Publications by authors named "Kaushik Joshi"

10 Publications

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Graphene reinforced carbon fibers.

Sci Adv 2020 Apr 24;6(17):eaaz4191. Epub 2020 Apr 24.

Department of Mechanical and Aerospace Engineering, University of Virginia, 122 Engineer's Way, Charlottesville, VA 22904, USA.

The superlative strength-to-weight ratio of carbon fibers (CFs) can substantially reduce vehicle weight and improve energy efficiency. However, most CFs are derived from costly polyacrylonitrile (PAN), which limits their widespread adoption in the automotive industry. Extensive efforts to produce CFs from low cost, alternative precursor materials have failed to yield a commercially viable product. Here, we revisit PAN to study its conversion chemistry and microstructure evolution, which might provide clues for the design of low-cost CFs. We demonstrate that a small amount of graphene can minimize porosity/defects and reinforce PAN-based CFs. Our experimental results show that 0.075 weight % graphene-reinforced PAN/graphene composite CFs exhibits 225% increase in strength and 184% enhancement in Young's modulus compared to PAN CFs. Atomistic ReaxFF and large-scale molecular dynamics simulations jointly elucidate the ability of graphene to modify the microstructure by promoting favorable edge chemistry and polymer chain alignment.
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http://dx.doi.org/10.1126/sciadv.aaz4191DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7182419PMC
April 2020

Unveiling Carbon Ring Structure Formation Mechanisms in Polyacrylonitrile-Derived Carbon Fibers.

ACS Appl Mater Interfaces 2019 Nov 1;11(45):42288-42297. Epub 2019 Nov 1.

Department of Mechanical and Aerospace Engineering , University of Virginia , 122 Engineer's Way , Charlottesville , Virginia 22904 , United States.

As the demand for electric vehicles (EVs) and autonomous vehicles (AVs) rapidly grows, lower-cost, lighter, and stronger carbon fibers (CFs) are urgently needed to respond to consumers' call for greater EV traveling range and stronger safety structures for AVs. Converting polymeric precursors to CFs requires a complex set of thermochemical processes; a systematic understanding of each parameter in fiber conversion is still, to a large extent, lacking. Here, we demonstrate the effect of carbonization temperature on carbon ring structure formation by combining atomistic/microscale simulations and experimental validation. Experimental testing, as predicted by simulations, exhibited that the strength and ductility of PAN CFs decreased, whereas the Young's modulus increased with increasing carbonization temperature. Our simulations unveiled that high carbonization temperature accelerated the kinetics of graphitic phase nucleation and growth, leading to the decrease in strength and ductility but increase in modulus. The methodology presented herein using combined atomistic/microscale simulations and experimental validation lays a firm foundation for further innovation in CF manufacturing and low-cost alternative precursor development.
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http://dx.doi.org/10.1021/acsami.9b15833DOI Listing
November 2019

Mirrored continuum and molecular scale simulations of the ignition of high-pressure phases of RDX.

J Chem Phys 2016 May;144(18):184111

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.

We present a mirrored atomistic and continuum framework that is used to describe the ignition of energetic materials, and a high-pressure phase of RDX in particular. The continuum formulation uses meaningful averages of thermodynamic properties obtained from the atomistic simulation and a simplification of enormously complex reaction kinetics. In particular, components are identified based on molecular weight bin averages and our methodology assumes that both the averaged atomistic and continuum simulations are represented on the same time and length scales. The atomistic simulations of thermally initiated ignition of RDX are performed using reactive molecular dynamics (RMD). The continuum model is based on multi-component thermodynamics and uses a kinetics scheme that describes observed chemical changes of the averaged atomistic simulations. Thus the mirrored continuum simulations mimic the rapid change in pressure, temperature, and average molecular weight of species in the reactive mixture. This mirroring enables a new technique to simplify the chemistry obtained from reactive MD simulations while retaining the observed features and spatial and temporal scales from both the RMD and continuum model. The primary benefit of this approach is a potentially powerful, but familiar way to interpret the atomistic simulations and understand the chemical events and reaction rates. The approach is quite general and thus can provide a way to model chemistry based on atomistic simulations and extend the reach of those simulations.
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http://dx.doi.org/10.1063/1.4948548DOI Listing
May 2016

Intermolecular Energy Transfer Dynamics at a Hot-Spot Interface in RDX Crystals.

J Phys Chem A 2016 Feb 25;120(4):477-89. Epub 2016 Jan 25.

Applied Research Institute, University of Illinois at Urbana-Champaign , Champaign, Illinois 61820, United States.

The phonon mediated vibrational up-pumping mechanisms assume an intact lattice and climbing of a vibrational ladder using strongly correlated multiphonon dynamics under equilibrium or near-equilibrium conditions. Important dynamic processes far from-equilibrium in regions of large temperature gradient after the onset of decomposition reactions in energetic solids are relatively unknown. In this work, we present a classical molecular dynamics (MD) simulation-based study of such processes using a nonreactive and a reactive potential to study a fully reacted and unreacted zone in RDX (1,3,5-trinitro-1,3,5-triazocyclohexane) crystal under nonequilibrium conditions. The energy transfer rate is evaluated as a function of temperature difference between the reacted and unreacted regions, and for different widths and cross-sectional area of unreacted RDX layers. Vibrational up-pumping processes probed using velocity autocorrelation functions indicate that the mechanisms at high-temperature interfaces are quite different from the standard phonon-based models proposed in current literature. In particular, the up-pumping of high-frequency vibrations are seen in the presence of small molecule collisions at the hot-spot interface with strong contributions from bending modes. It also explains some major difference in the order of decomposition of C-N and N-N bonds as seen in recent literature on initiation chemistry.
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http://dx.doi.org/10.1021/acs.jpca.5b06359DOI Listing
February 2016

Reactive simulation of the chemistry behind the condensed-phase ignition of RDX from hot spots.

Phys Chem Chem Phys 2015 Jul;17(28):18790-801

Illinois Applied Research Institute, University of Illinois at Urbana-Champaign, Champaign, IL 61820, USA.

Chemical events that lead to thermal initiation and spontaneous ignition of the high-pressure phase of RDX are presented using reactive molecular dynamics simulations. In order to initiate the chemistry behind thermal ignition, approximately 5% of RDX crystal is subjected to a constant temperature thermal pulse for various time durations to create a hot spot. After application of the thermal pulse, the ensuing chemical evolution of the system is monitored using reactive molecular dynamics under adiabatic conditions. Thermal pulses lasting longer than certain time durations lead to the spontaneous ignition of RDX after an incubation period. For cases where the ignition is observed, the incubation period is dominated by intermolecular and intramolecular hydrogen transfer reactions. Contrary to the widely accepted unimolecular models of initiation chemistry, N-N bond dissociations that produce NO2 species are suppressed in the condensed phase. The gradual temperature and pressure increase in the incubation period is accompanied by the accumulation of short-lived, heavier polyradicals. The polyradicals contain intact triazine rings from the RDX molecules. At certain temperatures and pressures, the polyradicals undergo ring-opening reactions, which fuel a series of rapid exothermic chemical reactions leading to a thermal runaway regime with stable gas-products such as N2, H2O and CO2. The evolution of the RDX crystal throughout the thermal initiation, incubation and thermal runaway phases observed in the reactive simulations contains a rich diversity of condensed-phase chemistry of nitramines under high-temperature/pressure conditions.
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http://dx.doi.org/10.1039/c5cp00950bDOI Listing
July 2015

Reactive molecular simulations of protonation of water clusters and depletion of acidity in H-ZSM-5 zeolite.

Phys Chem Chem Phys 2014 Sep;16(34):18433-41

Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA 16801, USA.

Using reactive molecular dynamics (RMD), we present an atomistic insight into the interaction between water molecules and acidic centers of H-ZSM-5 zeolite. The reactive force field method, ReaxFF, was used to evaluate the adsorption and diffusion of water as well as to study the protonation of water molecules inside zeolite channels. The existing Si/Al/O/H parameters were refitted against DFT calculations to improve the ReaxFF description of interaction between water molecules and the acidic sites of zeolites. The diffusion coefficient of water in the zeolite obtained from refitted parameters is in excellent agreement with experimental results. The molecular dynamics (MD) simulations indicate that protonation of water molecules and acidity of the zeolite catalyst depend on water loadings and temperature and the observed trends compare favorably with existing experimental and theoretical studies. At higher water loadings, protonation of water molecules is more frequent leading to formation and growth of protonated water clusters inside zeolite channels. From the analysis of various reaction channels that were observed during the simulations, we found that such water clusters have relatively short life due to frequent interchange of protons and water molecules among the water clusters. Such proton hopping events play a key role in moving the protons between different acidic centers of zeolite. These simulations show the capability of ReaxFF in providing atomistic details of complex chemical interactions between the water phase and solid acid zeolites.
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http://dx.doi.org/10.1039/c4cp02612hDOI Listing
September 2014

Exploring the conformational and reactive dynamics of biomolecules in solution using an extended version of the glycine reactive force field.

Phys Chem Chem Phys 2013 Sep;15(36):15062-77

CNR-Institute of Chemistry of Organometallic Compounds-UOS Pisa, Pisa, Italy.

In order to describe possible reaction mechanisms involving amino acids, and the evolution of the protonation state of amino acid side chains in solution, a reactive force field (ReaxFF-based description) for peptide and protein simulations has been developed as an expansion of the previously reported glycine parameters. This expansion consists of adding to the training set more than five hundred molecular systems, including all the amino acids and some short peptide structures, which have been investigated by means of quantum mechanical calculations. The performance of this ReaxFF protein force field on a relatively short time scale (500 ps) is validated by comparison with classical non-reactive simulations and experimental data of well characterized test cases, comprising capped amino acids, peptides, and small proteins, and reaction mechanisms connected to the pharmaceutical sector. A good agreement of ReaxFF predicted conformations and kinetics with reference data is obtained.
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http://dx.doi.org/10.1039/c3cp51931gDOI Listing
September 2013

Reactive adsorption of ammonia and ammonia/water on CuBTC metal-organic framework: a ReaxFF molecular dynamics simulation.

J Chem Phys 2013 Jan;138(3):034102

Department of Chemical and Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, USA.

We report ReaxFF molecular dynamics simulations for reactive adsorption of NH(3) on dehydrated CuBTC metal-organic framework. If the temperature is moderate (up to 125 °C), the dehydrated CuBTC demonstrates a good hydrostatic stability for water concentrations up to 4.0 molecules per copper site. However, if the temperature increases to 550 K, the dehydrated CuBTC will collapse even at a small water concentration, 1.0 H(2)O molecule per copper site. When NH(3) molecules are adsorbed in the channel and micropores of CuBTC, they prefer to chemisorb to the copper sites rather than forming a dimer with another NH(3) molecule. The formation of equimolar Cu(2)(NH(2))(4) and (NH(4))(3)BTC structures is observed at 348 K, which is in good agreement with previous experimental findings. The dehydrated CuBTC framework is partially collapsed upon NH(3) adsorption, while the Cu-Cu dimer structure remains stable under the investigated conditions. Further calculations reveal that the stability of CuBTC is related to the ammonia concentration. The critical NH(3) concentration after which the dehydrated CuBTC starts to collapse is determined to be 1.0 NH(3) molecule per copper site. Depending on whether NH(3) concentration is below or above the critical value, the dehydrated CuBTC can be stable to a higher temperature, 378 K, or can collapse at a lower temperature, 250 K. H(2)O∕NH(3) mixtures have also been studied, and we find that although water molecules do not demonstrate a strong interaction with the copper sites of CuBTC, the existence of water molecules can substantially prevent ammonia from interacting with CuBTC, and thus reduce the amount of chemisorbed NH(3) molecules on CuBTC and stabilize the CuBTC framework to some extent.
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http://dx.doi.org/10.1063/1.4774332DOI Listing
January 2013

ReaxFF molecular dynamics simulation of thermal stability of a Cu3(BTC)2 metal-organic framework.

Phys Chem Chem Phys 2012 Aug 13;14(32):11327-32. Epub 2012 Jul 13.

Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA.

The thermal stability of a dehydrated Cu(3)(BTC)(2) (copper(II) benzene 1,3,5-tricarboxylate) metal-organic framework was studied by molecular dynamics simulation with a ReaxFF reactive force field. The results show that Cu(3)(BTC)(2) is thermally stable up to 565 K. When the temperature increases between 600 K and 700 K, the framework starts to partially collapse. The RDF analysis shows that the long range correlations between Cu dimers disappear, indicating the loss of the main channels of Cu(3)(BTC)(2). When the temperature is above 800 K, we find the decomposition of the Cu(3)(BTC)(2) framework. CO is the major product, and we also observe the release of CO(2), O(2), 1,3,5-benzenetricarboxylate (C(6)H(3)(CO(2))(3), BTC) and glassy carbon. The Cu dimer is stable up to 1100 K, but we find the formation of new copper oxide clusters at 1100 K. These results are consistent with experimental findings, and provide valuable information for future theoretical investigations of Cu(3)(BTC)(2) and its application in adsorption, separation and catalytic processes.
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http://dx.doi.org/10.1039/c2cp41511aDOI Listing
August 2012

Effect of formic acid addition on water cluster stability and structure.

J Phys Chem A 2011 May 20;115(18):4657-64. Epub 2011 Apr 20.

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.

Computational chemistry simulations were performed to determine the effect that the addition of a single formic acid molecule has on the structure and stability of protonated water clusters. Previous experimental studies showed that addition of formic acid to protonated pure water results in higher intensities of large-sized clusters when compared to pure water and methanol-water mixed clusters. For larger, protonated clusters, molecular dynamics simulations were performed on H(+)(H(2)O)(n), H(+)(H(2)O)(n)CH(3)OH, and H(+)(H(2)O)(n)CHOOH clusters, 19-28 molecules in size, using a reactive force field (ReaxFF). Based on these computations, formic acid-water clusters were found to have significantly higher binding energies per molecule. Addition of formic acid to a water cluster was found to alter the structure of the hydrogen-bonding network, creating selective sites within the cluster, enabling the formation of new hydrogen bonds, and increasing both the stability of the cluster and its rate of growth.
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http://dx.doi.org/10.1021/jp1110746DOI Listing
May 2011