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Time Dependence (time + dependence)
Selected AbstractsModelling survival in acute severe illness: Cox versus accelerated failure time modelsJOURNAL OF EVALUATION IN CLINICAL PRACTICE, Issue 1 2008John L. Moran MBBS FRACP FJFICM MD Abstract Background, The Cox model has been the mainstay of survival analysis in the critically ill and time-dependent covariates have infrequently been incorporated into survival analysis. Objectives, To model 28-day survival of patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), and compare the utility of Cox and accelerated failure time (AFT) models. Methods, Prospective cohort study of 168 adult patients enrolled at diagnosis of ALI in 21 adult ICUs in three Australian States with measurement of survival time, censored at 28 days. Model performance was assessed as goodness-of-fit [GOF, cross-products of quantiles of risk and time intervals (P , 0.1), Cox model] and explained variation (,R2', Cox and ATF). Results, Over a 2-month study period (October,November 1999), 168 patients with ALI were identified, with a mean (SD) age of 61.5 (18) years and 30% female. Peak mortality hazard occurred at days 7,8 after onset of ALI/ARDS. In the Cox model, increasing age and female gender, plus interaction, were associated with an increased mortality hazard. Time-varying effects were established for patient severity-of-illness score (decreasing hazard over time) and multiple-organ-dysfunction score (increasing hazard over time). The Cox model was well specified (GOF, P > 0.34) and R2 = 0.546, 95% CI: 0.390, 0.781. Both log-normal (R2 = 0.451, 95% CI: 0.321, 0.695) and log-logistic (R2 0.470, 95% CI: 0.346, 0.714) AFT models identified the same predictors as the Cox model, but did not demonstrate convincingly superior overall fit. Conclusions, Time dependence of predictors of survival in ALI/ARDS exists and must be appropriately modelled. The Cox model with time-varying covariates remains a flexible model in survival analysis of patients with acute severe illness. [source] Magnetic viscosity and activation volume in chromium substituted Pb,M hexaferritePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2005J. C. Faloh-Gandarilla Abstract PbFe11CrO19 polycrystalline samples were prepared by the chemical coprecipitation method. X-ray diffraction and Mössbauer spectroscopy confirm the formation of the M-type hexagonal structure. Time dependence of the magnetization was recorded on the demagnetization curve of the hysteresis loop. It is well described by a simple logarithmic law but also non-logarithmic behavior was detected. A two-peak dependence of the viscosity coefficient S with the applied field was encountered. Considering reversible susceptibility measurements and a M(H) curve at constant dM/dt in the irreversible susceptibility determination, the calculated activation volume as a function of the applied field shows two well defined zones. The zones observed in the activation volumes are related to the two local maxima that the total susceptibility exhibits. It points to the fact that the viscosity study is sensible enough to characterize this system, encountering two apparent activation volumes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Extension of the time-dependent dynamical diffraction theory to `optical phonon'-type distortions: application to diffraction from coherent acoustic and optical phononsACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2003Peter Sondhauss An extension of the time-dependent Takagi,Taupin theory to `optical phonon'-type distortions is presented. By splitting the susceptibility into the contributions from each atom in a unit cell, modifications to the structure factor as well as lattice parameter are taken into account. The result is a compact, surprisingly simple, equation with a strong formal similarity to the classical Takagi,Taupin equation, with the latter included as a special case. Time dependence is explicitly retained and thus the analysis is applicable to situations where the crystal is modified on time scales comparable with that for the X-rays to traverse an extinction depth. A comparison is made between the influence of coherent acoustic and optical phonons on the diffraction of X-rays. Numerical and perturbative analytical solutions of the generalized Takagi,Taupin equation are presented in the presence of such phonons. [source] A Semivirtual Watershed Model by Neural NetworksCOMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 2 2001James C. Y. Guo A semivirtual watershed model is presented in this study. This model places the design rainfall distribution on the input layer and the predicted runoff hydrograph on the output layer. The optimization scheme developed in this study can train the model to establish a set of weights under the guidance of the kinematic wave theory. The weights are time-dependent variables by which rainfall signals can be converted to runoff distributions by weighting procedures only. With the consideration of time dependence, the computational efficiency of virtual watershed models is greatly enhanced by eliminating unnecessary visitations between layers. The weighting procedure used in the semivirtual watershed model expands the rational method from peak runoff predictions to complete hydrograph predictions under continuous and nonuniform rainfall events. [source] AMP-activated protein kinase in contraction regulation of skeletal muscle metabolism: necessary and/or sufficient?ACTA PHYSIOLOGICA, Issue 1 2009T. E. Jensen Abstract In skeletal muscle, the contraction-activated heterotrimeric 5,-AMP-activated protein kinase (AMPK) protein is proposed to regulate the balance between anabolic and catabolic processes by increasing substrate uptake and turnover in addition to regulating the transcription of proteins involved in mitochondrial biogenesis and other aspects of promoting an oxidative muscle phenotype. Here, the current knowledge on the expression of AMPK subunits in human quadriceps muscle and evidence from rodent studies suggesting distinct AMPK subunit expression pattern in different muscle types is reviewed. Then, the intensity and time dependence of AMPK activation in human quadriceps and rodent muscle are evaluated. Subsequently, a major part of this review critically examines the evidence supporting a necessary and/or sufficient role of AMPK in a broad spectrum of skeletal muscle contraction-relevant processes. These include glucose uptake, glycogen synthesis, post-exercise insulin sensitivity, fatty acid (FA) uptake, intramuscular triacylglyceride hydrolysis, FA oxidation, suppression of protein synthesis, proteolysis, autophagy and transcriptional regulation of genes relevant to promoting an oxidative phenotype. [source] The Political Survival of Foreign MinistersFOREIGN POLICY ANALYSIS, Issue 2 2009Alejandro Quiroz Flores The survival of governments ultimately depends on the survival of its components. These components are politicians whose goal is to stay in office. There has been extensive research on the survival of leaders, but not on the survival of other politicians in government; and even less on how the survival of one affects the survival of the others. The purpose of this article is to take the first step in this direction by analyzing and precisely measuring the impact of the tenure of leaders on the tenure of foreign ministers. This article provides a systematic and formal investigation of the variables that affect the duration in office of foreign ministers. The investigation is based on a new data base on the tenure of more than 7,500 foreign ministers spanning three centuries. Although evidence shows that political institutions have significant impact on the tenure of foreign ministers, internal coalition dynamics such as affinity and loyalty toward a leader, uncertainty, and time dependence are better predictors of their political survival. [source] Genetic analysis of phenotypes derived from longitudinal data: Presentation Group 1 of Genetic Analysis Workshop 13GENETIC EPIDEMIOLOGY, Issue S1 2003Konstantin Strauch Abstract The participants of Presentation Group 1 used the GAW13 data to derive new phenotypes, which were then analyzed for linkage and, in one case, for association to the genetic markers. Since the trait measurements ranged over longer time periods, the participants looked at the time dependence of particular traits in addition to the trait itself. The phenotypes analyzed with the Framingham data can be roughly divided into 1) body weight-related traits, which also include a type 2 diabetes progression trait, and 2) traits related to systolic blood pressure. Both trait classes are associated with metabolic syndrome. For traits related to body weight, linkage was consistently identified by at least two participating groups to genetic regions on chromosomes 4, 8, 11, and 18. For systolic blood pressure, or its derivatives, at least two groups obtained linkage for regions on chromosomes 4, 6, 8, 11, 14, 16, and 19. Five of the 13 participating groups focused on the simulated data. Due to the rather sparse grid of microsatellite markers, an association analysis for several traits was not successful. Linkage analysis of hypertension and body mass index using LODs and heterogeneity LODs (HLODs) had low power. For the glucose phenotype, a combination of random coefficient regression models and variance component linkage analysis turned out to be strikingly powerful in the identification of a trait locus simulated on chromosome 5. Haseman-Elston regression methods, applied to the same phenotype, had low power, but the above-mentioned chromosome 5 locus was not included in this analysis. Genet Epidemiol 25 (Suppl. 1):S5,S17, 2003. © 2003 Wiley-Liss, Inc. [source] Saturation and time dependence of geodynamo modelsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010M. Schrinner SUMMARY In this study we address the question under which conditions a saturated velocity field stemming from geodynamo simulations leads to an exponential growth of the magnetic field in a corresponding kinematic calculation. We perform global self-consistent geodynamo simulations and calculate the evolution of a kinematically advanced tracer field. The self-consistent velocity field enters the induction equation in each time step, but the tracer field does not contribute to the Lorentz force. This experiment has been performed by Cattaneo and Tobias and is closely related to the test field method by Schrinner et al. We find two dynamo regimes in which the tracer field either grows exponentially or approaches a state aligned with the actual self-consistent magnetic field after an initial transition period. Both regimes can be distinguished by the Rossby number and coincide with the dipolar and multipolar dynamo regimes identified by Christensen and Aubert. Dipolar dynamos with low Rossby number are kinematically stable whereas the tracer field grows exponentially in the multipolar dynamo regime. This difference in the saturation process for dynamos in both regimes comes along with differences in their time variability. Within our sample of 20 models, solely kinematically unstable dynamos show dipole reversals and large excursions. The complicated time behaviour of these dynamos presumably relates to the alternating growth of several competing dynamo modes. On the other hand, dynamos in the low Rossby number regime exhibit a rather simple time dependence and their saturation merely results in a fluctuation of the fundamental dynamo mode about its critical state. [source] Identifying rotation and oscillation in surface tension measurement using an oscillating droplet methodHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2008Shumpei Ozawa Abstract We proposed a new approach to identify the frequencies of droplet rotation and m=±2 oscillation that degrade the accuracy of surface tension measurement by an oscillating droplet method. Frequencies of droplet rotation and m=±2 oscillation can be identified by a phase unwrapping analysis of time dependence of the deflection angle for the maximum diameter of the droplet image observed from above. The present method was validated, using test data with given frequencies. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 421,430, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20214 [source] Non-local dispersive model for wave propagation in heterogeneous media: one-dimensional caseINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 3 2002Jacob Fish Abstract Non-local dispersive model for wave propagation in heterogeneous media is derived from the higher-order mathematical homogenization theory with multiple spatial and temporal scales. In addition to the usual space,time co-ordinates, a fast spatial scale and a slow temporal scale are introduced to account for rapid spatial fluctuations of material properties as well as to capture the long-term behaviour of the homogenized solution. By combining various order homogenized equations of motion the slow time dependence is eliminated giving rise to the fourth-order differential equation, also known as a ,bad' Boussinesq problem. Regularization procedures are then introduced to construct the so-called ,good' Boussinesq problem, where the need for C1 continuity is eliminated. Numerical examples are presented to validate the present formulation. Copyright © 2002 John Wiley & Sons, Ltd. [source] Kinetics of aerobic and anaerobic oxidations of ethanol by Fenton's reagentINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2008Mordechai L. Kremer For the first time, the time dependence of [H2O2] and [Fe2+] was followed during the aerobic oxidation of ethanol by Fenton's reagent. It was found that part of the ethanol was oxidized by dissolved O2 via the transient formation of H2O2. A model was set up based on FeO2+ as the key intermediate. Both one- and two-equivalent oxidations of ethanol occur, the former producing radical species derived from ethanol. No free radicals derived from H2O2 play part in the system. The relevant rate constants or their ratios were determined. The mechanism accounted successfully also for the anaerobic oxidation of ethanol. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 541,553, 2008 [source] On the use of "fast kinetics" to determine the mechanism of ligand substitution at a solvated transition-metal intermediateINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2004Richard H. Schultz Substitution of the weakly-coordinated solvent molecule at a solvated transition-metal intermediate is frequently investigated by "fast kinetic" methods. In typical experiments, the kinetics of the reaction are determined by following the time dependence of the changes in the reaction mixture's UV-visible or infrared spectrum following photolytic creation of the intermediate. We consider the two limiting mechanisms (associative and dissociative), as well as the case of competition between them, and show that under typical "fast kinetics" experimental conditions, the different mechanisms are kinetically indistinguishable. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 427,433, 2004 [source] Influence of the carrier diffusion process on the transient response of vertical-cavity surface-emitting lasersINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 1 2003M. S. Torre Abstract We investigate the transverse mode dynamics of weakly index-guided vertical-cavity surface-emitting lasers (VCSEL). The turn-on time of transverse modes are calculated by implementing a model for the VCSEL dynamics including diffusion and transport/capture phenomena. It takes into account the spatial dependence of the two carrier density profiles associated with the confined carriers in the quantum wells, and with the unconfined carriers in the barrier region. Devices of different aperture diameter under different excitation conditions are also studied. The model displays the correct turn-on time dependence on the injection current density when compared with the experimental data available. We show that the turn-on time of the modes increases when capture time increases and escape time decreases and also when diffusion increases. Copyright © 2002 John Wiley & Sons, Ltd. [source] Dynamical scaling in fractal structures in the aggregation of tetraethoxysilane-derived sonogelsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5-1 2010Dimas R. Vollet Dynamical scaling properties in fractal structures were investigated from small-angle X-ray scattering (SAXS) data of the kinetics of aggregation in silica-based gelling systems. For lack of a maximum in the SAXS intensity curves, a characteristic correlation distance , was evaluated by fitting a particle scattering factor model valid for polydisperse coils of linear chains and f -functional branched polycondensates in solution, so the intensity at q = ,,1, I(,,1, t), was considered to probe dynamical scaling properties. The following properties have been found: (i) the SAXS intensities corresponding to different times t, I(q, t), are given by a time-independent function F(q,) = I(q, t),,D/Q, where the scattering invariant Q has been found to be time-independent; (ii) , exhibited a power-law behavior with time as ,,t,, the exponent , being close to 1 but diminishing with temperature; (iii) I(,,1, t) exhibited a time dependence given by I(,,1, t) ,t,, with the exponent , found to be around 2 but diminishing with temperature, following the same behavior as the exponent ,. In all cases, ,/, was quite close to the fractal dimension D at the end of the studied process. This set of findings is in notable agreement with the dynamical scaling properties. [source] Understanding the Chemistry of the Development of Latent Fingerprints by Superglue FumingJOURNAL OF FORENSIC SCIENCES, Issue 5 2007Stephen P. Wargacki Ph.D. Abstract:, Cyanoacrylate fuming is a widely used forensic tool for the development of latent fingerprints, however the mechanistic details of the reaction between the fingerprint residue and the cyanoacrylate vapor are not well understood. Here the polymerization of ethyl-cyanoacrylate vapor by sodium lactate or alanine solutions, two of the major components in fingerprint residue, has been examined by monitoring the time dependence of the mass uptake and resultant polymer molecular weight characteristics. This data provides insight into the molecular level actions in the efficient development of latent fingerprints by superglue fuming. The results show that the carboxylate moiety is the primary initiator of the polymerization process and that a basic environment inhibits chain termination while an acidic environment promotes it. The results also indicate that water cannot be the primary initiator in this forensic technique. [source] Tracer studies of high-shear granulation: II.AICHE JOURNAL, Issue 9 2001Population balance modeling A population balance framework developed describes the tracer studies in Part I. A two internal coordinate population balance equation (PBE) links the evolution with time of granule-size and tracer-mass distributions to underlying rate processes. A new analytical PBE was developed for the tracer distribution and novel numerical techniques, including a new discretized population balance equation for breakage or grinding. Also developed is a general differential technique for extracting rate constants from measurements of particle-size distributions. Granulation in a high-shear mixer proceeds after nucleation, not studied here, with very high initial breakage rates but a relatively unchanging aggregation rate constant. The breakage function is bimodal on a mass basis and the selection rate decays exponentially over about 20 s. A heterogeneous strength hypothesis was used to account for this time dependence. Aggregation rates are the highest for interactions between small and large granules and may be quantitatively given by the Equipartition of kinetic energy kernel developed from the theory of collisions between gas molecules. The model can describe granule-size and tracer-mass distributions simultaneously with great accuracy. The need to replace time as a driving force variable in the kinetics for these systems is identified. [source] Application of ultrasonic shear rheometer to characterize rheological properties of high protein concentration solutions at microliter volumeJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 6 2005Atul Saluja Abstract The purpose of this work was to conduct preliminary rheological analysis on high protein concentration solutions by using the technique of ultrasonic shear rheometry at megahertz frequencies. The work was aimed at establishing the viability of the technique for analyzing protein solution rheology as well as obtaining an initial understanding of the effect of solution conditions on solution rheology of a model protein. Bovine serum albumin (BSA) was used for this study, and rheological analysis was conducted at 20 ,L sample volume between pH 2.0 and 9.0 at different ionic strengths at 25°C using 5 and 10 MHz quartz crystals. Significant differences in storage modulus among solutions at pH 5.0, 7.0, and 9.0 could only be detected at 10 MHz, and the errors associated with measurements were smaller as compared to those at 5 MHz for all the solutions studied. Solutions at pH 2.0 and 3.0 showed a time-dependent change in solution rheology. For solutions at pH 5.0, 7.0, and 9.0, which did not show time dependence in solution rheology, loss modulus data at lower concentrations correlated well with the dilute solution data in the literature. At higher concentrations, pH 5.0 solutions exhibited a higher loss modulus than pH 7.0 and pH 9.0 solutions. Storage modulus decreased with increasing ionic strength, unlike loss modulus, which did not show any change, except at pI of protein when no effect was observed. The results show the potential of high frequency rheometry for analyzing subtle differences in rheology of pharmaceutically relevant protein solutions at microliter volume. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:1161,1168, 2005 [source] Field measurements of the water content in the top soil using a new capacitance sensor with a flat sensitive volumeJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 2 2005Bernhard Ruth Abstract Water content directly near the soil surface plays an essential role for degradation of natural organic material and agrochemicals by soil microbes. Furthermore, the water losses by evaporation depend sensitively on the top-soil water content. Rain, irrigation, evaporation, and the water flow between the soil horizons together with the natural inhomogeneity of soils cause a high spatial gradient and a pronounced time dependence of the water content in the top soil. To understand processes in top soil such as redox gradients, the knowledge on ecological conditions in the top soil, which is subject to rapid changes, is essential. In order to meet the requirements for such field measurements, a capacitance sensor with a depth resolution of 1,cm and an active area of 7.5,cm × 14,cm was constructed and operated by a special electronic circuit. Field measurements using these sensors at 1,cm depth showed the high dynamics when measurements were carried out every 10,min. As simultaneous measurements of the soil temperature at 1,cm depth exhibit large temperature variations during the day, its influence on the measurements must be compensated for. As the data, measured during drying periods, allow the assessment of the temperature coefficient, the water content at a reference temperature can be calculated. The course of the water content reflects precipitation events and quantifies the drying of the soil, providing these parameters for process evaluation. Furthermore, the diurnal variation exhibits the drying during the day and the possible rewetting from deeper horizons during the night. Freilandmessungen des Wassergehalts im Oberboden mit einem neuen Kapazitätssensor mit flachem sensitiven Volumen Der Wassergehalt direkt an der Bodenoberfläche spielt für den mikrobiellen Abbau natürlicher organischer Substanz und von Agrochemikalien eine bedeutende Rolle. Darüber hinaus hängen die Wasserverluste durch Evaporation empfindlich vom Wassergehalt an der Bodenoberfläche ab. Regen, Bewässerung, Evaporation und die Wasserbewegung zwischen den Bodenhorizonten, sowie die natürliche Inhomogenität des Bodens verursachen einen großen Gradienten und eine ausgeprägte Zeitabhängigkeit des Wassergehalts und entsprechender Stofftransformationsprozesse im Oberboden. Für das Verständnis der Prozesse im Oberboden, wie z.,B. der Redox-Gradienten, ist die Kenntnis der ökologischen Bedingungen in dem sich schnell verändernden Oberboden unerlässlich. Um die Anforderungen für solche Feldmessungen zu erfüllen, wurde ein Kapazitätssensor mit einer Tiefenauflösung von 1,cm und einer aktiven Fläche von 7.5,cm × 14,cm konstruiert und mit einem speziellen elektronischen Schaltkreis betrieben. Feldmessungen in der Tiefe von 1,cm zeigen eine große Dynamik, wenn alle 10 min ein neuer Messwert erfasst wird. Da simultane Messungen der Bodentemperatur in 1,cm Tiefe hohe Variationen zeigen, muss deren Einfluss auf die Messung kompensiert werden. Da die Messungen während der Trockenperioden die Abschätzung des Temperaturkoeffizienten erlauben, kann der Wassergehalt bei einer Referenztemperatur errechnet werden. Die Messergebnisse korrespondieren mit Regenereignissen und erfassen die Austrocknung des Bodens, so dass damit Parameter für die Prozessberechnung zur Verfügung gestellt werden. Der Tagesgang zeigt Austrocknung während des Tages und die mögliche Wiederbefeuchtung aus tieferen Horizonten während der Nacht. [source] Kinetic analysis of the cross reaction between dithioester and alkoxyamine by a Monte Carlo simulation,JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2007Yong Ao Abstract A model reaction of dithioester and alkoxyamine is proposed to probe the reversible addition,fragmentation chain transfer (RAFT) process. The kinetics of the model reaction is analyzed and compared with that of pure alkoxyamine homolysis with a Monte Carlo simulation. Although the pure alkoxyamine obeys the law of persistent radical effect, the model reaction results in higher concentration of the persistent radical during the main period of the reaction. However, for a very fast RAFT process or a very low addition rate constant, the time dependence of the persistent radical concentration is quite close to that of pure alkoxyamine. Furthermore, the cross termination between the intermediate and alkyl radicals causes a retardation effect for the model reaction when the intermediate is relatively long-lived. The Monte Carlo simulation indicates that it is feasible to measure the individual rate constants of the RAFT process, such as the rate constant of addition, with a large excess of alkoxyamine. In addition, the special feature of the system with different leaving groups in the alkoxyamine and dithioester is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 374,387, 2007 [source] Specimen charging in X-ray absorption spectroscopy: correction of total electron yield data from stabilized zirconia in the energy range 250,915,eVJOURNAL OF SYNCHROTRON RADIATION, Issue 2 2005Dimitrios Vlachos The effects of specimen charging on X-ray absorption spectroscopy using total electron yield have been investigated using powder samples of zirconia stabilized by a range of oxides. The stabilized zirconia powder was mixed with graphite to minimize the charging but significant modifications of the intensities of features in the X-ray absorption near-edge fine structure (XANES) still occurred. The time dependence of the charging was measured experimentally using a time scan, and an algorithm was developed to use this measured time dependence to correct the effects of the charging. The algorithm assumes that the system approaches the equilibrium state by an exponential decay. The corrected XANES show improved agreement with the electron energy-loss near-edge fine structure obtained from the same samples. [source] Wettability of Silica Substrates by Silver,Copper Based Brazing Alloys inVacuoJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000Jorge López-Cuevas The sessile drop method has been used to determine the time dependence of the contact angle at 850°C in vacuo for Ag,28 wt% Cu, Ag,35 wt% Cu,1.5 wt% Ti, and Ag,27 wt% Cu,12 wt% In,2 wt% Ti on vitreous and devitrified fused quartz substrates. Nonwetting behavior (, > 90°) was observed for Ag,28 wt% Cu on both substrates with no evident effect of time at temperature. The silica substrate structure, whether crystalline or amorphous, as well as its surface condition, whether smooth or rough, made no significant difference. In contrast, with Ag,35 wt% Cu,1.5 wt% Ti and Ag,27 wt% Cu,12 wt% In,2 wt% Ti the contact angle continuously decreased with time for both silica substrates, and the structure and surface condition of the substrates had a negligible effect in the case of Ag,27 wt% Cu,12 wt% In,2 wt% Ti, which produced essentially the same contact angles on both silica substrates at a given time of hold at 850°C. The contact angles produced by Ag,35 wt% Cu,1.5 wt% Ti on devitrified fused quartz were consistently higher than those produced on the vitreous substrates, with increasing holding time at 850°C. This is attributable to the presence of extensive cracks in the ,-cristobalite layer at the surface of the devitrified substrates, which obstruct wetting and spreading. These results, when correlated with the wettability of preoxidized silicon carbide by the same alloys reported in previous work, could account for the adverse effect on wetting of the high-temperature silica films formed on the surface of the SiC in that work. [source] Averaged Periodogram Spectral Estimation with Long-memory Conditional HeteroscedasticityJOURNAL OF TIME SERIES ANALYSIS, Issue 4 2001Marc Henry The empirical relevance of long-memory conditional heteroscedasticity has emerged in a variety of studies of long time series of high frequency financial measurements. A reassessment of the applicability of existing semiparametric frequency domain tools for the analysis of time dependence and long-run behaviour of time series is therefore warranted. To that end, in this paper the averaged periodogram statistic is analysed in the framework of a generalized linear process with long-memory conditional heteroscedastic innovations according to a model specification first proposed by Robinson (Testing for strong serial correlation and dynamic conditional heteroscedasticity in multiple regression. J. Economet. 47 (1991), 67,84). It is shown that the averaged periodogram estimate of the spectral density of a short-memory process remains asymptotically normal with unchanged asymptotic variance under mild moment conditions, and that for strongly dependent processes Robinson's averaged periodogram estimate of long memory (Semiparametric analysis of long memory time series. Ann. Stat. 22 (1994), 515,39) remains consistent. [source] Similarities in the Structural Organization of Major and Minor Ampullate Spider SilkMACROMOLECULAR RAPID COMMUNICATIONS, Issue 9-10 2009Periklis Papadopoulos Abstract Minor and major ampullate spider silks are studied under varying mechanical stress by static and time-resolved FT-IR spectroscopy. This enables one to trace the external mechanical excitation on a microscopic level and to determine for the different moieties the time dependence of the molecular order parameters and corresponding band shifts. It is concluded that the hierarchical nanostructure of both types of silk is similar, being composed of highly oriented nanocrystals, which are interconnected by amorphous chains that obey the worm-like chain model and have a Gaussian distribution of pre-strain. By that it is possible to describe the mechanical properties of both silks by two adjustable parameters only, the center and width of the distribution. For major ampullate silk, the observed variability is small in pronounced contrast to the findings for minor ampullate. [source] Self-Similar Wave of Swelling/Collapse Phase Transition along Polyelectrolyte GelMACROMOLECULAR THEORY AND SIMULATIONS, Issue 8 2004Andrey Yu. Abstract Summary: Theoretical analysis of the possibility of collapse/swelling phase transition propagation along a polyelectrolyte gel thread has been performed. A differential equation that determines the time dependence of the degree of swelling of polymer thread under the radial mechanical force has been obtained. It formally coincides with the equation of diffusion of an impurity in a medium with a certain source density that depends on excluded-volume and Coulomb interactions, osmotic pressure of counterions, and entropy of the subchains. Two stationary points of this equation correspond to contracted and swollen states. It has been shown that once activated, the phase transition between these stationary points could propagate along the thread in the form of a wave with positive speed if the initial state is characterized by a higher level of free energy than the final state. The results of numerical calculations illustrating the process of phase transition propagation along polyelectrolyte gel thread show that propagation of the swelling/collapse phase transition takes place in the form of the stepwise self-similar wave with steep front. The sharp boundary between swollen and collapsed zones is observed and does not tend to dissipate as propagation proceeds. [source] Diffusion time dependence of the apparent diffusion tensor in healthy human brain and white matter diseaseMAGNETIC RESONANCE IN MEDICINE, Issue 6 2001Chris A. Clark Abstract The diffusion time dependence of the brain water diffusion tensor provides information regarding diffusion restriction and hindrance but has received little attention, primarily due to limitations in gradient amplitude available on clinical MRI systems, required to achieve short diffusion times. Using new, more powerful gradient hardware, the diffusion time dependence of tensor-derived metrics were studied in human brain in the range 8,80 ms, which encompasses the shortest diffusion times studied to date. There was no evidence for a change in mean diffusivity, fractional anisotropy, or in the eigenvalues with diffusion time in healthy human brain. The findings are consistent with a model of unrestricted, but hindered water diffusion with semipermeable membranes, likely originating from the extracellular space in which the average extracellular separation is less than 7 microns. Similar findings in two multiple sclerosis plaques indicated that the size of the water diffusion space in the lesion did not exceed this dimension. Magn Reson Med 45:1126,1129, 2001. © 2001 Wiley-Liss, Inc. [source] Amplitude,shape approximation as an extension of separation of variablesMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 3 2008N. Parumasur Abstract Separation of variables is a well-known technique for solving differential equations. However, it is seldom used in practical applications since it is impossible to carry out a separation of variables in most cases. In this paper, we propose the amplitude,shape approximation (ASA) which may be considered as an extension of the separation of variables method for ordinary differential equations. The main idea of the ASA is to write the solution as a product of an amplitude function and a shape function, both depending on time, and may be viewed as an incomplete separation of variables. In fact, it will be seen that such a separation exists naturally when the method of lines is used to solve certain classes of coupled partial differential equations. We derive new conditions which may be used to solve the shape equations directly and present a numerical algorithm for solving the resulting system of ordinary differential equations for the amplitude functions. Alternatively, we propose a numerical method, similar to the well-established exponential time differencing method, for solving the shape equations. We consider stability conditions for the specific case corresponding to the explicit Euler method. We also consider a generalization of the method for solving systems of coupled partial differential equations. Finally, we consider the simple reaction diffusion equation and a numerical example from chemical kinetics to demonstrate the effectiveness of the method. The ASA results in far superior numerical results when the relative errors are compared to the separation of variables method. Furthermore, the method leads to a reduction in CPU time as compared to using the Rosenbrock semi-implicit method for solving a stiff system of ordinary differential equations resulting from a method of lines solution of a coupled pair of partial differential equations. The present amplitude,shape method is a simplified version of previous ones due to the use of a linear approximation to the time dependence of the shape function. Copyright © 2007 John Wiley & Sons, Ltd. [source] Diffusion-weighted MRI measurements on stroke patients reveal water-exchange mechanisms in sub-acute ischaemic lesionsNMR IN BIOMEDICINE, Issue 6 2009J. Lätt Abstract The aim of this study was to investigate the diffusion time dependence of signal- versus - b curves obtained from diffusion-weighted magnetic resonance imaging (DW-MRI) of sub-acute ischaemic lesions in stroke patients. In this case series study, 16 patients with sub-acute ischaemic stroke were examined with DW-MRI using two different diffusion times (60 and 260,ms). Nine of these patients showed sufficiently large lesions without artefacts to merit further analysis. The signal- versus - b curves from the lesions were plotted and analysed using a two-compartment model including compartmental exchange. To validate the model and to aid the interpretation of the estimated model parameters, Monte Carlo simulations were performed. In eight cases, the plotted signal- versus - b curves, obtained from the lesions, showed a signal,curve split-up when data for the two diffusion times were compared, revealing effects of compartmental water exchange. For one of the patients, parametric maps were generated based on the extracted model parameters. These novel observations suggest that water exchange between different water pools is measurable and thus potentially useful for clinical assessment. The information can improve the understanding of the relationship between the DW-MRI signal intensity and the microstructural properties of the lesions. Copyright © 2009 John Wiley & Sons, Ltd. [source] Kinetics of PME/Pi in pig kidneys during cold ischemiaNMR IN BIOMEDICINE, Issue 7 2007Dominik von Elverfeldt Abstract Quality assessment of renal grafts via 31P magnetic resonance spectroscopy (MRS) has been investigated since 1986. As ATP concentrations decay rapidly during cold ischemia, the ratio of phosphomonoesters (PME) to inorganic phosphate (PiO) within the organ (PME/PiO) is commonly used as a quality marker and is considered to be the most reliable parameter. MRS did not lead to any delay in the transplantation procedure since it was performed during the time necessary for immunological matching (cross-match). Differences in the time period until transplantation call for extrapolation of the measured ratio to the end of cold ischemia before correlating with graft performance after transplantation. Therefore, quantitative determination of PME/PiO kinetics is essential. As a model for metabolite decay in human renal grafts, pig kidneys obtained from a slaughterhouse were monitored for up to 80,h via 31P MRS at 2,T. By employing chemical shift imaging (CSI) with a spatial resolution of approximately 1,×,1,×,4,cm3, it was possible to reduce partial volume effects significantly. The improved spectral resolution gained through CSI enabled reliable PME/PiO ratios to be determined only from those voxels containing renal tissue. Spectra were fitted automatically using the magnetic resonance user interface (MRUI), with prior knowledge obtained from unlocalized spectra when necessary. A monoexponential time dependence of PME/PiO for histidine,tryptophane,alpha-ketoglutarate (HTK)-perfused kidneys during cold ischemia was observed, and the determined value of the decay constant , was 0.0099,±,0.0012,h,1. In University of Wisconsin solution (UW)-perfused kidneys, an , of 0.0183,±,0.0053,h,1 was determined. Determination of the decay constant enables a usable extrapolation of PME/PiO for quality assessment of UW perfusion and a reliable extrapolation for HTK-perfused human renal grafts. Copyright © 2007 John Wiley & Sons, Ltd. [source] The ,-particle excited scintillation response of YAG:Ce thin films grown by liquid phase epitaxyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 7 2009Petr Prusa Abstract Y3Al5O12:Ce (YAG:Ce) thin films were grown from PbO-, BaO-, and MoO3 -based fluxes using the liquid phase epitaxy (LPE) method. Photoelectron yield, its time dependence within 0.5,10 ,s shaping time, and energy resolution of these samples were measured under ,-particle excitation. For comparison a sample of the Czochralski grown bulk YAG:Ce single crystal was measured as well. Photoelectron yield values of samples grown from the BaO-based flux were found superior to other LPE films and comparable with that of the bulk single crystal. The same is valid also for the time dependence of photoelectron yield. Obtained results are discussed taking into account the influence of the flux and technology used. Additionally, , particle energy deposition in very thin films is modelled and discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Laser,target interaction during high-power pulsed laser deposition of superconducting thin filmsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 12 2007Ranran Fang Abstract We present a theoretical model to describe the high-power nanosecond pulsed laser ablation of multi-elemental oxide superconductors by considering both the vaporization effect and the plasma shielding effect. Using as an example a YBa2Cu3O7 target, the numerical solutions are obtained by solving the heat flow equations using a finite difference method. We obtain the time dependence of temperature, the transmitted intensity and the ablation rate of the target. The numerical results agree well with the experimental data and are much better than without considering the effects of vaporization and plasma shielding, which indicates that the two effects in high-power nanosecond laser ablation of superconductors must not be neglected. The present model will be helpful for the investigation of superconducting thin films prepared by pulsed laser deposition. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||||||||||||||||||||