Robocast Pb(Zr0.95Ti0.05)O3 Ceramic Monoliths and Composites

Robocasting, which is a computer-controlled slurry-deposition technique, was used to fabricate ceramic monoliths and composites of chemically prepared Pb(Zr_0.95Ti_0.05)O₃ ceramics. The densities and electrical properties of the robocast samples were equivalent to those obtained for cold isostatically pressed parts formed under a pressure of 200 MPa. Three-layer robocast composites that consisted of alternating layers of different sintered densities—93.9%/96.1%/93.9%—were fabricated using different levels of organic pore-former additions. Modification from a single-material to a multiple-material deposition robocaster was essential for the fabrication of composites that could withstand repeated cycles of saturated polarization switching under fields of 30 kV/cm. Furthermore, these composites withstood a poled ferroelectric-to-antiferroelectric phase transformation that was induced by a hydrostatic pressure of 500 MPa, during which strain differences on the order of 0.8% occurred between the composite elements.     

Ag2O-Doped Bi2Sr2Ca1Cu2Ox Superconductors Prepared via Melt-Quenching

Ag₂O-doped superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag₂O-doped, as-cast specimens exhibit superconductivity ( T _c= around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag₂O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag₂O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi₂Sr₂Ca₁Cu₂O _x ceramics sealed in metals or glasses. The addition of Ag₂O to Bi₂Sr₂Ca₁Cu₂O _x melt, however, had deleterious influences on the superconducting properties ( T _c and J _c) of the resultant ceramics when obtained by heat treatment in air.     

Interaction of Chemically Vapor Deposited YBa2Cu3Ox with Yttria-Stabilized Zirconia Substrates

High-resolution transmission electron microscopy and optical diffractograms have revealed that chemically vapor deposited films of superconducting YBa₂Cu₃O _x react to form an interaction layer with single-crystal yttriastabilized zirconia. The approximately 5 nm thick interlayer was identified as BaZrO₃. Zirconium was also found to diffuse through the entire YBa₂Cu₃O _x film.     

Lead-Free SrBi4Ti4O15 and Bi4Ti3O12 Material Fabrication Using the Microwave-Assisted Molten Salt Synthesis Method

In this paper, the microwave-assisted molten salt method (MAMSS) and molten salt method (MSS) were used to synthesize SrBi₄Ti₄O₁₅ (SBT). The phase constitution was determined by powder X-ray diffraction and the microstructure of powder was examined by scanning electron microscopy. In contrast to the conventional MSS method, MAMSS produces more distinct plate-like grains and synthesizes both SBT and Bi₄Ti₃O₁₂ (BTO) at 600°C with a 30-min soaking time. The increase of temperature and soaking time can make the plate-like grains of BTO more distinct.     

Electrical Properties of TiO2-K2Ti6O13 Porous Ceramic Humidity Sensor

Porous ceramic humidity sensors using pure TiO₂ doped with a few molar percent K₂CO₃ were studied. The conductance versus relative humidity (rh) sensitivity of the sensor sintered at 1200°C was as high as 4 orders of magnitude in the range of 15% to 95% rh at 200 Hz and 25°C. The sensors were reversible without repeated high-temperature thermal desorption processes. The electrical properties of the sensors under different rh were determined by both direct current (dc) and alternating current (ac) analysis techniques. The sensors could be polarized similar to electrolytes in a charging process as a result of the electrode and water molecular polarization effects, and the effects were enhanced with increasing rh. The conduction carriers of the sensors in a moist atmosphere were ions and electrons, and the dominant conduction carrier was ions. By complex impedance plots with a "non-Debye" capacitor concept, an equivalent circuit model was established which can simulate well all the electrical properties of the sensors in the range of 65% to 95% rh for all measured frequencies (5 Hz to 13 MHz). The relative dielectric dispersion k '(ω) increased with increasing rh and was almost proportional to ω^−1.85 at low frequencies. The relative dielectric absorption k "(ω) increased with increasing rh and was almost proportional to ω⁻¹ at low and median frequencies.     

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.     

Simultaneous Thermogravimetry and Evolved-Gas Analysis of YBa2Cu3O7 and LaBa2Cu3O7 Superconductors

Concurrent thermogravimetry (TG) and evolved-gas analysis (EGA) were done for YBa₂Cu₃O_7-z and LaBa₂Cu₃-O_7-z superconductors. The sample weights were monitored by thermobalance and the evolved O₂ and CO₂ species were monitored by quadruple mass spectrometer (QMS). No diffraction peak for the impurity phase containing a carbonate group was observed in the X-ray diffraction patterns for these samples, but the release of CO₂ was detected by EGA. CO₂ gas began to evolve from YBa₂Cu₃O_7-z at 543°C and from LaBa₂Cu₃O_7-z at 692°C. Preparation of high-quality YBa₂Cu₃O_7-z and LaBa₂Cu₃O_7-z superconductors is discussed on the basis of results of these thermal analyses.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

New Yellow Ceramic Pigment Based on Codoping Pyrochlore-type Y2Ti2O7 with V5+ and Ca2+

A new yellow pigment with the pyrochlore structure Ca_xY_{2− x} V _x Ti_{2− x} O₇ was prepared as a substitute for the decreasing variety of available yellow ceramic pigments due to the severe regulation of toxic lead and cadmium. The solubility limit of vanadium in this pigment was found to be 1.5 wt% as V₂O₅ or 0.13 as x in the above formula expression. Characterization of vanadium in the vanadium pyrochlore yellow pigment by electron spectroscopy for chemical analysis and electron spin resonance showed that the oxidation state of vanadium was V⁵⁺ and its yellow color mostly originated from V⁵⁺ substituted for Ti⁴⁺. Comparison of color characteristics of Ca_xY_{2− x} V _x Ti_{2− x} O₇ with those of commercial V–SnO₂ and V–ZrO₂ revealed that Ca _x Y_{2− x} V _x Ti₂₋O₇ had better color strength and brightness than the commercial pigments.     

Pyrolysis of Pb(Zr05Ti05)O3 Precursors: Avoiding Lead Partitioning

Three different chemical precursor routes were investigated to synthesize Pb(Zr₀₅Ti₀₅)O₃: mixing hexanoates, acetate complexing of alkoxides, and the synthesis of a mixed alkoxide by the reaction of titanium alkoxide and zirconium alkoxide with lead acetate. For each, elemental Pb and PbO were the first crystalline phase observed during pyrolysis conditions that involved rapid heating (e.g., to 400°C). The formation of Pb (and PbO) could be avoided by first heattreating hydrolyzed, mixed alkoxide precursor powders at 300°C for 1 h. This treatment was not effective for the two other precursors. It is concluded that both the carbonaceous content of the precursor (lowest for the hydrolyzed, mixed alkoxides) and the rate of hydrocarbon release during pyrolysis are critical to avoid the formation of elemental Pb during pyrolysis.     

Topotaxy between BaTiO3 and Ba6Ti17O40

The orientational relationship between intergrown lamellae of tetragonal (t) BaTiO₃ and monoclinic (m) Ba₆Ti₁₇O₄₀ was found to be ( 1 1 1 )_t||(001)_m, (001)_t||( 1 0 1 )_m, and [110]_t||[0 1 0]_m. On the basis of this relationship a model of a topotactic boundary between these two phases was constructed.     

Existence of the Ba2Ti5O12 Phase in Ca-Doped TiO2-Rich BaTiO3

A TEM investigation was conducted on the structure of a second phase precipitated between the grains of a polycrystalline TiO₂-rich BaTiO₃ which was doped with 8 mol% Ca. This phase was identified as Ca-stabilized Ba₂Ti₅O₁₂ with a 10-layer orthorhombic structure and unit-cell parameters a=0.990, b=1.131, and c=2.330 nm.     

Synthesis and Characterization of Sr2Ce2Ti5O16 in the System SrO-CeO2-TiO2

The ternary system SrO-CeO₂-TiO₂ was investigated using X-ray diffractometry. The formation of a new compound, Sr₂Ce₂Ti₅O₁₆, was established, and its compatibilities with SrO, SrCeO₃, and SrTiO₃ were studied. The results revealed the existence of a series of compounds Sr_6–12xCe_6xTi₅O₁₆ and solid solutions Sr_2+nCe₂Ti_5+nO_16+3n ( n ≤ 6).     

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.     

Tentative Phase Relationships in the System CaHfTi2O7-Gd2Ti2O7 with up to 15 mol% Additions of Al2TiO5 and MgTi2O5

The phase relationships in the CaHfTi₂O₇-Gd₂Ti₂O₇ (zirconolite-pyrochlore) pseudobinary system were investigated, after heating at 1500°C, because of their importance in the design of pyrochlore-rich titanate ceramics for immobilization of impure surplus plutonium. Up to 15 mol% of MgTi₂O₅ and Al₂TiO₅ were added to CaHfTi₂O₇-Gd₂Ti₂O₇ compositions to elucidate the effects of divalent and trivalent impurities on the phase stability within these systems. From X-ray diffractometry analysis, scanning electron microscopy, and energy dispersive X-ray spectrometry, phase formation and compositional stability limits were evaluated. The main phases observed in these systems were pyrochlore, perovskite, and polytypes of zirconolite. The formation of the 2 M -, 4 M -, and 3 O -zirconolite polytypes was dependent on the amount of aluminum or magnesium present. In the magnesium system, a large area of pyrochlore-only was observed, which indicated that divalent impurities of appropriate ionic size could be readily incorporated in the eightfold site of the pyrochlore. The locations of the tentative phase boundaries are discussed with respect to the chemical composition.     

Decomposition of Al2TiO5 and Al2(1-x)MgxTi(1+x)O5 Ceramics

The decomposition of Al_{2(1- x )}Mg _x Ti_{(1+ x )}O₅ ceramics in air has been studied between 900° and 1175°C for 0 lessthan equal to x lessthan equal to 0.6. The decomposition temperature versus composition x predicted using a thermodynamic model based on the regular solution approach is in satisfactory agreement with the experimental results. The decomposition kinetics has been studied at 1100°C for x = 0, 0.1, and 0.2 and follows a nucleation and growth mechanism. Random nucleation of the reaction products is hindered by the high elastic stresses that result from the molar volume change related to decomposition because of the small chemical driving force available. Decomposition occurs only at a limited number of sites, probably associated with the presence of impurities and/or glassy phase. The decomposition products grow as nodules formed by an Al₂O₃ (+ MgAl₂O₄ for x > 0) core and a TiO₂ shell. The growth is parabolic for x = 0 and linear for x = 0.1 and 0.2. The rate-controlling step in the decomposition mechanism of pure Al₂TiO₅ ( x = 0) is the transport of Al³⁺ ions through the TiO₂-rutile phase.     

Formation Mechanism of MoO3-Doped YSr2Cu3Oy Using the Citrate Process

The formation mechanism of the synthesis of MoO₃-doped YSr₂Cu₃O _y powders using the citrate process was investigated. It was shown that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ played a crucial role in forming the superconducting phase. It was found that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ interacted with water and decomposed when it was heavily milled and heated.     

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.