Dislocation Loop Formation in Nonstoichiometric (Ba,Ca)TiO3 and BaTiO3 Ceramics

Dislocation loops observed in nonstoichiometric and stoichiometric (Ba,Ca)TiO₃, and in stoichiometric BaTiO₃ sintered in a reducing atmosphere, were characterized by conventional transmission electron microscopy (TEM) under two-beam conditions and high-resolution TEM atomic structure analysis. Dislocation loops mostly lay on {100} planes with Burgers vectors of type 〈100〉. The dynamic behavior of these dislocation loops during the electron beam irradiation (EBI), however, was classified into two different types of dislocation loops: in A-site-excess (Ba,Ca)TiO₃, contrasts of dislocation loops faded completely away; in BaTiO₃ and B-site-excess (Ba,Ca)TiO₃, fine-line contrasts remained. Dislocation loops with Burgers vectors of type 1/2〈100〉 and the resultant crystallographic shear (CS) structure with a displacement vector of type 1/2〈110〉 after EBI were proposed to interpret residual line images. Disappearance of these line images in A-site-excess (Ba,Ca)TiO₃ strongly suggests preferential Ca ion site occupancy at the CS structure.     

C70 Nanowhiskers Fabricated by Forming Liquid/Liquid Interfaces in the Systems of Toluene Solution of C70 and Isopropyl Alcohol

C₇₀ whiskers with submicrometer diameters (C₇₀ nanowhiskers, or C₇₀NWs) have been successfully fabricated by forming liquid/liquid interfaces in systems that involve a toluene solution of C₇₀ and isopropyl alcohol. Transmission electron microscopy observations show that the C₇₀NWs have a 〈110〉 growth axis and that the intercluster distance of C₇₀ molecules in the C₇₀NWs are shortened by ∼3%, compared with that of face-centered cubic C₇₀ crystals in the 〈110〉 close-packed growth direction, which indicates the formation of strong chemical bonds between the C₇₀ molecules. It is suggested that the C₇₀ molecules are polymerized, with their short axis parallel to the growth axis of the C₇₀NWs, from observation via high-resolution transmission electron microscopy.     

π-Rotation Faults in Hexagonal BaTiO3

Planar defects in the metastably retained h-BaTiO₃ exhibiting α-fringe pattern have been characterized via transmission electron microscopy (TEM). The eligible fault vectors were determined by adopting the invisibility criteria of 2πg·R = 0 or 2 n π augmented by high-resolution imaging. Three stacking faults, F₁, F₂, and F₃, of the extrinsic nature have been fully analyzed. The eligible fault vectors for faults F₁ and F₃ contained a basal component respectively of ⅓[0001] and ⅙[0001] and a common prismatic component of ⅓〈10[1-macr]0〉. However, only three of the 〈10[1-macr]0〉 vectors are the eligible prismatic component for the fault vectors R_F1=⅓[0[1-macr]11], ⅓[10[1-macr]1], and ⅓[[1-macr]101], and R_F3=⅙[02[2-macr]1], ⅙[2[2-macr]01], and ⅙[[2-macr]021] that have fulfilled the invisibility criteria. On the other hand, all fault vectors R_F21=⅙〈[4-macr]223〉 for fault F₂, containing six vectors of the 〈[2-macr]110〉 family, is eligible. Unlike the faults of πR_F=⅙〈[2-macr]203〉 found in the D0₁₉ intermetallics of Ni₃Sn and Co₃W, neither fault F₁ nor F₃ is the π-rotation type. Fault F₂, however, is a π-rotation fault since a 60°-rotation clockwise about [0001] has produced another eligible fault vector.     

Dissociation of the 〈001〉 Dislocations and Their Interactions with Dislocation Loops in Tetragonal BaTiO3

Dislocations in pressureless-sintered BaTiO₃ ceramics have been analyzed using transmission electron microscopy. Subjected to effective sintering stresses, dislocations were generated and multiplied in plastically deformed BaTiO₃ crystals by the Frank–Read mechanism from both single- and double-ended sources. This is represented by dislocations encompassing a series of square-like borders that shared a common center. All border dislocations exhibited the characteristic scallop shape. True dislocation line directions ( u ) were determined by trace analysis and Burgers vectors ( b ) by contrast analysis for the dislocations dissociated from b =〈001〉 into two half-partials following the type (I) reaction of     by climb on {001}. Dislocation interactions between the main dislocations created from plastic deformation and dislocation loops of b =〈100〉 or 〈110〉 forming condensation of intrinsic Schottky vacancies were also found to obey the type (IV) reaction of     , the type (V) reactions of     . Migrating dislocations and loops interacting mutually in several stages, illustrated schematically, before arriving at the configuration described by types (IV) and (V) were observed and discussed.     

C60 Nanowhiskers in a Mixture of Lead Zirconate Titanate Sol–C60 Toluene Solution

Fine fibrous structures of C₆₀ with a diameter on the order of nanometers were discovered in a lead zirconate titanate sol ultrasonically mixed with a toluene solution of C₆₀. By transmission electron microscopy observations, they were identified as single-crystalline fibers of C₆₀ with submicrometer diameters, i.e., nanowhiskers of C₆₀. The C₆₀ nanowhiskers showed thin slablike TEM images, and the growth axis of the nanowhiskers was parallel to the 〈110〉 close-packed direction of a fcc crystal system of C₆₀.     

Cation Ordering and Domain Boundaries in Ca[(Mg1/3Ta2/3)1−xTix]O3 Microwave Dielectric Ceramics

Cation ordering and domain boundaries in perovskite Ca[(Mg_1/3Ta_2/3)_{1− x} Ti _x ]O₃ ( x =0.1, 0.2, 0.3) microwave dielectric ceramics were investigated by high-resolution transmission electron microscopy (HRTEM) and Rietveld analysis. The variation of ordering structure with Ti substitution was revealed together with the formation mechanism of ordering domains. When x =0.1, the ceramics were composed of 1:2 and 1:1 ordered domains and a disordered matrix. The 1:2 cation ordering could still exist until x =0.2 but the 1:1 ordering disappeared. Neither 1:2 nor 1:1 cation ordering could exist at x =0.3. The space charge model was used to explain the cation ordering change from 1:2 to 1:1 and then to disorder. A comparison between the space charge model and random layer model was also conducted. HRTEM observations showed an antiphase boundary inclined to the (111) _c plane with a projected displacement vector in the 〈001〉 _c direction and ferroelastic domain boundaries parallel to the 〈100〉 _c direction.     

Preparation of Grain-Oriented Sr0.5Ba0.5Nb2O6 Ferroelectric Ceramics by Magnetic Alignment

The properties of polycrystalline ceramics are strongly influenced by their crystallographic texture. In this study, highly grain-oriented tungsten bronze structure ferroelectric ceramics, Sr_0.5Ba_0.5Nb₂O₆, were successfully fabricated by magnetic alignment and gelcasting techniques using only the conventional solid-state-synthesized starting powder. Spherical Sr_0.5Ba_0.5Nb₂O₆ particles were aligned according to their anisotropic magnetic property in 40 vol% slurry in a 10 T magnetic field, and then in situ locked by polymerization via a gelcasting technique for 30 min. A 〈00 l 〉-axis orientation perpendicular to the magnetic field direction ( B ) was obviously observed in the green compact and sintered sample. The sintered Sr_0.5Ba_0.5Nb₂O₆ sample contained equiaxial grains and reached 98% theoretical density. Compared with the sample with randomly oriented grains, the magnetically aligned sample showed an enhanced with dielectric constant in the ⊥ B direction (1100 versus 750 at room temperature and 4300 versus 2800 at Curie temperature). This new method is readily applicable to other ceramics with tungsten bronze structure, and is expected to facilitate mass preparation of large and dense grain-oriented ceramic materials.     

High Strain, 〈001〉 Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties

Lead magnesium niobate–lead titanate, 0.675Pb(Mg_1/3Nb_2/3)O₃–0.325PbTiO₃ (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO₃ (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d ₃₃-coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d ₃₃-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT.     

Large Pyroelectric Effect in Relaxor-Based Ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

The pyroelectric properties of (1− x )Pb(Mg_1/3Nb_2/3)O_{3− x} PbTiO₃ (PMN− x PT) single crystals with various compositions and orientations have been investigated using a dynamic method. Excellent pyroelectric performances can be achieved in 〈111〉-oriented rhombohedral PMN− x PT (0.24≤ x ≤0.30) crystals, where the measurement direction corresponds to the polar axis of the crystal. At room temperature, the pyroelectric coefficient and the detectivity figure of merit ( F _d ) for the 〈111〉-oriented PMN–0.28PT single crystal are 8.55 × 10⁻⁴ C·(m²·K)⁻¹ and 9.89 × 10⁻⁵ Pa^−1/2 (100 Hz), respectively, superior to those of the widely used pyroelectric materials. They are also weak temperature dependent and nearly independent of frequency. These outstanding pyroelectric performances make the single crystals a promising candidate for uncooled infrared detectors and thermal imagers.     

Texture Development in Barium Titanate and PMN–PT Using Hexabarium 17-Titanate Heterotemplates

Bulk BaTiO₃ ceramics with 〈111〉-texture have been prepared by the modified templated grain growth method, using platelike Ba₆Ti₁₇O₄₀ particles as templates, and the mechanism of texture development is examined. The Ba₆Ti₁₇O₄₀ particles induce the abnormal growth of BaTiO₃ grains, and a structure similarity between {001} of Ba₆Ti₁₇O₄₀ and {111} of BaTiO₃ gives 〈111〉-texture to abnormally grown BaTiO₃ grains. Thus, the 〈111〉-texture develops in the BaTiO₃ matrix. The use of platelike Ba₆Ti₁₇O₄₀ particles has been extended to a 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ matrix, but the matrix phase is decomposed by extensive chemical reactions between the matrix and template phases.     

Growth of Single Crystals of BaTiO3 by Exaggerated Grain Growth

Grain-growth experiments with steep temperature gradients and with polycrystalline aggregates seeded with single crystals were carried out to explore the feasibility of growing large crystals of BaTiO₃ in the solid state. It is shown that both methods resulted in a large increase in size of crystals normally obtained by sintering but that the rapid grain-boundary motion resulted in high porosity. Definite orientation dependence of grain-boundary velocity is demonstrated, and growth in a 〈110〉 direction was found to be at least as fast as 2 mm per hour.     

High-Temperature Precipitation Hardening of Two-Phase Y2O3-Partially-Stabilized ZrO2 Single Crystals: A First Report

Precipitation hardening was observed in two-phase (cubic plus tetragonal) Y₂O₃-partia1ly-stabilized ZrO₂ single crystals deformed at 1400°C. Slip was activated on (001) 〈110〉, primarily in Luders bands.     

A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite

(Na_1/2La_1/2)(Mg_1/3Nb_2/3)O₃ undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (ɛ_r∼ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.     

Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO₂ ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO₂ paste (before sintering) was around 15 μm. After the SiO₂-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO₂ ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO₂ photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X 〈100〉 direction, between 490 and 630 GHz in the Γ– K 〈110〉 direction, and between 400 and 510 GHz in the Γ– L 〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.     

Microstructure and Electrical Properties of Sc2O3-Doped, Rare-Earth-Oxide-Doped, and Undoped BaTiO3

Polycrystalline BaTiO₃ prepared from alkoxy-derived high-purity submicron powders was studied. Highly dense bodies with uniform grain size were obtained typically by uniaxial cold-pressing at 3000 psi and isostatic pressing at 30,000 psi followed by sintering at 1300° to 1350°C in air for 0.5 to 1 h. Using the same consolidation parameters and intimate mixing of residual concentrations of highly active fine-particulate rare-earth oxides to act as grain-growth inhibitors, nearly theoretically dense bodies with a uniform microstructure and 1 to 1.5 μm grain size were obtained. Typical microstructures with well-defined 90° and 180° domain patterns characteristic of BaTiO₃: were observed. Also, an example of a checkerboard pattern resulting from a 〈111〉 ingrown twin plane in the structure which is independent of the Curie temperature was found. Electrical measurements on the undoped material indicated room-temperature dielectric constant and tan δ values of 5000±500 and 4×10⁻³, respectively. Very high k values and dissipation factors were observed with the La₂O₃- and Nd₂O₃-doped samples.     

Formation of BaTiO3 from BaCO3 and TiO2 in Air and in CO2

The formation of BaTiO₃ from equimolar BaCO₃ and TiO₂ (rutile) mixtures was studied in air and in CO₂. A small amount of BaTiO₃ is formed first directly from BaCO₃ and TiO₂ at the surface of contact. From then on it is a diffusion-controlled reaction, and both BaTiO₃ and Ba₂TiO₄ are produced, with Ba₂TiO₄ being formed in much larger amounts. In 1 atmosphere of CO₂, the intermediate Ba₂TiO₄ was suppressed up to a temperature of about 1100°C. in agreement with thermodynamic calculations. Ba₂TiO₄ reacts fast with 1 atmosphere of CO₂ below about 1100°C. to produce BaTiO₃and BaCO₃     

Investigation of Lamella-Twinned Crystallites and Dislocations in Paraelectric Phases of Bi2O3-Doped BaTiO3

Defects in the paraelectric phases of BaTiO₃ doped with Bi₂O₃ were analyzed by transmission electron microscopy under two-beam conditions. (111) twin structures were characterized by selected area diffraction and bright-field images. The orientation relationships of the (111) twins were determined using stereograms. Lamella-twinned crystallites included in the paraelectric phases were found in this system. Pure wedge fringes were analyzed in these grains using electron diffraction and imaging techniques. Double diffraction was observed in the overlapped regions of the matrix and the microtwin in the [113] direction, and high-density dislocation loops were seen in some grains. Weak-beam dark-field microscopy techniques were used to observe the dislocation loops, which predominately lay on {100} crystal planes with Burgers vectors a 〈100〉, and were found to be pure edge dislocations. Some dislocations were transformed into crystallographic shear planes.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

Conversion of SiO2 Diatom Frustules to BaTiO3 and SrTiO3

Diatom frustules were used as bio-templates to synthesize functional ceramics via solid–gas displacement reactions. Silica-based frustules were exposed to TiF₄ at 330°C to form TiOF₂, which was later converted to TiO₂ (anatase) by heat treatment in air at 600°C. The TiO₂ frustules were then exposed to molten Ba(OH)₂ or Sr(OH)₂ to form BaTiO₃ or SrTiO₃, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO₃ and SrTiO₃ frustules exhibit nearly phase pure, nanocrystalline perovskite structure.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.