Effect of Particle Sizes of Starting Materials on Microstructure Development in Textured Bi0.5(Na0.5K0.5)0.5TiO3

Microstructure development in Bi_0.5(Na_0.5K_0.5)_0.5TiO₃ prepared by a reactive-templated grain growth process was dependent on the sizes of platelike Bi₄Ti₃O₁₂ (BiT) and equiaxed TiO₂ particles used as starting materials. Calcined compacts were composed of large, platelike template grains and small, equiaxed matrix grains, the sizes of which were determined by those of the BiT and TiO₂ particles, respectively. Texture was developed by the growth of template grains at the expense of matrix grains during sintering, and a new mechanism of grain growth was proposed on the basis of microstructure observation. The grain growth rate was determined by the template and matrix grain sizes, and a dense ceramic with extensive texture was obtained using small BiT and TiO₂ particles.     

Fabrication of Na0.5Bi0.5TiO3–BaTiO3-Textured Ceramics Templated by Plate-Like Na0.5Bi0.5TiO3 Particles

Textured 0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃ (NBT–6BT) ceramics were fabricated by templated grain growth (TGG) using anisotropically shaped Na_0.5Bi_0.5TiO₃ (NBT) templates. Platelet NBT was synthesized by the topochemical technique, using precursor Na_0.5Bi_4.5Ti₄O₁₅ (NBIT). The NBT particles have an average length of 10–15 μm and a thickness of 1 μm, which are suitable templates for obtaining textured ceramics (especially NBT-based ceramics) by the TGG process. This study revealed that the NBT templates are effective in inducing grain orientation in NBT–6BT ceramics. For NBT–6BT ceramics textured with 5 vol% NBT templates, a Lotgering factor of 0.87 and a d ₃₃ of 299 pC/N are given.     

Reactive-Templated Grain Growth of Bi1/2(Na,K)1/2TiO3: Effects of Formulation on Texture Development

In this paper we report the effects of formulation on texture development for the "reactive-templated grain growth" (RTGG) of Bi_1/2(Na,K)_1/2TiO₃ (BNKT). The solids formulation for BNKT was systematically varied by prereacting to well—defined alkali and bismuth titanates (Na₂Ti₃O₇ (N₂T₃), K₂Ti₂O₅ (K₂T₂), and Bi₂Ti₄O₁₁ (B₂T₄)). Use of these precursors in different BNKT formulations determined that the amount of expansion associated with reacting dry-pressed compacts at 600−800°C could be influenced by formulation. Lotgering factors ( F _{00 l} ) derived from Θ/2Θ X-ray diffraction scans indicated that the formulation route strongly affected the {00 l } texture development in tape-cast and sintered specimens. Prereacting alkali carbonates with TiO₂ to form N₂T₃ and K₂T₂ inhibited texture development in RTGG-processsed BNKT. However, when Bi₂O₃ was prereacted to form B₂T₄, the measured F _{00 l} increased from 0.5 to 0.7.     

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.     

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.     

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.     

Diffuse Ferroelectric Phase Transition and Relaxor Behaviors in Ba-Based Bismuth Layer-Structured Compounds and La-Substituted SrBi4Ti4O15

The dielectric characteristics of BaBi₂Nb₂O₉, BaBi₄Ti₄O₁₅, BaBi₈Ti₇O₂₇, and La-substituted SrBi₄Ti₄O₄ were investigated to discuss their ferroelectric phase transition and relaxor behaviors. BaBi₂Nb₂O₉ showed typical relaxor behaviors, and a shift of T _m with increasing frequency was observed in BaBi₄Ti₄O₁₅ and SrBi_{4− x} La _x Ti₄O₁₅ ( x =0.8, 1.0) but they underwent a real paraelectric–ferroelectric phase transition on zero-field cooling, while BaBi₈Ti₇O₂₇ showed a normal ferroelectric nature. The reduced concentration and weakened coupling of the dipoles related to A-site bismuth are believed to be responsible for the appearance of short-range electric ordering and the relaxor behaviors in these bismuth layer-structured compounds.     

Comment on "Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20"

in a recent article of the Journal , Yu et al .¹ reported their experimental results on the effect of Al₂O₃ and Bi₂O₃ on the formation mechanism of Sn-doped Ba₂Ti₉O₂₀. They claimed that both Al₂O₃ and Bi₂O₃ can dramatically assist the formation of Sn-doped Ba₂Ti₉O₂₀ but are based on different mechanisms. They concluded that first, Bi₂O₃ melts above 830°C and accelerates the migration of the involved reactants to form Ba₂Ti₉O₂₀; second, Al₂O₃ can reduce the height of the potential energy barrier of the formation of Ba₂Ti₉O₂₀ due to the intergrowth of BaAl₂Ti₆O₁₆ phase. They explained their results from a point of view that the formation of Ba₂Ti₉O₂₀ is controlled by (1) the migration of reactants to the interfaces and (2) the height of the potential-energy barrier of the reaction at the interfaces. However, based on their results, we feel their conclusions are incautious and may be misleading, as will be discussed later.     

Lead-Free SrBi4Ti4O15 and Bi4Ti3O12 Material Fabrication Using the Microwave-Assisted Molten Salt Synthesis Method

In this paper, the microwave-assisted molten salt method (MAMSS) and molten salt method (MSS) were used to synthesize SrBi₄Ti₄O₁₅ (SBT). The phase constitution was determined by powder X-ray diffraction and the microstructure of powder was examined by scanning electron microscopy. In contrast to the conventional MSS method, MAMSS produces more distinct plate-like grains and synthesizes both SBT and Bi₄Ti₃O₁₂ (BTO) at 600°C with a 30-min soaking time. The increase of temperature and soaking time can make the plate-like grains of BTO more distinct.     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20

High-performance Ba₂Ti₉O₂₀ ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba₂Ti₉O₂₀ can be promoted strikingly by the participation of Bi₂O₃ and Al₂O₃. The effect of Bi₂O₃ on the formation of Ba₂Ti₉O₂₀ is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi₂O₃ above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba₂Ti₉O₂₀. The participation of Al₂O₃, on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba₂Ti₉O₂₀ by causing the formation of BaAI₂Ti₆O₁₆ crystals; these crystals intergrow with Ba₂Ti₉O₂₀ crystals and result in decreased stress.     

Synthesis and Characterization of Sr2Ce2Ti5O16 in the System SrO-CeO2-TiO2

The ternary system SrO-CeO₂-TiO₂ was investigated using X-ray diffractometry. The formation of a new compound, Sr₂Ce₂Ti₅O₁₆, was established, and its compatibilities with SrO, SrCeO₃, and SrTiO₃ were studied. The results revealed the existence of a series of compounds Sr_6–12xCe_6xTi₅O₁₆ and solid solutions Sr_2+nCe₂Ti_5+nO_16+3n ( n ≤ 6).     

Processing and Characterization of BaTi4O9

BaTi₄O₉ powder prepared by calcining BaCO₃ and TiO₂ powders was sintered to over 97% of theoretical density. Less than 5% Ba₂Ti₉O₂₀ occurred as a second phase in "pure" BaTi₄O₉, and Al₂O₃ impurities from processing formed isolated hollandite (∼BaAl₂Ti₆O₁₆) grains, which were identified by fringes in bright-field TEM images. For pure BaTi₄O₉ at 1 MHz, a dielectric loss (tan δ) of 5 × 10⁻⁴ and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.     

Templated Grain Growth of <001> Textured PMN-28PT Using SrTiO3 Templates

Highly textured PMN-28PT (0.72Pb(Mg_1/3Nb_2/3)O₃–0.28PbTiO₃) ceramics were produced by templated grain growth on <001> oriented platelet-shaped SrTiO₃ template particles with an aspect ratio of 10–15. The templates were aligned in PMN-28PT matrix powder via tape casting and fired in an O₂–PbO atmosphere at 1150°C for up to 15 h. This resulted in textured ceramics with a 40 micrometer grain size and without residual templates. The volume fraction of textured material (  f  ) and the orientation parameter ( r ) were quantified by fitting X-ray diffraction rocking curve data to the March–Dollase equation. Processing conditions were optimized to achieve the best possible values of f and r for the chosen templates and matrix powder. A texture fraction of at least 81 vol% and an orientation parameter of 0.2 were achieved when all random matrix grains were consumed (a perfect textured ceramic would show a texture fraction of 100 vol% and an orientation parameter of 0).     

Core-Shell Structure Formation in Nb2O5-Doped SrTiO3 by Oxygen Partial Pressure Change

A core-shell structure was observed in SrTiO₃ doped with 1.2 mol% of Nb₂O₅, after sintering in a reducing atmosphere (5H₂-95N₂) and then in an oxidizing atmosphere (air). In undoped and Al₂O₃-doped SrTiO₃ specimens, no core-shell structure formed after the same sintering treatments as those for SrTiO₃ doped with 1.2 mol% of Nb₂O₅. The measured chemical compositions of the core and shell regions of 1.2-mol%-Nb₂O₅-doped SrTiO₃ grains showed that the Sr/(Ti + Nb) ratio of the shell regions grown in air was ~1% less than that of core regions grown in 5H₂-95N₂, which was in good agreement with a value predicted by available defect equations. Therefore, the observed core-shell structure is thought to result from the formation of strontium vacancies in an oxidizing atmosphere.     

Exaggerated Grain Growth in ZrO2-Toughened Al2O3

Annealing of ZrO₂-toughened Al₂O₃ (ZTA) at elevated temperatures causes growth of both the intergranular ZrO₂ particles and the Al₂O₃"matrix" grains. Exaggerated ("breakaway") grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain-boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain-boundary phase.     

Preparation of a Highly Conductive Al2O3/TiN Interlayer Nanocomposite through Selective Matrix Grain Growth

An electroconductive TiN/Al₂O₃ nanocomposite was prepared by a selective matrix grain growth method, using a powder mixture of submicrosized α-Al₂O₃, nanosized γ-Al₂O₃, and TiN nanoparticles synthesized through an in situ nitridation process. During sintering, a self-concentration of TiN nanoparticles at the matrix grain boundary occurred, as a result of the selective growth of large α-Al₂O₃ matrix grains. Under suitable sintering conditions, a typical interlayer nanostructure with a continuous nanosized TiN interlayer was formed along the Al₂O₃ matrix grain boundary, and the electroconducting behavior of the material was significantly improved. Twelve volume percent TiN/Al₂O₃ nanocomposite with such an interlayer nanostructure showed an unprecedentedly low resistivity of 8 × 10⁻³Ω·cm, which was more than two orders lower than the TiN/Al₂O₃ nanocomposite without such an interlayer nanostructure.     

Solid Solutions within Selected SrO-LnO1.5-TiO2 Systems

A region of selected SrO-LnO_1.5-TiO₂ (Ln = La, Ce, Pr, or Nd) systems was studied experimentally using X-ray diffractometry (XRD). A series of solid solutions with composition Sr_{4 x} Ln_{2 x/ 3}Ti₄O₁₂ having tetragonally distorted per-ovskite structures was found to exist along the tie line connecting SrTiO₃ and Ln₂Ti₃O₉. Reactions of SrLn₂Ti₄O₁₂, representative compounds of the series, with SrO were also studied. Additionally, the solubility of TiO₂ in Ln₂O₃-(3TiO_{2- m} (Ln = La, Pr, or Nd) at 1300°C was investigated using XRD.     

Coherent Precipitation in the System MgO-Al2O3-Cr2O3

An isothermal section of the ternary system MgO–Al₂O₃-Cr₂O₃ was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl₂O₄–Cr₂O₃ and MgCr₂O₄-Al₂O₃, and the binary join MgAl₂O₄-MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al_1-xCr_x)₂O₃, with the corundum structure or through a metastable phase of the probable composition Mg(Al_1-xC_r)₂₆O₄₀. The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation.