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.     

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.     

Extended Defects in ZnO Ceramics Containing Bi4Ti3O12 Additive

Extended defects in ZnO ceramics containing, 6 wt% Bi₄Ti₃O₁₂ were studied by analytical electron microscopy. Apart from basal plane condensation stacking faults, which are also present in as-received ZnO, extended defects related to the presence of Bi₄Ti₃O₁₂ were observed. In samples sintered at 900°C they lie in the basal or in the prismatic     planes and they quite often form closed loops, whereas they form serpentine-shaped boundaries in samples sintered at 1200°C. Evidence is given that they are inversion boundaries. Their TEM image characteristics, as well as the unambiguous presence of Ti at the boundaries, suggest that they are formed due to the presence of 2-D coherent precipitates of Ti-rich (possibly Zn₂TiO₄-type spinel) phase.     

Preferred Orientation of Bi4Ti3O12 Thick Film

The preferred orientation of thick films prepared by paste printing is rarely observed because of their bulky polycrystalline nature. We found that a Bi₄Ti₃O₁₂ thick film with a thickness of ca. 20 μm showed c -axis-preferred orientation. Initially, the texture of the screen-printed film was found to have a random orientation, which was attributed to the equiaxed particle shape of the raw powder synthesized by the co-precipitation method. During subsequent heating, c -axis orientation emerged in which the degree of orientation was proportional to the film density. Analysis of the orientation distribution revealed that the progress of texturing was attributed to the film deformation, indicating that anisotropic shrinkage and morphological changes in particles during heating influenced the preferred orientation.     

Effects of Excess Bi2O3 on the Ferroelectric Behavior of Nd-Doped Bi4Ti3O12 Thin Films

Effects of excess Bi₂O₃ content on formation of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) films deposited by RF sputtering were investigated. The microstructures and electrical properties of BNT thin films are strongly dependent on the excess Bi₂O₃ content and post-sputtering annealing temperature, as examined by XRD, SEM, and P – E hysteresis loops. A small amount of excess bismuth improves the crystallinity and therefore polarization of BNT films, while too much excess bismuth leads to a reduction in polarization and an increase in coercive field. P – E loops of well-established squareness were observed for the BNT films derived from a moderate amount of Bi₂O₃ excess (5 mol%), where a remanent polarization 2P _r of 25.2 μC/cm² and 2E _c of 161.5 kV/cm were shown. A similar change in dielectric constant with increasing excess Bi₂O₃ content was also observed, with the highest dielectric constant of 304.1 being measured for the BNT film derived from 5 mol% excess Bi₂O₃.     

Electrical Properties of Superlattice-Structured Bi4Ti3O12–PbBi4Ti4O15 Single Crystals

Single crystals of superlattice-structured ferroelectrics composed of Bi₄Ti₃O₁₂ and PbBi₄Ti₄O₁₅ were grown and the properties of polarization hysteresis and leakage current along the a -axis were investigated. Oxidation treatment led to a marked increase in leakage current at room temperature, showing that electron hole acts as a detrimental carrier for electrical conduction. A well-developed polarization hysteresis with a remanent polarization of 41 μC/cm² was observed, which is suggested to originate from the peculiar ferroelectric displacement of Bi in the Bi₂O₂ layers.     

Grain-Oriented Bi4Ti3O12 Ferroelectric Ceramics Prepared by Magnetic Alignment

Grain-oriented ferroelectric ceramics are desirable for many applications, but developing a mass-production method for such kinds of ceramics remains a significant challenge. In the current study, we report a convenient approach combining magnetic alignment and gelcasting to prepare grain-oriented ceramics without applied pressure and templates. This method was found to be effective to prepare highly a–b plane-oriented Bi₄Ti₃O₁₂ ferroelectric ceramics and subsequently enhance the dielectric properties. We used the conventional ceramic process, i.e., solid-state synthesis, gelcasting forming technique, and pressureless sintering except for the application of a 10 T magnetic field. Indeed, such an approach should facilitate the mass production of large and dense grain-oriented ceramic materials.     

Control of Functional Microstructure in WO3-Doped Bi4Ti3O12 Ceramics

Bi₄Ti₃O₁₂ (BIT) coprecipitated powders sinter in the presence of a transitory liquid phase, which strongly enhances the diffusion of matter and also the anisotropic growth of BIT grains. However, by coating the surface of BIT particles with WO₃ the aspect ratio of the platelets can be controlled. This, in addition to the donor effect of tungsten, finally leads to a decrease of the electrical conductivity of these ferroelectric ceramics.     

Factors Affecting the Electrical Conductivity of Donor-Doped Bi4Ti3O12 Piezoelectric Ceramics

High-temperature piezoelectric ceramics based on W⁶⁺-doped Bi₄Ti₃O₁₂ (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rho_th) at 900°C/2 h, took place in the chemically prepared W⁶⁺-doped Bi₄Ti₃O₁₂ ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of rho_th, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W⁶⁺-doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO₂ produced a further decrease of the electrical conductivity.     

Bi4Ti3O12 Ceramics from Powders Prepared by Alternative Routes: Wet No-Coprecipitation Chemistry and Mechanochemical Activation

The processing of ferroelectric Bi₄Ti₃O₁₂ ceramics from powders prepared by wet no-coprecipitation chemistry (WNCC) and mechanochemical activation (MCA) has been investigated. Dense ceramics were obtained at sintering temperatures as low as 900°C. Exaggerated grain growth was observed for samples from WNCC, but not for those from MCA. Dielectric properties are discussed in relation to the type and concentration of defects, which is smaller for ceramic samples from WNCC. The activation energy of the dielectric relaxation for ceramics from MCA suggests that additional V _O^•• are present at the pseudoperovskite [Bi₂Ti₃O₁₀]²⁻ block in this case.     

Ferroelectric Thin Films of Bismuth-Containing Layered Perovskites: Part I, Bi4Ti3O12

Ferroelectric thin films of bismuth-containing layered perovskite Bi₄Ti₃O₁₂ have been fabricated by a metalorganic decomposition (MOD) method. Crack-free and crystalline films of ∼5000 Å thickness have been deposited on Pt/Ti/SiO₂/Si substrates. Different heat treatments have been studied to investigate the nucleation and growth of perovskite Bi₄Ti₃O₁₂ crystallites. If the same composition and final annealing temperature are used, films with different orientations are obtained by different heating schedules. These films show a large anisotropy in ferroelectric properties. Theoretical considerations are presented to suggest that nucleation control is responsible for texture and grain-size evolution. Moreover, the origin of the ferroelectric anisotropy is rooted in the two-dimensional nature of layered polarization.     

Decomposition Reactions in CaCu3Ti4O12 Ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics sintered in air at 1115°C for 3 and 24 h have been heat treated in N₂ at 1000°C. Surface layers develop on the outer regions of the ceramics, and a combination of X-ray diffraction and analytical electron microscopy has been used to establish the phase content of the layers. A model to explain the formation of the surface layers is proposed based on decomposition of CCTO into a mixture of CaTiO₃, TiO₂, and Cu₂O. The role of limited decomposition in the development of electrically inhomogeneous CCTO ceramics prepared at elevated temperatures in air is discussed.     

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.     

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.     

Molten Salt Synthesis of Bi4(V0.85Co0.15)2O11−δ (BICOVOX) Ceramic Powders

Cobalt (15 at.%)-doped bismuth vanadate, Bi₄(V_0.85Co_0.15)₂O_11−δ (BICOVOX) is known to have high oxygen ion conduction. However, the conductivity is highly anisotropic due to its layered structure. In this paper, we report the synthesis of BICOVOX powders with platelet habit by molten salt synthesis using a NaCl–KCl eutectic mixture as the reaction medium. The effects of treatment temperature and time on the morphology and phase content of the BICOVOX powders were investigated. By this method, it is possible to synthesize powders with platelets of major face dimension ∼50 μm and aspect ratio (major face dimension to thickness) as high as 15.     

Electric and Dielectric Properties of Nb-Doped CaCu3Ti4O12 Ceramics

Pure and Nb-substituted CaCu₃Ti_{4− x} Nb _x O_{12+ x /2} (CCTO, x =0, 0.02, 0.1, 0.2, 0.4) ceramics were prepared by a conventional solid-state sintering, and their electric and dielectric properties were investigated using an impedance analyzer. A single-phase CCTO was obtained up to x =0.2 Nb substitution and the lattice parameter increased with Nb substitution concentration. While the grain size decreased with Nb substitution, the resistivity of the grain boundary decreased. The dielectric constant increased with Nb substitution, and the highest value of ∼420 000 was observed in the x =0.2 Nb-substituted specimen at 10 kHz. The obtained electric and dielectric properties in Nb-substituted CCTO were discussed in terms of the internal barrier layer capacitor model, particularly focusing on a ratio of thickness of the grain boundary region to grain size.     

Low-Temperature Sputtering Deposition of Aligned Polycrystalline CaCu3Ti4O12 Nanorods

Vertically aligned polycrystalline CaCu₃Ti₄O₁₂ nanorods were obtained by rf sputtering at low substrate temperatures. The rods grow by electrostatic attraction and aggregation of charged clusters, independently of the substrate used. These nanostructures exhibit a strong nonlinear current × voltage behavior and high losses during cyclic testing, indicating a combination of Schottky's barrier and Joule's heating effects.     

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.