Liquid crystallinity: Medium effects on photopolymerization rates

Ordering of the medium in which free radical polymerization is conducted can, under appropriate circumstances, alter the kinetics of the chain-growth process, resulting in enhanced rates. Since in many systems medium changes occur at higher conversion, it is important to take into account any alterations in the polymerization rate induced by such phenomena. We have found that exotherms initiated by individual laser pulses can be useful in identifying and providing detailed information concerning rate acceleration in liquid crystalline systems. From measurements of the pulse intensities in the sample cell, estimates of the quantum efficiency and kinetic chain length have been made directly from the integrated areas of the exotherm curves. Recent refinements in our sample geometry have allowed us to generate reproducible efficient data as a function of the degree of conversion and order in the polymerization medium.     

Cross-section modelling effects on pressurised water reactor main steam line break analyses

Because of the strong asymmetric overcooling effects occurring during a PWR main steam line break (MSLB) event, an accurate analysis of this transient requires the use of 3-D kinetics methods. An assessment has been made of the relative performance of the two kinetics solvers currently employed at PSI for such analyses, viz. CORETRAN and SIMULATE-3 K. For the purpose, the simulation of a hypothetical MSLB in a real operated PWR MOX cycle has been considered, employing consistent 3-D core models with specified thermal-hydraulic boundary conditions at the lower and upper plenums. Although the employed cross-section library is in both codes based on the same set of homogenised 2-group cross-sections prepared with CASMO-4, significant differences are shown to occur due to the smaller moderator reactivity coefficient calculated in CORETRAN. It is found that this stems largely from differences in the cross-section formalism, i.e. the manner in which feedback dependencies are modelled and interpolated for the cross-section sets.In particular, the CORETRAN cross-section formalism induces an inadequate treatment of coupled feedback effects, principally between boron density and moderator temperature, which renders the MSLB dynamics predictions quite sensitive to the methodology employed during the cross-section preparation. As such, transient-specific cross-section libraries need to be produced for reliable MSLB analysis in this case. The cross-section model for SIMULATE-3 K, on the other hand, is shown to be adequate for accurately capturing the coupled reactivity effects occurring during an MSLB. In this case, the sensitivity of the results to other sources of uncertainties becomes more apparent, e.g. to those related to the neutron data and/or the thermal-hydraulic boundary conditions. Considering that many other state-of-the-art advanced kinetics solvers have cross-section formalisms similar to that of CORETRAN, effects of the type currently investigated need to be taken into account while developing methodologies for assessing neutronics-related uncertainties in best-estimate transient analysis.     

Effect of laser flux density on ZnCdS thin films

Effect of laser flux density on the structural quality and optical properties of Zn_0.6Cd_0.4S thin films synthesized by ultraviolet pulsed laser deposition have been studied. The stoichiometric composition of this alloy was estimated using lattice constant calculated via XRD data. Surface morphology of the samples was examined using AFM. Optical properties were studied at room temperature by transmittance, absorbance, and photoluminescence measurements. Studies revealed that there is an improvement in the structural quality with increasing the laser flux density in some range. However, too high laser flux density could lead to the degradation in structural quality of thin film. It was observed from the PL data that with increase in laser flux density there is a decrease in the band gap. Transmission data shows a transmittance of more than 70% in the visible region. TEM investigation of the samples reveals that the particles are spherical in shape with average diameter of 15 nm.