74 research outputs found
The Effect of Aluminium Surface Treatments on the Bonding Properties of Silica-Modified Epoxy Adhesive Joints: A Statistical Approach
A full factorial design is carried out to investigate the effects of different surface treatments, the inclusion of silica microparticles and the use of wash primer on the apparent shear strength and adherent strength of single-lap aluminium joints. Scanning electron microscopy, surface energy and roughness measurements are performed to characterise the aluminium surface. The results show that the use of wash primer decreases the apparent shear strength of the joints significantly. The cohesive failure of the primer is the main cause of the reduction in strength. On the other hand, the inclusion of 10 wt.% of silica microparticles in the adhesive layers increases the shear strength by 26%. Surfaces treated with NaOH for one minute, without using a wash primer, result in the most resistant joint. In contrast to the apparent shear strength, adherent strength is most effective when only degreasing is considered
The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 active stars
observed with the High Energy Transmission Grating Spectrometer on {\em
Chandra} in order to investigate their coronal plasma density. Densities where
investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII.
While Si XIII lines in all stars of the sample are compatible with the
low-density limit, Mg XI lines betray the presence of high plasma densities ( cm) for most of the sources with higher X-ray luminosity ( erg/s); stars with higher and tend to have higher
densities at high temperatures. Ratios of O VII lines yield much lower
densities of a few cm, indicating that the ``hot'' and
``cool'' plasma resides in physically different structures. Our findings imply
remarkably compact coronal structures, especially for the hotter plasma
emitting the Mg XI lines characterized by coronal surface filling factor,
, ranging from to , while we find
values from a few up to for the cooler plasma emitting the O
VII lines. We find that approaches unity at the same stellar surface
X-ray flux level as solar active regions, suggesting that these stars become
completely covered by active regions. At the same surface flux level,
is seen to increase more sharply with increasing surface flux. These
results appear to support earlier suggestions that hot K plasma in
active coronae arises from flaring activity, and that this flaring activity
increases markedly once the stellar surface becomes covered with active
regions.Comment: 53 pages, 19 figures, accepted for publication in Astrophysical
Journal. A version of the paper with higher quality figures is available from
http://www.astropa.unipa.it/Library/preprint.htm
Chandra Observations of the Pleiades Open Cluster: X-ray Emission from Late-B to Early-F Type Binaries
We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of
the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory detected 99 X-ray sources in a 17'X17' region,
including 18 of 23 Pleiades members. Five candidate Pleiades members have also
been detected, confirming their cluster membership. Fifty-seven sources have no
optical or near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5.
The unidentified X-ray sources are probably background AGN and not stars. The
Chandra field of view contains seven intermediate mass cluster members. Five of
these, HII 980 (B6 + G), HII 956 (A7 + F6), HII 1284 (A9 + K), HII 1338 (F3 +
F6), and HII 1122 (F4 + K), are detected in this study. All but HII 1284 have
high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties
comparable to non-flaring K-type stars. Since all five stars are visual or
spectroscopic binaries with X-ray properties similar to F-G stars, the
late-type binary companions are probably producing the observed coronal X-ray
emission. Strengthening this conclusion is the nondetection by Chandra of two A
stars, HII 1362 (A7, no known companion) and HII 1375 (A0 + A SB) with X-ray
luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7
stars with cooler companions. Despite the low number statistics, the Chandra
data appear to confirm the expectation that late-B and A stars are not strong
intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual
increase in coronal temperature with decreasing mass from F4 to K. M stars
appear to have somewhat cooler coronae than active K stars.Comment: 22 pages, 4 figures, 7 tables, to appear in Ap
The influence of rice husk ash addition on the properties of metakaolin-based geopolymers
This paper investigates the replacement of metakaolin (MK) with rice husk ash (RHA) in the production of alkali-activated binders or geopolymers. The influence of the RHA addition on compressive and flexural strength, as well as water absorption and apparent porosity were determined, in terms of the percentage of RHA in the mixture and molar ratios of the mixes. Fourier Transform Infrared (FTIR) spectroscopy and Energy Dispersive spectroscopy (EDS) were carried out to assess the changes in the microstructure of the geopolymer matrices with the RHA addition. Results have shown that RHA may be a supplementary precursor for geopolymers. The composition of the geopolymer matrices containing 0-40% RHA is very similar, which indicates that the additional Si provided by RHA is not incorporated to the geopolymer matrix. In addition, geopolymers with RHA content higher than 40% present a plastic behavior, characterized by extremely low strength and high deformation, which can be attributed to the formation of silica gel in formulations containing variable Si/Al ratio
Phylogeographic pattern and extensive mitochondrial DNA divergence disclose a species complex within the Chagas disease vector Triatoma dimidiata.
ABSTARCT: Previous studies have shown that "bioequivalent" generic products of vancomycin are less effective in vivo against Staphylococcus aureus than the innovator compound. Considering that suboptimal bactericidal effect has been associated with emergence of resistance, we aimed to assess in vivo the impact of exposure to innovator and generic products of vancomycin on S. aureus susceptibility. A clinical methicillin-resistant S. aureus (MRSA) strain from a liver transplant patient with persistent bacteremia was used for which MIC, minimum bactericidal concentration (MBC), and autolytic properties were determined. Susceptibility was also assessed by determining a population analysis profile (PAP) with vancomycin concentrations from 0 to 5 mg/liter. ICR neutropenic mice were inoculated in each thigh with ∼7.0 log(10) CFU. Treatment with the different vancomycin products (innovator and three generics; 1,200 mg/kg of body weight/day every 3 h) started 2 h later while the control group received sterile saline. After 24 h, mice were euthanized, and the thigh homogenates were plated. Recovered colonies were reinoculated to new groups of animals, and the exposure-recovery process was repeated until 12 cycles were completed. The evolution of resistance was assessed by PAP after cycles 5, 10, 11, and 12. The initial isolate displayed reduced autolysis and higher resistance frequencies than S. aureus ATCC 29213 but without vancomycin-intermediate S. aureus (VISA) subpopulations. After 12 cycles, innovator vancomycin had significantly reduced resistant subpopulations at 1, 2, and 3 mg/liter, while the generic products had enriched them progressively by orders of magnitude. The great capacity of generic vancomycin to select for less susceptible organisms raises concerns about the role of therapeutic inequivalence of any antimicrobial on the epidemiology of resistance worldwide
Adaptive Developmental Delay in Chagas Disease Vectors: An Evolutionary Ecology Approach
The developmental time of vector insects is important to their population dynamics, evolutionary biology, epidemiology of the diseases they transmit, and to their responses to global climatic change. In various triatomine species vectors of Chagas disease (Triatominae, Reduviidae), a delay in the molt of a small proportion of individuals has been observed, and from an evolutionary ecology approach, we propose the hypothesis that the developmental delay is an adaptation to environmental stochasticity through a spreading of risk (bet-hedging) diapause strategy. We confirmed, by means of a survey among specialists, the existence of the developmental delay in triatomines. Statistical descriptions of the developmental time of 11 species of triatomines showed some degree of bi-modality in nine of them. We predicted by means of an optimization model which genotype, coding for a given frequency of developmental diapause, is expected to evolve. We identified a series of parameters that can be measured in the field and in the laboratory to test the hypothesis of an optimal diapause frequency. We also discuss the importance of these findings for triatomines in terms of global climatic change and epidemiological consequences such as their resistance to insecticides
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