报告内容摘要：

Relaxor ferroelectric single crystals, particularly PMN-PT single crystals, have drastically improved the bandwidth and sensitivity of medical ultrasonic transducers, producing a much enhanced resolution for medical ultrasonic imaging systems. Based on physical principles, further increase the ultrasonic imaging resolution needs to increase the operating frequency of the system. However, the coercive field of PMN-PT single crystals is only about 1/5th of that of PZT ceramics. At high frequencies (> 20 MHz), the thickness of piezoelectric array elements becomes very thin so that the field applied on them may well exceed the coercive field of PMN-PT single crystals. If reducing the applied field, there will be insufficient ultrasonic energy for in depth imaging. Can relaxor-based single crystals be operated beyond their coercive field? To answer this critical question, we have carried out a combined theoretical and experimental investigation to study the evolution of dynamic hysteresis with respect to electrical field amplitude (E0) and frequency (f) for 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 (PMN-29PT) single crystal. We have also obtained the relationship between coercive field and frequency, which provide a useful guidance for the design and fabrication of higher frequency medical ultrasonic transducers using these relaxor-based single crystals. Our results showed that the electric field dependent scaling relationship in PMN-29PT single crystal can be divided to three regions, while the hysteresis area <A> follows the power law  in the low and high E0 regions. However, the power law is not obeyed in the intermediate region due to the complex collective contributions of 180o and non-180o domains. The hysteresis area decreases continually with the increase of frequency when E0 < Ec, but <A> first increases then decreases for high E0 situation (E0 ).

报告人简介：

Prof. Wenwu Cao received his Ph.D degree in condensed matter physics from the Pennsylvania State University in 1987. He became a faculty member of Penn State in 1990 after postdoctoral studies at the Materials Research Lab of Penn State and the Laboratory of Atomic and Solid State Physics, Cornell University. He currently holds a joint appointment between the Department of Mathematics and the Materials Research Institute of the Pennsylvania State University as a Professor of Mathematics and Materials Science. He is also a graduate faculty member in the Bioengineering Department and the Department of Materials Science and Engineering at Penn State. Up to date, he has published over 600 SCI papers with over 15000 citations and an H-factor 60. In addition, he co-authored 5 books in the field of numerical computation, smart materials and ferroelectric materials. His H-factor is 60. His research interest spans several fields, including piezoelectric materials, ultrasonic devices, medical imaging and sonodynamic therapy for cancer treatment.

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