Molecular dynamics modelling of diffusional formation of titanium carbide clusters in iron matrix

Molecular dynamics simulation was employed to study the atomic interactions in titanium carbides and iron matrix containing carbon and titanium, which are significant for understanding the formation of titanium carbide cluster during precipitate process. The atoms trajectory and diffusion coefficients of carbon in titanium carbide were analyzed to provide a vacancy-exchanging mechanism and clarify the carbon concentration dependence of carbon diffusion in titanium carbide. The dependence of the formation of titanium carbide cluster in iron matrix on carbon was determined from the study of atoms diffusivity, cluster formation and formation energy of titanium carbide cluster. The simulation results provided insight into the carbon diffusion process and improved the understanding of the formation of titanium carbide cluster.

A molecular dynamics simulation of alloy carbide clusters formation

 The formation of alloy carbide cluster in ferrite was investigated via molecular dynamics simulation, which disclosed the cluster property and formation mechanism. These together provided a better fundamental understanding of the cluster formation and firm information for the evolution of cluster and precipitate in high-strength low-alloy steel.

Organotin-oxo clusters

This thesis reports on the development and expansion of reliable synthetic di-and multi-tin precursors for the assembly of oligomeric organotin-oxo compounds in which the shape, dimension and tin nuclearity can be controlled. The reaction of polymeric diorganotin oxides, (R2SnO)m (R = Me, Et, n-Bu, n-Oct, c-Hex, i-Pr, Ph), with saturated aqueous NH4X solutions (X = F, Cl, Br, I, OAc) in refluxing 1,4-dioxane afforded in high yields dimeric tetraorganodistannoxanes, [R2(X)SnOSn(X)R2]2, and in a few cases diorganotin dihalides or diacetates, R2SnX2. This method appears to be particularly good for the synthesis of halogenated tetraorganodistannoxanes but a less suitable method for the preparation of dicarboxylato tetraorganodistannoxanes. Identification of [R2(OH)SnOSn(X)R2]2 (R = n-Bu; X = Cl, Br) and [R2(OH)SnOSn(X)R2][R2(X)SnOSn(X)R2] suggest a serial substitution mechanism starting from [R2(OH)SnOSn(OH)R2]2. A series of α, ω -bis(triphenylstannyl)alkanes, [Ph3Sn]2(CH2)n (n = 3-8, 10, 12) and some of their derivatives were synthesised and characterised. These α, ω-bis(triphenylstannyl)alkanes, [Ph3Sn]2(CH2)n were converted to the corresponding halides [R(Cl)2Sn]2(CH2)n (R = CH2SiMe3) and subsequently to the polymeric oxides {[R(0)Sn]2(CH2)n}m. Reaction of {[R(O)Sn]2(CH2)n}m with [R(Cl)2Sn]2(CH2)n. (n = 3, n' = 4 and n = 4, n' = 3) in toluene at 100°C results in a mixture of symmetric and asymmetric double ladders, where different spacer chain lengths (n and n') provide the source of asymmetry. The coexistence at high temperature of separate 119Sn NMR signals belonging to symmetric and asymmetric double ladders suggests an equilibrium that is slow on the 119Sn NMR time scale and the position of which is temperature dependent. However, 119Sn NMR spectroscopic experiments of {[R(0)Sn]2(CH2)3}m with [R(Cl)2Sn]2(CH2)n for longer spacers (n - 5, 6, 8, 10, 12) reveal that molecular self-assembly of symmetric spacer-bridged di-tin precursors of equal chain length is preferred over asymmetric species. An ether-bridged di-tin tetrachloride [R(Cl)2Sn(CH2)3]2O (R = CH2SiMe3) and its corresponding polymeric oxide {[R(O)Sn(CH2)3]2O}m were synthesised and characterised. Reaction of [R(Cl)2Sn(CH2)3]2O with {[R(O)Sn(CH2)3]2O}m results in a unique functionalised double ladder {{[RSn(Cl)](CH2)3O(CH2)3[RSn(Cl)]}O}4 whose structure in the solid state was determined by X-ray analysis. Identification of tetrameric functionalised double ladder as well as dimeric and monomeric species suggest the existence of an equilibrium in solution. The feasibility of the functionalised double ladder to form host-guest complexes with a variety of metal cations is investigated using electrospray mass spectrometry (ESMS). Evidence for such complexes is found only for sodium cations. The reaction between {[R(O)Sn]2(CH2)n}m (n = 3, 4, 8, 10) and triflic acid is described. The initial formed products [RSn(CH2)nSnR](OTf)4 are easily hydrolysed. For n = 3, self-assembly leads to a discrete double ladder type structure, {{[RSn(OH)](CH2)3[RSn(H2O)]}O}44OTf, which is the first example of a cationic double ladder. For n ≥ 3, hydrolysis gives polymeric products, as demonstrated by the crystal structure of {[(H2O)(OH)RSn]2(CH2)4-2OTf2H2O}m. Two spacer-bridged terra-tin octachlorides [R(Cl)2Sn(CH2)3Sn(Cl)2]2(CH2)n (R = CH2SiMes; n = 1, 8) and their corresponding polymeric oxides {[R(O)Sn(CH2)3Sn(O)]2(CH2)n}m were successfully synthesised and characterised. Attempts were made to synthesise quadruple ladders from these precursors. Reactions of [R(Cl)2Sn(CH2)3Sn(Cl)2]2CH2 with {[R(O)Sn(CH2)3Sn(O)]2CH2}m or (Y-Bu2SnO)3 result in, mostly insoluble, amorphous solids. Reactions of [R(Cl)2Sn(CH2)3Sn(Cl)2]2(CH2)8 with {[R(O)Sn(CH2)3Sn(O)]2(CH2)8}m or (t-Bu2SnO)s result in new tin-containing species which are presumably oligomeric. The synthesis of a series of alkyl-bridged di-tin hexacarboxylates [(RCO2)3Sn]2(CH2)n (n = 3, 4; R = Ph, c-C6H11, CH3, C1CH2) is also reported. The hydrolysis of these compounds is facile and complex. There appears to be no correlation between spacer chain length and hydrolysis product. However, the conjugate acid strength of the carboxylate does appear to be important. In general only insoluble amorphous polymeric organotin-oxo compounds were obtained.

Highly concentrated salt solutions : molecular dynamics simulations of structure and transport

Molecular dynamics (MD) simulations in NaI solutions, where the solvent has been represented by the Stockmayer fluid, were performed as a function of temperature, salt concentration, and solvent dipole strength. At higher temperatures contact ion pairs become more prevalent, regardless of solvent strength. An examination of the temperature dependence of the potential of mean force demonstrates the entropic nature of this effect. The transport properties calculated in the simulations are dependent on the balance between solvent dielectric constant and ion charge. In systems with a large solvent dipole moment, the ions appear to be independently mobile, and deviations from Nernst–Einstein behavior are small. In systems of smaller solvent dipole moment or greater ion charge, the ions form clusters, and large deviations from Nernst–Einstein behavior are observed.

Ion clustering in molecular dynamics simulations of sodium iodide solutions

Model systems of sodium iodide dissolved in dimethyl ether or 1,2-dimethoxyethane (glyme) were studied in order to investigate the structural and dynamic properties of ionic solutions in small and polymeric ethers. Full molecular dynamics simulations were performed at a range of different salt concentrations. An algorithm was designed which assigns ions to clusters and then calculates all the terms which contribute to ionic conductivity. In dilute solutions, free ions are the most common ionic species, followed by ion pairs. As the concentration increases, pairs become the most common species, with significant concentrations of clusters with 3 through 6 ions. Changing the solvent from dimethyl ether to glyme significantly decreases the ion clustering due to the chelate effect in which the two oxygens on a solvent stabilize an associated cation. The conductivity in stable systems is shown to be primarily the result of the movement of free ions and the relative movement of ions within neutral pairs. The Nernst-Einstein relation, commonly used in the discussion of polymer electrolytes, is shown to be inadequate to quantitatively describe conductivity in the model systems.

Characterization of structure and energy of TixCy cluster at early formation stage in iron matrix by molecular dynamics

The structure, energy and bonding property of TixCy clusters formed in iron matrix were studied through molecular dynamics (MD) simulation method. The initial clusters with 1D-linear, 2D-ring, and 3D-tetrahedral structures were determined and their stability was calculated. The effect of temperature on the stability of the clusters was also discussed. In addition, the dissociation path of TiC clusters in iron matrix and the corresponding energy variation were analyzed. © 2014 Elsevier B.V. All rights reserved.

Study of growth mechanism of TiC cluster in ferrite via molecular dynamics simulation

Abstract Atomistic simulations were used to investigate the evolution process of titanium carbide clusters to mature precipitates in ferrite. The typical kinetic of carbide cluster growth was studied in detail through analyzing the atomic interactions of a carbide cluster with scattered carbon atoms. The driving force required for cluster growth was calculated along with the atomic diffusivity in the iron matrix, exploring the change in response as two main growth steps. The growth kinetic improved the understanding of precipitate evolution at the atomic level.

Molecular dynamics study of a dual-cation ionomer electrolyte

The poly(N1222)xLi1-x[AMPS] ionomer system with dual cations has previously shown decoupled Li ion dynamics from polymer segmental motions, characterized by the glass transition temperature, which can result in a conductive electrolyte material whilst retaining an appropriate modulus (ie. stiffness) so that it can suppress dendrite formation, thereby improving safety when used in lithium metal batteries. To understand this ion dynamics behavior, molecular dynamics techniques have been used in this work to simulate structure and dynamics in these materials. These simulations confirm that the Li ion transport is decoupled from the polymer particularly at intermediate N1222+ concentrations. At 50 mol% N1222+ concentration the polymer backbone is more rigid than for higher N1222+ concentrations, but with increasing temperature Li ion transport is more significant than polymer or quaternary ammonium cation motions. Here we suggest an ion hopping mechanism for Li+, arising from structural rearrangement of ionic clusters that could explain its decoupled behavior. Higher temperatures favor an aggregated ionic structure as well as enhancing these hopping motions. The simulations discussed here provide an atomic-level understanding of ion dynamics that could contribute to designing an improved ionomer with fast ion transport and mechanical robustness.