Growth and optical characterisation of multilayers of InGaN quantum dots

We report on the growth (using metal-organic vapour phase epitaxy) and optical characterization of single and multiple layers of InGaN quantum dots (QDs), which were formed by annealing InGaN epilayers at the growth temperature in nitrogen. The size and density of the nanostructures have been found to be fairly similar for uncapped single and three layer QD samples if the GaN barriers between the dot layers are grown at the same temperature as the InGaN epilayer. The distribution of nanostructure heights of the final QD layer of three is wider and is centred around a larger size if the GaN barriers are grown at two temperatures (first a thin layer at the dot growth temperature, then a thicker layer at a higher temperature). Micro-photoluminescence studies at 4.2 K of capped samples have confirmed the QD nature of the capped nanostructures by the observation of sharp emission peaks with full width at half maximum limited by the resolution of the spectrometer. We have also observed much more QD emission per unit area in a sample with three QD layers, than in a sample with a single QD layer, as expected

A mechanism for bentonite buffer erosion in a fracture with a naturally varying aperture

In the deep geological disposal of nuclear waste in crystalline rock, erosion of the bentonite buffer may occur during periods of glaciation. Previous researchers have examined the mechanism and rates of extrusion and erosion for purified montmorillonite samples in smooth planar fractures. In this paper, we investigate the influence of using as delivered MX-80 material (i.e. including accessory minerals) and a naturally varying aperture on bentonite erosion. A bespoke fracture flow cell was constructed for this purpose and flow through conducted with deionised water. Throughout the experiment, gravimetric analysis was undertaken on the effluent and the swelling pressure of the bentonite monitored. Quantitative image analysis of the extrusion process was also undertaken. When the swelling pressure data was analysed, alongside both the oscillations in erosion rate and the area of the accessory mineral ring, a two-stage mechanism governing the erosion process became apparent. Once an accessory mineral ring had formed at the edge of the extruded material, further increases in swelling pressure resulted in a breach in the accessory mineral ring, triggering an erosive period. During which, the mineral ring was supplemented with additional minerals. The cycle repeated until the ring was sufficiently strong that it remained intact. This observed process results in erosion rates one order of magnitude less than those currently used in long-term safety case calculations

Quantification of the performance of iterative and non-iterative computational methods of locating partial discharges using RF measurement techniques

Partial discharge (PD) is an electrical discharge phenomenon that occurs when the insulation materialof high voltage equipment is subjected to high electric field stress. Its occurrence can be an indication ofincipient failure within power equipment such as power transformers, underground transmission cableor switchgear. Radio frequency measurement methods can be used to detect and locate discharge sourcesby measuring the propagated electromagnetic wave arising as a result of ionic charge acceleration. Anarray of at least four receiving antennas may be employed to detect any radiated discharge signals, thenthe three dimensional position of the discharge source can be calculated using different algorithms. These algorithms fall into two categories; iterative or non-iterative. This paper evaluates, through simulation, the location performance of an iterative method (the standardleast squares method) and a non-iterative method (the Bancroft algorithm). Simulations were carried outusing (i) a "Y" shaped antenna array and (ii) a square shaped antenna array, each consisting of a four-antennas. The results show that PD location accuracy is influenced by the algorithm's error bound, thenumber of iterations and the initial values for the iterative algorithms, as well as the antenna arrangement for both the non-iterative and iterative algorithms. Furthermore, this research proposes a novel approachfor selecting adequate error bounds and number of iterations using results of the non-iterative method, thus solving some of the iterative method dependencies

Ovarian cancer symptom awareness and anticipated time to help-seeking for symptoms among UK women.

OBJECTIVES: To determine levels of awareness of ovarian cancer symptoms and to identify barriers to help-seeking and predictors of a longer time to help-seeking in a UK female population-based sample. METHODS: A UK population-based sample of women [n=1000, including a subsample of women at higher risk due to their age (≥45 years, n=510)] completed the Ovarian Cancer Awareness Measure by telephone interview. Questions measured symptom awareness (using recall and recognition), barriers to medical help-seeking and anticipated time to help-seeking. Regression analyses identified predictors of a higher score on a scale of anticipated time to help-seeking. RESULTS: Most women (58% overall sample; 54% subgroup) were unable to recall any symptoms but 99% recognised at least one. Recognition was lowest for difficulty eating and persistently feeling full. In the sample overall, higher socio-economic status and higher endorsement of practical and service barriers independently predicted a longer anticipated time to help-seeking for more symptoms. White ethnicity was an additional predictor in the older subgroup. CONCLUSIONS: This study suggests awareness of ovarian cancer symptoms is low in the UK, and varies widely between symptoms. It identifies variables that may be involved in a longer time to help-seeking for possible ovarian cancer symptoms and highlights the need for more in-depth research into the factors related to time to help-seeking in real-world situations

Interparticle Forces of a Native and Encapsulated Metal-Organic Framework and Their Effects on Colloidal Dispersion

The colloidal properties of suspended metal-organic frameworks (MOFs) are critical for device fabrication and application. Herein, van der Waals attractive, electric double layer repulsive, and steric repulsive forces of a native and encapsulated MOF are quantified for the first time. The van der Waals attractive forces were investigated by conducting environmental ellipsometric porosimetry (EEP) and spectroscopic ellipsometry (SE) on submicron, optical-quality nanoparticle films. The repulsive forces were determined from colloid and material characterization measurements. These data were used to predict suspension properties via extended Derjaguin, Landau, Verwey, and Overbeek theory. The state of dispersion was quantified for comparison with theoretical predictions for nine solvents. The MOF encapsulated with a surface-selective modification showed superior suspension in hydrophobic solvents. These findings should expedite the formulation of MOF colloidal suspensions for future works

25-Hydroxyvitamin D Threshold for the Effects of Vitamin D Supplements on Bone Density:Secondary Analysis of a Randomized Controlled Trial

Most trials of vitamin D supplementation have shown no benefits on bone density (BMD), though severe vitamin D deficiency causes osteomalacia which is associated with profound BMD deficits. Recently, the ViDA-BMD study from New Zealand demonstrated a threshold of baseline 25-hydroxyvitamin D (30 nmol/L) below which vitamin D supplementation did benefit BMD. We have now re-examined data from a similar trial in Aberdeen to determine whether a baseline 25-hydroxyvitamin D threshold of 30 nmol/L is also observed in that database. The Aberdeen study recruited 305 postmenopausal women in late winter and randomized them to receive placebo, vitamin D 400 IU/day or vitamin D 1000 IU/day over one year. As previously reported, BMD loss at the hip was reduced by vitamin D 1000 IU/day only, and there was no significant treatment effect of either dose at the lumbar spine. In the present analysis, when the trial participants were grouped according to whether their baseline 25-hydroxyvitamin D was ≤30 nmol/L or above this threshold, significant treatment effects were apparent at both the spine and hip in those with baseline 25-hydroxyvitamin D ≤30 nmol/L, but no significant effects were apparent in those with baseline 25-hydroxyvitamin D above this level. There was evidence of a similar threshold for effects on parathyroid hormone, but no groups showed changes in bone turnover markers during the study. It is concluded that vitamin D supplements only increase bone density in adults with nadir 25-hydroxyvitamin D ≤30 nmol/L. This moves us further towards a trial-based definition of vitamin D deficiency in adults with adequate calcium intakes, and suggests that supplement use should be targeted accordingly. Future trials of vitamin D supplementation should focus on individuals with 25-hydroxyvitamin D concentrations in this range

Matrix Isolation and Computational Study of Isodifluorodibromomethane (F2 Cbr-Br): A Route to Br2 Formation in Cf2 Br2 Photolysis

The photolysis products of dibromodifluoromethane (CF2 Br 2) were characterized by matrix isolation infrared and UV/Visible spectroscopy, supported by ab initio calculations. Photolysis at wavelengths of 240 and 266 nm of CF2 Br2:Ar samples (∼1:5000) held at ∼5 K yielded iso- CF2 Br2 (F2 CBrBr), a weakly bound isomer of CF2 Br2, which is characterized here for the first time. The observed infrared and UV/Visible absorptions of iso- CF2 Br2 are in excellent agreement with computational predictions at the B3LYP/aug-cc-pVTZ level. Single point energy calculations at the CCSD(T)/aug-cc-pVDZ level on the B3LYP optimized geometries suggest that the isoform is a minimum on the CF2 Br2 potential energy surface, lying some 55 kcal/mol above the CF2 Br2 ground state. The energies of various stationary points on the CF2 Br 2 potential energy surface were characterized computationally; taken with our experimental results, these show that iso- CF2 Br 2 is an intermediate in the Br+ CF2 Br→ CF 2 + Br2 reaction. The photochemistry of the isoform was also investigated; excitation into the intense 359 nm absorption band resulted in isomerization to CF2 Br2. Our results are discussed in view of the rich literature on the gas-phase photochemistry of CF2 Br2, particularly with respect to the existence of a roaming atom pathway leading to molecular products. © 2010 American Institute of Physics

Spectroscopic And Computational Studies Of The Laser Photolysis Of Matrix Isolated 1,2-dibromoethanes: Formation And Fate Of The Bromoethyl Radicals

We report experimental and computational studies of the photolysis of atmospherically important 1,2-dibromoethanes (1,2-C(2)X(4)Br(2); X = H, F) in Ar matrixes at 5 K. Using the pulsed deposition method, we find that significant conformational relaxation occurs for 1,2-C(2)H(4)Br(2) (EDB; observed anti/gauche ratio =30:1) but not for 1,2-C(2)F(4)Br(2) (TFEDB; anti/gauche = 3:1), which is traced to a larger barrier to rotation about the C-C bond in the latter. Laser photolysis of matrix-isolated EDB at 220 nm reveals the growth of infrared bands assigned to the gauche conformer and C(2)H(4)-Br(2) charge transfer complex (both as major products), and the C(2)H(4)Br radical and C(2)H(3)Br-HBr complex as minor (trace) products. The presence of the C(2)H(4)-Br(2) complex is confirmed in the UV/visible spectrum, which shows an intense charge transfer band at 237 nm that grows in intensity upon annealing. In contrast to previous reports, our experimental and computational results do not support a bridged structure for the C(2)H(4)Br radical in either the gas phase or matrix environments. We also report on the laser photolysis of matrix-isolated TFEDB at 220 nm. Here, the dominant photoproducts are the anti and gauche conformers of the C(2)F(4)Br radical, the vibrational and electronic spectra of which are characterized here for the first time. The increase in yield of radical for TFEDB vs EDB is consistent with the stronger C-Br bond in the fluoro-substituted radical species. The photochemistry of the C(2)F(4)Br radical following excitation at 266 nm was investigated and found to lead C-Br bond cleavage and formation of C(2)F(4). The implications of this work for the atmospheric and condensed phase photochemistry of the alkyl halides is emphasized