Oxidation studies have been performed on titanium dibor‐ide (TiB2) powders, from room temperature up to 900°C. The studies were performed at low partial pressures of oxygen, at 10 and 0. 05 ppm of O2 in argon, simulating furnace atmosphere with flowing neutral gas of medium and high purity, and in air. It has been found that titanium borate (TiBO3) is formed in these processes. It has also been revealed that the oxidation process starts below 400°C and is reversible in this low temperature range from 100° to 400°C.
We have investigated certain aspects of the dielectric properties of anodic oxide films on tantalum in connection with Dignam's theory that a special current-driven polarization process is responsible for the transients shown by the ionic current on changing the electric field in the oxide. In his experiments, which were claimed to prove this theory, films were first held at potentials giving fields too low to produce significant ionic current. When voltage pulses were applied large enough to produce fields giving ionic current, extra current was observed to flow for a short time. This was taken as due to the decay of the dielectric polarization built up due to the ionic current, and partial numerical agreement with the model was found. In the present experiments the discharge current flowing after longer periods of voltage application or of ionic current flow was investigated with the aim of finding if more polarization was indeed produced by ionic current flow. The current was not found to be larger after discharge from fields giving ionic current, as required by Dignam's model. It is argued that structural changes in the glassy oxide are responsible for the transients.Anodic oxide films grow on tantalum by a process of high-field ionic conduction through the oxide. 1 Both oxygen and metal species move. The outer part of the films grows through metal ion transport and tends to contain anions from the electrolyte. At constant ionic current a steady state is believed to be reached with a constant field in the oxide and a constant rate of rise of voltage as the film grows. On changing the current ͑or field͒ a "transient" occurs before the oxide adjusts to a new steady state. The adjustment has been considered by some authors to be due to changes in the structure of the amorphous oxide, Vermilyea 2 being the first to emphasize the glassy aspects of the oxide structure. The discovery by Young 3 that, if a constant field is applied to a film which had been brought to a low ionic conductivity state by annealing at 100°C, the ionic current increases at constant field at a rate proportional to the square of the current, required some new feature in any model. This is a very striking effect. Dell'Oca and Young's data, 4 which confirmed Ref. 3, show the current increasing from about 1 to about 200 A/cm 2 over about 180 s. Previously the changes in the film had been assumed to be controlled by the field in the oxide. No model which does not allow for this effect can be considered realistic. One feature mooted to explain that the constant field transients involved some form of momentum transfer from moving ions. A model was proposed by Dignam, 5 the essential features of which involved ͑i͒ a special, previously unknown form of Debye dielectric polarization which relaxes to its equilibrium value at the existing field with a relaxation time inversely proportional to the ionic current, and ͑ii͒ that the ionic current is controlled by an effective ͑or cavity͒ field, such as is invoked in classical dielectric theory. ͑The essent...
Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite YBa2-Cu30 x have revealed local antiferromagnetic order for 6.0^x^6.4 with a Neel temperature TN that decreases rapidly with increasing oxygen content x. For slowly annealed samples with 6.35^x^6.5 the superconducting transition temperature T c increases smoothly with x from 25 K at x =6.348 to 60 K at x =6.507. Two such samples with x =6.348 and x =6.400 appear to "switch" from superconductivity to antiferromagnetic order at lower temperatures.PACS number: 76.75.+i, 74.70.Vy, 75.50.EeThe recent discovery 1 of superconductivity at the anomalously high temperature of T c «90 K in the perovskite YBa2Cu30 x (6.9^x^7.0) has stimulated theorists to explore new mechanisms for superconductivity. Several such theories 2 involve frustrated antiferromagnetic couplings between copper and/or oxygen ions in the CuC>2 planes that these materials have in common with the slightly lower-T c perovskite Lai.ssSro.isCuC^-s. It was therefore interesting to learn, both from neutron diffraction 3 and from positive-muon spin-rotation and -relaxation (/x + SR) and susceptibility measurements, 4 that the (insulating) parent compound La2Cu04-s exhibits antiferromagnetic (AFM) order below a Neel temperature TN that depends critically upon the oxygen deficiency 8 (TN increases with increasing <5), lanthanum stoichiometry, and/or doping of Sr for La. For instance, we have found that replacing La2 by La1.98Sro.02 reduces TN from around 250 K to less than 15 K. Similar AFM ordering was discovered in the oxygen-deficient YBa2Cu3-Ojc materials (insulating for x;$6.3) with use of muon spin rotation. 5 Although it has been more difficult to detect the antiferromagnetism in this material by other techniques, recent neutron-diffraction measurements on an oriented powder have now confirmed the presence of long-range AFM order. 6In this Letter we report on the competition between AFM and superconductivity (SO in YBa2Cu30 x as a function of x, using the technique of p. + SR. 7 ' 8 The positive muon serves as a microscopic probe of the local magnetic environment at interstitial sites and is sensitive to and can differentiate between AFM and SC in the bulk of the sample.The samples used in these measurements were sintered powders of YBa2Cu30 x prepared by two rather different methods. In the "quench" method, samples were heated in controlled atmospheres of oxygen and/or argon and allowed to equilibrate to the oxygen content determined by thermogravimetric analysis on test samples, using as a reference neutron-diffraction results from Jorgensen et al. 9 which show that as the oxygen content x is decreased, oxygen atoms are selectively removed first from the CuO "chains," leaving the CuC>2 "planes" essentially intact down to x =6.0 where the "chains" are fully deoxygenated. The samples were then dropped directly into liquid nitrogen and transported under argon to TRIUMF for p + SR experiments. In the "slow-anneal" method, a "mother batch" of YBa2Cu30jc powder was p...
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