Technique for producing highly planar Si/SiO0.64Ge0.36/Si metal–oxide–semiconductor field effect transistor channels

Si/Si0.64Ge0.36/Si heterostructures have been grown at low temperature (450 °C) to avoid the strain-induced roughening observed for growth temperatures of 550 °C and above. The electrical properties of these structures are poor, and thought to be associated with grown-in point defects as indicated in positron annihilation spectroscopy. However, after an in situ annealing procedure (800 °C for 30 min) the electrical properties dramatically improve, giving an optimum 4 K mobility of 2500 cm2 V – 1 s – 1 for a sheet density of 6.2 × 1011 cm – 2. The low temperature growth yields highly planar interfaces, which are maintained after anneal as evidenced from transmission electron microscopy. This and secondary ion mass spectroscopy measurements demonstrate that the metastably strained alloy layer can endure the in situ anneal procedure necessary for enhanced electrical properties. Further studies have shown that the layers can also withstand a 120 min thermal oxidation at 800 °C, commensurate with metal–oxide–semiconductor device fabrication

Qualitative Theory and Chemical Explanation

Roald Hoffmann and other theorists claim that we we ought to use highly idealized chemical models (``qualitative models'') in order to increase our understanding of chemical phenomena, even though other models are available which make more highly accurate predictions. I assess this norm by examining one of the tradeoffs faced by model builders and model users --- the tradeoff between precision and generality. After arguing that this tradeoff obtains in many cases, I discuss how the existence of this tradeoff can help us defend Hoffmann's norm for modelling

Online learning support in constructivist environments

Academic learning can essentially be seen as a constructivist process. This of course, is an alternative approach to traditional university teaching, insofar as learning is facilitated rather than taught. In consequence, learning in online constructivist environments has to be supported by appropriate resources and requires a number of specific skills from both tutor and learners, that is, online tutor skills, online learning skills and specifically designed online learning facilities

Statement Submitted to Environmental Protection Agency for the Public Meeting, February 22, 1978, San Francisco Regarding Modification of Secondary Treatment Requirements

The principal technical reason for having a waiver provision for secondary treatment for municipal discharges is that for some outfall systems the dilution is so high that very good ambient water quality can be achieved with less than secondary treatment. Therefore, the criteria for a waiver of the secondary treatment requirement must give full consideration to the dilution obtained by the outfall system. In a high performance outfall diffuser, such as those used by major dischargers in California and Hawaii, initial dilutions are typically 100:1, and may range up to 1000:1 in very favorable circumstances

Statement to Water Resources Subcommittee of House Committee on Public Works and Transportation for a Hearing, May 24 and 25, 1978, Washington, D.C. regarding Secondary Treatment Waivers and Ocean Outfalls

The principal technical reason for having a waiver provision for secondary treatment for municipal discharges is that for some outfall systems the dilution is so high that very good ambient water quality can be achieved with less than secondary treatment. Therefore, the criteria for a waiver of the secondary treatment requirement must give full consideration to the dilution obtained by the outfall system. In a high performance outfall diffuser, such as those used by major dischargers in California and Hawaii, initial dilutions are typically 100:1, and may range up to 1000:1 in very favorable circumstances

A genetic algorithm-assisted semi-adaptive MMSE multi-user detection for MC-CDMA mobile communication systems

In this work, a novel Minimum-Mean Squared-Error (MMSE) multi-user detector is proposed for MC-CDMA transmission systems working over mobile radio channels characterized by time-varying multipath fading. The proposed MUD algorithm is based on a Genetic Algorithm (GA)-assisted per-carrier MMSE criterion. The GA block works in two successive steps: a training-aided step aimed at computing the optimal receiver weights using a very short training sequence, and a decision-directed step aimed at dynamically updating the weights vector during a channel coherence period. Numerical results evidenced BER performances almost coincident with ones yielded by ideal MMSE-MUD based on the perfect knowledge of channel impulse response. The proposed GA-assisted MMSE-MUD clearly outperforms state-of-the-art adaptive MMSE receivers based on deterministic gradient algorithms, especially for high number of transmitting users

Risk Assessment of a Tunnelling Process Using Machinery Failure Mode and Effects Analysis (MFMEA)

Abstract In recent years, risk management associated with safety and reliability of the process especially in oil and gas industry has been widely used. For this purpose, different methods of risk analysis have been developed and successfully applied. Greater levels of complexity in tunnelling using TBM (Tunnel Boring Machine) especially in gassy tunnels with a large volume of water coming out of them, allow higher chances of failure that may increase the potential for tunnelling facilities to become more hazardous. When there is an ever increasing awareness of hazardous risks that need to be managed by the industrial community, the risks need to be analyzed. This paper presents the results of a study on risk management in a tunnel excavation with TBM. MFMEA was applied to analyze the risks of a tunnelling process. In order to apply MFMEA, 7 main systems and components involved in a tunnelling process were selected and split into subsystems. In total, 71 failure modes were then postulated for all subsystems. In the next step, the effects of every failure of each subsystem were listed. Safeguards or controls that might prevent or mitigate the effects of each failure were then listed. In the final step, essential 1 remedial actions to prevent or mitigate the failure were recommended. Risk Matrix was developed for each possible failure to be used for risk ranking. For this purpose the Risk Priority Number (RPN) was estimated for each failure mode to identify the most critical failures. The results revealed that, the failure of the ventilation system (RPN=480) is the most critical failure. The TBM failure due to bad rock condition (RPN=240) and rolling stock failure due to unleveled rails (RPN= 200) are the next significant critical failures. The findings from this study were applied to a long tunnel under construction and significantly reduced the accidents during the tunnelling period. Tracking of the accidents occurred during the next 2 years showed that MFMEA is a perfect method for risk management in tunnelling process as well. Key Words: Hazards Identification; Risk Analysis; Risk Management, Tunnelling

Spectrophotovoltaic orbital power generation

A subscale model of a photovoltaic power system employing spectral splitting and 1000:1 concentration was fabricated and tested. The 10-in. aperture model demonstrated 15.5% efficiency with 86% of the energy produced by a GaAs solar cell and 14% of the energy produced by an Si cell. The calculated efficiency of the system using the same solar cells, but having perfect optics, would be approximately 20%. The model design, component measurements, test results, and mathematical model are presented

Spectrophotovoltaic orbital power generation

A system with 1000 : 1 concentration ratio is defined, using a cassegrain telescope as the first stage concentration (270 x) and compound parabolic concentrators (CPC) for the second stage concentration of 4.7 x for each spectral band. Using reported state of the art (S.O.A.) solar cells device parameters and considering structural losses due to optics and beamsplitters, the efficiencies of one to four cell systems were calculated with efficiencies varying from approximately 22% to 30%. Taking into account cost of the optics, beamsplitter, radiator, and the cost of developing new cells the most cost effective system is the GaAs/Si system