16,019 research outputs found
Evaluation of true interlamellar spacing from microstructural observations
A method for evaluating true interlamellar spacing from micrographs is proposed for a multidomained lamellar structure. The microstructure of these materials is assumed to be composed of many domains with the lamellae aligned roughly parallel to each other within each domain and with the domains themselves randomly oriented relative to one another. An explicit expression for the distribution of apparent interlamellar spacing is derived assuming that the distribution of the true interlamellar spacing is Gaussian. The average interlamellar spacing is close to the peak interlamellar spacing observed in the distribution. The theoretical distributions are compared with experimental ones obtained by analyzing micrographs of PbTeâSb2Te3 lamellar composites
Zone Leveling Crystal Growth of Thermoelectric PbTe Alloys with Sb_(2)Te_3 WidmanstÀtten Precipitates
Unidirectional solidification of PbTe-rich alloys in the pseudobinary PbTe-Sb_(2)Te_3 system using the zone leveling technique enables the production of large regions of homogeneous solid solutions for the formation of precipitate nanocomposites as compared with Bridgman solidification. (PbTe)_(0.940)(Sb_(2)Te_3)_(0.060) and (PbTe)_(0.952)(Sb_(2)Te_3)_(0.048) alloys were successfully grown using (PbTe)_(0.4)(Sb_(2)Te_3)_(0.6) and (PbTe)_(0.461)(Sb_(2)Te_3)_(0.539) as seed alloys, respectively, with 1 mm h^(â1) withdrawal velocity. In the unidirectionally solidified regions of both alloys, Widmanstatten precipitates are formed due to the decrease in solubility of Sb_(2)Te_3 in PbTe. To determine the compositions of the seed alloys for the zone leveling experiments, the solute distribution in solidification in the PbTe-richer part of the pseudobinary PbTe-Sb_(2)Te_3 system has been examined from the concentration profiles in the samples unidirectionally solidified by the Bridgman method
Vortex-lattice melting in two-dimensional superconductors in intermediate fields
To examine the field dependence of the vortex lattice melting transition in
two-dimensional (2D) superconductors, Monte Carlo simulations of the 2D
Ginzburg-Landau (GL) model are performed by extending the conventional lowest
Landau level (LL) approximation to include several {\it higher} LL modes of the
superconducting order parameter with LL indices up to six. It is found that a
nearly vertical melting line in lower fields, which is familiar within the
elastic theory, is reached just by including higher LL modes with LL indices
less than five, and that the first order character of the melting transition in
higher fields is significantly weakened with decreasing the field.
Nevertheless, a genuine crossover to the consecutive continuous melting picture
intervened by a hexatic liquid is not found within the use of the GL model.Comment: 6 pages, 7 figures. To appear in Phys. Rev.
Rapid consolidation of powdered materials by induction hot pressing
A rapid hot press system in which the heat is supplied by RF induction to rapidly consolidate thermoelectric materials is described. Use of RF induction heating enables rapid heating and consolidation of powdered materials over a wide temperature range. Such rapid consolidation in nanomaterials is typically performed by spark plasma sintering (SPS) which can be much more expensive. Details of the system design, instrumentation, and performance using a thermoelectric material as an example are reported. The Seebeck coefficient, electrical resistivity, and thermal diffusivity of thermoelectric PbTe material pressed at an optimized temperature and time in this system are shown to agree with material consolidated under typical consolidation parameters
Effective thermal conductivity of polycrystalline materials with randomly oriented superlattice grains
A model has been established for the effective thermal conductivity of a bulk polycrystal made of randomly oriented superlattice grains with anisotropic thermal conductivity. The in-plane and cross-plane thermal conductivities of each superlattice grain are combined using an analytical averaging rule that is verified using finite element methods. The superlattice conductivities are calculated using frequency dependent solutions of the Boltzmann transport equation, which capture greater thermal conductivity reductions as compared to the simpler gray medium approximation. The model is applied to a PbTe/Sb_2Te_3 nanobulk material to investigate the effects of period, specularity, and temperature. The calculations show that the effective thermal conductivity of the polycrystal is most sensitive to the in-plane conductivity of each superlattice grain, which is generally four to five times larger than the cross-plane conductivity of a grain. The model is compared to experimental measurements of the same system for periods ranging from 287 to 1590 nm and temperatures from 300 to 500 K. The comparison suggests that the effective specularity increases with increasing annealing temperature and shows that these samples are in a mixed regime where both Umklapp and boundary scattering are important
Self-interactions in a topological BF-type model in D=5
All consistent interactions in five spacetime dimensions that can be added to
a free BF-type model involving one scalar field, two types of one-forms, two
sorts of two-forms, and one three-form are investigated by means of deforming
the solution to the master equation with the help of specific cohomological
techniques. The couplings are obtained on the grounds of smoothness, locality,
(background) Lorentz invariance, Poincar\'{e} invariance, and the preservation
of the number of derivatives on each field.Comment: LaTeX, 57 pages, final version, matching the published pape
Geometric Langevin equations on submanifolds and applications to the stochastic melt-spinning process of nonwovens and biology
In this article we develop geometric versions of the classical Langevin
equation on regular submanifolds in euclidean space in an easy, natural way and
combine them with a bunch of applications. The equations are formulated as
Stratonovich stochastic differential equations on manifolds. The first version
of the geometric Langevin equation has already been detected before by
Leli\`evre, Rousset and Stoltz with a different derivation. We propose an
additional extension of the models, the geometric Langevin equations with
velocity of constant absolute value. The latters are seemingly new and provide
a galaxy of new, beautiful and powerful mathematical models. Up to the authors
best knowledge there are not many mathematical papers available dealing with
geometric Langevin processes. We connect the first version of the geometric
Langevin equation via proving that its generator coincides with the generalized
Langevin operator proposed by Soloveitchik, Jorgensen and Kolokoltsov. All our
studies are strongly motivated by industrial applications in modeling the fiber
lay-down dynamics in the production process of nonwovens. We light up the
geometry occuring in these models and show up the connection with the spherical
velocity version of the geometric Langevin process. Moreover, as a main point,
we construct new smooth industrial relevant three-dimensional fiber lay-down
models involving the spherical Langevin process. Finally, relations to a class
of self-propelled interacting particle systems with roosting force are
presented and further applications of the geometric Langevin equations are
given
QP-Structures of Degree 3 and 4D Topological Field Theory
A A BV algebra and a QP-structure of degree 3 is formulated. A QP-structure
of degree 3 gives rise to Lie algebroids up to homotopy and its algebraic and
geometric structure is analyzed. A new algebroid is constructed, which derives
a new topological field theory in 4 dimensions by the AKSZ construction.Comment: 17 pages, Some errors and typos have been correcte
Topological Field Theories and Geometry of Batalin-Vilkovisky Algebras
The algebraic and geometric structures of deformations are analyzed
concerning topological field theories of Schwarz type by means of the
Batalin-Vilkovisky formalism. Deformations of the Chern-Simons-BF theory in
three dimensions induces the Courant algebroid structure on the target space as
a sigma model. Deformations of BF theories in dimensions are also analyzed.
Two dimensional deformed BF theory induces the Poisson structure and three
dimensional deformed BF theory induces the Courant algebroid structure on the
target space as a sigma model. The deformations of BF theories in
dimensions induce the structures of Batalin-Vilkovisky algebras on the target
space.Comment: 25 page
Thermal fluctuations and disorder effects in vortex lattices
We calculate using loop expansion the effect of fluctuations on the structure
function and magnetization of the vortex lattice and compare it with existing
MC results. In addition to renormalization of the height of the Bragg peaks of
the structure function, there appears a characteristic saddle shape ''halos''
around the peaks. The effect of disorder on magnetization is also calculated.
All the infrared divergencies related to soft shear cancel.Comment: 10 pages, revtex file, one figur
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