Coupled twist–bending static and dynamic behavior of a curved single-walled carbon nanotube based on nonlocal theory

The static and dynamic behavior of a curved single-walled carbon nanotube which is under twist–bending couple based on nonlocal theory is analyzed. The nonlocal theory is used to model the mechanical behavior of structure in small scale. The obtained differential equations are solved using a simply supported boundary condition and Navier analytical method. Moreover the twisted vibration and bending of curved nanotube is analyzed and also the armchair model is assumed in this study. The following parameters were studied in this paper: the effect of nonlocal parameter, the curved nanotube’s opening angel, the Young’s modulus and the mode number is studied. The results were verified with the previous literature which showed an excellent agreement. The results of this paper can be used as a benchmark for future investigations.

     

Speciation and determination of thallium by on-line microcolumn separation/preconcentration by flow injection-flame atomic absorption spectrometry using immobilized oxine as sorbent

A flow injection analysis (FIA) system incorporation a microcolumn of immobilized oxine on surfactant-coated alumina had been devised for performing rapid thallium enrichment/matrix removal in flame atomic absorption spectrometry (FAA). The preconcentration is based on the deposition of thallium(I) on microcolumn and subsequent elution with 500 microl of sodium thiosulfate (1 moll(-1)). In the presence of EDTA, only Tl(I) was retained on the microcolumn. Total thallium was determined after reduction of Tl(III) to Tl(I) by hydroxyl amine hydrochloride. A sample volume of 25 ml resulted in a preconcentration factor of 77. Precision at 30 microgl(-1) was 2.6% RSD (n=10). With 25 ml sampling volume a detection limit of 2.5 microgl(-1) was determined. The effect of potential interfering ions on the determination was studied. The method was applied for the determination of thallium in water, waste water, hair, nail, coal, and standard reference alloys. The accuracy was assessed through recovery experiment, independent analysis by Furnace-AAS, and analysis of certified reference alloys.     

Performance of ANFIS Coupled with PSO in Manufacturing Superior Wear Resistant Aluminum Matrix Nano Composites

In this study, ANFIS was combined with PSO in order to optimize the parameters in pressure assisted semi solid processing of A360 aluminum matrix nano composites. ANFIS was utilized to calculate the objective function, which was later minimized using PSO. Combination of EMS semi solid processing and pressure assistance during solidification resulted in improvement of microstructural features and tribological properties. Globular grain structure was formed in the pressure assisted EMS parts. Tribological properties were investigated using pin on disk. It was noted that wear properties of EMS parts were benefited from the refinement of the primary α-Al phase and uniform distribution of the particles. EMS composites showed higher hardness than conventional cast parts, consequently there was a lower real area of contact and therefore lower wear rate. Moreover, hard dispersoid made the virgin alloy plastically constrained and improved their wear resistance.     

A comparative study on abrasive wear behavior of semisolid–liquid processed Al–Si matrix reinforced with coated B4C reinforcement

A comparative study on abrasive wear behavior of the sol?Cgel coated B₄C particulate reinforced aluminum metal matrix composite has been carried out in the present investigation. In general, composites offer superior wear resistance as compared to the alloy irrespective of applied load and B₄C particles volume fraction. This is primarily due to the presence of the hard dispersoid which protects the matrix from severe contact with the counter surfaces, and thus results in less wear, lower coefficient friction and temperature rise in composite as compared to that in the alloy. The wear sliding test disclosed that the weight loss of the coated B₄C reinforced composites decreases with increasing volume fraction of B₄C particulates. The wear rate in all the samples increases marginally with applied load prior to reaching the critical load. It is ascribed to the increase in fracture of reinforcement, the penetration of hard asperities of the counter surface into the softer pin surface and micro cracking tendency of the subsurface. After the critical load there is a transition from smooth linear increase wear rate to sudden increase in wear rate. This is attributed to the significantly higher frictional heating and thus the localized adhesion and softening of the surface with the counter surface.     

Mechanical Properties of Squeeze-Cast A356 Composites Reinforced With B4C Particulates

In this study, different volume fractions of B₄C particles were incorporated into the aluminum alloy by a mechanical stirrer, and squeeze-cast A356 matrix composites reinforced with B₄C particles were fabricated. Microstructural characterization revealed that the B₄C particles were distributed among the dendrite branches, leaving the dendrite branches as particle-free regions in the material. It also showed that the grain size of aluminum composite is smaller than that of monolithic aluminum. X-ray diffraction studies also confirmed the existence of boron carbide and some other reaction products such as AlB₂ and Al₃BC in the composite samples. It was observed that the amount of porosity increases with increasing volume fraction of composites. The porosity level increased, since the contact surface area was increased. Tensile behavior and the hardness values of the unreinforced alloy and composites were evaluated. The strain-hardening behavior and elongation to fracture of the composite materials appeared very different from those of the unreinforced Al alloy. It was noted that the elastic constant, strain-hardening and the ultimate tensile strength (UTS) of the MMCs are higher than those of the unreinforced Al alloy and increase with increasing B₄C content. The elongation to fracture of the composite materials was found very low, and no necking phenomenon was observed before fracture. The tensile fracture surface of the composite samples was indicative of particle cracking, interface debonding, and deformation constraint in the matrix and revealed the brittle mode of fracture.     

Influence of the hard coated B4C particulates on wear resistance of Al–Cu alloys

In the present study, sliding wear tests were carried out on different sizes and volume fractions of coated B₄C particles reinforced 2024 aluminum alloy composites fabricated by a squeeze casting method. Microstructural examination showed that the B₄C distributions were generally homogeneous in the matrix while some particle clusterings were observed at relatively high particle containing composites. As compared to the 2024 Al matrix alloy, the hardness of the composites was found to be greater. It is observed that the wear resistance of the composites was significantly higher than that of the unreinforced aluminum alloy, and increased with increasing B₄C particles content and size. The hard B₄C particles act as a protrusion over the matrix, carries a major portion of the applied load and protect the abrasives from penetration into the specimen surface. Combination of rough and smooth regions is distinguished on the worn surface of the composites. The depth and number of grooves in composites decreased with increasing volume fraction of B₄C particles, and the worn surfaces of composites were relatively smooth.     

Evaluation of Metal Dusting Behavior in Reformer Tube Made of HP–Nb Alloy on the Outer Surfaces

The metal dusting behavior of Iron–Chromium–Nickel heat resistant HP–Nb steel specimen was investigated at the outer surfaces while the methane gas was passed inside the hole of the specimen. After an exposure of 130 h in a flowing methane (CH₄) gas at 680 °C, different dispersed corrosion products were formed on the outer surface of the specimen near the hole. Conventional metallography and scanning electron microscopy were used to identify the microstructure of the reaction products. Energy dispersive X-ray spectroscopy was used for microchemical analysis. The phases produced on the surface were identified by X-ray diffraction. Some of reaction products found as surface deposits on the outer surfaces of specimen near the hole contained Fe and Cr carbides, Fe, Cr and Ni oxides, scale of Ni, Fe particles and free C. Results revealed that carbon nano-filaments materials could be formed during disintegration of heat resistant HP–Nb steel under metal dusting environment.     

A Proposed Thermodynamic Model for Gas Hydrate Equilibrium in Electrolyte Solutions

In this work, the thermodynamic stability of gas hydrates is investigated in the presence of electrolyte solutions. The proposed model is based on the Van der Waals–Platteeuw model for gas hydrate equilibrium, and the Pitzer and Mayorga model is employed to calculate the water activity in the electrolyte solutions. Available values for the Pitzer model parameters are usually adjusted using experimental data at 25°C, which is usually higher than the gas hydrate formation temperature. In order to eliminate this problem, those adjustable parameters are re-optimized using experimental data from the literature at the lowest temperature. In the case of mixed electrolyte solutions and without using any adjustable parameters, a mixing rule is proposed to estimate the water activity. The new mixing rule is based on the ionic strength of the mixture and estimates the mixture water activity by using properties of the single electrolytes that constitute the mixture. The results show the proposed model can calculate hydrate equilibrium conditions with good accuracy, especially at low concentrations, which is the case for most industrial applications.     

Tribological behaviour of semisolid–semisolid compocast Al–Si matrix composites reinforced with TiB2 coated B4C particulates

The purpose of this paper is to provide a deeper understanding of the wear behavior of the sol–gel coated B₄C particulate reinforced A356 matrix composites. A typical microstructure of the composite consists of relatively large primary phase globules which are surrounded by B₄C particles. In fact the globules themselves are B₄C particles free and consequently the sample is not homogeneous on a scale smaller than the globule size. The wear sliding test disclosed that the wear rate of the coated B₄C reinforced composites is less than that of the unreinforced alloy and decreases with increasing volume fraction of B₄C particulates. As the hardness of the composites is higher, this reduces the cutting efficiency of the abrasives and consequently the abrasion wear loss. Once the particles fracture or loosen from the matrix alloy, they can be removed easily from the matrix, contributing to the material loss. Two kinds of debris present irregular-shaped flake, which has withstood a large of plastic deformation and then pull off from the surface. During the sliding wear, Iron is transferred to the surface of the composites from the steel counterface forming the iron-rich layer on the contact surfaces which increases with increasing the B₄C content and is substantially harder than the bulk material largely because it contains a fine mixture of Fe phase, Al and B₄C.