Chemical diffusion in non-stoichiometric cuprous oxide

The kinetics and thermodynamics of point defect diffusion in non-stoichiometric cuprous oxide, Cu_2−yO, has been studied as a function of temperature (1173–1373 K) and oxygen pressure (10²–7×10⁴ Pa) using microthermogravimetric reequilibration technique. It has been shown that the defect diffusion coefficient of cation vacancies, constituting the predominant point defects in this oxide does not change with their concentration, clearly indicating that these defects do not interact and are randomly distributed in the crystal lattice. Consequently, cation vacancy diffusion coefficient, D_V, being the direct measure of defect mobility, can be described by the following empirical equation:

     

Grain boundary diffusion metals versus non-stoichiometric compounds

This paper compares the current knowledge of grain boundary diffusion in three classes of materials: metals and metallic alloys, ordered intermetallic compounds, and ionic stoichiometric and non-stoichiometric compounds. Along with fundamentals, the recent progress in grain boundary diffusion studies in these materials is reviewed. The differences and common features of grain boundary diffusion in these materials are pointed out, and the origin of the differences is examined. It is concluded that, despite many differences, the basic features of grain boundary diffusion in metals, intermetallics and ionics are fairly similar. Those basic features are discussed through the paper, and some prospective topics of future research in the area are suggested.     

2D compound formation during copper dissolution: An electrochemical STM study

The reversible formation of a 2D-CuI film on Cu(1 0 0) is studied by means of cyclic voltammetry in combination with electrochemical scanning tunneling microscopy.Exposing the Cu(1 0 0) electrode surface to an acidic and iodide containing electrolyte (5 mM H₂SO₄/1 mM KI) leads to the formation of a well ordered c(p × 2)-I adsorbate layer at potentials close to the onset of the anodic copper dissolution reaction. Copper dissolution starts at slightly more positive potentials preferentially at step edges in the presence of the iodide adlayer via the removal of copper material from kink sites at step edges. This increase of mobile Cu⁺ ions causes the local exceeding of the CuI solubility product (pKL = 11.3), thereby giving rise to the nucleation and growth of a laterally well ordered 2D-CuI film. Key structural motifs of the growing CuI film are closely related to the (1 1 1) plane of bulk CuI. Quite intriguing, the 2D-CuI film does not act as a passive layer. Copper dissolution proceeds even in the presence of this binary compound via an inverse step flow mechanism.     

An electrochemical determination of oxygen adsorption on copper

The adsorption of oxygen on polycrystalline and thin film copper samples was studied with the solid state electrochemical cell: Cu,Cu₂O¦7.5wt%CaO/ZrO₂¦Cu in ultrahigh vacuum. The oxygen partial pressure in the bulk of the copper sample was controlled electrochemically by applying a voltage across the cell, while the oxygen coverage at the copper free surface was monitored by Auger Electron Spectroscopy (AES). This technique has enabled us to establish much lower oxygen partial pressures at high temperatures than normally attainable in ultrahigh vacuum. In this paper we report the results of reversible oxygen adsorption isotherms on polycrystalline copper at 928, 970 and 1093 K. The results agree reasonably well with the deductions of earlier surface energy measurements and indicate a surprising degree of stability for chemisorbed oxygen on polycrystalline copper. Isosteric heats of adsorption are calculated with and without the inclusion of the earlier surface energy measurements and are compared to previous differential heats of adsorption determined calorimetrically.     

Reactively evaporated copper sulphide films

Thin films of covellite (CuS) have been prepared for the first time. The films were prepared by reactively evaporating copper and sulphur. X-Ray diffraction measurements indicate that the grains are oriented with (001) planes perpendicular to the substrate surface. Optical studies show that CuS is a semiconductor, contrary to what has been reported earlier. The films have a resistivity of ～10^?4 Ω cm and show p-type conductivity.     

An aes study of a copper-iron sulphide mineral

Synthetic bornite, Cu₅FeS⁴ has been studied by Auger electron spectroscopy. Sputtercleaned bornite shows a sulphur spectrum with three peaks at 138, 147 and 149 eV. These Auger transitions are different from those observed when sulphur is adsorbed on metal surfaces, where the peaks are at 139, 149 and 154 eV. The adsorption of oxygen on the surface of bornite at room temperature results in the formation of a layer of iron oxide and, in addition, the sulphur spectrum loses its fine structure and shows only a single peak at 148 eV. Under the influence of both the ion sputter beam and the electron beam, the surface composition of bornite shows large and rapid changes which are due mainly to movement of mobile Cu⁺ ions through the lattice, this movement being caused by surface charging effects.     

Copper diffusion in copper sulfide: a systematic study

Copper sulfide Cu_2−xS is a model mineral for chalcogenides because of the existence of a non-stoichiometric compounds series in the range Cu₂S - CuS in which properties change with x. For this reason, we have studied the influence of the mineral composition on the diffusion in this solid. Electrochemical Impedance Spectroscopy (EIS) applied to Cu_2-xS/cupric sulfate electrolyte was the main investigation technique. It enabled us to work at the equilibrium potential at which the composition is fixed and known. Changing the composition by electrochemically removing (or adding) a known amount of Cu, we were able to determine the chemical diffusion coefficient of copper in the composition range (from x=0 to 0.066). In this work, we present the results obtained in the chalcocite and djurleite phases. These results were compared to other values reported in the literature. From this systematic study we discuss various diffusion mechanisms. Our observations support that in chalcocite and djurleite Cu diffuses via a vacancy mechanism. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Oxide films of copper prepared by the oxidation of copper sulphide films

Copper sulphide films prepared by reactive evaporation, when heated in air at 500 K, oxidized to Cu₂O after a series of intermediate chemical transitions. Golden yellow coloured Cu₂O films showed a large absorption before the fundamental absorption edge. The optical band gap was found to be (2.29±0.02)eV. When these Cu₂O films were further heated they got converted to CuO and the optical band gap was found to be (2.17±0.02)eV.     

Hologram recording in cadmium and copper doped cadmium sulphide

Photoinduced changes in the optical absorption of large crystals of cadmium sulphide doped with cadmium and copper, are used to store Bragg-angle holograms by means of a He-Ne laser. The recordings are stable in the dark at room temperature and can be erased either optically or thermally. Diffraction efficiencies of over 1% at a write energy of about 1J/cm² are obtained.     

Muon diffusion in nanocrystalline copper

A systematic μSR study on nano‐Cu has demonstrated that the diffusion of μ⁺ in nanocrystalline metals is influenced by both features of the nanostructure, i.e., by the very small grain size and by the comparatively large fraction of grain boundaries. The former feature yields a size effect of the phonon‐assisted muon tunneling, but only at particle diameters below 20 nm. The latter feature, in samples with crystallite sizes above 20 nm diameter, i.e., with bulk diffusional behaviour, establishes a connection between μ⁺ diffusion coefficient and particle size: if one of these quantities is known, the other could be evaluated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nanostructured copper filaments in electrochemical deposition

In this Letter we report a novel self-organized copper electrodeposition in an ultrathin layer of CuSO4 electrolyte. The macroscopic fingering branches of the deposit consist of long copper filaments covered with periodic corrugated nanostructures. The mechanism of the nanostructure formation is explored and the origin of the significant descent of the branching rate in electrodeposition is discussed. We suggest that this growth phenomenon provides deeper insights into the role of diffusion and migration on pattern formation in electrodeposition.     

Neutron scattering study of short-range correlations and ionic diffusion in copper selenide

The paper reports the results of a neutron scattering study of Cu_{2 - d}Se {\hbox{C}}{{\hbox{u}}_{{2 - \delta }}}{\hbox{Se}} superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Se↔Cu (8c,32f) and Cu (8c)↔Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.     

A study of oxygen ion conductivity in doped non-stoichiometric oxides

In doped non-stoichiometric oxides that show oxygen ion conductivity, association commonly takes place between the dopant cations and the compensating oxygen vacancies. The activation energy thus comprises two parts, a migration enthalpy and an association enthalpy. We have determined the effects of structure and host cation type on the migration enthalpies, and the effect of dopant cation size on the association enthalpies. This we did by a variety of methods including theoretical calculations and experiment. We have further reviewed the literature in order to verify our calculations and we conclude that size terms in the association enthalpies are the most important factor in the determination of the magnitude of oxygen ion conduction.     

Investigation on self activated and copper activated zinc sulphide

The diamagnetic susceptibility of zinc sulphide decreases on heat treatment and becomes less temperature dependent. The susceptibilities or the differences in susceptibilities of the pure and activated samples do not obey Curie Law. The susceptibility follows a relation of the typeX _T =X ₀ +X ₁ e ^?E/KT in which bothX ₀ andX ₁ have the same sign.X ₀ is interpreted as true diamagnetic susceptibility of the sample and gradually increases in value with activation,X ₁ depends on the particle size and is found to decrease with the increase in particle size. This is interpreted as being due to surface electrons.“E” as measured in this way is found to be of the order of 0·01 ev. in all cases. Thus“E” seems to be the average surface trap depth of zinc sulphide. It has also been found that in zinc sulphide with low copper concentration the luminescent centres are diamagnetic; copper ions are present presumably in substitutional positions. For zinc sulphide containing high concentration of copper, the presence of paramagnetic centres are detected. It is suggested that in this case copper is present not only at the regular lattice positions acting as luminescent centre but also settles at special lattice sites like dislocation jogs.     

An ultrasonic study of ion diffusion in β-alumina

Ultrasonic attenuation in β-alumina is anisotropic and attributed to Na⁺ ion diffusion. Two relaxation processes, with activation energies 0.16 and 0.02 eV, explain the data. An attempt frequency of ? 0.6 × 10¹¹ Hz is indicated for the 0.16 eV process.     

Muon diffusion in copper bolow 2K

Transverse field SR experiments were performed on several different samples of copper in the temperature range below 2K, including isotope separated copper, Fe and Si doped polycrystalline copper and monocrystalline copper. No strong isotope dependent effect was observed. A⁶³Cu and a natural copper sample of identical purity both yield 0.16 s^–1 for the low-temperature plateau, while an increased linewidth inthe⁶⁵Cu case may be related to the strong effects of Fe impurities. Careful transverse field measurements on large single crystals at 0.08K reveal non-Gaussian lineshapes in accordance with the picture of diffusing muons at this temperature. This allows us to reject several of the existing models for muon behaviour in copper below 2K.     

Characterization of nanocrystalline products prepared by mechanochemical reduction of copper sulphide

The mechanochemical processing of the copper sulphide with iron in a high-energy mill was studied. The nanosized copper of 10 nm crystallite size and the hexagonal pyrrhotite 1C were identified among products of the reaction by methods of XRD and Mössbauer spectroscopy. In the surface layer of products after the mechanochemical processing copper in its mono— and bivalent forms was identified by XPS method.     

Muon diffusion and level crossing in copper

We have studied the quantum diffusion of positive muons in pure copper over the temperature range 12 mK≤T≤150 K using weak longitudinal field μSR. Below 150 K, this technique has proved to be the most sensitive to the muon hop rate. Our final results for the behaviour of the muon hop rate are well explained within the framework of theories for the quantum diffusion of light interstitials in metals of Kondo, Yamada and others. In addition, the use of level-crossing resonance has allowed us to measure the electric quadrupole interaction strength (and sign) of the copper nuclei, ω_Q= −3.314(7) μS⁻¹. These results have enabled us to show that the muon occupies the same octahedral site at all the temperatures studied, ruling out the possibility of metastable muon sites contributing to any significant portion of the muon polarization.     

Morphological and optical characterization of thermally evaporated copper sulphide thin films

Thin films of Cu₂S on opaque gold layers and quartz substrates at the temperature of 393 K were deposited by a thermal evaporation technique. The surface morphology of the Cu₂S thin films at different thicknesses is investigated by AFM. It is seen that all the films are composed of highly coordinated spherical nano-sized particles well adhered to the substrate. The transmittance and reflectance spectra of Cu₂S thin films on the quartz substrate were recorded by a UV–visible spectrophotometer. The results show that the thermally evaporated Cu₂S thin films have the characteristic transmittance and reflectance suitable for optoelectronic applications. The stoichiometry and surface morphology of a grown Cu₂S thin film were confirmed by energy-dispersive X-ray spectroscopy (EDAX) and scanning electron microscopy (SEM), respectively. The dependence of the refractive index and the extinction coefficient on the photon energy for both the surface film and the opaque gold layer have been determined by ellipsometry. From the spectral behaviour of the absorption coefficient at two distinct absorption regions, a dual-band scheme of optical absorption for a Cu₂S thin film is described. The indirect and direct edges of Cu₂S are found to be about at 0.91 eV and 2.68 eV, respectively.