Colossal dielectric permittivity materials: Myths and reality

Since their discovery, 13 years ago there has been a lot of interest in the colossal dielectric permittivity ceramic materials. These materials derived from or are related to the calcium-copper titanate (CaCu
₃
Ti
₄
O
₁₂
) exhibit large values of permittivities (ε
_r
> 10
⁵
), well above those obtained with ferroelectric ceramics. Along the large values of permittivity, good thermal stability is exhibited over a large temperature range (-100 to 200°C) making this family of materials a potential candidate for microelectronics and power electronics integration. The discussion over the physical origin of the large dielectric constants tends, by a large consensus, to describe the permittivity as the result of the formation of an Internal Barrier Layer Capacitance (IBLC). This would mean that internal barriers control the full electrical characteristics of the ceramics. Nevertheless, despite large attempts to control and diminish the dielectric and conduction losses of these materials, the whole electrical characteristics do not allow them yet to jump from fundamental research to real applications. In this paper, we present a state of the art of the colossal dielectric constant materials, providing a review of the different mechanisms that aim to explain the large values of permittivity. An analysis of the different proposed mechanisms and their limitations will be discussed. We will also present recent developments made by our group on the characterization of these ceramic materials that bring light into new characterization and analysis strategies to describe these materials and may, in some ways, explain why they are still not in use in today's highly demanding applications.