Transient grating measurements of excitonic dynamics in single-walled carbon nanotubes: The dark excitonic bottleneck

Transient grating measurements affirm the excitonic model for single-walled carbon nanotubes (SWNT) by identifying the dark exciton (D) as the population relaxation bottleneck in semiconducting-SWNT (S-SWNT). The data allow the reconstruction of the kinetics of excitonic cascade and cooling, from band continuum to vibrational cooling in the ground electronic state. In S-SWNT, the intraband relaxation occurs in 40 fs, localization into the 2g exciton occurs in 50 fs, followed by the excitonic cascade: 2g --> 1u --> D --> 1g with time constants of 175 fs, 3 ps, 300 ps, respectively. Fluorescence from the 1u state is quenched by efficient population transfer to 1D dark exciton. In metallic tubes, cooling is completed on the time scale of 1 ps.     

Optimization of the gas separation performance of polyurethane–zeolite 3A and ZSM-5 mixed matrix membranes using response surface methodology

In the present work, the response surface method software was used with five measurement levels with three factors.These were applied for the optimization of operating parameters that affected gas separation performance of polyurethane–zeolite 3A, ZSM-5 mixed matrix membranes.The basis of the experiments was a rotatable central composite design(CCD).The three independent variables studied were: zeolite content(0–24 wt%), operating temperature(25–45 ℃) and operating pressure(0.2–0.1 MPa).The effects of these three variables on the selectivity and permeability membranes were studied by the analysis of variance(ANOVA).Optimal conditions for the enhancement of gas separation performances of polyurethane–3A zeolite were found to be 18 wt%, 30 ℃ and 0.8 MPa respectively.Under these conditions, the permeabilities of carbon dioxide, methane, oxygen and nitrogen gases were measured at 138.4, 22.9, 15.7 and 6.4 Barrer respectively while the CO_2/CH_4, CO_2/N_2 and O_2/N_2 selectivities were 5.8, 22.5 and 2.5, respectively.Also, the optimal conditions for improvement of the gas separation performance of polyurethane–ZSM 5 were found to be 15.64 wt%, 30 ℃ and 4 bar.The permeabilities of these four gases(i.e.carbon dioxide, methane, oxygen and nitrogen) were 164.7, 21.2, 21.5 and 8.1 Barrer while the CO_2/CH_4, CO_2/N_2 and O_2/N_2 selectivities were 7.8, 20.6 and 2.7 respectively.     

Effect of Nanoparticles on the Hysteresis Loop in Mixed Convection within a Two-Sided Lid-Driven Inclined Cavity Filled with a Nanofluid

The present work deals with numerical modeling of mixed convection flow in a two-sided lid driven inclined square enclosure filled with water-Al₂O₃ nanofluid. The limiting cases of a cavity heated from below and cooled from above and the one differentially heated are recovered respectively for inclination angles 0° and 90°. The moving walls of the cavity are pulled in opposite directions with the same velocity and maintained at constant but different temperatures while the remaining walls are kept insulated. The numerical resolution of the studied problem is based on the lattice Boltzmann method. A parametric study is conducted and a set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles and enclosure inclination angle on fluid flow and heat transfer characteristics. The governing parameters of this problem are the Richardson number (varied from 0.1 to 10⁶), the nanoparticles volume fraction (varied from 0 to 0.04) and the inclination angle (varied from 0° to 180°). The critical conditions leading to the transition from monocellular flow to multicellular flow and vice versa are determined. In the common ranges of Richardson number and inclination angle where both monocellular and tri-cellular patterns coexist, the heat transfer is seen to be strongly reduced by the latter.     

Utilisation of water extract of green chilli pepper in the manufacture of low‐fat fresh cheese

The effect of the water extract of green chilli pepper (WECP) on some properties of low‐fat fresh cheese was studied. Cheese was manufactured from a mixture of reconstituted skim milk powder, whey protein concentrate and sodium chloride and fortified with WECP at concentrations of 0, 1, 2 and 3%. The addition of WECP significantly decreased the total and lactic acid bacteria counts as well as the yeasts and moulds counts in the fortified cheeses. The flavour was improved in cheeses made using 1 and 2% WECP, and the cheese manufactured with 2% WECP had the highest flavour and total scores.     

Insights into the effect of catalyst loading on methane steam reforming and controlling regime for metallic catalytic monoliths

The influence of Ru/LaAl₂O₃ catalyst loading (100–200 mg) was investigated over high cell density Fecralloy^® monoliths (461 cpsi, 1367 cpsi) for methane steam reforming (SMR). A uniform and well-attached catalyst layer was developed by in-situ washcoating method and the developed catalysts were analyzed by using various physico-chemical characterization techniques. The results confirmed the impact of catalyst loading on the geometric and hydraulic properties of monoliths, and methane conversion was improved by increasing both the catalyst loading and cell density. As per characteristic time analysis, no external and washcoat diffusion regimes were observed and SMR was found to be in kinetic controlling regime. The methane conversion was still limited by the amount of catalyst (200 mg) deposited onto the monoliths (40.9 μm for 461 cpsi, 26.9 μm for 1367 cpsi) which demonstrated the potential to deposit more catalyst up to the transition point of washcoat diffusion limitations. For same washcoat thickness of ～20.6 μm, the higher cell density 1367 cpsi monolith showed better catalytic activity towards SMR as compared to 461 cpsi monolith and this improvement is more prominent at lower temperature with a value of 13.6% higher methane conversion at 600 °C, WHSV = 55 NL h^?1 g_cat^?1 and S/C = 3.0.     

Framework for Analysis of Transmission Rate Scheduling for Wireless CDMA Data Networks

The optimal utilization of network resources and the capacity to fulfill quality of service requirements are key requirements for 3rd G networks operations. Several burst admission and transmission rate scheduling algorithms are proposed in the literature. In this study, we develop an analytical framework for the downlink transmission rate scheduling problem for CDMA networks employing discrete service bit rates. The framework uses the average downlink transmit power as the system state and develops a K-dimensional Markov chain representing all possible states in the system. In addition, the transition probabilities due to arrivals of burst requests are made a function of the power utilization and the average power required to support the new burst taking into account the path loss model. The study assumes a transmission rate assignment scheme where the maximum possible system bit rate is assigned given the current system state and the subscriber’s eligibility profile for particular system service rates. The analytical model provides performance metrics such as system throughput, average power utilization, average number of simultaneous transmissions, burst request blocking probability, and mean burst service time. While the developed model is applicable for a general CDMA based network with arbitrary discrete system service bit rates, the model is evaluated for the example of a cdma2000 1xRTT network. A comparison between simulation and analytic results to assess the accuracy of the model is provided.

Numerical simulation of sliding wear for a polymer–polymer sliding contact in an automotive application

Injection moulded polymer-based components are important for cost effective and fast production/assembly of auxiliary mechanisms in automotive industry. Wear is one of the critical factors, which influences the reliability and useful life in such mechanical components. Experimental determination of life parameters in terms of wear has both a cost and time impact. Therefore, the ability to predict wear at the development stage enables the designers to come up with a better design, longer useful life and more reliable products. This paper presents a numerical simulation of wear for a polymer–polymer sliding surface contact in dry conditions. Finite element analysis (FEA) is used as a tool to calculate nodal pressures at the contact area for small sliding steps. These pressures are then inputted to a customized wear calculating routine. The routine uses averaged wear coefficients (wear rates) obtained from custom designed experiments. The FE contact geometry is modified after each sliding step to account for the contact height decay thus determining wear volume loss over usage time and predicting the worn geometry.     

Identification of phenolic compounds and appraisal of antioxidant and antityrosinase activities from litchi (Litchi sinensis Sonn.) seeds

Antioxidant and antitryrosinase compounds from Litchi sinensis Sonn. seeds were extracted with five different types of polar solvents. The five extracts, namely ethanol extract (EE), 50% ethanol extract (50% EE), methanol extract (ME), 50% methanol extract (50% ME), and water extract (WE), were used for the evaluations of total phenolic content, antioxidant capabilities and antityrosinase activity. The 50% EE showed the highest total antioxidant capacity, scavenging the 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical and inhibitory activity against lipid peroxidation, and it was comparable to the activity of the synthetic antioxidant, butylated hydroxyl toluene. Fifty percent EE showed a better antityrosinase activity compared to the other extracts. After application of reverse phase high performance liquid chromatography, coupled to a diode array detector and electrospray ionisation mass spectra, five phenolic compounds, namely, gallic acid, procyanidin B2, (−)-gallocatechin, (−)-epicatechin and (−)-epicatechin-3-gallate were identified from 50% EE. This study suggests that litchi seed can potentially be used as a readily accessible source of natural antioxidants.     

Structure–properties of electron beam irradiated and dicumyl peroxide cured low density polyethylene blends

Low density polyethylene (LDPE) and its blends with styrene butadiene rubber and ethylene propylene diene monomer were cured, using electron beam irradiation as well as dicumyl peroxide (DCP). The effect of blending was found to change the melt viscosity when compared with the virgin LDPE. Also, the influence of curing system on the various properties was investigated. It was found that either irradiation or DCP curing enhanced the gel content and crosslink density as well as the mechanical and hot set properties. The obtained data showed that the curing greatly influence the stress strain and hot set properties, in which there is a variation in such properties for both curing methods in spite of the gel contents were comparable. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers