Nadia Pellerin

N2+/N2 ratio and temperature measurements based on the first negative N2+ and second positive N2 overlapped molecular emission spectra

The and molecular emission spectra are frequently observed simultaneously in plasmas containing nitrogen. Relative band intensities of these systems are very sensitive to a variation of the ratio and temperature. The spectrum, emitted between 3800 and 4000 A, has been used to measure rotational and vibrational temperatures, and to estimate the ratio when the electron temperature is known, in different plasma sources (Glidarc, ac discharge between tips). The proposed method is based on a point-to-point comparison of an experimentally measured spectrum with the computer-simulated one.

Migration and segregation of sodium under β-irradiation in nuclear glasses

Raman, 11B Nuclear Magnetic Resonance (NMR) and X-ray Photoelectron (XPS) spectroscopy investigations have been made on a series of β-irradiated aluminoborosilicate glasses. This work shows the following changes of the glass structure under β-irradiation: (i) an increase of glass polymerization, (ii) an increase of boron in a trigonal environment and (iii) no modification in the aluminum environment during irradiation. These glass modifications under β-irradiation can be correlated with the migration of sodium outside of the Si and B glass network formers. Moreover, XPS experiments show a strong decrease of the sodium concentration at the surface of these irradiated glasses. This study presents therefore strong evidences of migration and segregation processes of sodium during a β-irradiation in the bulk for these simplified aluminoborosilicate glass compositions.

Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR

Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of real materials and often serves as the key to the tuning of their properties and their final applications. Although researchers can easily assess local ordering using two-dimensional imaging techniques with resolution that approaches the atomic level, the diagnosis, description, and qualification of local order in three dimensions is much more challenging. Solid-state nuclear magnetic resonance (NMR) and its panel of continually developing instruments and methods enable the local, atom-selective characterization of structures and assemblies ranging from the atomic to the nanometer length scales. By making use of the indirect J-coupling that distinguishes chemical bonds, researchers can use solid-state NMR to characterize a variety of materials, ranging from crystalline compounds to amorphous or glassy materials. In crystalline compounds showing some disorder, we describe and distinguish the contributions of topology, geometry, and local chemistry in ways that are consistent with X-ray diffraction and computational approaches. We give examples of materials featuring either chemical disorder in a topological order or topological disorder with local chemical order. For glasses, we show that we can separate geometric and chemical contributions to the local order by identifying structural motifs with a viewpoint that extends from the atomic scale up to the nanoscale. As identified by solid state NMR, the local structure of amorphous materials or glasses consists of well-identified structural entities up to at least the nanometer scale. Instead of speaking of disorder, we propose a new description for these structures as a continuous assembly of locally defined structures, an idea that draws on the concept of locally favored structures (LFS) introduced by Tanaka and coworkers. This idea provides a comprehensive picture of amorphous structures based on fluctuations of chemical composition and structure over different length scales. We hope that these local or molecular insights will allow researchers to consider key questions related to nucleation and crystallization, as well as chemically (spinodal decomposition) or density-driven (polyamorphism) phase separation, which could lead to future applications in a variety of materials.

Plasma diagnostics in gas metal arc welding by optical emission spectroscopy

The plasma column in a metal inert gas welding process is investigated by optical emission spectroscopy and high-speed imaging. The concentration and repartition of iron vapours are measured and correlated with the plasma and electrode geometric configuration. Plasma temperatures and electron densities are also measured for each studied position in the plasma. The temperatures are calculated using two different methods, allowing validation of the local thermodynamic equilibrium state of the plasma. The results show a maximum temperature of 12 500 K in the upper part of the arc, away from the arc axis. The iron concentration reaches a maximum of 0.3% close to the anode and strongly decreases along both the vertical and radial directions.The plasma thermophysical properties, calculated from this plasma composition, are then discussed regarding the metal transfer mode.

Spin-counting NMR experiments for the spectral editing of structural motifs in solids

Scalar couplings, recoupled or full dipolar interactions can be used to characterize multinuclear structural molecular motifs in solids, by counting the neighbouring spins in solid-state NMR, opening new ways for the differentiation of overlapping spectral responses which is a limiting factor in many high resolution experiments carried out on disordered systems.

Carbon in YBa2Cu3O7−x: origin and effects

Abstract Residual carbon content in powders, sintered ceramics and textured samples was determined. Its source was shown to be essentialy the starting material precursors. Carbon content may be reduced to below 600 ppm if no liquid phase is formed during precursor processing. The case of samples processed by solidification from a liquid is important. In samples textured on MgO substrate, carbon is supplied by this substrate. In this case, liquid phase plays an active role on the carbon retention in the YBa2Cu3O7−x structure during crystallisation. Magnetic susceptibility behaviour of textured samples was analysed and discussed in relation to the presence of carbon. A qualitative correlation between the superconducting transition width and the carbon content was established. A sharp superconducting transition at 91.5 K was achieved when carbon content was less than 600 ppm.

Nucleation and growth mechanisms of textured YBaCuO and the influence of Y2BaCuO5

texturation has been achieved with success on Y203 without application of an external thermal gradient for the purpose of a better understanding of nucleation and growth mechanisms. We have studied the effects of thermal parameters such as the maximal applied temperature and crystallization speed. Their respective contributions to the peritectic recrystallization (211 consumption), and to the textured domain size have been made precise by using X-ray data and semi-quantitative analysis on the micrographs. The interface reaction between the Y203 substrate and 123 has been analyzed. The substrate promotes a seeded type growth of 123 owing to an interracial 211 layer. The mechanism of crystallization has been analyzed in the light of existing theoretical models. Our data confirm a crystallization of 123 directly from a liquid as in a peritectic reaction where a major part is played by the yttrium diffusion in the liquid. Y atoms are supplied by the dissolution of 211 particles. According to Uhlmann- Chalmers-Jackson (UCJ) theory, coarsened 211 grains are trapped by the solidification front in contrast to particles smaller than a critical radius that are consumed in the peritectic reaction. (This process explains why such large 211 particles are ob- tained in MTG contrary to QMG or MPMG methods. ) Moreover, EPR results show a preferential orientation of 211 inclusions according to the 123 matrix which could be favorable to flux pinning in the superconducting state. 211 oriented inclusions make possible, to some extent, heterogeneous nucleation at platelet-211 junctions. On the other hand, microstructural studies show that the liquid/solid interface is rather unstable being frequently cellular or dendritic, consistent with a model proposed by Alexander et al.

Analysis of infrared reflectivity of Pr2NiO4 single crystal

The reflectivity spectra of a single crystal of stoichiometric and non-stoichiometric Pr2NiO4+δ (0 < δ < 0.3, measured by thermogravimetry) are reported for the polarizations parallel and perpendicular to the c axis. The crystal structure is found to depend upon the oxygen stoichiometry, which itself controls the transition from insulator to semiconducting. The semiconducting behaviour is checked by the temperature dependence of resistivity. In the non-stoichiometric compound, the phonons are partially screened by the plasma background, which is limited to the conducting basal (a, b) plane, as in La2CuO4. An annealing process at 650 K under 5% of hydrogen in argon provides the stoichiometric compound which is an insulator and shows an orthorhombic distortion with respect to the tetragonal structure.

The microstructure of interfaces in melt-textured YBa2Cu3O7−x: mechanisms of formation

Microstructures of textured YBa2Cu3O7-x ceramics were examined by transmission electron microscopy. The samples were synthesized under a residual thermal gradient by using modified melt textured growth on a Y oxide substrate. Defects running parallel to stacking (ab) planes are in fact growth defects associated with 211 trapped particles. Microanalysis of such areas proves the presence of solidified liquid in these defects. The growth is cellular or dendritic and the texture has many characteristics typical of a single crystal. Interfaces formed between 211 and 123 were studied and analysed with reference to the crystallization mechanism. Two types of interface were evidenced: (A) those for which the matrix is deformed and (B) those for which structural (and chemical) modifications of the inclusions are the main characteristics. The interfaces belonging to group A may be classified according to the types of defect they create in the 123 matrix. Most coherent interfaces (interface planes parallel to (010)123, observed when (110)123 are parallel to (001)123) induce very few defects in the matrix; interfaces with ordinary coincidences give rise to strongly disordered areas. Interfaces with no planes in coincidence with the matrix are characterized by microprecipitates, which may be viewed as an alternative way for stress relaxation. Type B interfaces are found for small 211 particles; these induce more strain in the matrix than larger ones, and their composition and structure are also more defective.

Critical current density and activation energy in melt textured growth YBaCuO from magnetic measurements

Abstract Textured YBaCuO samples have been prepared using a modified melt textured growth (MTG) method. AC susceptibility data are presented showing the high quality of the textured samples. Static hysteresis loop measurements are realized for magnetic fields up to 5 T in a large temperature range (4.2 to 100 K). The anisotropic behavior is studied, the magnetic field being applied successively in the ( ab ) planes and along the c -axis. The critical current densities are obtained using the Bean model. Dynamic hysteresis loop measurements are performed at T ≥ 77 K and over the frequency range from 0.2 to 10 Hz using a field of 800 Oe. The magnetization relaxation effect appears to be significantly lower in the textured sample than in a single crystal. The characteristic activation energies calculated in the flux creep model are presented.