259 research outputs found
Literature review and experimental investigation of heat pipes
Tests on heat pipes determine operational limits, external boundary conditions, noncondensable gas effects, startup behavior, and geometric configurations. Experiment consists of design, construction, and testing of an apparatus for measuring wick properties, conventional heat pipes and coplanar heat pipes
Three-Dimensional Elastic Compatibility: Twinning in Martensites
We show how the St.Venant compatibility relations for strain in three
dimensions lead to twinning for the cubic to tetragonal transition in
martensitic materials within a Ginzburg-Landau model in terms of the six
components of the symmetric strain tensor. The compatibility constraints
generate an anisotropic long-range interaction in the order parameter
(deviatoric strain) components. In contrast to two dimensions, the free energy
is characterized by a "landscape" of competing metastable states. We find a
variety of textures, which result from the elastic frustration due to the
effects of compatibility. Our results are also applicable to structural phase
transitions in improper ferroelastics such as ferroelectrics and
magnetoelastics, where strain acts as a secondary order parameter
Droplet Fluctuations in the Morphology and Kinetics of Martensites
We derive a coarse grained, free-energy functional which describes droplet
configurations arising on nucleation of a product crystal within a parent. This
involves a new `slow' vacancy mode that lives at the parent-product interface.
A mode-coupling theory suggests that a {\it slow} quench from the parent phase
produces an equilibrium product, while a {\it fast} quench produces a
metastable martensite. In two dimensions, the martensite nuclei grow as
`lens-shaped' strips having alternating twin domains, with well-defined front
velocities. Several empirically known structural and kinetic relations drop out
naturally from our theory.Comment: 4 pages, REVTEX, and 3 .eps figures, compressed and uuencoded,
Submitted to Phys. Rev. Let
Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn
AuZn undergoes a shape-memory transition at 67 K. The de Haas van Alphen
effect persists to 100 K enabling the observation of a change in the quantum
oscillation spectrum indicative of a catastrophic Fermi surface reconstruction
at the transition. Coexistence of both Fermi surfaces at low temperatures is
suggestive of an intrinsic phase separation in the bulk of the material. In
addition, a Dingle analysis reveals a sharp change in the scattering mechanism
at a threshold cyclotron radius, which we suggest to be related to the
underlying microstructure that drives the shape-memory effect.Comment: 4 pages, 4 figure
Antiferromagnetic domain walls in lightly doped layered cuprates
Recent ESR data shows rotation of the antiferromagnetic (AF) easy axis in
lightly doped layered cuprates upon lowering the temperature. We account for
the ESR data and show that it has significant implications on spin and charge
ordering according to the following scenario: In the high temperature phase AF
domain walls coincide with (110) twin boundaries of an orthorhombic phase. A
magnetic field leads to annihilation of neighboring domain walls resulting in
antiphase boundaries. The latter are spin carriers, form ferromagnetic lines
and may become charged in the doped system. However, hole ordering at low
temperatures favors the (100) orientation, inducing a pi/4 rotation in the AF
easy axis. The latter phase has twin boundaries and AF domain walls in (100)
planes.Comment: 4 pages, 3 figures (1 eps). v2: no change in content, Tex shadow
problem cleare
Twin wall of cubic-tetragonal ferroelastics
We derive solutions for the twin wall linking two tetragonal variants of the
cubic-tetragonal ferroelastic transformation, including for the first time the
dilatational and shear energies and strains. Our solutions satisfy the
compatibility relations exactly and are obtained at all temperatures. They
require four non-vanishing strains except at the Barsch-Krumhansl temperature
TBK (where only the two deviatoric strains are needed). Between the critical
temperature and TBK, material in the wall region is dilated, while below TBK it
is compressed. In agreement with experiment and more general theory, the twin
wall lies in a cubic 110-type plane. We obtain the wall energy numerically as a
function of temperature and we derive a simple estimate which agrees well with
these values.Comment: 4 pages (revtex), 3 figure
A metabolomic approach to characterize the acid-tolerance response in Sinorhizobium meliloti
Omar Draghi W, Florencia Del Papa M, Barsch A, et al. A metabolomic approach to characterize the acid-tolerance response in Sinorhizobium meliloti. METABOLOMICS. 2017;13(6): 71.Introduction Sinorhizobium meliloti establishes a symbiosis with Medicago species where the bacterium fixes atmospheric nitrogen for plant nutrition. To achieve a successful symbiosis, however, both partners need to withstand biotic and abiotic stresses within the soil, especially that of excess acid, to which the Medicago-Sinorhizobium symbiotic system is widely recognized as being highly sensitive. Objective To cope with low pH, S. meliloti can undergo an acid-tolerance response (ATR(+)) that not only enables a better survival but also constitutes a more competitive phenotype for Medicago sativa nodulation under acid and neutral conditions. To characterize this phenotype, we employed metabolomics to investigate the biochemical changes operating in ATR(+) cells. Methods A gas chromatography/mass spectrometry approach was used on S. meliloti 2011 cultures showing ATR(+) and ATR(-) phenotypes. After an univariate and multivariate statistical analysis, enzymatic activities and/or reserve carbohydrates characterizing ATR(+) phenotypes were determined. Results Two distinctive populations were clearly defined in cultures grown in acid and neutral pH based on the metabolites present. A shift occurred in the carbon-catabolic pathways, potentially supplying NAD(P)H equivalents for use in other metabolic reactions and/or for maintaining intracellular-pH homeostasis. Furthermore, among the mechanisms related to acid resistance, the ATR(+) phenotype was also characterized by lactate production, envelope modification, and carbon-overflow metabolism. Conclusions Acid-challenged S. meliloti exhibited several changes in different metabolic pathways that, in specific instances, could be identified and related to responses observed in other bacteria under various abiotic stresses. Some of the observed changes included modifications in the pentose-phosphate pathway (PPP), the exopolysaccharide biosynthesis, and in the myo-inositol degradation intermediates. Such modifications are part of a metabolic adaptation in the rhizobia that, as previously reported, is associated to improved phenotypes of acid tolerance and nodulation competitiveness
Application of elastostatic Green function tensor technique to electrostriction in cubic, hexagonal and orthorhombic crystals
The elastostatic Green function tensor approach, which was recently used to
treat electrostriction in numerical simulation of domain structure formation in
cubic ferroelectrics, is reviewed and extended to the crystals of hexagonal and
orthorhombic symmetry. The tensorial kernels appearing in the expressions for
effective nonlocal interaction of electrostrictive origin are derived
explicitly and their physical meaning is illustrated on simple examples. It is
argued that the bilinear coupling between the polarization gradients and
elastic strain should be systematically included in the Ginzburg-Landau free
energy expansion of electrostrictive materials.Comment: 4 page
Fermi Surface as a Driver for the Shape-Memory Effect in AuZn
Martensites are materials that undergo diffusionless, solid-state
transitions. The martensitic transition yields properties that depend on the
history of the material and may allow it to recover its previous shape after
plastic deformation. This is known as the shape-memory effect (SME). We have
succeeded in identifying the primary electronic mechanism responsible for the
martensitic transition in the shape-memory alloy AuZn by using Fermi-surface
measurements (de Haas-van Alphen oscillations) and band-structure calculations.
This strongly suggests that electronic band structure is an important
consideration in the design of future SME alloys
Statics and dynamics of domain patterns in hexagonal-orthorhombic ferroelastics
We study the statics and the dynamics of domain patterns in proper
hexagonal-orthorhombic ferroelastics; these patterns are of particular interest
because they provide a rare physical realization of disclinations in crystals.
Both our static and dynamical theories are based entirely on classical,
nonlinear elasticity theory; we use the minimal theory consistent with
stability, symmetry and ability to explain qualitatively the observed patterns.
After scaling, the only parameters of the static theory are a temperature
variable and a stiffness variable. For moderate to large stiffness, our static
results show nested stars, unnested stars, fans and other nodes, triangular and
trapezoidal regions of trapped hexagonal phase, etc observed in electron
microscopy of Ta4N and Mg-Cd alloys, and also in lead orthovanadate (which is
trigonal-monoclinic); we even find imperfections in some nodes, like those
observed. For small stiffness, we find patterns like those observed in the
mineral Mg-cordierite. Our dynamical studies of growth and relaxation show the
formation of these static patterns, and also transitory structures such as
12-armed bursts, streamers and striations which are also seen experimentally.
The major aspects of the growth-relaxation process are quite unlike those in
systems with conventional order parameters, for it is inherently nonlocal; for
example, the changes from one snapshot to the next are not predictable by
inspection.Comment: 9 pages, 3 figures (1 b&w, 2 colour); animations may be viewed at
http://huron.physics.utoronto.ca/~curnoe/sim.htm
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