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排序方式: 共有198条查询结果,搜索用时 9 毫秒
1.
We present a new synthetic approach leading to the formation of polypyrrole architectures in submicron level and to silver/polypyrrole nanocomposites via an interfacial polymerization in a water/chloroform interface. The oxidizing agent was either Ag(I) or Fe(III). In the first case, silver nanoparticles resulted. The mean diameter of the polypyrrole structures is in the range of 200-300 nm according to the addition or not of various surfactants. The progress of the reaction was studied by UV-visible spectroscopy, which also revealed the formation of a polaron band during the growth of the oligomers. The crystal structure of the polymers was examined by X ray diffractometry and all samples appeared to be amorphous, while the samples were further characterized by thermogravimetric analysis and FT-IR spectroscopy. 相似文献
2.
Maryam Shakiba Angeli Gamez Imad L. Al-Qadi Dallas Neville Little 《International Journal of Pavement Engineering》2017,18(11):1027-1038
Realistic tire–pavement interface contact areas and stresses were incorporated into the Pavement Analysis using Nonlinear Damage Approach (PANDA) user interface (PUI). PANDA is a software library developed to simulate the complex thermo-viscoelastic–viscoplastic–viscodamage responses of the pavement to mechanical and environmental loads. The PUI is an interface generating a finite element representation of the pavement within PANDA. The application of realistic tire loading is necessary to calculate accurate pavement responses. The PUI incorporates a database of tire contact areas and stresses obtained from tire finite element simulations. The database includes tire interface characteristics with pavements for various applied loads, tire inflation pressures, vehicle speeds and scenarios of different rolling simulations. A parametric study was conducted to investigate the effect of simulations of tire contact stresses that match field measurements on viscoelastic and viscoplastic pavement responses. Pavement responses are greatly affected using realistic tire loading contact stresses and contact geometry as compared to simplified contact models. The impact on rutting and damage predictions cannot be ignored if reliable projections of pavement performance are to be made. This study confirms the importance of considering realistic three-dimensional contact stresses to design and analyse pavements. 相似文献
3.
Emad Kassem Lorena Garcia Cucalon Eyad Masad Dallas Little 《International Journal of Pavement Engineering》2018,19(12):1111-1124
The quality of the interfacial bonding between asphalt binder and aggregates plays a significant role in determining the durability of asphalt mixtures. Warm mix asphalt (WMA) modifiers have been used extensively in the last decade primarily to reduce production and compaction temperatures as well as to improve workability of asphalt mixtures. This study aimed to provide better understanding of the effects of these WMA modifiers on the interfacial bonding between asphalt binders and aggregates. The evaluation focused on measuring surface energy of binders in unaged and aged states and aggregates and then calculating energy parameters that describe the potential of a given asphalt-aggregate combination to resist fatigue cracking and moisture damage. Results show that the combination of asphalt-WMA additive, as well as the content applied of WMA additive has a significant impact on the fatigue cracking and moisture damage resistance. The results suggest that it is poor practice to use a given type and percentage of WMA modifier without regard for binder type. Instead, test methods are recommended to evaluate the compatibility of asphalt binder, WMA additive type/content, and aggregates for improved performance at different conditions. 相似文献
4.
Large signal-to-background (S/B) ratios for the Fe(CN)(6)(3)(-)(/4)(-) and IrCl(6)(2)(-)(/3)(-) redox couples in KCl have been observed in cyclic voltammetric measurements made at a conductive diamond thin-film electrode without any conventional surface pretreatment. The S/B ratios were a factor of ~16 and 8 larger at diamond than at freshly polished glassy carbon (GC) for Fe(CN)(6)(3)(-)(/4)(-) and IrCl(6)(2)(-)(/3)(-), respectively. The polycrystalline diamond film, grown on a p-Si(100) substrate, possessed significant cubic {100} faceting, as evidenced by AFM images, and was of high quality, as indicated by Raman spectroscopy. The high degree of electrochemical activity without surface pretreatment, the enhanced S/B ratios, and the excellent response stability demonstrate that diamond might be an attractive new electrode material for electroanalysis. 相似文献
5.
Eyad Masad Laith Tashman Dallas Little Hussein Zbib 《Mechanics of materials : an international journal》2005,37(12):1242-1256
This paper presents an approach for constitutive modeling of the viscoplastic behavior of asphalt mixes. This approach utilizes an anisotropic non-associated flow rule based on the Drucker–Prager yield surface. The selection of this yield surface is motivated by the field stress paths and material properties associated with permanent deformation at high temperatures. The efficacy of the model is demonstrated by analyzing data from compressive triaxial tests conducted at different confining pressures and strain rates for three different mixes. The model parameters are related to the experimental measurements of aggregate shape characteristics, aggregate surface energy, inherent anisotropic distribution of aggregates, and microstructure damage measured using X-ray computed tomography and image analysis techniques. Establishing the relationship between the model parameters and material properties is important in order to optimize the mix properties, and achieve desirable mix performance. 相似文献
6.
Hennig RG Trinkle DR Bouchet J Srinivasan SG Albers RC Wilkins JW 《Nature materials》2005,4(2):129-133
Impurities control phase stability and phase transformations in natural and man-made materials, from shape-memory alloys to steel to planetary cores. Experiments and empirical databases are still central to tuning the impurity effects. What is missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound. Pure titanium transforms from ductile alpha to brittle omega at 9 GPa, creating serious technological problems for beta-stabilized titanium alloys. Impurities in the titanium alloys A-70 and Ti-6Al-4V (wt%) suppress the transformation up to at least 35 GPa, increasing their technological utility as lightweight materials in aerospace applications. These and other empirical discoveries in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: the effect on the relative phase stability, and the energy barrier of the transformation. Ab initio methods calculate both changes due to impurities. We show that interstitial oxygen, nitrogen and carbon retard the transformation whereas substitutional aluminium and vanadium influence the transformation by changing the d-electron concentration. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti-6Al-4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations. 相似文献
7.
Dallas JL 《Applied optics》1994,33(27):6373-6376
To determine an optimum host for simultaneous short-pulse-width and large-slope-efficiency generation, the performance of Nd:YAG, Nd:YLF, Nd:YVO(4), and Nd(3+):Sr(5)(VO(4))(3)F [Nd:strontium fluorovanadate (S-VAP)] was characterized under a variety of end-pumped and frequency-modulation mode-locking conditions. The slope efficiency, threshold pump power, and pulse width were recorded for each laser and compared with theory. A figure of merit was defined, yielding Nd:YLF and Nd:YVO(4) as the experimentally determined crystals of choice. 相似文献
8.
9.
Ronald E. Davis Dallas G. Denery David A. Kendrick Raman K. Mehra 《Computational Economics》2012,39(1):71-76
A continuous-time control model was formulated and fitted to macroeconomic data by an expert control engineer who worked at
NASA-AMES in the 1980s, Rodney C. Wingrove. Two articles were prepared and made available to the aerospace industry at that
time, however the authors feel that wider distribution of his study posthumously is warranted at this time. This introduction
to the two Wingrove articles that follow provides background information on the technologies Wingrove was working with, as
well as subsequent developments in both macroeconomic and engineering modeling and analysis technology. Our purpose is to
stimulate further research along the same lines which can potentially lead to structural and/or policy rule improvements that
can prevent the extremely turbulent abberations that have been seen in recent years, and promote steady growth with low inflation
and low unemployment in a sustainable way. Of particular concern is the exploding national debt problem. 相似文献
10.
John F. Rushing Masoud K. Darabi Eisa Rahmani Dallas N. Little 《International Journal of Pavement Engineering》2017,18(2):138-159
This study presents the rutting performance results of full-scale pavement test sections subjected to F-15E and C-17 aircraft wheels at two different temperatures. Pavement structures for the tests were constructed under shelter in the U.S. Army Engineer Research and Development Center's (ERDC) pavement test facility. The full-scale test results are used to validate viscoelastic, viscoplastic and hardening-relaxation constitutive relationships implemented in the Pavement Analysis Using Nonlinear Damage Approach (PANDA) model. PANDA is a mechanistic-based model which incorporates nonlinear viscoelastic, viscoplastic, hardening-relaxation, viscodamage, moisture-induced damage and ageing constitutive relationships. Results of dynamic modulus and different repeated creep-recovery laboratory tests are analysed to extract the parameters associated with viscoelastic, viscoplastic and hardening-relaxation constitutive relationships implemented in PANDA. Once calibrated, PANDA is used to predict the rutting performance observed in full-scale pavement test sections. The simulation results illustrate that PANDA is capable of predicting the rutting of airfield pavements subjected to heavy aircraft wheel loads at intermediate and high temperatures. It is shown that PANDA successfully predicts the effect of shear flow and upheaval at the edges of the wheel. The data from simulation suggested that PANDA, once calibrated, can provide insight into the critical locations of tensile and compressive stresses within the pavement structure. PANDA simulations not only provide a tool for evaluating existing structures, but also can be used in designing more sustainable pavement structures and materials. 相似文献