Pengfei Ji | University of Missouri Columbia (original) (raw)

Papers by Pengfei Ji

Research paper thumbnail of Multiscale modeling of femtosecond laser irradiation on a copper film with electron thermal conductivity from ab initio calculation

Numerical Heat Transfer, Part A: Applications, 2017

By combining ab initio quantum mechanics calculation and Drude model, electron temperature and la... more By combining ab initio quantum mechanics calculation and Drude model, electron temperature and lattice temperature dependent electron thermal conductivity is calculated and implemented into a multiscale model of laser material interaction, which couples the classical molecular dynamics and two-temperature model. The results indicated that the electron thermal conductivity obtained from ab initio calculation leads to faster thermal diffusion than that using the electron thermal conductivity from empirical determination, which further induces deeper melting region, larger number of density waves travelling inside the copper film and more various speeds of atomic clusters ablated from the irradiated film surface.

Research paper thumbnail of Molecular dynamics simulation of condensation on nanostructured surface in a confined space

Applied Physics A, 2016

Understanding heat transfer characteristics of phase change and enhancing thermal energy transpor... more Understanding heat transfer characteristics of phase change and enhancing thermal energy transport in nanoscale are of great interest in both theoretical and practical applications. In the present study, we investigated the nanoscale vaporization and condensation by using molecular dynamics simulation. A cuboid system is modeled by placing hot and cold walls in the bottom and top ends and filling with working fluid between the two walls. By setting two different high temperatures for the hot wall, we showed the normal and explosive vaporizations and their impacts on thermal transport. For the cold wall, the cuboid nanostructures with fixed height, varied length, width ranging from 4 to 20 layers, and an interval of 4 layers are constructed to study the effects of condensation induced by different nanostructures. For vaporization, the results showed that higher temperature of the hot wall led to faster transport of the working fluid as a cluster moving from the hot wall to the cold wall. However, excessive temperature of the hot wall causes explosive boiling, which seems not good for the transport of heat due to the less phase change of working fluid. For condensation, the results indicate that nanostructure facilitates condensation, which could be affected not only by the increased surface area but also by the distances between surfaces of the nanostructures and the cold end. There is an optimal nanosctructure scheme which maximizes the phase change rate of the entire system.

Research paper thumbnail of Residual stress distribution and microstructure in the friction stir weld of 7075 aluminum alloy

Journal of Materials Science, 2015

ABSTRACT

Research paper thumbnail of The status quo analysis and policy suggestions on promoting China׳s hydropower development

Renewable and Sustainable Energy Reviews, 2015

Research paper thumbnail of Molecular dynamics simulation of condensation on nanostructured surface in a confined space

Understanding heat transfer characteristics of phase change and enhancing thermal energy transpor... more Understanding heat transfer characteristics of phase change and enhancing thermal energy transport in nanoscale are of great interest in both theoretical and practical applications. In the present study, we investigated the nanos-cale vaporization and condensation by using molecular dynamics simulation. A cuboid system is modeled by placing hot and cold walls in the bottom and top ends and filling with working fluid between the two walls. By setting two different high temperatures for the hot wall, we showed the normal and explosive vaporizations and their impacts on thermal transport. For the cold wall, the cuboid nanostructures with fixed height, varied length, width ranging from 4 to 20 layers, and an interval of four layers are constructed to study the effects of condensation induced by different nanostructures. For vaporization, the results showed that higher temperature of the hot wall led to faster transport of the working fluid as a cluster moving from the hot wall to the cold wall. However, excessive temperature of the hot wall causes explosive boiling, which seems not good for the transport of heat due to the less phase change of working fluid. For condensation, the results indicate that nanostructure facilitates condensation, which could be affected not only by the increased surface area but also by the distances between surfaces of the nanostructures and the cold end. There is an optimal nanostructure scheme which maximizes the phase change rate of the entire system.

Research paper thumbnail of Continuum-atomistic simulation of picosecond laser heating of copper with electron heat capacity from ab initio calculation

On the basis of ab initio quantum mechanics (QM) calculation, the obtained electron heat capacity... more On the basis of ab initio quantum mechanics (QM) calculation, the obtained electron heat capacity is implemented into energy equation of electron subsystem in two temperature model (TTM). Upon laser irradiation on the copper film, energy transfer from the electron subsystem to the lattice subsystem is modeled by including the electron–phonon coupling factor in molecular dynamics (MD) and TTM coupled simulation. The results show temperature and thermal melting difference between the QM-MD-TTM integrated simulation and pure MD-TTM coupled simulation. The successful construction of the QM-MD-TTM integrated simulation provides a general way that is accessible to other metals in laser heating.

Research paper thumbnail of Ab initio determination of effective electron–phonon coupling factor in copper

The electron temperature T e dependent electron density of states g(ε), Fermi–Dirac distribution ... more The electron temperature T e dependent electron density of states g(ε), Fermi–Dirac distribution f (ε), and electron–phonon spectral function α 2 F (Ω) are computed as prerequisites before achieving effective electron–phonon coupling factor G e–ph. The obtained G e–ph is implemented into a molecular dynamics (MD) and two-temperature model (TTM) coupled simulation of femtosecond laser heating. By monitoring temperature evolutions of electron and lattice subsystems, the result utilizing G e–ph from ab initio calculation shows a faster decrease of T e and increase of T l than those using G e–ph from phenomenological treatment. The approach of calculating G e–ph and its implementation into MD–TTM simulation is applicable to other metals.

Research paper thumbnail of Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superla... more The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers
(Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics. 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4850935]

Research paper thumbnail of Femtosecond laser processing of germanium: an ab initio molecular dynamics study

An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented i... more An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the structural change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by applying a Nosé–Hoover thermostat to the electronic subsystem for ∼100 fs and continuing with a microcanonical ensemble simulation of ∼200 fs. The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find that the irradiated germanium is distinguishable from the usual germanium crystal by analysing their melting and dynamic properties. (Some figures may appear in colour only in the online journal)

Research paper thumbnail of First-principles molecular dynamics investigation of the atomic-scale energy transport: From heat conduction to thermal radiation

First-principles molecular dynamics simulation based on a plane wave/pseudopotential implementati... more First-principles molecular dynamics simulation based on a plane wave/pseudopotential implementation of density functional theory is adopted to investigate atomic scale energy transport for semiconductors (silicon and germanium). By imposing thermostats to keep constant temperatures of the nanoscale thin layers, the initial thermal non-equilibrium between the neighboring layers is established under the vacuum condition. Models with variable gap distances with an interval of lattice constant increment of the simulated materials are set up and statistical comparisons of temperature evolution curves are made. The equilibration time from non-equilibrium state to thermal equilibrium state of different silicon or/and ger-manium layers combinations are calculated. The results show significant distinctions of heat transfer under different materials and temperatures combinations. Further discussions on the equilibrium time are made to explain the simulation results. As the first work of the atomic scale energy transport spanning from heat conduction to thermal radiation, the simulation results highlight the promising application of the first-principles molecular dynamics in thermal engineering.

Research paper thumbnail of Multiscale modeling of femtosecond laser irradiation on a copper film with electron thermal conductivity from ab initio calculation

Numerical Heat Transfer, Part A: Applications, 2017

By combining ab initio quantum mechanics calculation and Drude model, electron temperature and la... more By combining ab initio quantum mechanics calculation and Drude model, electron temperature and lattice temperature dependent electron thermal conductivity is calculated and implemented into a multiscale model of laser material interaction, which couples the classical molecular dynamics and two-temperature model. The results indicated that the electron thermal conductivity obtained from ab initio calculation leads to faster thermal diffusion than that using the electron thermal conductivity from empirical determination, which further induces deeper melting region, larger number of density waves travelling inside the copper film and more various speeds of atomic clusters ablated from the irradiated film surface.

Research paper thumbnail of Molecular dynamics simulation of condensation on nanostructured surface in a confined space

Applied Physics A, 2016

Understanding heat transfer characteristics of phase change and enhancing thermal energy transpor... more Understanding heat transfer characteristics of phase change and enhancing thermal energy transport in nanoscale are of great interest in both theoretical and practical applications. In the present study, we investigated the nanoscale vaporization and condensation by using molecular dynamics simulation. A cuboid system is modeled by placing hot and cold walls in the bottom and top ends and filling with working fluid between the two walls. By setting two different high temperatures for the hot wall, we showed the normal and explosive vaporizations and their impacts on thermal transport. For the cold wall, the cuboid nanostructures with fixed height, varied length, width ranging from 4 to 20 layers, and an interval of 4 layers are constructed to study the effects of condensation induced by different nanostructures. For vaporization, the results showed that higher temperature of the hot wall led to faster transport of the working fluid as a cluster moving from the hot wall to the cold wall. However, excessive temperature of the hot wall causes explosive boiling, which seems not good for the transport of heat due to the less phase change of working fluid. For condensation, the results indicate that nanostructure facilitates condensation, which could be affected not only by the increased surface area but also by the distances between surfaces of the nanostructures and the cold end. There is an optimal nanosctructure scheme which maximizes the phase change rate of the entire system.

Research paper thumbnail of Residual stress distribution and microstructure in the friction stir weld of 7075 aluminum alloy

Journal of Materials Science, 2015

ABSTRACT

Research paper thumbnail of The status quo analysis and policy suggestions on promoting China׳s hydropower development

Renewable and Sustainable Energy Reviews, 2015

Research paper thumbnail of Molecular dynamics simulation of condensation on nanostructured surface in a confined space

Understanding heat transfer characteristics of phase change and enhancing thermal energy transpor... more Understanding heat transfer characteristics of phase change and enhancing thermal energy transport in nanoscale are of great interest in both theoretical and practical applications. In the present study, we investigated the nanos-cale vaporization and condensation by using molecular dynamics simulation. A cuboid system is modeled by placing hot and cold walls in the bottom and top ends and filling with working fluid between the two walls. By setting two different high temperatures for the hot wall, we showed the normal and explosive vaporizations and their impacts on thermal transport. For the cold wall, the cuboid nanostructures with fixed height, varied length, width ranging from 4 to 20 layers, and an interval of four layers are constructed to study the effects of condensation induced by different nanostructures. For vaporization, the results showed that higher temperature of the hot wall led to faster transport of the working fluid as a cluster moving from the hot wall to the cold wall. However, excessive temperature of the hot wall causes explosive boiling, which seems not good for the transport of heat due to the less phase change of working fluid. For condensation, the results indicate that nanostructure facilitates condensation, which could be affected not only by the increased surface area but also by the distances between surfaces of the nanostructures and the cold end. There is an optimal nanostructure scheme which maximizes the phase change rate of the entire system.

Research paper thumbnail of Continuum-atomistic simulation of picosecond laser heating of copper with electron heat capacity from ab initio calculation

On the basis of ab initio quantum mechanics (QM) calculation, the obtained electron heat capacity... more On the basis of ab initio quantum mechanics (QM) calculation, the obtained electron heat capacity is implemented into energy equation of electron subsystem in two temperature model (TTM). Upon laser irradiation on the copper film, energy transfer from the electron subsystem to the lattice subsystem is modeled by including the electron–phonon coupling factor in molecular dynamics (MD) and TTM coupled simulation. The results show temperature and thermal melting difference between the QM-MD-TTM integrated simulation and pure MD-TTM coupled simulation. The successful construction of the QM-MD-TTM integrated simulation provides a general way that is accessible to other metals in laser heating.

Research paper thumbnail of Ab initio determination of effective electron–phonon coupling factor in copper

The electron temperature T e dependent electron density of states g(ε), Fermi–Dirac distribution ... more The electron temperature T e dependent electron density of states g(ε), Fermi–Dirac distribution f (ε), and electron–phonon spectral function α 2 F (Ω) are computed as prerequisites before achieving effective electron–phonon coupling factor G e–ph. The obtained G e–ph is implemented into a molecular dynamics (MD) and two-temperature model (TTM) coupled simulation of femtosecond laser heating. By monitoring temperature evolutions of electron and lattice subsystems, the result utilizing G e–ph from ab initio calculation shows a faster decrease of T e and increase of T l than those using G e–ph from phenomenological treatment. The approach of calculating G e–ph and its implementation into MD–TTM simulation is applicable to other metals.

Research paper thumbnail of Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superla... more The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers
(Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics. 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4850935]

Research paper thumbnail of Femtosecond laser processing of germanium: an ab initio molecular dynamics study

An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented i... more An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the structural change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by applying a Nosé–Hoover thermostat to the electronic subsystem for ∼100 fs and continuing with a microcanonical ensemble simulation of ∼200 fs. The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find that the irradiated germanium is distinguishable from the usual germanium crystal by analysing their melting and dynamic properties. (Some figures may appear in colour only in the online journal)

Research paper thumbnail of First-principles molecular dynamics investigation of the atomic-scale energy transport: From heat conduction to thermal radiation

First-principles molecular dynamics simulation based on a plane wave/pseudopotential implementati... more First-principles molecular dynamics simulation based on a plane wave/pseudopotential implementation of density functional theory is adopted to investigate atomic scale energy transport for semiconductors (silicon and germanium). By imposing thermostats to keep constant temperatures of the nanoscale thin layers, the initial thermal non-equilibrium between the neighboring layers is established under the vacuum condition. Models with variable gap distances with an interval of lattice constant increment of the simulated materials are set up and statistical comparisons of temperature evolution curves are made. The equilibration time from non-equilibrium state to thermal equilibrium state of different silicon or/and ger-manium layers combinations are calculated. The results show significant distinctions of heat transfer under different materials and temperatures combinations. Further discussions on the equilibrium time are made to explain the simulation results. As the first work of the atomic scale energy transport spanning from heat conduction to thermal radiation, the simulation results highlight the promising application of the first-principles molecular dynamics in thermal engineering.