[1] On grain growth in the presence of mobile particles

V.Yu. Novikov

The ability of second phase particles to migrate along with grain boundaries is shown to be determined not only by the particle mobility but also by the migration rate of the grain boundary where they locate. This leads to a duality in the mobile particle behaviour: they behave as either movable or immovable depending on the boundary migration rate. In the first case, they reduce the boundary mobility; in the second one they decrease the driving force for boundary migration. It is demonstrated by numerical modeling that mobile particles with low mobility can suppress grain growth even in nanocrystalline material, the limiting grains size being several times smaller than in the case of randomly distributed immobile particles. It is also shown that the Zener solution to the problem of the grain growth retardation by disperse particles is a specific case of the proposed approach.

[2] Neutron Larmor diffraction measurements for materials science

J. Repper et al

Neutron Larmor diffraction (LD) is a high-resolution diffraction technique based on the Larmor precession of polarized neutrons. In contrast to conventional diffraction, LD does not depend on the accurate measurement of Bragg angles, and thus the resolution is independent of the beam collimation and monochromaticity. At present, a relative resolution for the determination of the crystal lattice spacing d of Δd/dnot, vert, similar10-6 is achieved, i.e. at least one order of magnitude superior to conventional neutron or X-ray techniques. This work is a first step to explore the application of LD to high-resolution problems in the analysis of residual stresses, where both the accurate measurement of absolute d values and the possibility of measuring type II and III stresses may provide additional information beyond those accessible by conventional diffraction techniques. Data obtained from Inconel 718 samples are presented.

Sub-μm synchrotron tomography of multiphase metals using Kirkpatrick-Baez optics

G Requena et al

High resolution 3D imaging of heterogeneous metals is performed by high energy magnified synchrotron tomography using Kirkpatrick-Baez focusing optics achieving voxel sizes of (50-60 nm)3. Absorption and phase contrast are exploited applying holotomographic reconstructions. Microstructural features as small as not, vert, similar 180 nm are detected in ternary eutectic Al-Mg2Si-Si, SiC particle reinforced AlCu4, near β Ti-10V-2Fe-3Al, and in TiB reinforced α+β Ti-6Al-4V. The phase retrieval procedure yields enough contrast to segment the individual phases and analyze their shapes and 3D architecture.

Sequential growths of AlN and GaN layers on as-polished 6H–SiC(0001) substrates

Z J Reitmeier et al

Microstructures of surfaces and defects generated during initial and subsequent growths via metalorganic vapor-phase epitaxy of AlN(0001) films on 6H–SiC(0001) substrates and GaN(0001) films on AlN/SiC(0001) substrates have been investigated using atomic force microscopy and cross-sectional and plan-view transmission electron microscopy. Scratches present on the SiC surfaces did not appear to bias the nucleation of AlN. The lateral growth rate of AlN was greater than the vertical growth rate, leading to almost planar layers at 15 and 100 nm thicknesses. Partially coalesced islands were observed after nominally not, vert, similar15 nm of growth. Increasing the thickness to 100 nm resulted in complete island coalescence, formation of undulating films from the polishing scratches in the SiC substrate, a surface microstructure containing steps, terraces and small pits, and a reduced dislocation density relative to the 15 nm layers. The AlN/SiC interfaces contained steps and complex dislocation networks. GaN islands nucleated and grew on the AlN films. Complete coalescence of these islands occurred at thicknesses less than 100 nm. Dislocation density in the GaN films was reduced by increasing the thickness of either the AlN and or the GaN. Arguments are developed to account for these observations.

In Nature this week

August 10, 2007

Title: Single-molecule mass spectroscopy in solution using a solitary nanopore

Authors: Joseph W F Robertson, Claudio G Rodrigues, Vincent M Stanford, Kenneth A Rubinson, Oleg V Krasilnikov, and John J Kasianowicz

Source: PNAS, May 15, 2007, Vol. 104, No. 20, pp. 8207-8211

Abstract: We introduce a two-dimensional method for mass spectrometry in solution that is based on the interaction between a nanometer-scale pore and analytes. As an example, poly(ethylene glycol) molecules that enter a single {alpha}-hemolysin pore cause distinct mass-dependent conductance states with characteristic mean residence times. The conductance-based mass spectrum clearly resolves the repeat unit of ethylene glycol, and the mean residence time increases monotonically with the poly(ethylene glycol) mass. This technique could prove useful for the real-time characterization of molecules in solution.

Here is a review from the recent issue of Science by Simon J L Billinge and Igor Levin on the available experimental and theoretical methods for the determination of atomic structure at the nanoscale. Here is the abstract:

Emerging complex functional materials often have atomic order limited to the nanoscale. Examples include nanoparticles, species encapsulated in mesoporous hosts, and bulk crystals with intrinsic nanoscale order. The powerful methods that we have for solving the atomic structure of bulk crystals fail for such materials. Currently, no broadly applicable, quantitative, and robust methods exist to replace crystallography at the nanoscale. We provide an overview of various classes of nanostructured materials and review the methods that are currently used to study their structure. We suggest that successful solutions to these nanostructure problems will involve interactions among researchers from materials science, physics, chemistry, computer science, and applied mathematics, working within a “complex modeling” paradigm that combines theory and experiment in a self-consistent computational framework.

Take a look!