Phase evolution during the melting and recrystallization of ceramic core in the (Bi,Pb)-2223 tape

The phase evolution during melting and recrystallization of (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ ((Bi,Pb)-2223) core in a Ag-sheathed monofilamentary tape has been investigated. The tape was fabricated by PIT process with powders containing nearly pure (Bi,Pb)-2223 phase. Short samples were melted at 805 °C, 808 °C, 812 °C, 816 °C, 831 °C, slowly cooled at 1.5 °C/h under flowing 1.6% O₂ balanced with argon and quenched in air at room temperature. X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) were applied for the phase identification. The results show that (Bi,Pb)-2223 core is partially melted into a liquid and alkaline earth cuprates (AECs), mainly 2:1-AEC, at 805 °C, 808 °C, 812 °C, and well reforms directly from the melt during the slow cooling. More (Bi,Pb)-2223 phase is decomposed at temperatures higher than 816 °C, but cannot recrystallize, indicating that a partial melting at some temperatures around a given temperature range is essential to (Bi,Pb)-2223 phase reformation. The melt composition moves from that between “2223” and “2212” stoichiometries towards 2212-like stoichiometry with increasing temperature. This seems to lead to the conclusion that (Bi,Pb)-2223 phase decomposes incongruently into a 2212-like liquid and (Ca,Sr)-cuprates. 2:1-AEC plays the most important role in (Bi,Pb)-2223 melt-recrystallization process. Our results also reveal that plate-like shape (Bi,Pb)-2223 grains can be obtained via melting and recrystallization if the optimum processing conditions are used.     

The temperature dependence of the short wavelength transmittance limit of vacuum ultraviolet window materials—I. Experiment

The short wavelength transmittance limit or cut-off wavelength, λ_co, of LiF, MgF₂, CaF₂, LaF₃, BaF₂, sapphire, synthetic crystal quartz and fused quartz has been measured from about 100°C to about 10°K. λ_co is not a well denned quantity, so for the purpose of this experiment it has been arbitrarily taken as the wavelength where transmittance could just be measured, usually 0.1-0.5 per cent. With one exception λ_co shifted to shorter wavelengths as the sample was cooled; the shift varied from about 40 to 80 Å over the temperature range from 100°C to 10°K, depending on the material, with the largest shift occurring in BaF₂;. The exception was LaF₃ which showed no measurable change in λ_co wth temperature. Over the temperature range from 20° to 100°C the slope of λ_co, with temperature for all materials was fairly constant, but below 20°C it decreased, approaching zero as the temperature approached 20°-10°K. In the case of synthetic crystal quartz, for example, the slope changed from about 0.28 Å/°K at room temperature to about 0.055 Å/°K at 80°K.     

Ln1−xSrxCoO3(Ln = Sm, Dy) for the electrode of solid oxide fuel cells

The perovskite-type oxides were synthesized in the series of Ln_1−xSr_xCoO₃(Ln = Sm, Dy). The formation of solid solutions in Dy_{1 − x}Sr_xCoO₃ was limited, compared with that in Sm_{1 − x}Sr_xCoO₃. The electrical conductivities of the sintered samples were measured as a function of x in the temperature range 30 to 1000 °C. The highest conductivity of around 500 S/cm at 1000 °C was found in Sm_0.7Sr_0.3CoO₃. The reactivity of all the samples with YSZ was examined at 800–1000 °C for 96 h. The Sr-doped perovskite oxides were more reactive with YSZ and produced SrZrO₃ at 900 °C after 96 h. However, no reaction product between SmCoO₃ and YSZ was observed at 1000 °C for 96 h. The cathodic polarization of the oxide electrodes, sputtered on yttria stabilized zirconia (YSZ), was studied at 800–1000 °C in air. SmCoO₃ shows no degradation of the electrode performance at higher temperatures. The thermal expansion measurements on the sintered samples were carried out from room temperature to 1000 °C. Large thermal expansion coefficients were found in these samples.     

Studies of ionic conductivity and structural phase transitions of Na3H(SO4)2 crystal

The complex electrical impedance of Na₃H(SO₄)₂ along the b_m-axis has been measured from 25°C to 316°C in the frequency range 4 kHz–40 MHz. The temperature dependence of the electrical conductivity shows remarkable changes in the temperature range 160°C–260°C. The sample crystal becomes a fast ionic conductor above 260°C. The conduction mechanisms of proton and sodium ions in the different phases are analyzed in detail with respect to the structural features of the sample crystal.     

A comparison of FePt thin films with HfO2 or MnO additive

The influence of oxide additives on the magnetic and structural properties of FePt L1₀ thin films has been studied. FePt films with HfO₂ additive grown on a 5 nm MgO buffer showed a primarily random texture for both as-deposited and annealed samples. The average grain size was limited to 10 nm and the perpendicular coercivity was 1.3 kOe for a 10 nm thick FePt +20% HfO₂ film annealed at 650°C for 10 min. In direct contrast, MnO additive neither limited grain size nor L1₀ ordering in annealed FePt films. A 10 nm thick FePt+20% MnO film grown on a 5 nm MgO buffer showed a unique discontinuous microstructure composed of clusters of (0 0 1) textured L1₀ grains after being annealed at 650°C for 10 min. The average size of the grains making up these clusters was 50 nm and the perpendicular coercivity of the film exceeded 7 kOe.     

Oxygen tracer diffusion in La1 − xSrxCoO3

Tracer diffusion of ¹⁸O in dense, polycrystalline La_1−xSr_xCoO₃ for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10⁵ Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La_0.9Sr_0.1CoO₃ is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/RT] cm²/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La_0.8Sr_0.2CoO₃ exhibits a higher diffusivity at 500 °C than does La_0.9Sr_0.1CoO₃.     

Dilatometry of the high-temperature proton conductor Ba3Ca1.18Nb1.82O9 − δ

Firstly, the production of solid bulk specimens of the proton conductor Ba₃Ca_1.18Nb_1.82O_{9 − gd} (BCN18) of known water content up to [H^.] = n_H/n_Ba = 0.16 is described. Secondly, measuring the length change of such samples versus water content [H^.] demonstrated that the sample lengths increased linearly with a slope of (Δl/l)/[H^.] = (2.13 ±0.07) × 10⁻². Thirdly, the density of bulk samples was found to decrease linearly with water content [H^.]. This decrease was in good agreement with the above values derived from the length change. Fourthly, high temperature dilatometry showed that samples reach the thermodynamic solubility values in water vapor atmospheres only at temperatures above 700 °C. Two time scales were observed for the time-dependence of the elongation upon exposure to water vapor. A fast process occurred within minutes above 700 °C, a slow one took several hours. The fast one was identified as the chemical diffusion of the diffusion pair H^. and V_o^. which is required for the water uptake and loss of BCN18. The chemical diffusivity of water is described by the parameters D₀ = (0.022 ± 0.002) cm²/s and ΔH* = (0.79 ± 0.05) eV.     

Surface tension and density measurements for indium and uranium using a sessile-drop apparatus with glow discharge cleaning

The sessile-drop method is used to measure the surface tension and density of liquid indium and uranium under high vacuum. Measurements are made over the temperature range 156–500°C for In and at the melting point for U. Surface oxides are efficiently removed with a glow discharge system. Drop profiles are captured by photograph and processed using nonlinear regression to yield the surface tension and density. In this regression procedure, normal distances from calculated profiles to data points are minimized. For indium, the density and surface tension measurements yield _mp = 7.05 × 10³kg/m³, d/dT = −0.776 kg/m³·°C, and γ_mp = 0.568 N/m, dγ/dT = −9.45 × 10⁻⁵ N/m·°C. The results for uranium at the melting point are _mp = 17.47 × 10³ kg/m³ and γ_mp = 1.653 N/m.     

The effect of air exposure on palladium–copper composite membranes

It was found that when electrolessly deposited thin Pd and Pd–Cu membranes were exposed to air at temperatures above 350 °C, their H₂ flux increased substantially immediately after the air exposure, then decreased to a new steady-state value. While this was a quasi-reversible change for the H₂ flux, the flux of insoluble species, such as N₂, irreversibly increased with every air exposure but by a much smaller extent. The extent of these changes was found to be dependent on the exposure time and the temperature of the tests. Thus, we decided to investigate the effect of gas exposures on the properties of these materials.

Palladium and palladium–copper films, prepared by electroless deposition on ceramic supports, and commercial foils were exposed to air, hydrogen and helium at 500 and 900 °C for times varying from 1 h to 1 week with the objective of determining the effect of the different exposure conditions on the surface morphology, the flux of different penetrants and the crystalline structure of the materials. Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to study the changes occurring in the films under those conditions.

It was observed that the exposure of both the electroless films and the foils to hydrogen and air markedly modified their surface morphology. The hydrogen exposure tended to smooth the surface features whereas the oxygen exposure created new surface features such holes and large peaks. Additionally it was found that the air exposure produced some oxidation of the film to create PdO.

These results suggested that a common hypothesis stating that air oxidation just cleans the surface of the membrane might not be sufficient to explain all of those changes. A contributing effect of air exposure may be the increase in surface area due to the formation of palladium oxide. However, the extent of the surface area increase was insufficient to explain the increase in steady-state H₂ flux.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.     

Thermoluminescence of KMgF3(RE) crystals

Optical properties of pure and RE doped KMgF₃ crystals were studied. Thermoluminescence curves are independent of the introduced impurity for all dopant besides europium. Emission, radiation damage and TL are found to be significantly dependent on the types of Eu-dopant added to the melt: europium fluoride or oxide. An intensive high temperature TL peak at 390°C in KMgF₃(Eu₂O₃) may be used as a dosimetric one not only for ionizing but also for UV radiation.     

High and low thermal nitridation of SiO2 thin films

Thin thermal SiO₂ films on crystalline silicon substrate were nitrided at low ammonia pressures (10^-6P_NH310^-1 mbar) for times varying from 1 to 10 h, by means of two techniques. (i) Surface nitridation has been achieved by thermal activation at high temperature (HT), in the range 800–1100°C. (ii) A new process at low temperature (LT), was employed at T ≈ 30°C, under electron-beam irradiation; the nitridation-reaction rate depends on the electron energy (reaching a maximum within the energy range 1 to 2 keV), and on the electron flux. Conduction and electron trapping on the nitrided oxide films depends on the chemical compositions and on the amount of nitrogen incorporated into the bulk of the films and/or at the SiO₂-Si interface.     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

The effect of annealing under hydrostatic pressure on the visible photoluminescence from si-ion implanted SiO2 films

We have studied the influence of the hydrostatic pressure during annealing on the intensity of the visible photoluminescence (PL) from thermally grown SiO₂ films irradiated with Si⁺ ions. Post-implantation anneals have been carried out in an Ar ambient at temperatures T_a of 400°C and 450°C for 10 h and 1130°C for 5 h at hydrostatic pressures of 1 bar–15 kbar. It has been found that the intensity of the 360, 460 and 600 nm PL peaks increases with rising hydrostatic pressure during low-temperature annealing. The intensity of the short-wavelength PL under conditions of hydrostatic pressure continues to rise even at T_a=1130°C. Increasing T_a leads to a shift in the PL spectra towards the ultraviolet range. The results obtained have been interpreted in terms of enhanced, pressure-mediated formation of ≡Si–Si≡ centres and small Si clusters within metastable regions of the ion-implanted SiO₂.     

Cross section of the charged current reaction C(ve, e)Ng.s.

The charged current nuclear transition ¹²C(v_e, e⁻)¹²N_g.s. has been observed in the KARMEN experiment. The flux average cross section for v_e from μ⁺ decay at rest is determined to be σ = [8.1±0.9(stat.)±0.75 (syst.)]×10⁻⁴²cm². For the first time also the energy dependence of the cross section has been measured for neutrino energies up to 50 MeV.     

Magnetic behaviour of some Mn.III2.VI4 compounds and their alloys

Measurement of the magnetic susceptibility χ as a function of temperature were made on polycrystalline samples from the alloy systems Cd_1−zMn_zGa₂Se₄, Zn_1−zMn_zGa₂Se₄ and Cd_1−zMn_zIn₂Te₄ which had been subjected to various heat treatments. The 1/χ versus T curves indicated that for the Zn---Se alloys, for all values of z, samples slowly cooled to room temperature were antiferromagnetic showing ideal Curie-Weiss behaviour, but for samples quenched from 700°C the behaviour was a mixture of antiferromagnetic and spin-glass. For the Cd---Se alloys, samples from the range (0.6 < z < 1.0) showed very similar behaviour, but in the range (0 < z < 0.6) even the very slowly cooled samples showed a mixture of antiferromagnetic and spin-glass behaviour. For the Cd---Te alloys, all samples, however heat-treated, showed spin-glass form. Values of the Curie-Weiss constant Ф were determined from all of the 1/χ versus T curves, and by comparison with the T(z) phase diagrams for the different alloy systems, the values of Ф were correlated with the ordering of the Mn atoms on the cation lattice. It is shown that the experimental values of Ф can give a very convenient way of determining the type of ordering and the degree of order in such alloys.     

Microstructure and transport properties of YBa2Cu3O7−δ films produced by laser ablation from a BaF2/Y2O3/CuO target

The effects of varying the temperature and duration of the post-deposition anneal in watersaturated oxygen were investigated for YBa₂Cu₃O_7−δ films of varying thickness. The films were produced by laser ablation from pressed powder targets consisting of BaF₂,Y₂O₃, and CuO mixtures. This technique produces superconducting films with a highly textured surface. The films were fabricated on SrTiO₃ substrates and were analyzed with X-ray diffraction, scanning electron microscopy, and temperature dependent resistivity. Critical current density (J_c) measurements were performed in magnetic fields up to 1 T. For film thickness on the order of 900 nm, completely c-axis oriented films were obtained with a 60 min anneal at 850°C. Thinner films required less annealing, either shorter times or lower temperatures, to achieve similar results, indicating that the optimal annealing conditions are dependent on film thickness.     

Pulsed nuclear magnetic resonance of gamma-irradiated lithium hydride

Some of the effects of ⁶⁰Co gamma irradiation on lithium hydride are described. Volume increase and nuclear magnetic resonance data are given for samples irradiated from 40 to 395°C. Maximum swelling occurs between 160 and 200°C; negligible swelling occurs above 300°C. Motionally narrowed proton and ⁷Li nuclear magnetic resonance signals appear on irradiation and increase with increasing swelling. These decomposition products, which are trapped inside the LiH, can amount to more than one-tenth the total sample at doses of 50 Grad. At this point, 25 volume per cent swelling has occurred and the growth rate has subsided. The hydrogen nuclear magnetic resonance signal has been shown to come from H₂ molecules by observation of the ortho-para conversion on cooling. Hydrogen densities derived independently from the longitudinal relaxation time and the swelling data are in reasonable agreement. The corresponding gas pressures range from 750 to 5000 atm. The H₂ is thought to be in bubbles which cause the volume growth, and recent electron microscopy results support this view. The ⁷Li signal has a Knight shift, and the lithium is present as metal particles. Above 200°C, the H₂ and Li back-react rapidly. Above 300°C this reaction takes place as fast as the decomposition, which was caused by the irradiation.     

Characteristics of TL and PTTL glow curves of gamma irradiated pure Li2B4O7 single crystals

The TL glow curve of X-ray irradiated pure and Cu-doped Li₂B₄O₇ shows that the most intense TL peak is at 160°C. In the present work the characteristics of the TL and PTTL glow curves from gamma irradiated pure Li₂B₄O₇ single crystal samples (prepared by Mitui Kinzoku Kougyo, Japan) have been studied. The samples were irradiated with different gamma doses (from 0.5 up to 500 Gy) using a ⁶⁰Co gamma ray source at a dose rate of 78 Gy h⁻¹. The TL glow curve shows three intense peaks (at 160°C, 260 and 305°C) and three weak ones (at 110, 140 and 220°C). The most intense TL peak is at 160°C, which agrees with the TL glow curve from X-ray irradiated samples [Kutomi Y. and Tomita A. (1990) TSEE and TL of Li₂B₄O₇:Cu single crystals. Radiat. Prot. Dosim. 33, 347–350]. The 305°C peak in gamma irradiated samples also appears to be very intense, which indicates its possible use in high-dose high-temperature dosimetry. Further, the characteristics of the PTTL glow curve have been studied as a function of ultraviolet exposure and number of repeated PTTL cycles.