Showing papers by "Missouri University of Science and Technology published in 2006"

Toxicity of Cerium Oxide Nanoparticles in Human Lung Cancer Cells

Abstract: With the fast development of nanotechnology, the nanomaterials start to cause people's attention for potential toxic effect. In this paper, the cytotoxicity and oxidative stress caused by 20-nm cerium oxide (CeO2) nanoparticles in cultured human lung cancer cells was investigated. The sulforhodamine B method was employed to assess cell viability after exposure to 3.5, 10.5, and 23.3 microg/ml of CeO2 nanoparticles for 24, 48, and 72 h. Cell viability decreased significantly as a function of nanoparticle dose and exposure time. Indicators of oxidative stress and cytotoxicity, including total reactive oxygen species, glutathione, malondialdehyde, alpha-tocopherol, and lactate dehydrogenase, were quantitatively assessed. It is concluded from the results that free radicals generated by exposure to 3.5 to 23.3 microg/ml CeO2 nanoparticles produce significant oxidative stress in the cells, as reflected by reduced glutathione and alpha-tocopherol levels; the toxic effects of CeO2 nanoparticles are dose dependent and time dependent; elevated oxidative stress increases the production of malondialdehyde and lactate dehydrogenase, which are indicators of lipid peroxidation and cell membrane damage, respectively.

Kinetics and mechanisms of the conversion of silicate (45S5), borate, and borosilicate glasses to hydroxyapatite in dilute phosphate solutions.

Abstract: Bioactive glasses with controllable conversion rates to hydroxyapatite (HA) may provide a novel class of scaffold materials for bone tissue engineering. The objective of the present work was to comprehensively characterize the conversion of a silicate bioactive glass (45S5), a borate glass, and two intermediate borosilicate glass compositions to HA in a dilute phosphate solution at 37 degrees Celsius. The borate glass and the borosilicate glasses were derived from the 45S5 glass by fully or partially replacing the SiO(2) with B(2)O(3). Higher B(2)O(3) content produced a more rapid conversion of the glass to HA and a lower pH value of the phosphate solution. Whereas the borate glass was fully converted to HA in less than 4 days, the silicate (45S5) and borosilicate compositions were only partially converted even after 70 days, and contained residual SiO(2) in a Na-depleted core. The concentration of Na(+) in the phosphate solution increased with reaction time whereas the PO(4) (3-) concentration decreased, both reaching final limiting values at a rate that increased with the B(2)O(3) content of the glass. However, the Ca(2+) concentration in the solution remained low, below the detection limit of atomic absorption, throughout the reaction. Immersion of the glasses in a mixed solution of K(2)HPO(4) and K(2)CO(3) produced a carbonate-substituted HA but the presence of the K(2)CO(3) had little effect on the kinetics of conversion to HA. The kinetics and mechanisms of the conversion process of the four glasses to HA are compared and used to develop a model for the process.

Neural Network Control of Nonlinear Discrete-Time Systems

Abstract: BACKGROUND ON NEURAL NETWORKS NN Topologies and Recall Properties of NN NN Weight Selection and Training NN Learning and Control Architectures References Problems BACKGROUND AND DISCRETE-TIME ADAPTIVE CONTROL Dynamical Systems Mathematical Background Properties of Dynamical Systems Nonlinear Stability Analysis and Controls Design Robust Implicit STR References Problems Appendix 2.A NEURAL NETWORK CONTROL OF NONLINEAR SYSTEMS AND FEEDBACK LINEARIZATION NN Control with Discrete-Time Tuning Feedback Linearization NN Feedback Linearization Multilayer NN for Feedback Linearization Passivity Properties of the NN Conclusions References Problems NEURAL NETWORK CONTROL OF UNCERTAIN NONLINEAR DISCRETE-TIME SYSTEMS WITH ACTUATOR NONLINEARITIES Background on Actuator Nonlinearities Reinforcement NN Learning Control with Saturation Uncertain Nonlinear System with Unknown Deadzone and Saturation Nonlinearities Adaptive NN Control of Nonlinear System with Unknown Backlash Conclusions References Problems Appendix 4.A Appendix 4.B Appendix 4.C Appendix 4.D OUTPUT FEEDBACK CONTROL OF STRICT FEEDBACK NONLINEAR MIMO DISCRETE-TIME SYSTEMS Class of Nonlinear Discrete-Time Systems Output Feedback Controller Design Weight Updates for Guaranteed Performance Conclusions References Problems Appendix 5.A Appendix 5.B NEURAL NETWORK CONTROL OF NONSTRICT FEEDBACK NONLINEAR SYSTEMS Introduction Adaptive NN Control Design Using State Measurements Output Feedback NN Controller Design Conclusions References Problems Appendix 6.A Appendix 6.B SYSTEM IDENTIFICATION USING DISCRETE-TIME NEURAL NETWORKS Identification of Nonlinear Dynamical Systems Identifier Dynamics for MIMO Systems NN Identifier Design Passivity Properties of the NN Conclusions References Problems DISCRETE-TIME MODEL REFERENCE ADAPTIVE CONTROL Dynamics of an mnth-Order Multi-Input and Multi-Output System NN Controller Design Projection Algorithm Conclusions References Problems NEURAL NETWORK CONTROL IN DISCRETE-TIME USING HAMILTON-JACOBI-BELLMAN FORMULATION Optimal Control and Generalized HJB Equation in Discrete-Time NN Least-Squares Approach Numerical Examples Conclusions References Problems NEURAL NETWORK OUTPUT FEEDBACK CONTROLLER DESIGN AND EMBEDDED HARDWARE IMPLEMENTATION Embedded Hardware-PC Real-Time Digital Control System SI Engine Test Bed Lean Engine Controller Design and Implementation EGR Engine Controller Design and Implementation Conclusions References Problems Appendix 10.A Appendix 10.B INDEX

Real-Time Implementation of a STATCOM on a Wind Farm Equipped with Doubly Fed Induction Generators

Abstract: Voltage stability is a key issue to achieve the uninterrupted operation of wind farms equipped with doubly fed induction generators (DFIGs) during grid faults. This paper investigates the application of a static synchronous compensator (STATCOM) to assist with the uninterrupted operation of a wind turbine driving a DFIG, which is connected to a power network, during grid faults. The control schemes of the DFIG rotor- and grid-side converters and the STATCOM are suitably designed and coordinated. The system is implemented in real-time on a real time digital simulator. Results show that the STATCOM improves the transient voltage stability and therefore helps the wind turbine generator system to remain in service during grid faults.

A Comparison of Diode-Clamped and Cascaded Multilevel Converters for a STATCOM With Energy Storage

Abstract: The progression of distributed generation within a bulk power system will lead to the need for greater control of transmission-line power flows. Static synchronous compensators (STATCOMs) provide a power-electronics-based means of embedded control of transmission-line voltage and power flows. The integration of energy storage with a STATCOM can extend traditional STATCOM capabilities to four-quadrant power flow control and transient stability improvement. This paper discusses energy storage systems (ESSs) integrated with conventional and multilevel bidirectional power converters for a hybrid STATCOM/ESS. Conventional, diode-clamped, and cascaded multilevel converter-based STATCOM/ESSs are developed, and their performances for a variety of power system applications are compared using battery energy storage. The advantages and disadvantages of each topology are presented. Both simulation and experimental results are provided to validate the conclusions

Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Abstract: Zirconium diboride (ZrB 2 ) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B 2 O 3 and ZrO 2 ) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B 4 C were evaluated as additives to facilitate the removal of ZrO 2 Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB 2 (surface area ∼ 1 m 2 /g) or attrition-milled ZrB 2 (surface area ∼ 7.5 m 2 /g) with WC and/or B 4 C as a sintering aid. ZrB 2 containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B 4 C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.

Pressureless Sintering of Zirconium Diboride

Abstract: Zirconium diboride (ZrB 2 ) ceramics were sintered to a relative density of ∼ 98% without applied external pressure. Densification studies were performed in the temperature range of 1900 -2150 C. Examination of bulk density as a function of temperature revealed that shrinkage started at ∼2100°C, with significant densification occurring at only 2150°C. At 2150°C, isothermal holds were used to determine the effect of time on relative density and microstructure. For a hold time of 540 min at 2150 C, ZrB 2 pellets reached an average density of 6.02±0.04 g/cm 3 (98% of theoretical) with an average grain size of 9.0 ±5.6 μm. Four-point bend strength, elastic modulus, and Vickers' hardness were measured for sintered ZrB 2 and compared with values reported for hot-pressed materials. Vickers' hardness of sintered ZrB 2 was 14.5±2.6 GPa, which was significantly lower when compared with 23 GPa for hot-pressed ZrB 2 . Strength and elastic modulus of the ZrB 2 were 444±30 MPa and 454 GPa, which were comparable with values reported for hot-pressed ZrB 2 . The ability to densify ZrB 2 ceramics without hot pressing should enable near-net shape processing, which would significantly reduce the cost of fabricating ZrB 2 components compared with conventional hot pressing and machining.

Extended operation of flying capacitor multilevel inverters

Abstract: Recent research in flying capacitor multilevel inverters (FCMIs) has shown that the number of voltage levels can be extended by changing the ratio of the capacitor voltages. For the three-cell FCMI, four levels of operation are expected if the traditional ratio of the capacitor voltages is 1:2:3. However, by altering the ratio, the inverter can operate as a five-, six-, seven-, or eight-level inverter. According to previous research, the eight-level case is referred to as maximally distended (or full binary combination schema) since it utilizes all possible transistor switching states. However, this case does not have enough per-phase redundancy to ensure capacitor voltage balancing under all modes of operation. In this paper, redundancy involving all phases is used along with per-phase redundancy to improve capacitor voltage balancing. It is shown that the four- and five-level cases are suitable for motor drive operation and can maintain capacitor voltage balance under a wide range of power factors and modulation indices. The six-, seven-, and eight-level cases are suitable for reactive power transfer in applications such as static var compensation. Simulation and laboratory measurements verify the proposed joint-phase redundancy control.

Base-Level Buffers and Buttresses: A Model for Upstream Versus Downstream Control on Fluvial Geometry and Architecture Within Sequences

Abstract: The effects of downstream base-level control on fluvial architecture and geometry are well explored in several broadly similar sequence-stratigraphic models. Cretaceous Dakota Group strata, U.S. Western Interior, have characteristics reflecting combined downstream and upstream base-level controls that these models cannot address. Particularly, three layers of amalgamated channel-belt sandstone within this group thicken and are continuous for distances (≤ 300 km) along dip that stretch the reasonable lengths for which these models are intended to apply. As well, architecture in up-dip reaches records repeated valley-scale cut-and-fill cycles. This contrasts with equivalent strata down dip which record channel-scale lateral migration with no such valley-scale cycles apparent. We here introduce the concept of "buffers and buttresses" to address these observations. We assume that river longitudinal profiles are each anchored down dip to some physical barrier (e.g., the sea strand, etc.) that we refer to as a "buttress." Buttress shift is considered the primary downstream control on base level. Profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high-frequency variability in upstream base-level controls (e.g., discharge, etc). All these potential profiles however are bounded above by the profile of highest possible aggradation, and below by the profile of maximum possible incision. These upper and lower profiles are "buffers," and they envelop the available fluvial preservation space. Thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance. Dakota valley-scale surfaces record repeated cut-and-fill cycles driven by up-dip controls and are confined between thick stable buffers. Equivalent strata down dip record lateral reworking within a thinner channel-scale buffer zone that was positioned by downstream controls. Regression exposed slopes similar to the buffer zone, thus buffers were stable for long distances and durations. This prompted dip-extensive lateral reworking of strata into upstream valley-scale and downstream channel-scale sheets. Buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs. downstream base-level controls on geometry and architecture. The model lends general insights into dip-oriented variations in fluvial architecture, production of sheet vs. lens geometry, total preservation volumes for fluvial systems, and variations in these factors related to contrasting climatic conditions and basin physiography. The model can be amended to existing sequence stratigraphic approaches in order to capture dip-oriented variations in sequence architecture.

Effect of friction stir processing on the microstructure of cast A356 aluminum

Abstract: Friction stir processing (FSP) was applied to cast A356 Al to modify the as-cast microstructure. FSP homogenizes and. refines the cast microstruclure, completely eliminates porosity, and creates a microstructure with fine Si particles (0.25-0.42 mu m) distributed in a fine grain aluminum matrix (3-4 mu m). Further, FSP parameters significantly influence microstructural development in the processed zone of cast A356 Al. Generally, higher tool rotation rate creates a more homogeneous microstructure. At lower tool rotation rates, a macroscopically visible banded structure characterized by a low density of coarse particles, was detected in the nugget zone. However, FSP parameters did not significantly influence the Al matrix grain size. The varied distribution pattern, size, and volume fraction of Si particles at different locations within the FSP zone indicates inhomogeneous material flow. A post-FSP T6 heat treatment did not alter the Si particle distribution, but did significantly coarsen the Si particles. (c) 2006 Elsevier B.V. All rights reserved.

Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing

Abstract: Friction stir processing (FSP) has been applied to cast aluminum alloy A356 plates to enhance the mechanical properties through microstructural refinement and homogenization. The effect of tool geometry and FSP parameters on resultant microstructure and mechanical properties was investigated. The FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix with significant microstructural refinement. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Higher tool rotation rate was the most effective parameter to refine coarse Si particles, heal the casting porosity, and consequently increase strength. The effect of tool geometry was complicated and no systematic trend was observed. For a standard pin design, maximum strength was achieved at a tool rotation rate of 900 rpm and traverse speed of 203 mm/min. Post-FSP aging increased strength for materials processed at higher tool rotation rates of 700 to 1100 rpm, but exerted only a marginal effect on samples prepared at the lower rotation rate of 300 rpm. Two-pass FSP with 100 pct overlapping passes resulted in higher strength for both as-FSP and post-FSP aged conditions.

College drinking behaviors: mediational links between parenting styles, impulse control, and alcohol-related outcomes.

Abstract: Mediational links between parenting styles (authoritative, authoritarian, permissive), impulsiveness (general control), drinking control (specific control), and alcohol use and abuse were tested. A pattern-mixture approach (for modeling non-ignorable missing data) with multiple-group structural equation models with 421 (206 female, 215 male) college students was used. Gender was examined as a potential moderator of parenting styles on control processes related to drinking. Specifically, the parent-child gender match was found to have implications for increased levels of impulsiveness (a significant mediator of parenting effects on drinking control). These findings suggest that a parent with a permissive parenting style who is the same gender as the respondent can directly influence control processes and indirectly influence alcohol use and abuse.

Integrated Guidance and Control of Missiles With $theta hbox - D$ Method

Abstract: A new suboptimal control method is proposed in this study to effectively design an integrated guidance and control system for missiles. Optimal formulations allow designers to bring together concerns about guidance law performance and autopilot responses under one unified framework. They lead to a natural integration of these different functions. By modifying the appropriate cost functions, different responses, control saturations (autopilot related), miss distance (guidance related), etc., which are of primary concern to a missile system designer, can be easily studied. A new suboptimal control method, called the thetas-D method, is employed to obtain an approximate closed-form solution to this nonlinear guidance problem based on approximations to the Hamilton-Jacobi-Bellman equation. Missile guidance law and autopilot design are formulated into a single unified state space framework. The cost function is chosen to reflect both guidance and control concerns. The ultimate control input is the missile fin deflections. A nonlinear six-degree-of-freedom (6-DOF) missile simulation is used to demonstrate the potential of this new integrated guidance and control approach

Random lasing in weakly scattering systems

Abstract: We present detailed experimental and numerical studies of random lasing in weakly scattering systems. The interference of scattered light, which is weak in the passive systems, is greatly enhanced in the presence of high gain, providing coherent and resonant feedback for lasing. The lasing modes are confined in the vicinity of the pumped volume due to absorption of emitted light outside it. In the ballistic regime where the size of the gain volume is less than the scattering mean free path, lasing oscillation occurs along the direction in which the gain volume is most extended, producing directional laser output. The feedback for lasing originates mainly from backscattering of particles near the boundaries of the pumped region. It results in nearly constant frequency spacing of lasing modes, which scales inversely with the maximum dimension of the gain volume.

Oxidation of Zirconium Diboride–Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen

Abstract: The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO2 was used to produce an oxygen partial pressure of ∼10−10 Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO2 produced a non-protective oxide surface scale.

Electrochemical Durability of Carbon Nanotubes in Noncatalyzed and Catalyzed Oxidations

Abstract: Noncatalyzed and catalyzed electrochemical oxidations of multiwalled carbon nanotubes (CNTs) were studied with the aim to understand their durability as catalyst support in polymer electrolyte membrane (PEM) fuel cells. Bare and Pt-deposited CNTs were investigated in 1.0 M sulfuric acid at a constant potential of 1.2 V. Carbon black (CB, Vulcan XC-72R) was also studied under the same experimental conditions. The carbons were oxidized at room temperature in the form of thin-film electrodes with time durations up to 48 h. Cyclic voltammetry was used to monitor the surface oxide redox reactions as a way to quantify the degree of surface oxidation on the carbons. It was found that the redox current peaks were stabilized after 8 h for CNTs, but they continue to increase for CB, showing that CNTs are more resistant to electrochemical oxidation. Similar trends were observed in the catalyzed oxidation with Pt-deposited carbons. However, much larger currents were observed, demonstrating that catalyzed oxidation had indeed occurred. The observed durability demonstrated that CNTs would be a better catalyst support in PEM fuel cells in which the commonly used CB often undergoes severe electrochemical corrosion.

Using Social Development Lenses to Understand E-Government Development

Abstract: As governments at different levels and all around the world are increasingly using the Web to enhance and improve their services, understanding e-government development and exploring factors that affect e-government development have become important research topics. The purpose of this research is to investigate factors explaining e-government development in terms of social development lenses. Based on growth and regional development theories, the paper hypothesizes that income level, development status, and region are three factors that differentiate e-government development in countries. Group comparison tests are conducted using secondary data from the United Nations and the United Nations Development Programme. The results support the hypotheses that significant differences in e-government development exist between countries with respect to the three categorical variables mentioned above. In addition, the paper applies planned post-hoc tests to further investigate the differences. The results of this research are valuable to e-government scholars and practitioners. As the research involves data from more than a hundred countries, the research contributes to understanding e-government development factors on a global scale.

Bio-inspired Algorithms for the Design of Multiple Optimal Power System Stabilizers: SPPSO and BFA

Abstract: Damping intra-area and interarea oscillations are critical to optimal power flow and stability in a power system. Power system stabilizers (PSSs) are effective damping devices, as they provide auxiliary control signals to the excitation systems of generators. The proper selection of PSS parameters to accommodate variations in the power system dynamics is important and is a challenging task particularly when several PSSs are involved. Two classical bio-inspired algorithms, which are small-population-based particle swarm optimization (SPPSO) and bacterial foraging algorithm (BFA), are presented in this paper for the simultaneous design of multiple optimal PSSs in two power systems. A classical PSO with a small population of particles is called SPPSO in this paper. The SPPSO uses the regeneration concept, introduced in this paper, to attain the same performance as a PSO algorithm with a large population. Both algorithms use time domain information to obtain the objective function for the determination of the optimal parameters of the PSSs. The effectiveness of the two algorithms is evaluated and compared for damping the system oscillations during small and large disturbances, and their robustness is illustrated using the transient energy analysis. In addition, the computational complexities of the two algorithms are also presented.