A molecular dynamics study of segregation and diffusion in FCC nanocrystals using the sutton-chen potential

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dc.contributor.advisorTerblans, J. J.
dc.contributor.advisorSwart, H. C.
dc.contributor.authorVan der Walt, Cornelia
dc.date.accessioned2017-08-07T06:45:47Z
dc.date.available2017-08-07T06:45:47Z
dc.date.issued2017-02
dc.description.abstractEnglish: English: Nanotechnology research has expanded notably, with a wide range of applications from catalysis in fuels, to optics. A key factor in manufacturing these particles is understanding diffusion and segregation of dopants and impurities in the nanocrystals, as segregation of these impurities influences which atom is exposed to the surface of the nano-particle, and able to react. Understanding these processes in terms of the shape and size of the particle, as well as the effects of temperature, are all important factors for nano-material manufacture. Molecular Dynamics software is uniquely able to study the dynamics inside particles of up to several thousand atoms. The Sutton-Chen potential, in particular, is able to simulate the reactions of face-centred cubic (FCC) metals and model bulk modulus, elastic constants, lattice parameters, surface energies, phonon dispersion, cohesion energy and vacancy formation energy. It is ideally suited for studying the diffusion and segregation dynamics of the large clusters of atoms that make up nanocrystals. In this study, a Molecular Dynamics model using the Sutton-Chen potential was built. This model implements the Verlet Velocity scheme to simulate the kinetics of the atoms, and uses the Berendsen thermostat to keep the system at a constant temperature. The model was tested on six FCC metals, namely Al, Ni, Cu, Pd, Ag and Pt, and, making use of periodic boundaries in order to simulate bulk crystals, calculated the cohesion energy to confirm the effectiveness of the model. The model further confirmed surface orientation dependence for low index surfaces. The relationship for vacancy formation energy Ev(111) > Ev(100) > Ev(110) of applied to all the FCC metals studied. The effects of temperature on other diffusion-related energies in the crystals were also studied. It was further found that the diffusion activation energy of FCC metals has the same relationship of Q(110) < Q(100) < Q(111) Equipped with this information, the model was used for in-depth analysis of Cu, and later Ag, nano-cubes, -rhombicuboctahedrons and -octahedrons. A thorough analysis of the surface orientation dependence, size dependence, shape dependence and temperature dependence of key energies involved in diffusion, created a complete picture of nanoparticle stability and surface reactivity. It was found that larger particles are more stable, and that surface reactivity indicates that nano-rhombicuboctahedrons are more reactive than perfect cubes, and that octahedrons are the least surface-reactive. The final part of this study calculated the segregation energy in Ag/Cu systems to confirm the ability of the mixed Sutton-Chen potential to simulate segregation in alloys. In the Ag/Cu system, Ag is known to segregate to the surface, while Cu desegregates, and the model was able to demonstrate this. As this model can successfully reproduce that segregation, it can become a powerful tool for the study of diffusion dynamics in FCC alloy nano-materials.en_ZA
dc.description.abstractAfrikaans: Nanotegnologie navorsing het geweldig uitgebrei, met 'n wye verskeidenheid van toepassings van katalise in brandstof, tot optika. 'n Belangrike faktor in die vervaardiging van hierdie deeltjies is die begrip van diffusie en segregasie van doteermiddels en onsuiwerhede in nano-kristalle, aangesien segregasie van hierdie onsuiwerhede beïnvloed watter atoom word blootgestel op die oppervlak van die nano-deeltjie, en in staat is om te reageer. Begrip van hierdie prosesse in terme van die vorm en grootte van die deeltjies, sowel as die effekte van temperatuur is almal belangrike faktore vir die vervaardiging van nanomateriale. Molekulêre Dinamika sagteware is uniek in staat daarin om die dinamika binne deeltjies van tot 'n paar duisend atome te bestudeer. Die Sutton-Chen potensiaal, in besonder, is in staat om die reaksies, volumemodulus, elastisiteitskonstantes, roosterkonstantes, oppervlakenergie, fononverstrooiing, kohesie-energie en leemte-vormingsenergie van vlakgesentreerde kubiese metale te modelleer. Dit is ideaal vir die bestudering van diffusie en segregasie dinamika van groot groepe atome waaruit die nano-kristalle bestaan. In hierdie studie is 'n Molekulêre Dinamika model, wat van die Sutton-Chen potensiaal gebruik maak, ontwikkel. Hierdie model maak gebruik van die Verlet Snelheid skema om die kinetika van die atome te simuleer, en maak gebruik van die Berendsen termostaat om die sisteem teen 'n konstante temperatuur te hou. Die model is getoets op ses FCC metale, naamlik Al, Ni, Cu, Pd, Ag en Pt, en deur van periodiese-randvoorwaardes gebruik te maak om grootmaatkristalle te simuleer, die kohesie-energie is bereken om die doeltreffendheid van die model bevestig. Die model bevestig verder die oppervlak afhanklikheid vir lae-indeks oppervlaktes. Die verhouding vir leemte-vormingsenergie van Ev(111) >Ev(100)>Ev(110) is van toepassing op al die FCC metale wat bestudeer was. Die effek van temperatuur op ander diffusie-verwante energieë in die kristalle is ook bestudeer. Dit is verder bevind dat die diffusie-aktiveringsenergie vir FCC metale het dieselfde verhouding dat Q(110)< Q(100) <Q(111) Met hierdie inligting is die model gebruik vir in-diepte analise van Cu, en later Ag, nano-kubussse, -rombiese-oktaëders en -oktaëders. 'n Deeglike ontleding van die oppervlakafhanklikheid, grootteafhanklikheid, vormafhanklikheid en temperatuurafhanklikheid van die energieë betrokke by diffusie en segregasie het 'n volledige beeld van nano-deeltjie se stabiliteit en oppervlak-reaktiwiteit gegee. Daar is bevind dat groter deeltjies meer stabiel is, en die oppervlak-reaktiwiteit getoon het dat nano-rombiese-oktaëders meer reaktief is as perfekte kubusse, en dat oktaëders die minste oppervlak-reaktief is. In die laaste gedeelte van hierdie studie is die segregasie energie in 'n Ag/Cu sisteem bereken ten einde die vermoë te bevestig dat die gemengde Sutton-Chen potensiaal segregasie in legerings kan simuleer. In die Ag/Cu sisteem, is Ag bekend daarvoor dat dit segregeer na die oppervlak terwyl Cu de-segregeer, en die model was in staat om dit te demonstreer. Aangesien hierdie model suksesvol segregasie kan weergee, kan dit 'n kragtige instrument vir die studie van diffusie dinamika in FCC allooi nano-materiale word.af
dc.description.sponsorshipNano Cluster of the University of the Free Stateen_ZA
dc.description.sponsorshipNational Research Foundationen_ZA
dc.identifier.urihttp://hdl.handle.net/11660/6516
dc.language.isoenen_ZA
dc.publisherUniversity of the Free Stateen_ZA
dc.rights.holderUniversity of the Free Stateen_ZA
dc.subjectAlloyen_ZA
dc.subjectAluminiumen_ZA
dc.subjectBerendsen thermostaten_ZA
dc.subjectBinding energyen_ZA
dc.subjectCohesion energyen_ZA
dc.subjectCopperen_ZA
dc.subjectDiffusionen_ZA
dc.subjectDiffusion activation energyen_ZA
dc.subjectDiffusion mechanismen_ZA
dc.subjectDissipative particle dynamicsen_ZA
dc.subjectGibbs free energyen_ZA
dc.subjectMigration energyen_ZA
dc.subjectMolecular dynamicsen_ZA
dc.subjectNanocrystalen_ZA
dc.subjectNanocubeen_ZA
dc.subjectNano particleen_ZA
dc.subjectNickelen_ZA
dc.subjectPalladiumen_ZA
dc.subjectPlatinumen_ZA
dc.subjectSchottky defecten_ZA
dc.subjectSegregationen_ZA
dc.subjectSegregation energyen_ZA
dc.subjectSilveren_ZA
dc.subjectSurface orientationen_ZA
dc.subjectSutton-chen potentialen_ZA
dc.subjectTemperature dependanceen_ZA
dc.subjectVacancy formation energyen_ZA
dc.subjectVelocity verlet schemeen_ZA
dc.subjectVerlet algorithmen_ZA
dc.subjectFCC metalsen_ZA
dc.subjectThesis (Ph.D. (Physics))--University of the Free State, 2017en_ZA
dc.titleA molecular dynamics study of segregation and diffusion in FCC nanocrystals using the sutton-chen potentialen_ZA
dc.typeThesisen_ZA
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