Well-defined phosphonated homo- and copolymers via direct ring opening metathesis polymerization

Phosphonated polymers with a well-defined molecular weight, composition and architecture have been prepared via ring opening metathesis polymerization (ROMP) of phosphonated and non-phosphonated norbornene imides at room temperature for the first time. ROMP was proven to be living and versatile. This enabled preparation of a broad range of phosphonated homopolymers, statistical copolymers, AB diblock as well as ABA and BAB triblock copolymers based on poly(norbornene imide)s with low polydispersity (1.09–1.32). Complete hydrolysis of phosphonated poly(norbornene imide)s under mild conditions yielded the phosphonic acid derivatives. Thermogravimetric analysis indicated high thermal and thermo-oxidative stability of the polymers. Free standing and transparent films with good mechanical stability were obtained from the phosphonic acid functional homopolymers, diblock and triblock copolymers. Combining these basic properties with the advantages mentioned above makes ROMP a promising pathway for accessing a wide diversity of phosphonated macromolecular structures. These new phosphonated polymers will open new perspectives in advanced application areas, which require a high level of control over polymer structure.     

Imaging the Electric Potential within Organic Solar Cells

The charge transport in organic solar cells is investigated by surface potential measurements via scanning Kelvin probe microscopy. Access to the solar cell's cross‐section is gained by milling holes with a focused ion beam which enables the direct scan along the charge transport path. In a study of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, the open circuit voltage is built up at the top contact. A comparison of the potential distribution within normal and inverted solar cells under operation exhibits strongly different behaviors, which can be assigned to a difference in interface properties.     

Models for long-range current flow in bulk oxide superconductors

Mitigating the weak-link behavior at grain boundaries, which limits critical current density (J_c) in polycrystalline high-transition-temperature (high-T_c) superconducting materials, is required for all applications in magnetic fields. Although in general oxide superconductors in polycrystalline form have very poor in-field J_c, conductors containing the bismuth-based compounds Bi₂Sr₂CaCu₂O₈ (Bi-2212)andBi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) are very important exceptions. Long-range, strongly linked conduction has been demonstrated in just two additional polycrystalline materials: TlBa₂Ca₂Cu₃O₁₀ (Tl-1223) deposits prepared by spray pyrolysis, and YBa₂Cu₄O₈ (Y-124) conductor made by the oxidation of metallic precursor method. Determining the characteristics of these materials that permit strongly linked conduction is of critical importance to the development of better conductors. Models of strongly linked polycrystalline conductors are reviewed. Recent studies reveal that small-angle boundaries are present in much higher numbers than expected from calculations based on macroscopic texture, indicating a strong influence of grain boundary energy.It is proposed that strongly linked current flows through a connected network of low energy grain boundaries. Evidence has been obtained to support this model in Bi-2223, Y-124 and Tl-1223.     

Analysis of plane failures in compound slopes

ABSTRACT

Plane failure in rock and soil slopes is usually caused by the intersection of a discontinuity plane with a slope face. This discontinuity can be any one of many different types of faulting, fracturing, bedding, foliation, schistosity, etc. Generally, these discontinuities are weaker than the surrounding intact material and provide a surface along which movement can occur.

When designing a stable slope, it is often desirable to find the worst conditions for a particular slope configuration and base the final design around those conditions. The worst case for most dry slopes usually includes a tension crack at an unknown location. This paper focuses on plane failure analyses of compound slopes using limit equilibrium techniques. It provides new equations to calculate the position of tension cracks in compound slopes.     

Crystallization of Zr-Ni metallic glasses

The crystallization of Zr-Ni metallic glasses of composition between 55 and 70 at % Zr has been investigated using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The samples were prepared by splat quenching in an arc-hammer device. Transformation temperatures, effective activation energies, and enthalpy changes are reported as a function of composition. Results of XRD patterns obtained as a function of annealing temperature in the DSC are presented. A high temperature exothermic DSC peak and XRD patterns indicate the presence of a metastable phase which occurs between 57 and 63.5 at % Zr. The results tend to support suggestions of a connection between the short range structure of the glass and the crystalline phase to which it transforms. It was found that the metastable phase, whose presence is strongest at 57 to 59 at % Zr, and the process of phase separation around the eutectic composition (63.5 at % Zr) play important roles in the crystallization process.     

Acknowledgement of refereeing work

Acknowledgement

Acknowledgement of refereeing work     

Prevalence of enteropathogenic and enterotoxigenic Escherichia coli in children with and without diarrhoea in Esteli, Nicaragua

OBJECTIVE: To assess some of the employment experiences of people with diabetes mellitus and to compare their experiences with those of a non-diabetic sibling control group. DESIGN: A questionnaire about employment experiences was administered to diabetic subjects aged 16-39 years, and an abbreviated questionnaire was administered to their eligible siblings. SETTING: The Illawarra area of New South Wales. PARTICIPANTS: The names of diabetic subjects were obtained from the Illawarra diabetes register. RESULTS: Interviews were conducted with 184 of 226 (81.4%) eligible diabetic subjects and with 70 eligible siblings. There were no significant differences between the diabetic subjects and their siblings with respect to educational achievements and rates of employment. Siblings reported a mean of 2.6 days sickness absenteeism in the year prior to the survey. Diabetics were absent from work for a mean of 4.5 days for reasons not related to their diabetes and for a mean of 2.6 days for diabetic causes. Within the diabetic group, 50% felt that having diabetes would make it more difficult to find another job, 33.7% felt that diabetes would influence their search for alternative employment and 19.6% felt that at some stage they had been refused employment because of their diabetes. Fifteen per cent of diabetics were aware of an example of discrimination and 24.2% of diabetics in employment had at some stage tried to hide their diagnosis from their employer. CONCLUSIONS: Diabetics do not appear disadvantaged compared with their siblings with respect to employment participation but are more likely to be absent from work due to sickness. However, many diabetic subjects had experienced a job refusal, had tried to hide their diagnosis from employers, were aware of examples of discrimination and were very negative about future employment prospects.     

A new process with a potential to rapidly texture bulk YBa2Cu3Ox superconductor

A new process has been developed to texture bulk YBa₂Cu₃O_x superconductors at temperatures about 100°C below the peritectic temperature. The texture is achieved in this process by directional recrystallization of compacts fabricated from quenched YBCO powders. Isothermal recrystallization of quenched precursors at 890°C for 3 min is found to result in the formation of more than 75% of YBa₂Cu₃O_x phase without any Y₂BaCuO₅. Recrystallization at higher temperatures leads to rapid formation of fine Y₂BaCuO₅ precipitates in addition to YBa₂Cu₃O_x. The formation of YBa₂Cu₃O_x increases with increasing heating rates. Directional recrystallization at 10 mm/h results in a well-textured microstructure with the YBa₂Cu₃Ox grains oriented parallel to the sample length. The magnetic field dependence of the critical current density at 77K of the directionally recrystallized material compares well with that of melt-textured YBCO and is superior to that of magnetically aligned and sintered YBCO.     

Optimization of critical current density of bulk YBCO superconductor prepared by coprecipitation in oxalic acid

YBa₂Cu₃O_7–x(YBCO) superconductor powder was prepared from mixtures of solutions containing yttrium, barium, and copper nitrates by coprecipitation in oxalic acid. Single-phase YBCO was obtained from a solution mixture of 14 vol% excess of barium nitrate and 12 vol% excess of copper nitrate relative to the stoichiometry of YBCO. The optimal pH in the coprecipitation process was determined to be 6.6–6.7 which was obtained by using 12 vol % excess oxalic acid anhydrous solution of the required oxalic acid to convert all metal cations to oxalates and adding a dilute ammonium hydroxide solution. The measurement of critical current density, J _c, of bulk YBCO prepared by the coprecipitation, showed a trend that the J _c increased together with the degree of orthorhombic distortion of YBCO phase which depended on the sample density and the content of impurity phases.