534 research outputs found
Terahertz hot electron bolometer waveguide mixers for GREAT
Supplementing the publications based on the first-light observations with the
German Receiver for Astronomy at Terahertz frequencies (GREAT) on SOFIA, we
present background information on the underlying heterodyne detector
technology. We describe the superconducting hot electron bolometer (HEB)
detectors that are used as frequency mixers in the L1 (1400 GHz), L2 (1900
GHz), and M (2500 GHz) channels of GREAT. Measured performance of the detectors
is presented and background information on their operation in GREAT is given.
Our mixer units are waveguide-based and couple to free-space radiation via a
feedhorn antenna. The HEB mixers are designed, fabricated, characterized, and
flight-qualified in-house. We are able to use the full intermediate frequency
bandwidth of the mixers using silicon-germanium multi-octave cryogenic
low-noise amplifiers with very low input return loss. Superconducting HEB
mixers have proven to be practical and sensitive detectors for high-resolution
THz frequency spectroscopy on SOFIA. We show that our niobium-titanium-nitride
(NbTiN) material HEBs on silicon nitride (SiN) membrane substrates have an
intermediate frequency (IF) noise roll-off frequency above 2.8 GHz, which does
not limit the current receiver IF bandwidth. Our mixer technology development
efforts culminate in the first successful operation of a waveguide-based HEB
mixer at 2.5 THz and deployment for radioastronomy. A significant contribution
to the success of GREAT is made by technological development, thorough
characterization and performance optimization of the mixer and its IF interface
for receiver operation on SOFIA. In particular, the development of an optimized
mixer IF interface contributes to the low passband ripple and excellent
stability, which GREAT demonstrated during its initial successful astronomical
observation runs.Comment: Accepted for publication in A&A (SOFIA/GREAT special issue
Jamming under tension in polymer crazes
Molecular dynamics simulations are used to study a unique expanded jammed
state. Tension transforms many glassy polymers from a dense glass to a network
of fibrils and voids called a craze. Entanglements between polymers and
interchain friction jam the system after a fixed increase in volume. As in
dense jammed systems, the distribution of forces is exponential, but they are
tensile rather than compressive. The broad distribution of forces has important
implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure
Synthesis of intermetallic hydrogen storage materials based on TI-CR by glow discharge plasma
The TiCr2 intermetallic compound with the C36 hexagonal Laves phase has been synthesized by glow discharge plasma. The hydrogen absorption-desorption properties of the obtained alloy were investigated. It is shown that hydrogen storage capacity of material was 0.43 wt % at 85 °C and 8 atm
Indications, clinical outcome and survival of rotating hinge total knee arthroplasty in a retrospective study of 63 primary and revision cases.
PURPOSE
The purpose of this study is to report and compare outcome data of both primary and revision cases using a rotating hinge knee (RHK) implant.
METHODS
This study retrospectively analyzed 63 cases (19 primary, 44 revisions) at a mean follow-up of 34 ± 8 months after RHK implantation. Outcome parameters were stability, range of motion (ROM), loosening, Hospital of Special Surgery Score (HSS), Knee Society Score (KSS), Oxford Knee Score (OKS), EQ-5D-3L, and Visual Analog Scale (VAS) for overall function. Revision rates and implant survival are reported.
RESULTS
Eleven percent showed medio-lateral instability < 5 mm, a mean ROM of 115° ± 17° and radiologic loosening occurred in 8% (2% symptomatic). PROMS showed the following results: HSS 79 ± 18, KSS 78 ± 27, OKS 26 ± 10, EQ-5D index 0.741 ± 0.233 and VAS 70 ± 20. Primary cases revealed better outcomes in HHS (p = .035) and OKS (p = 0.047). KSS, EQ-5D index and VAS did not differ between primary and revision cases (p = 0.070; p = 0.377; p = 0.117). Revision rate was 6.3% with an implant survival of 96.8%.
CONCLUSIONS
RHK arthroplasty can be performed with good clinical outcome and low revision rate in revision and complex primary cases. RHK is an option in cases where standard arthroplasty and even implants with a higher degree of constraint have reached their limits.
LEVEL OF EVIDENCE
Level III, retrospective cohort study
Yield conditions for deformation of amorphous polymer glasses
Shear yielding of glassy polymers is usually described in terms of the
pressure-dependent Tresca or von Mises yield criteria. We test these criteria
against molecular dynamics simulations of deformation in amorphous polymer
glasses under triaxial loading conditions that are difficult to realize in
experiments. Difficulties and ambiguities in extending several standard
definitions of the yield point to triaxial loads are described. Two
definitions, the maximum and offset octahedral stresses, are then used to
evaluate the yield stress for a wide range of model parameters. In all cases,
the onset of shear is consistent with the pressure-modified von Mises
criterion, and the pressure coefficient is nearly independent of many
parameters. Under triaxial tensile loading, the mode of failure changes to
cavitation.Comment: 9 pages, 8 figures, revte
Prüfung von Perkolations- und Elutionsversuchen zur Schadstofffreisetzung aus mineralischen Ersatzbaustoffen
In einem groß angelegten Ringversuch
wurden die DIN-Normen DIN 19528 und
DIN 19529 verifiziert. In dieser Arbeit
haben wir die darin vorgesehenen
Experimente durchgeführt, das Versuchsprogramm
erweitert und die
Ergebnisse mittels verschiedener mathematischer
Verfahren ausgewertet.
Schüttelversuche, wie in DIN 19529
vorgesehen, haben wir durch Ansätze
mit weiteren Wasser-/Feststoffverhältnissen
erweitert. Diese Ergebnisse
wurden zur Ermittlung der das
Verteilungsverhalten bestimmenden Parameter
mittels MuBaX (Iden und Durner,
2007) ausgewertet. Die Säulenversuche
nach DIN 19528 wurden ebenfalls in
ihrer zeitlichen Dauer erweitert und durch
Fließunterbrechungen ergänzt. Für die
Abschätzung der kurz- bis mittelfristigen
Freisetzungen wurden reproduzierbare
und zwischen den unterschiedlichen
Methoden vergleichbare Ergebnisse
erzielt
Shear yielding of amorphous glassy solids: Effect of temperature and strain rate
We study shear yielding and steady state flow of glassy materials with
molecular dynamics simulations of two standard models: amorphous polymers and
bidisperse Lennard-Jones glasses. For a fixed strain rate, the maximum shear
yield stress and the steady state flow stress in simple shear both drop
linearly with increasing temperature. The dependence on strain rate can be
described by a either a logarithm or a power-law added to a constant. In marked
contrast to predictions of traditional thermal activation models, the rate
dependence is nearly independent of temperature. The relation to more recent
models of plastic deformation and glassy rheology is discussed, and the
dynamics of particles and stress in small regions is examined in light of these
findings
A Universal Method to Generate Hyperpolarisation in Beams and Samples
Sizable hyperpolarisation, i.e. an imbalance of the occupation numbers of
nuclear spins in a sample deviating from thermal equilibrium, is needed in
various fields of science. For example, hyperpolarised tracers are utilised in
magnetic resonance imaging in medicine (MRI) and polarised beams and targets
are employed in nuclear physics to study the spin dependence of nuclear forces.
Here we show that the quantum interference of transitions induced by radio-wave
pumping with longitudinal and radial pulses are able to produce large
polarisations at small magnetic fields. This method is easier than established
methods, theoretically understood and experimentally proven for beams of
metastable hydrogen atoms in the keV energy range. It should also work for a
variety of samples at rest. Thus, this technique opens the door for a new
generation of polarised tracers, possibly low-field MRI with better spatial
resolution or the production of polarised fuel to increase the efficiency of
fusion reactors by manipulating the involved cross sections.Comment: 22 pages, 9 figures, splitted into two parts: main paper and method
Sol-gel deposited Sb-doped tin oxide films
The structural, electrical and optical properties of single sol-gel derived antimony-doped tin oxide (ATO) films sintered at 550°C have been measured. The reproducibility of both the preparation and the characterization procedures have been tested by a round-robin test involving eight laboratories within a Concerted European Action (CEA) project. The resistivity measured as a function of Sb content has been obtained by electric and reflectance and transmission measurements. Their differences are discussed in terms of structural and grain boundary effects. An increase of Sb content results in a decrease of the crystallite size (7.0 to 5.4 nm) and a greater influence of the grain boundary
Growth, microstructure, and failure of crazes in glassy polymers
We report on an extensive study of craze formation in glassy polymers.
Molecular dynamics simulations of a coarse-grained bead-spring model were
employed to investigate the molecular level processes during craze nucleation,
widening, and breakdown for a wide range of temperature, polymer chain length
, entanglement length and strength of adhesive interactions between
polymer chains. Craze widening proceeds via a fibril-drawing process at
constant drawing stress. The extension ratio is determined by the entanglement
length, and the characteristic length of stretched chain segments in the
polymer craze is . In the craze, tension is mostly carried by the
covalent backbone bonds, and the force distribution develops an exponential
tail at large tensile forces. The failure mode of crazes changes from
disentanglement to scission for , and breakdown through scission
is governed by large stress fluctuations. The simulations also reveal
inconsistencies with previous theoretical models of craze widening that were
based on continuum level hydrodynamics
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