2,406 research outputs found
Safety considerations for fabricating lithium battery packs
Lithium cell safety is a major issue with both manufacturers and end users. Most manufacturers have taken great strides to develop the safest cells possible while still maintaining performance characteristics. The combining of lithium cells for higher voltages, currents, and capacities requires the fabricator of lithium battery packs to be knowledgable about the specific electrochemical system being used. Relatively high rate, spirally wound (large surface area) sulfur oxychloride cells systems, such as Li/Thionyl or Sulfuryl chloride are considered. Prior to the start of a design of a battery pack, a review of the characterization studies for the cells should be conducted. The approach for fabricating a battery pack might vary with cell size
Dust Coagulation and Settling in Layered Protoplanetary Disks
Previous models of dust growth in protoplanetary disks considered either
uniformly laminar or turbulent disks. This Letter explores how dust growth
occurs in a layered protoplanetary disk in which the magnetorotational
instability generates turbulence only in the surface layers of a disk. Two
cases are considered: a completely laminar dead zone and a dead zone in which
turbulence is ``stirred up'' from the MRI acting above. It is found that dust
is depleted from high altitudes in layered disks faster than in those cases of
a uniformly laminar or turblent disks. This is a result of the accelerated
growth of particles in the turbulent regions and their storage in the lower
levels where they escape energetic collisions which would result in disruption.
Thus the regions of a protoplanetary disk above a dead zone would become
rapidly depleted in small dust grains, whereas the outer regions, where the MRI
is active throughout, will maintain a small dust poplulation at all heights due
to the disruptive collisions and vertical mixing from turbulence. This
structure is similar to that which has been inferred for disks around TW Hydra,
GM Auriga, and CoKu Tau/4, which are depleted in dust close to the star, but
are optically thick at larger heliocentric distances.Comment: 4 pages, 3 figures, accepted to ApJ Letter
The Evolution of the Water Distribution in a Viscous Protoplanetary Disk
(Abridged) Astronomical observations have shown that protoplanetary disks are
dynamic objects through which mass is transported and accreted by the central
star. Age dating of meteorite constituents shows that their creation,
evolution, and accumulation occupied several Myr, and over this time disk
properties would evolve significantly. Moreover, on this timescale, solid
particles decouple from the gas in the disk and their evolution follows a
different path. Here we present a model which tracks how the distribution of
water changes in an evolving disk as the water-bearing species experience
condensation, accretion, transport, collisional destruction, and vaporization.
Because solids are transported in a disk at different rates depending on their
sizes, the motions will lead to water being concentrated in some regions of a
disk and depleted in others. These enhancements and depletions are consistent
with the conditions needed to explain some aspects of the chemistry of
chondritic meteorites and formation of giant planets. The levels of
concentration and depletion, as well as their locations, depend strongly on the
combined effects of the gaseous disk evolution, the formation of rapidly
migrating rubble, and the growth of immobile planetesimals. We present examples
of evolution under a range of plausible assumptions and demonstrate how the
chemical evolution of the inner region of a protoplanetary disk is intimately
connected to the physical processes which occur in the outer regions.Comment: 45 pages, 7 figures, revised for publication in Icaru
Post-Impact Thermal Evolution of Porous Planetesimals
Impacts between planetesimals have largely been ruled out as a heat source in
the early Solar System, by calculations that show them to be an inefficient
heat source and unlikely to cause global heating. However, the long-term,
localized thermal effects of impacts on planetesimals have never been fully
quantified. Here, we simulate a range of impact scenarios between planetesimals
to determine the post-impact thermal histories of the parent bodies, and hence
the importance of impact heating in the thermal evolution of planetesimals. We
find on a local scale that heating material to petrologic type 6 is achievable
for a range of impact velocities and initial porosities, and impact melting is
possible in porous material at a velocity of > 4 km/s. Burial of heated
impactor material beneath the impact crater is common, insulating that material
and allowing the parent body to retain the heat for extended periods (~
millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma,
matching a wide range of measurements of metallographic cooling rates from
chondritic materials. While the heating presented here is localized to the
impact site, multiple impacts over the lifetime of a parent body are likely to
have occurred. Moreover, as most meteorite samples are on the centimeter to
meter scale, the localized effects of impact heating cannot be ignored.Comment: 38 pages, 9 figures, Revised for Geochimica et Cosmochimica Acta
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