Solid-Solution Ranges of the n= 2 and n= 3 Superconducting Phases in Bi2(SrxCa1 −x)n+1CunOy and the Effect on Tc

The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi₂(Sr _x Ca_{1 − x} ) _{n +1}Cu _n O _y system were established along tie lines with compatible phases via electron probe microanalysis on bi- (or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T _c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c -axis parameter.     

Crystal Structure and Characterization of Pure and Ag-Doped (La1−xYx)2Ba2CaCu5Oz (0≤x≤0.5) Superconductors

Pure and 5 wt% of Ag-doped (La_{1− x} Y _x )₂Ba₂CaCu₅O_z superconducting compounds for x =0–0.5 have been prepared. Analysis of X-ray diffraction patterns shows that the samples are essentially in the single-phase form and they could be refined using the P 4 /mmm space group in a tetragonal cell. The typical lattice parameters are found to be a = b =3.879 Å, c =11.646 Å for x =0 and a = b =3.856 Å, c =11.576 Å for x =0.5 samples. The detailed crystal structure parameters are presented. The average grain size values from scanning electron microscope images are found to be in the order of 1–3 μm. Temperature variations of ac susceptibility and electrical resistivity have been measured. Superconducting transitions with diamagnetic T _c ranging from 60 to 75 K have been observed, with the maximum T _c for the x =0.25 sample. Ag-doped samples exhibit improved inter-granular coupling and homogeneous oxygenation.     

Alignment of YBa2Cu3O7-x and Ag─YBa2Cu3O7-x Composites at ∼930°C by Eutectic Formation

The present work describes a new technique to synthesize aligned YBa₂Cu₃O_{7- x} and Ag─YBa₂Cu₃O_{7- x} superconducting composites from Ba- and Cu-deficient compositions (relative to YBa₂Cu₃O_{7- x} ) plus BaCuO₂. For YBa₂Cu₃O_{7- x} , high transition temperature midpoint T_c (91 K), temperature of zero resistivity T ₀ (90 K), and critical current density J_c (>3000 A°Cm⁻² at 77 K) were achieved by using this technique. This procedure provides the potential for using a reliable and reproducible densification and alignment technique alternative to partial or full melting. The composite is highly aligned, with an average grain size of ∼1 to 2 mm and domains of width greater than 5 mm. The initial phase assemblage consists of YBa₂Cu₃O_{7- x} (123) as the major phase plus YBa₂CuO₅ (211) CuO as minor phases. The BaCuO₂ is added to the Ba- and Cu-deficient starting composition in order to assist in the formation of a CuO-rich liquid as well as to compensate for the Ba and Cu deficiences in 123. Since the liquid forms at ∼900°C and is compatible with 123, it can be used to facilitate alignment of 123 at ∼930°C. The addition of Ag to the system results in eutectic formation with the (solidified) liquid, substantial filling of the pores during sintering, and improved alignment.     

Synthesis of Y-Doped BaTiO3–(Bi1/2K1/2)TiO3 Lead-Free Positive Temperature Coefficient of Resistivity Ceramics and Their PTC Effects

As a lead-free positive temperature coefficient of resistivity (PTCR) material, (1– x mol%) BaTiO₃– x mol% (Bi_1/2K_1/2) TiO₃– y mol% Y₂O₃–0.5 mol% TiO₂ (BT– x BKT–2 y Y–0.5TiO₂) systems were prepared by the conventional solid-state reaction method. All samples containing <2 mol% BKT sintered in air possessed relatively low room-temperature resistivity (ρ₂₅) and high positive temperature coefficient (PTC) effect. However, when the BKT content exceeded 2 mol%, the sample was not semiconductive after sintering in air. The effects of sintering schedule on the properties of PTCR ceramics were discussed. The results showed that the optimum composition of BT–1BKT–0.2Y–0.5TiO₂, sintered at 1330°C for not-soaking and then fast quenched in air, achieved rather low ρ₂₅ of 28 Ω·cm and a high jump of resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) of 4.0 orders of magnitude with T _c about 155°C. The ρ₂₅ of the as-sintered sample could be further reduced to about 10 Ω·cm by annealing in N₂ at 450°C for 30 min, accompanied decrease on the PTC effect.     

Phase Structure and Electrical Properties of (K0.48Na0.52)(Nb0.95Ta0.05)O3-LiSbO3 Lead-Free Piezoelectric Ceramics

(1− x )(K_0.48Na_0.52)(Nb_0.95Ta_0.05)O₃– x LiSbO₃ [(1− x )KNNT− x LS] lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases was identified in the composition range of 0.03< x <0.05. The ceramics near the MPB exhibit a strong compositional dependence and enhanced electrical properties. The (1− x )KNNT– x LS ( x =0.04) ceramics exhibit good electrical properties ( d ₃₃=250 pC/N, k _p=45.1%, k _t =46.3%, T _c=348°C, T _{o − t} =74°C, P _r=25.9 μC/cm², E _c=10.7 kV/cm, ɛ_r∼1352, tan δ∼3%). These results show that (1− x )KNNT– x LS ceramic is a promising lead-free piezoelectric material.     

Characterization of a Large-Scale Nondoped YBa2Cu3O7−x Superconductor Prepared by Plastic Forming Without High-Pressure Molding

A slurry containing YBa₂Cu₃O_{7− x} particles and a fine YBa₂Cu₃(OH) _x colloid solution was prepared, and a large-scale bulk YBa₂Cu₃O_{7− x} superconductor (about 50 mm × 35 mm × 2 mm) was produced by plastic forming without high-pressure molding. The samples molded from the slurry were dried and then fired at 1223 K in air. X-ray diffraction data indicated that the samples had the characteristic orthorhombic YBa₂Cu₃O_{7− x} structure. Measurements of electrical resistance were carried out between 300 and 50 K by the standard four-probe DC electrical measurement. The samples began superconducting at an onset temperature around 92 K, and the full-transition temperature (critical temperature) ( T _c) was 88.7±1.4 K. The critical current density ( J _c) measured at 77 K was about 440 A/cm², the value of J _c was improved by the heat treatment under an oxygen atmosphere, and J _c=1.6 × 10³ A/cm² was observed. Under the magnetic field (B=1 T), the sample held its superconductivity, and demonstrated that this method can be used to produce the magnetic shielding used in magnetic resonance imaging diagnosis.     

K/Na Ratio Dependence of the Electrical Properties of [(KxNa1−x)0.95Li0.05](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K _x Na_{1− x} )_0.95Li_0.05](Nb_0.95Ta_0.05)O₃ (K _x NLNT) ( x= 0.40–0.60) lead-free piezoelectric ceramics were prepared by conventional solid-state sintering. The effects of K/Na ratio on the dielectric, piezoelectric, and ferroelectric properties of the K _x NLNT ceramics were studied. The experimental results show that the electrical properties strongly depend on the K/Na ratio in the K _x NLNT ceramics. The K _x NLNT ( x =0.42) ceramics exhibit enhanced properties ( d ₃₃∼242 pC/N, k _p∼45.7%, k _t∼47%, T _c∼432°C, T _o−t =48°C, ɛ_r∼1040, tanδ∼2.0%, P _r∼26.4 μC/cm², E _c∼10.3 kV/cm). Enhanced electrical properties of the K _x NLNT ( x =0.42) ceramics could be attributed to the polymorphic phase transition near room temperature. These results show that the K _x NLNT ( x =0.42) ceramic is a promising lead-free piezoelectric material.     

Effect of Praseodymium on Superconductivity and Magnetism in Y1–xPrxBa2Cu4O8 Prepared by Nitrite Pyrolysis Method

X-ray diffraction patterns show that most samples of Y_1-x Pr_xBa₂Cu₄O₈ examined in the present study contained a single YBa₂ Cu₄O₈ (1-2-4) superconductive phase for x<0.7.Lattice parameters a and b increased with Pr concentration, suggesting that most of the Pr is trivalent in Y_1-x Pr_x-Ba₂Cu₄O₈. The zero-resistance temperature, T _co, decreases monotonically from 80 K at x=0 to 12 K at x=0.65, and superconducting transition widths tend to broaden for x>0. The room-temperature resistivity changes linearly until x=0.7 and increases abruptly at x=-0.75. The critical concentration, x_cr, thus was estimated to be 0.7. The effective magnetic moments of Pr in Y _1-x Pr_xBa₂Cu₄O₈ were 3.63., 3.35, and 3.23, μ_B for x=0.2, 0.4 and 0.6, respectively. In the R_0.8 Pr_0.2Ba₂Cu₄O₈ system, the depression of T_c weakly depends on the ionic radius of rare-earth elements. Similarities and differences between Y _1-x Pr_xBa₂Cu₄O₈ and Y_1-xPr_x-Ba₂Cu₃O_7-y also were noted and are discussed in this paper.     

Melt Processing of YBa2Cu3O7-x

Sintering YBa₂Cu₃O_{7- x} bulk forms at 1050°C followed by annealing at 980°C causes the development of a thick oriented surface layer (Lotgering factor = 0.7). The thickness of the layer depends on the thermal treatment, which is a two-step sintering process. Firing at 1050°C for 2.5 h followed by 30 h at 980°C leads to the development of a 0.1-mm-thick surface layer, with clear indication that longer annealing would result in a thicker film. Some orientation develops during un-axial compaction of the powders. Lotgering orientation factor calculation from X-ray diffraction analysis. SEM, and TEM were used to characterize the microstructure of these samples. T _c was similar to that of conventionally processed high-density samples, between 83 and 87 K. Some thermal treatments resulted in samples that displayed high resistivity above T _c , possibly caused by segregation of Cu to the grain boundaries.     

High Q Microwave Dielectric Ceramics in (Ni1−x Znx)Nb2O6 System

(Ni_{1− x} Zn _x )Nb₂O₆, 0≤ x ≤1.0, ceramics with >97% density were prepared by a conventional solid-state reaction, followed by sintering at 1200°–1300°C (depending on the value of x ). The XRD patterns of the sintered samples (0≤ x ≤1.0) revealed single-phase formation with a columbite ( Pbcn ) structure. The unit cell volume slightly increased with increasing Zn content ( x ). All the compositions showed high electrical resistivity (ρ_dc=1.6±0.3 × 10¹¹Ω·cm). The microwave (4–5 GHz) dielectric properties of (Ni_{1− x} Zn _x )Nb₂O₆ ceramics exhibited a significant dependence on the Zn content and to some extent on the morphology of the grains. As x was increased from 0 to 1, the average grain size monotonically increased from 7.6 to 21.2 μm and the microwave dielectric constant (ɛ'_r) increased from 23.6 to 26.1, while the quality factors ( Q _u× f ) increased from 18 900 to 103 730 GHz and the temperature coefficient of resonant frequency (τ_f) increased from −62 to −73 ppm/°C. In the present work, we report the highest observed values of Q _u× f =103 730 GHz, and ɛ'_r=26.1 for the ZnNb₂O₆-sintered ceramics.     

Fundamental Material Aspects Underlying the Preparation of High-Temperature Superconducting (Bi,Pb)2+xSr2Ca2Cu3O10+d Ceramics

Phase equilibria of the quasi-quinary system Bi₃O₃-PbO-SrO-CaO-CuO were studied between 650° and 900°C in air with emphasis on the high-temperature superconducting phase (Bi,Pb)_{2+ x} Sr₂Ca₂Cu₃O_{10+ d} (2223). The 2223 phase lies in equilibrium with a number of nonsuperconducting phases and also with the superconducting phase (Bi,Pb)₂Sr₂CaCu₂O_{8+ d} (2212). The single-phase region was found to be very limited. The Pb solubility of the 2223 phase is strongly temperature dependent. The phase relations are very sensitive to variations of the cation concentration and temperature. This effect significantly influences the preparation of 2223 ceramics.     

Preparation and Characterization of a Large-Scale YBa2Cu3O7−x Superconductor Prepared by Plastic Forming without a High-Pressure Molding: Effect of Polyvinyl Alcohol (PVA) Addition on the Superconducting Properties

The preparation of large-scale YBa₂Cu₃O_{7− x} superconductor samples was investigated. This method is based on plastic forming using a slurry consisting of YBa₂Cu₃O_{7− x} particles and a sol solution made up of multimetallic hydroxide particles (YBa₂Cu₃(OH) _x colloidal particles) and poly(vinyl) alcohol (PVA). The effects of adding PVA on the product, the crystallinity, and the superconducting properties of the sample were investigated. It was found that PVA acted as a protective colloid in the sol solution and stabilized YBa₂Cu₃(OH) _x colloidal particles, and that the role of PVA changed from a thickener to a flocculant during drying so that the formability/workability of the green sheet sample was improved and large samples (about 80 mm × 80 mm × 3 mm) without large cracks were obtained after firing. The samples became superconducting at 91.5±0.5 K ( T _con) and the full transition temperature ( T _coff) was 88.5±1.5 K. The critical current density ( J _c) of the sample prepared from the slurry containing 1 wt% PVA was 713±150 A/cm² at 77 K. This J _c value was improved to 2300 A/cm² by heat treatment at 773 K under an oxygen atmosphere.     

Microstructure, Density, and Dielectric Properties of Lead-Free (K0.44Na0.52Li0.04)(Nb0.96−xTaxSb0.04)O3 Piezoelectric Ceramics

Lead-free (K_0.44Na_0.52Li_0.04) (Nb_0.96−xTa_xSb_0.04)O₃ piezoelectric ceramics were prepared by the conventional solid-state sintering method. The grain growth of the ceramics was inhibited and the relative density was improved with Ta substituting for Nb. Increasing x led to different variations of dielectric properties before and after poling, and prevented the occurrence of orthorhombic–tetragonal phase transition (at T _{o − t} ). All the ceramics show an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics. Significantly enhanced dielectric and piezoelectric properties were obtained in the ceramics with x =0.20. The ceramics are very promising lead-free materials for electromechanical device applications.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Influence of Ca2PbO4 on Phase Formation and Electrical Properties of (Bi,Pb)2Sr2Ca2Cu3O10/Ag Superconducting Composites

(Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) precursor powders with large and small amounts of Ca₂PbO₄ phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca₂PbO₄ on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature ( T _m,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca₂PbO₄. Tapes with a small amount of Ca₂PbO₄ had a higher T _m,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca₂PbO₄. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current-carrying capacity of the tapes were significantly improved by reducing the Ca₂PbO₄ content of the precursor powders. These results are of practical significance for the commercialization of Bi-based high-temperature superconductors.     

Superconducting Properties of Barium-Doped (Bi,Pb)2Sr2Ca2Cu3Ox Glass-Ceramics

The Ba-doped superconducting (Bi,Pb)₂Sr_{2- x} Ba _x Ca₂Cu₃O _y and (Bi,Pb)₂Sr₂Ca_{2- x} Ba _x Cu₃O _y (0 ≦ x ≦ 1.0) were prepared by using a melt-quenching method, and the effect of Ba additions on the glass-forming ability and the crystalline phase was examined. The glass-forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO₃ as well as CaO was observed in the melt-quenched sample of (Bi,Pb)₂SrBaCa₂Cu₃O _y . BaPbO₃ crystals were precipitated in all glass-ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high- T _c phase, and (Bi,Pb)₂Sr_1.4Ba_0.6Ca₂Cu₃O _y glass-ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a T _c of 103 K, although BaPbO₃ and the low- T _c phase were still largely present.     

Electrical Resistivities of Monocrystalline and Polycrystalline TiB2

The electrical resistivity of monocrystalline and polycrystalline TiB₂, was measured under an inert atmosphere by a four-point ac impedance technique over the range 298 to 1373 K. The results are expressed in the form ρ-ρ₂₉₈= m(T -298). The following values of ρ₂₉₈ (μω.cm) and m (nω.cm-K^-1) were determined: for polycrystalline TiB₂ (69% dense) 18.2 and 95; for polycrystalline TiB₂ (99% dense) 7.4 and 42; and for monocrystalline TiB₂, 6.6 and 34.9.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Chemical Solution Deposition of Transparent and Metallic La0.5Sr0.5TiO3+x/2 Films Using Topotactic Reduction

Metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films were prepared by the chemical solution deposition (CSD) method using topotactic reduction processing. The use of Si powder as the reducing agent was facile and allowed easy manipulation. It was observed that metallic (resistivity at 300 K ∼2.43 mΩ cm) and transparent (∼80% transmittance at visible light) La_0.5Sr_0.5TiO_{3+ x /2} films could be obtained with an annealing temperature of 900°C, which was significantly lower than the hydrogen reduction temperature (∼1400°C). The successful preparation of metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films using CSD has provided a feasible route for depositing other perovskite-structured functional layers on La_0.5Sr_0.5TiO_{3+ x /2} films using this low-cost all CSD method.     

Devitrification of High-SiO2 Glasses of the System Al2O3-SiO2

Pertinent works of previous investigators are reviewed briefly. A superior technique for the preparation of experimental glasses of high purity and chemical homogeneity is presented. Devitrification growth-rate data and traction-viscosity data are reported for high-silica glasses of the system Al₂O₃-SiO₂. Glasses with added Al₂O₃ were found to devitrify to cristobalite more rapidly than ultrapure silica glasses at a given temperature, and yet their fluidities (i.e., reciprocal viscosities) were found to be less. The devitrification growth rate, g, was found to be of zero order and to follow the temperature dependence In( g/T ) = -(B/T) + In A over the temperature range 1561°K. T 1730°K.