Effect of partially oxidized Ti powders on the microstructure and PTCR characteristics of (Ba,Sr)TiO3

Rutile TiO₂ (a=4.594 å and c=2.958 å) phase was formed on the outer region of Ti powders after oxidation at 600 °C for 1–300 h. Porous (Ba,Sr)TiO₃ ceramics were fabricated by adding partially oxidized Ti powders (4–8 vol %) into (Ba,Sr)TiO₃ powders, and showed excellent positive temperature coefficient of resistivity (PTCR) characteristics after paste-baking treatment at 580 °C in air. The PTCR characteristics of the porous ceramics were mainly attributed to the adsorption of oxygen at the grain boundaries. The microstructure and electrical properties of the porous (Ba,Sr)TiO₃ ceramics containing the partially oxidized Ti powders oxidized at 600 °C for different oxidation times (1–300 h) were investigated.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Effect of Bi/Pb ratio and annealing temperature onTc and formation of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.80.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

Effects of zirconium on the structural and dielectric properties of (Ba,Sr)TiO3 solid solution

The effects of zirconium on (Ba, Sr)TiO₃ solid solution are studied by preparing (Ba_0.75Sr_0.25) (Zr_y Ti_1-y)O₃ (BSZT) ceramics with ranging from 0 to 0.15. With increased incorporation of zirconium the lattice parameters increase with decreasingc/a ratio, while a linear relationship is found between (a ² c)^1/3 values and the content of zirconium. The Curie temperature (T _c) is also lowered linearly with increasing . The Curie temperature of the (Ba_1-xSr_x (Zr_yTi_1-y)O₃ system can be estimated byT _c = 125 – 270x – 350_y (°C). Observations of microstructures also show that a small amount of zirconium promotes the grain growth, while a higher content will then inhibit it. Dielectric properties of BSZT dielectrics and BSZT-based boundary layer (BL) capacitors are also correlated to have similar temperature dependence, where the maximum dielectric constant at room temperature is found aty = 0.1.     

High-T c superconductivity in theRBa2−xSrxCu3O7 system forR ≡ Y,Gd, Sm and Nd

RBa_2–x Sr_xCu₃O₇ samples whereRY, Gd, Sm and Nd were synthesized in oxygen between 930 and 1070 °C. The effect of the Ba substitution by Sr on the sample optimum synthesis temperature, microstructure and electrical properties was investigated for 0x1.0. The magnitude of the resistivity increase and of theT _c decrease due to the Ba substitution by Sr is found to depend on theR ionic radius. Moreover, the Sr addition has a decreasing effect onJ _c for allR elements.     

Dielectric properties of (Na0.5Bi0.5)1 − xMexTiO3 ceramics near morphotropic phase boundary

Ceramic samples of modified ferroelectric sodium-bismuth titanate (Na_0.5Bi_0.5)_0.87 Me_0.13TiO₃ (Me = Pb, Sr and Pb + Sr), were prepared using conventional solid state reaction techniques. The studies of powder X-ray diffraction of the obtained compounds revealed their rhombohedral symmetry at room temperature, the increase of lattice constant and the increase of rhombohedral lattice distortion (except for the material with Sr dopand, where distortion decreases). Temperature (at room temperature to 400°C) and frequency (at 20 Hz to 1 MHz) dielectric measurements reveal that A-site cations addition of Pb and/or Sr have resulted in the increase of relative electric permittivity. However, the temperature T _m (when the electric permittivity is a maximum) increases after Pb or Sr doping and it decreases after (Pb + Sr) doping. The pyroelectric and current loop measurements have shown that all samples were ferroelectric. The results of these measurements also allowed us to determine the temperature variation of the remanent and spontanous polarizations. The polarizations are found to decrease after Pb or Sr doping and increase after (Pb + Sr) doping. The piezoelectric constants (d ₃₃ and d ₃₁) and electromechanical coupling factors (k ₃₃ and k ₃₁) were obtained from resonance-antiresonance measurements method. The best piezoelectric and electromechanical properties have NBT doped by Pb. This ceramic may be good candidate for device applications. The diffuse ferroelectric phase transition of the investigated materials, similarly as for pure NBT, has been revealed. The properties of these materials (especially in diffuse phase transition range) can be explained by the behaviour of polar regions.     

BPSCCO Superconductor with Addition of Ag2O Synthesized Under Electrical Field

Ag₂O-doped (1.2% wt.) nitrate freeze-dried powders (Bi : Pb : Sr : Ca : Cu = 1.7 : 0.3 : 2 : 2.5 : 3.5) were processed under an external electrical field and 17.5 MPa pressure at 800°C, for 4 min in vacuum. Final heat treatments (HT) were applied at 835–850°C for 70 h. in air (Bi, Pb)₂Sr₂CaCu₂O _x (2212-phase) was formed by electrical field processing in just 4 min. Electrical field application enhanced (Bi, Pb)₂Sr₂Ca₂Cu₃O _y (2223-phase) formation during final HT. Ag₂O additions to field sintered BSCCO ceramics increased the amount of 2223-phase and the zero resistance critical temperature (T _e(R=0)) by 4 k.     

Annealing and irradiation studies of MoC x thin films

On single-phase B1 molybdenum carbide thin films prepared by the rf sputtering technique at substrate temperatures between 40 and 800°C isochronous annealing and He-irradiation investigations were performed. The transition temperature to superconductivityT _c ,the lattice parametera ₀, and the specific resistivity were measured as a function of annealing temperature and fluence, respectively. Both during annealing and irradiation MoC _x behaves quite differently from typical refractory compounds such as NbC and NbN. During annealingT _c drops pronouncedly between 250 and 400°C. The B1 structure transforms to -Mo₂C between 600 and 900°C. During irradiationT _c rises slightly and then drops by only 0.5 K. The lattice parametera ₀ changes only by 0.2%.On leave of absence from the Institute of Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia.     

Preparation, Structure, and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + δ Ceramics with Si3N4 Additions

The microstructure, phase composition, texture, and superconducting properties (T _c, T _c, j _c(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} ceramics (sintering at 840°C for 36 h) with ultrafine Si₃N₄ additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si₃N₄ is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.     

Ageing effect on microstructural and mechanical properties of mullite-ZrO2-TiO2 composites

The microstructural and mechanical properties of mullite-zirconia composites with TiO₂ (0.25 and 1.0 mol) additions have been studied, after ageing the samples over a wide temperature range (1000 to 1500° C) for long periods of time (100 to 200 h). In the sample with 0.25 mol TiO₂ addition, changes in mullite composition and in the solid state compatibility at temperatures below 1450° C were detected. In the sample containing 1 mol TiO₂, decomposition of Al₂TiO₅ occurs atT1200° C. Both compositions exhibit no increment in zirconia average grain size during ageing and, concomitantly, there is no strength degradation until higher temperatures (>1400° C) are reached, which become more drastic when Al₂Ti₅ is present.     

Structure determination at room temperature and phase transition studies aboveT c in ABi4Ti4O15 (A = Ba, Sr or Pb)

The room temperature structure of three compounds belonging to the Aurivillius family (n = 4), ABi₄Ti₄O₁₅ (A = Ba, Sr or Pb) has been analysed. BaBi₄Ti₄O₁₅ crystallizes in a tetragonalI4/mmm space group whereas SrBi₄Ti₄O₁₅ and PbBi₄Ti₄O₁₅ crystallize in the orthorhombic space group A2₁am. The starting model for the Sr and Pb analogues was derived fromab initio methods and refined using the Rietveld method. The cations Ba and Sr are disordered over the Bi sites while the Pb cation is found exclusively in the [Bi₂O₂]²⁺ layers. The TiO₆ octahedra are tilted with the Ti-O bonds forming zigzag chains along the “c” axis. The displacement of Bi atoms along the “a” axis might be responsible for ferroelectricity in these compounds. The high temperature X-ray data aboveT _c indicate no structural transition for A = Ba and Pb while A = Sr transforms to the tetragonal structure.     

Kondo effect and impurity interactions in the resistivity of diluteZnMn alloys

The electrical resistivity of diluteZnMn alloys (c=1.7–2400 ppm Mn) has been investigated in the temperature range from 0.05 to 14 K. For the most dilute sample, single-impurity Kondo behavior is found, well described by the Hamann formula withT _K=0.9 K,S=3/2. ForT<50 mK, aT ² law with _R =0.3 K is expected. In the dilute limit the Kondo slope is –(1/c)d()/d(logT)=3.7±0.2 µ-cm/at % dec. In the more highly concentrated alloys, the slope decreases with increasing c and the lnT-like variation of the Kondo resistivity roughly terminates near a temperatureT _W(c),T _W being related to the average Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction strength between the Mn impurities. For 20 ppm c 1000 ppm, the resistivity slightly decreases at low temperatures and a broad resistivity maximum is observed atT _m (c), withT _m c ^0.7. ForT 2 _m, the resistivity dependence is linear inT, and for the most concentrated alloy aT ^3/2 orT ² dependence is measured at the lowest temperatures attainable. The investigation of the transition temperatureT _c (c) to superconductivity ofZnMn results in a critical concentrationc _cr=18 ppm Mn. The concentration dependence ofT _c below 0.3 K suggests the presence of the Kondo effect, although impurity interactions may also influenceT _c in this temperature range.Supported by Deutsche Forschungsgemeinschaft.     

Optimization of superconducting critical temperatures by control of cation and anion stoichiometry in Bi2Sr2CaCu2Oδ-based solid solutions

T _c data are reported for powders of cation-stoichiometric Bi₂Sr₂CaCu₂O and for nonstoichiometric samples based on the three mechanisms BiSr, SrCa and Sr vacancy. For each, the T _c values depend critically on the final oxygen contents, which were varied by heating samples in either O₂ or N₂ at different temperatures. Stoichiometric Bi₂Sr₂CaCu₂O has the highest T _c, 92 K, obtained after heating in O₂ at 820 °C. Heating in O₂ at lower temperatures gives rise to overdoping and T _c decreases to 60 K. The other cation compositions show a smaller maximum T _c but also less reduction in T _c on overdoping. Under-doped samples, with reduced T _c values were obtained on heating in N₂. These data, together with selected literature results, lead to a unified picture of the variation of T _c with cation composition and oxygen content.     

Lattice Anomalies in (La,Sr)2CuO4 Under Epitaxial Strain Probed by Polarized X-Ray Absorption Spectroscopy

Local lattice anomalies in optimally doped T-(La,Sr)₂CuO₄ single crystal like thin films (T _c = 43.4 K) grown by molecular-beam epitaxy have been studied by the in-plane polarized Cu K-edge extended X-ray absorption fine structure (EXAFS). The results indicate temperature-dependent local atomic displacements which are anomalous at the T _c and below a higher temperature T _s as demonstrated by a change in the mean square relative displacement of the Cu–O bond , i.e., a sharp drop at the T _c and a gradual deviation from a noncorrelated Debye-like behavior below T _s where the spatial inhomogeneity appears. We find that the magnitude of the Cu–O displacement changes at the T _c, is enhanced by compressive strain while the tendency of charge segregation is suppressed. The results suggest that the uniaxial pressure effects stabilize the system by decreasing the onset temperature and magnitude of spatial heterogeneity.     

Superconducting properties of Pb/Ag multilayers

The electrical resistivity, ac susceptibility, critical current density, and Josephson tunneling of Pb/Ag multilayers have been measured in wide temperature ranges in order to study the proximity effect. A resistivity drop was found to occur atT _c of Pb in versusT measurement and, at low temperature, the multilayers show proximity effect. The I-V characteristic of the multilayer shows voltage steps, indicating a resistive state has occurred in the sample. The superconducting properties of the multilayer were analyzed with the bilayer theory of the proximity effect.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

A thermally-stimulated-current study of ethylene-methyl-methacrylate copolymers

Two ethylene-methyl methacrylate (EMMA) copolymers were studied by infrared (i.r.) spectroscopy, Differential Scanning Calorimetry (DSC) and Thermally Stimulated Currents (TSCs). The critical temperature, T _c, relaxation time, _c (measured in seconds), and Degree of Disorder (DOD) of the polymers were also measured by Relaxation Map Analysis (RMA). The i.r. and DSC results showed that these two copolymers are random copolymers with a melting point, T _m, at 82.6 °C for a sample containing 18 wt % methyl methacrylate (EMMA18), and at 53.0 °C for a sample having a MMA content of 38 wt % (EMMA38). From TSC spectra analysis, EMMA18 showed a glass-transition point, T _g, at – 41.0 °C, whereas EMMA38 had a T _g at – 36.0 °C together with a -relaxation at – 120 °C. Both samples have their space charges in the range 5–30 °C. The RMA showed that EMMA18 has T _c = – 5.74 °C, log _c = – 2.90 ( in seconds) and DOD = 54.65. EMMA38, however, has T _c = – 1 5.71 °C, log _c = – 1.59 and DOD = 48.69. The absence of -relaxation in EMMA18 was explained by evidence that it has a higher DOD. While the low mechanical properties of EMMA38 were mainly attributed to the contribution from the short chain segments, demonstrated by its low T _c.     

Preparation of nanocrystalline SrBi2Ta2O9 powders using sucrose-PVA as the polymeric matrix

Nanocrystalline powders of SrBi₂Ta₂O₉ (Strontium Bismuth Tantalate) have been prepared through evaporation of a polymer-based aqueous precursor solution. The precursor solution was obtained by homogeneous dispersion of the water-soluble metal salts (i.e., strontium nitrate, bismuth nitrate and tartarate complex of tantalum) in a polymeric matrix created by an aqueous solution mixture of sucrose and polyvinyl alcohol. Complete evaporation of the precursor solution (at 200°C) resulted in a fluffy, porous, carbonaceous mass, which on calcination at 750°C/2 h yield the single-phase SrBi₂Ta₂O₉ powders with average particle size 35 nm. The compacted powders, after sintering at 1000°C/4 h, show density of 96.8% of its theoretical value and dielectric constant value of 862 with Curie temperature (T _c) at 287°C, when measured at 100 KHz.     

Superconducting and mechanical properties of YBCO/Ag composites fabricated at high pressures up to 5.4 GPa

The YBCO/Ag superconducting composites were fabricated by compressing powders of YBCO and silver mixed at various volume proportions 0 to 100% under pressures of 2.0 and 5.4 GPa and subsequently sintering at 900 °C for 10 h in a flowing oxygen atmosphere. The superconducting transition temperature, particularlyT _c (R=0), was found to retain the value above 85 K up to 50 vol % Ag while the normal resistivity at 300 K to reduce below 10 cm. The highestJ _c of 400 Acm^–2 was attained for the sample containing 20 vol % Ag pressed at 5.4 GPa. The mechanical properties were also improved by the addition of silver: the compressive strength is increased by two to three times and the maximum strain, which is a measure of ductility, reached a value almost four times as large as that in the pure YBCO prepared under the ordinary pressure of 0.5 GPa. The persistence of the superconducting characteristics up to 70 vol % Ag is discussed in terms of the microstructure taken with the scanning electron microscope.     

Thermal shock of quartz lascas

As a pre-treatment to grinding, quartz lascas (crushed pieces) were thermally shocked into room-temperature water by quenching from temperatures between 50 and 800 °C. Comminuted particles exhibited two distinctive geometries, granular forT _q(quench) <T _c (573 °C) and needle-like whenT _q>T _c. The needle-like shapes become thinner and longer with increasing temperature aboveT _c. The differences in shape are believed to result from the differences in the crack generation patterns which are governed by the thermoelastic properties in the -phase and -phase of the quartz during the thermal shock process. Crack densities induced by the thermal shock were measured as a function ofT _q. For the temperature range of 200 °C<T _q<T _c andT _c<T _q<800 °C, the resulting crack densities were determined to be governed by the rate of crack nucleation, which is characterized by an Arrhenius-type equation. The activation energies associated with the crack nucleation rates for the two regions were determined to be 14 and 39 kJ mol^–1, respectively.