High-Temperature Proton-Conducting LaNbO4-Based Materials: Powder Synthesis by Spray Pyrolysis

Submicrometer lanthanum ortho-niobate (LaNbO₄ (LN))-based powders have been prepared by spray pyrolysis of an aqueous solution containing stable La–EDTA and Nb–malic acid complexes. The powders had a particle size of ∼0.1 μm, a narrow particle size distribution, and high purity after calcination above 800°C. The powders possessed excellent sintering properties resulting in >98% dense materials at 1200°C. The present route is shown to be excellent for the large-scale preparation of high-quality LaNbO₄-based powders.     

Phase Transitions and Transient Liquid-Phase Sintering in Calcium-Substituted Lanthanum Chromite

This paper investigates sintering and phase transitions of La_0.7CaxCrO₃(0.25≥x≥0.35), a material useful as electrical interconnections in solid oxide fuel cells. Heating of the quenched, metastable single-phase chromite resulted in exsolution of CaCrO₄ due to Ca solubility limitations below 1200°C. A transient liquid phase formed between 850° and 1000°C as the CaCrO₄ melted, causing partial densification in materials having 0.25 < x < 0.30. A slight increase in Ca content induced an additional liquid-phase sintering event, likely due to melting of Ca₃(CrO₄)₂, which facilitated near-complete densification by 1250°C. After enhancing sintering, the secondary phases redissolved into the chromite.     

{111} Twin Formation and Abnormal Grain Growth in Barium Strontium Titanate

Two series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba, Sr)TiO₃). In the first series, the phase equilibria in the BaTiO₃–SrTiO₃–TiO₂ system were determined. XRD and WDS analysis, done in the BaTiO₃-rich region, of 45(Ba,Sr)TiO₃–10TiO₂ samples annealed at 1250°C for 200 h in air showed that (Ba,Sr)TiO₃ was in equilibrium with Ba₆Ti₁₇O₄₀ (B₆T₁₇) and Ba₄Ti₁₃O₃₀ phases with strontium solubility (Sr/(Ba + Sr)) of ∼0.02 and 0.20, respectively. In the second series the microstructures of samples consisting of a mixture of (Ba,Sr)TiO₃ and 2.0 mol% TiO₂, were observed after sintering at 1250°C for 100 h in air. {111} twins formed only in the samples with faceted B₆T₁₇ second phase particles, similar to the case of BaTiO₃. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B₆T₁₇. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution.     

Synthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors

Powders of composition Ba_0.65Sr_0.35TiO₃ were prepared from catecholate precursor phases, BaTi(C₆H₄O₂)₃ and SrTi (C₆H₄O₂)₃. The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000.     

The Role of Li2CO3 and PbO in the Low-Temperature Sintering of Sr, K, Nb (SKN)-Doped PZT

It is demonstrated that the use of ∼0.9 mol% Li₂CO₃ (LC) as a sintering aid for Sr, K, Nb modified Pb(Zr_{1− x} ,Ti _x )O₃ (PZT) ceramics is effective only in the presence of excess PbO (∼2 mol%). It is shown that LC and PbO react to form the compound, Li₂PbO₃ (LPO) which has a melting temperature of ∼836°C. Using dilatometry, we were able to correlate shrinkage during heating of a green ceramic to the melting of the LPO. Consequently, complete densification and sizeable grain growth are achieved by solution-precipitation of the ceramic through the liquid phase. Importantly, sintering is not particularly effective with such small additions of either LC or PbO alone. In confirmation of this model, the LPO compound was presynthesized and used as the only sintering aid in the same PZT composition. The densification behavior of this mixture compared well with the case of separate additions.     

Hot Isostatic Pressing of Transparent Nd:YAG Ceramics

This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO₂ doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO₂ levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO₂, especially at higher temperatures, because the SiO₂ solubility limit is exceeded.     

Incorporation of Yttrium in Barium Titanate Ceramics

In this work, X-ray diffractometry and scanning electron microscopy techniques were used to study the incorporation of yttrium in BaTiO₃, with the following nominal compositions: (Ba_{1- x} Y _x )TiO₃(0.015 ≤ to x ≤ to 0.08), (Ba_{1- x} Y _x )Ti_{1- x /4}-(□_Ti) _{x /4}O₃(0.005 ≤ to x ≤ to 0.1), and Ba(Ti_{1- y} Y _y )O_3-delta(0.028 ≤ to y ≤ to 0.258). The phase assemblage and the lattice parameters indicated a slight solubility (∼1.5 at.%) of yttrium at the Ba sites at 1440°C but a high solubility (∼12.2 at.%) of yttrium at the Ti sites at 1515°C. When BaTiO₃was heavily doped with yttrium at the Ti sites (a yttrium concentration ( y ) of <0.059), the crystallographic structure was tetragonal, whereas for y greater than equal to 0.059, the crystallographic structure was cubic.     

Reactions between Strontium-Substituted Lanthanum Manganite and Yttria-Stabilized Zirconia: II, Diffusion Couples

Formation of secondary phases and diffusion of cations in diffusion couples of yttria-stabilized zirconia and lanthanum manganite substituted with 0 to 60 mol% strontium have been studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. Only the primary phases were observed after 120 h at 1200°C, while formation of secondary phases was identified already after 1 h heat treatment at 1350°C. The phase composition of the reaction layer altered from La₂Zr₂O₇ to SrZrO₃ at increasing Sr content in La _x Sr_{1- x} MnO₃. The thickness of the reaction layer was increasing with heat treatment time. In diffusion couples of La_0.4Sr_0.6MnO₃ formation of manganese oxide was observed in the perovskite layer after 1 h heat treatment at 1350°C, while isolated grains of SrZrO₃ relatively deep inside the zirconia were observed after longer heat treatment time. Diffusion of Mn into zirconia was observed preferenced along grain boundaries in the early stage of the interface reaction.     

A Co-precipitation Technique of Preparing LaNbO4 Powders

A simple co-precipitation technique was successfully used for the preparation of pure ultra fine, single-phase LaNbO₄ (LN). Aqueous sodium hydroxide was used to precipitate La³⁺ and Nb⁵⁺ cations as hydroxides simultaneously under basic conditions. For comparison, LN powders were also prepared by the traditional solid-state method. It is observed that the co-precipitation technique produces LaNbO₄ on heating at temperatures as low as ∼300°C, whereas complete phase formation occurs only at 900°C or higher in the solid-state method. The phase contents and lattice parameters were studied using powder X-ray diffraction (XRD). The XRD results show that phases obtained by both methods were different. The average particle size and morphology of these powders were analyzed by scanning electron microscope.     

Microstructure and Grain-Boundary Effect on Electrical Properties of Gadolinium-Doped Ceria

The microstructural evolution and grain-boundary influence on electrical properties of Ce_0.90Gd_0.10O_1.95 were studied. The nanoscale powders synthesized from a semibatch reactor exhibited 50% green density and 92% sintering density at 1200°C (∼200°C lower than previous studies). Impedance spectra as a function of temperature and grain size were analyzed. The Ce_0.90Gd_0.10O_1.95 with finest grain size possessed highest overall grain-boundary resistance; this contribution was eliminated at temperatures >600°C, regardless of grain size. The grain conductivity was independent of grain size and was dependent on temperature with two distinct regimes, indicative of the presence of Gd'_Ce− V _o^∘∘ complexes that dissociated at a critical temperature of ∼580°C. The activation energy for complex dissociation was ∼0.1 eV; the value for the grain-boundary was ∼1.2eV, which was size independent.     

Novel Niobium-Doped Titania Varistor with Added Barium and Bismuth

Characteristics of the sintered compacts and the microstructures and electrical properties of Nb-doped TiO₂ varistors with added Ba and Bi were studied at various sintering temperatures ranging from 1250° to 1400°C. Adding both Ba and Bi to Nb-doped TiO₂ ceramics resulted in a maximum intergranular phase and a minimum weight loss at 1350°C. In contrast, adding either Ba or Bi alone produced no such maxima and minima. The intergranular phases included mainly a Bi₂Ti₂O₇ crystal phase, apt to occur at a triple junction, and a Ba-rich amorphous phase that surrounded the grains, but discontinuously. The intergranular phases varied consistently with variation in electrical properties. The optimum conditions for the most efficient boundary barrier layer, with the lowest weight loss and the highest resistivity at low frequencies, were 1350°C with both Ba and Bi addition. The highest values for α (∼9.5), V_gb10 (∼0.8 V), and E_B (∼0.42 e V) support that finding. The effective relative dielectric constant, K _eff∼ 20 000, also was obtained under optimum conditions. The single addition of either Ba or Bi, however, produced nearly the opposite results, as discussed in this paper.     

Influence of Nd2O3 Doping on the Reaction Process and Sintering Behavior of BaCeO3 Ceramics

The influence of Nd₂O₃ doping on the reaction process and sintering behavior of BaCeO₃ is investigated. Formation of BaCeO₃ is initiated at 800°C and completed at 1000°C. When Nd₂O₃ is added to the starting materials, the formation of BaCe_1–xNd_xO_3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe_1-xNd_xO_3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd₂O₃ content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO₃ is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO₃ with Nd₂O₃ markedly enhances its sinterability. The enhancement of the sinterability of Nd₂O₃-doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering.     

The Solid Solubility of Holmium in BaTiO3 Under Reducing Conditions

The solid solubility of Ho³⁺ in BaTiO₃ fired under the reducing conditions of a nitrogen atmosphere containing 10% of hydrogen was studied by quantitative electron-probe microanalysis using wavelength-dispersive spectroscopy. The solubility was found to depend mainly on the starting composition. In the TiO₂-rich samples, the solubility of the Ho³⁺ donors at the Ba lattice sites was measured to be ∼4 mol% Ho_Ba (Ba_0.96Ho_0.04TiO_3−δ at 1400°C), which is much higher than after sintering in air. In the BaO-rich compositions, the solubility of the Ho³⁺ acceptors at the Ti sites (solubility limit     at 1500°C) decreased compared with that in air. Like with sintering in air, under reducing conditions, the highest solubility of holmium in BaTiO₃ was determined when it was incorporated at both lattice sites (solubility limit     ).     

Amorphous Intergranular Films in Silicon Nitride Ceramics Quenched from High Temperatures

High-temperature microstructures of an MgO-hot-pressed Si₃N₄ and a Yb₂O₃+ Al₂O₃-sintered/annealed Si₃N₄ were obtained by quenching thin specimens from temperatures between 1350° and 1550°C. Quenching materials from 1350°C produced no observable changes in the secondary phases at triple-grain junctions or along grain boundaries. Although quenching from temperatures of ∼ 1450°C also showed no significant changes in the general microstructure or morphology of the Si₃N₄ grains, the amorphous intergranular film thickness increased substantially from an initial ∼ 1 nm in the slowly cooled material to 1.5–9 nm in the quenched materials. The variability of film thickness in a given material suggests a nonequilibrium state. Specimens quenched from 1550°C revealed once again thin (1-nm) intergranular films at all high-angle grain boundaries, indicating an equilibrium condition. The changes observed in intergranular-film thickness by high-resolution electron microscopy can be related to the eutectic temperature of the system and to diffusional and viscous processes occurring in the amorphous intergranular film during the high-temperature anneal prior to quenching.     

Magneto-Dielectric Properties of Mg–Cu–Co Ferrite Ceramics: I. Densification Behavior and Microstructure Development

The densification, grain growth, and microstructure development of Mg–Cu–Co ferrite ceramics (MgFe_1.98O₄, Mg_{1− x} Cu _x Fe_1.98O₄, with x =0.10–0.30 and Mg_{0.90− x} Co _x Cu_0.10Fe_1.98O₄, with x =0.05–0.20) were studied. The primary objective was to develop magneto-dielectric materials for miniaturization of high frequency and very-high frequency antennas. It was found that magnesium ferrite (MgFe_1.98O₄) is a promising magneto-dielectric material. However, due to its poor densification, it could not be fully sintered at a temperature below 1200°C. High-temperature sintering resulted in undesirable electrical and dielectric properties, due to the formation of Fe²⁺ ions. The poor densification and slow grain growth rate of MgFe_1.98O₄ can be considerably improved by incorporating Cu, due to the occurrence of liquid-phase sintering at a high temperature. A critical concentration of Cu was observed for Mg_{1− x} Cu _x Fe_1.98O₄, above which both densification and grain growth were maximized or saturated. The presence of Co did not have a significant influence on the densification and grain growth of the Mg-based ferrite ceramics.     

2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: III, The Role of Atmosphere

The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at ∼830°C, and their conversion to the 2223 phase can be completed in the temperature range 810°C ≤ T < 830°C. A Ca₂PbO₄-like phase can precipitate from the liquid phase at ∼830°C during cooling. A (Sr,Ca)₁₄Cu₂₄O₄₁ phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca₂PbO₄-like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa₂Cu₃O _x superconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, air, and oxygen.     

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr_0.2Ba_0.8)TiO₃ materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al₂O₃–SiO₂–TiO₂ (AST). The ultimate PTCR properties with ρ_ht/ρ_rt as great as 10^5.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO₃ is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.     

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.     

Effect of MnO on the Phase Development, Microstructures, and Dielectric Properties of 0.95Na0.5K0.5NbO3–0.05LiTaO3 Ceramics

Lead-free piezoelectric ceramics in the system 0.95Na_0.5K_0.5NbO₃–0.05LiTaO₃ were modified with ≤1 mol% MnO. Maximum densities occurred at a sintering temperature of 1050°C. Characteristic changes in the relative intensity of X-ray diffraction peaks were consistent with Mn ions substituting on the perovskite lattice to produce a change from orthorhombic to a mixture of tetragonal and orthorhombic phases. Grain growth during secondary recrystallization was also affected, leading to increased grain sizes. The dielectric constant increased from ∼600 in unmodified ceramics to ∼1040 in ceramics prepared with 0.5 mol% MnO.     

Shift of the Curie Point of Barium Titanate Ceramics with Sintering Temperature

Definite increases in the Curie point (T_C) of undoped and lanthanum- (La-) doped (<0.5 at.%) barium titanate (BaTiO₃) ceramics sintered at elevated temperatures in the range of 1300°-1450°C were observed. Both undoped and 0.3 at.% La-doped BaTiO₃ (chosen as a typical doping concentration to yield semiconducting materials) ceramics showed almost the same T_C behavior; their T_C values increased by ∼3.5°C as the sintering temperature was increased from 1300° to 1450°C. Semiconducting 0.3 at.% La-doped materials increased in room-temperature bulk resistivity and TC with increased sintering temperature. The bulk resistivity of the La-doped materials, which was obtained from complex impedance analysis, increased from ∼2 omega cm for the material sintered at 1350°C to ∼6 ω cm at 1450°C. The phenomenon of bulk resistivity increase with sintering temperature was observed in the materials with a doping concentration of ≥ 0.2 at.% La, but was not observed in those doped with <0.2 at.% La. The mechanisms of TC and the bulk resistivity increase observed in the present materials with increased sintering temperature are discussed based on various models found in the literature, particularly in terms of the defect chemistry in semiconducting BaTiO₃ ceramics and the influence of liquid phases present during sintering.