Influence of Low Fractions of Styrene/Butyl Acrylate Polymer Latexes on Some Properties of Ordinary Portland Cement Mortars

The influence of the chemical structure of styrene/butyl acrylate latexes on some properties of ordinary Portland cement mortars has been investigated for low fractions of polymer with respect to the amount of cement. The structural parameters that were varied included the presence of carboxylic surface charges, the nature of the surfactant system, the molecular weight of the non‐ionic emulsifier, and the particle surface coverage. The first part of the study showed that sterically stabilised latexes remain colloidally and chemically stable for several hours in the cement interstitial medium. Moreover, no interaction could be shown between cement and latex components. Then, for a fraction of about 5wt.‐% of polymer, no significant influence of the presence of the latex was observed on the end‐use properties of the mortars. However, it clearly appears that the use of a fraction of anionic stabiliser is highly detrimental to the paste workability, whereas this latter property is improved by the presence of non‐ionic surfactant due to steric repulsion forces. Finally, the microstructure analysis confirms that the particles are homogenously dispersed in the mineral matrix and that they remain smooth and spherical.

     

Room-temperature single-phase Li insertion/extraction in nanoscale Li(x)FePO4

Classical electrodes for Li-ion technology operate by either single-phase or two-phase Li insertion/de-insertion processes, with single-phase mechanisms presenting some intrinsic advantages with respect to various storage applications. We report the feasibility to drive the well-established two-phase room-temperature insertion process in LiFePO4 electrodes into a single-phase one by modifying the material's particle size and ion ordering. Electrodes made of LiFePO4 nanoparticles (40 nm) formed by a low-temperature precipitation process exhibit sloping voltage charge/discharge curves, characteristic of a single-phase behaviour. The presence of defects and cation vacancies, as deduced by chemical/physical analytical techniques, is crucial in accounting for our results. Whereas the interdependency of particle size, composition and structure complicate the theorists' attempts to model phase stability in nanoscale materials, it provides new opportunities for chemists and electrochemists because numerous electrode materials could exhibit a similar behaviour at the nanoscale once their syntheses have been correctly worked out.     

Design of experiment approach applied to reducing and oxidizing tolerance of anode supported solid oxide fuel cell. Part I: Microstructure optimization

The main drawback of Ni/YSZ anode supports for solid oxide fuel cell application is their low tolerance to reducing and oxidizing (RedOx) atmosphere changes, owing to the Ni/NiO volume variation. This work describes a structured approach based on design of experiments for optimizing the microstructure for RedOx stability enhancement. A full factorial hypercube design and the response surface methodology are applied with the variables and their variation range defined as: (1) NiO proportion (40-60 wt% of the ceramic powders), (2) pore-former proportion (0-30 wt% corresponding to 0-64 vol.%), (3) NiO particle size (0.5-8 μm) and (4) 8YSZ particle size (0.6-9 μm).To obtain quadratic response models, 25 different compositions were prepared forming a central composite design. The measured responses are (i) shrinkage during firing, (ii) surface quality, (iii) as-sintered porosity, (iv) electrical conductivity after reduction and (v) expansion after re-oxidation. This approach quantifies the effect of all factors and their interactions. From the quadratic models, optimal compositions for high surface quality, electrical conductivity (>500 S cm⁻¹ at room temperature) and RedOx expansion (<0.2% upon re-oxidation) are defined. Results show that expansion after re-oxidation is directly influenced by the sample porosity whereas, surprisingly, the NiO content, varied between 40 and 60 wt%, does not show any impact on this response.     

Distributions of the growth rate of the germ tubes and germination time of Penicillium chrysogenum conidia depend on water activity

The effects of water activities for sporulation (a(wsp)) and germination (a(wge)) on the distributions of the growth rate of the germ tubes (mu) and the germination time (t(G)) of Penicillium chrysogenum conidia were determined by monitoring the length of the same germ tubes throughout the experiments automatically. No relationship between the individual t(G)'s and mu's could be established. Irrespective of the water activity for germination, mu was greater and t(G) was less for conidia produced at 0.95a(wsp) than that at 0.99a(wsp). At 0.99 a(wge) the mean and the standard deviation of t(G) were smaller than those obtained at 0.95a(wge). At 0.99a(wge), normal distributions for mu and t(G) were exhibited, but not at 0.95a(wge). The cumulative frequencies were used to reconstruct the germination curves. Great differences in the percentage of spores capable of germination (P(G)) and in the mean germination times between conidia produced at 0.95a(wsp) and at 0.99a(wsp) were clearly exhibited at 0.95a(wge), thus demonstrating the paramount influence of sporulation conditions on germination kinetics.     

The use of nanocarbons as chemical filters for the selective detection of nitrogen dioxide and ozone

The gas filtering abilities of different nanocarbon materials such as nanocones/nanodiscs, and nanofibres, either as-prepared or modified by physical (annealing, grinding) or chemical (fluorination) treatment are reported. The aptitude to filter nitrogen dioxide and ozone, two of the most significant gaseous pollutants of the atmosphere, have been correlated to both the BET specific surface area studied by N2 adsorption at 77 K, and the presence of chemical functional groups at the surface. Valuable information regarding the mechanisms of gas-nanocarbon interaction has been obtained, in terms of chemisorption and physisorption. A prototype microsystem is proposed for the selective measurement of nitrogen dioxide and ozone concentration by means of organic semiconductor gas sensors.     

Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions

A literature review of critical heat flux (CHF) experimental visualizations under subcooled flow boiling conditions was performed and systematically analyzed. Three major types of CHF flow regimes were identified (bubbly, vapor clot and slug flow regime) and a CHF flow regime map was developed, based on a dimensional analysis of the phenomena and available experimental information. It was found that for similar geometric characteristics and pressure, a Weber number (We)/thermodynamic quality (x) map can be used to predict the CHF flow regime.Based on the experimental observations and the review of the available CHF mechanistic models under subcooled flow boiling conditions, hypothetical CHF mechanisms were selected for each CHF flow regime, all based on a concept of wall dry spot overheating, rewetting prevention and subsequent dry spot spreading. Even though the selected concept has not received much attention (in term or theoretical developments and applications) as compared to other more popular DNB models, its basis have often been cited by experimental investigators and is considered by the authors as the “most-likely” mechanism based on the literature review and analysis performed in this work. The selected modeling concept has the potential to span the CHF conditions from highly subcooled bubbly flow to early stage of annular flow and has been numerically implemented and validated in bubbly flow and coupled with one- and three-dimensional (CFD) two-phase flow codes, in a companion paper. [Le Corre, J.M., Yao, S.C., Amon, C.H., in this issue. A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one and three-dimensional computer codes. Nucl. Eng. Des.].     

A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one- and three-dimensional computer codes

Based on a review of visual observations at or near critical heat flux (CHF) under subcooled flow boiling conditions and consideration of CHF triggering mechanisms, presented in a companion paper [Le Corre, J.M., Yao, S.C., Amon, C.H., 2010. Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions. Nucl. Eng. Des.], a model using a two-dimensional transient thermal analysis of the heater undergoing nucleation was developed to mechanistically predict CHF in the case of a bubbly flow regime. The model simulates the spatial and temporal heater temperature variations during nucleation at the wall, accounting for the stochastic nature of the boiling phenomena. It is postulated that a high local wall superheat occurring underneath a nucleating bubble at the time of bubble departure can prevent wall rewetting at CHF (Leidenfrost effect). The model has also the potential to evaluate the post-DNB heater temperature up to the point of heater melting.Validation of the proposed model was performed using detailed measured wall boiling parameters near CHF, thereby bypassing most needed constitutive relations. It was found that under limiting nucleation conditions; a peak wall temperature at the time of bubble departure can be reached at CHF preventing wall cooling by quenching. The simulations show that the resulting dry patch can survive the surrounding quenching events, preventing further nucleation and leading to a fast heater temperature increase. The model was applied at CHF conditions in simple geometry coupled with one-dimensional and three-dimensional (CFD) codes. It was found that, within the range where CHF occurs under bubbly flow conditions (as defined in Le Corre et al., 2010), the local wall superheat underneath nucleating bubbles is predicted to reach the Leidenfrost temperature. However, a better knowledge of statistical variations in wall boiling parameters would be necessary to correctly capture the CHF trends with mass flux (or Weber number).     

A predictive current control applied to a permanent magnet synchronous machine, comparison with a classical direct torque control

This paper presents a new predictive current control (PCC) for permanent magnet synchronous machines (PMSM) drivers. Based on a hybrid model including an inverter model and a PMSM model, the proposed scheme selects more than one inverter configuration and allows to reach exactly the reference state vector. It appears attractive when compared to conventional direct torque control (DTC). Hysteresis comparators are not used. The switching frequency is fixed and it does not have to be very high. The control scheme has been verified experimentally with a 1.6 kW PMSM drive. Comparative studies with DTC control are performed with the same test bench. The results show that the proposed control obtains the same fast torque dynamics if not better than DTC control schemes but with smaller ripples.     

PL characterization of GaN scintillator for radioluminescence-based dosimetry

Via photoluminescence (PL) measurements, we have investigated GaN (which is a widely employed material in optoelectronics) to be used as scintillator for developing implantable dosimetric probes. The studied Si-doped n-type GaN samples have common dominant band-edge emission at room temperature, with a strong doping-dependence on their PL spectra. Si-doped GaN with 10¹⁸ cm⁻³ concentration exhibits a non-negligible yellow luminescence (YL) or red luminescence broad band contribution, which is probably due to Ga vacancy native defect. However, heavily-doped GaN (1.5 × 10¹⁹ cm⁻³) has much more intense band-edge emission, with no significant contributions of lower-energy bands. The dominant band-edge emission peak remains unchanged at 3.4 eV for both concentrations, and may be accounted for by considering combined effects of band-gap narrowing and Fermi level rising. It has also been demonstrated by back-side PL collection that, due to bulk self-absorption, the dominant peak is shifted to 3.25 eV DAP (donor–acceptor-pair) or e–A (conduction-band-to-acceptor) band. At low temperatures (up to 200 K), PL intensity of heavily-doped GaN is dominated by a DBE (donor-bound exciton) line (D⁰X_A at 3.467 eV for T = 10 K). When further increasing the temperature, band-to-band recombination becomes dominant with red-shift of the emission peak in connection with gap narrowing. The shift in peak wavelength from 10 K to room temperature is about 5 nm, while the corresponding PL integrated intensity is decreased only by 30%. Irradiating samples at room temperature to a 200 Gy dose with 6 MeV photon and electron beams causes a slight increase on defect-related DAP or e–A line and YL broad band, but the effect is not noticeable for heavily-doped GaN.     

RedOx study of anode-supported solid oxide fuel cell

The common technology for solid oxide fuel cells (SOFC) is based on a cermet (ceramic-metal composite) anode of nickel with yttria stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this technology is the tolerance of the anode towards reduction and oxidation (“RedOx”) cycles.In this study, two techniques are used to quantify the anode expansion after a RedOx cycle of the nickel at different temperatures. The first method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. In the second method, the anode porosity is measured using scanning electron microscopy (SEM) image quantification. The same measurement techniques are used to quantify anode expansion after consecutive RedOx cycles at constant temperature.The quantification technique is then applied to cells tested in real stack conditions. The cell corners can undergo several RedOx cycles depending on stack design and fuel utilization. The study of such zones allows estimating the number of cycles that the anode experienced locally.