Study On Electrical Properties And Defect Chemistry Of Tb-CaCu₃Ti₄O₁₂ And Nb-BaTiO₃ Ceramics

CaCu₃Ti₄O₁₂?CCTO?discovered first by Subramanian et al belongs to ACu3Ti4O12 family.CCTO exhibits the most special behavior and a high dielectric permmitivity?e'?of about 12,000 at 1 kHz.The e' value is nearly constant from room temperature to 300 °C and frequency-independent in the range of 102 106 Hz.These results indicate that CCTO is a potential candidate for capacitor applications.Barium titanate?BaTiO₃?is a very attractive material in the field of electroceramics and microelectronics due to its good characteristics.Its high e' and low dielectric loss?tan d?make BaTiO₃ an excellent option for many applications.Doped BaTiO₃ has been discovered to have wide application in semiconductors,positive temperature coefficient resistors?PTCR?,ultrasonic transducers,piezoelectric devices,and has become one of the most important ferroelectric ceramics.The effects of the sintering time and site occupations of doping ions on the high e' and high tan d of CCTO were studied.The structure,microstructure,dielectric,electrical properties,and point defects were studied for Nb-doped BaTiO₃ ceramics.The main contents are as follows:1.The effects of different sintering times on the microstructure,dielectric and electrical properties of CCTO were investigated.The results show that the dielectric properties of CCTO ceramics are affected by microstructure;e' increases as the average grain size increases.The study on the impedance and admittance spectra shows that the giant dielectric properties at low frequencies are mainly affected by grain-boundary capacitance(C_gb),while the dielectric platform at high frequencies is mainly affected by grain capacitance?C_g?,and the dielectric relaxation process is closely related to grain resistance?Rg?.As the sintering time is prolonged,tan d reduces and is inversely proportional to the total resistance and is controlled by grain-boundary resistance(R_gb).2.The tan d of Ca_1-3x/2Tb_xCu₃Ti₄O₁₂ ceramics with a single-phase cubic structure increases relative to CCTO.This is attributed to the modification of chemical structure of Tb doping ions on CCTO grain boundaries and lowering of the barrier heights of grain boundary??_B?,which allows more charge carriers to pass through the grain boundary barrier.The tan d is therefore increased.3.The strong suppression of grain growth in Ca_1-3x/2Tb_xCu₃Ti_4-xTb_xO₁₂?CTCTT?ceramics is attributed to the solute drag mechanism.The Raman spectra?RS?investigations indicate that the multiple-phonon effect occurs in CTCTT.The XPS results reveal that the coexistence of mixed valence states of Cu²⁺ / Cu⁺,Ti⁴⁺ / Ti³⁺,and Tb³⁺ / Tb⁴⁺.The Tb doping in CCTO gives rise to a decrease in e' of CTCTT and an increase in tan d.The analyses on the defect chemistry and impedance spectra?IS?confirm that the decrease in e' and the increase in tan d are mainly due to the decrease in conductivity in the CTCTT grains owing to the acceptor effect of Tb⁴⁺ on the Ti site,which affected the interior barrier layer capacitance?IBLC?effect.4.The plate-shaped and fine grains coexist in Ba(Ti_1-x/4Nb_x)O₃?x = 0.03?ceramic?BTN?with a tetragonal perovskite structure.The BTN ceramic with x = 0.05 exhibits a cubic structure,high density??r = 98 %?,a uniform and fine-grained Nb-rich microstructure,mixed valence states of Nb⁵⁺?percentage: 80 %?,Nb4+? 11 %?,and Nb³⁺? 9 %?.Ba vacancies and Ti ones also exist in BTN.The electron compensation from the Nb⁵⁺ donor effect prevails in BTN.The highly-densified BTN ceramic with x = 0.05 is a semiconductor with a room-temperature resistivity of ?RT = 2 × 104 ?cm,rather than a conventional dielectric.