Dynamic softening behavior of Al−18Si alloy produced by spray forming

The high-temperature deformation behavior and dynamic softening transition from dynamic recrystallization (DRX) to dynamic recovery (DRV) of Al-18 wt.%Si alloy was studied by torsion tests in a temperature range of 573∼773 K and a strain rate range of 0.001∼1/sec. The alloy was produced by spray-forming followed by hot extrusion. The dependence of flow stress (σ) on strain rate and temperature (T) could be described by the power Arrhenius relationship,. The activation energy and stress exponent values were higher at the low temperature region than those at the high temperature region. The alloy showed a flow curve of DRX in the temperature range of 573∼673 K, while at 673∼773 K, the alloy exhibited a flow curve of DRV. These results were analyzed by observing the shapes of the flow curves throughout the deformation condition. Also, the transition behavior from DRX to DRV has been investigated through observation of deformed microstructure and flow curves during hot deformation.     

Stabilized conforming nodal integration in the natural‐element method

A stabilized conforming nodal integration scheme is implemented in the natural neighbour method in conjunction with non‐Sibsonian interpolation. In this approach, both the shape functions and the integration scheme are defined through use of first‐order Voronoi diagrams. The method illustrates improved performance and significant advantages over previous natural neighbour formulations. The method also shows substantial promise for problems with large deformations and for the computation of higher‐order gradients. Copyright © 2004 John Wiley & Sons, Ltd.     

Crack and cavity nucleation at interfaces during creep

Athermal nucleation of microcracks and thermal nucleation of cavities during creep deformation are reviewed with an emphasis on effects of solute segregation to grain boundaries and cavity surfaces. The magnitude and the duration of stress concentration at a triple grain junction or at a grain boundary inclusion are estimated for transient Coble creep and steady state power-law creep conditions. Stable configurations of wedge-type microcracks are predicted by a Griffith-like crack model. The rate for thermal nucleation of cavities is obtained by the Fokker-Planck equation for vacancy clusters. Cracks and cavities are interdependent, and cavity nucleation occurs continuously throughout the three creep stages. The local stress concentration enhances microcrack and cavity nucleation. The cavity nucleation rate is generally increased as a result of solute segregation to the surfaces and interfaces and/or gas precipitation into cavity volume. This enhanced nucleation is more effective in a system with mobile solutes than with immobile solutes. Immobile solute or trace elements may affect the nucleation rate also by changing the grain boundary diffusivity. Experimental techniques for quantitative analyses of cavity nucleation processes are discussed. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Application of multiway ICA for on-line process monitoring of a sequencing batch reactor

Multiway principal component analysis has been shown to be a powerful monitoring tool in many industrial batch processes. However, it has the shortcomings that all batch lengths should be equal, the measurement variables must be normally distributed and that future values of the current batch must be estimated to allow on-line monitoring. In this work, it is shown that multiway independent component analysis (MICA) can be used to overcome these drawbacks and obtain better monitoring performance. The on-line MICA monitoring of batch processes is based on a new unfolding method and independent component analysis (ICA). ICA provides better monitoring performance than PCA in cases with non-Gaussian data because it is not based on the assumption that the latent variables are normally distributed. The MICA algorithm does not require any estimation of future batch values and can also be applied to non-equal batch length data sets. This article describes the application of on-line MICA monitoring of a sequencing batch reactor (SBR). It is successfully applied to an 80L SBR for biological wastewater treatment, which is characterized by a variety of disturbance sources with non-Gaussian characteristics. The SBR poses an interesting challenge from the point of process monitoring characterized by non-stationary, batchwise, multiscale, and non-Gaussian characteristics. The results of the bench-scale SBR monitoring clearly showed the power and advantages of MICA monitoring in comparison to conventional monitoring methods.     

Uncertainty analysis of absolute concentration measurement with continuous-wave cavity ringdown spectroscopy

To evaluate the uncertainty of concentration measurement using cavity ringdown spectroscopy, we analytically derived expressions for uncertainty for parameters, such as temperature, laser frequency, and ringdown time deviation, from the model equation. The uncertainties that are due to systematic errors in a practical cavity ringdown system were assessed through an experimental study of the PQ(35) transition in an A band of molecular oxygen. We found that, except for the line strength that is regarded as a reference value independent of the measurement, the laser frequency jitter is the largest uncertainty source in the system. Some practical requirements for minimizing the uncertainty in concentration measurements are discussed. We also demonstrated determination of the line strength of the PQ(35) transition line of oxygen to be 8.63(3) x 10(-27) cm(-1) with a relative uncertainty of less than 0.4%.     

Pressure effects on high temperature steam oxidation of zircaloy-4

The pressure effects on the oxidation of Zircaloy-4 (Zry-4) cladding in high temperature steam have been analyzed. A double layer autoclave was made for the high pressure/high temperature oxidation tests. The experimental test temperature range was 700–900°C, and pressures were 0.1–1.5 MPa. Steam partial pressure was found to be a more important factor than total pressure. Steam pressure enhances the oxidation rate of Zry-4 exponentially and depends on the temperature, the maximum being between 750–800°C. A preexisting oxide layer decreases the enhancement by about 40–60%. The acceleration of the oxidation rate caused by high pressure steam seems to originate from the formation of cracks due to an abrupt transformation of the tetragonal phase in the oxide, where the instability of the tetragonal phase comes from the reduction of surface energy due to steam.     

ITO/ATO/TiO2 triple-layered transparent conducting substrates for dye-sensitized solar cells

A novel transparent conductive oxide film based on the triple-layered indium tin oxide (ITO)/antimony-doped tin oxide (ATO)/titanium oxide (TiO₂) has been developed for dye-sensitized solar cells by using radio frequency magnetron sputtering technique. Effects of the absence and presence of TiO₂ layer and the ITO layer thickness were investigated. Deposition of ATO layer was found to stabilize the thermal instability of ITO. Little change in sheet resistance and optical transmittance was observed by introduction of insulating thin TiO₂ layer on top of the ATO layer, whereas photovoltaic performance was significantly influenced. The conversion efficiency was improved from 4.57% without TiO₂ layer to 6.29% with TiO₂ layer. The enhanced photovoltaic performance with addition of TiO₂ layer was attributed mainly to the improved adhesion and partially to the reduced electron loss at the ITO/ATO conductive layer. Increase in the ITO layer thickness resulted in a slight decrease in photocurrent due to the reduced optical transmittance. When compared with the conventional fluorine-doped tin oxide (FTO), the ITO/ATO/TiO₂ conductive material exhibited similar photocurrent density but higher photovoltage and fill factor, resulting in better conversion efficiency.