117,324 research outputs found
Investigation of dry reforming of methane over Mo-based catalysts
No full textAn investigation of methane dry reforming over Mo–Ni based catalysts is carried out in a fixed bed catalytic reactor at different temperatures. Two Mo–Ni catalysts supported on alumina are prepared with 20%Mo–10%Ni and 20%Mo–2%Ni, respectively, in which the nickel is used for its highly resistance at high temperature during dry reforming of methane (DRM) reaction. Experimental results shows that an increase in temperature favours the CH4 conversion and determined a higher H2/CO ratio. A small amount of deposited coke is observed because of the abundant presence of CO2 in the reaction medium and only for 2% Ni catalysts. A kinetic model is proposed for the DRM with Mo–Ni based catalysts, in which the reaction mechanism routes and the operating conditions such as the reaction temperature and the CH4/CO2 molar ratio are accounted for. The results of the mathematical model allow a consistent description of the experimental data, in terms of gas outlet composition. The absence of the methane decomposition reaction, responsible of carbon deposition that is known to lead to catalyst deactivation, is the main result that is adequately predicted by the model
Analysis of 4H-SiC MOSFET with distinct high-k/4H-SiC interfaces under high temperature and carrier-trapping conditions
No full textIn this work, the reliability of different oxide/4H-SiC interfaces under high temperature and carrier-trapping conditions are investigated carefully. In more detail, the carrier-trapping and temperature effects are considered in the electrical characterization of a low breakdown 4H-SiC-based MOSFET by using in turn SiO2, Si3N4, AlN, Al2O3, Y2O3 and HfO2 as gate dielectric. A gate oxide with a high relative permittivity notably improves the transistor performance. In addition, HfO2 assures the MOSFET best immunity behaviors. The obtained results are explained in terms of the carrier channel mobility, device on-state resistance, and oxide electric field. By using HfO2, however, an increased gate leakage current is calculated. This drawback is overcome by inserting a thin interfacial layer (2 nm-thick) in the HfO2/4H-SiC MOS structure. In particular, two alternative gate stacked dielectrics, involving either SiO2 or Al2O3, have proven their effectiveness in preserving the transistor on-state figures of merit while limiting the gate leakage current in the whole explored gate voltage range. To support the prediction capabilities of the presented modeling analysis, the simulations results are compared with experimental data from literature resulting in a good agreement. Low power MOSFETs are used in several applications for which reliability and durability are as critical as performance. For example, referring to power optimizers for photovoltaic (PV) modules, which fall under the low-load and low-voltage category of DC–DC converters, these devices significantly increase the energy generated by each single PV module operating under harsh conditions and stressing environments. In addition, they have to ensure high reliability over the long term of operation
Temperature and SiO2/4H-SiC interface trap effects on the electrical characteristics of low breakdown voltage MOSFETs
No full textThe temperature and carrier-trapping effects on the electrical characteristics of a 4H silicon carbide (4H-SiC) metal–oxide–semiconductor field effect transistor (MOSFET) dimensioned for a low breakdown voltage (BVDS) are investigated. Firstly, the impact of the temperature is evaluated referring to a fresh device (defects-free). In particular, the threshold voltage (Vth), channel mobility (μch), and on-state resistance (RON) are calculated in the temperature range of 300 K to 500 K starting from the device current–voltage characteristics. A defective MOSFET is then considered. A combined model of defect energy levels inside the 4H-SiC bandgap (deep and tail centers) and oxide-fixed traps is taken into account referring to literature data. The simulation results show that the SiO2/4H-SiC interface traps act to increase RON, reduce μch, and increase the sensitivity of Vth with temperature. In more detail, the deep-level traps in the mid-gap have a limited effect in determining RON once the tail traps contributions have been introduced. Also, for gate biases greater than about 2Vth (i.e., VGS > 12 V) the increase of mobile carriers in the inversion layer leads to an increased screening of traps which enhances the MOSFET output current limiting the RON increase in particular at low temperatures. Finally, a high oxide-fixed trap density meaningfully influences Vth (negative shifting) and penalizes the device drain current over the whole explored voltage range
Improving the efficiency of a-Si:H/c-Si thin heterojunction solar cells by using both antireflection coating engineering and diffraction grating
No full textIn this paper, we present an analytical study of the impact of light trapping and multilayer antireflection coating (ARC) on the electrical characteristics of n(a-Si:H)/i(a-Si:H)/p(c-Si)/p+(C-Si) heterojunction solar cells with intrinsic thin layer (SHJ). The developed analytical model considers a triangular texture morphology of the solar cell top surface and a double ARC layer. This model serves as a fitness function to optimize the device reliability against the interfacial traps using Grey Wolf optimization approach (GWO). The optimized solar cell reveals a high short circuit current I SC = 47.9 mA, an open circuit voltage V OC = 0.56 V, fill factor FF = 74.72% and a conversion efficiency improvement in the order of 30% over conventional planar solar cells efficiency. Not only the optimized SHJ solar cell exhibits higher performance in terms of figure of merits, but also shows superior interfacial traps reliability at the amorphous/crystalline interface. This reliability enhancement is due to a better surface texture control and intrinsic thin film layer tuning provided by GWO approach
Chemical Constituents of the Essential Oil from Salvia Verbenaca ssp. Clandestina from Algerian Pre-Sahara
No full textThe essential oil obtained by hydrodistillation from the aerial parts of Salvia verbenaca (L.) Briq. ssp. clandestina (L.) Pugsl. (Lamiaceae) growing wild in Bou Saâda, pre-Saharan region of Algeria, was analyzed by GC-MS. Sixty-four compounds were detected, representing 95.6% of the whole oil, among them forty five compounds are identified in this sample for the first time. The essential oil of S. verbenaca ssp. clandestena showed the predominance of sesquiterpenes (56.4%) followed by monoterpene derivatives (35.5%). The main constituents were β-pinene (10.2%), spathulenol (8.7%), caryophylene oxide (6.1%), α- pinene (5.2%), germacrene D (5%) and α-gurjunene (4.9%). Chemical composition of the essential oil from our sample may be categorized as sesquiterpene and monoterpene chemotype among the four chemotypes identified for Salvia species
Numerical simulations of the electrical transport characteristics of a Pt/n-GaN Schottky diode
No full textIn this paper, using a numerical simulator, we investigated the current-voltage characteristics of a Pt/n-GaN thin Schottky diode on the basis of the thermionic emission (TE) theory in the 300 to 500 K temperature range. During the simulations, the effect of different defect states within the n-GaN bulk with different densities and spatial locations is considered. The results show that the diode ideality factor and the threshold voltage decrease with increasing temperature, while at the same time, the zero-bias Schottky barrier height (Fb0) extracted from the forward current density-voltage (J-V) characteristics increases. The observed behaviors of the ideality factor and zero-bias barrier height are analyzed on the basis of spatial barrier height inhomogeneities at the Pt/GaN interface by assuming a Gaussian distribution (GD). The plot of apparent barrier height (Fb,App) as a function of q/2kT gives a straight line, where the mean zero-bias barrier height (b0 ) and the standard deviation (s0) are 1.48 eV and 0.047 V, respectively. The plot of the modified activation energy against q/kT gives an almost the same value ofb0 and an effective Richardson constant A* of 28.22 A cm%2 K%2, which is very close to the theoretical value for n-type GaN/Pt contacts. As expected, the presence of defect states with different trap energy levels has a noticeable impact on the device electrical characteristics
Analysis of the Forward I–V Characteristics of Al-Implanted 4H-SiC p-i-n Diodes with Modeling of Recombination and Trapping Effects Due to Intrinsic and Doping-Induced Defect States
No full textIn this paper, the impact of silicon carbide intrinsic defect states, such as Z1/2 and EH6/7 centers, on the forward current–voltage curves of aluminum (Al)-implanted 4H-SiC p-i-n diodes is investigated by means of a physics-based device simulator. During the simulations, an explicit carrier trap effect due to an electrically active defect concentration produced by the Al+ ion implantation process in the anode region was also taken into account. The obtained current–voltage characteristics are compared with those measured experimentally for several samples at different current levels. It is found that intrinsic defect densities as high as the epilayer doping may lead to undesirable device properties and instability of the forward bias behavior. The diode ideality factor and the series resistance increase with the increase of defects and could be controlled by using high-purity epi-wafers. Furthermore, due to their location in the bandgap and capture cross-sections, the impact of Z1/2 centers on the device electrical characteristics is more severe than that of EH6/7 centers
Improved InxGa1_xP/GaAs /Ge tandem solar cell using light trapping engineering and multi-objective optimization approach
No full textIn this paper, an analytical model for studying the effect of light trapping mechanism on tandem solar cell performance is developed. The proposed model considers diffraction grating morphology and antireflection coating of the InxGa1_xP/GaAs/Ge tandem solar cell. The main photovoltaic figures of merit of the InxGa1_xP/GaAs/Ge tandem solar cell are investigated. The obtained results prove the outstanding capability of the light trapping mechanism to improve device performance. An efficiency of 32.5% was obtained. A short circuit current density (JSC) of 28 mA/cm2, an open-circuit voltage (VOC) of 1.288 V, and a fill factor (FF) of 87.7% were calculated. In addition, the developed model serves as a fitness function to optimize the light trapping capability using a multi-objective particle swarm optimization (MOPSO) approach. The optimized tandem solar cell design exhibits higher performance characterized by JSC =35.3 mA/cm2, VOC = 1.305 V, and a conversion efficiency of 41.7% which outweighs that of the conventional planar solar cell. Therefore, the proposed design methodology efficiently minimize the reflectance via establishing an intensive light trapping mechanism at the front of both subcells and opens promising opportunities to enhance the tandem solar cell performances
Simulation Study of Carbon Vacancy Trapping Effect on Low Power 4H-SiC MOSFET Performance
No full textThe carbon vacancy in 4H-SiC is an important recombination center of the minority carrier and a direct consequence of SiC-based device degradation. In 4H-SiC, this defect acts as the primary carrier-lifetime killer. Whether, low-energy electron radiation exposure or high temperature processing in an inert ambient gas will produce the carbon vacancy defect. Despite, the extensiveness of the studies concerning the defect’s modeling and characterization, numerous essential questions remain. Amongst them, we have the impact of these defects on the performance of 4H-SiC MOSFET. Herein, the influence of intrinsic defect states, namely, Z1/2 and EH6/7 centers, on the 4H-SiC MOSFET electrical outputs is examined via 2D numerical simulation. The obtained results show that the traps act to increase the device on-state resistance (RON), reduce the channel mobility, increase the threshold voltage (Vth). Besides, the increase of the temperature leads to less influence of the traps on the threshold variation. Furthermore, due to their locations in the bandgap, the impact of both Z1/2 and EH6/7 centers at room temperature on the device electrical outputs is extreme. For high temperature the EH6/7 have the severest impact because of the cross section temperature dependency
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