材料在自然界中种类很少,而且多数在极低的温度下才有较强的磁电耦合效应,而利用铁电材料的电致伸缩性和磁性材料的磁致伸缩性通过界面之间的弹性耦合制备的复合型多重铁性材料在室温下具有较大的磁电耦合效应。由于这种磁电耦合主要来自于两相之间的弹性耦合,因而,分析两相界面之间、薄膜与衬底界面之间的弹性相互作用,详细考虑外电场和外磁场对材料内部应力场的影响,才能合理选择相互匹配的材料,获得更大的磁电耦合性能。本文研究了复合型多重铁性材料的耦合机理,详细考虑了尺寸效应、表面效应、体积份数、厚度以及各种应力应变对材料性能的影响,同时也关注如何利用这一耦合性能实现一些功能结构材料的多场多自由度调控,最后,对材料的断裂性能做了初步的分析。以下是本文的主要研究思路、方法和结论:1)基于Landau理论,详细分析了1-3型纳米铁电铁磁复合材料中各种应力应变之间的相互作用关系及其对系统自由能的影响。各种应力应变包括铁电相的电致伸缩应变和铁磁相的磁致伸缩应变以及两相界面之间和薄膜与衬底之间由于晶格不匹配产生的错配应力和由于位错产生的释放应力之间的弹性相互作用。建立电极化和磁极化随时间演化的金兹堡-朗道动力方程,通过分岔理论预测了两相材料的相变点,利用数值方法给出了稳态时电极化强度和磁极化强度以及两相中的应变状态,并基于Landau-Khalatnikor方法,数值模拟了耦合场下的电极化强度和磁极化强度的蝶形曲线;2)基于Ginzburg-Landau理论结合Timeshenko弹性力学方法对介电/磁性合金(Ba0.6Sr0.4TiO3/terfenol-D)层状复合材料的介电性能进行研究。利用外磁场诱发的磁性合金的磁致伸缩,可以在一定范围内调节钛酸锶钡的小场介电性能,而通过改变Ba/Sr成份比例和层厚比可以实现较大范围内的调控;3)利用磁电耦合效应,设计了铁电/铁磁层状弯曲型驱动器,并用Hsueh的方法对铁电/磁性合金层状复合材料的弯曲性能进行研究。结果表明,由外磁场调控terfenol-D层的磁致伸缩可以调控结构的竖向相对弯曲位移,最高可达55%;4)基于能量的方法,从宏观尺度研究了铁磁纤维增强型铁电基体复合结构的断裂韧性。考虑力、电和磁多场作用下的磁电耦合的性能,得出了关于铁电极化强度和磁极化强度的能量释放密度函数。结构的断裂韧性很大程度上依赖于铁电材料和铁磁材料的极化大小、两相的成份比和界面弹性相互作用。更多还原

【Abstract】 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.更多还原