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We investigated the resistance response to trace hydrogen gas in the air between the asymmetrically shaped point contact- (Pt tip) and plane contact- (Al thin-film) electrodes formed in-plane on a rutile TiO2 single crystal at 673 K. It was found that the polarity of the resistance response, that is, the increase or the decrease of the resistance by exposing to hydrogen, reverses depending on the bias voltage polarity. This reversal mechanism of the resistance response was analyzed from the electrical conduction properties and the depth profile of the oxygen tracer (18O) in the electric field-induced diffusion. The central mechanism is proposed to be the oxygen transfer reaction at the Pt/TiO2 interface, accompanied by the modulation of the positively charged-oxygen vacancy concentration, thus the resistance between the two terminals. It is also proposed that the proton hinders the transfer reaction at the interface; and hence, the resistance increases by exposing to hydrogen with biasing the positive voltage to the Pt tip electrode. The application of this reversal of the resistance response for separating hydrogen and ethanol in the trace gas sensing is discussed, which is quite limited for a conventional semiconductor gas sensor. [ABSTRACT FROM AUTHOR]

Although a semiconductor gas sensor is promising in the future gas sensing technology, the material design for highly sensitive and selective gas sensing is quite challenging. The crystal plane dependence of gas sensing has been studied using various nano‐crystal samples with a controlled crystal facet to obtain a clue for designing the sensing material, but the contact effect between nano‐crystal particles has often hindered the straightforward analysis. In the present study, the crystal plane dependence of ethanol (EtOH) sensing by ZnO, one of the most widely studied target gas‐sensing material combinations, was investigated from an analytical approach. We applied a newly developed low‐energy He+ ion scattering spectroscopy (He+ LEIS) combined with the pulsed jet technique for analyzing the surface structure of ZnO during the EtOH sensing. In this novel technique, the free gas jet is periodically blown onto the sample surface to simulate the gas‐sensing surface in a vacuum as one under the realistic working condition, while the background pressure is maintained low enough for operating He+ LEIS. It was found that the divalent oxygen adatom on the outermost surface is consumed during the EtOH sensing while no change occurs in the concentration of the lattice oxygen on the Zn‐terminated (0001) surface. The availability of this oxygen adsorbate determines the dissociative EtOH adsorption probability on the surface; hence it determines the EtOH sensing sensitivity, which is substantially higher on Zn‐terminated (0001) than other dominant low‐index surfaces, such as O‐terminated (000 1‾), (10 1‾ 0), and (11 2‾ 0). [ABSTRACT FROM AUTHOR]

We have investigated the compressive behaviour of closed-cell aluminium foams using a high-resolution X-ray CT. The microstructures of cell walls or Plateau borders in the foams were visualized in 3D using the local tomography technique which is a high-resolution CT method to reconstruct a region of interest within a large sample. The shapes and the 3D distribution of micropores, particles, and regions of solute segregation in the foams are evaluated, comparing the cell walls with the Plateau borders. Under compressive loads, the damage behaviour of such microstructures has been observed using an in situ test rig. It is found that the microcracks were mainly initiated from the cell walls and the micropores with large diameters were also damaged. The crack initiation sites are classified from the results. In addition, a method for non-destructive characterization of elastic and plastic deformation in the foams, which is called a 3D microstructure gauge (MG) method, is presented. Thousands of micropores as markers on each load were automatically matched by the information of those volumes and surface areas. The local strain mapping by the MG indicates that the edges of the micropores with large diameters have large strain under compression and this is consistent with the crack analyses. [ABSTRACT FROM AUTHOR]

The preferential penetration of a liquid metal along grain boundaries (GBs) in polycrystalline metals is a well-known phenomenon. Gallium-decorated GB networks in rolled aluminium alloys have been visualized three-dimensionally using the high-resolution synchrotron radiation computer tomography (SRCT). The distribution of gallium concentration along GBs was measured using X-ray absorption. Statistical correction for blurring was performed to raise accuracy of the measurement, and then compared with orientation mapping by the SEM/EBSP method on the surface of a tomographic specimen. The pancake-like grain microstructure formed by a rolling process causes the anisotropy of penetration direction. Although the gallium penetrated into high-angle GBs, all of the high-angle GBs are not necessarily decorated by the gallium. The reason for this may be explained by considering geometrically possible penetration paths that seem to be dependent on local grain arrangement and GB structure through each path. The dependence of the gallium concentration on the rotation axis of misorientation has been found along the high-angle GBs. Especially, GBs with a specific misorientation (<221> as a misorientation axis) showed high gallium concentration. [ABSTRACT FROM AUTHOR]