1,458 research outputs found
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Polyamide Nanocomposites for Selective Laser Sintering
Current polyamide 11 and 12 are lacking in fire retardancy and high strength/high
heat resistance characteristics for a plethora of finished parts that are desired and required
for performance driven applications. It is anticipated that nanomodification of polyamide
11 and 12 will result in enhanced polymer performance, i.e., fire retardancy, high strength
and high heat resistance for polyamide 11 and 12. It is expected that these findings will
expand the market opportunities for polyamide 11 and 12 resin manufacturers.
The objective of this research is to develop improved polyamide 11 and 12 polymers
with enhanced flame retardancy, thermal, and mechanical properties for selective laser
sintering (SLS) rapid manufacturing (RM). A nanophase was introduced into the
polyamide 11 and 12 via twin screw extrusion to provide improved material properties of
the polymer blends. Arkema RILSANĀ® polyamide 11 molding polymer pellets and
Degussa VESTAMIDĀ® L1670 polyamide 12 were examined with three types of
nanoparticles: chemically modified montmorillonite (MMT) organoclays, surface
modified nanosilica, and carbon nanofibers (CNFs) to create polyamide 11 and 12
nanocomposites.
Wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM)
were used to determine the degree of dispersion. Injection molded test specimens were
fabricated for physical, thermal, mechanical properties, and flammability tests. Thermal
stability of these polyamide 11 and 12 nanocomposites was examined by TGA.
Mechanical properties such as tensile, flexural, and elongation at break were measured.
Flammability properties were also obtained using the Cone Calorimeter at an external
heat flux of 50 kW/m2. TEM micrographs, physical, mechanical, and flammability
properties are included in the paper. Polyamide 11 and 12 nanocomposites properties are
compared with polyamide 11 and 12 baseline polymers. Based on flammability and
mechanical material performance, selective polymers including polyamide 11
nanocomposites and control polyamide 11 were cryogenically ground into fine powders
and fabricated into SLS parts.Mechanical Engineerin
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Innovative Selective Laser Sintering Rapid Manufacturing using Nanotechnology
The objective of this research is to develop an improved nylon 11 (polyamide 11) polymer
with enhanced flame retardancy, thermal, and mechanical properties for selective laser sintering
(SLS) rapid manufacturing (RM). A nanophase was introduced into nylon 11 via twin screw
extrusion to provide improved material properties of the polymer blends. Atofina (now known
as Arkema) RILSANĀ® nylon 11 injection molding polymer pellets was used with three types of
nanoparticles: chemically modified montmorillonite (MMT) organoclays, nanosilica, and carbon
nanofibers (CNF) to create nylon 11 nanocomposites. Wide angle X-ray diffraction (WAXD)
and transmission electron microscopy (TEM) were used to determine the degree of dispersion.
Fifteen nylon 11 nanocomposites and control nylon 11 were fabricated by injection molding.
Flammability properties (using a cone calorimeter with a radiant flux of 50 kW/m2
) and
mechanical properties such as tensile strength and modulus, flexural modulus, elongation at
break were determined for the nylon 11 nanocomposites and compared with the baseline nylon
11. Based on flammability and mechanical material performance, five polymers including four
nylon 11 nanocomposites and a control nylon 11 were cryogenically ground into fine powders
for SLS RM. SLS specimens were fabricated for flammability, mechanical, and thermal
properties characterization. Nylon 11-CNF nanocomposites exhibited the best overall properties
for this study.Mechanical Engineerin
Cardiovascular Damage in Alzheimer Disease: Autopsy Findings From the Bryan ADRC
Autopsy information on cardiovascular damage was investigated for pathologically confirmed Alzheimer disease (AD) patients (n = 84) and non-AD control patients (n = 60). The 51 relevant items were entered into a grade-of-membership model to describe vascular damage in AD. Five latent groups were identified āI: early-onset AD,ā āII: controls, cancer,ā āIII: controls, extensive atherosclerosis,ā āIV: late-onset AD, male,ā and āV: late-onset AD, female.ā Expectedly, Groups IV and V had elevated APOE Ļµ4 frequency. Unexpectedly, there was limited atherosclerosis and frequent myocardial valve and ventricular damage. The findings do not indicate a strong relationship between atherosclerosis and AD, although both are associated with the APOE Ļµ4. Instead, autopsy findings of extensive atherosclerosis were associated with possible, not probable or definite AD, and premature death. They are consistent with the hypothesis that brain hypoperfusion contributes to dementia, possibly to AD pathogenesis, and raise the possibility that the APOE allele Ļµ4 contributes directly to heart valve and myocardial damage
Structure and Magnetism of Neutral and Anionic Palladium Clusters
The properties of neutral and anionic Pd_N clusters were investigated with
spin-density-functional calculations. The ground state structures are
three-dimensional for N>3 and they are magnetic with a spin-triplet for 2<=N<=7
and a spin nonet for N=13 neutral clusters. Structural- and spin-isomers were
determined and an anomalous increase of the magnetic moment with temperature is
predicted for a Pd_7 ensemble. Vertical electron detachment and ionization
energies were calculated and the former agree well with measured values for
anionic Pd_N clusters.Comment: 5 pages, 3 figures, fig. 2 in color, accepted to Phys. Rev. Lett.
(2001
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Evaluation of Lateglacial temperatures in the Southern Alps of New Zealand based on glacier modelling at Irishman Stream, Ben Ohau Range
Climate proxy records from the middle to high latitude Southern Hemisphere indicate that a Lateglacial (15,000-11,500 years ago) climate reversal, approximately coeval with the Antarctic Cold Reversal (ACR), interrupted a warming trend during deglaciation. In New Zealand, some palaeoclimate proxy records indicate a cool episode during the ACR (ca 14,500-12,500 years ago), while others do not express a significant change in climate. Recently published moraine maps and ages present an opportunity to improve the palaeoclimate interpretation through numerical modelling of glaciers. We use a coupled energy-balance and ice-flow model to quantify palaeoclimate from past glacier extent constrained by mapped and dated moraines in the headwaters of Irishman Stream, a high-elevation catchment in the Southern Alps. First, a suite of steady-state model runs is used to identify the temperature and precipitation forcing required to fit the modelled glacier to well-dated Lateglacial moraine crests. Second, time dependent glacier simulations forced by a nearby proxy temperature record derived from chironomids are used to assess the fit with the glacial geomorphic record. Steady-state experiments using an optimal parameter set demonstrate that the conditions under which the 13,000 year old moraine formed were 2.3-3.2 *C colder than present with the range in temperature corresponding to a +/- 20% variance in precipitation relative to the present-day. This reconstructed climate change relative to the present-day corresponds to an equilibrium-line altitude of ca 2000 +/- 40 m above sea level (asl), which is ca 400 m lower than present. Time-dependent simulations of glacier length produce ice advance to within 100 m of the 13,000 year old terminal moraine, indicating that the chironomid-based temperature forcing and moraine record provide consistent information about past climate. Our results, together with other climate proxy reconstructions from pollen records and marine sediment cores, support the notion that temperatures during the ACR in New Zealand were ~2-3 *C cooler than today
An Analysis of Resting-State Functional Transcranial Doppler Recordings from Middle Cerebral Arteries
Functional transcrannial Doppler (fTCD) is used for monitoring the hemodynamics characteristics of major cerebral arteries. Its resting-state characteristics are known only when considering the maximal velocity corresponding to the highest Doppler shift (so called the envelope signals). Significantly more information about the resting-state fTCD can be gained when considering the raw cerebral blood flow velocity (CBFV) recordings. In this paper, we considered simultaneously acquired envelope and raw CBFV signals. Specifically, we collected bilateral CBFV recordings from left and right middle cerebral arteries using 20 healthy subjects (10 females). The data collection lasted for 15 minutes. The subjects were asked to remain awake, stay silent, and try to remain thought-free during the data collection. Time, frequency and time-frequency features were extracted from both the raw and the envelope CBFV signals. The effects of age, sex and body-mass index were examined on the extracted features. The results showed that the raw CBFV signals had a higher frequency content, and its temporal structures were almost uncorrelated. The information-theoretic features showed that the raw recordings from left and right middle cerebral arteries had higher content of mutual information than the envelope signals. Age and body-mass index did not have statistically significant effects on the extracted features. Sex-based differences were observed in all three domains and for both, the envelope signals and the raw CBFV signals. These findings indicate that the raw CBFV signals provide valuable information about the cerebral blood flow which can be utilized in further validation of fTCD as a clinical tool. Ā© 2013 SejdiÄ et al
Attenuation of UVR-induced vitamin D<sub>3</sub> synthesis in a mouse model deleted for keratinocyte lathosterol 5-desaturase
An influenza A virus can evolve to use human ANP32E through altering polymerase dimerization
Human ANP32A and ANP32B are essential but redundant host factors for influenza virus genome replication. While most influenza viruses cannot replicate in edited human cells lacking both ANP32A and ANP32B, some strains exhibit limited growth. Here, we experimentally evolve such an influenza A virus in these edited cells and unexpectedly, after 2 passages, we observe robust viral growth. We find two mutations in different subunits of the influenza polymerase that enable the mutant virus to use a novel host factor, ANP32E, an alternative family member, which is unable to support the wild type polymerase. Both mutations reside in the symmetric dimer interface between two polymerase complexes and reduce polymerase dimerization. These mutations have previously been identified as adapting influenza viruses to mice. Indeed, the evolved virus gains the ability to use suboptimal mouse ANP32 proteins and becomes more virulent in mice. We identify further mutations in the symmetric dimer interface which we predict allow influenza to adapt to use suboptimal ANP32 proteins through a similar mechanism. Overall, our results suggest a balance between asymmetric and symmetric dimers of influenza virus polymerase that is influenced by the interaction between polymerase and ANP32 host proteins
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Using ensemble reforecasts to generate flood thresholds for improved global flood forecasting
Global flood forecasting systems rely on predefining flood thresholds to highlight potential upcoming flood events. Existing methods for flood threshold definition are often based on reanalysis datasets using a single threshold across all forecast lead times, such as in the Global Flood Awareness System. This leads to inconsistencies between how the extreme flood events are represented in the flood thresholds and the ensemble forecasts.
This paper explores the potential benefits of using river flow ensemble reforecasts to generate flood thresholds that can deliver improved reliability and skill, increasing the confidence in the forecasts for humanitarian and civil protection partners. The choice of dataset and methods used to sample annual maxima in the threshold computation, both for reanalysis and reforecast, are analysed in terms of threshold magnitude, forecast reliability and skill for different flood severity levels and lead times. The variability of threshold magnitudes, when estimated from the different annual maxima samples, can be extremely large, as can the subsequent impact on forecast skill. Reanalysis-based thresholds should only be used for the first few days, after which ensemble-reforecast-based thresholds, that vary with forecast lead time and can account for the forecast bias trends, provide more reliable and skilful flood forecasts
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