Non‐Ohmic Properties and Electrical Responses of Grains and Grain Boundaries of Na1/2Y1/2Cu3Ti4O12 Ceramics

The dielectric and non‐Ohmic properties of Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramics sintered under various conditions to obtain different microstructures were investigated. Microstructure analysis confirmed the presence of Na, Y, Cu, Ti, and O and these elements were well dispersed in the microstructure. Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramics exhibited non‐Ohmic characteristics with large nonlinear coefficients of about 5.7–6.6 irrespectively of sintering conditions. The breakdown electric field of fine‐grained ceramic with the mean grain size of ≈1.7 μm (≈5600 V/cm) was much larger than those of the course‐grained ceramics with grain sizes of ≈9.5–10.4 μm (≈1850–2180 V/cm). Through optimization of sintering conditions, a low loss tangent of about 0.03 and very high dielectric permittivities of 18 000–23 000 with good temperature stability were successfully accomplished. The electrical responses of the grains and grain boundaries can, respectively, be well described using admittance and impedance spectroscopy analyses based on the brickwork layer model. A possible mechanism for the origin of semiconducting grains is discussed. The colossal dielectric response was reasonably described as closely correlated with the electrically heterogeneous microstructure by means of strong interfacial polarization at the insulating grain‐boundary layers. The non‐Ohmic properties of Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramics were primarily related to their microstructure, i.e., grain size and volume fraction of grain boundaries.     

Effects of Ta5+ doping on microstructure evolution,dielectric properties and electrical response in CaCu3Ti4O12 ceramics

The effects of Ta⁵⁺ substitution on the microstructure, electrical response of grain boundary, and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The mean grain size decreased with increasing Ta⁵⁺ concentration, which was ascribed to the ability of Ta⁵⁺ doping to inhibit grain boundary mobility. This can decrease dielectric constant values. Grain boundary resistance and potential barrier height of CaCu₃Ti₄O₁₂ ceramics were reduced by doping with Ta⁵⁺. This results in enhancement of dc conductivity and the related loss tangent. Influence of charge compensations on microstructure and intrinsic electrical properties of grain boundaries resulting from the effects of replacing Ti⁴⁺ with Ta⁵⁺ are discussed. The experimental data and variation caused by the substitution of Ta⁵⁺ can be described well by the internal barrier layer capacitor model based on space charge polarization at the grain boundaries.     

A Novel Route to Greatly Enhanced Dielectric Permittivity with Reduce Loss Tangent in CaCu3−xZnxTi4O12/CaTiO3 Composites

Dielectric and nonlinear properties of a binary compound derived from Ca₂Cu₂Ti₄O₁₂ were greatly improved by doping with Zn²⁺ to deliberately create CaCu_3–xZn_xTi₄O₁₂/CaTiO₃ composites. Ca₂Cu_1.8Zn_0.2Ti₄O₁₂ composition can exhibit an enhanced ε′, ~6,513, with a strong reduction in tanδ to ~0.015 (at 1 kHz). The nonlinear coefficient and breakdown field strength were significantly enhanced. The dielectric and nonlinear properties were described based on the effect of Zn²⁺ substitution on electrical response of internal interfaces.     

Effects of La3+ doping ions on dielectric properties and formation of Schottky barriers at internal interfaces in a Ca2Cu2Ti4O12 composite system

Influences of La³⁺ substitution on the dielectric properties and formation of Schottky barriers at internal interfaces of a Ca₂Cu₂Ti₄O₁₂ (CaTiO₃/CaCu₃Ti₄O₁₂) composite system were investigated. It was found that electrostatic potential barrier height was greatly reduced by doping with La³⁺, leading to a large decrease in the total resistance of internal interfaces between grains. This observation was attributed to the creation of conduction electrons, which were possibly induced by electrical charge compensation of La³⁺ substitution into Ca²⁺ sites. Variations in the dielectric properties of La³⁺-doped CaTiO₃/CaCu₃Ti₄O₁₂ composite ceramics and nonlinear properties can be described based on the electrical responses at the internal interfaces between CaCu₃Ti₄O₁₂–CaCu₃Ti₄O₁₂ grains and CaTiO₃–CaCu₃Ti₄O₁₂ grains. Influence of possible charge compensation due to different levels of La³⁺ dopant on the formation of potential barriers was discussed.     

Enhancement of giant dielectric response in Ga-doped CaCu3Ti4O12 ceramics

The influences of Ga³⁺ doping ions on the microstructure, dielectric and electrical properties of CaCu₃Ti₄O₁₂ ceramics were investigated systematically. Addition of Ga³⁺ ions can cause a great increase in the mean grain size of CaCu₃Ti₄O₁₂ ceramics. This is ascribed to the ability of Ga³⁺ doping to enhance grain boundary mobility. Doping CaCu₃Ti₄O₁₂ with 0.25 mol% of Ga³⁺ caused a large increase in its dielectric constant from 5439 to 31,331. The loss tangent decreased from 0.153 to 0.044. The giant dielectric response and dielectric relaxation behavior can be well described by the internal barrier layer capacitor model based on Maxwell?Wagner polarization at grain boundaries. The nonlinear coefficient, breakdown field, and electrostatic potential barrier at grain boundaries decreased with increasing Ga³⁺ content. Our results demonstrated the importance of ceramic microstructure and electrical responses of grain and grain boundaries in controlling the giant dielectric response and dielectric relaxation behavior of CaCu₃Ti₄O₁₂ ceramics.     

Giant dielectric behavior of monovalent cation/anion (Li+, F−) co-doped CaCu3Ti4O12 ceramics

Giant dielectric behavior and electrical properties of monovalent cation/anion (Li⁺, F⁻) co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction route were systematically investigated. Substitution of Li⁺ and F⁻ led to a significantly enlarged mean grain size. A reduced loss tangent (tanδ ~0.06) with the retainment of an ultra-high dielectric permittivity (ε′ ~7.7-8.8 × 10⁴) was achieved in the co-doped ceramics, while the breakdown electric field and nonlinear coefficient of CaCu₃Ti₄O₁₂ ceramics were increased by co-doping with (Li⁺, F⁻). The variations in nonlinear electrical properties and giant dielectric response, as well as the dielectric relaxation, were well explained by the Maxwell-Wagner polarization model for an electrically heterogeneous microstructure, in which a Schottky barrier height at the grain boundaries (GBs) was formed. ε′ was closely correlated to the GB capacitance. Significantly decreased tanδ value and enhanced nonlinear properties were related to a significant increase in the GB resistance, which was attributed to the significantly increased potential barrier height and conduction activation energy at the GBs. The semiconducting nature of the grains was also studied using X-ray photoelectron spectroscopy and found to originate from the presence of Cu⁺ and Ti³⁺ ions.