108 research outputs found
Analysis of acoustic emission during the melting of embedded indium particles in an aluminum matrix: a study of plastic strain accommodation during phase transformation
Acoustic emission is used here to study melting and solidification of
embedded indium particles in the size range of 0.2 to 3 um in diameter and to
show that dislocation generation occurs in the aluminum matrix to accommodate a
2.5% volume change. The volume averaged acoustic energy produced by indium
particle melting is similar to that reported for bainite formation upon
continuous cooling. A mechanism of prismatic loop generation is proposed to
accommodate the volume change and an upper limit to the geometrically necessary
increase in dislocation density is calculated as 4.1 x 10^9 cm^-2 for the
Al-17In alloy. Thermomechanical processing is also used to change the size and
distribution of the indium particles within the aluminum matrix. Dislocation
generation with accompanied acoustic emission occurs when the melting indium
particles are associated with grain boundaries or upon solidification where the
solid-liquid interfaces act as free surfaces to facilitate dislocation
generation. Acoustic emission is not observed for indium particles that require
super heating and exhibit elevated melting temperatures. The acoustic emission
work corroborates previously proposed relaxation mechanisms from prior internal
friction studies and that the superheat observed for melting of these
micron-sized particles is a result of matrix constraint.Comment: Presented at "Atomistic Effects in Migrating Interphase Interfaces -
Recent Progress and Future Study" TMS 201
Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 2: crystallinity and supra molecular structure
Test methods including OM, SEM, TEM, DSC, SAXS, WAXS, and IR were used to characterise supra-molecular structure in three batches of polyethylene (PE), which had weight-average relative molar masses ¯¯¯¯ M w of approximately 0.6 × 106, 5 × 106, and 9 × 106. They were applied to compression mouldings made by the polymer manufacturer. Electron microscopy showed that powders formed in the polymerization reactor consisted of irregularly shaped grains between 50 and 250 μm in diameter. Higher magnification revealed that each grain was an aggregate, composed of particles between 0.4 and 0.8 μm in diameter, which were connected by long, thin fibrils. In compression mouldings, lamellar thicknesses ranged from 7 to 23 nm. Crystallinity varied between 70 and 75 % in reactor powder, but was lower in compression mouldings. Melting peak temperatures ranged from 138 to 145 °C, depending on processing history. DMTA showed that the glass transition temperature θg was −120 °C for all three grades of polyethylene. IR spectroscopy found negligibly small levels of oxidation and thermal degradation in mouldings. Optical microscopy revealed the presence of visible fusion defects at grain boundaries. It is concluded that relatively weak defects can be characterized using optical microscopy, but there is a need for improved methods that can detect less obvious fusion defects
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Unconventional Hall response in the quantum limit of HfTe5
Interacting electrons confined to their lowest Landau level in a high magnetic field can form a variety of correlated states, some of which manifest themselves in a Hall effect. Although such states have been predicted to occur in three-dimensional semimetals, a corresponding Hall response has not yet been experimentally observed. Here, we report the observation of an unconventional Hall response in the quantum limit of the bulk semimetal HfTe5, adjacent to the three-dimensional quantum Hall effect of a single electron band at low magnetic fields. The additional plateau-like feature in the Hall conductivity of the lowest Landau level is accompanied by a Shubnikov-de Haas minimum in the longitudinal electrical resistivity and its magnitude relates as 3/5 to the height of the last plateau of the three-dimensional quantum Hall effect. Our findings are consistent with strong electron-electron interactions, stabilizing an unconventional variant of the Hall effect in a three-dimensional material in the quantum limit
Observation of a three-dimensional fractional Hall response in HfTe5
Interacting electrons in two dimensions can bind magnetic flux lines to form
composite quasiparticles with fractional electric charge, manifesting
themselves in the fractional quantum Hall effect (FQHE). Although the FQHE has
also been predicted to occur in three dimensions, it has not yet been
experimentally observed. Here, we report the observation of fractional plateaus
in the Hall conductivity of the bulk semimetal HfTe5 at magnetic fields beyond
the quantum limit. The plateaus are accompanied by Shubnikov-de Haas minima of
the longitudinal electrical resistivity. The height of the Hall plateaus is
given by twice the Fermi wave vector in the direction of the applied magnetic
field and scales with integer and particular fractional multiples of the
conductance quantum. Our findings are consistent with strong electron-electron
interactions, stabilizing a fractionalized variant of the Hall effect in three
dimensions.Comment: 35 pages with 17 figure
Anomalous Shubnikov-de Haas effect and observation of the Bloch-Gr\"uneisen temperature in the Dirac semimetal ZrTe5
Appearance of quantum oscillations (QO) in both thermodynamic and transport
properties of metals at low temperatures is the most striking experimental
consequence of the existence of a Fermi surface (FS). The frequency of these
oscillations and the temperature dependence of their amplitude provides
essential information about the FS topology and fermionic quasiparticle
properties. Here, we report the observation of an anomalous suppression of the
QO amplitude seen in resistivity (Shubnikov de-Haas effect) at sub-kelvin
temperatures in ZrTe5 samples with a single small FS sheet comprising less than
5% of the first Brillouin zone. By comparing these results with measurements of
the magneto-acoustic QO and the recovery of the usual Lifshitz-Kosevich
behavior of the Shubnikov de-Haas (SdH) effect in ZrTe samples with a
multi-sheet FS, we show that the suppression of the SdH effect originates from
a decoupling of the electron liquid from the lattice. On crossing the so-called
Bloch-Gr\"uneisen temperature, T, electron-phonon scattering becomes
strongly suppressed and in the absence of Umklapp scattering the electronic
liquid regains Galilean invariance. In addition, we show, using a combination
of zero-field electrical conductivity and ultrasonic-absorption measurements,
that entering this regime leads to an abrupt increase of electronic viscosity
Tensile Deformation of Oriented Poly(ε-caprolactone) and Its Miscible Blends with Poly(vinyl methyl ether)
The structural evolution of micromolded poly(ε-caprolactone)
(PCL) and its miscible blends with noncrystallizable poly(vinyl
methyl ether) (PVME) at the nanoscale was investigated as a function of
deformation ratio and blend composition using in situ synchrotron smallangle
X-ray scattering (SAXS) and scanning SAXS techniques. It was
found that the deformation mechanism of the oriented samples shows a
general scheme for the process of tensile deformation: crystal block slips
within the lamellae occur at small deformations followed by a stressinduced
fragmentation and recrystallization process along the drawing
direction at a critical strain where the average thickness of the crystalline
lamellae remains essentially constant during stretching. The value of the
critical strain depends on the amount of the amorphous component
incorporated in the blends, which could be traced back to the lower
modulus of the entangled amorphous phase and, therefore, the reduced network stress acting on the crystallites upon addition of
PVME. When stretching beyond the critical strain the slippage of the fibrils (stacks of newly formed lamellae) past each other
takes place resulting in a relaxation of stretched interlamellar amorphous chains. Because of deformation-induced introduction of
the amorphous PVME into the interfibrillar regions in the highly oriented blends, the interactions between fibrils becomes
stronger upon further deformation and thus impeding sliding of the fibrils to some extent leading finally to less contraction of the
interlamellar amorphous layers compared to the pure PCLNational Natural Science Foundation of China (21204088 and 21134006). This
work is within the framework of the RCUK/EPSRC Science Bridges China project of UK−China Advanced Materials Research Institute (AMRI)
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