International audienceWe report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by gas immersion laser doping. The doping concentration c(B) has been varied up to similar to 10 at. % by increasing the number of laser shots to 500. No superconductivity could be observed down to 40 mK for doping level below similar to 2 at. %. The critical temperature T(c) then increased steeply to reach similar to 0.6 K for c(B) similar to 8 at. %. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field mu(o)H(c2) (0) was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. [E. Bustarret et al., Nature (London) 444, 465 (2006)]

This article begins by briefly reviewing the more important contributions to the area of modeling heat and mass transfer, and particle growth during the polymerization of olefins on Ziegler–Natta catalysts. It is shown that these models are capable of identifying the critical areas involved in heat and mass transfer, and of modeling polymerizations where the observed activity is less than approximately 5,000 g of polymer per gram of catalyst per hour (g/g/h). However, it is not possible to use these models “as‐is” to model more modern catalysts whose activity levels can surpass the 50,000 g/g/h mark because they predict prohibitively large concentration gradients inside the growing particles during slurry polymerizations, and temperature gradients outside the particles during polymerization in the gas phase. An analysis of the mass and heat transfer Peclet numbers (Pe) reveals that certain simplifying assumptions may not always be valid. Pe values in the transition range suggest that convection inside the particles during polymerization in the liquid phase may help to explain why observed mass transfer rates are higher than the predicted rates. In an opposite vein, a Pe analysis shows that conductive heat transfer may play an important role at length scales characteristic of those in the early stages of polymerization. A new mechanism for heat transfer at reduced length scales is proposed. © 1995 John Wiley & Sons, Inc.

The aim of this study is to investigate the synthesis of 2,5‐furandicarboxylic acid (FDCA)‐based copolyamides. Indeed, FDCA monomer may be a potential bio‐based alternative to phthalic acids. A series of polyamides and copolyamides, PA 6‐I(x)/6‐F(y), are synthesized in a pilot scale reactor by melt polycondensation of salts based on FDCA, isophthalic acid (IPA), and 1,6‐hexamethylenediamine. The chemical structure and composition of the resulting copolymers are extensively characterized by NMR (1H, 13C, and 2D), MALDI‐TOF as well as size exclusion chromatography and solution viscosimetry. Their thermal properties are studied by differential scanning calorimetry and TGA and also by a molecular modeling technique. It is pointed out that, during the synthesis, FDCA‐rich polymers overcome a massive decarboxylation of FDCA, preventing the production of high molar mass polymers. It is also found that all polymers are amorphous and that the glass transition temperatures decrease as the amount of FDCA in the polymer increases. This is confirmed by the results from molecular dynamics. In addition, it is observed that the copolymers become more hygroscopic when the amount of FDCA in the copolyamides increases. However, the water uptake of PA 6‐I(x)/6‐F(y) with 90 > x > 50 is lower than those of PA6 and PA 66 polyamides. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45901.

UV cure kinetics of the system bisphenol A dimethacrylate and a phenylphosphine oxide initiator are studied as a function of incident intensity (I) and initiator concentration (PI). The rate of polymerization, d(ln[M(t)])/dt, is found to be proportional to the intensity and initiator concentration raised to an exponential power of approximately 0.7 rather than the classical value of 0.5. The system does not obey classical steady-state kinetics but rather the rate of the reaction reaches a maximum shortly after gel and then decreases rapidly well before the reaction is quenched during the glass transition. These nonclassical results are proposed to be due to the coexistence of a varying ratio of the traditional bimolecular kinetics and a unimolecular trapped radical termination process due to the changing spatial/ dynamic heterogeneity arising from microgel formation in these systems. A model which focuses on the changing concentration of mobile active radicals is proposed. It uses four fitting parameters to describe changes in the total number of radicals minus the number of trapped immobile radicals. The model assumes a constant propagation rate constant, k p, as is generally assumed in traditional crosslinking reactions up to the reaction quench in the final stage of the cure. The model is similar in mathematical form to a more complex multiparameter free volume molecular approach in which both k p, k t, and a trapping rate constant change with reaction advancement. Our model is based on the changing mobile radical concentration, which reflects the changes in the spatial heterogeneity of these systems due to the formation of microgels at the beginning of the reaction. It predicts the initial rapid buildup of the total radical concentration to a constant value and then the existence of an increasing proportion of trapped radicals up to reaction quenching in the glass transition. It accurately describes the UV cure kinetics at varying incident intensities and varying initiator concentrations and its predictions of the changes in the total and trapped radical concentrations are similar to ESR measurements on other systems.