| The main interest of this thesis lies in how to improve and what is responsible for the change of the ferroelectric, piezoelectric and dielectric properties of intergrowth BLSFs Bi4Ti3O12-SrBi4Ti4O15 (BIT-SBTi) ceramics. By means of constructing intergrowth and A-site and B-site doping, we explored their effects on the crystal structure, ferroelectric, dielectric and piezoelectric properties and analysed the possible mechanism that determined the variation dependence of the remanent polarization(2Pr) , coercive field (Ec) and piezoelectric coefficient(d33).There are two kinds of A-site ions sitting in the perovskite unit of BIT-SBTi, that is, trivalent Bi3+ and divalent Sr2+ and B-site ions is quadrivalent Ti4+. By using the traditional solid-state reaction, we prepared ceramic samples of Nd-doped SrBi8–xNdxTi7O27 (BTN-SBTN) and Ce-doped SrBi8–x Ce3x/4Ti7O27 (BTC-SBTC). As for B-site doping, we prepared the V-doped and Nb-doped BIT-SBTi intergrowth. X-ray diffraction patterns (XRD) of all our samples show that the basic crystal structures of these samples were not changed by A-site or B-site doping and all the samples exhibited random orientation. In BTN-SBTN sample, as the Nd content increases from x = 0.00 to 0.50, the 2Pr increases steadily and reaches the maximum value of 32.3μC·cm?2, which is over 70% higher than that of the undoped BIT-SBTi, then decreases with further substitution. The Ec keeps almost unchanged at 103 kV?cm?1 when x = 0.50, then decreases monotonically than that of undoped BIT-SBTi. The variation of 2Pr is dominated by the compromise between the restraint of space charge and the relief of lattice distortion. Piezoelectric coefficient d33 was increased from nondoped 6 pC·N–1 to 11 pC·N–1 at x=0.50. Interestingly, the optimal doping values for ferroelectric and piezoelectric properties are the same. We try to give an explanation from the phenomenological points of view.In Ce-doped BTC-SBTC samples, a different 2Pr dependence was found. The Ce-doping decreases the 2Pr from 15.2μC·cm?2 to 3.1μC·cm?2 when x<0.6, and after x >0.6 the 2Pr increases from 3.1μC·cm?2 to 8.4μC·cm?2, meanwhile the Ec and Tc exhibit the same tendency with the 2Pr. Piezoelectric coefficient d33 was increased from nondoped 8.8 pC·N–1 to 16.1 pC·N–1 at x=0.2. It may be due to the fact that Ce presents two different chemical valence, namely, +1, +3 and +4. In different doping value, the doped Ce may change its valence, which should be responsible for the variation of electrical properties.As for high-valent B-site doping, we found similar variation tendency in V-doped and Nb-doped BIT-SBTi. In V-doped BIT-SBTi case, the 2Pr increases initially and reaches its maximum value of 29.0μC·cm?2 when V content is 0.03, which is about 150% as large as that of non-doped one, then decreases with further doping. The coercive field (Ec) shows a weak V content dependency. Piezoelectric coefficient d33 was increased from nondoped 6.8 pC·N–1 to 12.2 pC·N–1 at x=0.03. In Nb-doped BIT-SBTi case, the 2Pr increases initially and reaches its maximum value of 34.9μC·cm?2 when Nb content is 0.06, which is 175% as large as that of non-doped one, the coercive field (Ec) shows a weak Nb content dependency. Piezoelectric coefficient d33 was increased from 9.3 pC·N–1 to 18.6 pC·N–1. It is considered that the enlarged 2Pr could be mainly attributed to the restraint of oxygen vacancies as well as the weakening of their mobility. But when doping content exceeds the optimal value, the increasing Bi vacancy can deteriorate the integrality of the crystal cell and consequently lead to a decrease of 2Pr. It is the competition mechanism of the opposite effects regulates the variation tendency of 2Pr . |
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