Bibliothek

Notizen: This article describes the modeling of transmission line transformers using methods which are analogous to the methods used to analyze both the ac frequency and transient response of conventional transformers. Transformers in which the lines used to construct them are wound inductively, in order to suppress parasitic short circuit paths within the transformers, are analyzed. It is shown that by using this technique the resulting inductive isolation of the secondary circuit from the primary substantially reduces pulse droop and pulse distortion. Despite the apparent complexity of these transformers, a method by which circuit models of these transformers can be deduced is given. From these models very simple equivalent circuits can be derived which can then be used to calculate accurately the performance characteristics of the transformers and, in particular, predict the pulse distortion characteristics of these devices. Different winding configurations are also considered and it is shown that, by the use of mutually coupled winding of the transmission lines in the transformer, it is possible to minimize pulse droop. Finally it is shown that the modeling technique can be used, in modified form, to analyze the ac frequency response of this type of transformer. © 1999 American Institute of Physics.

Notizen: We have developed a novel multitip scanning Kelvin probe which can measure changes in biological surface potential ΔVs to within 2 mV and, quasisimultaneously monitor displacement to 〈1 μm. The control and measurement subcomponents are PC based and incorporate a flexible user interface permitting software control of each individual tip, measurement, and scan parameters. We review the mode of operation and design features of the scanning bio-Kelvin probe including tip steering, signal processing, tip calibration, and novel tip tracking/dithering routines. This system uniquely offers both tip-to-sample spacing control (which is essential to avoid spurious changes in ΔVs due to variations in mean spacing) and a dithering routine to maintain tip orientation to the biological specimen, irrespective of the latter's movement. These features permit long term (〉48 h) "active" tracking of the displacement and biopotentials developed along and around a plant shoot in response to an environmental stimulus, e.g., differential illumination (phototropism) or changes in orientation (gravitropism). © 1999 American Institute of Physics.

Notizen: Current research on the Earth's upper atmosphere requires molecular parameters of unprecedented detail and accuracy. For example, state-of-the-art models of the vacuum-ultraviolet (VUV) absorbing properties of the atmosphere call for absorption cross sections with details on the scale of the Doppler linewidths. As a consequence, spectroscopic data at resolving powers of the order of 106 are needed. Current particular needs are for ultra-high-resolution absorption cross-section data for some bands of NO and O2 in the 170–185 nm spectral region. To meet these requirements, an existing, portable, VUV Fourier-transform (FT) spectrometer will be moved to a synchrotron-radiation facility (Photon Factory, KEK, Japan) and VUV photoabsorption cross sections of NO and O2 will be measured. It has been demonstrated so far with emission line sources that the FT spectrometer will perform to ≤139 nm. Extension of work on absorption measurements to shorter wavelengths and with ultrahigh resolution requires an improved light source. Synchrotron radiation from a storage ring facility is the best alternative. © 1995 American Institute of Physics.

Notizen: The design, construction, and operational characteristics of a 200 kV transmission line pulse transformer is described. The transformer is wound using a new winding method that enhances the isolation of the output of the transformer from the input. As a result, pulse droop is substantially reduced, minimizing pulse distortion in the transformer. The ways in which both pulse rise time and droop can be further improved are investigated using a simple model for the transformer. The frequency response performance of the transformer is also described and modeled. As a result, it is shown that this type of transformer has the potential to be used as a high-frequency, continuously excited power transformer. © 1996 American Institute of Physics.

Notizen: Abstract. Agricultural soils, having been depleted of much of their native carbon stocks, have a significant CO2 sink capacity. Global estimates of this sink capacity are in the order of 20-30 Pg C over the next 50-100 years. Management practices to build up soil C must increase the input of organic matter to soil and/or decrease soil organic matter decomposition rates. The most appropriate management practices to increase soil C vary regionally, dependent on both environmental and socioeconomic factors.In temperate regions, key strategies involve increasing cropping frequency and reducing bare fallow, increasing the use of perennial forages (including N-fixing species) in crop rotations, retaining crop residues and reducing or eliminating tillage (i.e. no-till). In North America and Europe, conversion of marginal arable land to permanent perennial vegetation, to protect fragile soils and landscapes and/or reduce agricultural surpluses, provides additional opportunities for C sequestration.In the tropics, increasing C inputs to soil through improving the fertility and productivity of cropland and pastures is essential. In extensive systems with vegetated fallow periods (e.g. shifting cultivation), planted fallows and cover crops can increase C levels over the cropping cycle. Use of no-till, green manures and agroforestry are other beneficial practices. Overall, improving the productivity and sustainability of existing agricultural lands is crucial to help reduce the rate of new land clearing, from which large amounts of CO2 from biomass and soil are emitted to the atmosphere.Some regional analyses of soil C sequestration and sequestration potential have been performed, mainly for temperate industrialized countries. More are needed, especially for the tropics, to capture region-specific interactions between climate, soil and management resources that are lost in global level assessments.By itself, C sequestration in agricultural soils can make only modest contributions (e.g. 3-6% of total fossil C emissions) to mitigating greenhouse gas emissions. However, effective mitigation policies will not be based on any single ‘magic bullet’ solutions, but rather on many modest reductions which are economically efficient and which confer additional benefits to society. In this context, soil C sequestration is a significant mitigation option. Additional advantages of pursuing strategies to increase soil C are the added benefits of improved soil quality for improving agricultural productivity and sustainability.

Notizen: Abstract. Nitrogen (N) leaching losses from a shallow limestone soil growing a five course combinable croprotation (oilseed rape, wheat, peas, wheat, barley) were measured from 1990 until 1995 using porous ceramic cups, at 60 cm depth, and drainage estimates. The crops were grown with three husbandry systems and two levels of N fertilizer. The husbandry systems were designed to reflect local practice (Standard), the best possible techniques to reduce N loss (Protective) and an Intermediate system which was a compromise between the two. Nitrogen was applied at full and half recommended rates. Drainage started during September in four years and November in one year, with above average drainage in three years. Losses of N were largest after peas (58 kg/ha) and oilseed rape (42 kg/ha), and least (17 kg/ha) before peas sown in spring after a cover crop. Over five years, the Protective management system, which used early sowing and shallow cultivation wherever possible, lost least N (31 kg/ha/y) and the Standard system, with conventional drilling dates and ploughing as the primary cultivation, lost most (49 kg/ha/y). Halving the N fertilizer decreased N loss by 11 kg/ha/y, averaged over the rotation. None of the treatments gave mean drainage water nitrate concentrations of less than 50 mg/l, averaged over the five years. Changes to arable cropping alone will not eliminate the need for other measures to control nitrate concen-trations in public drinking water supplies.

Notizen: Abstract. An effective fertilizer recommendation system requires information on seasonal, soil-related and cultural variations in soil mineral nitrogen (N) and nutrient requirements of the crop. This can be provided by dynamic N turnover models, such as listed by Plentinger & Penning De Vries (1996). In this paper, we describe a survey of farmer opinion designed to ascertain what farmers want from such a decision support system. Over 100 farmers were surveyed. Surveyed farmers requested that default values be available for all model inputs. Inputs should be entered both by windows-based menu (for clarity) and tabular format (for speed), have user-selected units, and be fully supported by context-sensitive help. The system should have a hierarchical structure allowing access to fixed parameters, and be compatible with commonly used farm recording packages. Recommendations should be provided both for the field (single and optional application rates), and in tabular format across the whole farm. Simulations should be easily rerun using more recent crop and weather data. Turnover processes underlying recommendations should be illustrated by flow diagrams of flux between pools, pie charts of fertilizer fate, bar charts of movement down the soil profile and graphical plots of changes in N status against time.

Notizen: Light-emitting diodes (LEDs) of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) were fabricated using high surface area hole-injecting network electrodes made in situ from blends of polyaniline (PANI) in low molecular weight polyester resin. Due to the high interface area between the PANI network and the active luminescent layer and enhancement of the local electric field at the rough surface of the PANI network, charge carrier injection was significantly improved. LEDs using the PANI network electrode have lower device operating voltage, higher quantum efficiency, and significantly increased brightness. Polymer LEDs fabricated with the PANI-CSA network as anode and aluminum:lithium alloy (0.2% Li) as the cathode turn on at 1.7 V, and achieve a brightness in excess of 400 cd/m2 at 3 V with a measured external quantum efficiency of 2.23% photons/electron. © 1995 American Institute of Physics.

Notizen: In this letter, we present the results of an experimental study of low-temperature-grown GaAs, which clearly resolves separately both carrier trapping and recombination processes. We extend our previous model to account for the observed carrier dynamics, and show how the material growth and annealing conditions can be adjusted to optimize the material properties for all-optical device applications. © 1997 American Institute of Physics.