Mössbauer study of Fe-substituted orthorhombic and tetragonal YBa2(Cu1-xFex)3O7-σ system

Mössbauer study of orthorhombic and tetragonal YBa₂(Cu_1-xFe_x)₃O_7-σ;x=0.02, 0.04, 0.08 has been done to investigate the two inequivalent Cu-sites. Fe substituting Cu having pyramidal oxygen co-ordination is in Fe³⁺ state while Fe substituting Cu having square planar co-ordination is in Fe³⁺ and Fe⁴⁺ states in dynamic equilibrium.     

Growth and photorefractive properties of near-stoichiometric In:Fe:Cu:LiNbO3 crystals

The near-stoichiometric LiNbO₃ crystal co-doped with In₂O₃, Fe₂O₃, and CuO has been grown from a Li-rich melt (Li/Nb = 1.38, atomic ratio) by the Czochralski method in air atmosphere for the first time. The OH⁻ absorption spectra were characterized to investigate the structure defects of the crystals. The appearance of the 3506 cm⁻¹ absorption peak manifests that the composition of the grown crystal is close to the stoichiometric ratio. The photorefractive properties were also measured by the two-wave coupling experiments. The results show that the near-stoichiometric In:Fe:Cu:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient than those obtained in the congruent In:Fe:Cu:LiNbO₃ crystal under the same experimental conditions. The material of near-stoichiometric In:Fe:Cu:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.     

The role of copper species on Cu/γ-Al2O3 catalysts for NH3-SCO reaction

UV-vis spectra, XRD, H₂-TPR, TEM and ESR were used to characterize a series of Cu/γ-Al₂O₃ catalysts, which were prepared by incipient wetness impregnation using copper nitrate, copper acetate or copper sulfate as precursors, to study the role of Cu species on Cu/γ-Al₂O₃ catalysts for NH₃-SCO reaction. It was found that the mixture of CuO phase and CuAl₂O₄ phase formed on various Cu/γ-Al₂O₃ catalysts, and the Cu species and dispersion had significant influence on the Cu/γ-Al₂O₃ activity. Highly dispersed CuO phase on the support would be related with its high activity for the NH₃-SCO reaction.     

Electronic structure analysis of Mn- and Fe-codoped In2O3 by photoemission yield measurements

The valence band electronic structures of Mn- and/or Fe-doped In₂O₃, i.e., In₂O₃:Mn, In₂O₃:Fe, and In₂O₃:(Mn, Fe), are investigated by photoemission yield measurements. Significant changes are observed in the threshold energy of photoemission, depending on the doped magnetic ions, which indicates that an additional occupied band appears above the top of the valence band of In₂O₃ owing to doping with Mn and/or Fe ions. It is confirmed that the order of the threshold energies of photoemission, E_PET, is E_PET(In₂O₃:Mn)<E_PET(In₂O₃:(Mn, Fe))<E_PET(In₂O₃:Fe)<E_PET(In₂O₃). To gain a better understanding of these results, first-principles molecular orbital calculations are also carried out, which successfully explain the observed changes in the photoemission threshold energies.     

Magnetic phase present in YBa2(Cu1−x Fe x )3O6+δ oxides

The Mössbauer study of⁵⁷Fe: YBa₂Cu₃O_6+δ oxides was very important to establish the preferential occupation of Cu(1) site by Fe at very low concentrations. Recent determination of antiferromagnetic ordering for Cu moments in Cu(2) sites (T≈450 K) and our early observation of a small proportion of a magnetic phase at room temperature for Fe:YBa₂Cu₃O₆ lead us to perform systematic studies of YBa₂(Cu_1?x Fe _x )₃O₆ withx=0.005, 0.03, 0.05, 0.10 and 0.15 in order to obtain information about the iron occupation of Cu(2) sites.     

Mössbauer studies on high temperature superconductors

⁵⁷Fe Mössbauer spectroscopy as well as X-ray diffractometry and DTA measurements were used to study the preparation of YBa₂(Cu(⁵⁷Fe))₄O₈ cuprates, the degradation of 1-2-3 type superconductor by hydrogenation and the suppression of superconductivity in a PrBa₂(Cu(⁵⁷Fe))₃O_7–d compound. 1-2-3 type compounds could only be prepared instead of a 1-2-4 one using the same or similar methods reported by Cava et al. or Pooke et al. We have demonstrated that the change in the relative intensity of doublet D2 (associated with iron at five-coordinated Cu(1) site) in 1-2-3 superconductors can sensitively reflect not only the change in the oxygen content, but also the effect of hydrogenation. At hydrogenation performed by gas loading at room temperature, hydrogen can remain localized near the O(5) at the Cu(1) site, while a decrease of oxygen content can be observed according to the degradation of the superconductor caused by hydrogenation at 220 °C. The isomer shifts, observed with unusual values for 1-2-3 superconductors, of doublets of the PrBa₂(Cu(⁵⁷Fe))₃O_7–d material were associated with the mechanism of suppression of superconductivity in the Pr-containing 1-2-3 compound.     

Properties of nanocrystalline Al–Cu–O films reactively sputtered by DC pulse dual magnetron

The article reports on the effect of the addition of copper in the Al₂O₃ film on its mechanical and optical properties. The Al–Cu–O films were reactively co-sputtered using DC pulse dual magnetron in a mixture of Ar + O₂. One magnetron was equipped with a pure Al target and the second magnetron with a composed Al/Cu target. The amount of Al and Cu in the Al–Cu–O film was controlled by the length of pulse at the Al/Cu target. The Al–Cu–O films with ≤16 at.% Cu were investigated in detail. The addition of Cu in Al₂O₃ film strongly influences its structure and mechanical properties. It is shown that (1) the structure of Al–Cu–O film gradually varies with increasing Cu content from γ-Al₂O₃ at 0 at.% Cu through (Al_8−2x,Cu_3x)O₁₂ nanocrystalline solid solution to CuAl₂O₄ spinel structure, (2) the Al–Cu–O films with ≥3 at.% Cu exhibit (i) relatively high hardness H increasing from ∼15 GPa to ∼20 GPa, (ii) enhanced elastic recovery W_e increasing from ∼67% to ∼76% with increasing Cu content from ∼5 to ∼16 at.% Cu and (iii) low values of Young's modulus E* satisfying the ratio H/E* > 0.1 at ≥5 at.% Cu, and (3) highly elastic Al–Cu–O films with H/E* > 0.1 exhibit enhanced resistance to cracking during indentation under high load.     

Active metal brazing of Al2O3 to Kovar® (Fe–29Ni–17Co wt.%) using Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%)

The application of an active braze alloy (ABA) known as Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%) to join Al₂O₃ to Kovar® (Fe–29Ni–17Co wt.%) has been investigated. This ABA was selected to increase the operating temperature of the joint beyond the capabilities of typically used ABAs such as Ag–Cu–Ti-based alloys. Silica present as a secondary phase in the Al₂O₃ at a level of ~5 wt.% enabled the ceramic component to bond to the ABA chemically by forming a layer of Si₃Ti₅ at the ABA/Al₂O₃ interface. Appropriate brazing conditions to preserve a near-continuous Si₃Ti₅ layer on the Al₂O₃ and a continuous Fe₃Si layer on the Kovar® were found to be a brazing time of ≤15 min at 1025 °C or ≤2 min at 1050 °C. These conditions produced joints that did not break on handling and could be prepared easily for microscopy. Brazing for longer periods of time, up to 45 min, at these temperatures broke down the Si₃Ti₅ layer on the Al₂O₃, while brazing at ≥1075 °C for 2–45 min broke down the Fe₃Si layer on the Kovar® significantly. Further complications of brazing at ≥1075 °C included leakage of the ABA out of the joint and the formation of a new brittle silicide, Ni₁₆Si₇Ti₆, at the ABA/Al₂O₃ interface. This investigation demonstrates that it is not straightforward to join Al₂O₃ to Kovar® using Copper ABA®, partly because the ranges of suitable values for the brazing temperature and time are quite limited. Other approaches to increase the operating temperature of the joint are discussed.     

The study of Mössbauer effect in EuBa2Cu3O7-y

EuBa₂(Cu_1-xFe_x)₃O_7-x has been investigated by the⁵⁷Fe and¹⁵¹Eu Mössbauer effect. The⁵⁷Fe Mössbauer spectra of the EuBa₂(Cu_1xFe_x)₃O_7-y without or with DC electric current (the current strength I=0.5A) around the superconducting transition temperature have been measured. The results indicate that the isomer shift (IS) and the quadrupole splitting (QS) of the Fe replacing the Cu(2) vary neither with increasing the Fe content nor with the small DC eletric, current passing the superconductor and that the IS and the QS of the Fe replacing the Cu(1) vary with the Fe content. Especially, the IS and the QS of the Fe (D3) replacing the Cu(1) are changed when the small electric current passes the superconductor at 80K.     

Al Kα and Cu Kα1 excited XPS of vanadium oxide and VF3 powders: Measurement of the V 1s – KLL Auger parameters

Al Kα (hν = 1486.6 eV) excited XPS shows that powder samples of V₂O₃, V₂O₅ and VF₃ are surface contaminated and that the V₂O₃ can be cleaned by heating in vacuum at 400 °C. The greater sampling depth of Cu Kα₁ (hν = 8047.8 eV) excited XPS allows measurement of the bulk V 1s – KL₂L₃ Auger parameters (APs) for these materials. The APs of VF₃ and V₂O₅, relative to V metal, fall into the range of values expected for metal fluorides and oxides with non-local final state core-hole screening, whereas the AP of V₂O₃ is significantly closer to that of V metal. We ascribe this to a greater final state valence orbital occupation following photionisation in V₂O₃, part of which results from metal-like screening.     

Mössbauer spectroscopy in highT c superconductors

Mossbauer spectroscopy has recently been applied to study the new high T_c superconducting compounds RBa₂Cu₃O_z, using isotopes of rare earths mainly¹⁵⁵Gd and¹⁵¹Eu, and⁵⁷Fe, with different amounts of Fe ions replacing Cu. It was shown that magnetic moments on the rare earth site do not interfere with superconductivity. Fe at low concentrations (<1%) was found to replace Cu mainly in the Cu(1) site, and the Mossbauer spectra reveal different quadrupole doublets-fol lowing the different oxygen coordination around the Fe ion. The change of the relative intensities of the different doublets with z can easily be followed. For higher iron concentrations, it seems that increasing amounts of iron replace Cu in the Cu(2) site. For z<6.5, the iron reflects the magnetic ordering of Cu in this site, and the ordering temperature as function of z can be obtained. The agreement between neutron diffraction and Mossbauer measurements prove that Fe is a good probe for the magnetic behaviour of the Cu(2) ions. At low temperatures, Fe Mossbauer spectra of Fe in the Cu(1) site are also magnetically broadened, for all z. Superconducting-magnetic phase diagrams are also obtained in Y_1−x Pr_x Ba₂ Cu₃O_z as function of x and z. For z=7.1, T_N changes sharply with x. T_N=300, 230 and 35 K for x=0.8, 0.6 and 0.4 respectively, whereas for z=6.1 T_N changes very little with x. Mossbauer measurements performed on 5 at %⁵⁷Fe doped in CalaBaCu₃O_z show that most of the la occupy the Ba site. For z=7 about half the iron in the Cu(2) sites are magnetically ordered, with H_eff=520 kOe and T_N=400 K, even though the sample is superconducting with T_c=35 K. The possibility of coexistence between superconductivity and magnetic order in these systems will be discussed.     

57Fe and57Co Mössbauer studies of highT c Y-Ba-Cu oxides

The⁵⁷Co emission Mössbauer spectra from YBa₂Cu₃O_6.92 (1-2-307) and YBa₂Cu₃O_6.00 (1-2-306) have been measured and compared with the⁵⁷Fe absorption spectra from YBa₂Cu_2.95Fe_0.05O_7?δ in order to clarify decisively the site assignments for the⁵⁷Fe quadrupole-split doublets in these compounds. Mössbauer spectra obtained from both specimens consist of four components whose hyperfine interaction parameters well agree with each other. It is shown that the Co and Fe atoms mainly substitute at Cu1 chain sites in 1-2-307, but in 1-2-306 the Co atoms occupy randomly the Cu2 plane sites and indicate magnetically-split sextet which converts to a paramagnetic doublet of S-state Fe³⁺ in 1-2-307 by a post-annealing in O₂ gas.     

Reaction mechanism of spinel CuFe2O4 with CO during chemical-looping combustion: An experimental and theoretical study

Spinel CuFe₂O₄ is a promising oxygen carrier due to its synergistic enhanced performance. A fundamental understanding of the reaction mechanism between oxygen carrier and fuels is important for a rational design of highly efficient oxygen carrier. The reaction mechanism of spinel CuFe₂O₄ with CO during chemical-looping combustion (CLC) was studied based on thermogravimetric analyses (TGA) and density functional theory (DFT) calculations. Two distinct reaction stages were clearly observed. CuFe₂O₄ was mainly transformed into Cu and Fe₃O₄ with a rapid reaction rate in the initial stage, and then product Fe₃O₄ was slowly reduced to FeO or even to Fe. The reactivity of CuFe₂O₄ is much higher than that of Fe₂O₃, which is ascribed to the existence of Cu. The enhanced oxygen evolution activity of CuFe₂O₄ at low temperature is validated by both the experimental and theoretical methods. Three types of surface oxygen coordinated with different metal atoms show different reactivity. Two kinds of reaction pathways are involved in CO oxidation over CuFe₂O₄. In the one-step reaction pathway, CO directly reacts with the oxygen bonding to two octahedral Cu and one octahedral Fe atoms to form a CO₂ molecule without an energy barrier, which corresponds to the surface oxygen consumption observed in TGA experiments. In the possible two-step reaction pathway, CO first adsorbs on the surface, and then reacts with the oxygen bound to one octahedral Cu and two octahedral Fe atoms to generate CO₂ by surmounting an energy barrier of 10.84 kJ/mol, which is the most kinetically and thermodynamically favorable pathway.     

Formation of single-wall carbon nanohorn aggregates hybridized with carbon nanocapsules by laser vaporization

Single-wall carbon nanohorn (SWNH) aggregates hybridized with carbon nanocapsules (CNCs) were fabricated at a high yield (∼70%). The carbon was laser-vaporized for 2 s into an Ar gas atmosphere with one of the following: Fe, Al, Si, Co, Ni, Cu, Ag, La₂O₃, Y₂O₃, and G₂O₃. By optimizing the Ar gas pressure and metal content, we were able to produce hybridized SWNH structures for Fe, Co, Ni, Cu, and Ag. Possible mechanisms for governing hybrid production, which occurs with smaller CNCs (<100 nm) with only certain metals and carbide, are discussed on the basis of thermal and catalytic graphitization. PACS 61.46.Df; 68.37.Lp; 81.16.Mk     

Nonvolatile photorefractive holographic recording in Sc:Ce:Cu:LiNbO3

In this paper experimental studies of nonvolatile photorefractive holographic recording in Ce:Cu:LiNbO₃ crystals doped with Sc(0,1,2,3 mol%) were carried out. The Sc:Ce:Cu:LiNbO₃ crystals were grown by the Czochralski method and oxidized in Nb₂O₅ powders. The nonvolatile holographic recording in Sc:Ce:Cu:LiNbO₃ crystals was realized by the two-photon fixed method. We found that the recording time of Sc:Ce:Cu:LiNbO₃ crystal became shorter with the increase of Sc doping concentration, especially doping with Sc(3 mol%), which exceeds the so-called threshold, and there was little loss of nonvolatile diffraction efficiencies between Sc(3 mol%):Ce:Cu:LiNbO₃ and Ce:Cu:LiNbO₃ crystals.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Manipulating Bonding at a Cu/(0001)Al2O3 Interface by Different Substrate Cleaning Processes

Growth of Cu films on (0001)Al₂O₃ substrates can result in metallic Cu—Al or ionic-covalent Cu—O bonds at atomically abrupt interfaces. The type of bonding depends on the substrate cleaning procedure prior to film growth. Cu films deposited on Ar⁺-ion sputter-cleaned substrates exhibit interfacial Cu-L_2,3, Al-L_2,3 and O-K energy-loss near-edge structures that indicate the formation of metallic Cu—Al bonds at the Cu/Al₂O₃ interface. In contrast, growth on chemically cleaned -Al₂O₃ substrates results in interfacial energy-loss near-edge structures that suggest Cu—O bonding at the interface. The experimental electron energy-loss spectroscopic results are compared to calculated spectra, and the mechanisms causing the changes in the atomic and electronic structure are addressed.     

The effect of PT on phases and electrical properties in Ba-Y-Cu-O system

Several new phases containing platinum in Y-Ba-Cu-O system are reported in this paper. When Ba-Y-Cu-O system containing platinum is sintered at 950°C, two hexagonal phases YBa₂(Pt, Cu)₃O_9-δ and Ba₄Pt_1+x Cu_2-x Cu_2-x O_9-δ (x = 0.1 – 0.9) are found, respectively. The super-K₂NiF₄ structure Ba _x Y_2-x (Cu, Pt)O₄ is obtained above 1000°C in the Y-Ba-Cu-O system containing platinum. Electrical properties of these phases are also reported.     

Structural and superconducting behaviour of Mg- and Zn-substituted YBa2Cu3O7-δ

Effects of doping YBa₂Cu₃O_7-δ with Mg and Zn on its crystallographic structure and superconducting behaviour have been investigated. Orthorhombic structure is retained upto Mg/Cu ratio of 0.12 (the highest investigated here), but T_c decreases rapidly with the level of Mg-doping. The effects produced by Mg-doping are very similar to those due to Zn-doping investigated by the present authors as well as by others. Neutron diffraction measurements have been carried out on two substituted materials: YBa₂Cu_2.82Mg_0.18O_7-δ and YBa₂Cu_2.82Zn_0.18O_7-δ. Both Mg- and Zn-dopants show strong tendency of occupying Cu(II)-sites in the unit cell. In the case of Mg-doping, however, a strikingly different feature is the concomitant depletion of oxygen at the O₃-sites by an amount nearly equal to the Mg-concentration.     

Fabrication, characterization and photoelectrochemical properties of Fe2O3 modified TiO2 nanotube arrays

TiO₂ nanotube (NT) arrays modified by Fe₂O₃ with high sensibility in the visible spectrum were first prepared by annealing anodic titania NTs pre-loaded with Fe(OH)₃ which was uniformly clung to the titania NTs using sequential chemical bath deposition (S-CBD). The photoelectrochemical performances of the as-prepared composite nanotubes were determined by measuring the photo-generated currents and voltages under illumination of UV-vis light. The titania NTs modified by Fe₂O₃ showed higher photopotential and photocurrent values than those of unmodified titania NTs. The enhanced photoelectrochemical behaviors can be attributed to the modified Fe₂O₃ which increases the probability of charge-carrier separation and extends the range of the TiO₂ photoresponse from ultraviolet (UV) to visible region due to the low band gap of 2.2 eV of Fe₂O₃.