Microstructure and Grain Boundary Structure of Na+-Diffused (Sr,Ca)TiO3 Capacitor-Varistor Ceramics

The microstructure of (Sr,Ca)TiO₃ capacitor-varistor materials has been investigated by employing electron microscopy techniques (TEM, STEM, HREM, EDX, and EPA). The material is found to contain (Sr,Ca)TiO₃ grains (∼30 μm) having perovskite crystal structure with domains, a Na⁺-diffused layer at the grain boundaries which is dependent on thermal diffusion conditions, and multiple-grain junctions in which the Ti _n O_2n–1 Magneli phase coexists with an amorphous intergranular phase. In addition, wider grain boundaries (10–30 nm), thin grain boundaries (∼1 nm), and clean grain boundaries which are free from intergranular phase were observed, and the effects of different grain boundaries on the diffusion of Na⁺are discussed.     

Roles of Ba/Ti Ratios in the Dielectric Properties of BaTiO3 Ceramics

The effect of the Ba/Ti ratio on microstructure, dielectric/ferroelectric properties, and domain width was studied using optical microscopy, ɛ( T ) curves, D – E hysteresis, and transmission electron microscopy. Although Ti-excess samples showed abnormal grain growth and a decrease of room-temperature permittivity due to a liquid phase at grain boundaries, its ferroelectric properties were similar to those of stoichiometric BaTiO₃ ceramics. However, in Ba-excess samples, an increase of permittivity and ferroelectric properties different from those of stoichiometry were found. Changes in domain width and ferroelectric transition behavior indicated that the variation of dielectric properties was related to the internal stress. It is proposed that this internal stress originated from differences in the thermal expansion coefficient between the matrix and the second phase.     

Microstructural Development in Partially Stabilized ZrO2 in the System CaO-ZrO2

The microstructures of 3 zirconias partially stabilized with CaO were investigated using scanning electron microscopy and qualitative and quantitative X-ray analysis. The structure was closely related to the heat treatments involved in fabrication. A bimodal structure with small grains of pure ZrO₂ dispersed along the grain boundaries of larger cubic solid-solution grains developed during slow cooling from 1850° to 1300°C. The presence of a liquid phase greatly enhances the growth of the pure ZrO₂ phase. An anneal at 1300°C induces precipitation of fine ZrO₂ particles within the solid-solution grains. The relative mechanical strengths of the materials are explained in terms of the weakening of the grain boundaries associated with the transformation of the grain-boundary phase on cooling.     

Microstructural Development in SnO2-Doped ZnO–Bi2O3 Ceramics

The effects of adding small quantities of SnO₂ to the basic ZnO–Bi₂O₃ varistor composition were studied in terms of phase reactions, microstructural development, and the formation of inversion boundaries. Scanning and transmission electron microscopy studies showed that the inversion boundaries, triggered by the addition of SnO₂, cause anisotropic grain growth in the early stages of sintering. ZnO grains that include inversion boundaries grow exaggeratedly, at the expense of normal grains, until they dominate the microstructure. Higher additions of SnO₂ lead to an increase in number of grains with inversion boundaries and to a more fine-grained microstructure. The increasing amount of secondary phases is also related to a higher level of SnO₂ addition; however, the influence of these phases on ZnO grain growth is subordinate to the role of inversion boundaries.     

Dielectric Properties and Microstructures of Ba(Ti,Zr)O3 Multilayer Ceramic Capacitors with Ni Electrodes

The microstructures and dielectric properties of multilayer ceramic capacitors based on reoxidized Ba(Ti_0.88,Zr_0.12)O₃ (BTZ) materials with Ni electrodes were studied using transmission electron microscopy. Dielectric measurements showed that the BTZ materials exhibited frequency relaxation effects. Although X-ray diffraction showed a single pseudocubic phase, split and elongated electron diffraction spots were observed using selected area diffraction (SAD). There were no super-lattice diffraction spots in the SAD pattern. The microstructures of BTZ dielectric materials were observed at dynamical diffraction conditions, and multidomain structures coexisting in one grain were imaged with high contrast. Bright field and centered dark field images revealed the pseudocubic (100) and (110) domain walls had developed in some regions of the same grain with normal ferroelectric macro-domain features, and bend contours and distorted domain walls were seen. Defects with the features of low angle grain boundaries, dislocations, and phase boundaries were also observed. Uneven distribution of internal stress and coexistence of multiphases and multidomains in individual grains were considered to be responsible for the frequency relaxor behavior observed in these materials. A model of the evolution of the microstructures with the decrease of temperature is presented.     

Faceting of High-Angle Grain Boundaries in Titanium-Excess BaTiO3

The grain boundaries in BaTiO₃ with excess Ti of 0.5, 0.3, and 0.1 at.% sintered at 1300° or 1250°C have been examined by scanning electron microscopy (SEM), electron backscattered diffraction pattern (EBSP), and transmission electron microscopy (TEM). In the 0.1% Ti-excess specimen, large grains growing abnormally form high-angle grain boundaries when they impinge on each other as verified by EBSP. A large fraction of these grain boundaries are faceted with hill-and-valley shapes. In the 0.5% Ti-excess specimen, large grains growing abnormally are elongated in the directions of their {111} double twins. These grains often form flat grain boundaries parallel to their {111} planes with the fine matrix grains, and the grain-boundary segments between the large impinging grains with high misorientation angles are often also parallel to the {111} planes of one of the grains. These grain boundaries are expected to be singular. Most of the grain boundaries between the randomly oriented fine-matrix grains in the 0.3 at.% Ti-excess specimen are also faceted with hill-and-valley shapes at finer scales when observed under TEM. The facet planes are parallel to {111}, {011}, and {012} planes of one of the grain pairs and are also expected to be singular. These high-angle grain boundaries lying on low index planes of one of the grain pairs are similar to those observed in other oxides and metals.     

Differential Negative Resistance and Piezoresistivity in Thin Semiconducting BaTiO3 Ceramic Bars

Thin, semiconducting barium titanate (BaTiO₃) ceramic bars, with a diameter of 10 to 20 μm, consisting of single grains joined together in series have been prepared to investigate the piezoresistivity in the materials, which was evaluated from their current ( I )-voltage ( V ) characteristics under the loading condition of various bending stresses. I-V characteristics of single grain boundaries in some of the materials were found to exhibit distinct differential negative resistance (DNR) at room temperature with its feature changing with stress. The DNR appeared on the I-V curves at an electric field of several volts per one grain, and has been confirmed to be connected with the transition of current between two conduction states in the grain boundary region. The obtained results indicate that this phenomenon cannot be interpreted by a rise in the temperature of the materials up to their positive temperature coefficient of resistivity (PTCR) region above the Curie point by Joule heating due to current flow, that is their self-heating effect. This newly observed DNR phenomenon has thus been tentatively interpreted by the morphological change in the ferroelectric domain structure in the vicinity of grain boundaries under mechanical and electric stresses, on an assumption that different configurations of ferroelectric domains yield different conduction states in the grain boundary due to a difference in the degree of surface acceptor charge compensation or the anisotropic carrier mobilities in the crystal.     

Piezoelectric electron acoustic study of domain structures in ferroelectric ceramics BaTiO3

The domain structures of ferroelectric ceramics BaTiO₃ have been observed by piezoelectric electron acoustic probe without any special processing to the sample. It is found that multi-distribution of domain structures could exist in a single grain and a domain structure could cross several grains. Experimental results show that the orientations of arrangements of domain structures on the boundary of two neighboring grains are the same as far as possible and the width of domain depends on the magnitude of grain. The origins of electron acoustic image contrasts of BaTiO₃ were mainly determined by the electronic properties of ferroelectric domains.     

Two Types of Domain Boundaries in Lanthanum Magnesium Niobate

Microstructural studies of the domain boundaries in the complex perovskite compound lanthanum magnesium niobate (La[Mg_2/3Nb_1/3]O₃, LMN) were conducted using transmission electron microscopy. Both the 1:1 chemical ordering of B-site cations and the tilting of oxygen octahedra affected the domain boundaries. Two types of domain boundaries were observed. In addition to the presence of antiphase boundaries, which were insensitive to the crystallographic planes, ferroelastic domain boundaries that were caused by the phase transition due to the tilting of oxygen octahedra also were present. In some grains, only one type of oxygen tilting was present, which resulted in a single domain in one grain. Two or three domains were observed in a grain where the walls were parallel to the {110} plane. Many domains also were observed in a grain that had boundaries whose linear characteristics were gradually reduced.     

Evidence of Cu Deficiency: A Key Point for the Understanding of the Mystery of the Giant Dielectric Constant in CaCu3Ti4O12

The compositions of the grains and the grain boundaries in CaCu₃Ti₄O₁₂ (CCTO) have been characterized by a high-resolution field emission transmission electron microscope equipped with a high-angle annular-dark-field detector and energy-dispersive X-ray spectroscopy (EDS). The excess ions and their clusters at grain boundaries led to the observed strain contrast. These ions are further clarified as Cu ions by Z-contrast (Z: atomic number) image and EDS. The quantitative EDS analyses further indicate that the grain boundaries of CCTO are Cu rich and the grains are Cu deficient. The Ca- or Ti-rich amorphous phases were observed at corners or edges of the grains of CCTO.     

Effect of Powder Coating on Stabilizer Distribution in CeO2-Stabilized ZrO2 Ceramics

The phase and microstructure relationship of 12 mol% CeO₂-stabilized ZrO₂ ceramics prepared from coated powder was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersed X-ray spectroscopy (EDS). As compared with the sample prepared with co-precipitated method, which exhibited a similar grain size distribution, the EDS analysis revealed that the powder coating induced a wide distribution of CeO₂ solubility, which decreases monotonically with the increase of grain size. This variation of stabilizer content from grain to grain rendered many large grains in the monoclinic phase. Stronger cerium segregation to grain boundaries was observed between large grains, which often form thin amorphous films there. The inhomogeneous CeO₂ distribution keeps more tetragonal ZrO₂ grains close to the phase boundary to facilitate the transforming toughness. Addition of an Al₂O₃ precursor in coated powders effectively raises the overall CeO₂ stabilizer content in the grains and preserves more transformable tetragonal phase in the microstructure, which further enhanced the fracture toughness. The dependence of CeO₂ solubility on grain size may be explained in a simple coating-controlled diffusion and growth process that deserves further investigation.     

BaTiO3陶瓷的晶粒壳—芯结构与畴

用分析电镜和高分辨电镜观测了BaTiO_3电容器陶瓷的晶粒壳-芯结构与铁电畴。首次发现本实验的BaTiO_3陶瓷的晶粒壳-芯结构由Nb原子的不均匀分布而形成。晶粒芯为贫Nb的铁电相区,而晶粒壳处由富Nb的顺电相组成。并且,壳-芯晶粒含量和壳-芯区域的大小与BaTiO_3陶瓷的性能可能存在一定关系。研究结果表明,壳-芯边界不同于通常晶粒边界结构,而在铁电畴的畴界处,存在一维方向几埃到数十埃的原子紊乱过渡区。     

On the phase and grain boundaries in dual phase carbide/boride ceramics from micro to atomic level

The phase and grain boundary characteristics of recently developed fine-grained dual-phase high-entropy (Ti-Zr-Nb-Hf-Ta)C/(Ti-Zr-Nb-Hf-Ta)B₂ was investigated throughout all the accessible length scales using scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The system exhibits relatively homogeneous grain size distribution where the average size is approximately 0.97 µm, with chemical composition (Ti_0.14 Zr_0.2 Nb_0.2 Hf_0.2 Ta_0.26)C + (Ti_0.38 Zr_0.18 Nb_0.22 Hf₀.₁₁₅ Ta_0.105)B₂. SEM analyses revealed no micro-crack formation and second – phase segregation at the boundaries or micro-pores at the triple – points. Investigation down to the sub-nanometer scale revealed that the phase and grain boundaries were typically clean and sharp with an indistinct 1 – 1.5 nm thin gradient of metallic elements at boride/boride and carbide/carbide interfaces. The sharp phase and grain boundaries do exhibit elemental enrichment from a trace amount of Fe being incorporated in interstitial positions of carbide and boride grains locally at boride/carbide boundaries or are present in boride and carbide grains in the form of continuous thin layer at boride/boride and carbide/carbide interfaces with the probably origin from starting powders.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Transmission Electron Microscopic Observation of the Martensitic Phase Transformation in Tetragonal ZrO2

The present work involves the observation of phase transformation in Y₂O₃-containing tetragonal ZrO₂ polycrystals (Y-TZP) using electron microscopy. The observations indicate that the martensitic phase transformation ususally starts from grain boundaries. Transformation was also observed at the tip of a microcrack and its adjacent region. Our observations also indicate that the tetragonal ZrO₂ grains do not all transform at the same time at the fracture surfaces.     

Dielectric breakdown behavior of ferroelectric ceramics: The role of pores

Dielectric breakdown is a fundamental issue for ferroelectric ceramics. In this work, a phase-field method is introduced to study the breakdown behavior of ferroelectric ceramics with pores randomly distributed. Effects of the position and the size of pores on the breakdown behavior are analyzed. Results indicate that the position of pores, for example in grains or at grain boundaries, has a significant influence on the breakdown strength of ferroelectric ceramics. The nominal breakdown strength of ferroelectric ceramics with 2 % pores at grain boundaries is almost 50 % higher than 2 % pores in grains. Further, for ferroelectric ceramics with a certain porosity, the smaller the pore size, the higher the breakdown strength. As the nominal pore size decreases from 2.5 to 1, the nominal breakdown strength is enhanced from 0.73 to 1.16. Such results agree well with the widely accepted Gerson-Marshall model and previously published experimental observations.     

Role of Liquid Phase in PTCR Characteristics of (Ba0.7Sr0.3)TiO3 Ceramics

The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba_0.7Sr_0.3)TiO₃ (BST) with the addition of AST (4Al₂O₃· 9SiO₂· 3TiO₂) is investigated in this paper. The AST–BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO₃ phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO₃. Finally, the effect of precipitated (Ba,Sr)TiO₃ on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.     

Structural,electrical and ferroelectric characteristics of Bi(Fe0.9La 0.1)O3

Nowadays, much attention is paid for the development of lead-free complex or mixed metal oxides, which can be utilized for multi-functional devices. This communication provides the information on synthesis (by mixed oxide route) and physical properties (structural, electrical and ferroelectric) of the polycrystalline sample of Bi(Fe_0.9La_0.1)O₃ Analysis of the phase formation and basic crystal data of the material using X-ray diffraction (XRD) technique shows an orthorhombic symmetry with well-defined cell parameters. It has been shown that a small amount (10%) of La substitution at the Fe site of BiFeO₃ suppresses the impurity phase usually observed during phase formation of BiFeO₃. The average crystallite size, calculated through applying Scherrer's technique, was found to be 68?nm. For the study of surface morphology (grain size and distribution) of the compound, the scanning electron microscope (SEM) was used. The grains of different dimension were found homogeneously distributed at the entire surface of the sample. The La substitution strongly affects the capacitive (dielectric) and resistive (electrical) characteristics of bismuth ferrite in a wide range of frequency and temperature. The contributions of grains and grain boundaries in the capacitive as well as in the resistive properties of the material at different temperatures and frequencies were studied by means of the impedance spectroscopy technique. This study has provided numerous useful and interesting data which may find potential industrial applications.     

Dislocations and Stacking Faults in Ti3SiC2

The ternary carbide Ti₃SIC₂ fabricated by a reactive hot-press route is investigated by transmission electron microscopy. The material consists mainly of large elongated grains with planar boundaries, and is characterized by a low defect density. Dislocations are observed in the grains and at grain boundaries. Perfect dislocations with b = 1/3<1120> lying in (0001) basal planes are present. These basal plane dislocations are mobile and multiply as a result of room-temperature deformation. All of the stacking faults observed lie in the basal planes.     

Mechanism for the Role of Magnesia in the Sintering of Alumina Containing Small Amounts of a liquid Phase

Microstructural morphology in A1₂0₃ containing trace amounts (∼0.25 vol%) of a silicate-based amorphous phase was studied, as a finction of MgO additions, using transmission electron microscopy. Incipient abnormal grains develop during hot-pressing of undoped Al₂O₃ powder and are characterized by a large aspect ratio (>3) and long basal plane {0001} facets. The facets are completely wet by a thin (∼3 nm) amorphous phase in contrast with grain ends, which appear to be devoid of an amorphous phase. The shape of the grains is believed to result from differences in mobility between clean (i.e., unwet) grain boundaries and intergranular films which are liquid at the firing temperature. Doping with trace additions of MgO (Mg/A1=250 ppm) results in smaller, more uniform grain structures. Many grains in the MgO doped material exhibit all the features of the incipient abnormal grains in the undoped material, with the exception of the large aspect ratio. It is concluded that the role of MgO in such a system is a grain-growth inhibitor and a microstructural stabilizer. It is proposed that the additive operates by reducing the mobility difference between clean (unwet) boundaries and grain boundaries wetted by thin amorphous films. It is believed the additive accomplishes this primarily by reducing the mobility of the clean grain boundaries via a solid-solution pinning mechanism.