A novel approach to study the conductivity behavior of CaCu 3 Ti 4 O 12 using scanning probe microscopy technique
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esearch Letter
A novel approach to study the conductivity behavior of CaCu3Ti4O12 using scanning probe microscopy technique M.S. Ivanov, Department of Physics, CFisUC, University of Coimbra, P-3004-516 Coimbra, Portugal F. Amaral, College of Health Technology of Coimbra, Polytechnic Institute of Coimbra, P-3040-162 Coimbra, Portugal; I3N and Physics Department, University of Aveiro, P-3810-193 Aveiro, Portugal V.A. Khomchenko, Department of Physics, CFisUC, University of Coimbra, P-3004-516 Coimbra, Portugal L.C. Costa, I3N and Physics Department, University of Aveiro, P-3810-193 Aveiro, Portugal J.A. Paixão, Department of Physics, CFisUC, University of Coimbra, P-3004-516 Coimbra, Portugal Address all correspondence to M.S. Ivanov at [email protected] (Received 26 January 2018; accepted 26 July 2018)
Abstract Herein, we show that scanning probe microscopy (SPM) is an effective tool permitting to disclose the nature of the colossal dielectric permittivity characteristic of CaCu3Ti4O12 (CCTO) compound. SPM data confirm the existence of micro- and nanoscale barrier layer capacitance mechanisms which simultaneously contribute to the electrical conductivity of the material. The former mechanism is associated with the potential grain-to-grain barriers. The latter mechanism involves the barriers created by intragrain structural defects. The results of the SPM study shed new light on the origin of the colossal dielectric constant in CCTO.
Introduction Complex oxides exhibiting colossal dielectric behavior (ε’ > 103) are recognized as promising materials which can be used as miniaturized passive components for micro- and nanoelectronic capacitive elements with improved efficiency.[1–5] Calcium copper titanate [CaCu3Ti4O12 (CCTO)], a nonferroelectric material, whose unusual dielectric properties have been reported in 2000,[2] attracts the attention due to its colossal dielectric constant (CDC) (ε’ > 105) observed both for ceramics and single crystals. Temperature stability of its dielectric properties is ensured by the absence of non-polar-to-polar transition (the latter complicates the explanation of CDC behavior). Theoretical and experimental data obtained during the last decade suggest that the extrinsic barrier mechanisms associated with the electric charge at the grain-to-grain boundaries,[3] depletion layers of Schottky contacts,[4,5] and/or plane defects[6,7] play an important role in the appearance of the giant dielectric permittivity. The microscale barrier layer capacitance (MBLC) model proposed by Sinclair and West[3] suggests that insulating surfaces are formed on the CCTO semiconducting grains during sintering process. According to MBLC approach, CCTO microstructure can be compared with a brick layer structure, where two pathways for the electric current should be considered. Since grain boundaries are supposed to be much more resistive than grains, the MBLC model implies that the impedance response of CCTO polycrystalline samples can be represented by the
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following equation: Z=
Rg Rgb + , 1 + jvRg Cg
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