Changes In Oxygen Stoichiometry Research Articles

Oxygen stoichiometry and mobility effects on domain wall motion in ferroelastic YBa2Cu3O7-δ

Abstract The purpose of this paper is to examine the interrelationship between oxygen stoichiometry and mobility with ferroelastic properties in YBa2Cu3O7-δ. Changes in oxygen stoichiometry, δ, affect the orthorhombic-tetragonal phase transformation behavior and consequently can be expected to directly affect the ferroelastic domain structure and switching properties. A free energy formulation is used to derive an expression for the energy γT of a {110} twin wall in YBa2Cu3O7-δ in terms of the spontaneous strain, coercive stress, and twin wall separation. An upper limit of 40 mJ/m2 was estimated. Ferroelastic viscosity, η, of YBa2Cu3O7-δ was assumed to be an activated process which depends on both the diffusivity and concentration of oxygen according to the relation η = η0(0.6-δ)exp(-E/kT) for δ < 0.6. Two types of domain reorientation mechanisms are proposed, a cooperative shear process of oxygen along a twin wall and a stress-assisted short-range oxygen hopping process in the bulk of the domain, both of...

Effect of He + irradiation on the magnetic properties of superconducting YBa 2Cu 3O 7 thin films

Irradiation induced defects have been thought to create strong pinning centres leading to high critical current density J c of YBaCuO. We report here for the first time, the effect of He + ion irradiation on J c and T c of laser ablated epitaxial thin films of YBaCuO (∼ 100 nm) on (100) SrTiO 3, studied through magnetization and transport measurements. J c ( H = 0) of the unirradiated film (T c ∼ 89 K) at 10 K decreases rapidly from 3.25 × 10 7 A/cm 2 to 0.9 × 10 7 A/cm 2 for the film with 5 × 10 15/cm 2 dose. While T c has a nearly stable value (upto 2 × 10 15/cm 2 dose), J c contrary to neutron irradiation studies, drops by a factor of more than three, followed by a slower decrease. Transport measurements also corroborate such behaviour. One possible mechanism of vortex pinning in thin films could be the surface pinning. In this case the initial decrease in J c with He + irradiation could be due to a decrease in surface pinning while at high dose level, the film quality appears to have deteriorated (T c ∼ 81 K) possibly due to a change of oxygen stoichiometry. Various possibilities are examined to explain the observed results.

Raman spectra of bismuth cuprate high-Tc superconductors and 3d-metal-substituted phases.

Raman spectra are presented for Bi{sub 2}Sr{sub 2}CuO{sub 6+{ital y}}, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{ital y}}, and compounds formed by cationic substitution of Cu by Fe, Mn, and Co. For {ital n}=1 materials, the spectra show a greater number of modes than those predicted by group theory using the undistorted pseudotetragonal unit cell. These extra modes, however, are consistent with group-theory predictions based on the space group of the supercell of Bi{sub 2}Sr{sub 2}CoO{sub 6+{ital y}}, which is caused by commensurate modulation over several periods of the smaller unit cell. Similar analysis is made of the supercell of Bi{sub 2}Sr{sub 2}SrFe{sub 2}O{sub 8+{ital y}}; predictions are consistent with the rearrangement of low-frequency modes as compared with Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{ital y}}. Accompanying changes in oxygen stoichiometry cause strong shifts in the corresponding high-frequency oxygen stretch modes in both {ital n}=1 and 2 materials. Additional substitution of strontium by calcium in {ital n}=2 compounds leads to a more disordered structure, with consequent broadening of all modes in the spectrum. Overall, consideration of the supercell structure where it exists is essential in interpreting Raman spectra from the superconducting cupric oxides and their substituted compounds.

Structural, magnetic and electrical properties of strontium-doped neodymium nickelate

We have measured some of the physical properties of the Nd 2−xSr xNiO 4+y family below room temperature and find a complicated system in which orthorhombic lattice distortion, oxygen stoichiometry, resistivity and magnetic susceptibility change with strontium doping. As the strontium concentration is increased the materials become stoichiometric and tetragonal without reduction and the conductivity of the samples increases although remaining semiconducting. Some compounds demonstrate an unusual low temperature susceptibility which is either temperature independent or falls slightly below ~ 15 K.

Effect of post-sintering quenching parameters on Tc and crystal structure of the Bi2Sr2CaCu2O8+δ (2212) phase

Effects of post-sintering cooling rate from liquid quenching to slow air cooling were investigated for the low- T c 2212 phase in the Bi-Sr-Ca-Cu-O system. The fastest cooling rate yielded the 2212 granular phase with the highest superconductive T c . The change in T c is attributed mainly to the change in oxygen stoichiometry. The difference between the granular and bulk T c 's was smallest after slow cooling, even though the granular T c then had its lowest value. No variation in lattice parameters was observed by powder X-ray diffraction. When the samples were quenched, structural imperfections were introduced; these could not be eliminated even by long-term annealing and slow cooling in air. The lowering of T c was inhibited by the presence of these imperfections in the crystal structure of the 2212 phase. It is emphasized that the true granular superconductive properties are best observed by magnetic measurements since bulk resistivity measurements may lead to erroneous conclusions.

Microwave Sintering of Nanocrystalline TiO2

ABSTRACTNanocrystalline TiO2 compacts having initial approximate mean grain sizes of 14 nm and approximate green densities of 70% of theoretical were sintered by short-time exposure in a 2.45 GHz microwave cavity to maximum temperatures of 800, 1000 or 1200 ºC. Sample densities were measured before and after exposure to microwaves using Archimede's method. Transmission electron microscopy and x-ray diffraction were utilized to monitor grain growth and phase changes. Rutherford backscattering was used to monitor any changes in oxygen stoichiometry. The results of this study indicate that enhanced densification behavior is obtained for microwavesintered samples relative to samples sintered using conventional pressureless-sintering techniques.

Thermogravimetric study of the recovery of oxygen-deficient superconducting YBa2Cu3O7−δ oxides in ambient oxygen

The oxides of oxygen-deficient YBa2Cu3O7−δ with δ≂0.32 in both powder and chunk form were annealed in ambient oxygen to investigate the behavior of oxygen indiffusion. Thermogravimetric measurements were used to monitor in situ weight gain during the annealing at constant heating rates between room temperature and 580 °C. The temperature for achieving a definite oxygen stoichiometry changes with the heating rates, which were varied from 0.1 to 15 °C/min. By measuring the rate dependence, the activation energy of oxygen diffusion in the powder oxide was determined to be 1.25±0.05 eV, in good agreement with the value obtained by previous resistivity measurements. However, the activation energy obtained from the chunk oxide is higher, i.e., 1.52±0.06 eV. A longer diffusion distance in the chunk oxide which requires diffusion across grains might be responsible for this discrepancy.

Kinetics of the oxidation of Ba2 YCu3Ox ceramics

The kinetics of the oxidation of dense and porous samples of Ba2 YCu3Ox ceramic have been determined by gravimetric analysis at 400–700 °C. At 600 °C and above, the rate decreases as the thickness of the oxidized layer increases. At 500 °C and below, the kinetics show a linear relation that indicates that the oxidized layer does not protect the ceramic. Dilatometric, microscopic, and high-temperature x-ray data suggest that fractures in the oxide layer at the lower temperatures are caused by the large volume decrease that accompanies the change in oxygen stoichiometry.

High-temperature ceramic oxide superconductors

High-temperature superconductivity in oxides of the type $La_{2-x}Ba_x(Sr_x)CuO_4$, $YBa_2Cu_30_{7-\delta}$, $La_{3-x}Ba_{3+x}Cu_60_{14}$ and Bi (Tl)- Ca-Sr(Ba)-Cu-0 systems is discussed, with special emphasis on the experimental findings from the author's laboratory. The importance of holes on oxygen and of the $Cu^{1+} (d^{10})$ state is examined. A transition is shown to occur from chain- to sheet-superconductivity in $YBa_2CU_3O_{7-\delta}$ accompanying a change in oxygen stoichiometry. Some of the important material parameters and technological applications are briefly presented. There is every hope that materials with $T_c$ close to room temperature will be discovered in the near future. All the high $T_c$ oxides found hitherto have perovskite-related structures with two-dimensional Cu-0 sheets.

Oxygen stoichiometry in high superconducting critical temperature perovskites by thermogravimetry

Small changes in oxygen stoichiometry are shown to have a dramatic effect on the superconducting transition temperature, T c , of the new oxide superconductors. The ease with which superconducting perovskite oxides loose and take up oxygen with reversible structure changes was established. The results of this study support earlier thermoanalytical work that the optimum physical properties are achieved by a very precise balance of the oxidation state of metal ions and oxygen stoichiometry which originate during synthesis and/or subsequent processing. We describe a method for establishing the oxygen stoichiometry of La 1.825 Sr 0.175 CuO 4−y and REBa 2Cu 3O 7−x, where RE = Y or a rare earth element, by measuring the weight change as a function of the temperature and ambient.

Laser patterning of metal oxide superconductor films by reactive solid-state transformation

A planar submicrometer-resolution patterning method has been developed for fabrication of thin-film Ba/sub 2/YCu/sub 3/O/sub x/ devices without photoresist, water, or solvent exposure. The method is based on a rapid transformation from the superconductive to a dielectric phase. The phase change is induced by controlled changes in the oxygen stoichiometry which are induced thermally by local-area laser irradiation of the thin film in a gaseous ambient. Both extended-area pattern projection and scanned-beam direct writing have been demonstrated with a spatial resolution in the submicrometer range and are presently limited by the grain size of available films. Negligible thickness loss is observed in patterning. The method circumvents lithographic techniques which tend to degrade the electronic quality of Ba/sub 2/YCu/sub 3/O/sub x/ superconductors. >

Reversible Laser Modification of High Temperature Superconducting Y-Ba-Cu-O Films

ABSTRACTWe describe a reversible technique for locally modifying the oxygen stoichiometry and electrical transport properties of superconducting thin films. A focused argon ion laser beam is scanned across the surface of a YBa2Cu3O7-x thinfilm, contained in a vacuum, at incident power levels well below those necessary for ablation. The change in oxygen stoichiometry is monitored in-situ by the room temperature electrical resistance. We have measured the superconducting properties of these locally modified films. The resulting R vs T curve for the composite structure (film/laserstripe/film) shows the expected double transition. The first transition, corresponding to the unmodified film, occurs at 87 K while the second transition, corresponding to the modified stripe, occurs at a lower temperature and is a function of the laser induced change in the room temperature electrical resistance. The critical current for the composite structure is depressed from the original film. The laser writing can be erasedor bleached out by room temperature exposure to an oxygen plasma.

Ba2YCu3O7-σ; Effect of Oxygen Stoichiometry

ABSTRACTThe effect of changes of oxygen stoichiometry on the properties of Ba2YC3O7-σ is discussed. Crystallographic, susceptability, resistivity, and specific heat data from 1 set of samples prepared by Zr gettered annealing are described and discussed.

Self-compensation in lanthanumdoped calcium titanate

The reversible change of oxygen content observed by gravimetric measurements on lanthanum (donor)-doped calcium titanate between specified states of reduction and oxidation is proportional to the lanthanum concentration. These measurements indicate that the lanthanum addition is compensated by additional oxygen up-take in the oxidized state. The range of this reversible change in oxygen stoichiometry is up to more than an order of magnitude larger than the oxygen non-stoichiometry of the undoped CatiO3. There is excellent agreement between the calculated amounts of oxygen necessary to compensate the donor-dopant and the measured reversible oxygen weight-change between the oxidized and reduced state of equilibration. A single-phase region from La0.06Ca0.94Ti0.063+Ti0.064+O3 to La0.94Ca0.94TiO3.03 has been confirmed. The gravimetric measurements can be explained by a model involving a shear structure.

Oxygen non-stoichiometry of tantalum-doped SrTiO 3

The reversible change of oxygen content in tantalum(donor)-doped SrTiO 3 between specified states of oxidation and reduction was measured gravimetrically. These measurements indicate that the maximum reversible weight change is proportional to the dopant concentration. The donor dopants are compensated in the oxidized state by additional oxygen uptake. The range of this reversible change in oxygen stoichiometry is up to more than an order of magnitude larger than the oxygen non-stoichiometry of the undoped SrTiO 3. The results indicate excellent agreement between the calculated amount of oxygen necessary to compensate the donor dopant (on the assumption that each added oxygen will neutralize two Ta 5+ ions) and the measured reversible oxygen weight change between the oxidized and reduced states of equilibration. A single-phase region for tantalum concentrations up to 10 at.% was confirmed. The observed gravimetric measurements can be explained by a model involving a shear structure.

Self-compensation in tantalum-doped TiO2

The reversible change of oxygen content observed by gravimetric measurements on tantalum-doped rutile (TiO2) between specified states of oxidation and reduction is proportional to the dopant concentration. These measurements indicate that donordopants are ionically compensated by additional oxygen uptake in the oxidized state. The range of this reversible change in oxygen stoichiometry is up to more than an order of magnitude larger than the oxygen non-stoichiometry of the undoped TiO2. Self-compensation has been measured in TiO2 with up to 10 at% Ta+5 and a single-phase region from TaO0.1+5Ti0.1+3Ti0.8+4O2 to Ta0.1+5Ti0.9+4O2.05 has been confirmed. The gravimetric measurements can be explained by either the creation of metal-deficit point defects (self-compensation) or a model involving a shear structure.

Self-compensation in lanthanum-doped strontium titanate

Gravimetric measurements on pure and lanthanum-doped SrTiO 3 have shown that the reversible change of oxygen content, between specified states of oxidation and reduction, is proportional to the dopant concentration. These measurements indicate that the donor dopants are electronically compensated by additional oxygen uptake in the oxidized state. The range of this reversible change in oxygen stoichiometry is up to more than an order of magnitude larger than the oxygen nonstoichiometry of the undoped SrTiO 3. A single-phase region from La 0.2Sr 0.8Ti 4+ 0.8Ti 3+ 0.2O 3 to La 0.2Sr 0.8Ti 4+O 3.10 has been confirmed for lanthanum-doped SrTiO 3. The gravimetric measurements can be explained by a model involving a shear structure.

Self-compensation in niobium-doped TiO 2

Gravimetric measurements on pure and niobium-doped TiO 2 have shown that the reversible change of oxygen content, between specified states of oxidation and reduction, is proportional to the dopant concentration. These measurements indicate that the donor-dopants are electronically compensated by additional oxygen uptake in the oxidized state. The range of this reversible change in oxygen stoichiometry is up to more than an order of magnitude larger than the oxygen nonstoichiometry of the undoped oxides. Self-compensation has been measured in TiO 2 with donor-dopant concentrations of up to 8 at% Nb +5 over the oxygen partial pressure range 10 0–10 −15 atm. For the case of Nb +5-doped TiO 2 the monoclinic phase “TiNb 2O 7” was found to exsolve under oxidizing conditions for Nb +5 concentrations >8 at%. The exsolved phase transformed into niobium-doped rutile under reducing conditions. The gravimetric measurements for Nb +5 concentrations <8 at% can be explained by the creation of metal-deficit point defects (self-compensation) or a model involving a shear structure.