192 research outputs found
Exchange Effects in the Invar Hardening: as a test case
An increase of the critical resolved shear stress of Invar alloys (Invar
hardening) with a lowering temperature is explained. The effect is caused by a
growth of the exchange interaction between dangling -electron states of
dislocation cores and paramagnetic obstacles (e.g., Ni atoms in FeNi alloys)
which occurs below the Curie temperature. The spins of the two electrons align
along the magnetization due to the exchange interaction with the surrounding
atoms of the ferromagnetic. The exchange interaction between the dislocations
and obstacles is enhanced in Invars due to a strong growth of the magnetic
moments of atoms under the action of elastic strains near the dislocation
cores. Parameters characterizing the exchange interaction are determined for
the case of the FeNi Invar. The influence of the internal
magnetic field on the dislocation detachment from the obstacles is taken into
account. The obtained temperature dependence of the critical resolved shear
stress in the FeNi Invar agrees well with the available
experimental data. Experiments facilitating a further check of the theoretical
model are suggested.Comment: 8 pages, 2 figure
Fine structure of the diamagnetic cavity boundary in comet Halley
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95105/1/jgra16786.pd
Local structure study of In_xGa_(1-x)As semiconductor alloys using High Energy Synchrotron X-ray Diffraction
Nearest and higher neighbor distances as well as bond length distributions
(static and thermal) of the In_xGa_(1-x)As (0<x<1) semiconductor alloys have
been obtained from high real-space resolution atomic pair distribution
functions (PDFs). Using this structural information, we modeled the local
atomic displacements in In_xGa_(1-x)As alloys. From a supercell model based on
the Kirkwood potential, we obtained 3-D As and (In,Ga) ensemble averaged
probability distributions. This clearly shows that As atom displacements are
highly directional and can be represented as a combination of and
displacements. Examination of the Kirkwood model indicates that the standard
deviation (sigma) of the static disorder on the (In,Ga) sublattice is around
60% of the value on the As sublattice and the (In,Ga) atomic displacements are
much more isotropic than those on the As sublattice. The single crystal diffuse
scattering calculated from the Kirkwood model shows that atomic displacements
are most strongly correlated along directions.Comment: 10 pages, 12 figure
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A model of melt pond evolution on sea ice
A one-dimensional, thermodynamic, and radiative model of a melt pond on sea ice is presented that explicitly treats the melt pond as an extra phase. A two-stream radiation model, which allows albedo to be determined from bulk optical properties, and a parameterization of the summertime evolution of optical properties, is used. Heat transport within the sea ice is described using an equation describing heat transport in a mushy layer
of a binary alloy (salt water). The model is tested by comparison of numerical simulations with SHEBA data and previous modeling. The presence of melt ponds on the sea ice surface is demonstrated to have a significant effect on the heat and mass balance. Sensitivity tests indicate that the maximum melt pond depth is highly sensitive to optical parameters and drainage. INDEX TERMS: 4207 Oceanography: General: Arctic and Antarctic
oceanography; 4255 Oceanography: General: Numerical modeling; 4299 Oceanography: General: General or
miscellaneous; KEYWORDS: sea ice, melt pond, albedo, Arctic Ocean, radiation model, thermodynami
Aerosol Characteristics at a High Altitude Location in Central Himalayas: Optical Properties and Radiative Forcing
Collocated measurements of the mass concentrations of aerosol black carbon
(BC) and composite aerosols near the surface were carried out along with
spectral aerosol optical depths (AODs) from a high altitude station, Manora
Peak in Central Himalayas, during a comprehensive aerosol field campaign in
December 2004. Despite being a pristine location in the Shivalik Ranges of
Central Himalayas, and having a monthly mean AOD (at 500 nm) of 0.059
0.033 (typical to this site), total suspended particulate (TSP) concentration
was in the range 15 - 40 micro g m^(-3) (mean value 27.1 8.3 micro g
m^(-3)). Interestingly, aerosol BC had a mean concentration of 1.36 0.99
micro g m^(-3), contributed to ~5.0 1.3 % to the composite aerosol mass.
This large abundance of BC is found to have linkages to the human activities in
the adjoining valley and to the boundary layer dynamics. Consequently, the
inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value
0.90 0.03), indicating significant aerosol absorption. The estimated
aerosol radiative forcing was as low as 4.2 W m^(-2) at the surface, +0.7 W
m^(-2) at the top of the atmosphere, implying an atmospheric forcing of +4.9 W
m^(-2). Though absolute value of the atmospheric forcing is quite small, which
arises primarily from the very low AOD (or the column abundance of aerosols),
the forcing efficiency (forcing per unit optical depth) was 88 W m^(-2),
which is attributed to the high BC mass fraction.Comment: 32 Pages, Accepted in JGR (Atmosphere
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