Traps centers and deep defects contribution in current instabilities for AlGaN/GaN HEMT's on silicon and sapphire substrates

AlGaN/GaN high electron mobility transistors (HEMTs) with Si and Al₂O₃ substrates reveals anomalies on I_ds-V_ds-T and I_gs-V_gs-T characteristics (degradation in drain current, kink effect, barrier height fluctuations, etc.). Stress and random telegraph signal (RTS) measurements prove the presence of trap centers responsible for drain current degradation. An explanation of the trapping mechanism responsible for current instabilities is proposed. Deep defects analysis performed by capacitance transient spectroscopy (C-DLTS), frequency dispersion of the output conductance (G_ds(f)), respectively, on gate/source and drain/source contacts and RTS prove the presence of deep defects localized, respectively, in the gate and in the channel regions. Defects detected by C-DLTS and G_ds(f) are strongly correlated, respectively, to barrier height inhomogeneities and kink anomalies. Gate current analysis confirms the presence of (G-R) centers acting like traps at the interface GaN/AlGaN. Finally, the localization of these traps defects is proposed.     

Extraction of Cadmium from Phosphoric Acid Media by Di(2‐ethylhexyl) Dithiophosphoric Acid

Di(2‐ethylhexyl) dithiophosphoric acid (D₂EHDTPA) was examined for cadmium removal from phosphoric acid solutions. High extraction performance was achieved. By means of slope and saturation methods, the stoichiometry of the complex was postulated. Both methods indicated the presence of anhydrous CdR₂ as being the metallic extracted complex, HR denoting the D₂EHDTPA molecule. The stripping of cadmium from the organic phase was ensured by aqueous solutions of HCl; quantitative stripping was achieved with either HCl (4 M) or an HCl‐NaCl mixture.     

Modified smectitic Tunisian clays used in the formulation of high performance lubricating greases

Two smectitic Tunisian clays were organically modified by exchange reaction with dioctadecyl dimethylammonium chloride (DODMA) and hexadecyl benzyl dimethylammonium chloride (HBDMA). The resulting organoclays were used in the formulation of lubricating greases. The results of the performance tests applied to the obtained greases showed that their physicochemical and mechanical characteristics are in conformity with the extreme-pressure (EP) greases specifications. The obtained greases present good EP properties with only 0.5% EP additives. The smectitic clays used could constitute a raw material in the formulation of high performance lubricating greases.     

Eco-friendly process for soft drink industries wastewater reuse as growth medium for Saccharomyces cerevisiae production

Soft drink industries suffer inadequate handling of their product losses generally considered as wastes. Those products contribute to the wastewater organic load augmentation and cause fastidious environmental impact. In this study, an industrial scale bioconversion process based on multistage fermentation was proposed to treat and reuse soft drink factories’ high-loaded effluents for valuable components production. An upstream segregation of non-consumed beverage was performed to reduce the organic load of the soft drink wastewater. Beverage characterization revealed an important sugar content. Such an organic compound is undoubtedly responsible of the high organic load of soft drink wastewater. Thus, the bioconversion of the sugar content of soft drink waste to single-cell proteins was proposed as a solution to reduce wastewater polluting load. Soft drink wastewater including rejected and returned products was tested to be used as a substrate for yeast biomass production using a commercial yeast strain of Saccharomyces cerevisiae. The effect of nutrient supplementation and the initial sugar concentration effect in culture media on the biomass production were investigated using batch and fed-batch process. Results indicated that supplementation is necessary for successful fermentation. Juices and nectars gave better sugar-biomass conversion yields (0.38–0.45 g g⁻¹). Depletion of the sugar contained in the soft drinks exceeded 96 % for all fermented media. Fed-batch culture revealed a biomass concentration improvement reaching 9.16 g L⁻¹ compared to batch biomass concentration resulting from batch cultures (5.2 g L⁻¹). The proposed process was shown to enable beverage industries to reduce water pollution generation through an on-site segregation procedure and a storage system to valorize product losses as source medium for single-cell protein production.

     

Cooperative Association/Re-association Approaches to Optimize Energy Consumption for Real-Time IEEE 802.15.4/ZigBee Wireless Sensor Networks

The major challenge of real-time Wireless Sensor Networks stills the optimization of both constraints: energy consumption, to get long network lifetime and the communication delay, to meet real-time requirements. In the context of IEEE 802.15.4/ZigBee networks, the association procedure has a direct effect on building paths optimizing those constraints. In this paper, we are interested on the definition of an ideal approach of load balancing to fairly distribute energy consumption among nodes in IEEE 802.15.4/ZigBee WSNs. This approach leads to conserve energy of each node in order to extend the network lifetime. To be closer to this ideal, we propose new dynamic association/re-association approaches allowing path alternation relative to association criteria and their threshold parameters. The implementation of those approaches in NS2 simulator highlights the efficiency of cooperative and dynamic association criteria particularly the one based on the sum of the inverses of remaining energy. Indeed, this approach gives better results with regard to energy distribution according to ideal approach which leads to a longer lifetime. It also performs lower latency for real time communication.     

Changes in Fatty Acid, Tocopherol and Xanthophyll Contents During the Development of Tunisian-Grown Pecan Nuts

Among oil compounds, fatty acids, tocopherols and xanthophylls (lutein and zeaxanthin) are of special interest due to their nutritional properties. The identification and quantification of these compounds in pecan nuts (Carya illinoinensis) could therefore be very useful to produce functional foods rich in compounds of this type. This paper reports studies on their accumulation and the effect of ripening on the content of these high value-added compounds. The total lipid content increased during the ripening. Saturated and polyunsaturated fatty acids decreased significantly, whereas, monounsaturated fatty acids increased during the ripening of pecan nut fruit. Maximum levels of total tocopherol (279.53 mg/kg oil) and xanthophyll (6.18 mg/kg oil) were detected at 20th weeks after the flowering date. These amounts decreased gradually as ripening advances. The early stages of pecan ripening seem to have nutritional and pharmaceutical interests. These results may be useful for evaluating the pecan nut quality and determining the optimal period when the pecans accumulated the maximum of these nutritional and healthy compounds.     

Simple analytical solution and efficiency improvement of polysilicon emitter solar cells

A simple analytical model has been developed to simulate the performance of solar cells with polysilicon contact on the front surface. The polysilicon layer with a columnar grain structure is modeled by an effective recombination velocity using a two-dimensional transport equation. A one-dimensional transport equation in the single-crystal emitter is solved, taking into account bulk recombination and non-uniformly doped emitter. Then, simple analytical expressions for the emitter reverse saturation current and light-generated current densities are obtained. The collection of the light-generated carriers in polysilicon layer has been discussed and an analytical solution of the light-generated current is derived. The results show that the polysilicon layer can result in a decrease in emitter reverse saturation current density and an increase in solar cell photovoltaic parameters. In fact, the emitter region should not be treated as a ‘dead layer’ because thin polysilicon layer front surface contact gives an improvement of about 60 mV for the open-circuit voltage, 3.6 mA/cm² for the photocurrent, and 3.9% for the cell efficiency.