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排序方式: 共有393条查询结果,搜索用时 31 毫秒
31.
Saliha Alehyen Fatima Bensajjay Mohammed El Achouri Lourdes Pérez Aurora Pinazo María Rosa Infante 《Journal of surfactants and detergents》2010,13(3):339-348
A new oligomeric surfactant: N,N,N′,N″,N″- pentamethyl diethyleneamine—N,N″-di-[tetradecylammonium bromide] referred to as 14-2-N(CH3)-2-14 was synthesized, purified and characterized by Elemental Analysis, 1H and 13C NMR and Electrospray. The micellar properties of this compound were determined by electrical conductivity and surface tension
methods. Optical microscopy was also employed to study the behavior of anhydrous surfactant and the binary water/surfactant
system as a function of temperature. The critical micellar concentration (cmc), degree of counterion binding and thermodynamic
parameters of micellization (standard molar Gibbs energy, enthalpy and entropy of micellization) were determined by electrical
conductivity measurements in the temperature range [24–54 °C]. Surface tension measurements also provide information about
the dependence of the surface tension at the cmc (γcmc), pC20 (negative logarithm of the surfactant’s molar concentration C20, required to reduce the surface tension by 20 mN/m, the surface excess (Γmax) at air/solution interface, the minimum area per surfactant molecule at the air/solution interface (Amin), surface pressure
at the cmc (Пcmc), critical packing parameter(CPP) and the standard free energies of micellization (
\Updelta Gm0\Updelta G_{m}^{0}) and of adsorption (
\Updelta G\textads0 \Updelta G_{\text{ads}}^{0} ). 相似文献
32.
Simin G. Koudymov A. Fatima H. Jianping Zhang Jinwei Yang Asif Khan M. Hu X. Tarakji A. Gaska R. Shur M.S. 《Electron Device Letters, IEEE》2002,23(8):458-460
The characteristics of a novel nitride based field-effect transistor combining SiO/sub 2/ gate isolation and an AlGaN/InGaN/GaN double heterostructure design (MOSDHFET) are reported. The double heterostructure design with InGaN channel layer significantly improves confinement of the two-dimensional (2-D) electron gas and compensates strain modulation in AlGaN barrier resulting from the gate voltage modulations. These decrease the total trapped charge and hence the current collapse. The combination of the SiO/sub 2/ gate isolation and improved carrier confinement/strain management results in current collapse free MOSDHFET devices with gate leakage currents about four orders of magnitude lower than those of conventional Schottky gate HFETs. 相似文献
33.
The present investigation deals with the effect of Al2O3 particle reinforcement on the lubricated sliding behavior of ZA-27 alloy. The composites with 3, 5, and 10 wt% of Al2O3 particles were produced by the compocasting procedure. Tribological properties of alloy and composites were studied, using
block-on-disk tribometer at different specific loads and sliding speeds. The test results revealed that composite specimens
exhibited significantly lower wear rate, but higher coefficient of friction than the matrix alloy specimens in all the combinations
of applied loads and sliding speeds. The improved antiwear characteristics of the composites were influenced by positive effects
of higher frictional heating on compatibility of the composite phases and suppressing micro-cracking tendency. Due to that,
effects of reinforcing hard particles were manifested through the reduced wear rate of composites, especially in conditions
of higher load, lower sliding speeds and higher Al2O3 particle content. In present wear tests, the significant forming of mechanically mixed layers was not noticed, what is confirmed
by the SEM microphotographs. 相似文献
34.
Fatima Z. BouanisLaurent Baraton Vincent HucDidier Pribat Costel S. Cojocaru 《Thin solid films》2011,519(14):4594-4597
We investigated the single-walled carbon nanotubes (SWCNTs) growth on Ru nanoparticle catalyst via hot filament assisted chemical vapor deposition (HFCVD) with two independent W filaments for the carbon precursor (methane) and the hydrogen dissociation respectively. The Ru nanoparticles were obtained following a two-step strategy. At first the growth substrate is functionalized by silanisation, then a self assembly of a ruthenium porphyrin complex monolayer on pyridine-functionalized metal oxide substrates. We have studied the impact of the filaments power and we optimized the SWCNTs growth temperature. The as grown SWCNTs were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. It was found that the quality, density and the diameter of SWCNTs depends on the filament and growth temperature. Results of this study can be used to improve the understanding of the growth of SWCNTs by HFCVD. 相似文献
35.
Kenzu Abdella Ruqaiya Ammar ThannonAisha Ibrahim Mehri Fatima Ahmed Alshaikh 《Journal of Constructional Steel Research》2011,67(5):826-832
Presented in this paper is a new stress-strain relation for stainless steel alloys that provides the stress as an explicit function of the strain. The relation is an approximate inversion of a recently proposed three-stage stress-strain relation based on a modified Ramberg-Osgood equation. The three-stage relation is a much more accurate formulation than the previous two-stage formulations and is applicable to both tensile as well as compressive stresses. The new relation is derived by making a rational function assumption on the fractional deviation of the actual stress-strain curve from an idealized linear elastic behaviour. The new expression is valid over the full range of the stress well beyond the elastic region. The validity of the inverted expression is tested over a wide range of material parameters. These tests demonstrate that, the new expression results in stress-strain curves which are both qualitatively and quantitatively in excellent agreement with the fully iterated numerical solution of the full-range stress-strain relation with a maximum error below 4%. 相似文献
36.
This paper presents the results of a study that consists of estimating the temperature distribution and air flow movement in a model room with a numerical model based on the Euler equations. Numerical results obtained for two scenarios of ventilation and heating are compared with the predictions of a Navier–Stokes model, as well as with experimental results. A comparison of the local thermal comfort indices PMV and PPD obtained experimentally and numerically is also presented. Results show that the Euler model is capable of properly estimating the temperature distribution, the air movement and the comfort indices in the room. Furthermore, the use of Euler equations allows a reduction of computational time in the order of 30% compared to the Navier–Stokes modeling. 相似文献
37.
Nano finish grinding of brittle materials using electrolytic in-process dressing (ELID) technique 总被引:4,自引:0,他引:4
Recent developments in grinding have opened up new avenues for finishing of hard and brittle materials with nano-surface finish,
high tolerance and accuracy. Grinding with superabrasive wheels is an excellent way to produce ultraprecision surface finish.
However, superabrasive diamond grits need higher bonding strength while grinding, which metal-bonded grinding wheels can offer.
Truing and dressing of the wheels are major problems and they tend to glaze because of wheel loading. When grinding with superabrasive
wheels, wheel loading can be avoided by dressing periodically to obtain continuous grinding. Electrolytic inprocess dressing
(ELID) is the most suitable process for dressing metal-bonded grinding wheels during the grinding process. Nano-surface finish
can be achieved only when chip removal is done at the atomic level. Recent developments of ductile mode machining of hard
and brittle materials show that plastically deformed chip removal minimizes the subsurface damage of the workpiece. When chip
deformation takes place in the ductile regime, a defect-free nano-surface is possible and it completely eliminates the polishing
process. ELID is one of the processes used for atomic level metal removal and nano-surface finish. However, no proper and
detailed studies have been carried out to clarify the fundamental characteristics for making this process a robust one. Consequently,
an attempt has been made in this study to understand the fundamental characteristics of ELID grinding and their influence
on surface finish. 相似文献
38.
The increasing pollution of the electromagnetic environment has prompted the study of array pattern nulling techniques. These techniques are very important in radar, sonar and communication systems for minimising degradation in signal-to-noise ratio performance due to undesired interferences. Adaptive array antennas backed by strong signal processing algorithms are able to automatically change the beam pattern in accordance with the changing signal environment. It not only directs maximum radiation in the direction of the desired mobile user but also introduces nulls for interfering directions while tracking the desired mobile user at the same time. The adaptation is achieved by multiplying the incoming signals with complex weights and then summing them together to obtain the desired radiation pattern. Adaptive array optimization is an NP-hardproblem. In this paper, a technique based on the coupling between tabu search and simulated annealing methods is presented to solve this problem. Several illustrative examples of patterns with imposed single and multiple null directions are given to show the versatility of the present method. 相似文献
39.
Sulphur volatiles are major factors in the perceived aroma of grapefruit juice, GFJ. The objective of this study was to develop a procedure to concentrate, separate, identify and quantify the major volatile sulphur compounds, VSC’s, in grapefruit juices. SPME parameters such as headspace atmosphere, fibre coating, extraction time and temperature were evaluated. High resolution capillary GC using ZB-5, DB-Wax and PLOT columns coupled with pulsed flame photometric detection, PFPD, were employed for separation and detection. Thirteen sulphur volatiles were identified including; hydrogen sulphide, sulphur dioxide, methanethiol, dimethyl sulphide, carbon disulphide, dimethyl disulphide, 2-methyl thiophene, 3-methyl thiophene, methional, dimethyl trisulphide, 3-mercaptohexylacetate, 2,8-epithio-cis-p-menthane and 1-p-menthene-8-thiol. Five additional VSC’s were tentatively identified. Canned reconstituted GFJ had more total sulphur volatiles and a greater number than fresh GFJ. Hydrogen sulphide comprised over 80% of total sulphur volatiles in fresh GFJ but only 5% in canned GFJ. 相似文献
40.
In present study, the effect of Al2O3 particle reinforcement on the sliding behavior of ZA-27 alloy composites was investigated. The composites with 3, 5, and
10 wt% of Al2O3 particles were produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied,
using block-on-disk tribometer under unlubricated sliding conditions at different specific loads and sliding speeds. The worn
surfaces of samples were examined by the scanning electron microscopy (SEM). The test results revealed that those composite
specimens exhibited significantly lower wear rate than the ZA-27 matrix alloy specimens in all combinations of applied loads
and sliding speeds. The difference in the wear resistance of composite with respect to the matrix alloy, increased with the
increase of the applied load/sliding speed and Al2O3 particle content. The highest degree of improvement of the ZA-27 alloy tribological behavior corresponded with change of
the Al2O3 particles content from 3 to 5 wt%. At low sliding speed, moderate lower wear rate of the composites over that of the matrix
alloy was noticed. This has been attributed to micro cracking tendency of the composites. Significantly reduced wear rate,
experienced by the composite over that of the matrix alloy at the higher sliding speeds and loads, could be explained due
to enhanced compatibility of matrix alloy with dispersoid phase and greater thermal stability of the composite in view of
the presence of the dispersoid. Level of wear rate of tested ZA-27/Al2O3 samples pointed to the process of mild wear, which was primarily controlled by the formation and destruction of mechanical
mixed layers (MMLs). 相似文献