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Time series of Earth observation (EO) data (Landsat Thematic Mapper (TM), U.S. National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) and European Remote-sensing Satellite synthetic-aperture radar (ERS SAR)) were obtained for a 2250 km2 mountainous basin in northern Sweden to validate snow-cover area (SCA) estimates produced by a conceptual model (HBV) during three melt seasons. SCA depletion curves derived for each image type, arid coincident images, reveal that the SCA estimate varies with the sensor. Discrepancies betweenTM and AVHRR appear to be an effect of spatial resolution. However, differences betweenTM and SAR are not simply related. Since more AVHRR thanTM data were available, a TM-equivalent SCA was derived from AVHRR by relating TM SCA to AVHRR channel 1 reflectance. The TM-equivalent SCA was used to test SCA simulated by HBV for the 1992 melt season. Although the modelled and TM-equivalent SCA were in reasonable agreement, the modelled SCA declined faster than the TM-equivalent SCA. Partial recalibration of model parameters controlling snowpack accumulation improved the match between the modelled and EO-derived SCA decline. The recalibrated parameters were verified using SCA maps generated for the 1996 and 1998 melt seasons. The adjusted parameter sets had little effect on the Nash-Sutcliffe R2 runoff fit but improved the volume fit in all three years.
Previous analyses of the history of Phanerozoic marine biodiversity suggested that the post-Paleozoic increase observed at the family level and below was caused, in part, by an increase in global provinciality associated with the breakup of Pangea. Efforts to characterize the Phanerozoic history of provinciality, however, have been compromised by interval-to-interval variations in the methods and standards used by researchers to calibrate the number of provinces. With the development of comprehensive, occurrence-based data repositories such as the Paleobiology Database (PaleoDB), it is now possible to analyze directly the degree of global compositional disparity as a function of geographic distance (geo-disparity) and changes thereof throughout the history of marine animal life. Here, we present a protocol for assessing the Phanerozoic history of geo-disparity, and we apply it to stratigraphic bins arrayed throughout the Phanerozoic for which data were accessed from the PaleoDB. Our analyses provide no indication of a secular Phanerozoic increase in geo-disparity. Furthermore, fundamental characteristics of geo-disparity may have changed from era to era in concert with changes to marine venues, although these patterns will require further scrutiny in future investigations.
The objective of this study was to develop a novel methodology that enables pig diets to be formulated explicitly for environmental impact objectives using a Life Cycle Assessment (LCA) approach. To achieve this, the following methodological issues had to be addressed: (1) account for environmental impacts caused by both ingredient choice and nutrient excretion, (2) formulate diets for multiple environmental impact objectives and (3) allow flexibility to identify the optimal nutritional composition for each environmental impact objective. An LCA model based on Canadian pig farms was integrated into a diet formulation tool to compare the use of different ingredients in Eastern and Western Canada. By allowing the feed energy content to vary, it was possible to identify the optimum energy density for different environmental impact objectives, while accounting for the expected effect of energy density on feed intake. A least-cost diet was compared with diets formulated to minimise the following objectives: non-renewable resource use, acidification potential, eutrophication potential, global warming potential and a combined environmental impact score (using these four categories). The resulting environmental impacts were compared using parallel Monte Carlo simulations to account for shared uncertainty. When optimising diets to minimise a single environmental impact category, reductions in the said category were observed in all cases. However, this was at the expense of increasing the impact in other categories and higher dietary costs. The methodology can identify nutritional strategies to minimise environmental impacts, such as increasing the nutritional density of the diets, compared with the least-cost formulation.
Throughout 1915 and 1916 a study was made of the dietaries of forty-seven labouring class families in Glasgow, and in February 1917, shortly after the publication of the Devonport Voluntary Ration, ten of the original families were investigated for a second time. The results of all these studies were published in the Proceedings of the Royal Society of Edinburgh, vol. xxxvii, p. 117, 1917.
In November 1917, after nine months of voluntary rationing, after the price of bread had been reduced from to per 2 lbs., and just before the introduction of the Rhondda Voluntary Ration, the diets of eight of the families were studied for a third time.
The dissociation channels of two prominent bound exciton complexes in wurtzite GaN thin films are determined via an extensive temperature dependent photoluminescence study. The shallow donor bound exciton dissociation at low temperatures (T ≤ 50 K) is found to be dominated by the release of a free exciton with thermal activation energy consistent with the exciton localization energy. At higher temperatures a second dissociation channel with activation energy EA = 28 ± 2 meV is observed. The dissociation of a bound exciton complex with exciton localization energy EXloc = 11.7 meV is also dominated by the release of a free exciton. In contrast to previous studies evidence is presented against the hypothesis of this emission being due to the exciton bound to an ionized donor. We find that it originates most likely from an exciton bound to a neutral acceptor.
InGaN epilayers have been investigated for use in photovoltaic solar cells for the past years. At present, almost all photovoltaic device structures reported have exhibited very low short circuit currents and thus very low solar conversion efficiency. This phenomenon has been attributed to point and extended defect chemistry in InGaN epilayers (e.g. vacancies, misfit dislocations, and V-defects), as well as to spinodal decomposition of the strained InGaN wurtzite lattice system. These defects become more dominant for higher indium concentration InGaN epilayers needed for multijunction photovoltaic device structures. In this work, we will report on the growth and characterization of indium-rich InGaN epilayers that have been grown by novel MOCVD growth technology, including the growth at superatmospheric reactor pressures.
Studies to quantify genetic variation in cassava germplasm, available within the national breeding programmes in Africa, have been limited. Here, we report on the nature and extent of genetic variation that exists within 1401 cassava varieties from seven countries: Tanzania (270 genotypes); Uganda (268); Kenya (234); Rwanda (184); Democratic Republic of Congo (DRC; 177); Madagascar (186); Mozambique (82). The vast majority of these genotypes do not exist within a formal germplasm conservation initiative and were derived from farmers' fields and National Agricultural Research Systems breeding programmes. Genotypes were assayed using 26 simple sequence repeat markers. Moderate genetic variation was observed with evidence of a genetic bottleneck in the region. Some differentiation was observed among countries in both cultivars and landraces. Euclidean distance revealed the pivotal position of Tanzanian landraces in the region, and STRUCTURE analysis revealed subtle and fairly complex relationships among cultivars and among landraces and cultivars analysed together. This is likely to reflect original germplasm introductions, gene flow including farmer exchanges, disease pandemics, past breeding programmes and the introduction of cultivars from the International Institute of Tropical Agriculture – Nigeria. Information generated from this study will be useful to justify and guide a regional cassava genetic resource conservation strategy, to identify gaps in cassava diversity in the region and to guide breeding strategies.
Thermoelectric materials with stable mechanical and chemical properties at high temperature are required for power generation applications. For example, gas temperatures up to 1000°C are normally present in the waste stream of industrial processes and this can be used for electricity generation. There are few semiconductor materials that can operate effectively at these high temperatures. One solution may be the use of wide bandgap materials, and in particular GaN-based materials, which may offer a traditional semiconductor solution for high temperatures thermoelectric power generation. In particular, the ability to both grow GaN-based materials and fabricate them into devices is well understood if their thermoelectric properties are favorable. To investigate the possibility of using III-Nitride and its alloys for thermoelectric applications, we synthesized and characterized room temperature thermoelectric properties of metal organic chemical vapor deposition grown GaN and InGaN with different carrier concentrations and indium compositions. The promising value of Seebeck coefficients and power factors of Si-doped GaN and InGaN indicated that these materials are suitable for thermoelectric applications.
Outputs of suspended sediment and bedload from the 7·7 km2 moorland Monachyle basin and the 6·8 km2, 40%-forested Kirkton basin near Balquhidder, and inputs from tributary streams and mainstream bank erosion, are compared. Sediment yield is about three times higher in the forested basin and varies more sensitively with streamflow, suggesting greater availability of erodible sediment. The output is predominantly of suspended sediment and is derived mainly from tributary streams. Initial observations following partial moorland ploughing and forest clearfelling in 1986 indicate that erosion of timber loading areas and logging roads is the main sediment source.
Snowmelt modelling is of potential value in flood forecasting, reservoir management, and understanding stream acidification. It involves meteorological extrapolation, snowmelt calculation, meltwater routing, and snowpack depletion. A simple conceptual model using air temperature can reproduce the general pattern of daily streamflow in basins of >100 km2 but is prone to parameter instability. At a point scale and with the benefit of automatic weather station data the energy balance approach is superior to temperature index methods, but the roughness length parameter is again unstable in time and space. Even in a small (0·4 km2) basin this approach has to be coupled with an adequate flow routing model. Current research is comparing alternative models and data inputs in an intermediate-sized basin.
Epitaxial ZnO layers heavily doped with Ga (GZO) were grown at 400 °C under metaland oxygen-rich conditions in terms of metal-to-reactive oxygen ratio by plasma-assisted molecular beam epitaxy (MBE). Several atomic force microscopy (AFM) techniques were used to characterize the surface morphology and electrical properties of these GZO films in ambient conditions. Local I-V spectra indicate that layers grown under both O-rich and metal-rich conditions are highly resistive until a relatively high voltage sweep (±12 V) is used. After removal of an insulating surface layer, conduction is possible at lower voltages, but eventually the film resistivity increases and it again becomes insulating. In addition to local I-V spectra, local charge injection and subsequent surface potential measurements were used to probe surface charging characteristics. For charge injection experiments, a reverse-bias voltage is applied to the sample while scanning in contact mode with a metallized tip. The resultant change in surface potential due to trapped charge is subsequently observed using scanning Kelvin probe microscopy (SKPM). The layers deposited in a metal-rich environment demonstrate the expected behavior, but the O-rich layers show anomalous negative and positive charging. Finally, surface photovoltage (SPV) measurements using above-bandgap UV illumination were performed. The GZO layers produce SPV values of 0.4 to 0.5 eV, where the films deposited in an O-rich environment have slightly higher SPV values and faster restoration.
InGaN/GaN/AlGaN multiple quantum well light emitting diodes (MWQ LED's) with different levels of p-doping in the contact layer have been characterized using surface photovoltage spectroscopy (SPS). Due to the high sensitivity of the SPS technique to the electric field, there is a strong correlation between the p-doping level in the contact layer and the magnitude of the SPS signal originating from the MQW region. The experimental results are confirmed by a numerical simulation.
The Structure I type binary metal clathrates of K/Si, Rb/Si and Cs/Sn have been synthesised and studied by powder X-ray diffraction and solid state NMR. Rietveld analysis shows that in all three materials some of the cages are empty, and that in the Cs/Sn clathrate there are vacancies in the Sn framework. The NMR results yield Knight shifts for 29Si and 39K and confirm that the Cs/Sn clathrate is not conducting. Many of the features of the NMR spectra can be understood in terms of the distributions of atom vacancies.
As the group III nitride semiconductor technology matures, an increasing number of devices are being fabricated with high Al fraction AlGaN. In this study, ohmic behavior is achieved using Ti/Al/Pt/Au contacts to n-Al0.4Ga0.6N, which is the highest Al fraction for which ohmic contact formation has been reported. The effect of contact composition, pretreatment, and annealing conditions is studied by 30 s isochronal annealing experiments between 500°C and 1000°C. A specific contact resistance ρC of (5±3) × 10−5 ωcm2 is obtained using Ti(26 nm)/Al(74 nm)/Pt(50 nm)/Au(50 nm) contacts to n-Al0.4Ga0.6N annealed in N2 at 800°C; however, this value is shown to be artificially high because the metal sheet resistance RM is 4 ω/⊏ causing an artifact in the data analysis. All contacts with ρC < 10−3 ωcm2 exhibit a local minimum in ρC after annealing at 800°C. The observed increase in ρC upon annealing at 850°C and 900°C, however, is not an artifact originating from a change in RM. The top Au layer is found to play an active role in forming ohmic contacts with low ρC, since omitting the Au layer yields an increase in ρC of two orders of magnitude after annealing at 800°C. Furthermore, leaving out the Au layer requires an annealing temperature of 700°C to result in linear I-V curves for currents up to 100 µA, as opposed to 500°C when the Au layer is present. The role of Au is further studied in Ti(26 nm)/Al(74 nm)/Ni(50 nm)/Au(50 nm) contacts, where Rutherford backscattering spectroscopy reveals Ga in the metal layer and/or Au buried deeper than the original semiconductor-metal interface, and x-ray diffraction indicates the formation of new phases to happen concurrently with a decrease in ρC of three orders of magnitude.
Optical absorption spectra of undoped, n-type, and semi-insulating 6H and 4H bulk silicon carbide (SiC) were obtained in the spectral region of 200 – 3200 nm (6.20 – 0.3875 eV). Several features were observed in the absorption spectra collected for various samples. A sharp peak below the band gap was observed in 4H SiC. The intensity of this peak was observed to increase in samples that exhibit larger absorption due to free carriers, which leads us to conclude that the defect responsible for this peak is also the source of the free carriers in the materials. Additionally, a series of optical absorption peaks separated by approximately 21 meV were observed around 0.9185 eV (1350 nm). These peaks are zero phonon lines of intraband transitions in the VSi 3d shell. The optical absorption near the band edge was observed to be sample dependent. The variation of the band gap as a function of temperature is also observed to be sample dependent.
Near-band-edge cathodoluminescence emission for some nominally undoped GaN samples decreases with time during electron bombardment in a scanning electron microscope. The rate of decrease depends on the incident beam voltage and current and on the size of the area being scanned, which is determined by the magnification used for the scanning electron microscope. Faster intensity decreases occur with lower beam voltages (10kV versus 20kV), higher beam currents (600nA or 200nA versus 35nA) and smaller scanned areas (120K magnification versus 15K magnification). For larger scanned areas, the maximum luminescence intensity occurs 10's or 100's of seconds after the start of electron bombardment, and the intensity then decreases with further bombardment. In some cases the size of the degraded region is larger than the bombarded area. These observations suggest that the degradation is caused by electromigration of impurities or other charged defects within the GaN resulting from local charging by electron bombardment.
This paper presents UV imaging results for a 256×256 AlGaN Focal Plane Array that uses a back-illuminated AlGaN heterostructure p-i-n photodiode array, with 30×30 μm2 unit cells, operating at zero bias voltage, with a narrow-band UV response between 310 and 325 nm. The 256×256 array was fabricated from a multilayer AlGaN film grown by MOCVD on a sapphire substrate. The UV response operability (>0.4×average) was 94.8%, and the UV response uniformity (σ/μ) was 16.8%. Data are also presented for back-illuminated AlGaN p-i-n photodiodes from other films with cutoff wavelengths ranging between 301 and 364 nm. Data for variable-area diagnostic arrays of p-i-n AlGaN photodiodes with a GaN absorber (cutoff=364 nm) show: (1) high external quantum efficiency (50% at V=0 and 62% at V=-9 V); (2) the dark current is proportional to junction area, not perimeter; (3) the forward and reverse currents are uniform (σ/μ=50% for forty 30×30 μm2 diodes at V=−40 V); (4) the reverse-bias dark current data versus temperature and bias voltage can be fit very well by a hopping conduction model; and (5) capacitance versus voltage data are consistent with nearly full depletion of the unintentionally-doped 0.4 μm thick GaN absorber layer and imply a donor concentration of 3-4×1016 cm−3.