Simulation Study On Solidification Processes And Microstructure Evolution Characteristics Of Liquid Metal Silver

The development of the rapid cooling theory and technology, the structure and en-ergy of nano-clusters, and the history of the molecular dynamics (MD) simulation arebrie?y reviewed in this thesis firstly. The MD principles and implementations for particu-lar ensembles from NVE to NPT which can be compare to experiment results directly, aresummarized compactly. Faults in some literatures for isobaric ensembles have been cor-rected; MD algorithms for random number, initial configuration and velocity, cell-list andso on are also improved. Basic Cluster Analysis (BCA) which is on the base of HA-pairs,is improved: extending CTIM to CTIM-2 that can express more basci clusters; combiningBasic Cluster Labeling (BCL)– and 3D graphics, the structure of phases and the symme-try of nanoparticles can be described clearly and tersely; and a corresponding 3D visualsoftware for structure analysis has been developed.The MD simulations for metal silver solidification at different cooling ratesγhavebeen performed with Quantum Sutton-Chen potential. The analysis reveals thatγis cru-cial for the final solid structure and the critical for amorphous forming is about 1013K/s.Whenγ<γc, the mixture of fcc and hcp basic clusters with different proportions evenhcp and fcc crystals can be formed. Moreover, both the scattering and layering states areaffirmed in mixtures; and the former usually occurs at higherγ. Layering crystals of 4Hand 12R whose Ising models are↑↑↓↓and↓↓↓↑respectively, have been obtained. Withγdecreasing, the crystallizing point Tc, the percentage of fcc basic cluster (1421 HA-pair),the degree of symmetry are all increasing; while the proportions of hcp cluster (1422HA-pair) rise firstly and then decline, and the summit appears at about 5.0×10¹² K/s.More meticulous calculations and analysis reveal that there are twice phase transi-tions in rapid crystal processes. The first is from liquid to metastable (transitional) bccphase, and the initial crystallizing point Tic increases withγdecreasing. The second isfrom the transitional bcc phase to more stable solid states whose structure depends uponthe cooling rate. This reveals that rapid cooling follows the'step rule'; taking into ac-count the barrier of forming free energy and the characteristic of rapid cooling, it is canbe made clear that for the crystal system of silver and copper, the proportions of hcp struc-tures ascend firstly and then descend withγdecreasing. In addition, the final structuresfrom different runs are usually different from each other even at an identicalγ; the energyof final states is not always decreasing withγdecrease; Therefore the kinetic acts as an important role in cooling processes and final structures.Most freezing processes for 2.5～10nm Ag nanodroplets containing non-magic num-ber of atoms at cooling rate 10³ and 10² K/ns show that there are a first order and a contin-uous phase transitions in turn. With the size increasing, the mean of Tis is increasing whilethe average of energy per atom in nanoparticles (at 273K) is decreasing and closing to thebulk case. The percentages of crystal, icosahedron, decahedron and other structures, thedependence of T_is on the size and its'chaos characteristic are all in good agreement withthe previous experiment results. Besides regular crystalline, decahedral and icosahedralnanoparticles, some very interesting novelty morphologies—surface-isomers—have alsobeen found with the help of BCA and 3D graphics. Surface-isomers can be constructedon the base of a regular crystal, decahedron, icosahedron and so on, by adding few layerswith specific atomic arrangement, such as ABCB, ABCBA. Some (100) facets whichare not close-packed introduced into surface-isomars, but they make the outmost surfacecloser to sphere than its'regular kernel, and reduce the total energy, enhance the stabil-ity. Therefore the surface-isomars should be stable enough to be observed in experiment,especially for those with well global (three- or five-fold) symmetry. Alloy nanoparticlesof AuPd and AuCu holding Umb-Dh structure have in fact been obtained in experiment.And the Fractal-Ih obtained in our simulations is close consistent with the recent calcula-tion about lead clusters.