Hydrogen Control of Dielectric Oxides for Multi-functional Sensors

Abstract

Dielectrics are insulating materials characterized by the ability to become polarized (i.e., dielectric permittivity) in the presence of an electric field. Nowadays, improving dielectric permittivity in complex oxides is of great interest in the aspects of fundamental science as well as technological applications. Note that such a huge dielectric permittivity, which is controllable by external stimuli, enables us to accomplish multi-functional nano-devices such as switchable passive sensors potentially. In this thesis, we demonstrate the hydrogenation control of dielectric responses in complex oxide ceramics for practical applications in dielectric-based gas sensors. First, an unusual polymorphic phase, which is an excellent platform to examine the protonation effect, is realized in oxide ceramics by cation substitution. Second, we systematically investigate the hydrogenation effect on the dielectric properties of dielectric oxide ceramics. Finally, by reversibly manipulating dielectric permittivity through protonation, we also provide a conceptual demonstration of the realization of dielectric-based humidity sensors with high performance. First, we present a room-temperature polymorphic phase transition in BaTiO3 ceramics induced by Ni substitution. As the Ni concentration increases, the initial tetragonal structure changes to a hexagonal structure. A possible mechanism of polymorphic transition in Ni-substituted BaTiO3 ceramics is discussed in conjunction with the microscopic analyses of the crystal structure and the electronic configuration. The oxygen vacancy defects are generated in oxide ceramics due to the substitution of Ni2+ to the Ti4+ cations. With the formation of oxygen vacancies, the Ti oxidation state is locally reduced from 4+ to 3+. Due to the reduction of the local Ti charge valency, the polar Ti-O hybridization in tetragonal BaTiO3 becomes weakened, leading to the structural phase transition in Ni-substituted BaTiO3 ceramics. As a result, a high-temperature 6H-hexagonal phase is realized in BaTiO3 ceramics by Ni substitution. ii Second, we demonstrate an ultrahigh dielectric permittivity in the 6H-hexagonal BaTiO3 ceramics by hydrogenation via the specific treatment under ambient environments. We observed a significant enhancement of low-frequency dielectric permittivity at room temperature when the as-sintered oxygen-deficient BaTiO3 ceramics were exposed to water-vapor-containing ambient environments. Intriguingly, the huge dielectric permittivity (i.e., on-state) in the treated ceramics is recovered to the initial value (i.e., off-state) of as-sintered ceramics by external stress, indicating the reversible manipulation of dielectric responses. When the as-sintered BaTiO3 ceramics are exposed to the ambient air containing water vapor, the dissociated protons (H+) from water molecules are introduced into the ceramics due to the high mobility. The introduction of hydrogen ions produces the heterogeneous distributions of the crystal structures and the electric polarizability microscopically leading to the enhancement of overall dielectric permittivity in the Ni-substituted BaTiO3 ceramics. For device applications, we examine the sensing capability of our Ni-substituted BaTiO3 ceramics for the realization of dielectric humidity sensors. The dielectric and associated electrical properties of Ni-substituted BaTiO3 are highly sensitive to ambient environments. The modification of dielectric responses under various ambient conditions (i.e., ambient air, vacuum, N2, CO2, water vapor, liquid water, and acetic acid) is systematically examined. Dielectric relaxation characteristics of Ni-substituted BaTiO3 ceramics are reversibly controllable by hydrogenation through the treatment under ambient environments and thermal annealing. We also evaluate the humidity sensing properties of Ni-substituted BaTiO3 in an actual sensor. The susceptibility of dielectric permittivity and related electrical conductivity to a humid atmosphere enables the realization of oxide-ceramic-based humidity sensors with high efficiency and sensitivity.