To understand antiwear phenomena in motor engines at the atomic level and provide evidence in sel... more To understand antiwear phenomena in motor engines at the atomic level and provide evidence in selecting future ashless wear inhibitors, we studied the thermal stability of the self-assembled monolayer (SAM) model for dithiophosphate (DTP) and dithiocarbamate (DTC) molecules on the iron oxide surface using molecular dynamics. The interactions for DTP, DTC and Fe2O3 are evaluated based on a force eld derived from tting to ab initio quantum chemical calculations of dimethyl DTP (and DTC) and Fe(OH)2(H2O)2-DTP (DTC) clusters. MD simulations at constant-NPT are conducted to asses relative thermal stabilities of the DTP and DTC with di erent pendant groups (n-propyl, i-propyl, npentyl, and i-pentyl). To investigate frictional process, we employ a steady state MD method, in which one of the Fe2O3 slabs maintained at a constant linear velocity. We obtain the time averaged normal and frictional forces from the interatomic forces. Then, we calculated the friction coe cient at the interface between SAMs of DTP and the con ned lubricant, hexadecane, to assess the shear stability of DTPs with di erent pendant groups.
Dendrimers are well defined, highly branched macromolecules that radiate from a central core and ... more Dendrimers are well defined, highly branched macromolecules that radiate from a central core and are synthesized through a stepwise, repetitive reaction sequence that guarantees complete shells for each generation, leading to polymers that are monodisperse. The synthetic procedures developed for dendrimer preparation permit nearly complete control over the critical molecular design parameters, such as size, shape, surface/interior chemistry, flexibility, and topology. Recent results suggest that dendritic polymers may provide the key to developing a reliable and economical fabrication and manufacturing route to functional nanoscale materials that would have unique properties (electronic, optical, opto-electronic, magnetic, chemical, or biological). In turn, these could be used in designing new nanoscale devices. In this paper, we determine the 3D molecular structure of various dendrimers with continuous configurational Boltzmann biased direct Monte Carlo method and study their energetic and structural properties using molecular dynamics after annealing these molecular representations.
Crystal engineering of metal-organic frameworks (MOFs) has allowed the construction of complex st... more Crystal engineering of metal-organic frameworks (MOFs) has allowed the construction of complex structures at atomic precision, but has yet to reach the same level of sophistication as organic synthesis. The synthesis of complex MOFs with multiple organic and/or inorganic components is ultimately limited by the lack of control over framework assembly in one-pot reactions. Herein, we demonstrate that multi-component MOFs with unprecedented complexity can be constructed in a predictable and stepwise manner under simple kinetic guidance, which conceptually mimics the retrosynthetic approach utilized to construct complicated organic molecules. Four multi-component MOFs were synthesized by the subsequent incorporation of organic linkers and inorganic clusters into the cavity of a mesoporous MOF, each composed of up to three different metals and two different linkers. Furthermore, we demonstrated the utility of such a retrosynthetic design through the construction of a cooperative bimetall...
We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain bo... more We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. It showed that hydrogen prefers to occupy the octahedral interstitial site in Pd matrix, however a stable H-vacancy complex with most H occupations would contain up to eight hydrogen atoms surrounding the vacancy at tetrahedral sites. Furthermore, H presence assists the pairing or formation of nearby vacancies, which in agreement with previous suggestions by both experiment and theory investigation. Also, this observation could imply about a hydrogen embrittlement (HE) mechanism through the connections of microvoid and cracks. The segregation of hydrogen at grain boundary, nevertheless, has shown a different effect. High H accumulation results in grain boundary extension, which is related the HE mechanism of grain decohesion observed by experiments.
We have been developing methods to obtain a fundamental understanding of the physico-chemistry of... more We have been developing methods to obtain a fundamental understanding of the physico-chemistry of sliding systems in contact that underlies the materials science and mechanics issues essential to tribology. Advances in theory and methods are now making it practical to consider fully first principles (de novo) predictions of surface and interface structures and their role in determining tribological properties of materials. Despite the progress, there remains an enormous gap between the distances and time scales of quantum mechanics (QM) simulations and the quantitative macroscopic continuum models essential in engineering design of tribosystems.
ABSTRACT In this work, we have characterized the thermomechanical properties of carbon nanotube b... more ABSTRACT In this work, we have characterized the thermomechanical properties of carbon nanotube based piezoelectric polymer nanocomposite using a hybrid force field for all atomistic molecular dynamic simulations. In addition, applicability of some of the well-known micromechanics composite theory in estimating carbon nanotube based polymer nanocomposite properties were assessed. We found that the primary reason for the strengthening effect of a nanocomposite with incorporation of a nanotube is the carbon-carbon bond and angle strength. The orientation of the nanotube in the polymer matrix is key to its reinforcement effect on a nanocomposite. We also observed that a perfect axial orientation does result in improving the axial modulus, but in the radial direction any strengthening for such a unidirectional composite does not seem possible without any bonding at the interface between the filler and the matrix material. The self-consistent field theory was found to be the closest with the atomistic simulation results for predicting mechanical properties of a polymer nanocomposite. The Halpin-Tsai model also demonstrated reasonable capability in predicting the strengthening effect. Mori-Tanaka model, however, underestimated the strengthening effect of carbon nanotube on the polymer matrix. It was also found that the existing composite theories are better at estimating low weight percentage nanotube strengthening effect.
Computational and Theoretical Polymer Science, 2001
Dendrimers and hyperbranched polymers represent a novel class of structurally controlled macromol... more Dendrimers and hyperbranched polymers represent a novel class of structurally controlled macromolecules derived from a branches-uponbranches structural motif. The synthetic procedures developed for dendrimer preparation permit nearly complete control over the critical molecular design parameters, such as size, shape, surface/interior chemistry,¯exibility, and topology. Dendrimers are well de®ned, highly branched macromolecules that radiate from a central core and are synthesized through a stepwise, repetitive reaction sequence that guarantees complete shells for each generation, leading to polymers that are mono-disperse. This property of dendrimers makes it particularly natural to coarsen interactions in order to simulate dynamic processes occurring at larger length and longer time scales. In this paper, we describe methods to construct 3-dimensional molecular structures of dendrimers (Continuous Con®guration Boltzmann Biased direct Monte Carlo, CCBB MC) and methods towards coarse graining dendrimer interactions (NEIMO and hierarchical NEIMO methods) and representation of solvent dendrimer interactions through continuum solvation theories, Poisson±Boltzmann (PB) and Surface Generalized Born (SGB) methods. We will describe applications to PAMAM, stimuli response hybrid star-dendrimer polymers, and supra molecular assemblies crystallizing to A15 colloidal structure or Pm6m liquid crystals. q
In this paper a novel algorithm for solution of constrained equations of motion with application ... more In this paper a novel algorithm for solution of constrained equations of motion with application to simulation of molecular dynamics systems is presented. The algorithm enables the solution of equation of motion with an internal coordinates model wherein the high frequency oscillations are freezed by explicit inclusion of hard constraints in the system as well as by clustering of atoms and thus it allows a much larger time step in the integration. For a molecular system with N clusters, the algorithm achieves the optimal sequential complexity of O(N). However, the main advantage of this new algorithm is its efficiency for massively parallel computation. In fact, this is the first known algorithm that achieves a both time- and processor-optimal parallel solution of constrained equations of motion, i.e., a computation time of O(Log N) by using O(N) processors. In addition to its theoretical significance, this algorithm is also very efficient for practical implementation on coarse grai...
We use a family of embedded atom model potentials all based on accurate quantum-mechanical calcul... more We use a family of embedded atom model potentials all based on accurate quantum-mechanical calculations to study the relation between Peierls stress and core properties of the 1/2a͗111͘ screw dislocation in bcc tantalum ͑Ta͒. We find that the Peierls stress ( P ) is a function of the core-polarization curvature ͑⌸͒ near the equilibrium core configuration. Our results suggest that the computationally available quantity ⌸ is a useful criterion for designing high-performance materials.
We use first-principles methods ͑no empirical parameters͒ to establish the phase diagram for the ... more We use first-principles methods ͑no empirical parameters͒ to establish the phase diagram for the B1͑NaCl͒, B2͑CsCl͒, and liquid phases of MgO. We used density-functional theory with the generalized gradient approximation to predict the equation-of-state ͓volume versus pressure "V( P)…͔ at 0 K for MgO in the low-density B1 ͑NaCl͒ phase and the high-density B2 ͑CsCl͒ phase. We find a pressure-induced phase transition at Pϭ400 GPa. We then fitted an MS-Q type force field ͑FF͒ to the quantum results. This FF, denoted as qMS-Q FF, was then used in molecular dynamics ͑MD͒ simulations to investigate the phase coexistence curves of the B1-B2 and B1-liquid phases. This leads to a first-principles phase diagram for MgO for pressures up to 500 GPa and temperatures up to 8000 K. The accuracy of the fit of the qMS-Q FF to the quantum mechanics validates the functional form of the qMS-Q FF in which the charges are obtained from charge equilibration ͑QEq͒ and the nonelectrostatic forces are described with simple two-body Morse potentials. Such qMS-Q FF using no empirical data should be useful for MD or Monte Carlo simulations of many other materials.
Two types of equilibrium core structures ͑denoted symmetric and asymmetric͒ for 1/2a͗111͘ screw d... more Two types of equilibrium core structures ͑denoted symmetric and asymmetric͒ for 1/2a͗111͘ screw dislocations in bcc metals have been found in atomistic simulations. In asymmetric ͑or polarized͒ cores, the central three atoms simultaneously translate along the Burgers vector direction. This collective displacement of core atoms is called polarization. In contrast, symmetric ͑nonpolarized͒ cores have zero core polarization. To examine the possible role of dislocation core in kink-pair formation process, we studied the multiplicity, structural features, and formation energies of 1/3a͗112͘ kinks in 1/2a͗111͘ screw dislocations with different core structures. To do this we used a family of embedded atom model potentials for tantalum ͑Ta͒ all of which reproduce bulk properties ͑density, cohesive energy, and elastic constants͒ from quantum mechanics calculations but differ in the resulting polarization of 1/2a͗111͘ screw dislocations. For dislocations with asymmetric core, there are two energy equivalent core configurations ͓with positive ͑P͒ and negative ͑N͒ polarization͔, leading to 2 types of ͑polarization͒ flips, 8 kinds of isolated kinks, and 16 combinations of kink pairs. We find there are only two elementary kinks, while the others are composites of elementary kinks and flips. In contrast, for screw dislocations with symmetric core, there are only two types of isolated kinks and one kind of kink pair. We find that the equilibrium dislocation core structure of 1/2a͗111͘ screw dislocations is an important factor in determining the kink-pair formation energy.
In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid... more In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid melting curve of MgO obtained using two-phase simulations is in good agreement with the published one obtained using the Clausius-Clapeyron equation in conjunction with separate single phase calculations of liquid and solid.
Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of ... more Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of Ta metal following the high compression ͑50 to 100 GPa͒ induced by high velocity (2 to 4 km/s͒ impact. We find that catastrophic failure in this system coincides with a critical behavior characterized by a void distribution of the form N(v)ϰV Ϫ , with ϳ2.2. This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter (, the ratio of the volume of the largest void to the total void volume͒ which changes rapidly from ϳ0 to ϳ1 when the metal fails and scales with as ϰ(Ϫ c )  with exponent ϳ0.4, where is the total void fraction. We found similar behavior for FCC Ni suggesting that this critical behavior is a universal characteristic for failure of solids in rapid expansion.
ABSTRACTLight weight, high strength fibers and films produced from stiff chain polymers are good ... more ABSTRACTLight weight, high strength fibers and films produced from stiff chain polymers are good candidates for use as structural materials. Over the last decade, considerable success has been achieved in synthesizing high strength fibers and films. Due to their thermal and oxidative stability aromatic heterocyclic stiff chain polymers such as ABPBO, ABPBT, and ABPBI are especially good candidates. We first describe the finite theory of elasticity as applied in atomistic modelling and simulations of anisotropic solids and then use this description to investigate the mechanical response of these crystalline polymers as a function of applied hydrostatic pressure and uniaxial tension and compression along the chain direction in molecular mechanics simulations. In addition to these finite stress-strain experiments, I will also present the results of the first elastic stiffness matrix calculations performed on these high performance polymers.
We study the structure and formation energy of kinks in 1/2a<111> screw dislocation in meta... more We study the structure and formation energy of kinks in 1/2a<111> screw dislocation in metallic Ta Embedded Atom Model Force Field parameterized using quantum mechanical computations. We studied a/3<112> kinks using a simulation cell containing four dislocations in a quadrupole arrangement. We impose periodic boundary conditions in the directions perpendicular to [111] and fixed boundaries in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a<111> dislocation lead to 8 distinguishable single kinks and 16 kink pairs. The different mismatches of core configurations along [111] direction cause variations in kink formation energy. The lowest formation energy of a kink pair is determined to be 0.73 eV. The geometric features of such kink pair have been studied with the help of structural analysis of the atomistic model. We also compare the activation energy for dislocation motion via the double kink mechanism with the activ...
The previously proposed polarizable charge equilibrium (PQEq) force field model is parameterized ... more The previously proposed polarizable charge equilibrium (PQEq) force field model is parameterized for studying lead titanate (PT), lead zirconate (PZ), and their alloys: lead zirconate titanate (PZT). Several molecular dynamics (MD) simulations are performed to assess the degree of accuracy of the model. The phase transition temperatures, which are generally inaccurate in MD, are shown to be similar to experimental measurements. Also, the calculation of the ferroelectric hysteretic behavior, including the spontaneous polarization, saturated polarization and coercive fields, with extended MD is shown to give a qualitatively correct comparison between PT and PZT. The accuracy of the electronic properties in PQEq leads to direct application to a range of interesting problems such as enhanced properties of piezo- and ferro-electric nanostructures and the kinetics of domain walls in these materials.
The maximum superheating and undercooling achievable at various heating ͑or cooling͒ rates were i... more The maximum superheating and undercooling achievable at various heating ͑or cooling͒ rates were investigated based on classical nucleation theory and undercooling experiments, molecular dynamics ͑MD͒ simulations, and dynamic experiments. The highest ͑or lowest͒ temperature T c achievable in a superheated solid ͑or an undercooled liquid͒ depends on a dimensionless nucleation barrier parameter  and the heating ͑or cooling͒ rate Q.  depends on the material: ϵ16␥ sl 3 /(3kT m ⌬H m 2 ) where ␥ sl is the solid-liquid interfacial energy, ⌬H m the heat of fusion, T m the melting temperature, and k Boltzmann's constant. The systematics of maximum superheating and undercooling were established phenomenologically as ϭ(A 0 Ϫb log 10 Q) c (1Ϫ c ) 2 where c ϭT c /T m , A 0 ϭ59.4, bϭ2.33, and Q is normalized by 1 K/s. For a number of elements and compounds,  varies in the range 0.2-8.2, corresponding to maximum superheating c of 1.06 -1.35 and 1.08 -1.43 at Q ϳ1 and 10 12 K/s, respectively. Such systematics predict that a liquid with certain  cannot crystallize at cooling rates higher than a critical value and that the smallest c achievable is 1/3. MD simulations (Q ϳ10 12 K/s) at ambient and high pressures were conducted on close-packed bulk metals with Sutton-Chen many-body potentials. The maximum superheating and undercooling resolved from single-and two-phase simulations are consistent with the c --Q systematics for the maximum superheating and undercooling. The systematics are also in accord with previous MD melting simulations on other materials ͑e.g., silica, Ta and ⑀-Fe͒ described by different force fields such as Morse-stretch charge equilibrium and embedded-atom-method potentials. Thus, the c --Q systematics are supported by simulations at the level of interatomic interactions. The heating rate is crucial to achieving significant superheating experimentally. We demonstrate that the amount of superheating achieved in dynamic experiments (Qϳ10 12 K/s), such as planar shock-wave loading and intense laser irradiation, agrees with the superheating systematics.
Dendrimers and hyperbranched polymers represent a revolution in methodology for directed synthesi... more Dendrimers and hyperbranched polymers represent a revolution in methodology for directed synthesis of monodisperse polymers with enormous possibility of novel architectures. They demonstrate ability to attain micelle-like structures with distinct internal and external character. Furthermore polyfunctional character of dendrimers allows varied response to environment and promise as selective sensors, carrier for drugs, encapsulation of toxic chemicals and metals. One of the key problems are the characterization of the structures. Theory and simulation can be essential to provide and predict structure and properties. Here, we present some recent advances in theory, modeling and simulation of dendritic polymers.
To understand antiwear phenomena in motor engines at the atomic level and provide evidence in sel... more To understand antiwear phenomena in motor engines at the atomic level and provide evidence in selecting future ashless wear inhibitors, we studied the thermal stability of the self-assembled monolayer (SAM) model for dithiophosphate (DTP) and dithiocarbamate (DTC) molecules on the iron oxide surface using molecular dynamics. The interactions for DTP, DTC and Fe2O3 are evaluated based on a force eld derived from tting to ab initio quantum chemical calculations of dimethyl DTP (and DTC) and Fe(OH)2(H2O)2-DTP (DTC) clusters. MD simulations at constant-NPT are conducted to asses relative thermal stabilities of the DTP and DTC with di erent pendant groups (n-propyl, i-propyl, npentyl, and i-pentyl). To investigate frictional process, we employ a steady state MD method, in which one of the Fe2O3 slabs maintained at a constant linear velocity. We obtain the time averaged normal and frictional forces from the interatomic forces. Then, we calculated the friction coe cient at the interface between SAMs of DTP and the con ned lubricant, hexadecane, to assess the shear stability of DTPs with di erent pendant groups.
Dendrimers are well defined, highly branched macromolecules that radiate from a central core and ... more Dendrimers are well defined, highly branched macromolecules that radiate from a central core and are synthesized through a stepwise, repetitive reaction sequence that guarantees complete shells for each generation, leading to polymers that are monodisperse. The synthetic procedures developed for dendrimer preparation permit nearly complete control over the critical molecular design parameters, such as size, shape, surface/interior chemistry, flexibility, and topology. Recent results suggest that dendritic polymers may provide the key to developing a reliable and economical fabrication and manufacturing route to functional nanoscale materials that would have unique properties (electronic, optical, opto-electronic, magnetic, chemical, or biological). In turn, these could be used in designing new nanoscale devices. In this paper, we determine the 3D molecular structure of various dendrimers with continuous configurational Boltzmann biased direct Monte Carlo method and study their energetic and structural properties using molecular dynamics after annealing these molecular representations.
Crystal engineering of metal-organic frameworks (MOFs) has allowed the construction of complex st... more Crystal engineering of metal-organic frameworks (MOFs) has allowed the construction of complex structures at atomic precision, but has yet to reach the same level of sophistication as organic synthesis. The synthesis of complex MOFs with multiple organic and/or inorganic components is ultimately limited by the lack of control over framework assembly in one-pot reactions. Herein, we demonstrate that multi-component MOFs with unprecedented complexity can be constructed in a predictable and stepwise manner under simple kinetic guidance, which conceptually mimics the retrosynthetic approach utilized to construct complicated organic molecules. Four multi-component MOFs were synthesized by the subsequent incorporation of organic linkers and inorganic clusters into the cavity of a mesoporous MOF, each composed of up to three different metals and two different linkers. Furthermore, we demonstrated the utility of such a retrosynthetic design through the construction of a cooperative bimetall...
We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain bo... more We have studied the fundamental process of hydrogen binding at interstitial, vacancy and grain boundary (GB) in palladium crystals using Density-Functional Theory. It showed that hydrogen prefers to occupy the octahedral interstitial site in Pd matrix, however a stable H-vacancy complex with most H occupations would contain up to eight hydrogen atoms surrounding the vacancy at tetrahedral sites. Furthermore, H presence assists the pairing or formation of nearby vacancies, which in agreement with previous suggestions by both experiment and theory investigation. Also, this observation could imply about a hydrogen embrittlement (HE) mechanism through the connections of microvoid and cracks. The segregation of hydrogen at grain boundary, nevertheless, has shown a different effect. High H accumulation results in grain boundary extension, which is related the HE mechanism of grain decohesion observed by experiments.
We have been developing methods to obtain a fundamental understanding of the physico-chemistry of... more We have been developing methods to obtain a fundamental understanding of the physico-chemistry of sliding systems in contact that underlies the materials science and mechanics issues essential to tribology. Advances in theory and methods are now making it practical to consider fully first principles (de novo) predictions of surface and interface structures and their role in determining tribological properties of materials. Despite the progress, there remains an enormous gap between the distances and time scales of quantum mechanics (QM) simulations and the quantitative macroscopic continuum models essential in engineering design of tribosystems.
ABSTRACT In this work, we have characterized the thermomechanical properties of carbon nanotube b... more ABSTRACT In this work, we have characterized the thermomechanical properties of carbon nanotube based piezoelectric polymer nanocomposite using a hybrid force field for all atomistic molecular dynamic simulations. In addition, applicability of some of the well-known micromechanics composite theory in estimating carbon nanotube based polymer nanocomposite properties were assessed. We found that the primary reason for the strengthening effect of a nanocomposite with incorporation of a nanotube is the carbon-carbon bond and angle strength. The orientation of the nanotube in the polymer matrix is key to its reinforcement effect on a nanocomposite. We also observed that a perfect axial orientation does result in improving the axial modulus, but in the radial direction any strengthening for such a unidirectional composite does not seem possible without any bonding at the interface between the filler and the matrix material. The self-consistent field theory was found to be the closest with the atomistic simulation results for predicting mechanical properties of a polymer nanocomposite. The Halpin-Tsai model also demonstrated reasonable capability in predicting the strengthening effect. Mori-Tanaka model, however, underestimated the strengthening effect of carbon nanotube on the polymer matrix. It was also found that the existing composite theories are better at estimating low weight percentage nanotube strengthening effect.
Computational and Theoretical Polymer Science, 2001
Dendrimers and hyperbranched polymers represent a novel class of structurally controlled macromol... more Dendrimers and hyperbranched polymers represent a novel class of structurally controlled macromolecules derived from a branches-uponbranches structural motif. The synthetic procedures developed for dendrimer preparation permit nearly complete control over the critical molecular design parameters, such as size, shape, surface/interior chemistry,¯exibility, and topology. Dendrimers are well de®ned, highly branched macromolecules that radiate from a central core and are synthesized through a stepwise, repetitive reaction sequence that guarantees complete shells for each generation, leading to polymers that are mono-disperse. This property of dendrimers makes it particularly natural to coarsen interactions in order to simulate dynamic processes occurring at larger length and longer time scales. In this paper, we describe methods to construct 3-dimensional molecular structures of dendrimers (Continuous Con®guration Boltzmann Biased direct Monte Carlo, CCBB MC) and methods towards coarse graining dendrimer interactions (NEIMO and hierarchical NEIMO methods) and representation of solvent dendrimer interactions through continuum solvation theories, Poisson±Boltzmann (PB) and Surface Generalized Born (SGB) methods. We will describe applications to PAMAM, stimuli response hybrid star-dendrimer polymers, and supra molecular assemblies crystallizing to A15 colloidal structure or Pm6m liquid crystals. q
In this paper a novel algorithm for solution of constrained equations of motion with application ... more In this paper a novel algorithm for solution of constrained equations of motion with application to simulation of molecular dynamics systems is presented. The algorithm enables the solution of equation of motion with an internal coordinates model wherein the high frequency oscillations are freezed by explicit inclusion of hard constraints in the system as well as by clustering of atoms and thus it allows a much larger time step in the integration. For a molecular system with N clusters, the algorithm achieves the optimal sequential complexity of O(N). However, the main advantage of this new algorithm is its efficiency for massively parallel computation. In fact, this is the first known algorithm that achieves a both time- and processor-optimal parallel solution of constrained equations of motion, i.e., a computation time of O(Log N) by using O(N) processors. In addition to its theoretical significance, this algorithm is also very efficient for practical implementation on coarse grai...
We use a family of embedded atom model potentials all based on accurate quantum-mechanical calcul... more We use a family of embedded atom model potentials all based on accurate quantum-mechanical calculations to study the relation between Peierls stress and core properties of the 1/2a͗111͘ screw dislocation in bcc tantalum ͑Ta͒. We find that the Peierls stress ( P ) is a function of the core-polarization curvature ͑⌸͒ near the equilibrium core configuration. Our results suggest that the computationally available quantity ⌸ is a useful criterion for designing high-performance materials.
We use first-principles methods ͑no empirical parameters͒ to establish the phase diagram for the ... more We use first-principles methods ͑no empirical parameters͒ to establish the phase diagram for the B1͑NaCl͒, B2͑CsCl͒, and liquid phases of MgO. We used density-functional theory with the generalized gradient approximation to predict the equation-of-state ͓volume versus pressure "V( P)…͔ at 0 K for MgO in the low-density B1 ͑NaCl͒ phase and the high-density B2 ͑CsCl͒ phase. We find a pressure-induced phase transition at Pϭ400 GPa. We then fitted an MS-Q type force field ͑FF͒ to the quantum results. This FF, denoted as qMS-Q FF, was then used in molecular dynamics ͑MD͒ simulations to investigate the phase coexistence curves of the B1-B2 and B1-liquid phases. This leads to a first-principles phase diagram for MgO for pressures up to 500 GPa and temperatures up to 8000 K. The accuracy of the fit of the qMS-Q FF to the quantum mechanics validates the functional form of the qMS-Q FF in which the charges are obtained from charge equilibration ͑QEq͒ and the nonelectrostatic forces are described with simple two-body Morse potentials. Such qMS-Q FF using no empirical data should be useful for MD or Monte Carlo simulations of many other materials.
Two types of equilibrium core structures ͑denoted symmetric and asymmetric͒ for 1/2a͗111͘ screw d... more Two types of equilibrium core structures ͑denoted symmetric and asymmetric͒ for 1/2a͗111͘ screw dislocations in bcc metals have been found in atomistic simulations. In asymmetric ͑or polarized͒ cores, the central three atoms simultaneously translate along the Burgers vector direction. This collective displacement of core atoms is called polarization. In contrast, symmetric ͑nonpolarized͒ cores have zero core polarization. To examine the possible role of dislocation core in kink-pair formation process, we studied the multiplicity, structural features, and formation energies of 1/3a͗112͘ kinks in 1/2a͗111͘ screw dislocations with different core structures. To do this we used a family of embedded atom model potentials for tantalum ͑Ta͒ all of which reproduce bulk properties ͑density, cohesive energy, and elastic constants͒ from quantum mechanics calculations but differ in the resulting polarization of 1/2a͗111͘ screw dislocations. For dislocations with asymmetric core, there are two energy equivalent core configurations ͓with positive ͑P͒ and negative ͑N͒ polarization͔, leading to 2 types of ͑polarization͒ flips, 8 kinds of isolated kinks, and 16 combinations of kink pairs. We find there are only two elementary kinks, while the others are composites of elementary kinks and flips. In contrast, for screw dislocations with symmetric core, there are only two types of isolated kinks and one kind of kink pair. We find that the equilibrium dislocation core structure of 1/2a͗111͘ screw dislocations is an important factor in determining the kink-pair formation energy.
In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid... more In answer to a Comment by Belonoshko [Phys. Rev. B 63, 096101 (2001)], we show that the B1-liquid melting curve of MgO obtained using two-phase simulations is in good agreement with the published one obtained using the Clausius-Clapeyron equation in conjunction with separate single phase calculations of liquid and solid.
Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of ... more Using molecular dynamics with an accurate many-body potential, we studied the rapid expansion of Ta metal following the high compression ͑50 to 100 GPa͒ induced by high velocity (2 to 4 km/s͒ impact. We find that catastrophic failure in this system coincides with a critical behavior characterized by a void distribution of the form N(v)ϰV Ϫ , with ϳ2.2. This corresponds to a threshold in which percolation of the voids results in tensile failure. We define an order parameter (, the ratio of the volume of the largest void to the total void volume͒ which changes rapidly from ϳ0 to ϳ1 when the metal fails and scales with as ϰ(Ϫ c )  with exponent ϳ0.4, where is the total void fraction. We found similar behavior for FCC Ni suggesting that this critical behavior is a universal characteristic for failure of solids in rapid expansion.
ABSTRACTLight weight, high strength fibers and films produced from stiff chain polymers are good ... more ABSTRACTLight weight, high strength fibers and films produced from stiff chain polymers are good candidates for use as structural materials. Over the last decade, considerable success has been achieved in synthesizing high strength fibers and films. Due to their thermal and oxidative stability aromatic heterocyclic stiff chain polymers such as ABPBO, ABPBT, and ABPBI are especially good candidates. We first describe the finite theory of elasticity as applied in atomistic modelling and simulations of anisotropic solids and then use this description to investigate the mechanical response of these crystalline polymers as a function of applied hydrostatic pressure and uniaxial tension and compression along the chain direction in molecular mechanics simulations. In addition to these finite stress-strain experiments, I will also present the results of the first elastic stiffness matrix calculations performed on these high performance polymers.
We study the structure and formation energy of kinks in 1/2a<111> screw dislocation in meta... more We study the structure and formation energy of kinks in 1/2a<111> screw dislocation in metallic Ta Embedded Atom Model Force Field parameterized using quantum mechanical computations. We studied a/3<112> kinks using a simulation cell containing four dislocations in a quadrupole arrangement. We impose periodic boundary conditions in the directions perpendicular to [111] and fixed boundaries in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a<111> dislocation lead to 8 distinguishable single kinks and 16 kink pairs. The different mismatches of core configurations along [111] direction cause variations in kink formation energy. The lowest formation energy of a kink pair is determined to be 0.73 eV. The geometric features of such kink pair have been studied with the help of structural analysis of the atomistic model. We also compare the activation energy for dislocation motion via the double kink mechanism with the activ...
The previously proposed polarizable charge equilibrium (PQEq) force field model is parameterized ... more The previously proposed polarizable charge equilibrium (PQEq) force field model is parameterized for studying lead titanate (PT), lead zirconate (PZ), and their alloys: lead zirconate titanate (PZT). Several molecular dynamics (MD) simulations are performed to assess the degree of accuracy of the model. The phase transition temperatures, which are generally inaccurate in MD, are shown to be similar to experimental measurements. Also, the calculation of the ferroelectric hysteretic behavior, including the spontaneous polarization, saturated polarization and coercive fields, with extended MD is shown to give a qualitatively correct comparison between PT and PZT. The accuracy of the electronic properties in PQEq leads to direct application to a range of interesting problems such as enhanced properties of piezo- and ferro-electric nanostructures and the kinetics of domain walls in these materials.
The maximum superheating and undercooling achievable at various heating ͑or cooling͒ rates were i... more The maximum superheating and undercooling achievable at various heating ͑or cooling͒ rates were investigated based on classical nucleation theory and undercooling experiments, molecular dynamics ͑MD͒ simulations, and dynamic experiments. The highest ͑or lowest͒ temperature T c achievable in a superheated solid ͑or an undercooled liquid͒ depends on a dimensionless nucleation barrier parameter  and the heating ͑or cooling͒ rate Q.  depends on the material: ϵ16␥ sl 3 /(3kT m ⌬H m 2 ) where ␥ sl is the solid-liquid interfacial energy, ⌬H m the heat of fusion, T m the melting temperature, and k Boltzmann's constant. The systematics of maximum superheating and undercooling were established phenomenologically as ϭ(A 0 Ϫb log 10 Q) c (1Ϫ c ) 2 where c ϭT c /T m , A 0 ϭ59.4, bϭ2.33, and Q is normalized by 1 K/s. For a number of elements and compounds,  varies in the range 0.2-8.2, corresponding to maximum superheating c of 1.06 -1.35 and 1.08 -1.43 at Q ϳ1 and 10 12 K/s, respectively. Such systematics predict that a liquid with certain  cannot crystallize at cooling rates higher than a critical value and that the smallest c achievable is 1/3. MD simulations (Q ϳ10 12 K/s) at ambient and high pressures were conducted on close-packed bulk metals with Sutton-Chen many-body potentials. The maximum superheating and undercooling resolved from single-and two-phase simulations are consistent with the c --Q systematics for the maximum superheating and undercooling. The systematics are also in accord with previous MD melting simulations on other materials ͑e.g., silica, Ta and ⑀-Fe͒ described by different force fields such as Morse-stretch charge equilibrium and embedded-atom-method potentials. Thus, the c --Q systematics are supported by simulations at the level of interatomic interactions. The heating rate is crucial to achieving significant superheating experimentally. We demonstrate that the amount of superheating achieved in dynamic experiments (Qϳ10 12 K/s), such as planar shock-wave loading and intense laser irradiation, agrees with the superheating systematics.
Dendrimers and hyperbranched polymers represent a revolution in methodology for directed synthesi... more Dendrimers and hyperbranched polymers represent a revolution in methodology for directed synthesis of monodisperse polymers with enormous possibility of novel architectures. They demonstrate ability to attain micelle-like structures with distinct internal and external character. Furthermore polyfunctional character of dendrimers allows varied response to environment and promise as selective sensors, carrier for drugs, encapsulation of toxic chemicals and metals. One of the key problems are the characterization of the structures. Theory and simulation can be essential to provide and predict structure and properties. Here, we present some recent advances in theory, modeling and simulation of dendritic polymers.
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Papers by Tahir Cagin