Probabilistic modeling of one dimensional water movement and leaching from highway embankments containing secondary materials

Predictive methods for contaminant release from virgin and secondary road construction materials are important for evaluating potential long-term soil and groundwater contamination from highways. The objective of this research was to describe the field hydrology in a highway embankment and to investigate leaching under unsaturated conditions by use of a contaminant fate and transport model. The HYDRUS2D code was used to solve the Richards equation and the advection–dispersion equation with retardation. Water flow in a Minnesota highway embankment was successfully modeled in one dimension for several rain events after Bayesian calibration of the hydraulic parameters against water content data at a point 0.32 m from the surface of the embankment. The hypothetical leaching of Cadmium from coal fly ash was probabilistically simulated in a scenario where the top 0.50 m of the embankment was replaced by coal fly ash. Simulation results were compared to the percolation equation method where the solubility is multiplied by the liquid-to-solid ratio to estimate total release. If a low solubility value is used for Cadmium, the release estimates obtained using the percolation/equilibrium model are close to those predicted from HYDRUS2D simulations (10–4–10–2 mg Cd/kg ash). If high solubility is used, the percolation equation over predicts the actual release (0.1–1.0 mg Cd/kg ash). At the 90th percentile of uncertainty, the 10-year liquid-to-solid ratio for the coal fly ash embankment was 9.48 L/kg, and the fraction of precipitation that infiltrated the coal fly ash embankment was 92%. Probabilistic modeling with HYDRUS2D appears to be a promising realistic approach to predicting field hydrology and subsequent leaching in embankments

Reducing Zero-point Systematics in Dark Energy Supernova Experiments

We study the effect of filter zero-point uncertainties on future supernova dark energy missions. Fitting for calibration parameters using simultaneous analysis of all Type Ia supernova standard candles achieves a significant improvement over more traditional fit methods. This conclusion is robust under diverse experimental configurations (number of observed supernovae, maximum survey redshift, inclusion of additional systematics). This approach to supernova fitting considerably eases otherwise stringent mission calibration requirements. As an example we simulate a space-based mission based on the proposed JDEM satellite; however the method and conclusions are general and valid for any future supernova dark energy mission, ground or space-based.Comment: 30 pages,8 figures, 5 table, one reference added, submitted to Astroparticle Physic

Galaxy Selection and Clustering and Lyman alpha Absorber Identification

The effects of galaxy selection on our ability to constrain the nature of weak Ly\alpha absorbers at low redshift are explored. Current observations indicate the existence of a population of gas-rich, low surface brightness (LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha absorption. Absorption arising in LSB galaxies may be attributed to HSB galaxies at larger impact parameters from quasar lines of sight, so that the observed absorption cross sections of galaxies may seem unreasonably large. Thus it is not possible to rule out scenarios where LSB galaxies make substantial contributions to Ly\alpha absorption using direct observations. Less direct tests, where observational selection effects are taken into account using simulations, should make it possible to determine the nature of Ly\alpha absorbers by observing a sample of ~100 galaxies around quasar lines of sight with well-defined selection criteria. Such tests, which involve comparing simulated and observed plots of the unidentified absorber fractions and absorbing galaxy fractions versus impact parameter, can distinguish between scenarios where absorbers arise in particular galaxies and those where absorbers arise in gas tracing the large scale galaxy distribution. Care must be taken to minimize selection effects even when using these tests. Results from such tests are likely to depend upon the limiting neutral hydrogen column density. While not enough data are currently available to make a strong conclusion about the nature of moderately weak absorbers, evidence is seen that such absorbers arise in gas that is around or between galaxies that are often not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa

Effect of Void Network on CMB Anisotropy

We study the effect of a void network on the CMB anisotropy in the Einstein-de Sitter background using Thompson &Vishniac's model. We consider comprehensively the Sacks-Wolfe effect, the Rees-Sciama effect and the gravitational lensing effect. Our analysis includes the model of primordial voids existing at recombination, which is realized in some inflationary models associated with a first-order phase transition. If there exist primordial voids whose comoving radius is larger than $\sim10h^{-1}$Mpc at recombination, not only the Sachs-Wolfe effect but also the Rees-Sciama effect is appreciable even for multipoles l\lsim1000 of the anisotropy spectrum. The gravitational lensing effect, on the other hand, slightly smoothes the primary anisotropy; quantitatively, our results for the void model are similar to the previous results for a CDM model. All the effects, together, would give some constraints on the configuration or origin of voids with high-resolution data of the CMB anisotropy.Comment: 23 pages, latex, 12 eps figures, some calculations and discussions are added, to appear in ApJ 510 (1999

Cosmological Models and Latest Observational Data

In this note, we consider the observational constraints on some cosmological models by using the 307 Union type Ia supernovae (SNIa), the 32 calibrated Gamma-Ray Bursts (GRBs) at $z>1.4$, the updated shift parameter $R$ from WMAP 5-year data (WMAP5), and the distance parameter $A$ of the measurement of the baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red galaxies with the updated scalar spectral index $n_s$ from WMAP5. The tighter constraints obtained here update the ones obtained previously in the literature.Comment: 10 pages, 5 figures, 1 table, revtex4; v2: discussions added, accepted by Eur. Phys. J. C; v3: published versio

Spatiotemporal dynamics of discrete sine-Gordon lattices with sinusoidal couplings

The spatiotemporal dynamics of a damped sine-Gordon chain with sinusoidal nearest-neighbor couplings driven by a constant uniform force are discussed. The velocity characteristics of the chain versus the external force is shown. Dynamics in the high- and low-velocity regimes are investigated. It is found that in the high-velocity regime, the dynamics is dominated by rotating modes, the velocity shows a branching bifurcation feature, while in the low-velocity regime, the velocity exhibits step-like dynamical transitions, broken by the destruction of strong resonances.Comment: 10 Revtex pages, 8 Eps figures, to appear in Phys. Rev.E 57(1998

Effect of in-plane magnetic field on the photoluminescence spectrum of modulation-doped quantum wells and heterojunctions

The photoluminescence (PL) spectrum of modulation-doped GaAs/AlGaAs quantum wells (MDQW) and heterojunctions (HJ) is studied under a magnetic field ($B_{\|}$) applied parallel to the two-dimensional electron gas (2DEG) layer. The effect of $B_{\|}$ strongly depends on the electron-hole separation ($d_{eh}$), and we revealed remarkable $B_{\|}$-induced modifications of the PL spectra in both types of heterostructures. A model considering the direct optical transitions between the conduction and valence subband that are shifted in k-space under $B_{\|}$, accounts qualitatively for the observed spectral modifications. In the HJs, the PL intensity of the bulk excitons is strongly reduced relatively to that of the 2DEG with increasing $B_{\|}$. This means that the distance between the photoholes and the 2DEG decreases with increased $B_{\|}$, and that free holes are responsible for the hole-2DEG PL.Comment: 6pages, 5figure

Effects of interface morphology and geometry on the thermoelectric properties of artificially structured ZnO-based thin-films

Thermoelectricity may play a major role in waste heat recovery of fossil fuel consuming devices. Unfortunately thermoelectric generators to date only have poor conversion efficiencies (5 %). One way to improve the efficiency is to improve the performance of the active thermoelectric material. For this the figure of merit Z is given by Z=(S^2 sigma)/kappa, where S denotes the Seebeck coefficient, sigma the electrical conductivity, and kappa; the thermal conductivity. Z can be improved by either increasing the numerator S^2 sigma; (the so called power factor) or decreasing the denominator. The typical and best understood thermoelectric materials so far are based on Te, such as Bi2Te3 or PbTe. Unfortunately, for a mass application of thermoelectric devices, estimations show that the tellurium resources will be consumed very quickly. Hence it is worth trying to develop novel thermoelectric materials which are more sustainable and “green”. Exemplarily the thermoelectric properties of ZnO as an ideal model system were investigated in the framework of this thesis. Main goal of the work was to get a better understanding of the influence of effects on the microscopic length scale (e.g. due to thin-films, grain boundaries, artificial structuring) on the macroscopic behavior of the sample. In this context the following results were found: Investigations of degenerately doped thin ZnO:Al films and subsequent annealing in air showed that at very high carrier concentrations, where the samples have metallic character, a sign reversal of S may occur. Although the sample is clearly n-type, small positive Seebeck coefficients can be measured, changing their sign with decreasing temperature. This is due to changes of the density of states at the Fermi-energy in a degenerately doped semiconductor. The energy filtering effect due to grain boundaries, e.g. the increase of the power factor with increasing carrier concentration only works to a certain extend: If the carrier concentration n exceeds a certain value, screening effects diminish the barrier height and width leading to a decrease of the power factor. Concerning the investigation of interfaces first measurements on a multilayer sample series of alternating ZnO/ZnS layers in in-plane geometry gave hints for the formation of interface layers of very high electrical conductivity between ZnO and ZnS, dominating the transport behaviour at large layer thicknesses (d > 100 nm). At smaller d, where d becomes comparable to the typical fluctuation length of the interface roughness, the transport path and hence the thermoelectric properties are strongly determined by the surface fluctuations. These results could be approved qualitatively by simulations within a Network Model (NeMo). Stronger impact on the thermoelectric parameters, especially on the thermal conductivity, were found in cross plane direction, i.e. perpendicular to the interfaces. Unfortunately measurements of multilayers in cross-plane direction are very difficult to perform. To overcome this problem lateral structuring of thin-films offers attractive possibilities. To realize bar structures of alternating materials the method of self-aligned pattern transfer was developed and employed. Measurements perpendicular to the interfaces show that the number of interfaces as well as their shape (i.e. length) and morphology has a strong influence on the power factor. Supported by numerous NeMo simulations the results indicated that the thermoelectric properties across the sample are dominated by the shortest path of electrical conductance. The transport path is strongly influenced by assuming space-charge regions of different width and conductivity. Best agreement between experiment and simulations has been achieved by replacing a certain fraction of the lowly conducting material with a highly conducting space-charge region. However, the origin of this highly conducting surface region requires further clarifications. The findings of this work suggest that due to its high Seebeck coefficients and the possibility to tune the electrical conductivity by doping, ZnO is a promising candidate for an environmentally friendly and sustainable n-type thermoelectric material. The fact that its thermal conductivity is quite high may be overcome by a combination with ZnS. However this back door shown by theory still needs to be approved by experiment.Thermoelektrizität kann eine wichtige Rolle bei der Nutzung der bei der Verbrennung fossiler Rohstoffe entstehenden Abwärme spielen. Leider weisen thermoelektrische Generatoren bisher nur geringe Wirkungsgrade (5%) auf. Eine Möglichkeit, die Effizienz zu verbessern, ist die Leistung des thermoelektrisch aktiven Materials zu verbessern. Kennzahl dafür ist der Gütefaktor Z Z=(S^2 sigma)/kappa, wobei S den Seebeck-Koeffizienten, sigma die elektrische Leitfähigkeit und kappa die thermische Leitfähigkeit bezeichnen. Z kann entweder durch Erhöhen des Zählers S^2 sigma (der sog. Leistungsfaktor) oder Verringern des Nenners verbessert werden. Die zurzeit typischen und am besten verstandenen thermoelektrischen Materialien basieren auf Tellur (Te), wie Bi2Te3 oder PbTe. Für eine breite Anwendung thermoelektrischer Bauteile zeigen allerdings Abschätzungen, dass die Tellurvorkommen schnell aufgebraucht sein werden. Somit macht es Sinn, neue nachhaltige und „grüne“ Materialien zu untersuchen. Beispielhaft wurden dafür innerhalb dieser Arbeit die thermoelektrischen Eigenschaften des idealen Modellsystems ZnO untersucht. Hauptziel dabei war es, die Auswirkungen der Effekte auf mikroskopischer Ebene (z. B. durch Dünnschichten, Korngrenzen, künstliche Strukturierung) auf das makroskopische Verhalten der Probe besser zu verstehen. In diesem Zusammenhang wurden folgende Ergebnisse gefunden: Untersuchungen an entartet dotierten - und anschließend an Luft getemperten ZnO:Al Schichten zeigen, dass bei sehr hohen Ladungsträgerkonzentrationen, bei denen die Proben metallischen Charakter aufweisen, ein Vorzeichenwechsel von S stattfindet. Obwohl die Proben klar n-Typ sind, konnten kleine positive Seebeck-Koeffizienten gemessen werden, die mit abnehmender Temperatur das Vorzeichen wechselten. Dies kann Änderungen in der Zustandsdichte am Ferminiveau dieses entarteten Halbleiters zugeschrieben werden. Der Energie-Filter Effekt bedingt durch Korngrenzen, d. h. das Ansteigen des Leistungsfaktors mit steigender Ladungsträgerkonzentration, konnte nur bis zu einem gewissen Grad beobachtet werden: Falls nämlich die Ladungsträgerkonzentration einen bestimmten Wert übersteigt, verringern sogenannte Abschirmungseffekte die Barrieren Höhe und - Breite, was wiederum zu einer Verkleinerung des Leistungsfaktors führt. Im Hinblick auf die Charakterisierung von Grenzflächen wurden erste Messungen an Übergittern aus alternierenden ZnO/ZnS Schichten in „in-plane“ Geometrie durchgeführt. Die Ergebnisse ließen auf die Ausbildung elektrisch hochleitender Grenzschichten zwischen ZnO und ZnS schließen, welche das Transportverhalten bei hohen Schichtdicken (d > 100 nm) dominieren. Zu geringeren Schichtdicken hin, wo d mit der typischen Oberflächenrauigkeit vergleichbar wird, sind die Transportpfade und damit auch die thermoelektrischen Eigenschaften stark durch Oberflächenfluktuationen bestimmt. Diese Ergebnisse konnten auch qualitativ durch Simulationen innerhalb eines Netzwerkmodells (NeMo) bestätigt werden. Ein stärkerer Einfluss auf die thermoelektrischen Parameter, insbesondere auf die Wärmeleitfähigkeit, wurde in der Literatur in „cross-plane“ Geometrie, d. h. senkrecht zur Grenze, gefunden. Unglücklicherweise sind Messungen an Übergittern in dieser Geometrie sehr schwer durchzuführen. Um dieses Problem zu umgehen bietet die laterale Strukturierung dünner Schichten attraktive Möglichkeiten. Zur Realisierung einer Stegstruktur aus abwechselnden Materialien wurde die Methode der selbstausrichtenden Strukturübertragung im Rahmen dieser Arbeit entwickelt und angewendet. Messungen senkrecht zu den Grenzen zeigen, dass die Anzahl der Grenzen sowie deren Gestalt (d. h. Länge) und Morphologie einen erheblichen Einfluss auf den Leistungsfaktor nehmen. Unterstützt von zahlreichen NeMo Simulationen zeigten die Ergebnisse, dass die thermoelektrischen Eigenschaften über die strukturierte Probe hinweg vom elektrisch kürzesten Transportpfad dominiert werden. Dieser wiederum hängt stark von der Annahme sogenannter Grenzflächenregionen verschiedener Breite und Leitfähigkeit ab. Beste Übereinstimmung zwischen Experiment und Simulationen wurde unter der Annahme erreicht, dass ein bestimmter Teil des schlecht leitenden Materials durch eine hochleitende Grenzflächenregion ersetzt wird. Der Ursprung dieser hochleitenden Region konnte jedoch noch nicht geklärt werden. Die Ergebnisse dieser Arbeit zeigen, dass aufgrund seiner hohen Seebeck-Koeffizienten und der Möglichkeiten durch Dotieren die elektrische Leitfähigkeit einzustellen, ZnO ein geeignetes Materialsystem für umweltfreundliche und nachhaltige thermoelektrische Anwendungen ist. Das Problem, dass es eine hohe Wärmeleitfähigkeit aufweist, könnte durch eine geeignete Kombination mit ZnS gelöst werden. Dieses von der Theorie gezeigte Hintertürchen konnte bislang jedoch noch nicht experimentell bestätigt werden