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.     

Effect of Seeding Sr2KNb5O15 on the Phase Formation and Microstructural Development in Reactive Sintering of Sr2NaNb5O15 Ceramics

The phase formation, densification behavior, and microstructure development of Sr₂NaNb₅O₁₅ (SNN) ceramics in both 10 wt% acicular Sr₂KNb₅O₁₅ (SKN) seed-containing and unseeded systems were investigated in this work. SNN ceramics were reactively sintered from SrNb₂O₆ and NaNbO₃ powders. The results show that the acicular SKN seeds not only accelerate SNN phase formation but also promote the densification at lower temperature. In reactive sintering, the acicular SKN seeds prepared by the molten salt synthesis method can give rise to the formation of a liquid phase and provide the structural framework for the grain growth of ceramics, leading to the formation of large anisotropic grains (>80 μm) in ceramics sintered at 1340°C. However, there are no such large anisotropic grains obtained in the SKN-free system. Observation of the large anisotropic grain growth is explained by the liquid-phase-assisted growth mechanism. For comparison, the microstructure evolution in the system with 10 wt% SKN seed, which was prepared by the conventional mixed-oxide method and without acicular morphology, was also investigated to further support the new growth mechanism.     

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.     

Phase Diagram for the SiF4 Join in the System SiO2-Al2O3-SiF4

Alumina reacts with 1 atm of SiF₄ below 660°± 7°C to form A1F₃ and SiO₂. At higher temperatures the product is a mixture of fluorotopaz and AIF₃. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF₄ at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF₄ above mixtures of fluorotopaz and AlF₃ is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF₄ to give acicular mullite, 2Al₂O₃^.1.07SiO₂. Alumina and mullite are stable in the presence of 1 atm of SiF₄ above 973° and 1056°C, respectively. The phase diagram of the system SiO₂-Al₂O₃-SiF₄ shows only gas-solid equilibria.     

Topochemical Synthesis of a High-Aspect-Ratio Platelet NaNbO3 Template

A high-aspect-ratio platelet sodium niobate (NaNbO₃) was synthesized by a topochemical reaction. Plate-like layered-perovskite Bi_2.5Na_3.5Nb₅O₁₈ (BNN5) as a precursor was first prepared, and then Bi³⁺ in the precursor phase was replaced by Na⁺ through topotactic conversion and perovskite NaNbO₃ formed at 950°C in NaCl molten salt. NaNbO₃ crystals had an average length of 15 μm and a thickness of 0.5 μm. Scanning electron microscope and high-resolution transmission electron microscope analyses indicated that the retro-synthesis process of the desired perovskite NaNbO₃ retained the morphological and structural features of the BNN5 precursor. It may be possible to synthesize other perovskite-structured templates using the same method.     

Phase Diagram for a Portion of the System Li2O-Nd2O3-P2O5

In the ternary system Li₂O-Nd₂O₃-P₂0₅, part of the phase diagram relevant to the growth of single LindP₄O₁₂ (LNP) crystals was examined. LNP melts incongruently and decomposes into NdP₃O₉ and liquid at the peritectic temperature of 970°C. For the crystal growth, an Li₂O-P₂O₅ mixture should be used as a flux. The melt compositions from which LNP nucleates were clarified.     

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.     

Stability of YBa2Cu3O7-x in Molten Chloride Salts

The decomposition products of YBa₂Cu₃O_7-x depend on the composition of the molten chloride salt for exposure at 1173 K in air. The presence of dichloride salts such as CuCl₂, CaCl₂, or MgCl₂ promote formation of CuO, Cu₂Y₂O₅, and loss of barium to the chloride salt as BaCl₂. Salts based on BaCl₂ or containing LiCl result in YBa₂Cu₃O_7-x decomposition products of Y₂BaCuO₅, CuO, and BaCl₂. High barium activity in the salt supports formation of the Y₂BaCuO₅ phase and reaction of CO₂ with the salt producing BaCO₃. Decomposition is most sluggish in binary NaCl-KCl salts where minimal amounts of reaction or decomposition products are observed.     

Effect of Ta2O5, Nb2O5, and HfO2 Alloying on the Transformability of Y2O3-Stabilized Tetragonal ZrO2

The addition of Ta₂O₅, Nb₂O₅, and HfO₂ enhanced the transformability of Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (Y-TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y-TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y-TZP suggested that an alloying oxide which increases the c/a axial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO₂-Y₂O₃-Ta₂O₅, -Nb₂O₅, and -HfO₂.     

Phase Relationships in the Si3N4–SiO2–Lu2O3 System

Phase relationships in the Si₃N₄–SiO₂–Lu₂O₃ system were investigated at 1850°C in 1 MPa N₂. Only J-phase, Lu₄Si₂O₇N₂ (monoclinic, space group P 2₁/ c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si₃N₄–SiO₂–Lu₂O₃ system. The Si₃N₄/Lu₂O₃ ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si₃N₄, and a new phase of Lu₃Si₅ON₉, having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si₃N₄–LuN–Lu₂O₃ system. The phase diagram suggests that Lu₄Si₂O₇N₂ is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications.     

Tungsten Bronze Field and Melt Growth of Crystals in the Na2O-BaO-Nb2O5 System

The extents of the liquidus and solidus fields were determined for tungsten bronze-type solid solutions in the Na₂O-BaO-Nb₂O₅ system by DTA and melt crystal growth experiments. Bronze-type solid solutions exist to 7.1Na₂O-34.9BaO-58Nb₂O₅ in the Nb₂O₅-rich region and from 12Na₂O-38BaO-50Nb₂O₅ to 4.6Na₂O-45.4BaO-50Nb₂O₅ along the NaNbO₂-BaNb₂O₆ join, which includes NaBa₂Nb₅O₁₅=10Na₂O-40BaO-50Nb₂O₆. There is little, if any, solid solubility of compositions with a deficiency of Nb₂O₅. Curie temperatures decline rapidly and dielectric constant peaks broaden with Nb₂O₅ substitution because the Nb:O ratio becomes greater than the octahedral 1:3 ratio. Useful ferroelectrics exist along the NaNbO₃-BaNb₂O₆ join where the Nb:O ratio is 1:3. Large striae-free crystals, with less optical scattering than Czochralski-grown crystals, were grown from unseeded Na₂O-rich melts (e.g. 15Na₂O-37.5BaO-47.5Nb₂O₅) cooled from 1520° to 1300°C at 2°C/h. Annealing effects on these crystals whose compositions lie on the NaNbO₃-BaNb₂O₆ join are discussed.     

Reactions in the System PbO-Ta2O5

Subsolidus phase relations in the binary system PbO-Ta₂O₅ were investigated by the quenching method. The following compounds were identified by X-ray diffraction patterns: PbO -2Ta2O₅, PbO Ta₂O₆, 3PbO 2Ta₂P₅, 2PbO Ta₂O₆, 5PbO -2Ta₂O₅, and 3PbOTa₂O₅ The 1:1 compound has rhombohedral symmetry when it is prepared below 1150°C. Above this temperature, it yields an orthorhombic phase. Compounds with the same ratio of lead oxide to pentoxide exist in the systems PbO-Ta₂O₆ and PbO-Nb₂O₅.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.     

Electrical Properties of Textured Potassium Strontium Niobate (KSr2Nb5O15) Ceramics Fabricated by Reactive Templated Grain Growth

Highly [001] textured KSr₂Nb₅O₁₅ (KSN) ceramics were fabricated by templated grain growth using acicular KSN template particles (5–15 wt%) and reactive matrix of SrNb₂O₆ and KNbO₃. Excess Nb₂O₅ (1–1.5 wt%) was added as a liquid former. Increasing sintering temperature and time resulted in increased texture with a maximum texture fraction of 0.98. Dielectric, ferroelectric, and piezoelectric measurements indicate anisotropic properties that are close to single crystal values in the textured ceramics with the highest P _r≈18 μC/cm², P _s≈25 μC/cm², and d ₃₃=65 pC/N obtained in the c -axis direction.     

Effect of Nb2O5 Alloying on Thermal Expansion Anisotropy of 2 mol% Y2O3-Stabilized Tetragonal ZrO2

Tetragonal ZrO₂ ( t -ZrO₂) solid solutions were prepared with addit ons of 2 mol% Y₂O₃ plus up to 0.45 mol% Nb₂O₅. The thermal expansion coefficients in both the a- and c -axis lattice directions increased with Nb₂O₅ alloying and the thermal expansion in the c -axis direction was greater than that in the a -axis direction over the entire composition range. This anisotropic thermal expansion behavior was related to the 4-fold coordination of Nb⁵⁺ with oxygen ions in t -ZrO₂ solid solutions in the system ZrO₂–Y₂O₃–Nb₂O₅. The fracture toughness continuously increased with Nb₂O₅ alloying and suggested that the c/a axial ratio is a more significant factor than the internal stress that arises from the thermal expansion anisotropy, in the determination of the transformability of t -ZrO₂ in this system.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

A New Method for Synthesizing Li2.06Nb0.18Ti0.76O3 Powder at Low Temperatures

Li_2.06Nb_0.18Ti_0.76O₃ powder has been successfully prepared at low temperatures via a facile and manageable, activated pretreatment on the inert raw Nb₂O₅. It is demonstrated that with triethanolamine, citric acid, and hydrogen peroxide, this simple pretreatment process could activate Nb₂O₅ efficiently. Pure Li₂TiO₃ solid solution phase was thus obtained by calcining the mixture of the activated Nb₂O₅, LiOH·H₂O, and Ti(C₄H₉O)₄ at temperatures as low as 650°C, which is about 200°C lower than that of the traditional solid-state method. To the best of our knowledge, this temperature is the lowest one for preparing Li₂TiO₃ solid solution. Additionally, the phase transformation and the morphology of the final powder are also discussed.     

Rapid Formation and Growth of Bixbyite-Type (In0.67Fe0.33)2O3 by 28 GHz Microwave Irradiation

(In_0.67Fe_0.33)₂O₃ with the bixbyite structure was synthesized via 28 GHz microwave irradiation, using multimode microwave heating equipment. Indium sesquioxide strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to drive a reaction with iron sesquioxide. A mixture of In₂O₃ and α-Fe₂O₃ powders (In:Fe ratio of 2:1) was irradiated with microwaves at a frequency of 28 GHz. The mixture was heated to 1400°C during the microwave irradiation. The formation of a solid solution was completed within a minute, which indicated a drastic enhancement of the reaction rate. Scanning electron microscopy revealed remarkable grain growth under microwave irradiation.