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|Title:||Semiconducting and dielectric properties of barium titanates, tantalates and niobates with perovskite structure|
|Department:||Materials Science and Engineering|
|Keywords:||Materials Science and Engineering;Materials Science and Engineering|
|Abstract:||<p>The dielectric and semiconducting properties of two types of ceramics (n-type BaTiO₃ and dielectric Ba(B'⅓ B''⅔ )O₃ where B' = Mg, Zn, Ni, and B'' = Nb₁ Ta) were characterized. Complex impedance analysis and dc conductivity measurements of samples prepared at various [Special Characters Removed] have ruled out oxygen chemisorption in favor of interfacial segregation of cation vacancies as the cause of the positive temperature coefficient of resistivity (PTCR) effect in n-type BaTiO₃ . The effect of preparation conditions, sintering atmosphere, stoichiometry, and post-sinter anneal on the defect chemistry of BaTiO₃ was studied using the electron paramagnetic resonance (EPR) technique. Several paramagnetic defects such as, Ti³⁺ , [Special Characters Removed] were detected and identified by EPR. Current-voltage characteristics (I-V ) of PTCR BaTiO₃ were analyzed in light of space-charge-limited-current, trap-filled-limited-current, Frenkel-Poole, small polaron, and double-Schottky barrier models. It was shown that for the double-Schottky barrier model, a partial stabilisation of the potential barrier is expected when the Fermi level is pinned at grain boundaries by a high density of the interface states. The deviation of I-V characteristics of BaTiO₃ in the region of the PTCR effect can be explained by dependence of the population of the interface electron states on applied voltage. Based on the Seebeck and Hall effect measurements, it was found that in the range of 100-300 K, the drift mobility of electrons in BaTiO₃ is not thermally activated, which supports the concept of conduction band electron transport rather than small radii polaron hopping. However, further study over a wider temperature range and on better quality crystals is required to unequivocally clarify the electron transport mechanism in BaTiO₃ . Phase composition, degree of cation ordering, and dielectric properties of complex perovskites with general formula Ba(B' ⅓ B''⅔ )O₃ where B' = Mg, Zn, Ni, and B'' = Nb₁ Ta were analyzed. It was shown that in Ba(Mg⅓ Ta⅔ )O₃ both intrinsic and extrinsic dielectric loss affect the Q-factor, whereas in Ba(Mg⅓ Nb⅔ )O₃ and Ba(Ni⅓ Nb⅔ )O₃ extrinsic factors such as the second phase and point defects dominate the dielectric loss at microwave frequencies.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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