The existence of a magnetodielectric (magnetocapacitance) effect is often used as a test for multiferroic behavior in new material systems. However, strong magnetodielectric effects can also be achieved through a combination of magnetoresistance and the Maxwell-Wagner effect, unrelated to true magnetoelectric coupling. The fact that this resistive magnetocapacitance does not require multiferroic materials may be advantageous for practical applications. Conversely, however, it also implies that magnetocapacitance per se is not sufficient to establish that a material is multiferroic.

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Magnetocapacitance without magnetoelectric coupling

The discovery of a giant dielectric constant of 10(5) in CaCu(3)Ti(4)O(12) has increased interest in this perovskite-type oxide Here we demonstrate that, in addition to high permittivity, CaCu(3)Ti(4)O(12) has remarkably strong nonlinear current-voltage characteristics without the addition of any dopants An intrinsic electrostatic barrier at the grain boundaries is responsible for the unusual nonlinear behaviour The nonlinear coefficient of CaCu(3)Ti(4)O(12) reaches a value of 900, which is even greater than that of the varistor material ZnO As a result, CaCu(3)Ti(4)O(12) may lead to efficient switching and gas-sensing devices

Strong nonlinear current-voltage behaviour in perovskite-derivative calcium copper titanate.

This review summarizes the current state of polymer composites used as dielectric materials for energy storage The particular focus is on materials: polymers serving as the matrix, inorganic fillers used to increase the effective dielectric constant, and various recent investigations of functionalization of metal oxide fillers to improve compatibility with polymers We review the recent literature focused on the dielectric characterization of composites, specifically the measurement of dielectric permittivity and breakdown field strength Special attention is given to the analysis of the energy density of polymer composite materials and how the functionalization of the inorganic filler affects the energy density of polymer composite dielectric materials

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Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage

Monitoring and control of temperature is of paramount importance in every part of our daily life. Temperature sensors are ubiquitous not only in domestic and industrial activities but also in laboratory and medical procedures. An assortment of temperature sensors is commercially available for such purposes. They range from metallic thermocouples to resistive temperature detectors and semiconductive ceramics, showing a negative temperature coefficient of resistance (NTCR). NTCR ceramic sensors occupy a respected market position, because they afford the best sensitivity and accuracy at the lowest price. Despite the enormous commercial success of NTCR thermistors, this area of advanced functional ceramics has not been recently reviewed. Nearly 100 years elapsed between the first report of NTCR behavior and the fabrication of NTCR devices. The manufacture of the first NTCR ceramic thermistors was problematic, as often the devices suffered from poor stability and nonreproducibility. Before NTCR ceramics could be seriously considered for mass production of thermistors, it was necessary to devote a large amount of R&D effort to study the nature of their semiconductivity and understand the influence of impurities/dopants and heat treatments on their electrical characteristics, particularly in their time dependence resistivity (aging). Simultaneously, from a technological viewpoint it was important to develop methods enabling reliable and permanent electrical contacts, and design suitable housing for ceramics, in order to preserve their electrical properties under conditions of variable oxygen partial pressure and humidity. These topics are reviewed in this article from an industrial perspective. Examples of common applications of NTCR thermistors and future challenges are also outlined.

Negative Temperature Coefficient Resistance (NTCR) Ceramic Thermistors: An Industrial Perspective

Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today’s most investigated material with colossal dielectric constant Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant

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Colossal dielectric constants in transition-metal oxides

There has been much recent interest in a so-called “giant-dielectric phenomenon” displayed by an unusual cubic perovskite-type material, CaCu3Ti4O12; however, the origin of the high permittivity has been unclear [M. A. Subramanian, L. Dong, N. Duan, B. A. Reisner, and A. W. Sleight, J. Solid State Chem. 151, 323 (2000); C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez, Science 293, 673 (2001); A. P. Ramirez, M. A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, and S. M. Shapiro, Solid State Commun. 115, 217 (2000)]. Impedance spectroscopy on CaCu3Ti4O12 ceramics demonstrates that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. The giant-dielectric phenomenon is therefore attributed to a grain boundary (internal) barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure. This barrier layer electrical microstructure with effective permittivity values in excess of 10 000 can be fa...

CaCu3Ti4O12: One-step internal barrier layer capacitor

Giant Barrier Layer Capacitance Effects in CaCu3Ti4O12 Ceramics

The influence of electrode material, dc bias, and pellet thickness on the electrical properties of fine- and coarse-grained $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ (CCTO) ceramics has been investigated using impedance spectroscopy. The low frequency arc observed in ${\mathrm{Z}}^{*}$ plots near room temperature is independent of the electron work function of the metal electrode. It shows significant variation with dc bias and pellet thickness for coarse-grained ceramics, but no such variations for fine-grained ceramics. The results demonstrate the importance of ceramic microstructure in controlling the electrical properties of CCTO ceramics and support the internal barrier layer capacitor (IBLC) model of Schottky barriers at the grain boundaries between semiconducting grains. The IBLC model explains the high permittivity and nonlinear current-voltage characteristics commonly reported for CCTO ceramics.

Characterization of grain boundary impedances in fine- and coarse-grained Ca Cu 3 Ti 4 O 12 ceramics

The electrical properties of a series of CaCu3Ti4O12 ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu3Ti4O12 ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N2 at 800°–1000°C but can be recovered to the original value by heat treatment in O2 at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N2 or O2 at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu3Ti4O12 at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

A review is given of the origins of high permittivity in two groups of materials, La-doped BaTiO3 and a new barrier layer capacitor material, CaCu3Ti4O12. Factors that influence permittivity include: dopant, doping mechanism, processing conditions and grain size. La-doped BaTiO3 has high permittivity due to its ferroelectric nature at low temperatures and a novel doping mechanism: A-site substitution linked to the creation of B-site vacancies for charge compensation. Permittivities of 25,000 have been achieved, which can be increased further to ∼36,000 by additional doping with Zr. The value of impedance spectroscopy to characterize materials that have heterogeneous electrical microstructures is illustrated with the example of CaCu3Ti4O12; the high permittivity is not a bulk effect, as widely stated in the literature, but is a thin layer effect typical of a barrier layer capacitor. By attention to processing conditions to achieve large grain sizes, effective permittivities as high as 300,000 have been obtained.

Timothy B. Adams

Papers

CaCu3Ti4O12: One-step internal barrier layer capacitor

Giant Barrier Layer Capacitance Effects in CaCu3Ti4O12 Ceramics

Characterization of grain boundary impedances in fine- and coarse-grained Ca Cu 3 Ti 4 O 12 ceramics

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

Novel high capacitance materials:- BaTiO3:La and CaCu3Ti4O12