The diffraction of elastic waves by an imperfect fusion bond

We consider diffraction by a semi-infinite crack located alonga fusion interface between two differing elastic media. Twotypes of crack, namely open and partially closed cracks, areinvestigated. An open crack is modelled by a stress-free contactboundary condition and a partially closed crack is modelledby a spring contact boundary condition. For the latter, thejump in the stress across the crack is assumed to be proportionalto the jump in the displacement across the crack. This situationarises in, for example, a K-weld where the fine grain of theparent material (for example, ferritic or forged austeniticsteel) is in stark contrast with the coarse-grained weld metal(for example, austenitic weld metal). In the metal weld thedirection of the grain axis varies through the metal. However,diffraction is a local phenomenon and so the austenitic steelis assumed to have a zonal axis so that it may be modelled bya transversely isotropic composite. The ferritic or forged austeniticsteel will be modelled as an isotropic material.     

Characterization and pH stability of insoluble products of the reaction between cyanide and ferrous sulphate

X-ray diffraction (XRD), infrared (IR) spectroscopy and⁵⁷Fe Mössbauer effect spectroscopy were used to characterize and monitor the pH stability of cyanide and ferrous sulphate reaction mixtures. Limited information was obtained from the XRD and IR measurements because of the amorphous nature of the samples. The Mössbauer studies show that considerable changes occur when there is an increase in pH at which the sample is prepared. Prussian-blue type compounds and iron oxide in the form of small particles crystallize out at low and high pH values, while additional complexes are formed at intermediate pH values of 6 to 9.⁵⁷Fe Mössbauer effect spectroscopy is thus a powerful technique for monitoring the environmental stability of cyanide waste streams that result when modern methods of extractive metallurgy of gold are employed on mineral ores.     

An investigation of the mechanism of detoxification in asbestos

⁵⁷Fe Mössbauer effect spectroscopy studies were conducted on native crocidolite and on a sample which had been detoxified by means of a chemical treatment with ferric salts. This allows for a comparison between the crystal chemistry of the untreated and treated samples. Significant chemical changes in the treated sample have been inferred from the Fe site-population analysis after conducting a temperature-dependent study of both samples in the range 300 K down to 90 K. These results, together with the results from complementary Electron Spin Resonance (ESR) studies, may help to elucidate the mechanism of toxicity in these asbetiforms.     

Minimal covers of finite sets

We enumerate the minimal covers of a finite set S, classifying such covers by their cardinality, and also by the number of elements in S which they cover uniquely.     

Selected studies of magnetism at high pressure

Summary Most previous studies of magnetism in various compounds under extreme conditions have been conducted over a wide pressure range at room temperature or over a wide range of cryogenic temperatures at pressures below 20 GPa (200 kbar). We present some of the most recent studies of magnetism over an extended range of temperatures and pressures far beyond 20 GPa,i.e. in regions of pressure-temperature (P-T) space where magnetism has been largely unexplored. Recent techniques have permitted investigations of magnetism in selected 3d transition metal compounds in regions ofP-T where physical properties may be drastically modified; related effects have often been seen in selected doping studies at ambient pressures. We present⁵⁷Fe and¹²⁹I M?ssbauer isotope studies covering the range 300–4 K to sub-megabar pressures in compounds such as Sr₂FeO₄, LaFeO₃ and FeI₂, representative of a broad class of 3d transition metal compounds. At ambient pressure the electronic structure of the transition metal atom in these antiferromagnetic insulators extends from 3d ⁴ to 3d ⁶ and has a distinct influence on the pressure evolution of their magnetic properties. M?ssbauer studies of these compounds are considered in conjunction with available structural and electrical transport data at pressure. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Mössbauer Investigations of MDAS Diamond Powder

Metal inclusions in synthetic MDAS diamond grits were investigated by Mössbauer spectroscopy as a function of the grit size. The larger grit sizes show a nicely developed sextet, which collapses to a paramagnetic singlet with decreasing grit size. This superparamagnetic behavior shows that the metal inclusions are of the order of 10 nm in size, and become larger with the grit size. One grit which was showing the collapse was further investigated from 300 K down to 4 K. Part of the paramagnetic singlet unfolds to a sextet with B_hf ～33T, while another part unfolds with a narrower hyperfine magnetic field of ～4T and is associated with the paramagnetic phase that coexists with a ferromagnetic phase in Invar at higher temperatures. A singlet with an isomer shift of about 1.1 mm/s is associated with similar findings in catalysts with iron particles supported on carbonaceous materials.     

Electron density measurements during a current perturbation of a wall stabilized argon arc

The paper describes the application of a laser interferometric technique to the measurement of electron density changes which occur when a d.c. argon arc is subjected to a current step or a.c. modulation. The frequency of the latter was varied from 50 to 30,000 Hz. A first attempt is made to derive mobility measurements from the data.     

The relation betweenT c and anharmonicity in Sn-base A15 superconductors

The absolute value of the recoil-free fraction was measured accurately at three reference temperatures in the range 300 K down to 4 K, in two Sn-baseA 15 superconductors with disparate superconducting properties. The higher-T _c compound Nb₃Sn (T _c≈18K) exhibits low-temperature anharmonicity; this is in contrast to the lattice-dynamics of the low-T _c isomorph V₃Sn (T _c≈4K) in which it is observed that harmonic binding of the Sn atoms is prevalent down to low temperatures. The difference in the superconducting properties of the two compounds is shown to correlate with the considerable difference in their lattice-dynamics.     

Low-temperature anharmonicity,strong electron-phonon interactions and heavy-fermion behaviour,in the A15 superconductor Nb3Sn

Absolute values of the Sn recoil-free fraction in the A15 superconductor Nb₃Sn were accurately measured in the range 300 K down to 50 K. Low—temperature anharmonicity is evident in this compound and no further lattice—dynamical anomalies were observed in the measured temperature range. The data has been used to estimate the dimensions of the highly anharmonic atomic potential-wells of the Sn atoms at low temperatures.