Microstructural Analysis in Processing of the High-Tc Superconductor Ba2YCu3O7-x in Air

In the synthesis of the superconducting compound Ba₂YCu₃O_7-x from a stoichiometric mixture containing BaCO₃, Y₂O₃, and CuO In air, a low-melting liquid phase is formed at about 890°C. The liquid phase was identified as a ternary eutectic located within the compatibility triangle Ba₂YCu₃O_7-x–BaCuO₂–CuO. The implication of this finding for the processing of Ba₂YCu₃O_7-x is discussed.     

Preparation of Orthorhombic Ba2YCu3O7 Powder by Single-Step Calcining

A single calcination step, solid-state process that provides orthohombic Ba₂YCu₃O₇ powder is described. BaCO₃, Y₂O₃, and CuO are used as precursor materials. The only phase identifiable by X-ray diffraction is the orthorhombic Ba₂YCu₃O₇. The use of a vacuum during the inital stages of the calcining process promotes complete decomposition of the carbonate, and no residual carbonate is observed. An oxygen atmosphere during the later stages of calcining ensures proper oxidation to Ba₂YCu₃O₇. The use of a similar combination vacuum-oxygen calcining schedule should also be beneficial in the preparation of chemically derived powders.     

Fabrication and Characterization of Pyroelectric Ba0.8Sr0.2TiO3 Thin Films by a Sol-Gel Process

A sol-gel process was used to prepare pyroelectric Ba_0.8Sr_0.2TiO₃ thin films with large columnar grains (100–200 nm in diameter) on Pt/Ti/SiO₂/Si substrates, via using a 0.05 M solution precursor. The relationship between dielectric constant and temperature (ɛ_r- T ) showed two distinctive phase transitions in the Ba_0.8Sr_0.2TiO₃ thin films. Both the remnant polarization and the coercive field decreased as the temperature increased from −73°C to 40°C. Its low dissipation factor (tan δ= 2.6%) and high pyroelectric coefficient ( p = 4.6 × 10⁻⁴ C·(m²·K)⁻¹ at 33°C), together with its good insulating properties, made the prepared Ba_0.8Sr_0.2TiO₃ thin films promising for use in uncooled infrared detectors and thermal imaging applications.     

Synthesis of Ba2NaNb5O15 Powders and Thin Films Using Metal Alkoxides

Transparent and highly oriented Ba₂NaNb₅O₁₅ (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction of NaOC₂H₅, Nb(OC₂H₅)₅, and barium metal. The BNN precursor included a molecular-level mixture of NaNb(OC₂H₅)₆ and Ba[Nb(OC₂H₅)₆]₂ in ethanol. The alkoxy-derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600°C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800°C via the low-temperature phase. Highly (002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO (100) substrates at 700°C by using BNN underlayer.     

Synthesis of Oriented Ba2NaNb5O15 (BNN) Thin Films from an Alkoxy-derived Precursor

Ba₂NaNb₅O₁₅ (BNN) thin films with a tungsten bronze structure were fabricated using a precursor solution that was synthesized from barium, sodium, and niobium alkoxides. Highly (002)-oriented BNN films were prepared successfully on Pt(100)/MgO(100) substrates at 700°C, using a BNN underlayer. X-ray pole-figure measurement showed that the BNN films that crystallized on Pt(100)/MgO(100) substrates had two in-plane orientations. The remanent polarization and coercive field of the BNN film (thickness of 1.0 µm) that was crystallized at 700°C were 12.3 µC/cm² and 101 kV/cm, respectively, at –150°C (123 K). BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.     

Rare-Earth Barium Stannates: Synthesis, Characterization, and Potential Use as Substrates for YBa2Cu3O7-δ Superconductor

A new group of complex perovskites, REBa₂SnO_5.5 (where RE = Pr, Sm, Eu, and Gd) have been synthesized and sintered as single-phase materials with high sintered density and stability by the solid-state reaction method. The structure of REBa₂SnO_5.5 was studied by the X-ray diffraction technique, and all of them were found to be isostructural and have a cubic perovskite structure. The dielectric constant and loss factor values of these materials are in a range suitable for substrate applications. The X-ray diffraction and resistivity measurements have shown that there is no detectable reaction between YBa₂Cu₃O_7-δ and REBa₂SnO_5.5, even when the substances are mixed thoroughly and sintered at 950°C for 15 h. Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline REBa₂SnO_5.5 substrates gave superconducting zero resistivity transition T _C(0)= 92 K, indicating the suitability of these new materials as substrates for YBa₂Cu₃O_7-δ films.     

Orientation Relationships of Reactively Grown Ba6Ti17O40 and Ba2TiSi2O8 on BaTiO3 (001) Determined by X-ray Diffractometry

Fresnoite grows at 700° and 800°C, and Ba₆Ti₇O₄₀ grows at 1200°C with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO₂ films react with the BaTiO₃ crystals, forming the phases Ba₂TiSi₂O₈ (fresnoite) and Ba₆Ti₁₇O₄₀. At 700° and 800°C, both phases grow with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures.     

Barium Rare-Earth Hafnates: Synthesis, Characterization, and Potential Use as Substrates for YBa2Cu3O7-delta Superconductor

A new group of complex perovskites Ba₂REHfO_5.5 (where RE = La, Pr, Nd, and Eu) has been synthesized and sintered as single-phase materials with high sintered density and stability using a solid-state reaction method for the first time. The structure of Ba₂REHfO_5.5 has been studied by X-ray diffactometry (XRD) and all of the perovskites are isostructural and have a cubic structure. The dielectric constant and loss factor values of these materials are in a range suitable for their use as substrates for YBa₂Cu₃O_7-delta superconductors. XRD and resistivity measurements show that there is no detectable reaction between YBa₂Cu₃O_7-delta and Ba₂REHfO_5.5, even when the two substances are mixed thoroughly and sintered at 950°C for 15 h. The addition of Ba₂REHfO_5.5 up to 20 vol% in YBa₂Cu₃O_7-delta-Ba₂REHfO_5.5 composite shows no detrimental effect on the superconducting transition temperature of YBa₂Cu₃O_7-delta. Thick films of YBa₂Cu₃O_7-delta fabricated on polycrystalline Ba₂REHfO_5.5 substrate have a superconducting zero resistivity transition of 92 K, indicating the suitability of these new materials as substrates for YBa₂Cu₃O_7-delta films.     

Anisotropy in an Oriented Ba2YCu3O7 Superconductor

A highly oriented Ba₂YCu₃O₇ ceramic disk was prepared with the c axis perpendicular to the flat faces. The use of high pressures during cold-pressing tends to enhance the orientation of the fired sample. A nitric acid procedure for forming the material produces a powder which orients most readily. Magnetic measurements show that the critical current is larger in the layers perpendicular to the pressing direction.     

Dense Perovskite, La1-xA'xFe1-yCoyO3-δ (A'= Ba, Sr, Ca), Membrane Synthesis, Applications, and Characterization

La_{1- x} A' _x Fe_0.8Co_0.2O_3-δ (A'= Ca, Sr, Ba) perovskite powders were synthesized to attain the desired properties of high O₂ flux and stability under reducing conditions. Steady-state oxygen permeation rates for La_{1- x} A' _x Fe_0.8-Co_0.2O_3-δ perovskite membranes in nonreacting experiments with air on one side and helium on the other side of the membrane were in the order A' _x = Ba_0.8 > Ba_0.6 > Ca_0.6 > Sr_0.6. Partial oxidation of methane to syngas (CO + H₂) was performed in a dense La_0.2Ba_0.8Fe_0.8Co_0.2O_3-δ membrane reactor at 850°C in which oxygen was separated from air and simultaneously fed into the methane stream. The reducing atmosphere affected the membrane reaction-side surface while barium enrichment occurred on the air-side surface. Oxygen continuously transported from the air side appeared to stabilize the membrane interior, and the reactor was operated for up to 850 h.     

Hot Deformation of YBa2Cu3O7-δ and Composite YBa2Cu3O7-δ/Ag

The response of ceramic superconductors and ceramic composites to compressive stresses at high temperatures has been examined. Monolithic YBa₂Cu₃O_7-δ and composite YBa₂Cu₃O₇-δ₆/Ag were tested at constant true strain rates from 10^-6 to 10^-3 s^-1 at temperatures from 800° to 950°C. Fine-grained monolithic YBa₂Cu₃O_7-δ appears to have a regime of superplastic deformation between temperatures of 850° and 950°C at strain rates from 10^-6 to 10^-4 S^-1. The addition of 20 vol% Ag to a coarser-grained material enhances the ductility of the ceramic and lowers the flow stress by a factor of 3 to 10. However, there is no evidence of superplasticity in the composite material in the range of temperature and strain rate where it was tested.     

Growth of Highly Oriented TlBa2Ca2Cu3O9—y Superconducting Films on Ag Substrates Using a Dip-Coated Barium Calcium Copper Oxide Sol—Gel Precursor

Thin films of Ba_zCa₂Cu₃O₇ a precursor of TIBa₂, Ca₂-Cu₃O_9-y, were prepared by sol-gel synthesis from an all alkoxide solution. The barium and calcium precursors were the respective metals reacted with 2-methoxyethanol, and the copper precursor was copper methoxide complexed by triethanolamine. Silver substrates were coated using the sol-gel solution by dip-coating. Subsequent processing included a low-temperature drying step (600°C), repeated coating to provide the desired thickness, heat treatment at 850°C in oxygen to remove carbon, and reaction at 860°C in a two-zone thallination furnace to produce a TIBa₂Ca₂-Cu₃O_9-y, superconducting film. These films were strongly c-axis-aligned, had a zero-resistance temperature (T₂) of 110 K, and a critical-current density (J_c) of 1.9 × 10₄ A/cmZ at 77 K and zero magnetic field.     

Effect of Praseodymium and Uranium Addition on the Jc of Ba2 YCu3O7

Ba₂YCu₃O₇ ceramics doped with either Pr or U at 0.000 17 to 1.7 wt% levels have been prepared. For each sample J _c (magn) has been measured with a vibrating sample magnetometer. No improvement in J _c was found for either dopant and it is concluded that neither provides the clusters necessary to produce suitable pinning sites.     

Synthesis of Ba2RCu4O8 (R = Y, Sm) Superconductors at Oxygen Pressure of 1 atm by Spray-Frozen/Freeze-Drying Method

Precursor powders for high- T_c Ba₂RCu₄O₈ (R = Y, Sm) superconductors were successfully prepared through spray-frozen/freeze-drying (SF/FD) of a mixed aqueous solution that included the respective compounds according to the stoichiometric composition. These SF/FD powders were used to obtain ceramic superconductors composed of single-phase orthorhombic Ba₂RCu₄O₈ (R = Y, Sm) by sintering at an oxygen pressure of 1 atm. Critical temperatures measured were 84 K and 76 K for Ba₂YCu₄O₈ and Ba₂SmCu₄O₈, respectively, comparable to the previously reported values for these superconductors prepared under high oxygen pressure (100 atm). The present result was assumed attributable to the highly homogeneous powders.     

Synthesis of Highly Oriented Lead Zirconate–Lead Titanate Film Using Metallo-organics

Crack-free Pb(Zr,Ti)O₃ (PZT) thin films with preferred orientation were prepared successfully on MgO (100), SrTiO₃ (100), and Pt/Ti/SiO₂/Si substrates from metal alkoxide solutions. Calcination of precursor films in a H₂O─-O₂ gas mixture was found to be effective not only for low-temperature crystallization of perovskite PZT, but also for obtaining the preferred orientation of PZT films. Single-phase PZT films with high preferred orientation were synthesized on MgO (100) and Pt/Ti/SiO₂/Si substrates at 550° and 600°C for 2 h, respectively. The PZT film on the Pt/Ti/SiO₂/Si substrate showed a permittivity of 520, tan δ of 0.03, a remanent polarization of 24 μC/cm², and a coercive field of 54 kV/cm.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Phase Equilibria in the TiO2-Rich Region of the System BaO-TiO2

Phase relations in the system BaO-TiO₂ from 67 to 100 mol% TiO₂ were investigated at 1200° to 1450°C in O₂. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ was confirmed. The compound BaTi₂O₅ is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba₆Ti₁₇O₄₀ and Ba₄Ti₁₃O₃₀ decompose in peritectic reactions, and BaTiO₃ and Ba₆Ti₁₇O₄₀ react to form a eutectic. Special conditions are required for the formation of Ba₂Ti₉O₂₀, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Hot Isostatic Pressing of YBa2Cu3O7-δ and YBa2Cu3O7–δ/Ag Composites

Hot isostatic pressing (HIP) can be used to produce fully dense shapes of high-temperature ceramic superconductors. Densification modeling of monolithic YBa₂Cu₃O_7-δ and the composite YBa₂Cu₃O_7-δ/Ag systems allows an understanding of the HIP process and has led to the development of successful protocols for HIP of these materials. Ag metal is the best encapsulation material found for both systems. HIP of monolithic YBa₂Cu₃O_7-δ requires a slow ramp of pressure in order to prevent decomposition into more basic oxides such as Y₂BaCuO₅ and CuO. HIP of composite YBa₂Cu₃O_7-δ/Ag requires careful powder processing to obtain dense material with a fine dispersion of Ag.     

Sol-Gel Route to Ferroelectric Layer-Structured Perovskite SrBi2Ta2O9 and SrBi2Nb2O9 Thin Films

Precursors for layer-structured perovskite thin films of SrBi₂Ta₂O₉ (SBT) and SrBi₂Nb₂O₉ (SBN) were prepared by the reactions of a strontium-bismuth double methoxyethoxide and tantalum or niobium methoxyethoxide in methoxyethanol, followed by partial hydrolysis. Several spectroscopic techniques, such as ¹H-, ¹³C-, and ⁹³Nb-NMR (nuclear magnetic resonance), and Fourier-transform infrared spectroscopy were used to analyze the arrangement of the metals and oxygen in the precursor molecules. The precursors contained Sr-O-M (where M is Ta or Nb) bonds (i.e., a strontium is connected to two MO₆ octahedra) and Sr-O-Bi bonds with a bismuth atom bonded to the oxygens of the MO₆ octahedron. The arrangement of metals and oxygens was considered to be similar to the layer-structured perovskite crystal sublattice. As a result, the sol-gel-derived SBT thin films crystallized, by rapid thermal annealing in an oxygen atmosphere below 550°C, and they exhibited preferred (115) orientation. The crystallinity improved and the crystallite size increased with temperature up to 700°C. In the case of SBN thin films, a low heating rate (2°C/min) was necessary for the control of the crystallographic (115) orientation, whereas a rate of 200°C/s (rapid thermal annealing) produced films that exhibited c -axis orientation. The (115) SBT thin film, heated to 700°C, exhibited improved ferroelectric properties.