The two-part series of papers presents the results of a study of the crushing behavior of open-cell Al foams under impact. In Part I, direct and stationary impact tests are performed on cylindrical ...foam specimens at impacts speeds in the range of 20–160m/s using a gas gun. The stress at one end is recorded using a pressure bar, while the deformation of the entire foam specimen is monitored with high-speed photography. Specimens impacted at velocities of 60m/s and above developed nearly planar shocks that propagated at well-defined velocities crushing the specimen. The shock speed vs. impact speed, and the strain behind the shock vs. impact speed representations of the Hugoniot were both extracted directly from the high-speed images. The former follows a linear relationship and the latter asymptotically approaches a strain of about 90% at higher velocities. The Hugoniot enables calculation of all problem variables without resorting to an assumed constitutive model. The compaction energy dissipation across the shock is shown to increase with impact velocity and to be significantly greater than the corresponding quasi-static value. Specimens impacted at velocities lower than 40m/s exhibited response and deformation patterns that are very similar to those observed under quasi-static crushing. Apparently, in this impact speed regime inertia increases the energy absorption capacity very modestly.
High-performance piezoelectrics are lead-based solid solutions that exhibit a so-called morphotropic phase boundary, which separates two competing phases as a function of chemical composition; as a ...consequence, an intermediate low-symmetry phase with a strong piezoelectric effect arises. In search for environmentally sustainable lead-free alternatives that exhibit analogous characteristics, we use a network of competing domains to create similar conditions across thermal inter-ferroelectric transitions in simple, lead-free ferroelectrics such as BaTiO3 and KNbO3. Here we report the experimental observation of thermotropic phase boundaries in these classic ferroelectrics, through direct imaging of low-symmetry intermediate phases that exhibit large enhancements in the existing nonlinear optical and piezoelectric property coefficients. Furthermore, the symmetry lowering in these phases allows for new property coefficients that exceed all the existing coefficients in both parent phases. Discovering the thermotropic nature of thermal phase transitions in simple ferroelectrics thus presents unique opportunities for the design of 'green' high-performance materials.
Diverse techniques have been applied over the past decade to quantify the uplift history of the central Andean Plateau (AP). In this study, opposing models for surface uplift are evaluated including: ...a rapid rise of ∼
2.5
km ∼
10–6
Ma and a slow and steady rise since ∼
40
Ma. These end member models are evaluated by synthesizing observations of the AP lithosphere and the history of deformation, sedimentation, exhumation, magmatism, uplift, and fluvial incision. Structural and geophysical studies estimate variable shortening magnitudes (∼
530–150
km) involving cover-to-basement rocks, an isostatically-compensated thick crust (∼
80–65
km), high heat flow, and zones of variable velocity and attenuation in the crust and mantle. These observations have invoked interpretations such as a hot/weak lithosphere, partial melt, crustal flow, and perhaps current, localized delamination, but do not provide strong support for massive delamination required by the rapid uplift model. Deformation and associated exhumation began ∼
60–40
Ma and generally migrated eastward with consistent long-term average shortening rates (∼
12–8
mm/yr) in Bolivia, favoring the slow uplift model. Volcanic and helium isotope evidence show an AP-wide zone of shallow mantle melting and thin lithosphere that has existed since ∼
25
Ma, which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10–6
Ma. Paleoaltimetry data suggest a rapid ∼
2.5
km elevation gain 10 to 6
Ma, but are equally consistent within error with a linear rise since ≥
25
Ma. Widespread fluvial incision (2.5–1
km) occurred along the western flank since ∼
11–8
Ma and may be associated with surface uplift as proposed by the rapid rise model. However, the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift. Implications of these results for reconstructions of AP evolution are that: (1) substantial deformation of a weak lithosphere is essential, (2) AP growth has taken significantly longer (≥
40
Myr) and was more uniform along strike (∼
1500
km) than previously appreciated, and (3) the slow and steady uplift model is most consistent with available constraints. We conclude that the rapid uplift model may be an overestimate and that a more protracted Cenozoic uplift history is tenable.
We present 3.7 arcsec (∼0.05 pc) resolution 3.2 mm dust continuum observations from the Institut de Radioastronomie Millimétrique Plateau de Bure Interferometer, with the aim of studying the ...structure and fragmentation of the filamentary infrared dark cloud (IRDC) G035.39–00.33. The continuum emission is segmented into a series of 13 quasi-regularly spaced (λobs ∼ 0.18 pc) cores, following the major axis of the IRDC. We compare the spatial distribution of the cores with that predicted by theoretical work describing the fragmentation of hydrodynamic fluid cylinders, finding a significant (a factor of ≳ 8) discrepancy between the two. Our observations are consistent with the picture emerging from kinematic studies of molecular clouds suggesting that the cores are harboured within a complex network of independent sub-filaments. This result emphasizes the importance of considering the underlying physical structure, and potentially, dynamically important magnetic fields, in any fragmentation analysis. The identified cores exhibit a range in (peak) beam-averaged column density (3.6 × 1023 cm−2 < N
H, c < 8.0 × 1023 cm−2), mass (8.1 M⊙ < M
c < 26.1 M⊙), and number density (6.1 × 105 cm−3 < n
H, c, eq < 14.7 × 105 cm−3). Two of these cores, dark in the mid-infrared, centrally concentrated, monolithic (with no traceable substructure at our PdBI resolution), and with estimated masses of the order ∼20–25 M⊙, are good candidates for the progenitors of intermediate-to-high-mass stars. Virial parameters span a range 0.2 < αvir < 1.3. Without additional support, possibly from dynamically important magnetic fields with strengths of the order of 230 μG < B < 670 μG, the cores are susceptible to gravitational collapse. These results may imply a multilayered fragmentation process, which incorporates the formation of sub-filaments, embedded cores, and the possibility of further fragmentation.
Challenges are identified for future control programs for Echinococcus granulosus and E. multilocularis. Full report of workshop proceedings at ICOPA XII, Melbourne, August 2010.
This paper is based ...on the experience of the authors, with the aim to define the challenges for Echinococcus granulosus (E.g./CE) diagnosis and control for those countries that may now or in the future be contemplating control of hydatid disease. A variety of methods are available for diagnosis in humans but a universal gold standard is lacking. Diagnosis in definitive hosts can avoid necropsy by the use of methods such as coproantigen detection but test performance is variable between populations. A sylvatic cycle adds challenges in some countries and the epidemiology of the parasite in these hosts is poorly understood. Control by solely administering praziquantel to dogs is not effective in developing countries where the disease is endemic. Additional avenues to pursue include the instigation of participatory planning, use of an existing vaccination for intermediate hosts and development of a vaccine and long-acting anthelmitic implants for definitive hosts. Promoting public acceptance of control of the dog population by humane euthanasia and reduced reproduction is also essential.
Recent studies of thin-film CdS/CdTe photovoltaic (PV) devices have suggested that a significantly higher device performance will not be achieved unless recombination in the CdTe is reduced. Although ...some control of CdTe recombination has been achieved historically through the careful incorporation of oxygen, chlorine, and copper, we believe a more promising avenue to higher device performance is by controlling the defects in the as-deposited CdTe. This is supported by theoretical studies that suggest much of the improvement associated with oxygen, chlorine, and copper is due to the interaction of these species with intrinsic defects related to cadmium and tellurium vacancies, interstitials, and anti-sites in the bulk as well as within the grain boundary regions. Several research projects at NREL are currently focused on altering CdTe deposition and post-deposition processes to allow for enhanced control of the as-deposited intrinsic defects. This paper discusses initial results in which process changes expected to alter the as-deposited defects are also observed to affect junction evolution and device functionality.
•All present CdTe photovoltaic devices may be limited by similar intrinsic mid-gap defects.•Previous theoretical studies suggest that the tellurium vacancy is an important mid-gap defect.•Recent experimental results indicate excess Te is also problematic.•More recent theoretical studies indicate tellurium anti-site defect may also be problematic.•Present studies are investigating both Cd- and Te-rich chemical potentials to control defects.
Nonlinear optics is an essential component of modern laser systems and optoelectronic devices. It has also emerged as an important tool in probing the electronic, vibrational, magnetic, and ...crystallographic structure of materials ranging from oxides and metals, to polymers and biological samples. This review focuses on the specific technique of optical second harmonic generation (SHG), and its application in probing ferroelectric complex oxide crystals and thin films. As the dominant SHG interaction mechanism exists only in materials that lack inversion symmetry, SHG is a sensitive probe of broken inversion symmetry, and thus also of bulk polar phenomena in materials. By performing in‐situ SHG polarimetry experiments in different experimental conditions such as sample orientation, applied electric field, and temperature, one can probe ferroelectric hysteresis loops and phase transitions. Careful modeling of the polarimetry data allows for the determination of the point group symmetry of the crystal. In epitaxial thin films with a two‐dimensional arrangement of well‐defined domain orientations, one can extract information about intrinsic material properties such as nonlinear coefficients, as well as microstructural information such as the local statistics of the different domain variants being probed. This review presents several detailed examples of ferroelectric systems where such measurements and modeling are performed. The use of SHG microscopic imaging is discussed, and its ability to reveal domain structures and phases not normally visible with linear optics is illustrated.
ABSTRACT
The current generation of (sub)mm-telescopes has allowed molecular line emission to become a major tool for studying the physical, kinematic, and chemical properties of extragalactic ...systems, yet exploiting these observations requires a detailed understanding of where emission lines originate within the Milky Way. In this paper, we present 60 arcsec (∼3 pc) resolution observations of many 3 mm band molecular lines across a large map of the W49 massive star-forming region (∼100 pc × 100 pc at 11 kpc), which were taken as part of the ‘LEGO’ IRAM-30m large project. We find that the spatial extent or brightness of the molecular line transitions are not well correlated with their critical densities, highlighting abundance and optical depth must be considered when estimating line emission characteristics. We explore how the total emission and emission efficiency (i.e. line brightness per H2 column density) of the line emission vary as a function of molecular hydrogen column density and dust temperature. We find that there is not a single region of this parameter space responsible for the brightest and most efficiently emitting gas for all species. For example, we find that the HCN transition shows high emission efficiency at high column density (1022 cm−2) and moderate temperatures (35 K), whilst e.g. N2H+ emits most efficiently towards lower temperatures (1022 cm−2; <20 K). We determine $X_{\mathrm{CO} (1-0)} \sim 0.3 \times 10^{20} \, \mathrm{cm^{-2}\, (K\, km\, s^{-1})^{-1}}$, and $\alpha _{\mathrm{HCN} (1-0)} \sim 30\, \mathrm{M_\odot \, (K\, km\, s^{-1}\, pc^2)^{-1}}$, which both differ significantly from the commonly adopted values. In all, these results suggest caution should be taken when interpreting molecular line emission.
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