High-temperature thermal storage systems for advanced solar receivers materials selections

Advanced space power systems that use solar energy and Brayton or Stirling heat engines require thermal energy storage (TES) systems to operate continuously through periods of shade. The receiver storage units, key elements in both Brayton and Stirling systems, are designed to use the latent heat of fusion of phase-change materials (PCMs). The power systems under current consideration for near-future National Aeronautics and Space Administration space missions require working fluid temperatures in the 1100 to 1400 K range. The PCMs under current investigation that gave liquid temperatures within this range are the fluoride family of salts. However, these salts have low thermal conductivity, which causes large temperature gradients in the storage systems. Improvements can be obtained, however, with the use of thermal conductivity enhancements or metallic PCMs. In fact, if suitable containment materials can be found, the use of metallic PCMs would virtually eliminate the orbit associated temperature variations in TES systems. The high thermal conductivity and generally low volume change on melting of germanium and alloys based on silicon make them attractive for storage of thermal energy in space power systems. An approach to solving the containment problem, involving both chemical and physical compatibility, preparation of NiSi/NiSi2, and initial results for containment of germanium and NiSi/NiSi2, are presented

The fractal structure of the universe : a new field theory approach

While the universe becomes more and more homogeneous at large scales, statistical analysis of galaxy catalogs have revealed a fractal structure at small-scales (\lambda < 100 h^{-1} Mpc), with a fractal dimension D=1.5-2 (Sylos Labini et al 1996). We study the thermodynamics of a self-gravitating system with the theory of critical phenomena and finite-size scaling and show that gravity provides a dynamical mechanism to produce this fractal structure. We develop a field theoretical approach to compute the galaxy distribution, assuming them to be in quasi-isothermal equilibrium. Only a limited, (although large), range of scales is involved, between a short-distance cut-off below which other physics intervene, and a large-distance cut-off, where the thermo- dynamic equilibrium is not satisfied. The galaxy ensemble can be considered at critical conditions, with large density fluctuations developping at any scale. From the theory of critical phenomena, we derive the two independent critical exponents nu and eta and predict the fractal dimension D = 1/nu to be either 1.585 or 2, depending on whether the long-range behaviour is governed by the Ising or the mean field fixed points, respectively. Both set of values are compatible with present observations. In addition, we predict the scaling behaviour of the gravitational potential to be r^{-(1 + eta)/2}. That is, r^{-0.5} for mean field or r^{- 0.519} for the Ising fixed point. The theory allows to compute the three and higher density correlators without any assumption or Ansatz. We find that the N-points density scales as r_1^{(N-1)(D-3)}, when r_1 >> r_i, 2 leq i leq N . There are no free parameters in this theory.Comment: Latex, 20 pages, no figures, to be published in the Astrophysical Journa

The dimension of loop-erased random walk in 3D

We measure the fractal dimension of loop-erased random walk (LERW) in 3 dimensions, and estimate that it is 1.62400 +- 0.00005. LERW is closely related to the uniform spanning tree and the abelian sandpile model. We simulated LERW on both the cubic and face-centered cubic lattices; the corrections to scaling are slightly smaller for the face-centered cubic lattice.Comment: 4 pages, 4 figures. v2 has more data, minor additional change

Analytical infrared intensities for periodic systems with local basis sets

We report a method for the efficient evaluation of analytic infrared (IR) intensities within generalized Kohn-Sham density functional theory using Gaussian orbitals and periodic boundary conditions. A discretized form of the Berry phase is used to evaluate a periodic dipole moment and its derivatives with respect to in-phase nuclear coordinate displacements. Benchmark calculations are presented for one-dimensional chains of water molecules and poly(paraphenylenevinylene).Comment: 16 pages, 3 figures, 4 tables, submitted to Phys. Rev.

Systematic Renormalization in Hamiltonian Light-Front Field Theory: The Massive Generalization

Hamiltonian light-front field theory can be used to solve for hadron states in QCD. To this end, a method has been developed for systematic renormalization of Hamiltonian light-front field theories, with the hope of applying the method to QCD. It assumed massless particles, so its immediate application to QCD is limited to gluon states or states where quark masses can be neglected. This paper builds on the previous work by including particle masses non-perturbatively, which is necessary for a full treatment of QCD. We show that several subtle new issues are encountered when including masses non-perturbatively. The method with masses is algebraically and conceptually more difficult; however, we focus on how the methods differ. We demonstrate the method using massive phi^3 theory in 5+1 dimensions, which has important similarities to QCD.Comment: 7 pages, 2 figures. Corrected error in Eq. (11), v3: Added extra disclaimer after Eq. (2), and some clarification at end of Sec. 3.3. Final published versio

Factorization fits to charmless strangeless B decays

We present fits to charmless strangeless hadronic B decay data for mean branching ratios and CP-violating asymmetries using the QCD factorization model of Beneke et al. Apart from one CP-violating parameter, the model gives a very good representation of 26 measured data. We find the CKM angle alpha = (93.5 +/- 8.4 -1.3) degrees and to be quite stable to plausible "charming penguin" corrections.Comment: 4 pages, LaTeX, Minor changes to text, references adde

High-Accuracy Calculations of the Critical Exponents of Dyson's Hierarchical Model

We calculate the critical exponent gamma of Dyson's hierarchical model by direct fits of the zero momentum two-point function, calculated with an Ising and a Landau-Ginzburg measure, and by linearization about the Koch-Wittwer fixed point. We find gamma= 1.299140730159 plus or minus 10^(-12). We extract three types of subleading corrections (in other words, a parametrization of the way the two-point function depends on the cutoff) from the fits and check the value of the first subleading exponent from the linearized procedure. We suggest that all the non-universal quantities entering the subleading corrections can be calculated systematically from the non-linear contributions about the fixed point and that this procedure would provide an alternative way to introduce the bare parameters in a field theory model.Comment: 15 pages, 9 figures, uses revte

The role of working hours, work environment and physical leisure activity on the need for recovery following a day's work among UK white-water raft guides: a within-subjects multilevel approach

Background: White-water raft guides are a growing workforce of the outdoor sector but little is known about how the working environment, workload and physical leisure activity impacts on the need for occupational recovery (the desire to replenish internal resources and recuperate in the time immediately following work) of those working in this physically demanding occupation. Methods: Longitudinal data were collected across an eight month working season at three month intervals. Multilevel analyses tested the within-subject associations between work environment, hours worked and physical leisure activity had on the need for recovery. Results: Working longer across the working season and participating in more physical leisure activity were directly associated with a lower need for occupational recovery. Furthermore, working on natural rivers significantly reduced the need for recovery experienced compared to work on man-made courses. This was regardless of the number of hours of worked in these environments. Discussion: Physical leisure activity may provide a distraction from work, allowing employees to replenish their physical and psychological energy, thus protecting themselves against work-related fatigue. The findings also expand upon the previous literature identifying that working in a natural environment reduces the risk of experiencing work-related fatigue