Thermodynamic and spectroscopic measurements of the electron-hole drop binding energy in Ge: An old problem revisited

One can expect to get the electron-hole (EHD) binding energy φ either from spectroscopic or from threshold measurements. While the value obtained by the former is in excellent agreement with theory, the results given by the latter did not seem to lead to φ in a simple way. We present here new optical threshold studies in which we think that the right conditions to obtain a direct thermodynamic measurement of φ have been mastered. Besides, our results present novel features like a sharp jump of the EHD luminescence at threshold above 2.4 K and observation of hysteresis in the whole temperature range of our work.     

Modelling photovoltaic modules with neural networks using angle of incidence and clearness index

A neural network for modelling photovoltaic modules using angle of incidence and clearness index is proposed. Engineers require methods to estimate the output of a photovoltaic plant depending on meteorological conditions. Therefore, models for the grid inverter and the generator must be provided, and their outputs must be combined. The connection between both models is related to the maximum power point of the generator and how it is tracked by the inverter. That maximum power point under specific conditions of irradiance and module temperature is determined by the I–V curve of the module, which must be simulated under those conditions. Algebraic procedures were used to simulate the I–V curve. Recently, neural networks have been used for the same purpose. Previous methods only take into account the irradiance and the module temperature. The model proposed is based on neural networks, and it uses not only the irradiance and the module temperature but also the angle of incidence and the instantaneous clearness index as additional inputs. The normalised clearness replaces the standard clearness index because it allows the removal of the hourly trend found in this last index. This new model improves the results obtained with previous ones as it can distinguish amongst samples with the same solar irradiance and temperature values but with different angle of incidence and instantaneous clearness index. Results show that this model could be used to improve the accuracy of the tools used to forecast the output of photovoltaic plants. Copyright © 2014 John Wiley & Sons, Ltd.     

Ethanolysis of rapeseed oil: Quantitation of ethyl esters,mono-, di-, and triglycerides and glycerol by high-performance size-exclusion chromatography

High-performance size-exclusion chromatography (HPSEC) was used to evaluate the influence of different variables affecting the transesterification of rapeseed oil (RSO) with anhydrous ethanol and sodium ethoxide as catalyst. The effect of temperature, ethanol/RSO molar ratio, catalyst concentration, and time can be interpreted by observing the variations of the reaction medium composition. HPSEC has made the quantitation of ethyl esters, mono-, di-, and triglycerides and glycerol possible. The best results for laboratory-scale reactions were obtained at 80°C with a 6:1 molar ratio of EtOH/RSO and 1% of NaOEt by weight of RSO.     

Correlations between relaxation time spectrum and melt spinning behavior of polypropylene. 1: Calculation of the relaxation spectrum as a log-normal distribution and influence of the molecular parameters

The linear viscoelastic behavior of commercial broad-distribution polypropylenes is studied. The samples underwent different controlled peroxide degradation rates, so they exhibit different molecular distributions. The relaxation spectrum is obtained by assuming that it takes the shape of a Log-Normal distribution. It is in agreement with the linear viscoelasticity theory by providing limiting values, contrary to other models. Moreover, the ratio τw/τn which takes place in this law is very useful to distinguish the synthesis differences of the samples when the Melt Flow Index alone cannot.     

Correlations between relaxation time spectrum and melt spinning behavior of polypropylene. II: Melt spinning simulation from relaxation time spectrum

Using an integral nonlinear constitutive equation including the Log-Normal relaxation spectrum, the fiber-spinning behavior of commercial polypropylenes is studied. The relaxation spectrum is strongly related to the molecular weight distribution. Moreover, by means of a finite element analysis, simulated stress and velocity profiles are obtained either in isothermal or nonisothermal cases. In the isothermal case, broadening the relaxation spectrum leads to more pronounced curvature of the velocity profile. In the nonisothermal case, the critical draw ratio is sensitive to the mean relaxation time which depends on both average molecular weight and distribution breadth. It is shown that increasing the average relaxation time leads to a less rapid growth of the velocity. Furthermore, this effect is less pronounced when the relaxation spectrum is wider.     

Détermination de constantes cinétiques du craquage catalytique par la modélisation du test de microactivité (MAT)

The microactivity test (MAT) for cracking catalyst test works with an approximately plug flow isothermal reactor. The yields at the outlet of such a reactor is numerically computed in the case of lumped kinetic with large molecular expansion and rapid catalyst deactivation expressed versus poison concentration. The poison (coke in cracking reaction) is considered as a product formed by several reaction routes. The results are applied to catalytic cracking kinetic with a four lump model (feedstock, gasoline, gas, coke). The comparison between experimental and computed yields, permits the adjustment of kinetic constants with a set of experimental results obtained from a laboratory scale reactor derived from the MAT. The simulation of concentration profile in the reactor permits a better understanding of the reaction courses.     

Tribological duality of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>

We report here on the friction behavior of fine- and coarse-grained Ti₃SiC₂ against steel and Si₃N₄ balls. Two successive friction regimes have been identified for both grain sizes and both counterparts. First, Type I regime is characterized by a relatively low (0.1–0.15) friction coefficient, and very little wear. Sliding occurs between a tribofilm on the ball and the Ti₃SiC₂ plane when against steel. Then, a Type II regime often follows, with increased friction coefficients (0.4–0.5) and significant wear. Compacted wear debris seems to act as a third body resulting in abrasion of the ball, even in the case of Si₃N₄. The transition between the two regimes occurs at different times, depending on various factors such as grain size, type of pin, and normal load applied. Some experiments under vacuum showed that the atmosphere plays also a major role. The reason for this evolution is not fully clear at that time, but its understanding is of major technological importance given the unusual good properties of this material.     

Impact d’une galerie étanche peu profonde sur l’écoulement d’une nappe

While many studies have been achieved on the interactions between groundwater and deep tunnels, in order to identify the evolution of pore pressure around the structure and to characterize the flow to its leaky parts, few studies have dealt with the impact of the carrying out of an impervious gallery in a shallow aquifer. The induced change in the piezometric level of the aquifer and the one in the hydraulic gradient of the flow however can, in this case, have significant consequences, in particular when the linear structure is located in an urban environment. This paper investigates, in steady state, the case of a straight tunnel having a horizontal axis perpendicular to the direction of the regional groundwater flow and a circular or square cross section. The aim is to determine the additional lost head Δh _s due to the tunnel (i.e. additional to that resulting from the regional flow, supposed to be uniform with a hydraulic gradient i ₀). In the context of a horizontal confined aquifer having a thickness 2B and of a tunnel of radius R located in the middle part of the aquifer, an analogy can be established with the flow above a hydraulic threshold resulting from a local rise of the elevation of the base of an aquifer, having a thickness B, on a width 2R and with a vertical maximum amplitude R. When neglecting the vertical component of the hydraulic gradient compared to its horizontal component, analytical solutions are developed for various hydraulic threshold shapes (rectangular, triangular and circular), based on the equivalence with a local change in the transmissivity of an aquifer keeping a constant thickness. The corresponding formulas take the form: $ {\frac{{\Updelta h_{s} - \Updelta h_{0} }}{{\Updelta h_{0} }}} = f(a) $ , with $ a = {\frac{R}{B}} $ and Δh ₀ = 2Ri ₀. The use of these formulas shows that the additional lost head Δh _s due to the hydraulic threshold is proportional to i ₀ and that, for values of the ratio a < 0.5, the change in the piezometric surface is small. These conclusions are therefore limited by the fact that the vertical conductivity is supposed to be very large. In order to remove this hypothesis, numerical simulations are achieved using the MODFLOW code. It is considered a confined aquifer of length 2L = 110 m and thickness B = 10 m, a ratio $ a = {\frac{R}{B}} = 0.25 $ and a horizontal hydraulic conductivity $ K_{H} = 10^{ - 5} \,{\text{m}}\,{\text{s}}^{ - 1} $ . In the case of an isotropic medium ( $ \alpha = {\frac{{K_{H} }}{{K_{V} }}} = 1 $ ), the simulations allow to check the linearity of the relationship between Δh _s and i ₀, with therefore a homogeneous variation in the proportionality coefficient compared to analytical solutions. Simulations also reveal that, in the case considered, the width of influence upstream and downstream L _i , corresponding to a value of the vertical component of the hydraulic gradient <1% of i ₀, is below 5.5R for the three hydraulic threshold shapes, and that it was few influenced by the hydraulic gradient i ₀. In the case of an anisotropy of the horizontal and vertical hydraulic conductivities, simulations reveal the significant importance of the anisotropy ratio $ \alpha = {\frac{{K_{H} }}{{K_{V} }}} $ when it is more than 1, the most common case, and indicate that the proposed analytical solutions give an asymptotic value of $ {\frac{{\Updelta h_{s} }}{{\Updelta h_{0} }}} $ for the isotropic case and for the values of the component α < 1. In the context of an unconfined aquifer, the hydraulic threshold model is not directly applicable. The model studied, using the Dupuit-Forchheimer assumption, is the one of a water table aquifer with a sloped base (slope value: p ₀). The simulations focus on an aquifer of length 2L = 85 m, with a tunnel of circular cross section having a diameter 2R = 5 m, bottom of which is located 5 m above the base of the aquifer, the isotropic hydraulic conductivity being equal to $ K = 10^{ - 5} \,{\text{m}}\,{\text{s}}^{ - 1} $ . The definitions of water heights d ₀ and d between the water table and the top of the tunnel are given in Fig. 7. The water table can be located above (fully submerged tunnel) or below (partially emerged tunnel) the top of the tunnel. The difference d ₀ ? d represents the half of the additional lost head Δh _s due to the tunnel. Simulations are performed for various values of p ₀ and d ₀. They provide the values of i ₀, d and Δh _s . In the case of a fully submerged tunnel (d > 0), a significant rise of the water table upstream of the tunnel is obtained only for high values of the hydraulic gradient (5 and 10%), but, even in this case, it remains less than the tenth of the wetted height of the aquifer h _m . It is also highlighted that the ratio $ {\frac{{\Updelta h_{s} }}{{i_{0} }}} $ varies as a linear function of (R + d) and that, in the studied case, there is no influence of the tunnel for d ≥ 4R. In the case of a partially emerged tunnel (d < 0), the aquifer is locally confined under the tunnel. It is suggested that an equivalence is possible with the case of a confined aquifer having a thickness equal to the wetted height in the unconfined aquifer. This is verified with one of the simulations. In the case of a partially emerged tunnel, the change in the water table due to the tunnel remains low.