Bulk and Grain Boundary Electrical Behaviours in Nb and Sn Doped Calcium Copper Titanium Oxide

Abstract

Three types of Calcium Copper Titanium oxide (CCTO), i.e. pure CCTO, niobium doped CCTO, and tin doped CCTO were obtained by mixed oxide route. The niobium and tin as a dopant were selected with the variation of doping concentration to create the stoichiometric formula CaCu3Ti4-xMxO12 (x = 0.05, 0.10, 0.15, 0.20, and 0.25; M = Nb, and Sn). Three different sintering times were applied for all the samples. X-ray diffraction, scanning electron microscopy,impedance spectroscopy at high and low temperature were used to determine the phases, dielectric constant, dielectric loss, resistivity and capacitance of bulk and grain-boundary, nonlinear coefficient of conduction, breakdown voltage-field, barrier height, and microstructural changes of CCTO. The purpose of this research is to investigate and provide the transition of dielectric properties from pure CCTO to non-linear behaviour in niobium and tin doped CCTO. The dielectric properties is represented by grain boundary thickness (tgb) and activation energy for electrical conductivity of bulk (Eab) and grain boundary (Eagb). The non-linear behaviours is represented by non-linear coefficient of conduction () and barrier height (B). The relative dielectric constant tends to decrease with increasing the amount of niobium and tin dopant. The activation energy of niobium and tin doped CCTO are in line with barrier height which are associated with conduction processes; the dopants are shown to reduce the conductivity of samples. The barrier height and grain boundary thickness tend to decrease with increasing the amount of niobium and tin dopant. The narrowest grain boundary thickness will increase rapidly the effective dielectric constant which is attributed to the formation of thin insulating boundaries with the conducting grains. The non-linear coefficient of conduction () of niobium and tin doped CCTO depend on the sintering hold time and the amount of dopant. The maximum value of alpha of niobium doped CCTO is ~600 at CCT10N4, and the maximum value of alpha of tin doped CCTO is ~830 at CCT15S4. Identically to the  value trend, the barrier height (B) also varies according to sintering hold time and the amount of dopant, and reaches maximum ~0.180 eV at CT05N4 in the range of ~0.173 to ~0.180 eV. There is no direct correlation between non-linear coefficient of conduction and barrier height in terms of sintering hold time for niobium and tin doped CCTO. The bulk activation energy of CCTO is in the range of 0.044 and 0.118 eV and the grain boundary activation energy is in the range of 0.389 and 0.706 eV. The large difference between bulk and grain boundary Ea is a strong indicator that different charge transport mechanisms exist for niobium and tin doped CCTO.

Date of Award	31 Dec 2014
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Colin Leach (Supervisor)