927 research outputs found
Pressure-induced amorphization and polyamorphism in one-dimensional single crystal TiO2 nanomaterials
The structural phase transitions of single crystal TiO2-B nanoribbons were
investigated in-situ at high-pressure using the synchrotron X-ray diffraction
and the Raman scattering. Our results have shown a pressure-induced
amorphization (PIA) occurred in TiO2-B nanoribbons upon compression, resulting
in a high density amorphous (HDA) form related to the baddeleyite structure.
Upon decompression, the HDA form transforms to a low density amorphous (LDA)
form while the samples still maintain their pristine nanoribbon shape. HRTEM
imaging reveals that the LDA phase has an {\alpha}-PbO2 structure with short
range order. We propose a homogeneous nucleation mechanism to explain the
pressure-induced amorphous phase transitions in the TiO2-B nanoribbons. Our
study demonstrates for the first time that PIA and polyamorphism occurred in
the one-dimensional (1D) TiO2 nanomaterials and provides a new method for
preparing 1D amorphous nanomaterials from crystalline nanomaterials.Comment: 4 figure
An Oort cloud origin for the high-inclination, high-perihelion Centaurs
We analyse the origin of three Centaurs with perihelia in the range 15 AU to
30 AU, inclinations above 70 deg and semi-major axes shorter than 100 AU. Based
on long-term numerical simulations we conclude that these objects most likely
originate from the Oort cloud rather than the Kuiper Belt or Scattered Disc. We
estimate that there are currently between 1 and 200 of these high-inclination,
high-perihelion Centaurs with absolute magnitude H<8.Comment: Accepted for publication in MNRA
The Physical Properties of High-Mass Star-Forming Clumps: A Systematic Comparison of Molecular Tracers
We present observations of HCO+ and H^13CO+, N2H+, HCS+, HCN and HN^13C, SO
and ^34SO, CCH, SO_2, and CH_3OH-E towards a sample of 27 high-mass clumps
coincident with water maser emission. All transitions are observed with or
convolved to nearly identical resolution (30"), allowing for inter-comparison
of the clump properties derived from the mapped transitions. We find N2H+
emission is spatially differentiated compared to the dust and the other
molecules towards a few very luminous cores (10 of 27) and the N2H+ integrated
intensity does not correlate well with dust continuum flux. We calculate the
effective excitation density, n_eff, the density required to excite a 1 K line
in T_kin=20 K gas for each molecular tracer. The intensity of molecular tracers
with larger effective excitation densities (n_eff > 10^5 cm^-3) appear to
correlate more strongly with the submillimeter dust continuum intensity. The
median sizes of the clumps are anti-correlated with the n_eff of the tracers
(which span more than three orders of magnitude). Virial mass is not correlated
with n_eff, especially where the lines are optically thick as the linewidths
may be broadened significantly by non-virial motions. The median mass surface
density and median volume density of the clumps is correlated with n_eff
indicating the importance of understanding the excitation conditions of the
molecular tracer when deriving the average properties of an ensemble of cores.Comment: 75 pages, 38 figure
Percolation transition and the onset of non exponential relaxation in fully frustrated models
We numerically study the dynamical properties of fully frustrated models in 2
and 3 dimensions. The results obtained support the hypothesis that the
percolation transition of the Kasteleyn-Fortuin clusters corresponds to the
onset of stretched exponential autocorrelation functions in systems without
disorder. This dynamical behavior may be due to the ``large scale'' effects of
frustration, present below the percolation threshold. Moreover these results
are consistent with the picture suggested by Campbell et al. in space of
configurations.Comment: 8 pages, 11 figures, revised versio
The local ordering of polar solvents around crystalline carbon nitride nanosheets in solution
The crystalline graphitic carbon nitride, poly-triazine imide (PTI) is highly unusual among layered materials since it is spontaneously soluble in aprotic, polar solvents including dimethylformamide (DMF). The PTI material consists of layers of carbon nitride intercalated with LiBr. When dissolved, the resulting solutions consist of dissolved, luminescent single to multilayer nanosheets of around 60–125 nm in diameter and Li+ and Br− ions originating from the intercalating salt. To understand this unique solubility, the structure of these solutions has been investigated by high-energy X-ray and neutron diffraction. Although the diffraction patterns are dominated by inter-solvent correlations there are clear differences between the X-ray diffraction data of the PTI solution and the solvent in the 4–6 Å
−1
range, with real space differences persisting to at least 10 Ã…. Structural modelling using both neutron and X-ray datasets as a constraint reveal the formation of distinct, dense solvation shells surrounding the nanoparticles with a layer of Br
−
close to the PTI-solvent interface. This solvent ordering provides a configuration that is energetically favourable underpinning thermodynamically driven PTI dissolution.
This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'
Temporary Acceleration of Electrons While Inside an Intense Electromagnetic Pulse
A free electron can temporarily gain a very significant amount of energy if
it is overrun by an intense electromagnetic wave. In principle, this process
would permit large enhancements in the center-of-mass energy of
electron-electron, electron-positron and electron-photon interactions if these
take place in the presence of an intense laser beam. Practical considerations
severely limit the utility of this concept for contemporary lasers incident on
relativistic electrons. A more accessible laboratory phenomenon is
electron-positron production via an intense laser beam incident on a gas.
Intense electromagnetic pulses of astrophysical origin can lead to very
energetic photons via bremsstrahlung of temporarily accelerated electrons
Lentiviral Engineered Fibroblasts Expressing Codon Optimized COL7A1 Restore Anchoring Fibrils in RDEB
Cells therapies, engineered to secrete replacement proteins, are being developed to ameliorate otherwise debilitating diseases. Recessive dystrophic epidermolysis bullosa (RDEB) is caused by defects of type VII collagen (C7), a protein essential for anchoring fibril formation at the dermal-epidermal junction (DEJ). Whilst allogeneic fibroblasts injected directly into the dermis can mediate transient disease modulation, autologous gene-modified fibroblasts should evade immunological rejection and support sustained delivery of C7 at the DEJ. We demonstrate the feasibility of such an approach using a therapeutic grade, self-inactivating-lentiviral vector, encoding codon optimized COL7A1, to transduce RDEB fibroblasts under conditions suitable for clinical application. Expression and secretion of C7 was confirmed, with transduced cells exhibiting supra-normal levels of protein expression and ex vivo migration of fibroblasts was restored in functional assays. Gene modified RDEB fibroblasts also deposited C7 at the DEJ of human RDEB skin xenografts placed on NOD-scid IL2Rgamma(null) recipients, with reconstruction of human epidermal structure and regeneration of anchoring fibrils at the DEJ. Fibroblast mediated restoration of protein and structural defects in this RDEB model strongly supports proposed therapeutic applications in man
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