Defect engineering in controlling the dielectric behavior of rutile tio2

Abstract

Defects have gained increasing attention to tailor/explore novel dielectric properties in metal oxide due to the correlation of defect complexes and the interaction between defects and crystal environment. For example, colossal permittivity (CP, >1000) was successfully achieved by electron-pinned defect-dipoles (EPDDs) with a concrete example In+Nb co-doped rutile titanium dioxide. This dissertation, therefore, aims to use defect engineering to control the dielectric behaviour of rutile titanium dioxide. Continuing works were first carried out on acceptor/donor ion co-doped rutile titanium dioxide. Ga, with much smaller ionic size than In, was used as acceptor. The 0.5% Ga+Nb co-doped case also shows similar CP (>10000) with optimized loss in the range of 0.05-0.1. Systematic dielectric analysis suggest the CP is not dominated by EPDDs, which is significantly different compared with the EPDD-dominated CP in In+Nb co-doped rutile titanium dioxide and no-EPDD-dominated CP in Al+Nb co-doped case. In addition to the acceptor, another donor Ta(V) with similar ionic size but one more electron shell structure was used to replace the Nb(V). M+Ta co-doped rutile titanium dioxide (M = In, Mg) were synthesized. Both samples show similarly low dielectric loss below 0.01 but different intrinsic CP level (i.e. {u0303}10000 in the former, {u0303}7000 in the later) at appropriate co-doping level. Theoretical calculations reveals different EPDDs formed in each material. These results suggest the important role of acceptor and donor in controlling the CP behaviours in these materials. A part from the system with slight acceptor/donor ion co-doping, rutile titanium dioxide with heavily co-substitutional acceptor/donor ions were analyzed to have a more comprehensive understanding of the defect chemistry, correlation behavior of defect complexes, and their related dielectric properties. Upon the success in synthesizing Al+Ta co-substituted rutile titanium dioxide with co-substitution level (x) up to 50%. Structural analysis suggests that the materials maintain rutile phase even for x up to 50% with signature of disorder. Defect analysis suggests the significantly low ratio of Ti(III)/Ta(V) as well as lower magnitude of permittivity (800{u0303}3000) in the system compared with that previously reported. The permittivity increases when x < 20% but decreases after over 20% followed by exponential decreases of relaxation frequency as well as increase of the thermal activation energy E (e.g. E increases from 33.4 meV to 131.4 meV when x increases from 5% to 50%). The system presents a typical dipole glass character when x > 20%. Finally, this dissertation reports for the first time, by using defect engineering, the linear polarisation behavior of binary oxide, e.g. rutile titanium dioxide, can be changed to a nonlinear one at room temperature. With slightly doping acceptor Mg into the host matrix, a combination of electrical measurements and theoretical calculations suggest that Mg-oxygen vacancy defect dipole can facilitate certain neighboring Ti(IV) with large move-off-center ability, offering the doped titanium dioxide an unexpected role as a new family of room temperature nonlinear dielectrics. Moreover, a poling induced internal electric field abnormally varies with the direction of poling electric field can be observable, which is quite different from that observed in ferroelectrics and relaxor ferroelectrics. Show more