Fabrication of Grain-Oriented Bi2WO6 Ceramics

Grain-oriented Bi₂WO₆ ceramics were fabricated by normal sintering techniques. Platelike crystallites were initially synthesized by a fused salt process using an NaCl-KCI melt. When calcined at <800°C, the Bi₂WO₆ crystallites are 3∼5 μ m in size and, at >850°C, =100 μm. After dissolving away the salt matrix, the Bi₂WO₆ particles were mixed with an organic binder and tapecast to align the platelike crystallites. Large particles were easily oriented by tapecasting but the sinterability of the tape was poor. Preferred orientation of small particles was increased by tapecasting and grain growth during sintering further improves the degree of orientation. Sintering above the 950°C phase transition, however, results in discontinuous grain growth and low densities. Optimum conditions for obtaining highly oriented ceramics with high density occur at sintering temperatures of 900°C using fine-grained powders which yield orientation factors of =0.88 and densities of 94% theoretical.     

Transformational Superplasticity of Bi2WO6 and Bi2MoO6

Transformational Superplasticity was studied in the compounds Bi₂WO₆ and Bi₂MoO₆. The magnitudes of transitional strain are related to the ( T_t/T_mP ), s of the phase transitions and are proportional to the externally applied stresses. Strain-rate sensitivities were similar, 0.85 and 0.86; however, the Bi₂WO₆ exhibited a strain-axis intercept and the Bi₂MoO₆ a stress-axis intercept. The grain-size effect present in the Bi₂WO₆ supports an accommodated grain-boundary sliding mechanism for the superplastic deformation process.     

Microwave Dielectric Properties of Low Firing Temperature Bi2W2O9 Ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi₂W₂O₉ ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi₂W₂O₉ powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi₂W₂O₉ to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna 2₁, with lattice parameters a =5.4401(8), b =5.4191(8), c =23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi₂WO₆ appears as a secondary phase. The measured relative permittivity of Bi₂W₂O₉ ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi₂W₂O₉ corrected for porosity was calculated as 41.3. Bi₂W₂O₉ ceramics fired up to 875°C exhibit moderate quality factors, Q × f _r, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi₂W₂O₉ ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.     

Hydrothermal Synthesis of Bismuth Sulfide Whiskers by a Template-Free Approach

Uniform Bi₂S₃ whiskers were synthesized via a template-free hydrothermal route using bismuth nitrate (Bi(NO₃)₃·5H₂O), thiourea (CS(NH₂)₂), and lithium hydroxide (LiOH) as starting materials. The resultant powders were characterized in detail by X-ray diffraction, scanning electron microscopy, and photoluminescent spectra techniques (PL), respectively. It was found that Bi₂S₃ whiskers could be easily synthesized in the presence of LiOH, whereas only irregular and aggregated particles were obtained without adding LiOH, and that elevating hydrothermal reaction temperature in a certain range would promote the preferred orientation growth of Bi₂S₃ crystallites. The PL spectra results evidenced that the optical properties of Bi₂S₃ crystallites were obviously influenced by their size and morphology.     

Transformational Plasticity in Bi2O3

Transformational plasticity associated with the monoclinic-to-cubic phase transition at 730°C in Bi₂O₃ was observed and characterized. This phenomenon is explained in terms of Greenwood and Johnson's model of internal stress-induced deformation, proceeding, in this instance, by a time-dependent, grain-size-sensitive creep mechanism, probably grain-boundary sliding. Criteria are proposed for choosing other prospective transformationally plastic ceramics; they are met by Bi₂WO₆, which also exhibits extensive transformation plasticity.     

Alkali Metal Ions-Assisted Controllable Synthesis of Bismuth Ferrites by a Hydrothermal Method

Various bismuth ferrite compounds have been controllably synthesized by a hydrothermal method assisted by alkali metal ions (K⁺, Na⁺, and Li⁺). The as-prepared powders were characterized by X-ray powder diffraction, energy-dispersive spectroscopy, and transmission electron microscope. It was suggested that different alkali metal ions can result in the formation of rhombohedral BiFeO₃, orthorhombic Bi₂Fe₄O₉, and cubic Bi₁₂(Bi_0.5Fe_0.5)O_19.5, respectively.     

Nanopowder Preparation and Dielectric Properties of a Bi2O3–Nb2O5 Binary System Prepared by the High-Energy Ball-Milling Method

The high-energy ball-milling (HEM) method was used to synthesize the compositions of BiNbO₄, Bi₅Nb₃O₁₅, and Bi₃NbO₇ in a Bi₂O₃–Nb₂O₅ binary system. Reagent Bi₂O₃ and Nb₂O₅ were chosen as the starting materials. The X-ray diffraction patterns of the three compositions milled for different times were studied. Only the cubic Bi₃NbO₇ phase, Nb₂O₅, and amorphous matters were observed in powders after being milled for 10 h. After heating at proper temperatures the amorphous matters disappeared and the proleptic phases of BiNbO₄ and Bi₅Nb₃O₁₅ could be obtained. The Scherrer formula was used to calculate the crystal size and the results of nanopowders are between 10 and 20 nm. The scanning electron microscopy photos of Bi₃NbO₇ powders showed drastic aggregation, and the particle size was about 100 nm. The dielectric properties of ceramics sintered from the nanopowders prepared by HEM at 100–1 MHz and the microwave region were measured. Bi₃NbO₇ ceramics showed a good microwave permittivity ɛ_r of about 80 and a Q × f of about 300 at 5 GHz. The triclinic phase of BiNbO₄ ceramics reached its best properties with ɛ_r=24 and Q × f =14 000 GHz at about 8 GHz.     

Low-Temperature Synthesis of Cubic and Rhombohedral Y6WO12 by a Polymerized Complex Method

Cubic Y₆WO₁₂, which has a defect fluorite-type structure with disordered cation and anion sublattices, was synthesized for the first time at the low temperature of 600°C by a polymerized complex method. The crystallization process of Y₆WO₁₂ from an amorphous precursor has been investigated in detail using TG-DTA, XRD, and TEM. Above 400°C, the cubic phase began to crystallize upon decomposition of the organic materials in the precursor. The crystallite size of the cubic Y₆WO₁₂ increased from about 5 nm at 600°C up to about 35–40 nm at 1000°C. Above 1200°C, the cubic phase transformed to rhombohedral Y₆WO₁₂, which is a stable phase.     

Effect of La Doping on the Phase Conversion, Microstructure Change, and Electrical Properties of Bi2Fe4O9 Ceramics

Undoped and La-doped Bi₂Fe₄O₉ ceramics were synthesized using a soft chemical method. It is observed that in calcining La-doped Bi₂Fe₄O₉, Bi(La)FeO₃ phase rather than Bi_{2− x} La _x Fe₄O₉ gradually increases with increasing La doping content. The phase conversion from mullite-type structure of Bi₂Fe₄O₉ to rhombohedrally distorted perovskite one of Bi(La)FeO₃ with increasing La doping content indicates that La doping can stabilize the structure of BiFeO₃. This is further evidenced that Bi₂Fe₄O₉ can be directly converted to Bi(La)FeO₃ by heating the mixtures of nominal composition of Bi₂Fe₄O₉/ x La₂O₃. Furthermore, the microstructure changes and the room temperature hysteresis loops and leakage current for Bi_{2− x} La _x Fe₄O₉ with x =0 and 0.02 were characterized.     

Microwave Dielectric Properties and Chemical Resistance of Low-Temperature-Sintered CaZrB2O6 Ceramics

Dolomite-type borate ceramics consisting of CaZrB₂O₆ were synthesized via a conventional solid-state reaction route; low-temperature sintering was explored using Bi₂O₃–CuO additives of 1–7 wt% for low-temperature co-fired ceramics applications. For several sintering temperatures, the microwave dielectric properties and chemical resistance of the ceramics were investigated. The CaZrB₂O₆ ceramics with 3 wt% Bi₂O₃–CuO addition could be sintered below 925°C, and the microwave dielectric properties of the low-temperature samples were ɛ_r=10.55, Q × f =87,350 GHz, and τ_f=+2 ppm/°C. The chemical resistance test result showed that both CaZrB₂O₆- and Bi₂O₃–CuO-added CaZrB₂O₆ ceramics were durable in basic solution but were degraded in acid solution.     

Criteria for Flame-Spray Synthesis of Hollow, Shell-Like, or Inhomogeneous Oxides

The influence of metal precursor and solvent composition on the morphology of SiO₂, Bi₂O₃, and other oxide particles made by flame spray pyrolysis (FSP) was investigated. Silica precursors with boiling points T _bp=299–548 K dissolved in xylene were used as well as different solvents ( T _bp=308–557 K) with tetraethyl-orthosilicate (TEOS) as the silica precursor. For Bi₂O₃, nonvolatile bismuth nitrate pentahydrate was dissolved in solvents with T _bp=338–468 K. Product powders were characterized by nitrogen adsorption, X-ray diffraction, and scanning and transmission electron microscopy. From these data as well as from the literature of FSP synthesis of Bi₂O₃, CeO₂, MgO, ZnO, Fe₂O₃, Y₂O₃, Al₂O₃, and Mg–Al spinel, it is inferred that hollow/inhomogeneous particles are formed at low combustion enthalpy densities and when the solvent boiling point is comparable or smaller than the precursor melting or decomposition point.     

Effects of Doping Trivalent Ions in Bismuth Borate Glasses

Bismuth borate glasses from the system: 40Bi₂O₃–59B₂O₃–1Tv₂O₃ (where Tv=Al, Y, Nd, Sm, and Eu) and three glasses of composition: 40Bi₂O₃–60B₂O₃, 37.5Bi₂O₃–62.5B₂O₃ and 38Bi₂O₃–60B₂O₃–2Al₂O₃ were prepared by melt quenching and characterized by density, UV-visible absorption spectroscopy and differential thermal analysis (DTA) studies. Bismuth borate glasses exhibit a very strong optical absorption band just below their absorption edge. Glasses were devitrified by heat treatment at temperatures above their glass transition temperatures and the crystalline phases produced in them were characterized by Fourier transform infrared (FTIR) absorption spectroscopy and X-ray diffraction (XRD). Bi₃B₅O₁₂ was found to be the most abundant phase in all devitrified samples. DTA studies on glasses and FTIR and XRD analysis on crystallized samples revealed that very small amounts of trivalent ion doping causes significant changes in the devitrification properties of bismuth borate glasses; rare-earth ions promote the formation of metastable BiBO₃–I and BiBO₃–II phases during glass crystallization.     

Processing and Dielectric Properties of Sillenite Compounds Bi12MO20−δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2)

Six sillenite compounds Bi₁₂MO_20-δ (M = Si, Ge, Ti, Pb, Mn, B_1/2P_1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of −20 ppm/K for the Bi₁₂SiO₂₀ and Bi₁₂(B_1/2P_1/2)O₂₀ compounds. The Q × f value is a maximum for Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi₁₂SiO₂₀ compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.     

Dielectric and Ferroelectric Properties of Nanocrystalline Bi2Ti2O7 Prepared by a Metallorganic Decomposition Method

Nanopowders of Bi₂Ti₂O₇ were synthesized by a metallorganic decomposition (MOD) technique. Pure Bi₂Ti₂O₇ nanocrystals formed after annealing at 550°C for 5 min. X-ray patterns show that Bi₂₀TiO₃₂ is a metastable phase during Bi₂Ti₂O₇ formation. It was found that there were two peaks in the curves of the dielectric response as a function of temperature for pressed nanocrystalline Bi₂Ti₂O₇ samples. The Curie temperature decreases with decrease of grain size whereas the ferroelectric-ferroelectric phase transition temperature increases. The hysteresis loops observed also suggest that Bi₂Ti₂O₇ might belong to a ferroelectric material.     

Fabrication of Textured Bi3NbTiO9 Ceramics

Highly textured Bi₃NbTiO₉ ceramics are fabricated by normal sintering from molten salt-synthesized plate-like crystallites. Fine Bi₃NbTiO₉ plate-like crystallites (∼1 μm) not only facilitate the densification, but also enhance texture in Bi₃NbTiO₉ ceramics. Weak-agglomerated platelets exhibit higher sinterability and can be densified at a temperature as low as 1000°C, which is about 100°C lower than that of equiaxed powders prepared by directly calcining Bi₃NbTiO₉ precursor. Meanwhile, the orientation degree of textured Bi₃NbTiO₉ ceramics increases with sintering temperature. Highly oriented Bi₃NbTiO₉ (orientation degree of ∼0.91) ceramic with a relative density of ∼92% is obtained at 1150°C. Because of the oriented grain microstructure, textured Bi₃NbTiO₉ ceramic exhibits anisotropic electrical properties.     

Nd/V Co-Doped Bi4Ti3O12 Powder Prepared By Molten Salt Synthesis

Powders of Nd/V-doped Bi₄Ti₃O₁₂ with a composition Bi_3.74Nd_0.26Ti_2.98V_0.02O_12.01 (BNTV), prepared by the molten salt synthesis, using chlorides as the fluxes, were successfully obtained. The influences of the molar ratio of KCl to NaCl, temperature, and soak time on the formation and morphology of BNTV were investigated. The results indicated that Na⁺ can enter the lattice of Bi₄Ti₃O₁₂ (BIT), leading to a decrease in the cell dimensions of BIT phase. The grain size and morphology of BNTV powders were considerably affected by the molar ratio of KCl to NaCl in the fluxes, in which the platelets showed the morphology of a rectangle and no regular shape in KCl and NaCl fluxes, respectively. The grain size of BNTV increased with increasing amount of NaCl in the fluxes, but the thickness decreased. The platelets prepared in the NaCl flux were faceted along (0 0 1) plane.     

Phase Formation and Crystal-Structure Determination in the Bi2O3–TiO2–TeO2 System Prepared in an Oxygen Atmosphere

Using X-ray diffraction analysis and scanning electron microscopy it was revealed that in an atmosphere of flowing oxygen in the temperature range 700°–800°C, three new compounds are formed in the Bi₂O₃–TiO₂–TeO₂ pseudoternary system. These compounds are Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂, and all the compounds include Te⁶⁺. All three crystal structures were solved and refined using X-ray powder diffraction data. Based on the results of the phase formation, a solid-state compatibility diagram is proposed.     

Sintering and Dielectric Characterization of Pseudoternary Compounds from the Bi2O3–TiO2–TeO2 System

Using X-ray diffraction analysis, scanning electron microscopy, thermogravimetry, and measurements of the dielectric properties up to the MW frequency range, the characterization of Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂ compounds, which all include Te⁶⁺, was performed. As the processes of Te⁶⁺ reduction and the evaporation of TeO₂-containing species contribute to the presence of secondary phases, the preparation of single-phase ceramics is rather difficult. To minimize the amount of secondary phases during the firing process, the pellets were muffled in a corresponding compound and then fired in an autoclave furnace under 10 bars of oxygen pressure. By sintering the Bi₂Ti₃TeO₁₂, Bi₂TiTeO₈, and Bi₆Ti₅TeO₂₂ between 840° and 1010°C, ceramics with ɛ_r ranging from 36 to 350, Q × f values from 220 to 12 500 GHz, and τ_f from +41 to +2600 ppm/K were obtained.     

Molten Salt Synthesis, Characterization, and Luminescence Properties of Gd2MO6:Eu3+ (M=W, Mo) Phosphors

Large-scale micro- and nano-sized Eu³⁺-doped gadolinium tungstate and molybdate Gd₂MO₆:Eu³⁺ (M=Mo, W) red phosphors with special morphologies have been successfully prepared via an efficient molten salt synthesis (MSS) method at 950°C for 6 h using NaCl and as the reaction medium. The results of X-ray powder diffraction patterns indicate that Gd₂MoO₆ and Gd₂WO₆ have monoclinic structures. The scanning electronic microscope and transmission electron microscope studies revealed that the samples of Gd₂WO₆:Eu³⁺ and Gd₂MoO₆:Eu³⁺ phosphors obtained by the MSS method in NaCl medium have a sphere-like morphology, and the corresponding samples obtained in KCl medium have rod-like morphologies. For the Gd₂MoO₆:Eu³⁺ phosphor obtained in NaCl and KCl media, the sphere-like particles had an average grain size of ∼100 nm, and the size of the rod-like particles ranged from 200 to 400 nm in length, and ∼150 nm in width, respectively. Compared with the same products obtained by the solid-state reaction we had reported, the luminescent properties of Gd₂MO₆:Eu³⁺ (M=Mo, W) are largely dependent on their morphology and crystal size prepared using different fluxes of NaCl and KCl. The reaction time and temperature also play an important role in the crystallization of the samples.     

Novel Low-Temperature Synthesis of Ferroelectric Neodymium-Doped Bismuth Titanate Nanoparticles

In this study, we report on the synthesis of nanopowders of ferroelectric Bi_3.5Nd_0.5Ti₃O₁₂ ceramic at temperatures below 500°C via a simple chemical method using citric acid as a solvent. The calcined powders were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Heating the as-dried powders in air first leads to crystallization of the Bi₂Ti₂O₇ phase at ∼310°C, followed by crystallization of the perovskite Nd-doped Bi₄Ti₃O₁₂ phase at ∼490°C as suggested by the peaks in the DSC analysis and confirmed by the evolution of phases in XRD patterns of the powders calcined at various temperatures. TEM of particles calcined at 550°C for 1 h in air showed an average particle size of 50–60 nm. The temperature dependence of capacitance of nanopowders calcined at 700°C for 1 h in air showed a Curie temperature of ∼615°C evincing a ferroelectric transition.