| Magnetoelectric(ME) effect is defined as a phenomenon in which an electric polarization response to an applied magnetic field, or a magnetic polarization response to an electric field. Since few years ago, researchers have paid attention to the laminated ME composites consisting of magnetostrictive materials and piezoelectric ceramics due to the large magnetic-electric conversion effects in these composites, and quickly they had been used in many intelligent ME devices.Until now, the study on ME effect centralizes in improving the magnetic-electric conversion effect. It has been proved that the major factors influencing ME effect include the material parameter, the value of the applied direct current(DC) magnetic field and the frequency of the alternating current(AC) magnetic field. However, there is a little research study the influences of DC magnetic field direction and temperature on ME effect. It is with this in mind that the author start with magnetic field direction, temperature and structural design, considering material parameter and the frequency of AC magnetic field, expanded the multi-field coupling model for ME effect, improved the magnetic-electric conversion effect of layered ME hybrid devices, and introduced a U-shaped ME structure.At first, based on elastic mechanics method, a model for magnetic-field-direction dependent ME effect in laminated composites has been introduced. The numerical results were compared to previous experimental data with excellent correlation. The results show that the DC direction has a significant effect on ME coefficient. When DC magnetic field is applied along length direction, there is an optimal angle corresponding to the best ME performance. The larger the DC magnetic field is, the larger the optimal angle is. When DC magnetic field is applied along width direction, the optimal angle is 0o all the time at any DC magnetic field. Different from former case, in later case the optimal magnetic field decreases with the DC angle. Meanwhile, the resonance frequency has no marked influences on the changing regulation of ME coefficient, while it can enhance ME coefficients by about 100 times. These results show that,in a practical application, the best ME performance should be obtained when the DC magnetic field direction is collinear with the AC magnetic field. This design can guarantee a lower DC magnetic field, except for a large magnetic-electric conversion effect.Then, based on equivalent circuit method, a mechanical-thermo-magneto ME model has been presented. The predictions from the model agree with the previous experimental data. The numerical results show that, when the pre-stress is small(about ?50MPa), magnetic-electric conversion effect can be enhanced by decreasing the operating temperature. However, when the pre-stress becomes large enough, only by increasing the operating temperature can the magnetic-electric conversion effect be improved. In any case, a small pre-stress and a low temperature will benefit the self-biased ME effect.Finally, a U-shaped ME structure has been introduced, and the corresponding theoretical model has also been presented. The theoretical results show that the shorter the length of magnetostrictive materials are and the longer the length of magnetostrictive materials are, the larger ME coefficient it can get. The experiment has proved that the maximum ME coefficient is 1.3V/ cm Oe when the frequency is 91 k Hz, and the low-frequency ME coefficient is 0.225V/ cm Oe.Above all, to improve magnetic-electric conversion effects, this thesis proposed two models, and achieved the optimal ME design plan. At the same time, to get around the bottleneck of the traditional laminated composite, a U-shaped ME structure is presented, and it have advantages for ME performance. The author believes that all these results in this thesis are of significance in both theory and application. |
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