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231 results on '"Harding, J. H."'

80. The Vkcentre in NaCl

Author

Testa, A., primary, Stoneham, A. M., additional, Catlow, C. R. A., additional, Song, K. S., additional, Harker, A. H., additional, and Harding, J. H., additional

Published
1991
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82. Mesoscopic modelling: materials at the appropriate scale.

Author

Stoneham, A. M. and Harding, J. H.

Subjects
*ATOMS, *MICROSTRUCTURE, *CERAMICS, *CONSTRUCTION materials, *ALLOYS
Abstract

There are successful and standard ways to model materials for macroscopic engineering purposes. Likewise, there are many effective ways for atomic scale modelling, though only for relatively few atoms. But many materials properties of major importance depend on what happens at the mesoscale, where microstructures of ceramics or of teeth or bones, the dislocation engineering of metal alloys, and the spaghetti structures of polymers all have a major influence on performance. We discuss the main issues involved, and how such systems can be modelled effectively. [ABSTRACT FROM AUTHOR]

Published
2009
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93. Modeling the Properties of Self-Assembled Monolayers Terminated by Carboxylic Acids

Author

Duffy, D. M. and Harding, J. H.

Abstract

Self-assembled monolayers of long-chain carboxylic acids are often used as substrates to promote the growth of oriented crystals. Recent work has shown that the length of the chain (odd or even number of carbon atoms) determines whether oriented growth is observed. We use molecular dynamics simulations to investigate whether the configuration of the headgroups is significantly different in the two cases. We conclude that there are differences between odd- and even-length chains, even at 300 K and in the presence of water for some packings of the monolayer. We discuss whether these differences are large enough to account for the different behavior.

Published
2005

94. Growth of Polar Crystal Surfaces on Ionized Organic Substrates

Author

Duffy, D. M. and Harding, J. H.

Abstract

Calcite crystals nucleate on the (01.2) face on a diverse range of organic substrates, including self-assembled monolayers, hydrogen-bonded ribbons, and polymer rafts. The (01.2) face of calcite is a polar surface. Therefore macroscopic crystal growth can only occur if the dipole moment is quenched. We demonstrate that the dipole moment can be quenched for a given polar direction by adsorption onto an organic substrate with arbitrary charge density. The density of ions in the outer calcium plane must be modified, by introducing rows of vacancies, to fulfill the condition of zero net dipole moment. Interfacial energies are calculated for interfaces between the polar (01.2) and (00.1) faces of calcite and stearic acid monolayers with a range of densities. It was found that, contrary to the experimental evidence, the (00.1) face has lower interfacial energy than the (01.2) face with monolayers with equivalent densities. We give an explanation for this discrepancy based on kinetic models.

Published
2004

95. Simulation of Organic Monolayers as Templates for the Nucleation of Calcite Crystals

Author

Duffy, D. M. and Harding, J. H.

Abstract

Living organisms can control the size, shape, and structure of minerals. Attempts to reproduce this biological control in the laboratory often use Langmuir monolayers of long-chain carboxylic acids. We use large-scale molecular dynamics simulations to calculate the interfacial energies of calcite crystals grown on stearic (octadecanoic) acid monolayers. In light of these simulations we discuss the argument that the orientation of the growing mineral is controlled by the organic substrate acting as a template which the mineral must fit in order to grow.

Published
2004

100. Vibrational entropies of defects in solids

Author

Harding, J. H. and Stoneham, A. M.

Abstract

We present a new method for calculating the perturbed phonon spectrum of defect crystals and show how this can be used to obtain entropies. The method considers a periodic structure in which a cluster of ions around the defect, placed in their relaxed positions, is embedded in a non-primitive unit cell. The normal modes of this structure may be used to calculate local modes and, since a reasonable representation of k space may be obtained by projection, considering only the Γ point of this cell, we may readily obtain values for the partition function and hence for thermodynamic quantities.

Published
1981
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