Assessment of Strain-Generated Oxygen Vacancies Using SrTiO3 Bicrystals.

Atomic-scale defects strongly influence the electrical and optical properties of materials, and their impact can be more pronounced in localized dimensions. Here, we directly demonstrate that strain triggers the formation of oxygen vacancies in complex oxides by examining the tilt boundary of SrTiO3 bicrystals. Through transmission electron microscopy and electron energy loss spectroscopy, we identify strains along the tilt boundary and oxygen vacancies in the strain-imposed regions between dislocation cores. First-principles calculations support that strains, irrespective of their type or sign, lower the formation energy of oxygen vacancies, thereby enhancing vacancy formation. Finally, current-voltage measurements confirm that such oxygen vacancies at the strained boundary result in a decrease of the nonlinearity of the I-V curve as well as the resistivity. Our results strongly indicate that oxygen vacancies are preferentially formed and are segregated at the regions where strains accumulate, such as heterogeneous interfaces and grain boundaries.

Cation disordering by rapid crystal growth in olivine-phosphate nanocrystals.

On the basis of Pauling's first rule for ionic bonding, the coordination number of cations with oxygen anions can be determined by comparison of their relative ionic size ratio. In contrast to simple oxides, various site occupancies by multicomponent cations with similar sizes usually occur in complex oxides, resulting in distinct physical properties. Through an unprecedented combination of in situ high-temperature high-resolution electron microscopy, crystallographic image processing, geometric phase analysis, and neutron powder diffraction, we directly demonstrate that while the initial crystallites after nucleation during crystallization have a very high degree of ordering, significant local cation disordering is induced by rapid crystal growth in Li-intercalation metal-phosphate nanocrystals. The findings in this study show that control of subsequent crystal growth during coarsening is of great importance to attain a high degree of cation ordering, emphasizing the significance of atomic-level visualization in real time.

Piezoelectric BaTiO3 thin film nanogenerator on plastic substrates.

The piezoelectric generation of perovskite BaTiO3 thin films on a flexible substrate has been applied to convert mechanical energy to electrical energy for the first time. Ferroelectric BaTiO3 thin films were deposited by radio frequency magnetron sputtering on a Pt/Ti/SiO2/(100) Si substrate and poled under an electric field of 100 kV/cm. The metal-insulator (BaTiO3)-metal-structured ribbons were successfully transferred onto a flexible substrate and connected by interdigitated electrodes. When periodically deformed by a bending stage, a flexible BaTiO3 nanogenerator can generate an output voltage of up to 1.0 V. The fabricated nanogenerator produced an output current density of 0.19 µA/cm(2) and a power density of ∼7 mW/cm(3). The results show that a nanogenerator can be used to power flexible displays by means of mechanical agitations for future touchable display technologies.

Effect of a sintering process on the electrical properties of SnO2 gas sensors.

SnO2 is a widely used sensor material that exhibits high sensitivity. It is known for its complex sensing characteristics, and its microstructure is an important parameter related to electrical properties and mechanical rigidity. Understanding of its microstructural effects is therefore essential to examine sensing mechanisms that may apply to industrial devices. In this work, the effect of a sintering process on the electrical properties is investigated. The sintering temperature and the relative density were chosen as process variables. The grain size of the specimen increased as the sintering temperature increased, whereas the relative density did not change. The apparent activation energy for conduction varied as the compacting pressure and the sintering conditions changed. Samples with a high density and large particle size showed low activation energy for conduction and low sensitivity at 375 degrees C. In an attempt to improve the properties, the powder was heat-treated at 1000 degrees C for 10 hours before compacting and sintering. The samples with heat-treated powder showed a slight decrease in the activation energy depending on the density and comparable sensitivity compared to non-treated powder at a high density. Pre-treatment of powder can be a simple means of thermal stability under high operating temperatures.

What We Should Consider for Full Densification when Sintering.

To fully densify a powder compact, we should avoid two things: (i) entrapment of insoluble gases within pores and (ii) entrapment of isolated pores within grains. This paper describes general directions for promoting full densification in view of the above two points. Emphasis is placed on ways to potentially prevent pore entrapment in terms of grain growth control. Currently available techniques that can enhance densification while suppressing grain growth are briefly described, and their major mechanisms are discussed.

Grain Growth Behavior of 0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3 Controlled by Grain Shape and Second Phase.

The grain growth behavior of 0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3 (mole fraction, NBT-5BTdid not appear in any of the NBT-5BT samples with excess Bi2O3. The amount of liquid phase increased as the amount of Bi2O3 increased. Therefore, the rate of grain growth could be decreased by the increasing the distance for the diffusion of atoms. These observations allowed us to conclude that the growth of Bi2O3-excess NBT-5BT grains is governed by the growth of facet planes via the two-dimensional nucleation grain growth mechanism during changing grain shape and amount of liquid.

Probing dopant segregation in distinct cation sites at perovskite oxide polycrystal interfaces.

Although theoretical studies and experimental investigations have demonstrated the presence of space-charge-induced dopant segregation, most work has been confined largely to the crystal-free surface and some special grain boundaries, and to the best of our knowledge there has been no systematic comparison to understand how the segregation varies at different types of interfaces in polycrystals. Here, through atomic-column resolved scanning transmission electron microscopy in real polycrystalline samples, we directly elucidate the space-charge segregation features at five distinct types of interfaces in an ABO3 perovskite oxide doped with A- and B-site donors. A series of observations reveals that both the interfacial atomic structure and the subsequent segregation behaviour are invariant regardless of the interface type. The findings in this study thus suggest that the electrostatic potential variation by the interface excess charge and compensating space charge provides a crucial contribution to determining not only the distribution of dopants but also the interfacial structure in oxides.

Unleashing the Full Potential of Magnetoelectric Coupling in Film Heterostructures.

A record-high, near-theoretical intrinsic magnetoelectric (ME) coupling of 7 V cm-1 Oe-1 is achieved in a heterostructure of piezoelectric Pb(Zr,Ti)O3 (PZT) film deposited on magnetostrictive Metglas (FeBSi). The anchor-like, nanostructured interface between PZT and Metglas, improved crystallinity of PZT by laser annealing, and optimum volume of crystalline PZT are found to be the key factors in realizing such a giant strain-mediated ME coupling.

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

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.