Qi, Yubo
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Publication Depolarization In Nanowires, Tribo–chemistry, And Negative Electrocaloric Effect(2017-01-01) Qi, YuboThis thesis contains investigations about several technologically--important scientific problems: depolarization process in BaTiO$_3$ nanowires, tribopolymer formation on metal surfaces, and negative electrocaloric effect in prototype ferroelectrics. Spontaneous electric polarization makes perovskite--based oxides of great interest for application to nonvolatile memory devices. However, the polarization of ferroelectric materials may not be infinitely stable. For successful technology application, the depolarization processes of nanoscale ferroelectric oxides must be better understood. Here, we report a combined experimental and theoretical investigation of the depolarization process of single--crystalline BaTiO$_3$ nanowires. By building an Modified Schottky emission model, we explain the both temperature and nanowire thickness dependent polarization decay rates successfully. Besides, Micro--electro--mechanical system and nano--electro--mechanical system (MEMS and NEMS) transistors are considered promising for size--reducing and power--maximizing electronic devices. However, the tribopolymer which forms due to the mechanical load to the surface contacts affects the conductivity between the contacts dramatically. This is one of the challenging problems that prevent widespread practical use of these otherwise promising devices. Here, we use density functional theory (DFT) to investigate the mechanisms of tribopolymer formation, including normal mechanical loading, the catalytic effect, as well as the electrochemical effect of the metal contacts. Our study illustrates the underlying mechanisms of tribopolymer formation clearly, which has great significance in designing tribopolymerization--suppressing strategies. The last work is about electrocaloric effect. The electrocaloric effect (ECE) refers to the phenomenon in which the temperature of a material changes reversibly under the application and removal of electric field. Here, we point out that negative unusual electrocaloric effect (uECE) with a fast cooling rate ($\approx$10$^{11}$ K/s) can be achieved by driving solid crystals to a high--temperature phase with a properly designed electric field pulse. We acquire and analyze these results by clarifying the mechanism of ECE. Our work offers a more general framework to understand the ECE and highlights the opportunities of electric--field engineering for controlled design of fast and efficient cooling technology.