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Based on the complex function theory and the homogenization principle, an universal approach of solving the dynamic stress concentration around a circular cavity in two-dimensional (2D) inhomogeneous medium is developed. The Helmholtz equation with variable coefficient is converted to the standard Helmholtz equation by means of the general conformal transformation method (GCTM) analytically. As an example, the inhomogeneous medium with density varying as a function of two spatial coordinates and the constant elastic modulus is studied. The dynamic stress concentration factors (DSCF) are calculated numerically. It shows that medium inhomogeneous parameters and wave numbers have significant influence on the dynamic stress concentration by the circular cavity in two-dimensional inhomogeneous medium.
Multilayer bulk heterojunction photovoltaic cells using chlorotricarbonyl rhenium (I) bis(phenylimino)acenaphthene (Re-DIAN) complex as photosensitizer were studied. The complex is sublimable, has lower band gap, good thermal stability and good processibility. It has broad absorption in UV-visible region. Therefore, Re-DIAN exhibits good photosensitising property for photovoltaic cells. Multilayer bulk heterojunction photovoltaic cells with simple structures were fabricated base on Re-DIAN complex. The active layer consists of a blend of Re-DIAN and fullerene that were co-deposited in the same layer by vacuum deposition. The photovoltaic properties of the devices were studied by irradiaton under AM1.5 simulated solar light. The effects of changing the co-deposition film thickness, amount of Re-DIAN photosensitizer, and hole transporting materials were studied.
The impact of growth temperature and Ga/N flux ratio on deep levels in GaN grown by molecular beam epitaxy (MBE) is systematically investigated using both deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) in a study designed to map out the presence and concentration of defects over a defined region of the MBE GaN growth phase diagram. A series of Si-doped GaN films were grown to cover a substrate temperature range and a Ga/N flux ratio range that spans from the N stable to the Ga droplet regimes along both variables. Identical growth templates were used to eliminate variations in dislocations between samples so that point defect variations could be tracked. For these samples, traps are detected at EC-Et=0.25, 0.60, 0.90, 1.35, 2.40, 3.04, and 3.28 eV. The near valence bands states at EC–3.04 and EC–3.28 eV are found to be strongly dependent on Ga/N flux with decreased concentrations as a function of increasing Ga flux toward the Ga droplet regime, but with little effect from growth temperature. The EC-1.35 eV level shows a strong dependence on growth temperature and only slight dependence on Ga/N flux ratio. In contrast, the concentration of the EC-Et=0.25, 0.90 eV levels increased with increasing Ga flux toward the Ga droplet regime, while the EC-Et=0.60 shows no dependence. The variation in concentration of the EC-2.40 eV level that has been related to VGa was difficult to quantify, but tends to increase towards nitrogen rich growth. The dependencies for the detected states with respect to growth temperature and Ga/N flux ratio suggest different physical point defect sources.
As contribution to a recent debate (James, 1998; Murphy et al., 1997, 1998) the proportion of twins following ovulation induction (OI) or assisted conception (AC) in 1994 in Oxfordshire and West Berkshire was estimated, and by extrapolation the natural twinning rate in England and Wales was judged to have maintained a plateau phase since the 1970s. Similar figures for 1995 and 1996 from the same study, and hence a more stable local estimate, are now provided. The proportions, as before, were estimated from women's responses to a questionnaire within a case-control study, with ascertainment from general practitioners' records or hospital case-notes for non-responders or for those excluded from the study originally. In 1994, 1995 and 1996 the proportion of twins following OI/AC was overall 27% (24%, 30% and 27% respectively). Restriction to the 87% locally resident made no difference. The national crude twinning rate for those years was overall 13·3 per 1000 maternities (12·8, 13·6 and 13·4 respectively).
An AlxGa1−xN/GaN two-dimensional electron gas structure with x = 0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm−2 and a mobility of 1.9 × 104 cm2/Vs at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov-de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov-de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3 × 10−12 s.
The contribution of spontaneous and piezoelectric polarization to the formation of a 2DEG in AlGaN/GaN heterostructures was investigated using undoped AlGaN/GaN structures. Hall measurements of 2DEG density on such structures with varying Al percentage (8%-27%) and varying thickness of the AlGaN layer (30-500Å) indicated that donor-like surface states at an energy of 1.42eV below the conduction band were the source of electrons in the 2DEG. Field effect transistors were fabricated on such undoped heterostructures. For an AlGaN/GaN structure with 0.27 Al mole fraction, power density in excess of 3.5 W/mm at 6 GHz with corresponding maximum PAE of 33.5% was obtained. These results exceed the best reported power performance of MBE grown GaN HFETs on sapphire, thus demonstrating the excellent capability of MBE grown GaN heterostructures for microwave power applications.
Temperature-variable Hall and Shubnikov- de Haas effects have been used to study persistent photoconductivity in an AlGaN/GaN heterojunction. At liquid helium temperatures, the mobility in this structure was close to 55000 cm2/Vs. A blue GaN-based light emitting diode was used to illuminate the sample. This illumination resulted in a persistent photocurrent, which allowed us to vary the carrier density and study the dependence of the mobility on the carrier concentration. Exposing the sample to this light resulted in an increase in the carrier density. For small increases in the density, the mobility also increased. However, unlike in previous reports by other authors, extended illumination resulted in an increase in the density and a decrease in the mobility. The initial increase in the mobility is attributed to increased screening due to the increase in the carrier density, while the decrease in the mobility may be attributed to alloy scattering.
Self-annealing properties of electroplated and sputtered copper films at room temperature were investigated in this study, in particular, the effect of copper film thickness, electrolyte systems used, as well as their level of organic additives for electroplating. Real-time grain growth was observed by transmission electron microscopy. Sheet resistance and X-ray diffraction measurements further confirmed the recrystallization of the electroplated copper film with time. The recrystallization of electroplated films was then compared with that of sputtered copper films.
An AlxGa1-xN/GaN two-dimensional electron gas structure with x = 0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm-2 and a mobility of 1.9 × 104 cm2/Vs at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov-de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov-de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3 × 10-12 s.
James questions the validity of the very tentative statement made in the final sentence of our paper. Our claim concerned the proportion of twins in Britain in the 1990s that might have arisen through subfertility treatment and was linked to the s
uggestion that the natural twinning rate might still be in decline. If this were true, we, like James, would regard that prospect with concern.
Trends in multiple birth rates are thought to have been substantially
affected by subfertility treatments in the last 25 years, but there are few
quantitative assessments of this. This paper examines trends in twin and
higher multiple birth rates separately in Scotland, England and Wales and
compares their course with corresponding multiple birth rates in the Oxford
Record Linkage Study area, where the proportions following subfertility
treatment are documented. National data on prescriptions for subfertility
treatments reinforce the view that they have had a major effect on the trends,
and currently perhaps 60% of triplet and higher order births
and 15% of twins
follow their use
The resistance and magnetoresistance of Gd/W Multilayers have been measured at temperatures between 300K and 4K and in fields of up to 7 Tesla. The temperature coefficent of resistance is strongly influenced by the thickness of the Gd layers, becoming negative at a Gd layer thickness of 0.8nm. The Magnetoresistance is dominated by the Gd below about 100K. Multilayers which show reduced saturation magnetisation at 4K also have an unusual form of magnetoresistance around 100K. We suggest that this may be due to antiferromagnetic alignment of the Gd and W Moments at low applied fields.
Optical Bragg reflectors consisting of the binaries AlAs and GaAs were investigated using double crystal topography and diffractometry. In undoped mirror stacks stress relaxation due to the formation of misfit dislocations was observed, which could be prevented by doping the stacks with 1018cm−3 silicon. In topographs taken in the substrate and different satellite reflections an unusual vanishing of the contrast of different segments of the misfit dislocations takes place, that shows these different segments to be located at different levels of the stack. The contrasts of the threading dislocations are quite similar in the substrate and the satellite reflections whereas the misfit dislocations change their contrast markedly.
A cash flow model is proposed as a way of analysing uncertainty in the future development of a general insurance company. The company is modelled alongside the market in aggregate so that the impact of changes in premium rates relative to the market can be assessed. An extensive computer model is developed along these lines, intended for use in practical applications by actuaries advising the management of genera1 insurance companies. Simulation methods are used to explore the consequences of uncertainty, particularly in regard to inflation and investments. Some comments are made on the role of actuaries in general insurance. Alternative approaches to describing the behaviour of an insurance firm in the market are considered.
The deposition rate of silicon dioxide from TEOS/02 capacitively-coupled plasmas increases with increasing applied rf power, increasing total pressure and decreasing wafer temperature. These dependences can be explained by a mechanism in which deposition occurs through both an ionassisted and an oxygen atom initiated pathway. The ion-induced deposition rate increases with increasing rf power density and decreasing total pressure, and is independent of wafer temperature. Assuming a well-mixed plasma, and using literature values for reaction rate parameters and rate forms for oxygen plasma reactions, the rate of atom-induced deposition was found to be independent of temperature and TEOS concentration and directly proportional to oxygen atom concentration. The model suggests that the apparent negative activation energy for deposition results from competition between deposition and thermally-activated atomic oxygen surface recombination. The derived deposition rate form is consistent with an Eley-Rideal mechanism in which the rate determining step for film deposition is oxidative attack of adsorbed TEOS or TEOS fragments by a activated oxygen.
1.1 The traditional approach to examining the financial status of a company is to look at the balance sheet and the profit and loss account. Such information is usually publicly available, it is certified by the auditors as having been drawn up according to relevant accounting standards and it is generally presumed to communicate reliable information.
1.2 In the case of a manufacturing or trading company the profit and loss account records purchases and sales and the balance sheet will include a valuation of stock in hand, since it is anticipated that this will give rise to future sales income. Working capital is required because products have to be manufactured or purchased before they can be sold. Profit is realized when the product is sold for more than it cost to buy it or to make it.