Dislocation-Containing Planar Faults in Metastably Retained Hexagonal Barium Titanate

Planar faults, designated F ₁– F ₆ and F _a , with intermixed dislocations in metastably retained hexagonal-BaTiO₃ ceramics, were found and analyzed by transmission electron microscopy. Only faults with one, two, and four extra c -layers parallel to the basal plane have been identified. Fault vectors R_F1=R_Fa=1/6[02 2 1], R_F2=R_F3=1/3[0 1 11], R_F4=1/3[01 1 2], and R_F'1=R_F5= R_F6=R_F'a=1/6[0 2 21] were determined adopting the 2πg·R=0 (or 2 n π) criteria in combination with high-resolution imaging. Further, the embedded dislocations were half-partials with Burgers vectors b=1/3〈10 1 0〉 determined by the g·b=0 effective invisibility criteria in conjunction with the eligible fault vectors. A rotation by 60° about the c -axis was found between fault segments with R_F1=1/6[02 2 1] and R_F'1=1/6[20 2 1] located on either side of a partial. Basal dislocations with b_B=1/3〈1 2 10〉 have dissociated into two prism-plane Shockley half-partials with b_Pr=1/3〈1 1 00〉 by glide in the fault plane (0002) according to 1/3〈1 2 10〉→1/3〈1 1 00〉+1/3〈0 1 10〉. The fault segment F'₁ encompassed by two half-partials is an extrinsic complex stacking fault.     

Migration of Half Partial Dislocations in the Planar Fault Plane of Hexagonal Barium Titanate

The prism plane half partials with Burgers vector b_Pr= 1/3 ×〈01 1 0〉 dissociated from the perfect basal dislocations with b_B= 1/3〈 1 2 1 0〉 embedded in an α-type extended planar stacking fault of hexagonal ( h -)BaTiO₃ have been analyzed by transmission electron microscopy. It is found that the dissociation of a series of basal dislocations occurred by glide in the fault plane (0002). However, migrating of the pair partials trailing behind in the fault plane was impeded by the leading pair. Under the applied stress in hot pressing, these partials were gradually piled up with successively decreasing separation between each pair. That has consequently led to partial separations ranging between ∼195 and 56 nm. One partial dislocation ( p ₁) belonging initially to the leading pair partials has moved to another plane by a mixed mechanism of glide in (0002) and climb down in (0 1 10) by 1/2[000 1 ] (i.e., (1/2)c). The climbing partial has become an obstacle hindering dislocation migration in the fault plane. Determining the secondary pyramidal plane (0 1 12) connecting the leading half partial allows us to propose that the p ₁ partial migrating by glide and climb has arrived at another planar fault plane (0005) during hot pressing. The stages of how half partials were created, separated in fault plane, and arrived gradually at the present configuration are proposed and schematically illustrated. The significance of climb-controlled dislocation glide mechanism in the hot pressing of h -BaTiO₃ is discussed.     

Crystallographic Orientation Relationships Between Hexagonal and Tetragonal Barium Titanate

The high-temperature hexagonal (h) phase has been found to retain metastability at room temperature and co-exist with the room-temperature tetragonal (t) phase in hot-pressed BaTiO₃ ceramics. The crystallographic orientation relationships between two polytypes have been analyzed by TEM and selected-area diffraction patterns (SADPs). The relationships are illustrated using transformation matrices for both the forward h→t phase transformation ( M _f) and the backward t→h phase transformation ( M _b) based on two independent microstructures containing h-particle in t-matrix and t-particle in h-matrix, respectively. Self-consistent transformation matrices are derived from the coincident reflections in corresponding SADPs. The forward and backward matrices are inverse matrix to each other.     

Microwave Dielectric Properties of Gallium-Doped Hexagonal Barium Titanate Ceramics

High-permittivity and low-loss ceramics with composition BaTi_0.92Ga_0.08O_2.96 have been prepared in the BaO–Ga₂O₃–TiO₂ system using the mixed-oxide route. This compound forms as the hexagonal polymorph (6 H ) of BaTiO₃ with the space group P 6₃/ mmc . The dielectric properties of dense ceramics have been studied, at microwave frequencies, with the ceramics fired at 1450°C under flowing oxygen gas; the results are a relative permittivity, ɛ_r, of ∼74 and a quality factor, Q · f _r, of ∼7815 at 5.5 GHz. The quality factor is strongly influenced by the sintering conditions (temperature and atmosphere), whereas the relative permittivity is not influenced significantly by ceramic processing for pellets ≥93% of the theoretical X-ray density. To our knowledge, this is the first report of microwave dielectric resonance in a perovskite-type BaTiO₃-based ceramic.     

Exaggerated Growth of Hexagonal Barium Titanate under Reducing Sintering Conditions

The influence of reducing sintering conditions on anisotropic grain growth in BaTiO₃ was studied above the BaTiO₃-Ba₆Ti₁₇O₄₀ eutectic temperature. The morphology and structure of exaggeratedly grown grains was examined by X-ray powder diffractometry, scanning electron micros-copy, and high-resolution transmission electron microscopy. The results show that all anomalously grown anisotropic grains were hexagonal BaTiO₃ in the form of platelike crystals with a/c ratios up to 10. The direction of preferential growth of hexagonal grains is crystallographically analogous with that of parallel (111) twins in a cubic phase. Ti³⁺ ions, induced by reducing atmosphere, play an important role in the formation of hexagonal stacking.     

Laser-Sintered Barium Titanate

Laser sintering of alkoxy-derived ultrafine BaTiO₃ powders was investigated. The temperature increases of the sample with laser irradiation were measured with a thermocouple. It was found that laser irradiation could generate enough heat to sinter ceramics. A slurry was prepared by mixing an alkoxy-derived BaTiO₃ powder, binder additives, solvent, and plasticizer. The slurry was tape cast and dried to give a green sheet. The green sheet was laser sintered and was then characterized by SEM, XRD, and density measurements. The effect of burnout before laser irradiation and the characteristic microstructure of laser-sintered BaTiO₃ are described.     

Ruddlesden–Popper Planar Faults and Nanotwins in Heteroepitaxial Nonstoichiometric Barium Titanate Thin Films

Nonstoichiometric 10-mol%-excess-BaO–BaTiO₃ (Ba_1.1TiO_3.1) thin film grown on a (100) SrTiO₃ substrate consisted of heteroepitaxial c -axis-oriented BaTiO₃ perovskite crystals including Ruddlesden–Popper planar faults and nanometer-scale multiple (111) twin lamellae. High-resolution electron microscopy and electron energy-loss spectroscopy revealed that nanotwins with coherent (111) Σ3 coincident site lattice boundaries were terminated in the BaTiO₃ matrix to form incoherent (211) Σ3 boundaries accommodating excess barium ions. Both Ruddlesden–Popper planar fault and incoherent (211) boundary formation were proposed as possible accommodation mechanisms of excess barium ions in the perovskite film.     

Devitrified Barium Titanate Dielectrics

Pure barium titanate was made into a glass by flame-spraying and after sufficient comminution was processed into specimens and fired. During this firing the glass devitrified to tetragonal barium titanate and contained crystals of sizes from 0.5 to below 0.2μ. The small crystals exhibit Curie temperatures from 120° to above 185°C. The dielectric Curie temperature of the devitrified ceramic is 150° to 155°C.     

Solution-Precipitation of Fine Powders of Barium Titanate and Strontium Titanate

Hydrolytic reactions of metal alkoxides offer a broad range of possibilities for their use in the preparation of ceramic powders. This paper reports a unique and novel process to prepare fine powders of BaTiO₃ and SrTiO₃ from stable precursor solutions by carefully controlling the pH and temperature. This simple route offers good control of stoichiometry and the powders are agglomerate-free with fine particles of size 0.06–0.1 µm and were well sintered at 1200° and 1350°C, respectively. The dielectric properties of the dense ceramics are also reported. The formation aspects of these perovskite phases are also discussed.     

Microstructure of Barium Titanate Ceramics

The microstructure of polycrystalline barium titanate as seen in polished and etched specimens is more complicated than that in the single crystal. The greater complexity is revealed in the form of a banded structure which is considered to be due to the stress configuration arising from mutual impingement of randomly oriented grains during the cubic-tetragonal phase transformation.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Characterization of Barium Titanate Powders: Barium Carbonate Identification

A range of methods for the detection of barium carbonate contaminant in barium titanate powder has been assessed, namely: X-ray diffraction (XRD), scanning electron microscopy (SEM), with EDS-X-ray microanalysis, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), gas chromatography (GC) for analysis of carbon, and X-ray photoelectron spectroscopy (XPS). The most satisfactory procedure for the detection of the small amounts of BaCO₃ commonly present is FT-IR. Surface analyses by XPS show that the carbonate is present as a discrete phase and is not a surface film on barium titanate particles.     

Barium Zirconate-Lead Titanate Ferroelectric Ceramics

Hot-pressed barium zirconate-lead titanate ceramics have been examined to determine crystal-line symmetry and dielectric, ferroelectric, and piezoelectric properties. Barium zirconate additions to lead titanate form solid solutions with a decreasing tetragonal c/a axial ratio until at 60PbTiO₃–40BaZrO₃ to 75PbTiO₃–25BaZrO₃ the ceramics have coercive forces low enough to permit polarization. High-coercive-force piezo-electric ceramics are formed with k_p up to 0.30 and d₃₃ up to 110 × 10⁻¹² coulombs per newton.     

Flextensional Barium Strontium Titanate Actuators

This paper describes the performance of the cymbal flextensional transducer using Dy-doped barium strontium titanate (BST) as the driver material. BST was first characterized for its dielectric and loss behavior as a function of temperature and electrical bias field. With no electrical bias, the transition temperature was measured to be near 20°C and have a dielectric constant >20 000. The strain of a BST disk was then measured and compared with other ceramics. At room temperature the strain and average effective piezoelectric d ₃₃ of this non-lead composition was slightly larger than Navy type I lead zirconate titanate (PZT-4) ceramic. The strain/field behavior was also measured as a function of temperature. Cymbal capped BST ceramic was found to have an amplified displacement of 28×, also very similar to type I ceramic. The stiffness of BST was found to be tunable by dc voltage and 2 to 3 times larger than that of PZT. This material has promise for applications in actuators and transducers with large generative force.     

Investigation of Rare-Earth Doped Barium Titanate

In this study the effect of additions of 0.0015 to 0.0030 mole fraction of rare-earth oxides on the d.-c. resistivity of sintered barium titanate was investigated. The substitution may be represented by ( X ₂O₃)_M (BaTiO₃)_1-M where X is the rare earth. The rare earths samarium, gadolinium, and holmium were introduced singly into the titanate, and the resistivity was measured as a function of temperature from −170° to +330°C. An anomalous increase near the tetragonal - cubic transition temperature at 120°C. occurred which in some cases amounted to an increase in the resistivity of 4000 times the value in the tetragonal phase. The thermoelectric power of the material changed sign at the Curie temperature. The tetragonal phase exhibited n -type behavior whereas the cubic phase was p -type. The rhombohedra1 and orthorhombic phases exhibited conduction activation energies of the order of 0.2 e.v. whereas that in the tetragonal phase was approximately 0.1 e.v.     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Humidity Response Characteristics of Barium Titanate

Humidity response characteristics of BaTiO₃ doped with lanthanum were examined using complex impedance measurements. A sample with relative density of 71% showed a nearly log-linear increase of conductivity with humidity at 118 Hz. The average capacitance of bulk changed little with humidity; however, the resistance showed a gradual decrease. The equivalent circuit explaining such an observation was presented.     

聚醚酰亚胺/改性钛酸钡复合材料制备及介电储能性能

采用Stober法对高介电常数陶瓷填料钛酸钡(BT)进行改性,得到二氧化硅(SiO2)包覆BT(BT@SiO2)填料,并采用溶液浇铸法制备具有高介电储能性能的聚醚酰亚胺(PEI)/BT@SiO2复合材料薄膜,研究了复合材料薄膜的表面结构、结晶行为、介电性能、储能性能等.X射线衍射(XRD)及透射电子显微镜结果表明,Si...     

Peroxo-oxalate Preparation of Doped Barium Titanate

The peroxo-oxalate complexation method is a method that can be used for the preparation of doped barium titanate. In this paper we focus on BaTi_0.91Zr_0.09O₃, which can be used for discharge capacitors in lamp starters. The preparation method described here is based on the complexation and subsequent precipitation in basic environment of Ba, Ti, and Zr ions with hydrogen peroxide and oxalate. The influence of several process parameters, like precipitation temperature and pH, on powder properties is described. A single-phase perovskite crystal structure is obtained after calcination starting from a chloride precursor solution using a precipitation temperature of 40°C and a pH of 9. Because the peroxo-oxalate process starts with inexpensive chlorides and is performed in air, the peroxo-oxalate process is suitable for the commercial production of doped barium titanate.