Self-Assembly in the Growth of Precious Opal

It is proposed that primary nucleation of amorphous microspherulites of hydrated silica in natural proto-precious-opal can be followed by a long range superlattice ordering process by means of electrostatic self-assembly. Necessary conditions in the thermodynamics are a high surface charge density on microspherulite surfaces, a long Debye length and an appropriate number density of nucleation centres. A further chemical requirement is a high alkaline environmental pH from 9 to 10. It is also proposed that the characteristic concentric spherical shell-like structure of spherulites, centred on primary nuclei, are due to sequential deposition of intrinsic salts which precipitate out when the corresponding solubility limits in the liquid are successively exceeded. It can be that the better-known sedimentation of microspherulites under gravity only plays part in the final stabilization period of overall growth.Comment: 12 pages pdf http://dx.doi.org/10.1016/j.jcrysgro.2009.09.04

Growth and structure of prismatic boron nitride nanorods

Prismatic boron nitride nanorods have been grown on single crystal silicon substrates by mechanical ball-milling followed by annealing at 1300 &deg;C. Growth takes place by rapid surface diffusion of BN molecules, and follows heterogeneous nucleation at catalytic particles of an Fe/Si alloy. Lattice imaging transmission electron microscopy studies reveal a central axial row of rather small truncated pyramidal nanovoids on each nanorod, surrounded by three basal planar BN domains which, with successive deposition of epitaxial layers adapt to the void geometry by crystallographic faceting. The bulk strain in the nanorods is taken up by the presence of what appear to be simple nanostacking faults in the external, near-surface domains which, like the nanovoids are regularly repetitive along the nanorod length. Growth terminates with a clear cuneiform tip for each nanorod. Lateral nanorod dimensions are essentially determined by the size of the catalytic particle, which remains as a foundation essentially responsible for base growth. Growth, structure, and dominating facets are shown to be consistent with a system which seeks lowest bulk and surface energies according to the well-known thermodynamics of the capillarity of solids.<br /

Ion-solid interaction: status and perspectives

The past half century has seen an explosive growth of the discipline that is characterized by the deposition of localized high energy densities in solid matter, by means of energetic ions produced e.g. in particle accelerators. First the fundamentals of Ion-Solid Interaction are outlined. A brief overview is given about the basic energy transfer mechanisms and the consequences of ion impact into solids, such as scattering, sputtering and radiation damage. The latter gives rise to numerous changes in the material's physical and chemical properties, part of which has proven to be detrimental, and part of which is beneficial for technological applications. Though meanwhile a good general understanding of this discipline has been obtained, there are still plenty of lacks of details in our knowledge. Some representative examples are given. As a conclusion of all these points, one may say that, in spite of its age, Ion-Solid Interaction is still a challenging scientifical field, and it still offers lots of promising applications

Standing Waves on the Moon

The existence of local annular and radial structures in the environment of lunar craters, and of a global selenographic tectonic pattern, is ascribed in part to standing “moonquakes” established after impact or eruption

Further discussion on the pressure dependence of fission track annealing in apatite: reply to the critical comment of Kohn et al

International audienc