98 research outputs found
The Structure of Eu-III
Previous x-ray diffraction studies have reported Eu to transform from the hcp
structure to a new phase, Eu-III, at 18 GPa. Using x-ray powder diffraction we
have determined that Eu remains hcp up to 33 GPa, and that the extra peaks that
appear at 18 GPa are from an impurity phase with space group R-3c . Above 33
GPa the diffraction pattern becomes very much more complex, signalling a
transition to a phase with a distorted hcp structure.Comment: 4 pages, 4 figures, AIRAPT-23 Conference, Mumbai, 201
Neutron diffraction of hydrogen inclusion compounds under pressure
When ice is compressed alongside a gas, crystalline 'host - guest' inclusion
compounds known as gas clathrate hydrates form. These compounds are of
interest not only for their environmental and possible technological impact as
gas storage and separation materials, but also for their ability to probe networks
not readily adopted by the pure `host' water molecules, and to study the
interactions between water and gas molecules. Despite the pressure dependent
crystal structures being fully determined for a large variety of `guest' gas species
there is still relatively little known about the crystal structures in small guest gas
systems such as H2 hydrate. The majority of structural studies have been done
with x-ray diffraction and report a number of conflicting structures or hydrogen
contents for the four known stable phases (sII, C0, C1 and C2). As this is a very
hydrogen rich system the most ideal method to study the structure is neutron
diffraction, which is able to fully determine the location of the hydrogen atoms
within the structure and would allow a direct measurement of any hydrogen
ordering within the host structure and the H2 content.
In this work the phase diagram of the deuterated analogue of the H2-H2O system
is explored at low pressures (below 0.3 GPa) with neutron diffraction. In the
pressure/temperature region where the sII phase is known to be stable, two
metastable phases were observed between the formation of sII from ice Ih and
that this transition sequence occurred in line with Ostwald's Rule of Stages. One
of these metastable phases was the C0 phase known to be stable in the H2-H2O
system above 0.5 GPa, and the other is a new structure not previously observed
in this system and is dubbed in this work as C-1 . Prior to this work the C0
phase has been reported with various structures that were determined with x-ray
diffraction, and here the crystal structure and H2 content at low pressure
are determined with neutron diffraction. The C0 phase was found to form a
similar host structure to those of the previous studies with spiral guest sites but
is best described with highly mobile H2 guests and a higher symmetry space
group which make it the same structure as the spiral hydrate structure (s-Sp)
recently observed in the CO2 hydrate system. In addition to this structure being
determined at pressure a sample of C0 was also recovered to ambient pressure
at low temperature and its structure/H2 content is presented as it was warmed
to decomposition. The crystal structure of the C-1 phase was determined to be
similar to ice Ih and a sample was recovered to ambient pressure to study its
decomposition behaviour. Evidence for a similar structure in the helium hydrate
system at low pressure is also reported here.
This work was then extended to higher pressures with the recent developments of
a hydrogen-compatible gas loader and large-volume diamond anvil cells. Several
test experiments on gas-loaded Paris-Edinburgh presses are described on systems
that are similar to hydrogen-water like urea-hydrogen and neon-water. And a
further preliminary high pressure study on the deuterated analogue of the H2-
H2O system in a diamond anvil cell between 3.6 and 28 GPa shows decomposition
behaviour as pressure was increased
One-dimensional chain melting in incommensurate potassium
Between 19 and 54 GPa, potassium has a complex composite incommensurate
host-guest structure which undergoes two intraphase transitions over this
pressure range. The temperature dependence of these host-guest phases is
further complicated by the onset of an order-disorder transition in their guest
chains. Here, we report single crystal, quasi-single crystal, and powder
synchrotron X-ray diffraction measurements of this order-disorder phenomenon in
incommensurate potassium to 47 GPa and 750 K. The so-called chain "melting"
transition is clearly visible over a 22 GPa pressure range, and there are
significant changes in the slope of the phase boundary which divides the
ordered and disordered phases, one of which results from the intraphase
transitions in the guest structure
Systematic and Controllable Negative, Zero, and Positive Thermal Expansion in Cubic Zr1–xSnxMo2O8
We describe the synthesis and characterization of a family of materials, Zr1–xSnxMo2O8 (0 < x < 1), whose isotropic thermal expansion coefficient can be systematically varied from negative to zero to positive values. These materials allow tunable expansion in a single phase as opposed to using a composite system. Linear thermal expansion coefficients, αl, ranging from −7.9(2) × 10–6 to +5.9(2) × 10–6 K–1 (12–500 K) can be achieved across the series; contraction and expansion limits are of the same order of magnitude as the expansion of typical ceramics. We also report the various structures and thermal expansion of “cubic” SnMo2O8, and we use time- and temperature-dependent diffraction studies to describe a series of phase transitions between different ordered and disordered states of this material
A Potential Role for Bat Tail Membranes in Flight Control
Wind tunnel tests conducted on a model based on the long-eared bat Plecotus auritus indicated that the positioning of the tail membrane (uropatagium) can significantly influence flight control. Adjusting tail position by increasing the angle of the legs ventrally relative to the body has a two-fold effect; increasing leg-induced wing camber (i.e., locally increased camber of the inner wing surface) and increasing the angle of attack of the tail membrane. We also used our model to examine the effects of flying with and without a tail membrane. For the bat model with a tail membrane increasing leg angle increased the lift, drag and pitching moment (nose-down) produced. However, removing the tail membrane significantly reduced the change in pitching moment with increasing leg angle, but it had no significant effect on the level of lift produced. The drag on the model also significantly increased with the removal of the tail membrane. The tail membrane, therefore, is potentially important for controlling the level of pitching moment produced by bats and an aid to flight control, specifically improving agility and manoeuvrability. Although the tail of bats is different from that of birds, in that it is only divided from the wings by the legs, it nonetheless, may, in addition to its prey capturing function, fulfil a similar role in aiding flight control
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Attention bias to emotional faces varies by IQ and anxiety in Williams syndrome
Individuals with Williams syndrome (WS) often experience significant anxiety. A promising approach to anxiety intervention has emerged from cognitive studies of attention bias to threat. To investigate the utility of this intervention in WS, this study examined attention bias to happy and angry faces in individuals with WS (N=46). Results showed a significant difference in attention bias patterns as a function of IQ and anxiety. Individuals with higher IQ or higher anxiety showed a significant bias toward angry, but not happy faces, whereas individuals with lower IQ or lower anxiety showed the opposite pattern. These results suggest that attention bias interventions to modify a threat bias may be most effectively targeted to anxious individuals with WS with relatively high IQ
A Potential Role for Bat Tail Membranes in Flight Control
Wind tunnel tests conducted on a model based on the long-eared bat Plecotus auritus indicated that the positioning of the tail membrane (uropatagium) can significantly influence flight control. Adjusting tail position by increasing the angle of the legs ventrally relative to the body has a two-fold effect; increasing leg-induced wing camber (i.e., locally increased camber of the inner wing surface) and increasing the angle of attack of the tail membrane. We also used our model to examine the effects of flying with and without a tail membrane. For the bat model with a tail membrane increasing leg angle increased the lift, drag and pitching moment (nose-down) produced. However, removing the tail membrane significantly reduced the change in pitching moment with increasing leg angle, but it had no significant effect on the level of lift produced. The drag on the model also significantly increased with the removal of the tail membrane. The tail membrane, therefore, is potentially important for controlling the level of pitching moment produced by bats and an aid to flight control, specifically improving agility and manoeuvrability. Although the tail of bats is different from that of birds, in that it is only divided from the wings by the legs, it nonetheless, may, in addition to its prey capturing function, fulfil a similar role in aiding flight control
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