Finite element analysis (FEA), a computer-based method for solving complex structural problems, was used to determine the wall stress distribution in three cases of model infrarenal abdominal aortic ...aneurysms representing common problems in determining risk of aneurysm rupture. The point of maximum circumferential wall stress in a spherical model aneurysm was located near the junction of the aneurysm and the nondilated aorta, while maximum longitudinal wall stress was located at the point of maximum diameter of the aneurysm. FEA showed that cylindrically shaped constant thickness model aneurysms had a higher maximum circumferential stress (sigma c = 11.9 X 10(5) dyn/cm2) and a comparable maximum longitudinal wall stress (sigma L = 6.6 X 10(5) dyn/cm2) when compared with spherical model aneurysms of the same diameter (sigma c = 8.1 X 10(5) dyn/cm2 and sigma L = 6.2 X 10(5) dyn/cm2). Analysis of the aorta to aneurysm diameter ratio (A:a gradient) indicated that aortic size is important in determining aneurysm wall stress and that the relationship between aortic size and wall stress is dependent upon aneurysm wall thickness. We conclude that the ability of the aneurysm wall to withstand stress in the longitudinal as well as the circumferential direction is an important factor determining aneurysm rupture. Finally, this investigation showed that FEA is a versatile tool for use in studying the mechanics of vascular structures, making it potentially more useful than size alone in estimating the clinical significance of abdominal aortic aneurysms.
In the United States, environmentally impaired rivers are subject to regulation under total maximum daily load (TMDL) regulations that specify watershed wide water quality standards. In California, ...the setting of TMDL standards is accompanied by the development of scientific and management plans directed at achieving specific water quality objectives. The San Joaquin River (SJR) in the Central Valley of California now has a TMDL for dissolved oxygen (DO). Low DO conditions in the SJR are caused in part by excessive phytoplankton growth (eutrophication) in the shallow, upstream portion of the river that create oxygen demand in the deeper estuary. This paper reports on scientific studies that were conducted to develop a mass balance on nutrients and phytoplankton in the SJR. A mass balance model was developed using WARMF, a model specifically designed for use in TMDL management applications. It was demonstrated that phytoplankton biomass accumulates rapidly in a 88 km reach where plankton from small, slow moving tributaries are diluted and combined with fresh nutrient inputs in faster moving water. The SJR-WARMF model was demonstrated to accurately predict phytoplankton growth in the SJR. Model results suggest that modest reductions in nutrients alone will not limit algal biomass accumulation, but that combined strategies of nutrient reduction and algal control in tributaries may have benefit. The SJR-WARMF model provides stakeholders a practical, scientific tool for setting remediation priorities on a watershed scale.
Aberration corrected HRTEM has been successfully applied to image the atomic motion at the edges and surfaces of nanoparticles of ceria of various types under electron irradiation. Here we identify ...movements not only on {100} facets, but also on {110} and even {111} facets, previously considered stable. However, the degree of movement varies strongly and HRTEM is evidently the preferred technique to measure relative stability at high spatial resolution as it does not require extended surfaces as in scanning probe microscopy (SPM) or chemical methods. The advantage of aberration correction shows in suppression of contrast from the carbon support films and the absence of delocalisation fringes at particle edges, apart from improving point resolution.
A variety of procedures associated with in vivo and in vitro embryo production, as well as cloning and transgenics, are in current use by both researchers and practitioners. Biohazards associated ...with these procedures could influence clinical proficiency and the outcome of basic research or result in unusual distribution of pathogens in populations of animals. By their nature, embryo technologies are vulnerable to contamination from numerous sources. Although pathogens can originate in the physical environments in which embryo technologies are applied, they are more likely to be introduced via animals or materials of animal origin. However, it is important to note that both the occurrence and consequences of contamination are heavily influenced by environmental circumstances. This paper represents a philosophical description of biohazards associated with three generations of embryo technologies using the cow as a model species. Emphasis is placed on sources of contamination, current or suggested preventive actions and the issue of environmental changes as they relate to the emergence of biohazards and the implementation of biosecurity measures. Some specific pathogens are discussed for illustration. In addition, details of the risks associated with introducing bovine viral diarrhoea virus in each of three generations of embryo technologies are described.
The Australian sheep blowfly, Lucilia cuprina, is the most important economic insect pest for the sheep industries in Australia and New Zealand. piggyBac‐mediated germ‐line transformation of L. ...cuprina was achieved with a helper plasmid that had the Drosophila melanogaster hsp70 promoter controlling expression of the transposase and a piggyBac vector with an EGFP marker gene. Two transformant lines were obtained, at a frequency of approximately 1–2% per fertile G0. One of these lines has a single copy of the transgene, the other most likely has four copies. This is the first report of germ‐line transformation of L. cuprina and is an important step towards the generation of engineered strains that would be suitable for male‐only release eradication/suppression programmes.
Surface processes in OMVPE – the frontiers Stringfellow, G.B; Shurtleff, J.K; Lee, R.T ...
Journal of crystal growth,
12/2000, Letnik:
221, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
Surface processes have long been known to be an important part of any epitaxial growth process. These processes are closely linked to the surface structure. However, until recently, the surface ...structure and the surface processes were difficult to study experimentally for conventional vapor-phase epitaxy (VPE) and liquid-phase epitaxy. Recently, optical techniques such as surface photo absorption (SPA) have been developed to the point that they give useful information about the surface reconstruction in situ during organometallic vapor-phase epitaxial (OMVPE) growth. Thus, they can in many cases be used to monitor the surface processes. A powerful method for controlling the surface structure during epitaxial growth using surfactants has recently emerged. This work describes the use of the surfactants Te, a donor, and As, Sb, and Bi, elements that are isoelectronic with P, on the properties of GaInP grown by OMVPE. These surfactants are found to significantly affect the microscopic arrangement of Ga and In atoms in the bulk solid by effecting a change in the surface structure. CuPt ordering is ubiquitous in III/V semiconductor alloys. It is significant because of the dependence of bandgap energy on the degree of order. The CuPt structure is formed due to the strain induced by the formation of
1
̄
1
0
P dimers on the surface. Each of the surfactants studied is found to result in disordering for layers grown using conditions that would otherwise produce highly ordered GaInP. Te yields disordered material with no change in the SPA spectra. However, the step velocity is found to increased markedly. Thus, the effect appears to be kinetic. Sb causes disordering due to a replacement of
1
̄
1
0
P dimers on the nominally (0
0
1) surface by larger Sb dimers, which reduces the strain-induced driving force for CuPt ordering at the surface. Thus, the effect is due to surface thermodynamics. For high Sb concentrations in the vapor, a triple-period ordered structure is formed. The appearance of this phase coincides with a distinct change in the surface reconstruction as indicated by SPA spectroscopy. Modulation of the TESb flow rate during growth was used to produce an abrupt order/disorder heterostructure with a bandgap energy difference of 135
meV with no significant change in solid composition at the interface. SPA results show that addition of As during growth also reduces the degree of order by displacing some of the
1
̄
1
0
P dimers on the surface. In this case, significant As concentrations in the solid of a few percent are observed. Thus, As is not an effective surfactant. Addition of Bi during growth results in a change in the surface reconstruction, as indicated by SPA spectroscopy, for Bi concentrations producing disordered GaInP. Unlike Sb and As, the Bi also causes a marked increase in the step velocity coincident with the loss of order. For singular (001) substrates, island formation is suppressed by Bi, resulting in the growth of much smoother layers. Modulation of the TMBi concentration during growth has been used to produce disorder/order heterostructures. The use of isoelectronic surfactants during growth to influence the properties of a semiconducting solid is a new and exciting development in control of the OMVPE growth process. It is expected that the use of isoelectronic surfactants to determine the surface reconstruction will find application in the growth of complex device structures. It also appears likely that this will be useful for controlling other characteristics of the growth process and the properties of the resultant semiconductor materials.
The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at ...mid-rapidity |yee| < 1 in minimum-bias Au+Au collisions at √sNN = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ρ spectral function for Mee < 1.1 GeV/c2. The integrated dielectron excess yield at √sNN = 19.6 GeV for 0.4 < Mee < 0.75 GeV/c2, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In + In collisions at √sNN = 17.3 GeV. For √sNN = 200 GeV, the normalized excess yield in central collisions is higher than that at √sNN = 17.3 GeV and increases from peripheral to central collisions. The measurements indicate that the lifetime of the hot, dense medium created in central Au + Au collisions at √sNN = 200 GeV is longer than those in peripheral collisions and at lower energies.
This paper explores the effects of surfactants isoelectronic with P on the step structure and ordering in GaInP grown by organometallic vapor phase epitaxy. Three elements, Bi, Sb, and As, were ...studied individually by adding them during epitaxial growth using the precursors trimethylbismuth, triethylantimony, and triethylarsenic. Atomic force microscopy was used to investigate the influence of the surfactants on the epilayer step structure. Bi produces a remarkable change in the step structure. A Bi concentration in the vapor of (Bi/III)
v=1.65×10
−2 increases the 1
1
0 step velocity by nearly an order of magnitude. The addition of Bi leads to the complete elimination of three-dimensional islands and a significant reduction in surface roughness for layers grown on singular (0
0
1) GaAs substrates. These observations suggest that the surfactant Bi alters the attachment kinetics of group III adatoms at descending and ascending steps. When a comparable amount of surfactant Sb is added (Sb/III)
v=1.2×10
−2 the 1
1
0 step velocity increases only slightly. However, this amount of Sb also produces smoother surfaces. Higher amounts of the surfactant Sb (Sb/III)
v=6.4×10
−2 produce surface undulations with a period of ∼120
nm. These undulations are related to formation of a lateral compositional modulation. No step structure change occurs when a high partial pressure of As is added during growth (As/III)
v=3.3×10
−1. Addition of each of the three isoelectronic elements has been observed to disorder GaInP layers that would otherwise be highly ordered.
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