An electrochemical study of copper diffusion in non-stoichiometric copper sulphide

The diffusion of copper in hexagonal chalcocite (Cu₂S) is not very well-known. In this work electrochemical cells with natural polycrystalline chalcocite working electrodes have been studied by electrochemical impedance spectroscopy (EIS) over a large frequency range at equilibrium potential. To study this phenomenon between 120 and 160°C a solid electrolyte RbCu₄Cl₅ has been used. The impedance spectra present two distinct regions. At high frequencies the general shape of the diagram in the Nyquist plane is a depleted are of a circle. Changes in electrolyte resistance, interfacial capacitance and transfer resistance have been studied as a function of temperature. At low frequencies, a diffusion impedance is observed attributed to the mobility of copper vacancies. Diffusion coefficients, with an activation energy of 1.7 eV, have been deduced from the impedance diagrams (4.7·10⁻⁵cm² s⁻¹ at 130°C). These results are compared with those obtained with orthorhombic chalcocite between 30 and 60 °C by EIS and using an electrochemical cell with a cupric liquid electrolyte. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Successive phase transitions in the highly ordered complex perovskite PbLu1/2Nb1/2O3

The structural phase transitions and the electrical behaviour of the complex perovskite PbLu_1/2Nb_1/2O₃ have been investigated using X-ray powder diffraction, dielectric constant measurements, differential scanning calorimetry and measurement of the polarisation as a function of applied electric field. The high-temperature paraelectric phase is highly ordered. A first-order paraelectric-antiferroelectric phase transition occurs at 270°C and an antiferroelectric-ferroelectric phase transition, characterised by dispersion in the curves of dielectric constant as a function of temperature, occurs at ≈ 30°C. The antiferroelectric phase is isostructural with the orthorhombic form of PbYb1/₂Nb1/₂O₃. The low-temperature ferroelectric phase also has an orthorhombic crystal structure.     

Investigation of the high-temperature phase transition of PHB/3-CI-PHB copolyester by means of deuteron NMR

The change of molecular mobility during the phase transition of PHB/3-C1-PHB copolymer (5:1) from the orthorhombic modification II into an orthorhombic modification which is close to hexagonal chain packing was investigated by means of ²H-NMR. It is shown that a flipping of the benzene rings takes place at temperatures far below the transition temperature. The phase transition starts after more than 60% of the benzene rings are flipping. Even in the newly formed phase close to the hexagonal chain packing, at temperatures up to 300°C, free rotation of the benzene rings can be excluded.     

Direct evidence for the decomposition of YBa2Cu3O7−x into orthorhombic and tetragonal phases at 200°C

The variations of room temperature resistivity (ρ ₃₀₀), powder a.c. susceptibility (χ′) and x-ray diffraction (XRD) line profiles of (013), (103) and (110); (003), (010) and (100) reflections in the step scanning mode were investigated on single phase samples of Y₁Ba₂Cu₃O_7−x (YBCO), as a function of ageing at 200°C up to 860 h. The average off-stoichiometryx=0.22 remained constant throughout the experiment.ρ ₃₀₀ showed a monotonic increase while the superconducting transition curves showed increasing sharpening with ageing time. XRD of the reflections investigated developed satellite structures or additional intensities at 2θ values different from those of main reflections. The results are consistent with the prediction and earlier observation of the decomposition of the off-stoichiometric YBCO into oxygen ordered orthorhombic and vacancy ordered tetragonal phases.     

Crystal structure of low-temperature rubidium monoaluminate modification

Rubidium monoaluminate RbAlO₂ has been studied by powder neutron diffraction and differential scanning calorimetry. A structural phase transition has been found at 1050°C. It is shown that the low-temperature modification RbAlO2 has the orthorhombic structure (Pnma, a = 0.5570(2) nm, b = 1.1189(4) nm, c = 1.5818(6) nm) close to the crystal structures of low-temperature modifications RbGaO₂ and RbFeO₂, not a face-centered cubic structure, as assumed previously.     

Étude par spectroscopie auger,diffraction d'électrons lents et rapides des premiers stades de la sulfuration de la face (100) de NiO par H2S

A structural study of the different stages during NiO(100) sulphurization by H₂S was carried out by RHEED, LEED and AES. On exposure to H₂S (PH₂S < 10^?5 Torr) The “clean” surface, obtained by UHV cleavage, was found to react with H₂S to produce islands of Ni(100) covered with an ordered c(2 × 2) S structure up to 300°C. Growth of Ni₃S₂ islands occurs on increasing the temperature and the exposure to H₂S.     

Low-temperature synthesis of morphology controlled metastable hexagonal molybdenum trioxide (MoO3)

We demonstrate a simple low-temperature chemical method to produce metastable hexagonal MoO₃ phase nanorods. The structure, chemical purity, thermal stability and optical properties of hexagonal MoO₃ are reported. Our results provide a more direct method to produce high quality and stable hexagonal MoO₃, which exhibits phase stability up to 400 ^°C at which point an irreversible phase transition occurs to form orthorhombic MoO₃.     

Effect of vapors of the simplest alcohols on the structure and properties of YBa2Cu3Oy compounds

With low-temperature treatments (200–300°C), the effect of alcohol vapors on the structural and electrophysical properties of YBa₂Cu₃O_y (123) having different oxygen content has been studied. It has been established that CuO and YBa₂Cu₃O_y are catalysts for the dehydration of alcohols to form water and aldehydes. Similar to the action of water vapors, the treatment in vapors of the simplest alcohols leads to hydration of 123 and its transformation into the pseudo-124 phase. In this case, planar stacking faults are formed to be centres of pinning and can improve the critical characteristics of high-temperature superconductors (HTSС) in magnetic fields. The influence of the alcohol vapors is more ‘mild’ as compared to the treatment in water vapors. At t?=?300°C, the alcohols are reducing agents of copper, followed by the decomposition of 123 into simple compounds.     

Preparation,structural characterization and conductivity of LiZr2(PO4)3

Amorphous LiZr₂(PO₄)₃ has been prepared at room temperature starting from aqueous solutions of ZrOCl₂, H₃PO₄, and LiOH and then crystallized by heating at temperatures between 600 and 900°C. The material obtained at 900°C has been characterized by X-ray powder diffractometry, DSC analysis, and ac conductivity. It is monoclinic from 20 up to about 300°C and orthorhombic at higher temperatures. A change in the activation energy for conduction (from 0.79 to 0.43 eV) and a weak endothermic effect (0.9–1.7 cal/g) are associated with the phase transition. The ac conductivity of sintered pellets is, on average, 7×10⁻⁴ S cm⁻¹ at 300°C.     

Structure and superconducting properties of nonstoichiometric YBa2Cu3Oy annealed at 300°C in a dry argon atmosphere

Using the full-profile X-ray analysis of diffraction lines, the behavior of YBa₂Cu₃O _y nonstoichiometric ceramics upon annealing at 300°C in a dry inert atmosphere has been investigated. It has been established that, in the YBa₂Cu₃O_6.3 sample with the initial tetragonal structure, this treatment leads to nucleation of an orthophase with a low degree of rhombicity, which becomes prevailing after annealing for 56 h. In this case, superconductivity with an initial temperature of the transition of ∼50 K occurs in the sample. In the YBa₂Cu₃O_6.7 sample with the initial orthorhombic structure, the orthophase with a somewhat enhanced rhombicity arises during annealing and becomes prevailing after annealing for 56 h; in this case, T _c of the system grows from 57 to 63 K. Data of the magnetic measurements for both cases can be interpreted as the presence of two orthorhombic phases with different temperatures of transition to the superconducting state in the samples after annealing.     

Effects of annealing temperature on GO–Cu2O composite films grown by electrochemical deposition for PEC photoelectrode

In this work, graphene oxide–cuprous oxide (GO–Cu₂O) composite films were grown on fluorine-doped tin oxide substrates by electrochemical deposition. We investigated the effects of the annealing temperature on the morphological, structural, optical and photoelectrochemical (PEC) properties of GO–Cu₂O composite films. As a result, our work shows that while GO–Cu₂O composite films exhibit the highest XRD (111) peak intensity at 300 °C sample, the highest photocurrent density value obtained was −4.75 mA/cm² at 200 °C sample (using 0.17 V versus a reversible hydrogen electrode (RHE)). In addition, a reduction reaction at 300 °C sample was observed using XPS analysis from the shift in the O1s peak in addition to a weaker O1s peak intensity.     

X-ray analysis of the actual structure of yttrium orthoborate YBO3

Detailed X-ray analysis of variations in the structure of yttrium orthoborate in the process of successive high-temperature isothermal anneals of an originally amorphous precursor state is performed. It is established that the diffraction reflex intensity distribution of YBO₃ measured at room temperature, obtained in the initial stages of crystallization, corresponds to the known low-temperature vaterite phase with the space group (sp. gr.) P6₃/m and, after a series of high-temperature anneals, it transforms into a distribution known for the vaterite modification with the sp. gr. P6₃/mmc and the same lattice parameters. This result is explained on the basis of the sphericity of X-ray waves and is connected with the transformation of the crystallites from a spherical shape upon low-temperature anneals to a dumbbell shape upon high-temperature anneals. As a result of in situ experiments conducted at 1250°C, it was established that the initial low-temperature hexagonal vaterite cell transforms above 1000°C into a monoclinic cell.

     

Structural phase transition in YBa2Cu3O7−δ: the role of dimensionality for high temperature superconductivity

We have performed detailed high temperature X-ray diffraction to study the nature of the structural phase of YBa₂Cu₃O_7−δ. The results indicate the existence of a reversible orthorhombic to tetragonal phase transition at a temperature close to 750°C. If the high temperature tetragonal phase is quenched-in at low temperatures the critical superconducting temperature is considerably reduced from 92.5 K. This suggests that the one dimensional CuO chains present in the orthorhombic structure are necessary for high temperature superconductivity.     

Mössbauer study of the microstructure during annealing of new microcrystalline Fe73.1Cu1.2Nb3.2Si12.5B10.0 alloy

We have studied the change of the microstructure of a new alloy of Fe_73.1Cu_1.2Nb_3.2Si_12.5B_10.0 during annealing process by using Mössbauer effect. The measured results show that the specimen annealed at 480°C contains a small amount of the precipitate of an ordered phase Fe_75+ySi_25?y and that at 540°C consist of an ordered phase Fe_75+ySi_25?y, as matrix, and an amorphous phase which is considered as a kind of grain boundaries.     

Structural and electrical characterization of the NASICON-type Li2FeZr(PO4)3 and Li2FeTi(PO4)3 compounds

In order to develop new electrolytes for all-solid-state rocking chair lithium batteries, the NASICON-type compounds Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ were investigated by powder X-ray diffraction technique and impedance spectroscopy. Li₂FeZr(PO₄)₃ is orthorhombic Pbna (a=8.706(3), b=8.786(2), c=12.220(5) Å) and Li₂FeTi(PO₄)₃ is orthorhombic Pbca (a=8.557(3), b=8.624(3), c=23.919(6) Å). They show no phase transitions from RT to 800 °C. In the same temperature range logσT vs. 1/T show no slope variations. The activation energies for the ionic conductivity were 0.62 and 0.64 eV for Li₂FeTi(PO₄)₃ and Li₂FeTi(PO₄)₃, respectively. In order to better evaluate the present results they were compared with those of α and β-LiZr₂(PO₄)₃ phases, which were also prepared and characterised. A change of activation energy from 0.47 eV to 1.03 eV was observed in the case of β phase, at about 300 °C; attributed to the β (orthorhombic) ? β′ (monoclinic) phase transition. In the α phase the activation energy 0.47 eV in the temperature range 150 – 850 °C. The Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ compounds can be interesting for applications as solid electrolytes in high temperature (>300 °C) lithium batteries.     

Effect of mechanical activation on the phase composition,structure, and polymorphism of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O6 + δ oxide

The mechanical activation of YBa₂Cu₃O_6+δ powders is studied comprehensively using thermal analysis, scanning electron microscopy, high-temperature x-ray diffraction, and x-ray photoelectron spectroscopy. It is shown that the size of the coherent scattering regions is substantially reduced by intensive grinding (from 83 to 17 nm) and the material transforms to a nanostructured state. An increase in the reactivity of the material is found, which shows up as an enhanced accumulation of intercalated water in its structure. At the same time, no significant increase in the volume of impurity phases in the powder or in the influence of impurities on the crystal lattice parameters was observed. High-temperature x-ray studies of the lattice parameters of the YBa₂Cu₃O_6+δ phase reveal a jump (at temperatures of 430–630°C) and a decrease (above 630°C) in the parameter c when the powder is mechanically activated. A significant reduction in the temperature of the transition from the orthorhombic to the tetragonal structural modification is observed. These effects are explained by a change in the charge states of oxygen ions belonging to the base plane of the YBa₂Cu₃O_6+δ oxide during mechanical activation.     

Phase transition in nanostructured LaMnO<Subscript>3</Subscript>

Nanocrystalline LaMnO₃ samples have been synthesized by the coprecipitation of lanthanum and manganese salt solutions followed by annealing at 800°C in air and then in an argon flow. With the use of X-ray and neutron diffraction analysis, it has been found that the resulting samples had an orthorhombic crystal structure (space group Pbnm). In the nanocrystalline LaMnO₃ samples, an order-disorder phase transition from the low-temperature phase to the high-temperature phase occurs at (220 ± 10)°C, which is much lower than the value of 477°C for bulk LaMnO₃. The phase transition is due to the removal of the Jahn-Teller distortion of the Mn³⁺O₆ oxygen octahedrons and is accompanied by a decrease in the unit cell volume of lanthanum manganite over a narrow temperature range.     

Microstructure,creep, and tensile behaviour of a Ti–15Al–33Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep and tensile deformation behaviour of a Ti–15Al–33Nb (at.%) alloy was studied. Monolithic sheet material was produced through conventional thermomechanical processing techniques comprising non-isothermal forging and pack rolling. Electron microscopy studies showed that depending on the heat-treatment schedule, this alloy may contain three constituent phases including:?β?(disordered body-centred cubic), α₂ (ordered hexagonal close-packed based on Ti₃Al) and O (ordered orthorhombic based on Ti₂AlNb). Heat treatments at all temperatures above 990°C, followed by water quenching, resulted in fully-β microstructures. Below 990°C, Widmanstätten O-phase or α₂-phase precipitated within the?β?grains. The fine-grained as-processed microstructure, which exhibited 90?vol.% β-phase, exhibited excellent strength (UTS?=?916?MPa) and ductility (?_f>12%). After heat treatment, greater volume fractions of the orthorhombic phase precipitated and resulted in lower ? _f values with UTS values ranging between 836–920?MPa. However, RT elongations of more than 2% were recorded for microstructures containing up to 63?vol.% O-phase. Specimens subjected to 650°C tensile experiments tended to exhibit lower strength values while maintaining higher elongation-to-failure. Tensile creep tests were conducted in the temperature range 650–710°C and stress range 49–275?MPa. The measured creep exponents and activation energies suggested that grain boundary sliding operates at intermediate stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behaviour are discussed in comparison to a Ti–12Al–38Nb O?+?β alloy.     

Field-ion microscope observation of ordering in the near-surface of Ni4Mo alloy using in situ annealing

Field-ion microscope observations were made on Ni₄Mo alloy to investigate surface effects on ordering during in situ annealing after quenching from high temperature α phase region. Surface ordering occurred preferentially at {200}_FCC and/or {111}_FCC facets (at 880–835°C). Surface layers with low degree of order ranging from ten to a few tens ångströms in thickness were formed (at 760–720°C). The degree of order in relatively large ordered domains decreased near the surface (at 825–810°C). In the interior of the specimens at a range of depths 100–300 Å from the surface, ordering behavior was essentially the same as that for bulk specimens (at 825–720°C). Homogeneously nucleated ordered domains coexisted with relatively large heterogeneously nucleated ones in the interior of the specimen (at 670°C). Certain relatively low index (FCC) planes faceted (at 880-720°C). Faceting originating from β structure (ordered structure) occurred on the surface (at 670°C). In addition, in situ formed deformation twin was observed (at 810–825°C).