In this study the formation processes of catalyst nanoparticles and single walled carbon nanotube... more In this study the formation processes of catalyst nanoparticles and single walled carbon nanotubes (SWCNTs) in a premixed flame doped with Fe(CO) 5 were first modelled using a three-step SWCNT growth model including a detailed surface chemistry model. The growth of SWCNTs was experimentally studied by the length measurement of the SWCNT using Raman radial breathing mode (RBM) and size measurements of the iron oxide catalyst particles using XRD and TEM. The flame chemistry and the formation of the catalyst particles were modelled in detail by means of a sectional model. In a post-processing step the SWCNT population balance growth model was numerically solved using a multivariate stochastic population balance solver. The model was able to capture the growth characteristics and revealed the role of the monolayer. The computational study on the adsorption, dissociation, and reactions of CO, H 2 and H 2 O on iron nanoparticles showed that carbon, hydrogen and oxygen atoms form at the surface of the catalyst. Their ratio, which is controlled by the surface reaction pathways, affects the growth of SWCNTs, the formation of monolayers and the phase transformation of catalyst particles.
In this paper, we conducted numerical simulation of the electroosmotic flow in a column of an aqu... more In this paper, we conducted numerical simulation of the electroosmotic flow in a column of an aqueous solution surrounded by an immiscible liquid. While governing equations in this case are the same as that in the electroosmotic flow through a microchannel with solid walls, the main difference is the types of interfacial boundary conditions. The effects of electric double layer (EDL) and surface charge (SC) are considered to apply the most realistic model for the velocity boundary condition at the interface of the two fluids. Effects on the flow field of 1-potential and viscosity ratio of the two fluids were investigated. Similar to the electroosmotic flow in microchannels, an approximately flat velocity profile exists in the aqueous solution. In the immiscible fluid phase, the velocity decreases to zero from the interface toward the immiscible fluid phase. The velocity in both phases increases with 1-potential at the interface of the two fluids. The higher values of 1-potential also increase the slip velocity at the interface of the two fluids. For the same applied electric field and the same 1-potential at the interface of the two fluids, the more viscous immiscible fluid, the slower the system moves. The viscosity of the immiscible fluid phase also affects the flatness of the velocity profile in the aqueous solution.
Since its invention in 1991, premixed combustion synthesis of fullerenic materials has been estab... more Since its invention in 1991, premixed combustion synthesis of fullerenic materials has been established as the major industrial process for manufacturing of these materials. Large-scale production of fullerenes such as C60, C70 and C84 has been implemented. More recently, combustion technology has been extended to the targeted synthesis of single-walled carbon nanotubes (SWCNT). Addition of catalyst precursor and operation at well-controlled fuel-rich but non-sooting conditions are required. Extensive parametric studies have allowed for the optimization of the formation of high-quality SWCNT. Purification techniques previously reported in the literature have been adjusted and used successfully for the nearly complete removal of metal and metal oxide. Material has been characterized using Raman spectroscopy, scanning (SEM) and transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Correlations between process conditions and nanotube properties such as length have been established. Product reproducibility and process scalability of the combustion process have been demonstrated. Sample preparation was found to affect significantly the apparent characteristics of nanotubes as seen in electron microscopy images.
A detailed continuum model is presented for predicting the growth characteristics of GaAs nanowir... more A detailed continuum model is presented for predicting the growth characteristics of GaAs nanowires during chemical beam epitaxy. The model describes the transport processes of Ga and As adatoms on the substrate and nanowire sidewalls, and through the nanoparticle and the nanowire-catalyst interface (NCI). The growth mechanisms of nanowires within the NCI are described using an extended step-flow kinetic model. The vapor-liquid-solid and vapor-solid-solid growth mechanisms are both described in the kinetic model. The growth rate of the nanowires, the surface and bulk concentrations of adatoms, and the role of transport processes of Ga and As adatoms during chemical beam epitaxy were investigated. The growth mechanisms of the nanowires were found to vary with increasing length of the nanowire.
ABSTRACT This paper assesses the Presumed Mapping Function (PMF) approach in the context of the S... more ABSTRACT This paper assesses the Presumed Mapping Function (PMF) approach in the context of the Stationary Laminar Flamelet Modelling (SLFM) of a reacting Double Scalar Mixing Layer (DSML). Starting from a prescribed Gaussian reference field, the PMF approach provides a presumed Probability Density Function (PDF) for the mixture fraction that is subsequently employed to close the Conditional Scalar Dissipation Rate (CSDR) upon doubly-integrating the homogeneous PDF transport equation. The PMF approach is unique in its ability to yield PDF and CSDR distributions that capture the effect of multiple fuel injections of different composition. This distinct feature overcomes the shortcomings of the classical SLFM closures (the $\beta$-distribution for the PDF and the counterflow solution for the CSDR). The current study analyses the impact of the binary (two-stream) and trinary (three-stream) PMF approaches on the structure of laminar flamelets in a DSML formed by the mixing of a fuel stream and an oxidiser stream separated by a pilot. The conditions of a partially-premixed methane/air piloted jet flame are considered. A parametric assessment is performed by varying the local mixing statistics and the findings are compared to those of the classical SLFM approach. Further, the influence of the PMF approach on flamelet extinction and transport by means of differential diffusion are thoroughly investigated. It is shown that the trinary PMF approach captures the influence of the pilot stream as it is capable of yielding bimodal CSDR and trimodal PDF distributions. It is further demonstrated that when the influence of the pilot is significant flamelets generated using the trinary CSDR closure extinguish at higher strain levels compared to flamelets generated using the binary and counterflow closures. Lastly, it is shown that the trinary PMF approach can be critical for the accurate SLFM computations of DSMLs when differential diffusion effects are important.
â–º Multizone model validated with simulated biogas fuel. â–º Model was parallelized using Fortra... more â–º Multizone model validated with simulated biogas fuel. â–º Model was parallelized using Fortran MPI. â–º Good pressure, heat release rate, and products of incomplete combustion prediction. â–º Poor NOx prediction and poor pressure trace behavior near misfire limit.
ABSTRACT In this study a flush wall scramjet combustor is tested in a supersonic incoming air flo... more ABSTRACT In this study a flush wall scramjet combustor is tested in a supersonic incoming air flow with the Mach number of 3 which is generated by an air vitiation heater producing the stagnation temperature of 1505 K. Using liquid kerosene as the fuel, the flame is stabilized by means of a centrally mounted O2 pilot strut after being ignited by a plasma torch. During experimental measurements, the fuel is injected with a constant equivalence ratio of 0.8 according to specified strut/wall injection ratios, i.e., a portion of the fuel amount is injected from the strut while the rest is injected from the wall. The strut and wall injectors are arranged at the same axial position. The combustion performance and wall temperature gradients are evaluated with various fuel feeding ratios between the wall and the strut. Experimental results show, when the equivalence ratio is constant and the axial injection position is fixed, the combustion characteristics vary significantly with the strut/wall fuel feeding ratio, especially when this ratio is close to its lowest and highest limits. Among the four fuel feeding ratios examined, the strut only injection mode and the average distributed strut/wall injection mode show the best combustion performance. However, the strut/wall injection mode produces a smaller wall temperature gradient compared to the strut only injection mode, which is due to the significant film cooling effect caused by the wall injected liquid kerosene.
ABSTRACT The bimetallic nanothermites, composed often of the core-shell nanoparticles or the nano... more ABSTRACT The bimetallic nanothermites, composed often of the core-shell nanoparticles or the nano-scale multilayers, provide a direct interface between two reactive metals and therefore facilitate the thermite reaction without generating excessive gases. This characteristic is extremely important for wire or material joining applications. In this study we investigated the energetic properties and diffusion characteristics of bimetallic atoms in the Al/Pd core-shell nanoparticles. A classic molecular dynamics approach was implemented using a recently reported force field for the Al/Pd binary system. The results suggested that, in order to preserve the core-shell structure of the Al/Pd nanoparticle before being utilized for the material joining purpose, it is essential to maintain an isothermal condition at a lower temperature such as 150K. In addition, a thicker Pd shell will further hinder the diffusion processes within the core-shell nanoparticle.
Pseudopotential lattice Boltzmann (LB) models have been recognized as efficient numerical tools t... more Pseudopotential lattice Boltzmann (LB) models have been recognized as efficient numerical tools to simulate complex fluid systems, including those at thermodynamic equilibrium states and with phase transitions. However, when the equation of state (EOS) of real fluids is implemented, the existing pseudopotential LB models suffer from thermodynamic inconsistency. This study presents a multipseudopotential interparticle interaction (MPI) scheme, which is fully consistent with thermodynamics and applicable to engineering applications. In this framework, multiple pseudopotentials are employed to represent dominant interaction potentials at different extents of the mean free path of particles. By simulating van der Waals and Carnahan-Starling fluids, it is demonstrated that the MPI scheme can correctly simulate the physical nature of two-phase systems on the lattice including the continuum predictions of liquid-vapor coexistence states and the sound speeds in liquid and vapor phases. It is also shown that the lattice interactions of the MPI scheme represent underlying molecular interactions as they vary in a broad range from strong short-distance repulsions to weak long-distance attractions during phase transitions. Consequently, the MPI is proved to be a reliable LB scheme as it avoids generating unphysical potentials in implementing the EOSs of real fluids and limiting the spurious velocities at the interface of two-phase systems. Additionally, a straightforward procedure is suggested and discussed to preset the MPI system with the two-phase properties of a selected fluid.
ABSTRACT The temperature dependent electrochemical properties of electrochemical double layer cap... more ABSTRACT The temperature dependent electrochemical properties of electrochemical double layer capacitors (EDLCs) assembled with two representative electrolytes, aqueous (H2SO4) and organic (Et4NBF4/PC), were investigated using a specified characterization procedure and interpreted by means of a recently developed equivalent circuit model. Over an operating temperature range between −30 °C and 60 °C, these parameters were characterized using the cyclic voltammetry (CV), galvanostatic charging/discharging measurements and electrochemical impedance spectroscopy (EIS). Based on experimental results, the correlations between internal resistance/capacitance and temperature were quantified. Moreover, the temperature dependency of impedances was analyzed through simulations of the EIS data by the equivalent circuit model which takes into account the important roles of the electrolyte properties, diffusion and interfacial processes within EDLCs. The simulated resistances were then utilized to discuss about the energy barriers of kinetic reactions and diffusion and to address the performance discrepancies of the two electrolytes.
The Phase Doppler Particle Analyzer (PDPA) technique is utilized for characterizing the droplet s... more The Phase Doppler Particle Analyzer (PDPA) technique is utilized for characterizing the droplet sizes and velocity profiles in an electrostatic spray flow which is atomized using the aerodynamic forces assisted with electrostatic forces. An electrostaticallyassisted twin-fluid atomization system is presented and spatial distributions of the droplet size and two-dimensional velocity components are measured simultaneously at each specified point. The droplets are firstly ejected from a liquid jet through the nozzle by aerodynamic shear forces, and the breakup mechanism of such an electrostatically-assisted twin-fluid atomization is investigated using the Rayleigh instability conditions through studying the specific surface charges on these droplets measured by a Faraday canister. The discrepancies between experiment data and Rayleigh limit for specific charges of droplets are presented and discussed due to energy transforming from electrical energy to surface energy, energy loss, Taylor instability breakup and nonexcess charge polarization. The measured average velocity profiles and their RMS (root mean square) velocity values for two different scale drops further reveal the effects of the aerodynamic and electrostatic forces on the atomization process.
Thermo-chemical properties of the Al nanoparticle and NiO nanowire composites modified by the mic... more Thermo-chemical properties of the Al nanoparticle and NiO nanowire composites modified by the microsized copper additive were investigated experimentally. Their onset temperatures of ignition and energy release data per mass were characterized using differential thermal analysis measurements. These microstructures and chemical compositions of reaction products were analyzed using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The fuel-rich Al/NiO/Cu composites produced two types of metallic spheres. Copper spheres were formed from melting and solidification of the copper additive, while AlNi composite spheres were identified by the energy dispersive X-ray spectroscopy and X-ray diffraction analyses. It was found that the amount of the copper additive did not significantly influence the onset temperature of thermite peaks, but caused a dramatic change in energy release. The aforementioned ignition and energetic properties were compared with these from the Al nanoparticle and CuO nanowire composites.
Combustion in a plug flow reactor (PFR) is similar to that occurring in the postflame region of l... more Combustion in a plug flow reactor (PFR) is similar to that occurring in the postflame region of laminar flames, and hence provides a well-defined environment to study those soot formation processes at a relatively larger residence time, which are identified as simultaneously occurring surface growth (typically by HACA mechanism) and PAH surface condensation together with the particle coagulation. The roles of HACA mechanism and PAH condensation are of great interest in recent research. A moving sectional aerosol dynamics model is implemented in this study for prediction of soot formation in a PFR. Coupled with the detailed gas phase chemistry, the soot model provides reasonable predictions of the molar concentrations of major species, PAHs and radicals, soot mass concentration, soot mean particle diameter, and the particle size distribution. The predicted size distribution shows the evolution of its shape from the power law to a bi-modal distribution. The study of the soot growth rate shows that both HACA surface growth and PAH surface condensation are important in the modeling of soot formation in the PFR. All available PAH radical species are included in surface condensation, which results in good prediction of mass growth rate without excessive depletion of the PAH species. However, the use of a single incipient species, pyrene, results in its concentration being underpredicted. This new soot model provides a solid approach to investigate further the soot formation in combustion with simultaneous particle nucleation, surface growth, PAH condensation, and coagulation.
Thermochemical properties and microstructures of the composite of Al nanoparticles and NiO nanowi... more Thermochemical properties and microstructures of the composite of Al nanoparticles and NiO nanowires were characterized. The nanowires were synthesized using a hydrothermal method and were mixed with these nanoparticles by sonication. Electron microscopic images of these composites showed dispersed NiO nanowires decorated with Al nanoparticles. Thermal analysis suggests the influence of NiO mass ratio was insignificant with regard to the onset temperature of the observed thermite reaction, although energy release values changed dramatically with varying NiO ratios. Reaction products from the fuel-rich composites were found to include elemental Al and Ni, Al 2 O 3 , and AlNi. The production of the AlNi phase, confirmed by an ab initio molecular dynamics simulation, was associated with the formation of some metallic liquid spheres from the thermite reaction.
Mineral Processing and Extractive Metallurgy Review, 2013
An improved method for the modeling of the impurity reduction factor during high temperature low ... more An improved method for the modeling of the impurity reduction factor during high temperature low pressure silicon treatment is suggested. The implementation of a modified logistic function in a mathematical model allows it to be utilized for any range of treatment temperatures. The new model defines the maximum reduction factor for each impurity element present in liquid silicon and defines
ABSTRACT Single-walled carbon nanotube (SWCNT) coated electrodes are fabricated from functionaliz... more ABSTRACT Single-walled carbon nanotube (SWCNT) coated electrodes are fabricated from functionalized suspensions with different pH values. Their electrochemical properties are characterized using cyclic voltammetry and electrochemical impedance spectroscopy. The effects of the pH value of the suspension on electrochemical performances are investigated through examining the microstructures of the SWCNT films and their BET specific areas and pore volume and size distributions. Both these specific areas and pore size distributions are found being important in determining the specific capacitances of SWCNT coated electrodes. An equivalent circuit model is developed for these electrodes using the measured electrochemical impedance data. It is found that the functionalized nanotube suspensions with higher pH values produce more desirable nanostructured electrodes with the increased specific capacitance and reduced electrode resistance. The role of the pseudo-capacitance which arises from the functionalization group is also discussed.
Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanop... more Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanoparticles passivated by nitrocellulose and 12 nm copper (II) oxide, were investigated with microstructure characterization, differential thermal analysis (DTA), and thermogravimetric analysis (TGA). Transmission electron microscopy observation confirmed that the passivation coating successfully hinders the oxidization. TGA revealed that the passivation shell does not influence the ignition temperature of the thermite reaction. Reaction chemistry of the nano-thermite was elucidated by heating the composite both in inert ambient and vacuum. It was found that the thermite reaction composes of three continuing steps: At 570 1C, Al is oxidized into Al 2 O 3 by reacting with CuO, which forms Cu 2 O and produces a significant amount of heat. Subsequently two endothermic reactions occur. Starting at 800 1C, alumina reacts with Cu 2 O and forms CuAlO 2 . Above this temperature CuAlO 2 will decompose and eventually produce alumina, Cu, and O 2 at 1000 1C. Since the nano-thermite reaction pathway differs greatly from bulk thermite reactions, these results are important to develop a nanothermite platform that can be used for a novel low cost, low temperature, and copper based microjoining and advance IC packaging.
The sintering process of iron nanoparticles produced in a flame environment is investigated by mo... more The sintering process of iron nanoparticles produced in a flame environment is investigated by molecular dynamic (MD) simulations. The thermodynamic characteristics and restructuring pathways are studied for two-body and three-body sintering processes. The melting point, energy, and structures are computed for nanoparticles before and after sintering. A simplified model is proposed to predict the equilibrium temperature of nanoparticles upon sintering. The MD results are used to explain the formation mechanisms of two size ranges of nanoparticles during the flame synthesis. The role of sintering during nanoparticle growth is analyzed.
In this study the formation processes of catalyst nanoparticles and single walled carbon nanotube... more In this study the formation processes of catalyst nanoparticles and single walled carbon nanotubes (SWCNTs) in a premixed flame doped with Fe(CO) 5 were first modelled using a three-step SWCNT growth model including a detailed surface chemistry model. The growth of SWCNTs was experimentally studied by the length measurement of the SWCNT using Raman radial breathing mode (RBM) and size measurements of the iron oxide catalyst particles using XRD and TEM. The flame chemistry and the formation of the catalyst particles were modelled in detail by means of a sectional model. In a post-processing step the SWCNT population balance growth model was numerically solved using a multivariate stochastic population balance solver. The model was able to capture the growth characteristics and revealed the role of the monolayer. The computational study on the adsorption, dissociation, and reactions of CO, H 2 and H 2 O on iron nanoparticles showed that carbon, hydrogen and oxygen atoms form at the surface of the catalyst. Their ratio, which is controlled by the surface reaction pathways, affects the growth of SWCNTs, the formation of monolayers and the phase transformation of catalyst particles.
In this paper, we conducted numerical simulation of the electroosmotic flow in a column of an aqu... more In this paper, we conducted numerical simulation of the electroosmotic flow in a column of an aqueous solution surrounded by an immiscible liquid. While governing equations in this case are the same as that in the electroosmotic flow through a microchannel with solid walls, the main difference is the types of interfacial boundary conditions. The effects of electric double layer (EDL) and surface charge (SC) are considered to apply the most realistic model for the velocity boundary condition at the interface of the two fluids. Effects on the flow field of 1-potential and viscosity ratio of the two fluids were investigated. Similar to the electroosmotic flow in microchannels, an approximately flat velocity profile exists in the aqueous solution. In the immiscible fluid phase, the velocity decreases to zero from the interface toward the immiscible fluid phase. The velocity in both phases increases with 1-potential at the interface of the two fluids. The higher values of 1-potential also increase the slip velocity at the interface of the two fluids. For the same applied electric field and the same 1-potential at the interface of the two fluids, the more viscous immiscible fluid, the slower the system moves. The viscosity of the immiscible fluid phase also affects the flatness of the velocity profile in the aqueous solution.
Since its invention in 1991, premixed combustion synthesis of fullerenic materials has been estab... more Since its invention in 1991, premixed combustion synthesis of fullerenic materials has been established as the major industrial process for manufacturing of these materials. Large-scale production of fullerenes such as C60, C70 and C84 has been implemented. More recently, combustion technology has been extended to the targeted synthesis of single-walled carbon nanotubes (SWCNT). Addition of catalyst precursor and operation at well-controlled fuel-rich but non-sooting conditions are required. Extensive parametric studies have allowed for the optimization of the formation of high-quality SWCNT. Purification techniques previously reported in the literature have been adjusted and used successfully for the nearly complete removal of metal and metal oxide. Material has been characterized using Raman spectroscopy, scanning (SEM) and transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Correlations between process conditions and nanotube properties such as length have been established. Product reproducibility and process scalability of the combustion process have been demonstrated. Sample preparation was found to affect significantly the apparent characteristics of nanotubes as seen in electron microscopy images.
A detailed continuum model is presented for predicting the growth characteristics of GaAs nanowir... more A detailed continuum model is presented for predicting the growth characteristics of GaAs nanowires during chemical beam epitaxy. The model describes the transport processes of Ga and As adatoms on the substrate and nanowire sidewalls, and through the nanoparticle and the nanowire-catalyst interface (NCI). The growth mechanisms of nanowires within the NCI are described using an extended step-flow kinetic model. The vapor-liquid-solid and vapor-solid-solid growth mechanisms are both described in the kinetic model. The growth rate of the nanowires, the surface and bulk concentrations of adatoms, and the role of transport processes of Ga and As adatoms during chemical beam epitaxy were investigated. The growth mechanisms of the nanowires were found to vary with increasing length of the nanowire.
ABSTRACT This paper assesses the Presumed Mapping Function (PMF) approach in the context of the S... more ABSTRACT This paper assesses the Presumed Mapping Function (PMF) approach in the context of the Stationary Laminar Flamelet Modelling (SLFM) of a reacting Double Scalar Mixing Layer (DSML). Starting from a prescribed Gaussian reference field, the PMF approach provides a presumed Probability Density Function (PDF) for the mixture fraction that is subsequently employed to close the Conditional Scalar Dissipation Rate (CSDR) upon doubly-integrating the homogeneous PDF transport equation. The PMF approach is unique in its ability to yield PDF and CSDR distributions that capture the effect of multiple fuel injections of different composition. This distinct feature overcomes the shortcomings of the classical SLFM closures (the $\beta$-distribution for the PDF and the counterflow solution for the CSDR). The current study analyses the impact of the binary (two-stream) and trinary (three-stream) PMF approaches on the structure of laminar flamelets in a DSML formed by the mixing of a fuel stream and an oxidiser stream separated by a pilot. The conditions of a partially-premixed methane/air piloted jet flame are considered. A parametric assessment is performed by varying the local mixing statistics and the findings are compared to those of the classical SLFM approach. Further, the influence of the PMF approach on flamelet extinction and transport by means of differential diffusion are thoroughly investigated. It is shown that the trinary PMF approach captures the influence of the pilot stream as it is capable of yielding bimodal CSDR and trimodal PDF distributions. It is further demonstrated that when the influence of the pilot is significant flamelets generated using the trinary CSDR closure extinguish at higher strain levels compared to flamelets generated using the binary and counterflow closures. Lastly, it is shown that the trinary PMF approach can be critical for the accurate SLFM computations of DSMLs when differential diffusion effects are important.
â–º Multizone model validated with simulated biogas fuel. â–º Model was parallelized using Fortra... more â–º Multizone model validated with simulated biogas fuel. â–º Model was parallelized using Fortran MPI. â–º Good pressure, heat release rate, and products of incomplete combustion prediction. â–º Poor NOx prediction and poor pressure trace behavior near misfire limit.
ABSTRACT In this study a flush wall scramjet combustor is tested in a supersonic incoming air flo... more ABSTRACT In this study a flush wall scramjet combustor is tested in a supersonic incoming air flow with the Mach number of 3 which is generated by an air vitiation heater producing the stagnation temperature of 1505 K. Using liquid kerosene as the fuel, the flame is stabilized by means of a centrally mounted O2 pilot strut after being ignited by a plasma torch. During experimental measurements, the fuel is injected with a constant equivalence ratio of 0.8 according to specified strut/wall injection ratios, i.e., a portion of the fuel amount is injected from the strut while the rest is injected from the wall. The strut and wall injectors are arranged at the same axial position. The combustion performance and wall temperature gradients are evaluated with various fuel feeding ratios between the wall and the strut. Experimental results show, when the equivalence ratio is constant and the axial injection position is fixed, the combustion characteristics vary significantly with the strut/wall fuel feeding ratio, especially when this ratio is close to its lowest and highest limits. Among the four fuel feeding ratios examined, the strut only injection mode and the average distributed strut/wall injection mode show the best combustion performance. However, the strut/wall injection mode produces a smaller wall temperature gradient compared to the strut only injection mode, which is due to the significant film cooling effect caused by the wall injected liquid kerosene.
ABSTRACT The bimetallic nanothermites, composed often of the core-shell nanoparticles or the nano... more ABSTRACT The bimetallic nanothermites, composed often of the core-shell nanoparticles or the nano-scale multilayers, provide a direct interface between two reactive metals and therefore facilitate the thermite reaction without generating excessive gases. This characteristic is extremely important for wire or material joining applications. In this study we investigated the energetic properties and diffusion characteristics of bimetallic atoms in the Al/Pd core-shell nanoparticles. A classic molecular dynamics approach was implemented using a recently reported force field for the Al/Pd binary system. The results suggested that, in order to preserve the core-shell structure of the Al/Pd nanoparticle before being utilized for the material joining purpose, it is essential to maintain an isothermal condition at a lower temperature such as 150K. In addition, a thicker Pd shell will further hinder the diffusion processes within the core-shell nanoparticle.
Pseudopotential lattice Boltzmann (LB) models have been recognized as efficient numerical tools t... more Pseudopotential lattice Boltzmann (LB) models have been recognized as efficient numerical tools to simulate complex fluid systems, including those at thermodynamic equilibrium states and with phase transitions. However, when the equation of state (EOS) of real fluids is implemented, the existing pseudopotential LB models suffer from thermodynamic inconsistency. This study presents a multipseudopotential interparticle interaction (MPI) scheme, which is fully consistent with thermodynamics and applicable to engineering applications. In this framework, multiple pseudopotentials are employed to represent dominant interaction potentials at different extents of the mean free path of particles. By simulating van der Waals and Carnahan-Starling fluids, it is demonstrated that the MPI scheme can correctly simulate the physical nature of two-phase systems on the lattice including the continuum predictions of liquid-vapor coexistence states and the sound speeds in liquid and vapor phases. It is also shown that the lattice interactions of the MPI scheme represent underlying molecular interactions as they vary in a broad range from strong short-distance repulsions to weak long-distance attractions during phase transitions. Consequently, the MPI is proved to be a reliable LB scheme as it avoids generating unphysical potentials in implementing the EOSs of real fluids and limiting the spurious velocities at the interface of two-phase systems. Additionally, a straightforward procedure is suggested and discussed to preset the MPI system with the two-phase properties of a selected fluid.
ABSTRACT The temperature dependent electrochemical properties of electrochemical double layer cap... more ABSTRACT The temperature dependent electrochemical properties of electrochemical double layer capacitors (EDLCs) assembled with two representative electrolytes, aqueous (H2SO4) and organic (Et4NBF4/PC), were investigated using a specified characterization procedure and interpreted by means of a recently developed equivalent circuit model. Over an operating temperature range between −30 °C and 60 °C, these parameters were characterized using the cyclic voltammetry (CV), galvanostatic charging/discharging measurements and electrochemical impedance spectroscopy (EIS). Based on experimental results, the correlations between internal resistance/capacitance and temperature were quantified. Moreover, the temperature dependency of impedances was analyzed through simulations of the EIS data by the equivalent circuit model which takes into account the important roles of the electrolyte properties, diffusion and interfacial processes within EDLCs. The simulated resistances were then utilized to discuss about the energy barriers of kinetic reactions and diffusion and to address the performance discrepancies of the two electrolytes.
The Phase Doppler Particle Analyzer (PDPA) technique is utilized for characterizing the droplet s... more The Phase Doppler Particle Analyzer (PDPA) technique is utilized for characterizing the droplet sizes and velocity profiles in an electrostatic spray flow which is atomized using the aerodynamic forces assisted with electrostatic forces. An electrostaticallyassisted twin-fluid atomization system is presented and spatial distributions of the droplet size and two-dimensional velocity components are measured simultaneously at each specified point. The droplets are firstly ejected from a liquid jet through the nozzle by aerodynamic shear forces, and the breakup mechanism of such an electrostatically-assisted twin-fluid atomization is investigated using the Rayleigh instability conditions through studying the specific surface charges on these droplets measured by a Faraday canister. The discrepancies between experiment data and Rayleigh limit for specific charges of droplets are presented and discussed due to energy transforming from electrical energy to surface energy, energy loss, Taylor instability breakup and nonexcess charge polarization. The measured average velocity profiles and their RMS (root mean square) velocity values for two different scale drops further reveal the effects of the aerodynamic and electrostatic forces on the atomization process.
Thermo-chemical properties of the Al nanoparticle and NiO nanowire composites modified by the mic... more Thermo-chemical properties of the Al nanoparticle and NiO nanowire composites modified by the microsized copper additive were investigated experimentally. Their onset temperatures of ignition and energy release data per mass were characterized using differential thermal analysis measurements. These microstructures and chemical compositions of reaction products were analyzed using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The fuel-rich Al/NiO/Cu composites produced two types of metallic spheres. Copper spheres were formed from melting and solidification of the copper additive, while AlNi composite spheres were identified by the energy dispersive X-ray spectroscopy and X-ray diffraction analyses. It was found that the amount of the copper additive did not significantly influence the onset temperature of thermite peaks, but caused a dramatic change in energy release. The aforementioned ignition and energetic properties were compared with these from the Al nanoparticle and CuO nanowire composites.
Combustion in a plug flow reactor (PFR) is similar to that occurring in the postflame region of l... more Combustion in a plug flow reactor (PFR) is similar to that occurring in the postflame region of laminar flames, and hence provides a well-defined environment to study those soot formation processes at a relatively larger residence time, which are identified as simultaneously occurring surface growth (typically by HACA mechanism) and PAH surface condensation together with the particle coagulation. The roles of HACA mechanism and PAH condensation are of great interest in recent research. A moving sectional aerosol dynamics model is implemented in this study for prediction of soot formation in a PFR. Coupled with the detailed gas phase chemistry, the soot model provides reasonable predictions of the molar concentrations of major species, PAHs and radicals, soot mass concentration, soot mean particle diameter, and the particle size distribution. The predicted size distribution shows the evolution of its shape from the power law to a bi-modal distribution. The study of the soot growth rate shows that both HACA surface growth and PAH surface condensation are important in the modeling of soot formation in the PFR. All available PAH radical species are included in surface condensation, which results in good prediction of mass growth rate without excessive depletion of the PAH species. However, the use of a single incipient species, pyrene, results in its concentration being underpredicted. This new soot model provides a solid approach to investigate further the soot formation in combustion with simultaneous particle nucleation, surface growth, PAH condensation, and coagulation.
Thermochemical properties and microstructures of the composite of Al nanoparticles and NiO nanowi... more Thermochemical properties and microstructures of the composite of Al nanoparticles and NiO nanowires were characterized. The nanowires were synthesized using a hydrothermal method and were mixed with these nanoparticles by sonication. Electron microscopic images of these composites showed dispersed NiO nanowires decorated with Al nanoparticles. Thermal analysis suggests the influence of NiO mass ratio was insignificant with regard to the onset temperature of the observed thermite reaction, although energy release values changed dramatically with varying NiO ratios. Reaction products from the fuel-rich composites were found to include elemental Al and Ni, Al 2 O 3 , and AlNi. The production of the AlNi phase, confirmed by an ab initio molecular dynamics simulation, was associated with the formation of some metallic liquid spheres from the thermite reaction.
Mineral Processing and Extractive Metallurgy Review, 2013
An improved method for the modeling of the impurity reduction factor during high temperature low ... more An improved method for the modeling of the impurity reduction factor during high temperature low pressure silicon treatment is suggested. The implementation of a modified logistic function in a mathematical model allows it to be utilized for any range of treatment temperatures. The new model defines the maximum reduction factor for each impurity element present in liquid silicon and defines
ABSTRACT Single-walled carbon nanotube (SWCNT) coated electrodes are fabricated from functionaliz... more ABSTRACT Single-walled carbon nanotube (SWCNT) coated electrodes are fabricated from functionalized suspensions with different pH values. Their electrochemical properties are characterized using cyclic voltammetry and electrochemical impedance spectroscopy. The effects of the pH value of the suspension on electrochemical performances are investigated through examining the microstructures of the SWCNT films and their BET specific areas and pore volume and size distributions. Both these specific areas and pore size distributions are found being important in determining the specific capacitances of SWCNT coated electrodes. An equivalent circuit model is developed for these electrodes using the measured electrochemical impedance data. It is found that the functionalized nanotube suspensions with higher pH values produce more desirable nanostructured electrodes with the increased specific capacitance and reduced electrode resistance. The role of the pseudo-capacitance which arises from the functionalization group is also discussed.
Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanop... more Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanoparticles passivated by nitrocellulose and 12 nm copper (II) oxide, were investigated with microstructure characterization, differential thermal analysis (DTA), and thermogravimetric analysis (TGA). Transmission electron microscopy observation confirmed that the passivation coating successfully hinders the oxidization. TGA revealed that the passivation shell does not influence the ignition temperature of the thermite reaction. Reaction chemistry of the nano-thermite was elucidated by heating the composite both in inert ambient and vacuum. It was found that the thermite reaction composes of three continuing steps: At 570 1C, Al is oxidized into Al 2 O 3 by reacting with CuO, which forms Cu 2 O and produces a significant amount of heat. Subsequently two endothermic reactions occur. Starting at 800 1C, alumina reacts with Cu 2 O and forms CuAlO 2 . Above this temperature CuAlO 2 will decompose and eventually produce alumina, Cu, and O 2 at 1000 1C. Since the nano-thermite reaction pathway differs greatly from bulk thermite reactions, these results are important to develop a nanothermite platform that can be used for a novel low cost, low temperature, and copper based microjoining and advance IC packaging.
The sintering process of iron nanoparticles produced in a flame environment is investigated by mo... more The sintering process of iron nanoparticles produced in a flame environment is investigated by molecular dynamic (MD) simulations. The thermodynamic characteristics and restructuring pathways are studied for two-body and three-body sintering processes. The melting point, energy, and structures are computed for nanoparticles before and after sintering. A simplified model is proposed to predict the equilibrium temperature of nanoparticles upon sintering. The MD results are used to explain the formation mechanisms of two size ranges of nanoparticles during the flame synthesis. The role of sintering during nanoparticle growth is analyzed.
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Papers by John Z Wen