Superconductivity in the (Yba2Cu3)1−xNaxO7−δ System

The (YBa₂Cu₃)_1−xNa_xO_7–δ system in the range of x = 0–0.8 was investigated. Experimental data suggest that the sodium doping with x 0.26 does not affect the critical transition temperature T_c, and the crystal structure maintains the orthorhombic lattice with a slightly smaller unit cell. However, sodium doping increases the sintering and grain growth kinetics, resulting in a higher superconducting phase volume and an enhanced Meissner effect. It also lowers the processing temperaturel. The experimental data also suggest that the sodium atoms diffuse into the superconducting YBa₂Cu₃O_7−δ crystallites, which stabilizes the orthorhombic phase. The transition temperature (ortho-rhombic to tetragonal) in sodium-doped materials increases with the increasing concentration of sodium.     

Solid Solutions Based on Ferroelectric PbNb2,O6

An X-ray and dielectric study of three series of PbNb₂O₆-type solid solutions was carried out. The PbNbZOs-type phase was present up to the following limits: (1) (Pb_1−xBi₄/_3x)(Ti_xNb₂-_x.)O₆, ×= 0.2; (2) (Pb_l-_xBi₄3_x)(Ti₂Nb_l—2)₂0₆, ×= 0.2; and (3) (Pb_1−xBi₂/_3x)Nb₂O₆×= 0.3. Orthorhombic distortion b/a and Curie temperature decreased with increasing x. Bi_4/3-Ti₂0₆ was most effective in this respect and Bi₂, ₃-Nb₂O₆ had the least influence. The important role played by ions of different valency located on crystallographically equivalent sites is pointed out.     

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.     

Novel Solution-Soaking Method for Adding Dopants to YBa2Cu3O7−x to Achieve Higher Sintered Densities

YBa₂Cu₃O_7−x was doped with various metal ions by a new technique in which a pellet (after binder burnout) was soaked in a solution containing the appropriate ions and then dried. The sintered density of the treated pellets depended on the dopant in the solution, and in many cases it was much higher than that obtained for pure YBa₂Cu₃O_7−x (93% to 96% as compared with 85% to 90%). A study of the microstructure revealed that, in those cases where higher sintered densities were obtained, the grain size was much smaller. The grain morphology, however, did not change. It is likely that the high concentration of dopant adsorbed on the grain surface during solution soaking enhanced the rate of sintering.     

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.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Structural and Dielectric Properties of Ag(Nb0.8Ta0.2)1−xSbxO3 (x≤0.08) Ceramics

Sb₂O₅ were selected to substitute (Nb_0.8Ta_0.2)₂O₅ and the effects of Sb substitution on the dielectric properties of Ag(Nb_0.8Ta_0.2)O₃ ceramics were studied. The perovskite Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics showed no obvious change with x value being no more than 0.08, and the pseudoperovskite unit cell parameters a = c , b and monoclinic angle β decrease with Sb concentration increasing. The dielectric properties of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics were found to be affected greatly by the substitution of Sb for Nb/Ta. The ɛ value of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics sintered at their densified temperature increased from 480 to 825 with x from 0 to 0.08, the tan δ value decreased sharply from 0.0065 to 0.0023 (at 1 MHz) with x increasing from 0 to 0.04, and then kept a stable lower tan δ value ∼0.0024 with x to 0.08. The temperature coefficient of capacitance values continuously decreased from a positive value of 1450 ppm/°C for x =0 to a negative value of −38.52 ppm/°C for x =0.08.     

Thermodynamic Evaluation for the Y2O3–BaO–CuOx System

The thermodynamic data for the Y₂O₃–BaO–Cu₂O–CuO quaternary system were optimized from measured thermodynamic data. A two-sublattice model for ionic solution was used to express the Gibbs free energy of the liquid phase, and a two-sublattice regular solution model was used for the nonstoichiometric YBa₂Cu₃O_6+δ superconducting compound. The optimized thermodynamic data were used to calculate the phase diagrams of the Cu₂O–CuO binary system and the CuO _x –Y₂Cu₂O₅ and CuO _x –BaCuO₂ quasi-binary systems. The results were in good agreement with reported measured data. The liquidus projection and isothermal and vertical sections of the Y₂O₃–BaO-CuO _x quasi-ternary system were calculated. The effect of oxygen pressure on some reaction temperatures was predicted by calculating them at various oxygen pressures, and the oxygen contents (6 +δ) in YBa₂Cu₃O_6+δ were calculated at various temperatures and oxygen pressures. The results were compared with experimental data.     

Formation of Grain-Boundary Carbon-Containing Phase During Annealing of YBa2Cu3O6+x

In situ annealing studies of YBa₂Cu₃O_6+x performed in an optical hot stage revealed that, at temperatures ∧450°, localized melting occurred. On subsequent cooling, a discrete second phase was observed at the YBa₂Cu₃O_6+x grain boundaries. Quantitative chemical analysis using X-ray wavelength dispersive spectroscopy indicated that the second phase was composed of a barium oxycarbonate. The source of the carbon in the second phase was identified to be CO₂ in the atmosphere.     

Effects of La on Dielectric and Piezoelectric Properties of Pb1−xLa2x/3(Nb0.95Ti0.0625)2 O6 Ceramics

Ceramics with the chemical compositions of Pb_{1− x} La_{2 x /3}(Nb_0.95Ti_0.0625)₂O₆ (0≤ x ≤0.060) (PLTN) were prepared by the conventional solid-state reaction method. X-ray diffraction analysis indicated that Ti and La doping not only decreased the rhombohedral–tetragonal phase transformation temperature, but also stabilized the orthorhombic phase of PLTN ceramics. All ceramics sintered at 1190°–1250°C had shown the pure orthorhombic ferroelectric phase. La doping suppresses grain growth and inhibits the formation of pores and cracks, resulting in an increase in relative density up to 97%. The amount of La doping to PLTN ceramics obviously affect ceramics' piezoelectric constant ( d ₃₃) and dielectric loss (tanδ). The sample with x =0.015 possesses high Curie temperature ( T _c=560°C), low dielectric loss (tanδ=0.0054), and excellent piezoelectric constant ( d ₃₃=92 pC/N), presenting a high potential to be used in high-temperature applications as piezoelectric transducers.     

Development of Nano-Composite Microstructures in ZrO2-Al2O3 via the Solution Precursor Method

Aqueous mixtures of zirconium acetate and aluminum nitrate were pyrolyzed and crystallized to form a metastable solid solution, Zr_{1- x} Al _x O_{2− x /2} ( x < 0.57). The initial, metastable phase partitions at higher temperatures to form two metastable phases, viz., t −ZrO₂+γ-Al₂O₃ with a nano-scale microstructure. The microstructural observations associated with the γ- →α-Al₂O₃ phase transformation in the t -ZrO₂ matrix are reported for compositions containing 10, 20, and 40 mol% A1₂O₃. During this phase transformation, the α-Al₂O₃ grains take the form of a colony of irregular, platelike grains, all with a common crystallographic orientation. The plates contain ZrO₂ inclusions and are separated by ZrO₂ grains. The volume fraction of A1₂O₃ and the heat treatment conditions influence the final microstructure. At lower volume fractions of A1₂O₃, the colonies coarsen to single, irregular plates, surrounded by polycrystalline ZrO₂. Interpenetrating microstructures produced at high volume fractions of A1₂O₃ exhibit very little grain growth for periods up to 24 h at 1400°C.     

Water Corrosion of YBa2Cu3O7−δ Superconductors

Solid-state sintering was used to make YBa₂Cu₃O_7−δ superconducting bulk materials. Corrosion of the YBa₂Cu₃O_7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain-boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (T_c) decreased with corrosion time. After 2 h of corrosion, T_c decreased from 87 to 81 K.     

Effect of Oxygen Loss on Densification When Hot Isostatic Pressing YBa2Cu3O7−δ

Hot isostatic pressing of the high-T_c superconductor YBa₂Cu₃O_7−δ can lead to loss of oxygen and transformation of the material from the high-T_c orthorhombic phase to the nonsuperconducting tetragonal phase. It is shown that glass encapsulation helps retain the orthorhombic structure, whereas steel encapsulation resulted in formation of the tetragonal phase. Reasons for this phenomenon are discussed. The equilibrium oxygen gas pressure for the oxygen decomposition reaction in YBa₂Cu₃O₇, however, prevents full densification of this material in glass when employing hot isostatic pressing conditions of 200 MPa and 845°C.     

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.     

Ionic Conductivity Enhancement of La2Mo2−xWxO9 Nanocrystalline Films Deposited on Alumina Substrates by the Sol–Gel Method

Dense, crack-free, and uniform La₂Mo_{2− x} W _x O₉ ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly-alumina substrates via a modified sol–gel method, with inorganic salt of La(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, and (NH₄)₆H₂W₁₂O₂₄ as precursors. Pure La₂Mo₂O₉ phase was confirmed by X-ray diffractometer when the annealing temperature was >500°C. The average grain size of the La₂Mo_{2− x} W _x O₉ films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin-coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W-doping can suppress the phase transition that occurs at 580°C in pure La₂Mo₂O₉ and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La₂Mo₂O₉ films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain.     

Atomistic Study of Polarons in YBa2Cu3O7

Simulation techniques are applied to the orthorhombic YBa₂Cu₃O₇ crystal in order to describe possible polaron and bipolaron species in the crystal. Perfect crystal properties are fitted well with a two- and three-body potential model. The O⁻ polaron state is significantly stabilized by the presence of holes in the CuO plane. A bipolaron O²⁻₂ is slightly stable on the O₄ and O₂ or O₃ lattice ion sites of oxygen. The oxidation reaction leading to O⁻ and involving Cu⁺ and oxygen vacancies is exothermic.     

Microstructure and Piezoelectric Properties of (1−x)(Na0.5K0.5)NbO3–xLiNbO3 Ceramics

(1− x )(Na_0.5K_0.5)NbO₃– x LiNbO₃ [(1− x )NKN– x LN] ceramics were produced by the conventional solid-state sintering method, and their microstructure and piezoelectric properties were investigated. The formation of the liquid phase and K₆Li₄Nb₁₀O₃₀ second phase that were observed in the (1− x )NKN– x LN ceramics was explained by the evaporation of Na₂O during the sintering. A morphotropic phase boundary (MPB) was observed in the specimens with 0.05< x <0.08. Promising piezoelectric properties were obtained for the specimens with x =0.07. Therefore, the piezoelectric properties of this 0.93NKN–0.07LN ceramic were further investigated and were found to be influenced by their relative density and grain size. In particular, grain size considerably affected the d ₃₃ value. Two-step sintering was conducted at different temperatures to increase the grain size. Piezoelectric properties of d ₃₃=240 (pC/N) and k _p=0.35 were obtained for the 0.93NKN–0.07LN ceramics sintered at 1030°C and subsequently annealed at 1050°C.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

X-Ray Study of Phase Transitions in Ferroelectric PbNb2O6 and Related Materials

The phase transitions in PbNb₂O₆ and in compositions of the type Pb_1+x (B_xNb_1-x)O₆, where B = Ti⁴⁺, Zr⁴⁺, or Sn⁴⁺, have been investigated between 25° and 650°C. using X-ray and dilatometric techniques. The modified PbNb₂o₆ compositions possess orthorhombic PbNb₂O₆-type structure, with the additional Pb²⁺ ions occupying vacant lattice sites. The lattice parameters a and c expand and b contracts during heating until, at the ferroelectric Curie temperature, a and b suddenly coincide and c expands slightly. Besides this phase change at the Curie temperature, the nonstoichiometric compounds show an additional phase transition in the range 450° to 300°C. depending on composition. The intermediate phase of Pb_l+x(Ti₂Nb_1-z)₂O₆ appears to possess orthorhombic symmetry.     

Pyrochlore Phases in the System ZnO–Bi2O3–Sb2O5: I. Stoichiometries and Phase Equilibria

Two cubic pyrochlore phases exist in the system ZnO–Bi₂O₃–Sb₂O₅. Neither has the supposed "ideal" stoichiometry, Zn₂Bi₃Sb₃O₁₄. One, P ₁, is a solid solution phase, Zn_{2+ x} Bi_{2.96−( x − y )}Sb_{3.04− y} O_14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P ₂, is a line phase, Zn₂Bi_3.08Sb_2.92O_13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P ₁ and P ₂ in the ZnO–Bi₂O₃–Sb₂O₅ phase diagram have been determined.