This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.

ÐMaterials involved in the catalytic formation of carbon nanotubes are for the ®rst time systematically studied by MoÈ ssbauer spectroscopy between 11 K and room temperature. Mg 1Àx Fe x Al 2 O 4 x 0:1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes±Fe/Fe 3 C±MgAl 2 O 4 composite powders by reduction in a H 2 ±CH 4 gas mixture. The oxides are defective spinels of general formulae Mg 2 1Àx Fe 2 xÀ3a Fe 3 2a q a Al 3 2 O 2À 4: Ferromagnetic a-Fe, ferromagnetic Fe 3 C and a g-Fe form, the latter possibly corresponding to a g-Fe±C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe 3 C in the post-reaction MoÈ ssbauer spectroscopy analysis. Re sumeÂÐDes mate riaux implique s dans la formation catalytique de nanotubes de carbone sont pour la premieÁ re fois syste matiquement e tudie s par spectroscopie MoÈ ssbauer entre 11 K et la tempe rature ambiante. Des solutions solides Mg 1Àx Fe x Al 2 O 4 x 0:1, 0.2, 0.3, 0.4) sont transforme es en poudres composites nanotubes de carbone±Fe/Fe 3 C±MgAl 2 O 4 par re duction dans un me lange gazeux de H 2 et de CH 4 . Les oxydes sont des spinelles lacunaires de formule ge ne rale Mg 2 1Àx Fe 2 xÀ3a Fe 3 2a q a Al 3 2 O 2À 4 : Du Fe-a ferro-magne tique, du Fe 3 C ferromagne tique et une forme de Fe-g, cette dernieÁ re correspondant probablement aÁ un alliage Fe±C-g, sont de tecte s dans les poudres composites. Des corre lations sont faites entre ces re sultats et la microstructure de la poudre. Il semble que les nanoparticules qui catalysent la formation des nanotubes de carbone sont de tecte es comme du Fe 3 C dans l'analyse par spectroscopie MoÈ ssbauer des produits obtenus apreÁ s la re action.

Purpose: The aims of this study were 1) to model the temporal profile of W′ recovery after exhaustion, 2) to estimate the contribution of changing V ˙O2 kinetics to this recovery, and 3) to examine associations with aerobic fitness and muscle fiber type (MFT) distribution. Methods: Twenty-one men (age = 25 ± 2 yr, V ˙O2peak = 54.4 ± 5.3 mL•min −1 •kg −1 ) performed several constant load tests to determine critical power and W′ followed by eight trials to quantify W′ recovery. Each test consisted of two identical exhaustive work bouts (WB1 and WB2), separated by a variable recovery interval of 30, 60, 120, 180, 240, 300, 600, or 900 s. Gas exchange was measured and muscle biopsies were collected to determine MFT distribution. W′ recovery was quantified as observed W′ recovery (W′ OBS ), model-predicted W′ recovery (W′ BAL ), and W′ recovery corrected for changing V ˙O2 kinetics (W′ ADJ ). W′ OBS and W′ ADJ were modeled using mono-and biexponential fitting. Root-mean-square error (RMSE) and Akaike information criterion (ΔAIC C ) were used to evaluate the models' accuracy. Results: The W′ BAL model (τ = 524 ± 41 s) was associated with an RMSE of 18.6% in fitting W′ OBS and underestimated W′ recovery for all durations below 5 min (P < 0.002). Monoexponential modeling of W′ OBS resulted in τ = 104 s with RMSE = 6.4%. Biexponential modeling of W′ OBS resulted in τ 1 = 11 s and τ 2 = 256 s with RMSE = 1.7%. W′ ADJ was 11% ± 1.5% lower than W′ OBS (P < 0.001). ΔAIC C scores favored the biexponential model for W′ OBS , but not for W′ ADJ . V ˙O2peak (P = 0.009) but not MFT distribution (P = 0.303) was associated with W′ OBS . Conclusion: We showed that W′ recovery from exhaustion follows a two-phase exponential time course that is dependent on aerobic fitness. The appearance of a fast initial recovery phase was attributed to an enhanced aerobic energy provision resulting from changes in V ˙O2 kinetics.

Various point defects in silicon are studied theoretically from the point view of positron annihilation spectroscopy. Properties of a positron trapped at a single vacancy, divacancy, vacancy-oxygen complexes (VO n ), and divacancy-oxygen complex are investigated. In addition to the positron lifetime and positron binding energy to defects, we also calculate the momentum distribution of annihilation photons ͑MDAP͒ for high momenta, which has been recently shown to be a useful quantity for defect identification in semiconductors. The influence of atomic relaxations around defects on positron properties is also examined. Mutual differences among the high momentum parts of the MDAP for various defects studied are mostly considerable, which can be used for the experimental defect determination. ͓S0163-1829͑98͒03039-2͔

Positronium emission from single crystalline Al2O3, MgO and vitreous a-SiO2 surfaces was studied as a function of the positron implantation energy E by means of Doppler broadening spectroscopy and Compton-to-peak ratio analysis. When the Ge-detector is in-line with the positron beam, the emission of para-positronium yields a red-shifted fly-away peak with intensity I-pPs(e). An analysis of I-pPs(e) versus E for Al2O3 and MgO where no Ps is formed in the bulk (f(Ps)=0) results in positron diffusion lengths L+(Al2O3)=(18+/-1) nm and L+(MgO)=(14+/-1) nm, and efficiencies for the emission of Ps by picking up of a surface electron of f(pu)(Al2O3)=(0.28+/-0.2) and f(pu)(MgO)=(0.24+/-0.2). For a-SiO2 the bulk Ps fraction is f(Ps)(a-SiO2)=(0.72+/-0.01), f(pu)(a-SiO2)=(0.12+/-0.01) and the diffusion lengths of positrons, para-positronium and ortho-positronium are L+(SiO2)=(8+/-2)nm, L-pPs(SiO2)=(14.5+/-2) nm and L-oPs(SiO2)=(11+/-2)=nm. Depending on the specimen-detector geometry the emission of Ps at low implantation energy may cause either an increase or a decrease of the width of the annihilation line shape at low implantation energies