K. J. Roberts

A fluorescence EXAFS study of the structure of copper-rich precipitates in Fe–Cu and Fe–Cu–Ni alloys

Abstract The influence of small amounts of copper on the neutron-irradiation-induced embrittlement of reactor pressure vessel steels is of considerable current interest. Previous work has shown that the embrittlement is associated with the formation of copper-rich precipitates, but uncertainties remain regarding their composition and form. This Letter reports the preliminary results from a structural investigation of such precipitates in solution-treated and thermally aged Fe–-Cu and Fe–-Cu–-Ni model alloys using fluorescence extended X-ray absorption spectroscopy. The most significant result is the first direct observation that small clusters in the peak hardness condition have a b.c.c. structure.

Modelling the morphology of molecular crystals; application to anthracene, biphenyl and β-succinic acid

Abstract The theoretical morphologies of the organic crystals anthracene, biphenyl and β-succinic acid are predicted using simple lattice geometry and atom-atom potential calculations. A number of atom-atom parameter sets are used and the morphologies derived from from these are compared with each other and with the observed morphologies. In these three cases the morphologies derived from energy calculations show little variation due to changes in parameter sets and give a better fit to the morphologies obtained from the lattice geometry model.

Investigation into the structures of some normal alkanes within the homologous series C13H28 to C60H122 using high-resolution synchrotron X-ray powder diffraction

High-resolution X-ray powder diffraction using synchrotron radiation has been used to determine the unit-cell parameters of some of the n-alkanes within the range C13H28–C60H122. These have, in general, been found to conform to the structural patterns predicted by Nyburg and Potworowski (Acta Crystallogr., Sect. B, 1973, 29, 347). Polymorphic behaviour has been observed for the n= even alkanes associated with increasing symmetry from a triclinic structure (space group P1), to a monoclinic structure (space group P21/a) and to an orthorhombic structure (mostly space group Pca21) as the chain length increases. The n-alkanes C38H78,C40H82 and C44H90 were found to crystallize in the polytypic orthorhombic polymorph (space group Pbca) proposed by Boistelle et al.(Acta Crystallogr., Sect. B, 1976, 32, 1240). The n= odd alkanes all conform to an orthorhombic structure (space group Pbcm). A comparative examination of the data, as a function of chain length, suggests the existence of a degree of structural disorder along the crystallographic c-axis for the longer homologues, which implies that incipient chain folding takes place.

In situ studies of a dispersed platinum on carbon electrode using X-ray absorption spectroscopy

Abstract In situ X-ray absorption spectroscopy measurements at the Pt L3 edge are reported for a high surface area platinum on carbon fuel cell type electrode in 1 M H2SO4 at 20°C. A purpose-built thin-layer cell was used in the transmission mode and spectra were recorded for the electrode at controlled potentials in the range 0.10–1.20 V/SCE. Changes in the near-edge spectra (XANES) and in the extended X-ray absorption fine structure (EXAFS) are reported and discussed in terms of the structural nature of the surface oxidation process.

Dislocations in energetic materials

An assessment has been made of the primary dislocation slip systems in the explosives pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) using a combination of dislocation etching and microhardness indentation techniques. Hardness measurements were made on all major habit faces as a function of temperature and load. These showed that, within the attainable temperature range (PETN 293 to 353 K, RDX 293 to 373 K) no change in hardness occurred which could be associated with the development of deformation mechanisms additional to those operating at room temperature. The hardness values of both materials were consistent with values obtained in some previous measurements (PETN 17 kg mm−2, RDX 39 kg mm−2). Solvent etching with acetone (5 sec at 274 K) proved to be an excellent method for revealing the emergent ends of growth and mechanically induced dislocations in PETN. Etching of microhardness indentations confirmed that observable slip traces comprised dislocations. These migrated up to 160μm (20g load) from the indentation point on both {110} and {101} faces. The alignments defined a {110} primary slip plane. Parallel experiments with RDX yielded evidence of highly localized slip around the indentation mark (90μm, 50g load). Two alignments of etch pits were noticeable. The better defined of these lay at the intersection of the (010) plane with the habit faces. The second could not be defined absolutely but most probably corresponds to the intersection of either the (011) or (012) plane with the surfaces. Consideration of the Burgers vectors of dislocations which are likely to glide in these planes lead us to speculate that the primary slip systems are, PETN {110} [001], and RDX (010) [001]. Such an assignment would be consistent with the relative hardness of the two materials.

Instrumentation for glancing angle x‐ray absorption spectroscopy on the Synchrotron Radiation Source

A versatile instrument for glancing‐angle x‐ray absorption spectroscopy (XAS) and x‐ray reflectivity in routine use on the Daresbury Synchrotron Radiation Source is described. This facility can be used for XAS measurements in both reflectivity and fluorescence modes as well as for qualitative studies of angular dependent x‐ray reflectivity. Reflectivity and XAS data obtained with this instrument on the oxidized surfaces of Cu thin films are presented.

An instrument for the examination of nucleation from solution and its application to the study of precipitation from diesel fuels and solutions ofn-alkanes

Abstract An automated apparatus has been designed and constructed to measure precipitation and dissolution of solids from solution at varying automatically predetermined rates of heating and cooling. The appearance and disappearance of crystals are detected by means of a fibre optic turbidity sensor attached to a Sybron/Brinkman colorimeter. Temperature is measured by a Pt resistance thermometer attached to a constant current source. Both of these measurements are recorded by a personal computer via an analog to digital converter. The temperature of the system and its variation is controlled from the personal computer via a digital to analog interface attached to the control head of a Haake F3Q cryostat. The system has been used for measurements of precipitation and dissolution temperatures for diesel waxes with and without nucleation additives. Studies have been made of the nucleation of a singlen-alkane from solution. From these measurements, saturation curves, orders of reactions, interfacial tensions and the critical radii of nuclei can be assessed.

Instrumentation and cell design for insitu studies of electrode surfaces using x‐ray synchrotron radiation

As an integral part of a program to develop x‐ray techniques for in situ structural studies of electrode surfaces, a series of experiments has been carried out using x‐ray diffraction (XRD) and x‐ray absorption spectroscopy (XAS) at the Daresbury SRS. These studies, which involve continuous control of the electrode potential and the need to probe the electrode surface with synchrotron radiation, demand careful attention to cell design. Three types of experiment are described in order to illustrate possible approaches to cell design and the versatility of in situ x‐ray techniques.

The nucleation of n-eicosane crystals from solution in n-dodecane in the presence of homologous impurities

Abstract Studies have been made of the nucleation of crystals of n-eicosane from solutions in n-dodecane both in the pure state and in the presence of the neighbouring homologues; n-octadecane, n-nonadecane, n-heneicosane and n-docosane. Impurity incorporation studies yielded distribution coefficients, λ ⪢ 1 in the early stages of precipitation decreasing to λ ⩽ 1 in the growth stage. This high degree of incorporation in the early stages implies that the homologues should play a significant part in the nucleation process. This conclusion is confirmed by nucleation studies which show that the addition of the homologous impurities yields an increase in the width of the metastable zone and in the evaluated surface free energy of the resulting solid. The higher molecular weight homologues yield a greater effect than the lower molecular weight homologues. The homologues are thus defined as efficient nucleation inhibitors. The characteristics of the product crystals are in accord with this interpretation.

Dislocations in energetic materials: IV. The crystal growth and perfection of cyclotrimethylene trinitramine (RDX)

Abstract Large (typically 10×15×20 mm 3 ) single crystals of cyclotrimethylene trinitramine (RDX) have been prepared from acetone solution by temperature lowering and solvent evaporation techniques. The crystal morphology and perfection are discussed in terms of growth conditions. The basic growth induced defect structure has been revealed by X-ray topography and has been found to comprise dislocations (density 10 to 10 3 lines cm −2 ) and weakly contrasted growth sector boundaries (typical strain 10 −5 ). Many of the dislocations observed had undergone post-growth motion. The extent of this motion was found to be less in crystals grown more slowly at lower temperatures.