Anomalous current-voltage behavior of CaCu3Ti4O12 ceramics

Marco A. L. Cordeiro, Flavio L. Souza, Edson R. Leite, Alexandre J. C. Lanfredi
2008 Applied Physics Letters  
This letter reports on an investigation of dc current-voltage measured in CaCu 3 Ti 4 O 12 ceramics. The experimental results show unusual I-V behaviors associated with the electrically inhomogeneous nature of the grains ͑mixture of semiconducting and insulating regions͒ and dependence on delay time. The data strongly suggest that both the localization of current in the conductive region and local Joule self-heating effects are essential elements to explain the abrupt drop in the electric
more » ... the electric field. In addition, the hysteresis behavior recorded during the I-V measurements shows strong dependence on delay time, i.e., the time needed for the polarization phenomena to occur. Oxide materials with high dielectric constants play a fundamental role in some technological applications, including the efficiency and miniaturization of microelectronic components. A surprising colossal dielectric constant ͑CDC͒ was recently discovered in CaCu 3 Ti 4 O 12 ͑CCTO͒ materials with a value in the range of 10 4 in polycrystalline samples and of more than 10 5 in single crystals in the kilohertz range and a slight temperature dependence between 100-600 K. 1-15 In addition, polycrystalline ceramics showed a strong nonlinear electrical behavior between current and potential. 4 In view of these properties, the scientific community has been focusing its attention on efforts to understand the dielectric and transport properties of these materials. In this letter, we report an anomalous current-voltage behavior strongly dependent on the time of applied current recorded during electrical measurements of polycrystalline CCTO samples. This anomaly is probably linked to local Joule self-heating associated with current localization in the conductive paths. Polycrystalline samples of CCTO were prepared by solid state reaction and sintering. The resulting powder was pressed into pellets ͑about 8 ϫ 1.5 mm 2 ͒ by isostatic pressing at 210 MPa and sintered at 1105°C in air for 1, 3, 12, and 24 h, reaching values of more than 96% of theoretical density. The electrical measurements and gold contacts were deposited by evaporation on the sample surfaces. The dielectric constant ͑Ј͒ and dielectric loss ͑Љ͒ versus the applied frequency in the range from 40 Hz to 5 MHz were measured at room temperature using an impedance analyzer ͑Agilent, 4294 A͒. Current-voltage behavior was determined using a Keithley 237 high voltage source-measure unit at room temperature. An analysis was made of the x-ray diffraction ͑XRD͒ diffraction patterns ͑Rigaku D/MAX 200͒ of the polycrystalline CCTO ceramics sintered at 1105°C for different periods of time ͑1, 3, 12, and 24 h͒, which revealed only a single CCTO perovskite phase in all the samples. The inset in Fig. 1͑a͒ shows the sample sintered for 12 h. The parameter cell could be indexed to a cubic cell with a lattice parameter a = 0.7395 nm, which is consistent with other works. 1,16 The scanning electron microscopy ͑SEM͒ micrograph ͑Zeiss 5000͒ of the pure CCTO sample sintered at 1105°C in air for 12 h revealed the presence of a bimodal grain distribution between 40-50 m and small grains from 20 to 1 m, as clearly visible in Fig. 1 . Furthermore, the samples of pure CCTO sintered at the same temperature for 3, 12, and 24 h show the existence of a second phase rich in CuO, which was indentified by energy dispersive spectroscopy analysis ͑not shown here͒. This second phase was observed as being discontinuous, e.g., there is a nonhomogeneous distribution of the CuO in the sintered ceramics. The microstructure obtained for the sintered polycrystalline CCTO in the present work is typical of CCTO ceramics and has been observed by several authors. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] The particles lying on the CCTO grains were attributed to Al 2 O 3 particles left behind by inefficient cleaning during preparation of the sample for morphological characterization. The dielectric properties of the sintered CCTO ceramics are depicted in Fig. 1͑b͒ ͑CCTO sample sintered at 1105°C for 12 h͒, which shows the frequency dependency of the CCTO dielectric constant and dielectric loss at room temperature. Values of dielectric constant exceeding 37 000 and a͒ Electronic
doi:10.1063/1.3023061 fatcat:7dluk2s2trdhtpiou7ekh6xvgq