Showing papers by "Clemson University published in 1998"

Phase transformations during cooling in α+β titanium alloys

Abstract: A simplified methodology for investigating the effects of cooling rate from elevated temperature on phase transformations observed in α+β titanium alloys is described. It involves adaptation and refinement of a circumferentially insulated and instrumented Jominy end quench bar, time–temperature profiles obtained during cooling at locations along the bar length providing a complete thermal history. The ability of this procedure to examine the phase transformation for α+β titanium alloys has been demonstrated in Ti–6Al–4V where varying cooling rates from 525 to 1.5°C s−1 are shown to result in a series of martensitic, massive and diffusional phase transformations. Cooling rates above 410°C s−1 result in a fully martensitic microstructure, a massive transformation being observed between 410 and 20°C s−1, this transformation being gradually replaced by diffusion controlled Widmanstatten α formation with decreasing cooling rate.

The Role of Relationship Quality in the Stratification of Vendors as Perceived by Customers

Abstract: Companies implement preferred supplier programs to reduce their vendor relationships to a reasonable few. Consequently, vendors who do not effectively manage their customer-based relationships are strong candidates for deletion from a customer’s list of long-term suppliers. The emergence of preferred supplier programs suggests that businesses are beginning to formally recognize and reward differences between their qualified vendors. Vendor stratification is proposed as a framework for understanding the evolution of preferred vendor programs. With the growing interest in relationship marketing, a study was conducted to empirically examine the extent to which businesses use relationship quality perceptions to differentiate their qualified vendors. The findings support the notion that relationship quality is a higher-order construct that can be used as a basis for developing vendor stratification systems. The article concludes with a discussion of the managerial and research implications of the study findings.

A Composite from Poly(m‐phenylenevinylene‐co‐2,5‐dioctoxy‐p‐phenylenevinylene) and Carbon Nanotubes: A Novel Material for Molecular Optoelectronics

Abstract: As research progresses towards smaller and more efficient devices, the need to develop alternative molecular scale electronic materials becomes apparent. Integrated electronic component fabrication from organics has been recognized theoretically as the ultimate goal. In order to gain a comprehensive insight into these materials, extensive research has been carried out on conjugated carbon systems over the last few decades to optimize their optical and electrical properties. For example, doping polyacetylene with I2 has been shown to result in a large increase in conductivity compared to the pristine material. However, doping polymers tends to retard their optical properties as regards luminescence by reducing their bandgaps and introducing trapping sites such as solitons, polarons, or bipolarons. The simple lesson over the years is that if materials are to be considered for luminescence, doping should not be carried out despite the desire to improve charge transport properties. We report here the first physical adopingo, to use the traditional term, using small concentrations of multiwalled nanotubes in a conjugated luminescent polymer, poly(m-phenylenevinylene-co-2,5-dioctoxyp-phenylenevinylene) (PmPV), in a polymer/nanotube composite. This can increase electrical conductivity of the polymer by up to eight orders of magnitude. The nanotubes appear to act as nanometric heat sinks, preventing the buildup of large thermal effects, caused either optically (photobleaching) or electrically, which degrade these conjugated systems. We also report that electroluminescence was achieved from an organic light-emitting diode (LED) using the composite as the emissive layer in the device. Since initial work on conjugated systems, attempts have been made to find an area where polymers and/or fullerenes could be used as active semiconductor components. Although many new and interesting materials have been synthesized to this end, very few have found a practical application. One exception is polyphenylenevinylene (PPV), first reported by Burroughes et al. as being the light-emitting semiconductor in a Schottky diode. This encouraged scientists to study a wide variety of conjugated systems, including derivatives of this polymer, in order to optimize the efficiency of light emission from such devices. Polymers for use in LEDs must possess a number of important qualities. A high quantum yield of photoluminescence is necessary and the material must remain undoped, as dopants act as trapping sites, quenching the radiative decay of excitons. It is essential therefore to find a polymer that is reasonably conductive while maintaining its luminescent properties. Most undoped polymers possess a very low conductivity and so require high aturn-ono fields to generate sufficient carriers in order to produce the excitons, which decay radiatively. This is, in practical terms, very inefficient as fields generally induce large thermal effects, consequently causing device breakdown. There are other problems that must be addressed, but elimination of these very basic ones should substantially improve efficiencies and soon lead to applications for these polymers. The polymer used in our studies is PmPV, whose structure is a variation of the more common PPV. In this case the substitution pattern leads to dihedral angles in the chain and, according to molecular mechanics energy minimization calculations, the polymer chain tends to coil, forming a helical structure. The calculated diameter of this helix in vacuum is ca. 20 Š, whilst the pitch is ca. 6 Š. Multiwalled nanotubes were produced by the arc discharge method, resulting in multiwalled nanotubes of 20 nm average diameter and lengths between 500 nm and 1.5 mm. The nanotube powder and PPV were mixed together in toluene and sonicated briefly. It is probable that the coiled polymer conformation allows it to surround layers of nanotubes, permitting sufficiently close intermolecular proximity for p±p interaction to occur. The color change was dramatic in that the polymer has a bright yellow color while the composite, at high nanotube concentrations, possesses a deep green color. Photoluminescence studies were carried out using an Ar laser at the pump wavelength of 457 nm. Electrical conductivity was measured using a twopoint probe sandwich geometry and Pt electrodes. The LED was fabricated by casting the composite onto indium tin oxide (ITO) then sputtering an aluminum electrode on top. As the polymer structure possesses helicity, it is not surprising that it is able to wrap itself around the nanotubes and keep them suspended in solution indefinitely. The actual texture of the composite can be observed in Figure 1,

Relationships of leaf dark respiration to leaf nitrogen, specific leaf area and leaf life-span: a test across biomes and functional groups.

Abstract: Based on prior evidence of coordinated multiple leaf trait scaling, we hypothesized that variation among species in leaf dark respiration rate (Rd) should scale with variation in traits such as leaf nitrogen (N), leaf life-span, specific leaf area (SLA), and net photosynthetic capacity (/4max)- However, it is not known whether such scaling, if it exists, is similar among disparate biomes and plant functional types. We tested this idea by examining the interspecific relationships between Rd measured at a standard temperature and leaf life-span, N, SLA and ^max for 69 species from four functional groups (forbs, broad-leafed trees and shrubs, and needle-leafed coni- fers) in six biomes traversing the Americas: alpine tun- dra/subalpine forest, was positively related to area-based leaf N within functional groups and for all species pooled, but not when comparing among species within any site. At all sites, mass-based Rd (Rd.mass) de- creased sharply with increasing leaf life-span and was positively related to SLA and mass-based Amax and leaf

Quality utility : a Compromise Programming approach to robust design

Abstract: In robust design, associated with each quality characteristic, the design objective often involves multiple aspects such as bringing the mean of performance on target and minimizing the variations. Current ways of handling these multiple aspects using either the Taguchi's signal-to-noise ratio or the weighted-sum method are not adequate. In this paper, we solve bi-objective robust design problems from a utility perspective by following upon the recent developments on relating utility function optimization to a Compromise Programming (CP) method. A robust design procedure is developed to allow a designer to express his/her preference structure of multiple aspects of robust design. The CP approach, i.e., the Tchebycheff method, is then used to determine the robust design solution which is guaranteed to belong to the set of efficient solutions (Pareto points). The quality utility at the candidate solution is represented by means of a quadratic function in a certain sense equivalent to the weighted Tchebycheff metric. The obtained utility function can be used to explore the set of efficient solutions in a neighborhood of the candidate solution. The iterative nature of our proposed procedure will assist decision making in quality engineering and the applications of robust design.

SOMETHING FROM ALMOST NOTHING: Carbon Dioxide Fixation in Chemoautotrophs

Abstract: The last decade has seen significant advances in our understanding of the physiology, ecology, and molecular biology of chemoautotrophic bacteria. Many ecosystems are dependent on CO2 fixation by either free-living or symbiotic chemoautotrophs. CO2 fixation in the chemoautotroph occurs via the Calvin-Benson-Bassham cycle. The cycle is characterized by three unique enzymatic activities: ribulose bisphosphate carboxylase/oxygenase, phosphoribulokinase, and sedoheptulose bisphosphatase. Ribulose bisphosphate carboxylase/oxygenase is commonly found in the cytoplasm, but a number of bacteria package much of the enzyme into polyhedral organelles, the carboxysomes. The carboxysome genes are located adjacent to cbb genes, which are often, but not always, clustered in large operons. The availability of carbon and reduced substrates control the expression of cbb genes in concert with the LysR-type transcriptional regulator, CbbR. Additional regulatory proteins may also be involved. All of these, as well as related topics, are discussed in detail in this review.

Patterns in the Diffusion of Strategies Across Organizations: Insights from the Innovation Diffusion Literature

Abstract: Firms often adopt strategies in spite of mixed evidence about the strategy's performance and of evidence that the strategy leads to inefficient outcomes. Here, we describe the conditions prompting the spread of inefficient strategies through a population of firms, as well as the characteristics of individual firms that affect their propensity to adopt efficient and inefficient strategies. We focus on one pattern that appears common to strategic adoptions: a pattern where the number of unsuccessful adoptions exceeds the number of successful adoptions. We note how the failure to consider diffusion patterns in empirical strategic research limits use of that research as a source of prescriptive theory.

Orientation of ECM protein deposition, fibroblast cytoskeleton, and attachment complex components on silicone microgrooved surfaces.

Abstract: The microfilaments and vinculin-containing attachment complexes of rat dermal fibroblasts (RDF) incubated on microtextured surfaces were investigated with confocal laser scanning microscopy (CLSM) and digital image analysis (DIA). In addition, depositions of bovine and endogenous fibronectin and vitronectin were studied. Smooth and microtextured silicone substrata were produced that possessed parallel surface grooves with a groove and ridge width of 2.0, 5.0, and 10.0 μm. The groove depth was approximately 0.5 μm. CLSM and DIA make it possible to visualize and analyze intracellular and extracellular proteins and the underlying surface simultaneously. It was observed that the microfilaments and vinculin aggregates of the RDFs on the 2.0 μm grooved substrata were oriented along the surface grooves after 1, 3, 5, and 7 days of incubation while these proteins were significantly less oriented on the 5.0 and 10.0 μm grooved surfaces. Vinculin was located mainly on the surface ridges on all textured surfaces. In contrast, bovine and endogenous fibronectin and vitronectin were oriented along the surface grooves on all textured surfaces. These proteins did not seem to be hindered by the surface grooves since many groove-spanning filaments were found on all the microgrooved surfaces. In conclusion, it can be said that microtextured surfaces influence the orientation of intracellular and extracellular proteins. Although results corroborate three earlier published hypotheses, they do not justify a specific choice of any one of these hypotheses. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 40, 291–300, 1998.

Responses of hardwood advance regeneration to seasonal prescribed fires in oak-dominated shelterwood stands

Abstract: Effects of seasonal prescribed fires of varying intensities on density, mortality, stem form, height, and height growth of hardwood advance regeneration were investigated. Three mixed-hardwood stands on productive upland sites were cut using a shelterwood technique, each forming a block of spring burn, summer burn, winter burn, and control treatments. Advance regeneration was inventoried from permanent plots before and after burning. Fires top-killed nearly all hardwood regeneration, forcing the rootstocks to sprout. Fire treatments reduced densities of all hardwood species relative to not burning, with spring and summer fires causing greater density reduction than winter burning. Among species, oak (Quercus spp. L.) and hickory (Carya spp. Nutt.) were more resilient sprouters than yellow-poplar (Liriodendron tulipifera L.) and red maple (Acer rubrum L.), especially as fire intensity increased. All prescribed fires improved oak stem form and stimulated height growth of hickory and oak. Overall, prescribed fires improved oak advance regeneration with spring burning providing the most benefit. This approach of following a shelterwood harvest with prescribed fire may be a viable method of regenerating oak-dominated stands on productive upland sites. Resume : Les effets du brslage dirigO, effectuO ‡ diffOrentes saisons et ‡ des intensitOs variables, sur la densitO, la mortalitO, la forme de la tige, la hauteur et la croissance en hauteur de la rOgOnOration feuillue prOOtablie ont OtO investiguOs. Trois peuplements de feuillus mOlangOs Otablis sur des sites riches situOs sur les plateaux furent coupOs selon la technique de la coupe progressive. Chaque peuplement constituait un bloc de traitements qui incluaient des brslages printanier, estival et hivernal ainsi quiun tOmoin. La rOgOnOration prOOtablie a OtO inventoriOe dans des parcelles permanentes avant et aprs le brslage. Le feu a tuO la cime de presque tous les semis de feuillus forAant leur systme racinaire ‡ drageonner. Le brslage a rOduit la densitO de toutes les espces feuillues comparativement ‡ liabsence de brslage. Les brslages printanier ou estival ont causO une plus forte rOduction que le brslage hivernal. Parmi les espces prOsentes, le chŒne ( Quercus spp. L.) et le caryer (Carya spp. Nutt.) avaient une meillleure capacitO de drageonner que le tulipier de Virginie (Liriodendron tulipifera L.) et liOrable rouge (Acer rubrum L.), particulirement lorsque liintensitO du feu augmentait. Dans tous les cas, le brslage dirigO a amOliorO la forme de la tige du chŒne et a stimulO la croissance en hauteur du caryer et du chŒne. Dans liensemble, le brslage dirigO a amOliorO la rOgOnOration prOOtablie du chŒne et le brslage effectuO au printemps a procurO le plus diavantages. Cette approche consistant ‡ utiliser le brslage dirigO aprs une coupe progressive pourrait siavOrer une mOthode viable pour rOgOnOrer les peuplements dominOs par le chŒne sur les sites riches des plateaux. (Traduit par la ROdaction)

Proton Conductivity in Nafion® 117 and in a Novel Bis[(perfluoroalkyl)sulfonyl]imide Ionomer Membrane

Abstract: A study of proton conductivity in a commercial sample of Nation® 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidity conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox‐initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four‐point‐probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a "liquid‐like" mechanism of proton conductivity whereby protons are transported as hydrated hydronium ions through water‐filled pores and channels in the ionomer.

Rainfall Microphysics and Radar Properties: Analysis Methods for Drop Size Spectra

Abstract: Analyses are performed of experimental drop size spectra to explore the relationships among integral parameters for rain. The data used in this work were acquired with an airborne optical 2D precipitation probe in TOGA COARE during a 4-month period in 1992–93. It is assumed that the experimental size spectra can be described by a gamma drop size distribution (DSD) of the form N(D) = N0Dμ exp(−ΛD) involving three parameters (N0, μ, Λ), which are determined using a new method of truncated moments. The method allows for truncation of the DSD at the large-diameter end of the spectrum due in part to instrumental effects and also in part to the trajectory of the aircraft through a rain streamer that has been sorted by wind shear. An effect analogous to truncation can occur at the small-diameter end of the size spectrum due to evaporation. However, truncation of the spectrum at the small-diameter end is not considered in this work. It is found that spectra with small space and timescales display conside...

A New Way of Using ac Voltammetry To Study Redox Kinetics in Electroactive Monolayers

Abstract: A new protocol for analyzing variable-frequency ac voltammetry (acv) data to obtain electron-transfer rate constants for redox-active groups immobilized in monolayer films on electrodes is described. The protocol involves plotting the ratio of the peak current to the background current as a function of the log of frequency and then fitting the plot to a calculated curve obtained using the Randles equivalent circuit model. The fitting procedure is particularly straightforward since it involves just two adjustable fitting parameters and can usually be performed manually in a spreadsheet. Advantages of the new method include the following: (1) it requires fewer adjustable fitting parameters than the conventional impedance analysis performed in the complex-plane format; (2) because there are fewer fitting parameters, there is less chance of the fit yielding a false convergence; (3) nonideal effects associated with a distribution of rate constants are apparently revealed in a particularly intuitive and easily...

Advances in modeling the effects of selected parameters on the SLS process

Abstract: Selective laser sintering (SLS) is a leading process for developing rapid prototype objects by selectively fusing layers of powder according to numerically defined cross‐sectional geometry. The process has the potential to become an indispensable industrial tool. However, continuous process improvement is necessary. Improved understanding of the parameter effects on the process response is expected to lead to process advances. In this work the analytical problem describing the energy delivery, heat transfer and sintering process along with other pertinent phenomena is studied. Physical experiments and implementation of a numerical simulation are conducted using Bisphenol‐A polycarbonate. The effects of selected parameters on the SLS process response are examined. The primary parameters of interest are the laser power, laser beam velocity, hatch spacing, laser beam spot size and scan line length. This work shows that the secondary process parameters, delay period and number of effective exposures have a significant influence on the process response.

Frequency-domain fluorescent diffusion tomography: a finite-element-based algorithm and simulations

Abstract: We present a finite-element-based algorithm for reconstruction of fluorescence lifetime and yield in turbid media, using frequency-domain data. The algorithm is based on a set of coupled diffusion equations that describe the propagation of both excitation and fluorescent emission light in multiply scattering media. Centered on Newton's iterative method, we implemented our algorithm by using a synthesized scheme of Marquardt and Tikhonov regularizations. A low-pass spatial filter is also incorporated into the algorithm for enhancing image reconstruction. Simulation studies using both noise-free and noisy data have been performed with the nonzero photon density boundary conditions. Our results suggest that quantitative images can be produced in terms of fluorescent lifetime and yield values and location, size, and shape of heterogeneities within a circular background region.

A Two-Phase Exact Algorithm for MAX-SAT and Weighted MAX-SAT Problems

Abstract: We describe a two-phase algorithm for MAX-SAT and weighted MAX-SAT problems. In the first phase, we use the GSAT heuristic to find a good solution to the problem. In the second phase, we use an enumeration procedure based on the Davis-Putnam-Loveland algorithm, to find a provably optimal solution. The first heuristic stage improves the performance of the algorithm by obtaining an upper bound on the minimum number of unsatisfied clauses that can be used in pruning branches of the search tree. We compare our algorithm with an integer programming branch-and-cut algorithm. Our implementation of the two-phase algorithm is faster than the integer programming approach on many problems. However, the integer programming approach is more effective than the two-phase algorithm on some classes of problems, including MAX-2-SAT problems.

Some cautionary notes on the use of principal components regression

Abstract: Many textbooks on regression analysis include the methodology of principal components regression (PCR) as a way of treating multicollinearity problems. Although we have not encountered any strong justification of the methodology, we have encountered, through carrying out the methodology in well-known data sets with severe multicollinearity, serious actual and potential pitfalls in the methodology. We address these pitfalls as cautionary notes, numerical examples that use well-known data sets. We also illustrate by theory and example that it is possible for the PCR to fail miserably in the sense that when the response variable is regressed on all of the p principal components (PCs), the first (p − 1) PCs contribute nothing toward the reduction of the residual sum of squares, yet the last PC alone (the one that is always discarded according to PCR methodology) contributes everything. We then give conditions under which the PCR totally fails in the above sense.

Displays of Bird Movements on the WSR-88D: Patterns and Quantification*

Abstract: The WSR-88D can readily detect birds in the atmosphere in both clear air and precipitation mode, and echo reflectivities of 30–35 dBZ may be realized during heavy migration events or when birds are departing a roosting site. This paper describes the appearance of birds on base reflectivity, base velocity, and velocity azimuth display wind profile products, and presents a calibration curve that relates decibel values of reflectivity to bird migration traffic rates. The recognition of bird displays in WSR-88D products is essential for the accurate interpretation of data gathered by the radar and its use in the development of forecasts. The findings also document the importance of the WSR-88D as a remote sensing tool for biological studies of birds and insects in the atmosphere and the application of such information in the avoidance of bird–aircraft collisions.

Robust neural-network control of rigid-link electrically driven robots

Abstract: A robust neural-network (NN) controller is proposed for the motion control of rigid-link electrically driven (RLED) robots. Two-layer NN's are used to approximate two very complicated nonlinear functions. The main advantage of our approach is that the NN weights are tuned online, with no off-line learning phase required. Most importantly, we can guarantee the uniformly ultimately bounded (UUB) stability of tracking errors and NN weights. When compared with standard adaptive robot controllers, we do not require lengthy and tedious preliminary analysis to determine a regression matrix. The controller can be regarded as a universal reusable controller because the same controller can be applied to any type of RLED robots without any modifications.

Influence of silicone (PDMS) surface texture on human skin fibroblast proliferation as determined by cell cycle analysis.

Abstract: In vivo biocompatibility of soft-tissue implants is often hampered by development of capsules that eventually might contract and impair implant function. It has been shown that capsule formation can be significantly reduced by using materials with textured surface elements in the micron range. In this study the interaction of human fibroblasts with silicone surfaces was analyzed using cell cycle analysis. Silicone was textured with 2, 5, and 10 microns wide grooves (2MU, 5MU, 10MU, respectively) or kept smooth (SMT). Cell cycle analysis was performed after staining of cells with propidium iodide. Cells proliferated on the fibronectin-preadsorbed silicone, as demonstrated by increased coverage and occurrence of subpopulations in the S and G2/M phase of the cell cycle. Cells on SMT went faster into the S phase than cells on textured silicones. Cells on 10MU showed less proliferation than cells on 2MU and 5MU. Besides the basic percentages of cells in the different cycle phases, DNA profiles were also influenced by incubation time and texture, especially with respect to the presence of hypodiploid populations and asymmetry of the G0/G1 peak. Finally scatter characteristics were influenced. 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay data did not reveal significant differences among the different samples. Fibronectin preadsorption of silicone only resulted in slightly higher MTT conversion. Cell cycle analysis proved to be a sensitive screening method for proliferation on the silicone surfaces and provided information beyond the normal G0/G1, S and G2/M subpopulations.

Mechanical Strength of Durable Press Finished Cotton Fabrics Part I: Effects of Acid Degradation and Crosslinking of Cellulose by Polycarboxylic Acids

Abstract: Severe tensile strength loss is the major disadvantage of durable press finished cotton fabrics. Such strength losses have been attributed to two main factors: acid-catalyzed depolymerization and crosslinking of cellulose molecules. In this research, we inves tigate the effects of acid degradation and cellulose crosslinking on the tensile strength of cotton fabric crosslinked by polycarboxylic acids. Multifunctional carboxylic acids such as butanetetracarboxylic acid (BTCA) are used as nonformaldehyde crosslinking agents for cotton fabrics. The strength loss caused by acid degradation is an irreversible process, and the magnitude of the loss is determined by the curing temperature and time, the dissociation constants of the acid, and the concentration and pH of the acid solution applied to the fabric. Crosslinking of cellulose molecules by a polycarboxylic acid causes a reversible fabric strength loss, which increases as the degree of crosslink ing increases. The magnitude of tensile strength loss caused ...

System and customer impact: considerations for series custom power devices

Abstract: The growing interest in power quality has led to a variety of devices designed for mitigating power disturbances, primarily voltage sags. One class of these devices is the series device. The size of a series device (SD) is usually described as having a certain percent RMS magnitude voltage injection capability. The required RMS magnitude is usually taken from measured data or the results of a circuit model simulation. These results can be graphically illustrated using magnitude versus duration matrices or curves. Due to the one cycle averaging effect of the RMS calculation, waveshape, transients, and phase shift are not adequately described, and thus does not accurately predict the necessary injection capability required of the series device. Careful consideration must also be taken when applying a series device onto a system. The series device acts as an additional energy source on the system. The device must coordinate with other protective devices to prevent damage to other loads on the system, particularly upstream loads. The series device requires a continuous current path to insert energy, so protective devices must be arranged accordingly. However, a load upstream of the series device and downstream of the protective device creates a potential for reverse current flow through the upstream load.

An adaptive protection scheme for advanced series compensated (ASC) transmission lines

Abstract: This paper presents an adaptive protection scheme for advanced series compensated (ASC) transmission lines. The scheme is based on Kalman and adaptive Kalman filters. The adaptive scheme utilizes the differences in the transient current signals for faults encountering and not encountering the ASC to determine the fault location with respect to the ASC and the faulted phases. Equations to determine the line impedance to the fault on these lines are developed. The adaptive scheme is tested for faults along and outside the protected line using EMTP simulated data.