Plate-Like Na0.5Bi0.5TiO3 Template Synthesized by a Topochemical Method

Large plate-like Na_0.5Bi_0.5TiO₃ (NBT) templates have been successfully synthesized from bismuth layer-structured ferroelectric Na_0.5Bi_4.5Ti₄O₁₅ (NBIT) particles by the topochemical method. Because of the highly anisotropic structure, plate-like NBIT particles were first synthesized by the molten-salt process. After the topochemical reaction with the complementary reactants (Na₂CO₃, and TiO₂) in NaCl flux, the layer-structured NBIT particles were transformed to the perovskite NBT templates. The resulting NBT templates are large and of plate-like shape. Our results also reveal that they are more effective in inducing grain orientation in the BNKT-BT ceramics as compared with BIT templates. For a BNKT-BT ceramic textured with 20 wt% of NBT templates, it exhibits a very high degree of grain orientation and gives a large Lotgering factor of 0.89.     

The Mechanochemical Synthesis of NaNbO3 Using Different Ball-Impact Energies

We studied the mechanochemical synthesis of NaNbO₃ from a starting-powder mixture of sodium carbonate and Nb₂O₅ using ball-impact energies of 15 and 370 mJ/hit. Based on the results of a quantitative phase analysis we propose a mechanism for the mechanochemical synthesis of NaNbO₃. During milling a condition is established, where the amounts of perovskite, X-ray diffraction -amorphous phase and residual carbonate in the mixture do not change with increasing milling time. Similarly, the NaNbO₃ crystallite size and the quantity of microstrains reach final values that are independent of the applied ball-impact energy.     

Synthesis of Micron-Scale Platelet BaTiO3

Micron-scale platelet barium titanate was synthesized using a twostep molten salt and topochemical technique. Plate-like BaBi₄Ti₄O₁₅ was first synthesized as a precursor by molten salt synthesis. Then, Bi³⁺ in the precursor was replaced by Ba²⁺ and formed perovskite-structure BaTiO₃ through a topochemical reaction. The BaTiO₃ single crystals have an average size of 5–10 μm and a thickness of 0.5 μm. The purpose of this article is to control the particle shape with desired structure. High aspect ratio BaTiO₃ platelets are suitable templates to obtain textured ceramics (especially Pb(Mg_1/3Nb_2/3)O₃–32.5PbTiO₃) by the templated grain growth process.     

Topochemical Synthesis of Plate-Like Na0.5Bi0.5TiO3 from Aurivillius Precursor

Plate-like Na_0.5Bi_0.5TiO₃ (NBT) particles with perovskite structure were synthesized by topochemical microcrystal conversion from plate-like particles of layer-structured Na_0.5Bi_4.5Ti₄O₁₅ (NBIT) at 950°C in NaCl molten salt. As the precursors of NBT, plate-like NBIT particles were first synthesized by molten salt process by the reaction of Bi₄Ti₃O₁₂, Na₂CO₃, and TiO₂. After the topochemical reactions, layer-structured NBIT particles were transformed to the perovskite NBT platelets. NBT particles with a thickness of approximately 0.5 μm and a length of 10–15 μm retained the morphology feature of the precursor. High-aspect-ratio NBT platelets are suitable templates to obtain textured ceramics (especially NBT-based ceramics) by (reactive) template grain growth process.     

Synthesis of High Aspect Ratio Platelet SrTiO3

A method for synthesis of high aspect ratio platelet seeds by growth of SrTiO₃ on Sr₃Ti₂O₇ core particles is reported. The aim of this study was to identify and control the morphology and size of SrTiO₃ particles via molten salt synthesis. Platelet and tabular morphologies with rectangular faces were obtained using rutile and anatase, respectively. Platelet SrTiO₃ particles with an edge length of 10–40 μm and a thickness of 1–4 μm were obtained. High aspect ratios (edge length to thickness) of 7–10 were measured for platelet particles as opposed to lower aspect ratios of 2–4 for tabular particles. Highly anisotropic platelets are suitable template candidates to achieve textured ceramics.     

Low-Temperature Synthesis of NaNbO3 Nanopowders and their Thin Films from a Novel Carbon-Free Precursor

We have demonstrated a synthetic procedure for nano-sized NaNbO₃ powder and its thin films using a carbon-free precursor prepared by the reaction of H₂O₂ with sodium and niobium alkoxides. A combination of X-ray powder diffraction, Raman spectra, and thermal gravity and differential thermal analysis, carbon analysis, fourier transform infrared spectra, transmission electron microscopy, and atomic force microscopy was used to characterize the resulting materials and precursor compounds. Results show that this procedure demonstrates the major advantages of low-temperature synthesis (∼400°C) and low weight loss during transformation into NaNbO₃ because of its carbon-free nature, which also provides the option of preparing nano-sized particles and dense, crack-free NaNbO₃ thin films.     

Thermodynamic Characterization of the Eutectic Phase Mixture NaNbO3/Na3NbO4. I: Literature Survey

The literature about the thermodynamic properties of NaNbO₃ and Na₃NbO₄ has mainly been governed by estimations. The only exceptions are two current calorimetric investigations on the standard enthalpy of formation of NaNbO₃ and, in addition, an old and inappropriately evaluated study on the carbon dioxide equilibrium gas pressure over the phase mixture NaNbO₃/Na₃NbO₄/Na₂CO₃. Upon reevaluating the latter results, first experimentally proven data on the difference of the Gibbs-free energies have been obtained (715°–822°C):      

Thermodynamic Characterization of the Eutectic Phase Mixture NaNbO3/Na3NbO4. II: Solid-State Electrochemical Investigation

The difference in the standard Gibbs-free energies of NaNbO₃ and Na₃NbO₄ has been determined by means of two electrochemical approaches based on potentiometric solid oxide electrolyte galvanic cells. The results are consistent with each other and prove the phase equilibrium between the niobates to be univariant. Quantitatively, the data can be represented by (525°–700°C)
The results are in between a wide range spanned by estimated literature data and are close to the findings of a previous manometric study.     

Nucleation and Crystallization of NaNbO3 from Glasses in the Na2O-Nb2O5-SiO2 System

Transparent glass-ceramics, in which the major phase was NaNbO₃, were obtained by heat treatment of glasses in the Na₂O-Nb₂O₅-SiO₂ system. The structure of the glass and the changes occurring during crystallization as a function of temperature and heating rate were examined by X-ray diffraction, transmission and replication electron microscopy, density, and other measurements. On heating, a rather abrupt formation of uniformly dispersed particles was observed. In the early stages of crystallization, these particles contained NaNbO₃ as loose, radially grown dendrites of identical crystal orientation which became dense during later stages of crystallization. The particle sizes ranged from 200 to 10,000 A, depending on the SiO₂ content of the glass. Transparency of the crystallized material was dependent on the particle size rather than on the amount of NaNbO₃ formed. The temperature at which crystallization occurred increased with the heating rate whereas the viscosity at crystallization decreased. For a given value of the rate of crystal formation per °C of temperature increase, the product (viscosity)ⁿ× (heating rate) was constant. The nucleation and growth phenomena which occurred in these glasses was attributed to microheterogeneities of higher Nb₂O₅ content which formed part of the glass structure.     

Grain Growth Control in NaNbO3–SrTiO3 Ceramics by Mechanosynthesis and Spark Plasma Sintering

The combination of non-conventional methods of synthesis (mechanosynthesis) and sintering (spark plasma sintering, SPS) has been used for the first time to process dense, fine-grained ceramics of the (1− x )NaNbO₃– x SrTiO₃ system. Dielectric properties have been measured across main phase transitions in the system for the submicrometer-structured ceramic materials processed by SPS, and are comparable with those of ceramics prepared by conventional sintering. This approach thus allows grain growth to be controlled while retaining properties, and provides the possibility of processing ceramics of alkaline niobate-based perovskite solid solutions with a homogeneous, fine-grained microstructure and good functionality.     

Bimodal Sintering of Al2O3/Al2O3 Platelet Ceramic Composites

The sintering behavior of an Al₂O₃ compact containing uniformly dispersed Al₂O₃ platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO₂ particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

High-pressure Synthesis of PbCrO3

A new compound with the composition PbCrO₃, with Cr in the valence state of 4, was synthesized at high pressures above a pressure-temperature line extending from about 50 kbars at 750°C to 60 kbars at 1450°C. PbCrO₃ can be quenched and retained at 1 atm but decomposes on heating above 275° C at the same pressure. PbCrO₃ is considered to be an equilibrium phase at high pressures because it was synthesized from mixtures of PbO: CrO₂ as well as from several other mixtures of compounds in the Pb-Cr-0 system. The new phase has the cubic perovskite structure and is the only known compound with Cr⁴⁺ in an octahedral site. PbCrO₃ crystallizes primarily as black cubes which are often twinned on (111).     

Formation Mechanisms of Platelet Sr3Ti2O7 Crystals Synthesized by the Molten Salt Synthesis Method

The molten salt synthesis (MSS) method is utilized to synthesize the anisotropic platelet Sr₃Ti₂O₇ (S3T2) single-crystal particles. The aim of this study is to identify the essence of platelet Sr₃Ti₂O₇ crystal growth and guide the synthesis of anisotropic platelet SrTiO₃ crystals as well as various technologically important materials. Based on the results of X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy, the formation mechanism of platelet Sr₃Ti₂O₇ crystals conforms to a nucleation–structure rearrangement–dissolution–diffusion in situ epitaxial growth mechanism model. First, SrCO₃ reacts with TiO₂ to form submicrometer SrTiO₃ nuclei. Then, most of the nuclei surrounded by salt ions aggregate and rearrange to form a large SrTiO₃ matrix. The structural rearrangement and the subsequent in situ epitaxial growth processes control the morphology, composition, and size of the final Sr₃Ti₂O₇ crystals. In the synthesis process, the conversion between SrTiO₃ and Sr₃Ti₂O₇ is as follows: and the crystallographic orientation relationship between Sr₃Ti₂O₇ and SrTiO₃ in the interface is (100)_S3T2//{100}_ST, (010)_S3T2//{010}_ST, and (001)_S3T2//{001}_ST.     

High-Pressure Synthesis of Perovskites Pb(Li1/4Nb3/4)O3 and Pb(Li1/4Ta3/4)O3

Complex perovskite-type compounds with the general formula Pb(B⁺_1/4B⁵⁺_3/4)O₃, where B⁺= Li⁺ and B⁵⁺= Nb⁵⁺ or Ta⁵⁺, were synthesized using a high-pressure technique and studied by X-ray powder diffraction. The X-ray patterns were indexed on the basis of a cubic cell with a ₀= 4.071 Å for Pb(Li_1/4 Nb_3/4)O₃ and a ₀= 4.052 Å for Pb(Li_1/4Ta_3/4)O₃. Electrical properties of the new perovskites were also studied.     

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.