805 research outputs found
Heat Transfer Mechanisms in Porous Materials and Contemporary Problems in Thermophysical Properties Investigations: Analyses and Solutions
This article is an overview of the topical problems in the investigation of thermophysical properties and the development of a database for porous materials. Determination of both apparent/measured and true thermophysical properties is discussed taking into account combined heat and mass transfer, latent heat effects during chemical and physical transformations, as well as structural changes. The approaches to the solution of these problems are demonstrated for a number of different classes of materials: Industrial refractories, ceramics, highly porous insulation; Moist materials and materials undergoing phase, chemical and structural transformations; Materials semitransparent for heat radiation.
The approaches being used in the development of a thermophysical properties database consist in a combination of theoretical and experimental methods.
The analysis, generalization, and extrapolation of available reference data can be conducted based on the models for classical (conduction, heat radiation, gas convection) and additional (novel) mechanisms and processes affecting the apparent thermophysical properties. The novel heat transfer mechanisms include: Heterogeneous heat and mass transfer processes occurring in pores existing at grain boundaries and in cracks, in particular, surface segregation and diffusion of impurities on pore surfaces and transport of gases produced from chemical reactions, evaporation, and sublimation. Microstructure changes due to non-uniform thermal expansion of particles and grains. These changes are caused by the mismatch of thermal expansion coefficients of different phases in the material and anisotropic thermal expansion of crystals
Nonlinear Micromechanical Casimir Oscillator
The Casimir force between uncharged metallic surfaces originates from quantum
mechanical zero point fluctuations of the electromagnetic field. We demonstrate
that this quantum electrodynamical effect has a profound influence on the
oscillatory behavior of microstructures when surfaces are in close proximity
(<= 100 nm). Frequency shifts, hysteretic behavior and bistability caused by
the Casimir force are observed in the frequency response of a periodically
driven micromachined torsional oscillator.Comment: 4 pages, 4 figures; added and rearranged references; added comments
on sensitivit
Theory of Neutron Diffraction from the Vortex Lattice in UPt3
Neutron scattering experiments have recently been performed in the
superconducting state of UPt3 to determine the structure of the vortex lattice.
The data show anomalous field dependence of the aspect ratio of the unit cell
in the B phase. There is apparently also a change in the effective coherence
length on the transition from the B to the C phases. Such observations are not
consistent with conventional superconductvity. A theory of these results is
constructed based on a picture of two-component superconductivity for UPt3. In
this way, these unusual observations can be understood. There is a possible
discrepancy between theory and experiment in the detailed field dependence of
the aspect ratio.Comment: 11 pages; uses REVTEX, APS and PRABIB styles; 2 Postscript figure
files include
Arithmetically Cohen-Macaulay Bundles on complete intersection varieties of sufficiently high multidegree
Recently it has been proved that any arithmetically Cohen-Macaulay (ACM)
bundle of rank two on a general, smooth hypersurface of degree at least three
and dimension at least four is a sum of line bundles. When the dimension of the
hypersurface is three, a similar result is true provided the degree of the
hypersurface is at least six. We extend these results to complete intersection
subvarieties by proving that any ACM bundle of rank two on a general, smooth
complete intersection subvariety of sufficiently high multi-degree and
dimension at least four splits. We also obtain partial results in the case of
threefolds.Comment: 15 page
Perturbation of Tunneling Processes by Mechanical Degrees of Freedom in Mesoscopic Junctions
We investigate the perturbation in the tunneling current caused by
non-adiabatic mechanical motion in a mesoscopic tunnel junction. A theory
introduced by Caroli et al. \cite{bi1,bi2,bi3} is used to evaluate second order
self-energy corrections for this non-equilibrium situation lacking
translational invariance. Inelastic signatures of the mechanical degrees of
freedom are found in the current-voltage characteristics. These give
rise to sharp features in the derivative spectrum, .Comment: 22 pages LaTeX + 3 uuencoded PS picture
The Canonical Model of a Singular Curve
We give refined statements and modern proofs of Rosenlicht's results about
the canonical model C' of an arbitrary complete integral curve C. Notably, we
prove that C and C' are birationally equivalent if and only if C is
nonhyperelliptic, and that, if C is nonhyperelliptic, then C' is equal to the
blowup of C with respect to the canonical sheaf \omega. We also prove some new
results: we determine just when C' is rational normal, arithmetically normal,
projectively normal, and linearly normal.Comment: 28 pages, no figures, IV Congresso Iberoamericano de Geometria
Complex
Moduli of mathematical instanton vector bundles with odd c_2 on projective space
The problem of irreducibility of the moduli space I_n of rank-2 mathematical
instanton vector bundles with arbitrary positive second Chern class n on the
projective 3-space is considered. The irreducibility of I_n was known for small
values of n: Barth 1977 (n=1), Hartshorne 1978 (n=2), Ellingsrud and Stromme
1981 (n=3), Barth 1981 (n=4), Coanda, Tikhomirov and Trautmann 2003 (n=5). In
this paper we prove the irreducibility of I_n for an arbitrary odd n.Comment: 62 page
Inversion of provenance data and sediment load into spatially varying erosion rates
Sediment fingerprinting methods aim to determine the relative contribution of different source areas in detrital sediments based on natural properties – fingerprints – of the source areas. Here, we use U/Th–Pb age signatures as fingerprints, assuming that the age signal is not altered during erosion–transportation–deposition events, and given that recent technological advances enable precise dating of large amounts of grains. We introduce a formal inversion method that allows to disentangle the amalgamation of source contributions in detrital zircon data and enables to convert this information into an erosion rate map starting from the spatial distribution of zircon age signatures. Relying on the least‐squares method and using prior and covariance information to deal with non‐uniqueness, we show, using synthetic and natural examples, that we are able to retrieve erosion rate patterns of a catchment when the age distribution and zircon fertility for each source area are well known. Moreover, we show that not only zircon age fingerprints but also other tracers such as mineral content can be used. Furthermore, we found that adding data from samples taken at the outlet of tributaries improves the estimation of erosion rate patterns. We conclude that the least squares inverse model applied to detrital data has great potential for investigating erosion rates
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