 ترتیب محصولات: پیش فرض
 نمایش 15 محصول در هر صفحه

A simple thermodynamic model for the doping and alloying of nanoparticles
0 تومانImpurity incorporation into nanoparticles is modeled using thermodynamics. For small particles, entropically driven impurity incorporation is reduced, rendering doping difficult. We show that the free energy of surface impurities in small nanoparticles is lower than core impurities, surface doping therefore occurs preferentially. A critical size for core doping is identified, below which it is energetically unfavorable. In all cases, core impurity concentration is reduced as particle size decreases. We show larger than bulk impurity concentrations are possible, corresponding to increased alloying.

An experimental and numerical study on heat transfer enhancement for gas heat exchangers fitted with porous media
0 تومانThe present experimental and numerical work investigates the effect of metallic porous materials, inserted in a pipe, on the rate of heat transfer. The pipe is subjected to a constant and uniform heat flux. The effects of porosity, porous material diameter and thermal conductivity as well as Reynolds number on the heat transfer rate and pressure drop are investigated. The results are compared with the clear flow case where no porous material was used. The results obtained lead to the conclusion that higher heat transfer rates can be achieved using porous inserts at the expense of a reasonable pressure drop. Also, it is shown that for an accurate simulation of heat transfer when a porous insert is employed its effective thermal conductivity should be carefully evaluated.

Fabrication of Nanostructured Electroforming Copper Layer by Means of an Ultrasonicassisted Mechanical Treatment
0 تومانElectroformed copper layer with nanostructure is obtained using a subsequent mechanical treatment under the conditions of ultrasonic vibration according to the demand of high performance material in aeronautics. The microstructure of the electroformed copper layer is observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile strength is evaluated with a tensile tester. It is found that bulk crystal of electroformed copper’s surface layer is changed to nanocrystals (about 10 nm in size) after the ultrasonicassisted mechanical treatment (UMT) but the whole monocrystalline structure still remains. The tensile strength exhibited by the new copper layer is two times better than the regular electroformed copper layer, while the fracture strain remains constant. In addition, the strengthening mechanism of UMT process is proved to be dislocation strengthening mechanism.

Innovative coupled fluid–structure interaction model for carbon nanotubes conveying fluid by considering the size effects of nanoflow and nanostructure
0 تومانIn this article, we reappraise the wellknown equation of motion for a pipe conveying viscous fluid. We utilize prominent principles of fluid mechanics such as Navier–Stokes’ equation as well as several benchmark references in the field of fluid–structure interaction (FSI) to reveal that the viscosity of the fluid flow should not appear explicitly in the equation of motion of pipe conveying fluid. Based on this result, we could develop an innovative model for one dimensional coupled vibrations of carbon nanotubes (CNTs) conveying fluid using slip velocity of the fluid flow on the CNT walls as well as utilizing sizedependent continuum theories to consider the size effects of nanoflow and nanostructure. Therefore, this innovative coupled FSI equation suggests that CNTs conveying nanoflow remain stable for higher velocities. In the other words, the critical average velocity of the fluid flow at which the divergence instability occurs, should be greater in comparison with the critical velocity predicted by the models used plug flow and classical continuum theories.

Mechanical properties of foamed concrete exposed to high temperatures
0 تومانThis paper reports the results of an experimental and analytical study to investigate the mechanical properties of unstressed foamed concrete exposed to high temperatures. Two densities of foamed concrete, 650 and 1000 kg/m3, were made and tested with additional tests being performed on densities of 800, 1200 and 1400 kg/m3 for additional data. The experimental results consistently demonstrated that the loss in stiffness for foamed concrete at elevated temperatures occurs predominantly after about 90 C, regardless of density as water expands and evaporates from the porous body. From a comparison of the experimental results of this research with a number of predictive models for normal strength concrete, this research has found that the mechanical properties of foamed concrete can be predicted using the mechanical property models for normal weight concrete given that the mechanical properties of foamed concrete come from Portland Cement CEM1.

Nonlinear Viscoelastic Analysis of Thick Walled Cylindrical Composite Pipes
0 تومانThis paper analyzes the effect of the polymer matrix nonviscoelastic behaviour in the mechanical behaviour of thick multilayered cylinders. The original contribution of this work is to provide novel approximate analytical solutions to compute the timedependent internal stress state throughout the pipe thickness within the framework of nonlinear viscoelasticity theory. The structures considered are thick, multilayered anisotropic infinitive long cylinders subjected to axisymmetric mechanical loading .Under such conditions there is an exact elastic solution which naturally satisfies equilibrium, straindisplacement, compatibility and boundary conditions for the stated constitutive equations and loading. Due to the continuous stress variations throughout the cylinder thickness, the proposed nonlinear viscoelastic solution assumes the averaged stress state to calculate the nonlinear elastic and viscoelastic factors in each layer. Furthermore the solution is obtained assuming that the creep strains, within each layer, are constant through the thickness. The proposed algorithm converges to the exact solution when the number of layers is artificially increased. For the linear viscoelastic case the proposed solution proved to match the exact known solution for isotropic viscoelastic materials. Finally several invented cases are run to illustrate the importance of the viscoelasticity phenomenon on the internal stress field throughout thick laminated cylinders.

Preparation of novel porous solids from aluminapillared fluorine micas by acidtreatment
0 تومانNew porous solids from aluminapillared fluorine micas (APMs), which were obtained from synthetic Na tetrasilicic fluorine mica [NaMg2.5Si4O10F2], were prepared by sulfuric acidtreatment under mild conditions at 25 C. The products were investigated by XRD, ICP, SEM, TEM and N2 adsorption–desorption isotherm at 77 K. XRD measurements indicated that the interlayer pillared structure having a large basal spacing collapsed during the early stages of the acidtreatment. ICP analyses indicated that Al3+ and Mg2+ ions were leached out from the pillared micas during the acidtreatment. The pore properties of the leached products were found to differ from those of the mother pillared micas: the acid leaching of the pillared micas leads to the formation of mesopores around 3.2 nm in diameter. The correlation between the change in pore properties and cation elution behavior suggests that the mesopore formation results from the leaching of Mg2+ ions from the octahedral sheet of the pillared micas. The leached products thus obtained retained the flaky morphology of the mother pillared micas. These results show that the mild acidtreatment using APMs provides a novel route for obtaining unique mesopore solids having the large particle sizes of the mother micas.

Recent advances in corrosion protective composite coatings based on conducting polymers and natural resource derived polymers
0 تومانConducting polymer (CP) coatings have been extensively investigated for corrosion protection of iron, steel and other metals owing to their superior performance in highly aggressive environments and ecofriendly characteristics. Corrosion protective coatings based on CP nanocomposites have opened a new area of research for obtaining low cost coatings with enhanced performance and tailored properties. This mini review highlights the latest developments in the corrosion protective performance of CP composite coatings with natural resource derived polymers. The presence of nanoscale dispersion of CP as filler significantly improves the barrier properties and lifetime of the organic polymeric coatings. These lowcost nanocomposite coatings are expected to play an important role in combating corrosion which can lead to drastic improvement in corrosion protection.

The Effect of Addition of Carbon Fibers on Mechanical Properties of High Strength Concrete
0 تومانHigh Strength Concrete (HSC)is dense, homogeneous and has the improved engineering properties and durability as conventional concrete. In recent years, HSC has gained wide application in the construction industry. High strength Concrete is a concrete having similar ingredients as conventional concrete, such as cement, fine aggregate, coarse aggregate and water. The paste of HSC requires high volume of cement content and less water to binder ratio. The stability and flowability of HSC is achieved by increasing the cement content or employment of mineral admixtures. However, increasing the cement content causes high cost, higher heat of hydration and higher drying shrinkage. This can be reduced by employing mineral admixture such as fly ash and ground granulated blast furnace slag etc. In the present investigation, cement content for HSC mix is replaced with fixed percentages of fly ash (10%) and carbon fiber are added in volume fraction (0 to 0.60%), also the Carbon Fiber Reinforced Polymer (CFRP) strip are placed in different layer (single, double and triple layer) with varying width of CFRP strip (0 to 80 mm). The hardened concrete properties of HSC were studied and the regression analysis was carried out on the experimental investigation. The study concludes that carbon fibers can be effectively used as a reinforcing material in HSC.

Thermodynamic modeling of fcc order/disorder transformations in the Co–Pt system
0 تومانThe present work reports on a thermodynamic modeling of the Co–Pt system with ordered fcc phases of L10 and L12 structures by means of the CALPHAD method. The liquid, hcp and fcc phases have been modeled as substitutional solutions where the interaction parameters are composition dependent in the form of the Redlich–Kister polynomial. The disordered and ordered fcc phases have been modeled in terms of the compound energy formalism with a single Gibbs energy function. The obtained phase equilibria and activities of Co and Pt agree well with the available experimental data. Firstprinciples calculations are performed to obtain the enthalpies of formation for the ordered fcc phases at 0 K. These calculated enthalpies of formations for the ordered phases are less negative than the enthalpies of the disordered state at low temperatures determined from the CALPHAD modeling. The Fe–Pt and Ni–Pt systems exhibit the same feature as that in the Co–Pt system, which is discussed in terms of the total magnetic moment of ordered fcc phases.