La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Probing of 1:2 Ordering in Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3 Ceramics by XRD and Raman Spectroscopy

The 1:2 ordering in Ba(Ni_1/3Nb_2/3)O ceramics sintered at 1350-1500°C has been investigated by using XRD and Raman spectroscopy. Both of the techniques show that the degree of the 1:2 ordering decreases as the sintering temperature increases. However, XRD discerns the 1:2 ordering only for the samples sintered at 1350-1400°C, whereas Raman spectroscopy discerns the 1:2 ordering for all the samples. Similar results have been obtained for Ba(Zn_1/3Nb_2/3)O₃ ceramics, where only the temperature range is slightly different. It is demonstrated that Raman spectroscopy can be a useful tool for probing of the 1:2 ordering in the A(B'^II_1/3B"^V_2/3)O₃-type complex perovskite compounds.     

Lanthanum-Modified (1 – x)(Bi0.8La0.2)(Ga0.05Fe0.95)O3·xPbTiO3 Crystalline Solutions: Novel Morphotropic Phase-Boundary Lead-Reduced Piezoelectrics

(1 – x )(Bi_0.8La_0.2)(Ga_0.05Fe_0.95)O₃· x PbTiO₃ (BLGF-PT) crystalline solutions have been fabricated by solid-state reactions. BLGF-PT has single perovskite phase structure with a rhombohedral–tetragonal (FE_r-FE_t) morphotropic phase boundary (MPB) at a PT content of x = 0.43. Lanthanum substitution has been found to increase the insulation resistance and decrease the coercive field down to 20 kV/cm, which results in significant improvements in dielectric and piezoelectric properties of BLGF-PT. The dielectric constant, loss tangent, Curie temperature, remnant polarization, piezoelectric d ₃₃ constant, and planar coupling factor of 1760, 0.05, 264°C, 33 μC/cm², 295 pC/N, and 0.36, respectively, have been achieved for BLFG-PT in the vicinity of the MPB. Compared with conventional Pb(Zr,Ti)O₃ (PZT) piezoelectric ceramics, the BLGF-PT is a competitive alternative piezoelectric material with decreased lead content.     

Preparation of PbZrO3–PbTiO3 Ferroelectric Thin Films by the Sol–Gel Process

Ferroelectric films, PbZr _x Ti_{1− x} O₃ ( x = 0 to 0.6), have been prepared from corresponding metal alkoxides partially stabilized with acetylacetone through the sol-gel process. The films dip-coated in an ambient atmosphere were heat-treated at 400°C for decomposition of residual organics and then at temperatures between 500° and 700°C for crystallization of the films. The perovskite phase precipitated at temperatures above 560°C, accompanied by an increase in dielectric constant. The dielectric constant of the films, which was comparable with that of sintered bodies, showed a maximum (∼620) at around x = 0.52 in PbZr _x Ti_{1− x} O₃. These films showed D – E hysteresis, with slightly higher values of coercive field, compared with those of sintered bodies.     

Characteristics of Si3N4-SiO2-Ce2O3 Compositions Sintered in High-pressure Nitrogen

Full-density Si₃N₄-SiO₂-Ce₂O₃ compositions were prepared by sintering with 2.5 MPa nitrogen pressure at temperatures of 1900° and 2090°C. Room-temperature flexural strengths near 700 MPa for sintered material compared favorably with the strength of hot-pressed material. At 1370°C, where flexural strengths as high as 363 MPa were obtained, it was observed that the coarsest structure was the strongest and the finest structure was the weakest. One of the compositions tested, Si₃N₄-8.7 wt% SiO₂-8.3 wt%-Ce₂O₃, was found to have excellent 200-h oxidation resistance at 700°, 1000°, and 1370°C, without incidence of 700° to 1000°C phase instability and cracking.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

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.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

Thermal Stability of Al3BC3

The thermal stability of Al₃BC₃ powder was analyzed. Nearly X-ray-pure Al₃BC₃ powder was obtained through the calcination of the aluminum, B₄C, and carbon mixture at 1800°C in Ar. In contrast to the former investigations, which reported the melting of so called "Al₈B₄C₇" at 1800°C, Al₃BC₃ did not melt up to 2100°C. Instead, it decomposed by the vaporization of aluminum. The decomposition occurred distinctly at 1400° and 1900°C in flowing Ar and a sealed carbon crucible, respectively. The results indicated that the decomposition temperature depended on the partial pressure of aluminum vapour in the atmosphere.     

High-Temperature Behavior of MoSi2 and Mo5Si3

The high-temperature stability and behavior of MoSi₂ was studied by heating dense sintered specimens under a vacuum of 10⁻⁵ mm Hg in the temperature range 1700° to 2000°C. The resulting material was examined using physical measurements, X-ray analysis, and metallographic techniques. The decomposition of MoSi₂ into Mo₅Si₃ is described. The Mo₅Si₃-MoSi₂ eutectic temperature was determined as 1900° C, and the melting points of MoSi₅ and Mo₅Si₃ were determined as 1980° and 2085° C, respectively.     

Negative Thermal Expansion in (HfMg)(WO4)3

A single-phase material (HfMg)(WO₄)₃ with an orthorhombic structure, A₂ (WO₄)₃-type tungstate, has been successfully prepared for the first time by the calcination of HfO₂, MgO, and WO₃, substituting Hf⁴⁺ and Mg²⁺ for A³⁺ cations in A₂(WO₄)₃. The new material shows a negative thermal expansion coefficient of approximately −2 ppm/°C from room temperature to 800°C. The mechanism of negative thermal expansion is assumed to be the same as that of Sc₂(WO₄)₃.     

Mechanical Activation-Assisted Synthesis of Pb(Fe2/3W1/3)O3

Perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) was prepared via a mechanical activation-assisted synthesis route from mixed oxides of PbO, Fe₂O₃, and WO₃. The mechanically activated oxide mixture, which exhibited a specific area of >10 m²/g, underwent phase conversion from nanocrystalline lead tungstate (PbWO₄) and pyrochlore (Pb₂FeWO_6.5) phases on sintering to yield perovskite PFW, although the formation of perovskite phase was not triggered by mechanical activation. When heated to 700°C, >98% perovskite phase was formed in the mechanically activated oxide mixture. The perovskite phase was sintered to a density of ∼99% of theoretical density at 870°C for 2 h. The sintered PFW exhibited a dielectric constant of 9800 at 10 kHz, which was ∼30% higher than that of the PFW derived from the oxide mixture that was not subjected to mechanical activation.     

Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

The effect of glass addition on the properties of BaO–TiO₂-WO₃ microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B₂O₃, SiO₂, 5ZnO–2B₂O₃, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B₂O₃ to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B₂O₃ glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B₂O₃.     

Decomposition Reactions in CaCu3Ti4O12 Ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics sintered in air at 1115°C for 3 and 24 h have been heat treated in N₂ at 1000°C. Surface layers develop on the outer regions of the ceramics, and a combination of X-ray diffraction and analytical electron microscopy has been used to establish the phase content of the layers. A model to explain the formation of the surface layers is proposed based on decomposition of CCTO into a mixture of CaTiO₃, TiO₂, and Cu₂O. The role of limited decomposition in the development of electrically inhomogeneous CCTO ceramics prepared at elevated temperatures in air is discussed.     

Surface Decomposition of Strontium-Doped Soft PbZrO3–PbTiO3

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZT) soft piezoelectric ceramics ( d ₃₃≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO₂, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO₂ and tetragonal PZT (PbZrO₃–PbTiO₃) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.     

Novel Deposition of Columnar Y3Al5O12 Coatings by Electrostatic Spray-Assisted Vapor Deposition

A novel and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) was used to deposit Y₃Al₅O₁₂ (YAG) coatings. Polycrystalline single-phase Y₃Al₅O₁₂ coatings were synthesized using the ESAVD method in an open atmosphere at 650°C, and then annealed at 700°–900°C for 1 h. The ESAVD process involves the decomposition and chemical reactions of charged aerosol in vapor phase. The low-temperature coating deposition characteristics of the ESAVD process using a suitable sol precursor decreases the reaction and crystallization temperatures for forming Y₃Al₅O₁₂ coatings. The microstructure of the Y₃Al₅O₁₂ coating prepared using the ESAVD method is columnar and such strain-resistance microstructure could be useful for thermal barrier coating applications.     

Character of Hydration of 3CaO.Al2O3

The C₃A compacts were hydrated and the reaction was studied by DTA, X-ray diffraction, mercury porosimetry, and volume change analysis. The hexagonal hydroaluminates C₂AH₈ and C₄AH₁₉ formed at 2°, 12°, and 23°C by a direct mechanism between C₃A and H₂O. The hydration reaction at 52° and 80°C was stopped by formation of C₃AH₆ around the C₃A grains. The rate of conversion of the hexagonal hydrates to cubic C₃AH₆ increased with temperature. Volume change analysis confirmed that C₃AH₆ grows epitaxially on the surface of the C₃A grain. The reaction at this surface and the passage of water through the layer of hexagonal hydroaluminates control the overall reaction rate. The conversion of the hexagonal hydrates to C₃AH₆ accelerates the reaction by removing the layer of products from around the C₃A grain by a solution mechanism. At 52° and 80°C, C₃AH₆ may form without the intermediate formation of the hexagonal hydrate.     

Raman Spectroscopic Study of Gallium-Doped Ba(Zn1/3Ta2/3)O3

Resonators of Ba(Zn_1/3Ta_2/3)O₃, sintered between 1450° and 1600°C, are characterized by Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. The quality factors of the resonators are found to depend on sintering temperature, and at 1600°C there is evidence of Zn loss from the surface. The frequency of the A_1g Raman mode changes from 800.9 cm⁻¹ for a sample with Q = 80000 (2 GHz), to 794.5 cm⁻¹ when Q = 44000 (2 GHz). Changes in the position of this and other Raman modes are thought to be due to distortions of the oxygen octahedra, brought about by Zn loss. The presence of a BaTa₂O₆ phase at the surface is confirmed by XRD and SEM.     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.