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Surface radiocarbon (Δ14C) in the North Pacific has been monitored using a commercial volunteer observation ship since the early 2000s. Here we report the temporal and spatial variations in Δ14C in the summer surface water when the surface ocean is vertically stratified over a 13-yr period, 2004–2016. The long-term Δ14C decreasing trend after the late 1970s in the subtropical region has continued to the present and the rate of decrease of the Kuroshio and Kuroshio Extension, North Pacific and California current areas is calculated to be –3.3, –5.2 and –3.3 ‰/yr, respectively. After 2012 the Δ14C of the Kuroshio and Kuroshio Extension area, however, has remained at an approximately constant value of around 50‰. The result may indicate that subtropical surface Δ14C in the western North Pacific has reached an equilibrium with atmospheric Δ14CO2. The Δ14C in the subarctic region is markedly lower than values in the subtropical region and it seems that the decreasing tendency of surface Δ14C has changed to an increasing tendency after 2010. The results may indicate that bomb-produced 14C, which has accumulated below the mixed layer in the past few decades, has been entrained into the surface layer by deep convection.
Highly alkaline environments induced by cement-based materials are likely to cause the physical and/or chemical properties of the bentonite buffer materials in radioactive waste repositories to deteriorate. Assessing long-term alteration of concrete/clay systems requires physicochemical models and a number of input parameters. In order to provide reliability in the assessment of the long-term performance of bentonite buffers under disposal conditions, it is necessary to develop and verify reactive transport codes for concrete/clay systems. In this study, a PHREEQC-based, reactive transport analysis code (MC-CEMENT ver. 2) was developed and was verified by comparing results of the calculations with in situ observations of the mineralogical evolution at the concrete/argillite interface. The calculation reproduced the observations such as the mineralogical changes in the argillite limited to within 1 cm in thickness from the interface, formation of CaCO3 and CSH, dissolution of quartz, decrease of porosity in the argillite and an increase in the concrete. These agreements indicate a possibility that models based on lab-scale (∼1 year) experiments can be applied to longer time scales although confidence in the models is necessary for much longer timescales. The fact that the calculations did not reproduce the dissolution of clays and the formation of gypsum indicates that there is still room for improvement in our model.
We investigated the effects of a postnatal fructose diet on the programmed hypertension and vascular and renal dysfunction in offspring from dams exposed to protein restriction. Pregnant Wistar rats were fed control and low-protein diets during the gestation and suckling periods. From the end of lactation, male offspring received standard chow or a 60% fructose diet: a control diet in the gestation and suckling periods and a control diet from the end of lactation, control-on-control (CC), 60% fructose diet-on-control (CF), control-on-low-protein diet (LPC) and 60% fructose diet-on-low-protein diet (LPF). The systolic blood pressure (SBP) was measured during treatment. At postnatal days 94–101, urinary 24 h nitrate/nitrite (NOx) content, protein levels of endothelial nitric oxide synthase (eNOS) and mRNA levels of endothelin-1 (ET-1), and NAD(P)H oxidase subunits in the aorta and kidney were examined. The SBP at postnatal days 97–101 increased in CF (137 ± 2 mmHg, P < 0.05), LPC (135 ± 1 mmHg, P < 0.05) and LPF (141 ± 2 mmHg, P < 0.05), compared with CC (124 ± 1 mmHg). The urinary NOx contents and eNOS phosphorylation in the aorta and kidney of CF, LPC and LPF decreased when compared with CC. In the aorta, the mRNA levels of NAD(P)H oxidase subunits p47phox in LPC and ET-1 in LPC and LPF increased. These results indicate that maternal protein restriction elevated the blood pressure, the downregulated nitric oxide production and eNOS phosphorylation, whereas the postnatal fructose diet made no significant difference to these alterations.
Nanoindentation creep and uniaxial tension were conducted on pure Mg with a grain size of about 2 μm at room temperature and the data were directly compared. Despite the differences in stress state, the two sets of data were found to match remarkably well with each other. An apparent stress exponent value of 4 was obtained and the deformation mechanism was discussed in light of dislocation slips and twinning in anisotropic Mg.
An experimental method is proposed for detecting the effects of positive natural selection on DNA polymorphisms. Since beneficial mutations are expected to increase in frequency faster than neutral mutations, variants which have reached high frequencies in a relatively short period could be linked to some beneficial mutation. D. melanogaster has a cosmopolitan polymorphic inversion -In(2L)t - whose age in some local populations has been estimated. Setting the age of In(2L)t as the upper limit for the age of variants, we searched for variants whose frequencies were possibly influenced by positive natural selection. We detected a single candidate whose frequency and distribution met the requirements imposed by our method.
The specific activities of α-amylase were measured for two sets of mutation accumulation lines, each set having originated from a different lethal-carrying second chromosome and SM1(Cy) chromosome and having been maintained by a balanced lethal system for about 300 generations. Significant variation was found to have accumulated among lines of both sets. Because of dysgenic crosses in the early generations of mutation accumulation, insertions or deletions of transposable elements in the Amy gene region were suspected of being the cause of this variation. In order to test this possibility, the structural changes in the 14 kb region of these chromosomes that includes the structural genes for α-amylase were investigated by restriction map analysis. We found that most part of the activity variation is due to replacements of a chromosomal region of SM1(Cy), including the structural genes for α-amylase, by the corresponding regions of the lethal chromosomes. One line also contained an insertion in this region but this line has an intermediate activity value. Thus, insertions of transposable elements into the Amy gene region were not found to be responsible for the new variation observed in α-amylase activity. If we remove those lines with structural changes from the analysis, the genetic variance of α-amylase specific activity among lines becomes non-significant in both sets of chromosomes.
The atomic structure of shear bands in Pd40Ni40P20 bulk metallic glass has been compared to an undeformed matrix phase using pair distribution functions (PDFs) derived from energy filtered nanobeam electron diffraction. Shear bands do not show any characteristic contrast in transmission electron microscopy (TEM) images when specimens are prepared with uniform thickness. PDFs from a shear band exhibit a slight decrease in the first peak, indicating a slight difference in packing density and short range order compared to the undeformed matrix.
We have investigated the compression stress–strain behaviors of Ti40Zr25Cu12Ni3Be20 bulk metallic glasses prepared by Cu mold casting from various melt temperatures. Plastic strain was found to vary sensitively on the temperature of melts and subsequent annealing conditions. To understand the origin of the plasticity change, the microstructures were characterized using transmission electron microscopy and a laser-assisted three-dimensional atom probe. The fully amorphous sample cast from 1273 K showed 0.6% plastic strain, and it was enhanced to 1.3% after isothermal annealing at 573 K. The sample cast from 1423 K showed 3.0% plastic strain, from which the presence of nanocrystals with a volume fraction of about 12% was confirmed. The sample cast from a higher temperature (1573 K) contained a larger fraction of crystals, which showed limited plastic strain. The effect of the volume fraction of the nanocrystals on the plasticity of bulk metallic glasses is discussed based on the experimental results.
The sublimation growth technique is highly attractive as a commercially viable GaN substrate technology on account of its simplicity and relatively high growth rates. Sublimation growth of GaN using GaN powder source, however, is hampered by formation of liquid Ga in the source. To overcome this limitation, an oxide transport process using a mixture of gallium oxide (Ga2O3) powder and graphite powder as precursors with nitrogen gas as carrier and ammonia as the source of nitrogen has been developed. GaN layers grown by this process were studied by optical microscopy, synchrotron white beam x-ray topography (SWBXT) and high resolution x-ray diffraction (HRXRD) to characterize their structural properties. Studies reveal that the GaN layers grown are single crystal but characterized by dislocation densities and impurities higher than those obtained using GaN powder source. Observed defect distribution is correlated with growth conditions to deduce optimal growth procedure.
We have studied the evolution of stress and microstructure of compositionally graded Al1-xGaxN (0 ≤ x ≤1) buffer layers on (111) Si substrates with varying thicknesses. In-situ stress measurements reveal a tensile-to-compressive stress transition that occurs near the half-thickness in each buffer layer. Cross-sectional transmission electron microscopy (TEM) shows a significant reduction in threading dislocation (TD) density in the top half of the buffer layer, suggesting that the compressive stress enhances the threading dislocation annihilation. The composition of the buffer layers varies linearly with thickness, as determined by X-ray energy dispersive spectrometry (XEDS). The composition grading-induced compressive stress offsets the tensile stress introduced by microstructure evolution, thus yielding a tensile-to-compressive stress transition at x ≈ 0.5.
A series of Al0.47Ga0.53N/GaN heterostructures with different AlN interlayer thicknesses ranging from 1nm to 50nm has been examined. It was found that when the interlayer thickness is greater than ∼5nm, it becomes possible to grow 250nm of Al0.47Ga0.53N without cracking. The interlayers are then believed to be sufficiently relaxed to place the AlGaN under compressive strain. The mechanisms for this relaxation have been studied using high angle annular dark field (HAADF) imaging, conventional transmission electron microscopy (TEM), energy-filtered TEM (EFTEM) and electron energy loss spectroscopy (EELS). It is found that relaxation takes place through both the small-scale cracking of the interlayer and the generation of misfit dislocations at the GaN/AlN interface. EELS and EFTEM have been used to probe the Al and Ga content of both the material filling the interlayer cracks, and the interlayer itself. This chemical analysis suggests Ga-rich AlGaN areas inside the interlayer cracks and also significant compositional variations in defect-free interlayer regions. It is observed that relaxation by the generation of misfit dislocations results in an increase in the threading dislocation density of the AlGaN layer, in part due to the bending up of misfit dislocations at crack walls.
Optical and structural properties of in situ Cu doped GaN thin films grown on sapphire substrates were optically investigated by means of Raman, photoluminescence (PL), and absorption spectroscopy. Different Cu concentrations in the films were analyzed by secondary ion mass spectroscopy (SIMS) and found to vary from 2×1016 cm-3 to 5×1017 cm-3. Raman studies confirmed high crystalline quality of GaN:Cu with no major structural damages due to Cu incorporation. PL investigation revealed that the origin of the emission around 2.4 eV is most likely due to Cu incorporation. The electrical conductivity of the samples was analyzed by Hall measurements and the found semi-insulating behavior was assigned to the compensation of intrinsic donors by the deep Cu acceptor states.
GaAs/GaN heterostructures were grown by molecular-beam epitaxy using GaN/supphire (0001) templates. In spite of a ∼20% lattice mismatch, epitaxial growth was realized, so that the GaAs films showed good adhesion and their surface had a large mirror-like area. The GaAs films were as thick as 1um. The surface profile was characterized by atomic-force microscopy, which gave an average roughness of 10 nm for a 5×5 μm scan. Micro-Raman characterization and transmission electron microscopy (TEM) showed that the epitaxial GaAs films had zincblende lattice with (111) orientation, whereas the GaN substrates had wurtzite symmetry. The GaAs/GaN interface was found to be flat and abrupt. A large number of defects have been observed which originated from relaxation of the large lattice mismatch. The defects included misfit dislocations and nanocavities at the interface, as well as dislocations and stacking faults in the bulk of the GaAs film. Sharp interference fringes and characteristic behavior were observed for the ψ and Δ parameters of spectroscopic ellipsometry in the range of 0.75-5.3 eV. Simulation of the optical properties of the GaAs/GaN/sapphire heterostructure indicated a reasonably good optical quality of the layers and interfaces. Photoluminescence (PL) spectra recorded at the temperatures from 17 to 300 K revealed wide and weak radiative bands. Non-radiative processes dominated in recombination of non-equilibrium carriers. The observed PL broadening originated from the band tails that were a result of the high density of charged defects.
Two-dimensional electro-thermal simulations of GaN-based metal-semiconductor field-effect transistor are performed in the framework of the drift-diffusion model. The dependence of the hot spot temperature in transistors with many gates on the gate-to-gate pitch is studied. The case of SiC substrate is compared to the case of sapphire substrate. The ambient temperature effect on transistor performance is simulated. The specific of a thermal breakdown in GaN-based devices is discussed. The results obtained can be useful for the optimization of the thermal design for field-effect transistors.