Patrick Chaudouët

Standard enthalpies of formation of some Mn–Y and Mn–Sc intermetallic compounds

Abstract The standard enthalpies of formation of Mn2Y, Mn23Y6 and Mn2Sc have been determined by aluminium drop solution calorimetry in an isothermal double cell Tian–Calvet high temperature microcalorimeter at 1123 K. The heats of dissolution of the elements and the intermetallic compounds have been measured at infinite dilution in liquid aluminium. Mn, Y and the three intermetallic compounds dissolved rapidly. It was not possible to determine the value for Sc directly due to the low solubility of this element in Al, which led to a precipitation of Al3Sc without further dissolution. The value has therefore been determined indirectly by the dissolution of two intermetallic compounds — Sc5Ge3 and Al2Sc — for which the enthalpies of formation are known from the literature.

Magneto-Optical Investigations of LZO Buffer Layer Thickness Effects on YBCO Microstructure in the Simple NiW/LZO/YBCO Structure

The NiRABiTS/La2Zr2O7MOD/YBa2Cu3 O7MOCVD structure is a promising architecture for (YBCO) coated conductors. We succeeded in growing highly textured superconducting YBCO films on LZO-buffered Ni-5 at%W (NiW) substrates by metalorganic chemical vapor deposition (MOCVD). A single LZO buffer layer grown by metalorganic decomposition (MOD) is sufficient to ensure structural compatibility between YBCO and Ni, and to protect the substrate from oxidation during YBCO deposition. Textured Ni substrates obtained by rolling technique have intrinsic defects such as grain boundaries and rolling scratches. It is not easy to highlight their effects on LZO and YBCO films by usual measurements like XRD or SEM. The combination of Electron Backscattered Diffraction (EBSD) and Magneto-Optics imaging (MO) is useful to image grain boundaries and flux distribution in coated conductors. It provides complementary information on superconducting film qualities. These two techniques allowed us to understand how the microstructure of the LZO buffer layer and thus of the YBCO film is linked to the current density in this simple heterostructure.

Static and dynamic properties of R(Rh1−x Ru x )2Si2 probed by μSR (R=U,Nd,Ce)

Muon spin relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of URh2Si2, U(Rh0.35Ru0.65)2Si2, NdRh2Si2 and CeRh2Si2. In order to obtain information on the localisation and diffusion properties of the muon, some measurements have been performed also on isostructural diamagnetic compounds. From our measurements. information on the magneto-crystalline anisotropy of the samples has been obtained. Depending on the compound, we found a static field distribution below the Néel temperature. We discuss the implications of this result on the magnetic properties of the various materials.

Reel-to-reel MOCVD for YBCO coated conductor

A reel-to reel MOCVD system has been developed for a fast and economic process to get high quality YBCO coated conductor tapes. Several oxides buffer layer sequences (based on YSZ, CeO/sub 2/, Y/sub 2/O/sub 3/...) were then grown epitaxially on two kinds of tape. Effective stacking for MOCVD reported here are YSZ/Y/sub 2/O/sub 3//YBCO and YSZ/CeO/sub 2//YBCO, respectively, on oxidized Ni RABiTS microalloys (0,1% W) and on SS/YSZ (IBAD) tapes. First, EBSD and AFM techniques have been used to characterize the substrates. Superconducting YBCO films were fully characterized by SEM, XRD and AC susceptibility measurements Epitaxial growth was depicted with a strong in-plane and out-of plane texture: FWHM of YBCO /spl phi/-scans were 8.5/spl deg/ on Ni tape and 9/spl deg/ on IBAD Hastelloy tapes. High performances were also measured by AC susceptibility, with J/sub c/ values of 0.6 MA/cm/sup 2/ at 77 K (0 T) on Ni tape and close to 1 MA/cm/sup 2/ on IBAD substrates, with a sharp transition (/spl Delta/T/sub c/ < 0.4 K) and a T/sub c/ onset at 90 K. Deposition of 2 meter buffered YSZ and CeO/sub 2//YBCO films have also been performed showing good in-plane alignment (/spl phi/-scan YSZ: FWHM = 11/spl deg/) with a tape velocity set at 4 m/h.

Magnesium oxide nanowires synthesized by pulsed liquid-injection metal organic chemical vapor deposition

Vertically aligned MgO nanowires were epitaxially grown at 600 °C on Au-coated MgO (001) substrates by metal organic chemical vapor deposition using Mg(tmhd)2 precursor. Discrete Au particles were embedded in the wires and distributed along the central axis. Scanning and transmission electron microscopy show that the orientation, diameter, and length of the wires strongly depend on the processing conditions such as oxygen partial pressure and reactive species flow rate as well as the starting Au thickness. Diameters down to 15–20 nm were obtained. The growth can be switched from vertical to horizontal wires by decreasing the period at which reactants are supplied.

Electron Back Scattering Diffraction (EBSD) as a Tool for the Investigation of 3C-SiC Nucleation and Growth on 6H or 4H

In this paper, we present the use of Electron Back Scattering Diffraction (EBSD) as a tool for the investigation and monitoring of 3C-SiC (β) nucleation and growth on 6Hor 4H-SiC (α) (0001) nominal surfaces. Experiments were performed under silicon rich gas phase, for temperatures varying from 1900 to 2100 °C and different supersaturations. For each condition, phase and orientation maps are imaged from the EBSD technique. Results are presented in terms of α-SiC/β-SiC ratios and β(1)/β(2) ratios where β(1) and β(2) are related to the two possible orientations for the 3C-SiC to epitaxially grow on the 4H-SiC (0001) nominal surface. We demonstrate that, depending on the experimental conditions, it is possible to control first the formation of 3C rather than 4H-SiC and to control a 3C orientation rather than the other and thus to limit the DPB density.

New YBCO superconducting wires obtained from narrow textured tubes

A conductor wire consisting of Ag/YBCO/LZO/Ni?5?at.%W RABITS has been processed as a cylindrical tube of millimetre diameter. Textured layers of LZO and YBCO were grown by chemical methods. Electron backscattered diffraction was used to characterize the texture of the LZO surface which was good enough to be used as a template for YBCO deposition. The YBCO layer was textured and had a superconducting transition at 88?K. However, the quality of both LZO and YBCO must be improved to consider practical applications in electrical current transport. On the other hand, this work demonstrates that epitaxial growth of oxides on a curved surface by using chemical methods is possible.

Growth and Characterization of Thick Polycrystalline AlN Layers by HTCVD

Thick polycrystalline AlN layers were grown at low pressure using high temperature chemical vapor deposition (HTCVD). The experimental setup consists of a graphite susceptor heated by an induction coil surrounding a vertical cold wall reactor. The reactants used were ammonia (NH(3)) and aluminum chloride (AlCl(x)) species formed in situ via chlorine (Cl(2)) reaction with high purity aluminum wire. AlN films were deposited on a 55 mm diameter graphite susceptor between 1200 and 1600 degrees C. AlN layers have been characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and electron backscattered diffraction. The influence of temperature on growth rate, surface morphology, grain size, and crystalline structure is presented. Growth rates of up to 230 mu m/h have been reached. A nonpolar preferred orientation of AlN films is stabilized at a higher temperature. The potential of investigation in this new range of experimental conditions, i.e., high temperature and high growth rate, as well as deposition of nonpolar AlN crystals, is very promising for epitaxial growth and extends the field of applications

Growth and structural characterization of YMnO3 thin films grown by pulsed liquid injection MOCVD on Si and SrTiO3 substrates

Optimum parameters for the growing of YMnO3 films by pulsed liquid injection metalorganic chemical vapor deposition have been studied. Si substrates were used for the optimization of the deposition process. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that polycrystalline single phase YMnO3 films can be obtained for an optimal ratio of Y and Mn on the injected solution and either amorphous, metastable orthorhombic, and/or hexagonal YMnO3 phases can be obtained depending on the deposition temperature and precursors ratio. In a second stage, YMnO3 films were grown on SrTiO3 substrates. Pure epitaxial orthorhombic YMnO3 phase was confirmed by XRD. The films microstructure, characterized by scanning electron microscopy and TEM, shows a columnar growth. Each columnar grain grows epitaxially with three possible orientations.

How to Grow 3C-SiC Single Domain on α-SiC(0001) by Vapor-Liquid-Solid Mechanism

We report on the heteroepitaxial growth of 3C-SiC layers by Vapor-Liquid-Solid (VLS) mechanism on various α-SiC substrates, namely on- and off-axis for both 4H and 6H-SiC(0001), Si and C faces. The Si-Ge melts, which Si content was varied from 25 to 50 at%, were fed by 3 sccm of propane. The growth temperature was varied from 1200 to 1600°C. It was found that singledomain 3C-SiC layers can be obtained on 6H-SiC off and on-axis and 4H-SiC on-axis, while the other types of substrate gave twinned 3C-SiC material. As a general rule, one has to increase temperature when decreasing the Si content of the melt in order to avoid DPB formation. It was also found that twinned 3C-SiC layers form at low temperature while homoepitaxy is achieved at high temperature.