Nanocrystalline Si-Based Resistive Humidity Sensors Prepared via HWCVD at Various Filament Temperatures
The growth of nanocrystalline Si on glass substrates via hot-wire chemical vapor deposition (HWCVD) at various filament temperatures and the fabrication of nanocrystalline Si-based resistive humidity sensors are reported. Experimental results indicate that the average diameter of nanocrystalline Si particles increases with increasing filament temperature. Measurements of the resistivity change of samples with humidity at 25°C show that the sensor response of samples fabricated at high filament temperatures is lower than that of samples fabricated at low filament temperatures. The sensitivity of the sensor increases with relative humidity.
Tailoring of the Gas Sensing Performance of TiO2 Nanotubes by 1-D Vertical Electron Transport Technique
Aligned TiO2 nanotubes (NTs), having diameter of 75 ± 10 nm and length of 280 ± 20 nm were synthesized by electrochemical anodization technique. Two types of device configurations, viz., planar/lateral and metal-insulator-metal (MIM)/vertical sensor structures were fabricated with TiO2 NTs as the sensing layer. Vertical 1-D electron transport technique of MIM configurations was found to be very effective over the random (across the connected tubes) electron transport kinetics of planar devices, in determining the vapor (ethanol) sensor characteristics like response time and operating temperature. The optimum operating temperature of ethanol sensing was found to be much lower (75 °C) in case of MIM compared with its planar counterpart (150 °C). In addition, MIM configuration offered four and six times faster response and recovery times, respectively, compared with the planar ones toward 100-ppm ethanol at 75 °C. Sensing performance of the device configuration was correlated with the corresponding grain boundary model and carrier transport time with the help of 1-D electron transport path constituted of TiO2 NT array.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME