Optimization of the electrophoretic deposition of YBa2Cu3O7−x superconducting large area coatings

Large area coatings (>10cm²) of the high temperature superconductor YBa₂Cu₃O_7–x (x = 0.1 – 0.2) (YBCO) have been prepared by scale up an electrophoretic deposition technique using silver sheets and Si-wafers coated with Ag or Au as substrates. Several parameters, like the kind of the solvent, the applied voltage, the distance between the electrodes, the initial concentration of the suspension and the temperature during the electrophoresis were investigated in order to attain high deposition rates, as well as uniform YBCO coatings with the proper stoichiometry. To obtain a strongly adherent and dense coating a subsequent appropriate sintering and annealing procedure has been developed. The coatings obtained were characterized for their stoichiometry and superconducting properties by X-ray diffraction (XRD), Raman spectroscopy and magnetic measurements. The homogeneity and thickness of the films and the average grain size of the deposited particles have been investigated by optical and scanning electron microscopy (SEM).     

Effect of substrate materials on the superconducting properties of electrophoretically deposited YBa2Cu3O7−x coatings

Coatings of the high-temperature superconductor YBa₂Cu₃O_7–x (x = 0.1–0.2) (YBCO), were deposited on commercial Ni,Ti and stainless steel substrates, by an electrophoretic deposition technique. Sintering and annealing procedures were followed in order to get strongly adherent superconducting films. The suitability of the used substrates was tested by measuring the stoichiometry and the superconducting properties of the deposited coatings by X-ray diffraction analysis (XRD), Raman spectroscopy and magnetization measurements with a superconducting quantum interference device (SQUID).     

Scanning Auger analysis for the clarification of oxidation and interdiffusion processes during laser ablation of YBa2Cu3O7-x on alloy/oxide substrates

Scanning Auger investigations are reported on layer systems consisting of a metallic substrate, an oxide buffer layer and the YBa₂Cu₃O_7–x (YBCO) high temperature superconductor. By bending the samples under UHV conditions the internal interfaces have been made accessible for the Auger analysis. The examination of the interfaces have shown that an oxidation of the substrate alloy had taken place during the deposition of the YBCO in spite of the buffer layer already being present. Additionally, segregated sulphur has been found on the substrate/buffer interface, which can lead to a loss of adhesion of the layer system. By line scan investigations on samples prepared by ball cratering a carbon enriched zone of approximately 50 nm thickness has been detected at the interface of buffer and YBCO.     

Stability and transport properties of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>

La₂Mo₂O₉ samples were prepared from freeze-dried powder precursors and characterized by XRD, TG/DTA, SEM, electrical and electrochemical measurements. Pellets with different density were obtained by sintering at temperatures between 900 and 1100 °C to obtain nearly dense samples with grain sizes in the range 1–8 m. The electrical conductivity was measured using impedance spectroscopy. The capacitance and relaxation frequencies of the main contributions to the spectra were used to ascribe the contributions of grain interiors and internal interfaces, and their temperature dependence. A coulometric titration technique was used to evaluate the change of oxygen stoichiometry under moderately reducing conditions, and to estimate the stability limits under strongly reducing conditions. An ion-blocking method was used to evaluate the onset of n-type conductivity, and a combination of these results with total conductivity measurements was used to obtain the ionic transport number. A combination of oxygen stoichiometry changes and ion-blocking results was used to obtain estimates of mobility.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

The effects of gamma radiation and different substitution on the spectro-mechanical properties of La-ferrite polycrystals

Various samples of different substituted La-ferrites (La_{1–x 1}A_{x 1} (Fe_{1–x 2}B_{x 2})O₃) have been prepared by the usual ceramic and high temperature sintering technique. On these specimens several measurements were carried out, namely, Cu–K X-ray analysis, true density, bulk density, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), IR absorption spectra, electronic absorption spectra and crushing strength before and after -radiation dose (4.5×10⁷ rad). Results reported were promising, interpreted and discussed in detail on the basis of the interaction of -radiation and different dopant cations with La-ferrite lattices. Finally, a suggested lattice structure symbol was put forward.     

Silver incorporation into cathodes for solid oxide fuel cells operating at intermediate temperature

Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La_0.65Sr_0.3MnO_3– perovskite phase is already formed at 900 °C. The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen.     

Interface analysis of the system Si/YBa2Cu3O7?x

Summary The influence of different preparation conditions and substrate surface orientations on the superconducting properties of thin YBa₂Cu₃O_7–x (YBCO) films on silicon was studied. Comparative electrical and surface spectroscopic measurements were performed. SAM and SIMS depth profile analysis show an enrichment of barium at the interface between the superconductor and silicon for samples with T_c<76 K. Comparison with XPS data obtained for thin silicon films on YBCO indicates the formation of barium and yttrium silicates at the interface under these conditions.     

Electrophysical Properties of a Poly(ethylene terephthalate) Film Modified in a Cyclohexane Plasma

The structure of a thin layer applied on a poly(ethylene terephthalate) (PET) film using ion-assisted chemical vapor deposition of cyclohexane was studied by electron spectroscopy for chemical analysis and IR spectroscopy. It was found that the film was composed of linear (–₂–)_n chains bearing six-membered cyclohexane rings, including those containing carbonyl groups as substituents, and carbon chains free of H atoms. The plasma-synthesized cyclohexane film was found to be semiconducting. The deposition of a film 10–120 nm in thickness on the surface of PET 30 m in thickness resulted in an increase in the bulk conductivity over the temperature range 20–200°C and in a considerable increase in the electric strength.     

Synthesis,spectroscopy and electrochemistry of mixed-ligand complexes of platinum(II) and palladium(II)

Mixed-ligand complexes [MX₂(MBPY)] (M = Pd^II or Pt^II; X = Cl^–, I^–, N^– ₃ or NO₂ ^–; MBPY = 4,4-dimethyl-2,2-bipyridine) have been prepared and characterised by elemental analyses, conductivity measurements, i.r., electronic absorption and ¹H-n.m.r. spectroscopic techniques. The cyclic voltammograms of these complexes suggest the involvement of the metal d orbital in the one-electron oxidation process and the * orbital of the MBPY in the one-electron reduction process. [Pt(MBPY)(N₃)₂] shows solution-state luminescence at room temperature.     

Quantitative depth profile analysis of high-Tc-superconductors with sputtered neutral mass spectrometry (SNMS)

Summary SNMS, a new surface sensitive as well as a bulk analysis technique, has been applied to the quantitative compositional depth profiling analysis of conductive and insulating thin films. Its performance is demonstrated by investigation of YBCO superconductor bulk analysis and sputter deposited layers on single crystalline substrates of SrTiO₃ and Al₂O₃. A stable stoichiometric volume has been observed in bulk material and thin films respectively. Inside a range of 500 nm from the surface, the composition deviates strongly from the stoichiometric concentration of Y and Ba. Comparative measurements with a freshly cleaved piece of YBCO bulk material which was immediately loaded into the ultrahigh vacuum analysis chamber show constant and stoichiometric concentration of all faced elements (Y, Ba, Cu, O) from the beginning of surface etching. The ratio of Y to Ba and Cu varied by less than 2%, the ratio Cu to oxygen by less than 5% from the nominal composition 1-2-3 (4).

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Quantitative Tiefenprofilanalyse von HT_c-Supraleitern mit SNMS

     

Short-Chain Di-Ureasil Ormolytes Doped with Potassium Triflate: Phase Diagram and Conductivity Behavior

Di-urea cross-linked poly(oxyethylene)/siloxane hybrids, synthesized by the sol-gel process and containing a wide concentration range of potassium triflate, KCF₃SO₃, have been analyzed by x-ray diffraction and differential scanning calorimetry. The pseudo-phase diagram proposed has been taken into account in the interpretation of the complex impedance measurements. The xerogels prepared are obtained as transparent, thin monoliths. At room temperature the highest conductivity found was 2 × 10^{–6 –1} cm^–1.     

Oxidation of Methanol on Pt,Pt–Ru,and Pt–Ru–Mo Electrocatalysts Dispersed in Polyaniline: An in situ Infrared Reflectance Spectroscopy Study

The electrochemical oxidation of methanol was investigated on a Pt–Ru–Mo catalyst with an in situ infrared reflectance spectroscopy. The electrocatalysts were prepared by an electrochemical deposition and dispersed in a conducting three-dimensional matrix of polyaniline (PAni). We observed that CO₂ is produced from methanol oxidation at 350 mV vs. RHE on PAni/Pt–Ru–Mo, which is 100 mV less negative than on PAni/Pt–Ru and 200 mV less than on PAni/Pt. The results suggest that Pt–Ru–Mo is less sensitive to CO_ADS poisoning than Pt–Ru and much more sensitive than Pt. Large differences are observed concerning the average wavenumber of _ADS between Pt–Ru–Mo, Pt–Ru, and Pt.     

Effects of sintering additives on the mixed transport properties of ceria-based materials under reducing conditions

Commercial Ce_0.8Gd_0.2O_2– nanopowders and alternative precursors synthesized by a freeze-drying method were used to obtain samples with and without Co addition as a sintering agent. Two percent Co-doped samples were sintered at 1150 °C and 1500 °C and undoped samples were sintered at 1500 °C or 1600 °C to obtain samples with relative densities in the range 92–94%. The total conductivity and the relative roles of bulk and grain boundary conductivity were studied by impedance spectroscopy. These results demonstrated that additions of Co play a very significant effect on the grain boundary behaviour, which is spoiled when the sintering temperature is excessive. Significant differences in grain boundary behaviour were also found between samples prepared from different precursor powders. The electronic conductivity of these materials was evaluated by an ion-blocking method and revealed that samples containing Co additions as a sintering additive possess somewhat lower n-type conductivity under identical conditions of temperature and oxygen partial pressure. The differences tend to vanish when these Co-doped samples are fired at high temperatures (1500 °C).Abbreviations CGO Ce_0.8Gd_0.2O_1.9 - FD freeze-dried - RH rhodiaPresented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

Mechanism of silicon film deposition in the RF plasma reduction of silicon tetrachloride

Plasma-chemical reduction of SiCl₄ in mixtures with H₂ and Ar has been studied by optical emission spectroscopy (OES) and laser interferometry techniques. It has been found that the Ar:H₂ ratio strongly affects the plasma composition as well as the deposition (r _D) and etch (r _E) rates of Si: H, Cl films and that the electron impact dissociation is the most important channel for the production of SiCl_x species, which are the precursors of the film growth. Chemisorption of SiCl_x and the reactive surface reaction SiCl_x+H–SiCl_(x–1)0+HCl are important steps in the deposition process. The suggested deposition model givesr _D [SiCl_x][H], in agreement with the experimental data. Etching of Si: H, Cl films occurs at high Ar: H₂ ratio when Cl atoms in the gas phase become appreciable and increases with increasing Cl concentration. The etch rate is controlled by the Cl atom chemisorption step.     

Röntgenographische Untersuchungen in den Systemen Chrom-Silicium (Germanium)-Arsen

Zusammenfassung Durch Sintern und Schmelzen hergestellte Proben in den Dreistoffen: Cr–Si–As und Cr–Ge–As werden nach Homogenisieren röntgenographisch untersucht. Die Gleichgewichte werden für einen isothermen Schmitt ermittelt. Die Phase Cr₄As ist sowohl mit Cr₃Si wie mit Cr₃Ge lückenlos mischbar. In den T1-Phasen erfolgt ein merklicher Si/As- bzw. Ge/As-Austausch. Eine Stapelvariante, die in der Nachbarschaft des Defekt-Digermanids Cr₁₁Ge₁₉ auftritt, nämlich Cr₃₇Ge₅₉As₄ wird beobachtet und als isotyp mit Rh₁₀Ge₁₇ erkannt.

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Samples prepared by sintering and melting of the respective powder mixtures Cr–Si–As and Cr–Ge–As have been examined by X-rays after homogenization; the phase equilibria for an isothermic section were established. Cr₄As is forming solid solutions with the corresponding phases Cr₃Si and Cr₃Ge. An extended Si/As and Ge/As-substitution takes place within the T1-phase. In the vicinity of the defect-digermanide Cr₁₁Ge₁₉ a new stacking type of formula Cr₃₇Ge₅₉As₄ has been observed; the crystal structure has been recognized as to be that of Rh₁₀Ga₁₇.

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Mit 3 Abbildungen     

Synthesis of Calcium Phosphates by PO(OH)x(OBut)3−x and CaO2C2H4

In this work, an alkoxide solution route to synthesize Ca phosphates was developed. For the precursors, a CaO₂C₂H₄ solution was prepared by dissolving Ca metal powder into ethylene glycol, and a PO(OH)_x(OBut)_3–x solution was prepared by dissolving P₂O₅ inton -butanol under reflux conditions. In order to obtain a mixed solution of the two precursors, acetic acid was used as an additive. The experimental results show that (1) -2CaO · P₂O₅, -3CaO · P₂O₅, and hydroxyapatite can be easily synthesized by converting the corresponding mixed solutions to powder products in a hot plate, and calcining the as prepared products at 1100°C; (2) acetic acid behaves as a good agent for controlling the reactions between the two precursors by modifying the CaO₂C₂H₄ species in solution and decreasing the reactivity of the PO(OH)_x(OBut)_3–x species.     

Crystal structure and high-pressure properties of γ-Mo2N determined by neutron powder diffraction and X-ray diffraction

We prepared samples of cubic γ-MoN_x (x∼0.5) by high-pressure-high-temperature synthesis. N atom site occupancies within the defect rock salt structure were determined from time-of-flight neutron diffraction and powder X-ray diffraction data by Rietveld analysis. The results show that N atoms occupy only octahedral sites within the structure. The semi-metallic compound is a superconductor, with determined by SQUID magnetometry. The compressibility of the material was determined by synchrotron X-ray diffraction measurements at high pressure in the diamond anvil cell. The vibrational density of states was studied by Raman scattering spectroscopy.     

Structural studies of diamine complexes of cobalt(II) and nickel(II) thiocyanates and selenocyanates

Summary Complexes of the types ML₂X₂ [M = cobalt(II) or nickel(II); L = hydrazine, ethylenediamine (en) or o-phenylenediamine (opd) and X = SCN^– or SeCN^–] and NiL₂(NCS)₂MCl₂ [M = cadmium(II) or mercury(II)] have been prepared and characterised by elemental analysis, molar conductance, molecular weight determination, magnetic susceptibility, electronic and i.r. spectral measurements.     

Outer-sphere electron-transfer reactions with the participation of platinum(IV) haloammines

The kinetics of some outer-sphere electron-transfer reactions with the participation of the complexes [Pt(NH₃)_nX_6–n]² (n=6–0, X=Cl^–, Br^–) and dipyridyl complexes of Os(II), Ru(II), Ir(III), and Cr(II) have been investigated by means of luminescence-quenching measurements and flash photolysis. Estimates of the values of the Pt(IV)/Pt(III) one-electron potential and the change in the free energy of activation of electron self-exchange processes of the type Pt(IV) Pt(III) have been obtained on the basis of an analysis of the dependence of the rate constant on the change in the free energy accompanying the electrontransfer process.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 455–462, July–August, 1990.     

Influence of the Sol-Gel Method on a NiO/Al2O3 Catalyst for CH4/O2 to Syngas Reaction

NiO/-Al₂O₃ catalysts were prepared by the impregnation and the sol-gel method. The sintering and loss of nickel, carbon deposition and 80 h life-test for POM reaction over the catalyst were investigated by a flow-reactor and by XPS, TG, EXAFS and AAS analysis. The results indicated that the sol-gel method could enhance the ability to suppress carbon deposition and to resist nickel sintering and loss over nickel-based catalysts.