The preparation of high transition temperature superconducting Pb-Bi-Ge alloy filaments using the method of glass-coated melt spinning

The melt spinning of Pb-Bi-Ge alloys with Pyrex glass was investigated as a means of producing a superconducting long filament with highT _c. Continuous filaments with maximumT _c of more than 10 K of Pb_100-x-y Bi _x Ge _y (15x37 and 725) were obtained from the molten state at 1500 K with a winding speed of 0.95 m sec^–1. The Pb₄₉Bi₃₃Ge₁₈ filament, which was 34×10^–6 m diameter and a ductile material with a tensile strength of 20 MPa and elongation of 2.7%, exhibited superconductivity at the highestT _c of 14.3 K. These filaments were found to be polycrystalline with a grain size of more than 5000×10^–10 m and had a mixed structure of germanium (diamond) (h c p) and bismuth phases. The germanium element distributed homogeneously in the filament.     

Lead zinc niobate pyrochlore: Structure and dielectric properties

In Pb(B _x B_1–x )O₃ ceramic compositions, it is customary to find a mixture of cubic pyrochlore and perovskite phases after calcination. Based on X-ray diffraction analysis, we concluded that both phases are made up of the same structural unit of BO₆ octahedra. The B and B ions occupy the B sites randomly to the extent that local charge equilibrium is maintained. Thus a general formula for the pyrochlore phase can be expressed as Pb(B _x B_1–x )O₃ · O_p where O p 0.5. An extensive study of the Pb(Zn _x Nb_1–x )O_3.5–1.5x pyrochlore system was made by varying the zinc concentration. We interpret changes in the X-ray diffraction pattern and the lattice constant as indicative of the changing occupancy of the seventh oxygen sites in order to maintain local charge balance. The best combination of the dielectric properties, with a dielectric constant of130 and a factor greater than 1000 at10 MHz, is achieved at a composition of 0.3 x 0.4 and a sintering temperature of 980° C. The temperature coefficient of the dielectric constant measures –0.75 × 10^–3° C^–1. It decreases to –0.54 × 10^–3° C^–1 when5 mol% of PbTiO₃ was mixed with the nominal pyrochlore compositions and sintered. Thus, it may be possible to effect a larger change in the temperature coefficient by judiciously including selective amounts of a second phase which has the best compensating temperature coefficient.     

Dielectric properties and microstructural behaviour of B-site calcium-doped barium titanate ceramics

Dielectric properties and microstructural behaviour of Ba_1–x Sr _x Ti_1–y Ca _y O_3–y ceramics, where strontium and calcium were doped on the barium and titanium sites, respectively, within the range 0x0.24 and 0y0.05, were investigated. Calcium addition decreased the tetragonality,c/a, increased the unit cell volume, and lowered the Curie temperature, which were all attributed to the occupancy of Ca²⁺ ions on titanium sites. When sintered at a low oxygen partial pressure of 10^–9 MPa, a resistivity higher than 10¹¹ cm was maintained for the formulations containing B-site calcium substitution more than 0.5 mol %. With increasing the amount of calcium addition, the Curie peak was depressed and completely broadened for the compositions with calcium addition more than 3 mol %, where the average grain size was smaller than 1 m. Co-firing with nickel electrodes in a reducing atmosphere also depressed the Curie peak and inhibited the grain growth due to the diffusion of nickel into the dielectrics.     

Some properties of Sb2Te3−xSex for nonvolatile memory based on phase transition

Composition dependence of properties of Sb₂Te_3–x Se _x in the range 0x<3 were studied using differential thermal analysis and X-ray diffraction. Sb₂Te_3–x Se _x form solid solution for 0<x1.25 and 2.75x<3. A systematic study of crystallization temperature in Sb₂Te_3–x Se _x (0x2.75) thin films prepared by flash evaporation was carried out. In preliminary experiments for some compositions, more than 10³ repetitions between amorphous and crystalline states were attained by the application of electric pulses.     

Soft magnetic properties of NiFeTa thin films for biasing film applications in magnetoresistive elements

The effects of Ta addition on the magnetic properties of permalloy thin films have been investigated. The alloy compositions on a weight basis are (Ni₈₁Fe₁₉)_1-x Ta _x with 0 x 0.105, and the films are sputtered onto a glass substrate at between room temperature and 300 C. The saturation magnetization and anisotropic magnetic field decrease with increasing Ta content. The saturation magnetization is 0.75 T at 5 wt % Ta. The coercivity remains constant at 125 Am^-1. The electrical resistivity increases linearly with increasing Ta content, then saturates at approximately 7.5 wt% Ta. The saturation resistivity is approximately 1.00 m. The magnetoresistivity ratio (/) decreases with increasing Ta content, mainly due to increased electrical resistivity (). The magnetostriction changes from negative to positive with increasing Ta content and reaches nearly zero at 2 wt% Ta. The NiFeTa films containing 5–6 wt% Ta have potential for use as the soft-biasing film in magnetoresistive elements.     

Isochoric pvT measurements of SF6 in the density range 100 to 1200 kg · m−3

The pvT properties of SF₆ were determined by an isochoric method. The pressure p and the temperature T were measured in the pressure range 1.5 MPa p 9.0 MPa and in the temperature range 293 K T 340 K. The results are compared with data in the literature. The adsorption of SF₆ on glass and iron surfaces and its influence on the pvT measurements are also briefly discussed.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Crystallization kinetics of Zr-rich FexZr0−x0−x0−x(20 ⩽ x ⩽ 40) metallic glasses

The crystallization kinetics of the melt-spun Fe-Zr metallic glasses in the iron-rich region has been investigated by means of DSC and X-ray diffraction. The crystallization mode changes with iron concentration. In the lower iron region, 20 x 25, the Fe _x Zr100–x glasses crystallize into -Zr and Ti₂Ni-type FeZr₂ with an accompanying sharp and large exotherm at the first crystallization step and immediately after this step, they transform into orthorhombic FeZr₃. On the other hand, the alloys with 35 x 40 exhibit a gradual exotherm which initiates from a temperature far below the definite crystallization temperature (T _x). The Fe-Zr metallic glasses in this concentration region crystallize polymorphously into the oxygenstabilized Ti₂Ni-type FeZr₂ with accompanying relatively small and composite exotherms. The annealing at a temperature where the gradual exotherm occurs for the alloys with 30 x 40 does not cause any changes of X-ray halo pattern but results in the reduction of the heat of exotherm due to the crystallization.     

Crystallization kinetics of bulk amorphous Se80−xSbxTe20

Crystallization kinetics of the Se_80–x Sb _x Te₂₀ (0x9) alloys have been studied using differential scanning calorimetry. The activation energies for the glass transition and that for crystallization have been determined from the heating rate dependence of the glass transition temperature and the peak crystallization temperature. The results have been analysed using the modified Kissinger's and Matusita's equations for the non-isothermal crystallization of materials. The variation of glass transition temperature with composition suggests that a small amount of Sb ( 4 at %) leads to an increase in the chain length of Se-Te, whereas further increase in Sb atomic per cent increases the number of Se-Te chains in the alloys.     

Investigation of structural transport and magnetic properties of the system Li0.25Cu0.5Fe2.25- x Al x O4

Experimental data on X-ray electrical conductivity, thermoelectric coefficient, magnetic hysteresis and infrared absorption spectra of the system Li_0.25Cu_0.5Fe_2.25–x Al _x O₄ are presented. All the compounds, 0x2.25, showed cubic symmetry. Lattice constant values progressively decreased on increasing the Al³⁺ content. X-ray intensity calculations, magnetic hysteresis and infrared spectroscopy studies indicated the presence of Li⁺, Al³⁺ and Fe³⁺ at tetrahedral and octahedral sites, while Cu²⁺ is present only at the octahedral site. The activation energy and threshold frequency increased with increasing values ofx. The compounds withx1.50 aren-type, and those withx2.0 arep-type semiconductors. Magnetic hysteresis indicated that compounds withx1.50 are ferrimagnetic, and those withx2.0 are antiferrimagnetic. High coercive force,H _c, values and remanence ratios (J _R/J_S) showed that all the compounds exceptx2.0 exhibit single-domain behaviour. The probable ionic configuration for the system is suggested as Li _0.15 ⁺ Al _0.5 ³⁺ Fe _0.35 ³⁺ [Li _0.1 ⁺ Cu _0.5 ²⁺ Fe _0.4 ³⁺ Al³⁺]O ₄ ^2– .     

Y xBayCuzO7− δ solid solutions

Y_xBa_yCu_zO_7– ceramics forming at isobaric conditions were studied by x-ray diffraction analysis, dynamic magnetic measurements and potentiometric titration. It was established that compositions: 0.8 x 1.2 y=2, z=3; x-1, 1.8 y 2.2, z=3; x=1, y=2, 2.7 z 3.4: are in the homogeneity range of 123. It was found that compositions with nonstoichiometric cations rations have minimum T_c.     

Nuclear ferromagnetic ordering of141Pr in the diluted Van Vleck paramagnets Pr1−x Y x Ni5

We report on the first investigation of the effect of magnetic dilution on nuclear magnetic spin interactions in metals; we studied this effect in the diluted Van Vleck paramagnets Pr_1–xY_xNi₅. In addition, we investigated the electronic magnetic properties of these intermetallic compounds. For this purpose, we have measured the nuclear and electronic susceptibilities of Pr_1–xY_xNi5 with x = 0 to 0.2 at 50K T 8 mK and at 2.2 K T 300 K. We observe a linear decrease of the electronic Van Vleck susceptibility and of the hyperfine enhancement factor by 13% (per mole Pr³⁺). The hyperfine-enhanced nuclear susceptibility decreases by about a factor of three when going fromx = 0 tox = 0.2. The samples show nuclear ferromagnetic transitions of¹⁴¹Pr with nuclear Curie temperatures T_c decreasing from 370 K to 100 K and Weiss temperatures decreasing from 218 K to 13 K in this concentration range. These data are compared with the results of mean-field calculations. In addition, we report on measurements of the nuclear spin relaxation time of these compounds, for which we find values of a few msec at millikelvin temperatures and a critical speeding-up at T_c. Our data give hints that for x > 0.2 the character of the nuclear magnetic transition may change, possibly to a nuclear spin glass freezing.     

X-ray,electrical conductivity and magnetic hysteresis studies of the chalcogenide system Zn1−xCuxCr2Se4

X-ray diffraction studies performed here indicated solid-solution formation between ZnCr₂Se₄ and CuCr₂Se₄, and all the compounds could be indexed on the basis of cubic spinel lattice. The compositions between 0 x 0.8 showed semi-conducting behaviour, while CuCr₂Se₄ (x=1.0) exhibited metallic conductivity. Seebeck measurements showd P-type behaviour for 0 x 1. Magnetic hysteresis indicated a ferromagnetic nature for all the compounds except ZnCr₂Se₄ (x=0). The shapes of/ _i plotted againstT exhibited single domain behaviour, and Curie temperatures showed an increase with increasing copper concentration.     

Photoelectric properties of undoped and lithium-doped Zn1−xMgxTe alloys

The room temperature photoelectric response of undoped and lithium-doped Zn_1–xMg_xTe (0 x 0.50) alloys has been measured in the wavelength range 0.50 3.0 m. The response curve for undoped samples is characterized by a single peak in the band edge region. The peak shifts with composition in accordance with the expected shift in the energy band gap. Lithium-doped samples show an additional peak centred at 1.04 eV for all compositions. This peak is attributed to photo-generated holes in the split-off band created as the result of electronic transitions to shallow acceptor impurities.     

Fermi liquid behaviour of the viscosity of3He-4He mixtures

We have investigated³He-⁴He mixtures at³He-concentrations 0.98%x9.5% by the vibrating wire technique in the temperature range 1 mKT 100 mK and at pressures 0 bar p 20 bar. In the degenerate regime of the mixtures the Landau theory of Fermi liquids predicts a temperature dependence of the viscosity proportionalT ^–2. We report on the first observation of this behaviour at 3 mKT 10 mK for all investigated concentrations and pressures. At temperatures below about 20 mK slip corrections had to be taken into account due to the increase of the quasiparticle mean free path at very low temperatures. The low-temperature cut-off in T ² = constant indicates the transition into the ballistic regime of the mixtures, where the mean free path of the quasiparticles exceeds the radius of the vibrating wire. Our results for the pressure dependence of the viscosity as well as for its magnitude show substantial differences from predictions based on pseudopotential theory. However, a calculation of with the quasiparticle interaction potential of recent solubility measurements in mixtures agrees well with our experimental data, in particular the pressure independence of .     

Superconducting properties of the lithium-substituted YBa2Cu4O8 prepared by the sol-gel method

The effects of the lithium substitution for copper on the properties of the superconducting YBa₂Cu₄O₈ were studied. Single-phase YBa₂Cu_4–x LixO₈ was successfully prepared by the sol-gel method under ambient pressure over a composition range of 0x 0.08, while impurity phases appeared at x=0.10 and 0.20. The lattice constants of YBa₂Cu_4–x Li _x O₈ were almost invariant with increasing lithium content x. The superconducting transition temperature decreased monotonically with increasing x in the range, 0x0.08. The suppression of superconductivity was discussed in terms of the impurity effect in the CuO₂ planes.     

High pressure synthesis and electrical properties of BaTiO3−xFx

The substitution of fluorine for oxygen in BaTiO₃ was investigated by the reaction of BaO₂, BaF₂, TiO₂ and Ti under the conditions of 3.0 GPa and 1300° C for 2 H. Batio_3–x F _x solid solutions were obtained as a single phase in the region of 0 x0.1. The X-ray diffraction data showed that the symmetrical change from tetragonal to cubic form occurred atx=0.08 at room temperature. Also, the solid solutions were characterized by TG-DTA analysis, ESCA spectroscopy and diffusion reflectance spectroscopy. As a result, the existence of Ti³⁺ was verified. The electrical resistivities of BaTiO_3–x F _x were in the range from 4cm for x=0.05 to 3Ocm forx=0.1 at 300 K and the relationship between In and 1000 T^–1 was linear. The thermoelectric powers at 300 K were –250V K^–1 forx=0.05 and –130VK^–1 forx=0.1. The electrical property of the solid solution was discussed on the basis of a conventional band model which involved localized donor levels associated with oxygen vacancies.     

Substitution of Cu by Fe in the superconducting ceramic oxide EuBa2Cu3Oy: Structure, electrical conductivity, and microwave response

A series of EuBa₂(Cu_1–x Fe _x )₃O_y (0.0x0.15) ceramics were prepared and examined by thermogravimetric analysis, X-ray powder diffraction, electrical resistance measurements, and a magnetically modulated microwave absorption technique. The oxygen content (y=6.98±0.03), determined by thermogravimetric analysis in a reducing atmosphere, was independent of the iron concentration forx0.12. The introduction of iron into the EuBa₂Cu₃O _y lattice, via substitution into copper sites, effects a progressive decrease in the superconducting transition temperature with increasing iron concentration. Normal-state conductivity similarly progresses from a low resistivity and negative temperature coefficient behavior, to a semiconducting-like response at the high iron concentrations. X-ray powder diffraction measurements indicate that material containing 6% iron and above is tetragonal, yet it remains superconducting with up to at least 12% iron.     

EPR and magnetic susceptibility studies of xCr2O3-(1-x) 3B2O3-PbO glasses

By using X-ray diffraction analysis, EPR and magnetic measurements, the chromium-ion distribution in xCr₂O₃-(1–x) [3B₂O₃-PbO] glasses with 0x35 mol% Cr₂O₃ was studied. EPR investigation evidenced the presence of both Cr³⁺ and Cr⁵⁺ ions, the latter being in small proportion, in agreement with the atomic magnetic moment values. For concentrations x20 mol% Cr₂O₃ the isolated Cr³⁺ ions coexist with those coupled by super-exchange magnetic interactions, the isolated ones prevailing only for x<3 mol% Cr₂O₃. For x>20 mol % Cr₂O₃ microcrystalline precipitates were detected, giving rise to an antiferromagnetic transition with Néel temperature, N _N310 K and paramagnetic Curie temperature, _p=–480 K.     

Critical review of the thermodynamics of the Na-Al-O system

The free energies of formation of various phases in the Na-Al-O systems are critically reviewed for the composition range between pure alumina and sodium-aluminate. The results clearly indicate that in the temperature range between 600 and 1100 K neither-alumina nor'-alumina (Na₂O.5Al₂O₃) are stable with respect to sodium; the most stable phase in that range being sodium aluminate. Least square fitting and statistical analysis of all the available data on the free energy of formation of-alumina deemed accurate yields G _form ⁰ (from oxides) = –8772+1.53T (J/mole of O) (600 T 1200K) If the composition of-alumina is assumed to be Na₂0.11Al₂O₃ then G _form ⁰ (Na_1/17Al_11/17O, from elements) = –565100+109.6T (J mole of O) (600 T 1200 K) If the composition is assumed to be 19 then: G _form ⁰ (Na_1/14Al_9/14O, from elements) = –564000+109.6T (J/mole of O) (600 T 1200K) Based on the only study to date, the best estimate for the free energy of formation of-alumina is G _form ⁰ (Na_1/8Al_5/8O, from the elements) = –567860+114T (J/mole of O) (600 T 1200 K)The uncertainty (95% confidence band) is estimated to be about ±200 J/mole of O. Based on these results-alumina should be thermodynamically stable at lower temperatures, but become unstable at temperatures >900 K and should, kinetics permitting, dissociate into-alumina and sodium aluminate. Experimental evidence, however exists that conclusively shows that-alumina is thermodynamically stable at much higher temperatures than 900 K which leads to the conclusion that G °_form for-alumina reported to date is too high and should be more negative.     

Heat capacity and magnetic properties in the (PdCO y )H x -system at low temperatures

Results of heat capacity measurements on (PdCo _y )H _x (y 2.5 at %; 0 x 0.8) in the temperature range from 2 to 12 K are reported. The dependence of the coefficient of the electronic heat capacity on the Co content and on the x value is considered. For the binary alloy PdCo _y a decreasing spin wave contribution to the heat capacity (T ^3/2) is found, which agrees well with the known ferromagnetic properties of the alloy. In contrast, for the phase of (PdCo _y )H _x a contribution to the heat capacity can be separated, probably caused by the thermal excitation of magnetic clusters. The temperature dependence of this cluster contribution can be well explained by means of an Einstein function. Conclusions are drawn with regard to the intercluster interaction and to the number of atoms per cluster.