Semiconducting Barium Titanate Ceramics Prepared by Boron-Containing Liquid-Phase Sintering

Semiconducting BaTiO₃ ceramics have been prepared by adding BN as a sintering aid. Density as high as 93% of theoretical and grain size as large as 16 μm are obtained after sintering at 1160°C. Most significant is that the semiconducting BaTiO₃ is obtained at sintering temperatures as low as 1100°C. The low-temperature-sintered BaTiO₃ exhibits a positive temperature coefficient. (PTC) anomaly above 120°C with a resistivity maximum at a temperature as high as 400°C, which is much higher than that of the conventional BaTiO₃. The incorporation of B into the perovskite structure is negligible. Also, the presence of B at a grain boundary after sintering is believed to enhance the PTC effect.     

A Novel Approach to Sintering Nanocrystalline Barium Titanate Ceramics

A novel approach to pressureless sintering based on the combination of rapid-rate sintering, rate-controlled sintering, and two-step sintering under a controlled atmosphere is proposed. This combined sintering method facilitates control of grain and pore morphology. The application of this sintering approach for pure nanocrystalline barium titanate powder enables the suppression of grain growth during the intermediate and final stages of sintering and the production of fully dense ceramics with 108 nm grain size. The grain growth factor is 3.5, which is three and 17 times smaller than rate-controlled and conventional sintering, respectively.     

Improvement of Physical Properties for KNN‐based Ceramics by Modified Two‐Step Sintering

A modified two‐step sintering (MTSS) technique was attempted to fabricate dense (K,Na)NbO₃‐based lead‐free ceramics through the solid‐state reaction route. (K_0.50Na_0.50)_0.98Li_0.02(Nb_0.795Ta_0.18Sb_0.025)O₃ and (K_0.45Na_0.55)_0.98Li_0.02Nb_0.77Ta_0.18Sb_0.05O₃ were chosen as the test chemical compositions, and high relative density values over 97.5% were successfully obtained in their corresponding ceramics. Consequently, the d₃₃ values were enhanced considerably and reached 353 and 436 pC/N, respectively. The latter d₃₃ value is a record one ever achieved in the (K,Na)NbO₃‐based ceramic materials so far. Furthermore, observation analyses of microstructure and domain patterns were carried out and showed that partial depoling occurs more easily in large grains than in small ones. In particular, it was found that the (K_0.50Na_0.50)_0.98Li_0.02(Nb_0.795Ta_0.18Sb_0.025)O₃ ceramic densified by MTSS has a quite uniform grain‐size distribution with comparatively small grains and exhibits the very excellent time‐aging stability. The study results suggest that microstructure with high relative density and suitably suppressed grain sizes is desirable for further acquiring the superior KNN‐based ceramics of both excellent piezoelectric properties and good depoling stabilities.     

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.     

Phase Transitions in Nanocrystalline Barium Titanate Ceramics Prepared by Spark Plasma Sintering

The bulk dense nanocrystalline BaTiO₃ (BT) ceramics ranging from 20 to 100 nm have been successfully prepared by the spark plasma sintering (SPS) method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometer scale for BT ceramics. The results reveal that the SPS technique provides exceptional opportunity to compact ceramics to full density with nanograin size. It is also demonstrated that all structural modifications in nanocrystalline BT and low-symmetry structures still exist in 20 nm nanograin BT ceramics. The ferroelectric properties of crystalline structures were investigated by scanning force microscopy in piezoresponse mode. Piezoelectric hysteresis loop was recorded, demonstrating that 20 nm BT ceramics has a remanent polarization and is switchable by an electric field. Thus, if a critical grain size exists for ferroelectricity, it is less than 20 nm for polycrystalline BT ceramics.     

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.     

Shift of the Curie Point of Barium Titanate Ceramics with Sintering Temperature

Definite increases in the Curie point (T_C) of undoped and lanthanum- (La-) doped (<0.5 at.%) barium titanate (BaTiO₃) ceramics sintered at elevated temperatures in the range of 1300°-1450°C were observed. Both undoped and 0.3 at.% La-doped BaTiO₃ (chosen as a typical doping concentration to yield semiconducting materials) ceramics showed almost the same T_C behavior; their T_C values increased by ∼3.5°C as the sintering temperature was increased from 1300° to 1450°C. Semiconducting 0.3 at.% La-doped materials increased in room-temperature bulk resistivity and TC with increased sintering temperature. The bulk resistivity of the La-doped materials, which was obtained from complex impedance analysis, increased from ∼2 omega cm for the material sintered at 1350°C to ∼6 ω cm at 1450°C. The phenomenon of bulk resistivity increase with sintering temperature was observed in the materials with a doping concentration of ≥ 0.2 at.% La, but was not observed in those doped with <0.2 at.% La. The mechanisms of TC and the bulk resistivity increase observed in the present materials with increased sintering temperature are discussed based on various models found in the literature, particularly in terms of the defect chemistry in semiconducting BaTiO₃ ceramics and the influence of liquid phases present during sintering.     

Effects of Initial Powder Size on the Densification of Barium Titanate Ceramics Prepared by Microwave‐Assisted Sintering

The effects of initial powder size on microwave‐assisted sintering (MWS) were investigated. BaTiO₃ powders with an average particle size of 50, 100, and 500 nm were prepared and sintered with MWS and conventional heating‐based sintering (CS). Samples of the 50 ‐ and 100‐nm‐sized BaTiO₃ powders were mechanically milled to study the effects of powder crystallinity on microwave absorption during the MWS process. The MWS of the 50‐nm‐sized BaTiO₃ powder resulted in a relative mass density of more than 90% when sintered at 1050°C, whereas the same density was achieved at 1200°C with CS. This difference between the optimal sintering temperatures, which is caused by the absorption of microwaves, was not observed when the 500‐nm‐sized BaTiO₃ powder was used. The sinterability of the BaTiO₃ ceramics prepared through the MWS of mechanically milled, 50‐nm‐sized powders decreased with increasing milling time. However, the sinterability was much higher than that of the BaTiO₃ ceramics prepared through the MWS of the 100‐ and 500‐nm‐sized unmilled powders. In conclusion, microwave absorption has significant effects on the sintering behavior of ~50‐nm‐sized powders, but is negligible for 500‐nm‐sized powders.     

Domain Twinning in Barium Titanate Ceramics

The patterns found in metallographically prepared barium titanate ceramics are shown to be due to 90° domains. Sets of domains are bounded by (110) planes. Forsbergh's solution of the "square-net" pattern is confirmed.     

Grain Size Effects on Piezoelectric Properties and Domain Structure of BaTiO3 Ceramics Prepared by Two‐Step Sintering

A series of highly dense barium titanate (BaTiO₃) ceramics with the average grain size (GS) from 0.29 to 8.61 μm are successfully prepared by two‐step sintering, and the GS effect on piezoelectric coefficient (d₃₃) is systematically discussed in this work. It is found that when GS above 1 μm, d₃₃ can be enhanced with decreasing GS, reaching a maximum value of 519 pC/N around 1 μm due to the high activity of domain wall mobility. Subsequently, d₃₃ rapidly drops with a further decrease in GS owing to the reduced domain density. The results suggest that it is possible to prepare high‐performance BaTiO₃ ceramics by controlling the GS and domain configuration properly, which brings great revitalization to the BaTiO₃‐based piezoceramics.     

Electron Emission from Barium Strontium Titanate Ceramics

This paper focuses on understanding the influence of materials' properties on the ferroelectric electron emission. Ferroelectric ( x =1.0 and 0.8) and paraelectric ( x =0.67 and 0.5) compositions of barium strontium titanate (Ba _x Sr_{(1− x )}TiO₃) system were chosen for study based on their different ferroelectric and dielectric properties. Similar emission current waveforms were obtained from four compositions with negative triggering voltage applied to the rear electrode of the samples. It was difficult to explain the experimental results using the spontaneous polarization-switching model. The mechanism of electron emission from Ba _x Sr_{(1− x )}TiO₃ ceramics was ascertained to surface plasma emission.     

Stabilized Barium Titanate Ceramics for Capacitor Dielectrics

The effects of additions of La₂O₃ and La₂O₃-3TiO₂ on the electronic properties of barium titanate have been investigated. It has been found that small additions of these components markedly increase the lifetime of this material subjected to dc fields of 50 v per mil at 200°C.     

Semiconducting–Insulating Transition for Highly Donor-Dopod Barium Titanate Ceramics

BaTiO₃ ceramics doped with La (0.01–0.84 at.%) were prepared only with the addition of La and stoichiometric TiO₂. As a result, even when BaTiO₃ was doped with 0.53 at.% La, it could be converted to a semiconductor by sintering at 1540°C for 2 h in air and cooled slowly in the furnace. Differential thermal analysis data clearly demonstrated that the Curie point in the materials shifted toward lower temperatures with increased content of La substituted at the Ba site up to a critical concentration that varied with the sintering temperature. The obtained results suggest that the semiconducting–insulating transition for highly donor-doped BaTiO₃ was closely related to the incorporation of donor into the grains and to the resultant grain size, which were significantly affected by the sinterability of the BaTiO₃ starting powders and sintering conditions used.     

A Microstructure Study of Barium Titanate Ceramics

Polishing and etching techniques suitable for the metallographic examination of polycrystalline barium titanate ceramics are described. Interpretation of the photomicrographs reveals information which may be correlated with the electromechanical properties.     

Incorporation of Yttrium in Barium Titanate Ceramics

In this work, X-ray diffractometry and scanning electron microscopy techniques were used to study the incorporation of yttrium in BaTiO₃, with the following nominal compositions: (Ba_{1- x} Y _x )TiO₃(0.015 ≤ to x ≤ to 0.08), (Ba_{1- x} Y _x )Ti_{1- x /4}-(□_Ti) _{x /4}O₃(0.005 ≤ to x ≤ to 0.1), and Ba(Ti_{1- y} Y _y )O_3-delta(0.028 ≤ to y ≤ to 0.258). The phase assemblage and the lattice parameters indicated a slight solubility (∼1.5 at.%) of yttrium at the Ba sites at 1440°C but a high solubility (∼12.2 at.%) of yttrium at the Ti sites at 1515°C. When BaTiO₃was heavily doped with yttrium at the Ti sites (a yttrium concentration ( y ) of <0.059), the crystallographic structure was tetragonal, whereas for y greater than equal to 0.059, the crystallographic structure was cubic.     

Dependence of Crystallization Behavior on Particle Size in Barium Strontium Titanate Glass‐Ceramics

Differential thermal analysis studies on the crystallization kinetics and phase developments of barium strontium titanate glass‐ceramics have been performed for a series of glass particles with different particle sizes. The crystallization behavior was deduced to be influenced strongly by the particle size of the glass samples. These studies have revealed the initial formation at lower temperatures of metastable fresnoite Ba₂TiSi₂O₈ (BTS) phase followed by its transformation at higher temperatures to feldspar BaAl₂Si₂O₈ (BAS) and perovskite (Ba,Sr)TiO₃ (BST) phases. The metastable BTS phase was proved to crystallize predominately by surface crystallization while the feldspar BAS phase showed significant evidence of internal crystallization. And for the perovskite BST phase, crystallization mechanism changes from surface to internal type at a critical particle size of 75 μm. In addition, activation energy and the Avrami parameter for crystallization have been determined for the three phases by the employment of glass samples with two typical particle sizes.     

Centrifugal Sintering of a Barium Titanate Thick Film

A highly packed barium titanate film with a thickness of 30 μm was prepared under 1000 g _n ( g _n , standard acceleration of free fall) via centrifugal sintering. Here, BaTiO₃ particles were used as the source material, and LiF flux was co-added as a grain growth enhancer. The film was originally printed on a substrate by screen printing, and subsequently sintered. As the amount of flux increased, the film density also increased with remarkable grain growth. However, it was difficult to remove pores in conventional sintering even by the heavy addition of flux such as 20 wt%. In contrast, centrifugal sintering successfully compacted films (90% of theoretical density). The centrifugally sintered film possessed a relatively smooth surface and showed no flux segregation. These features of a centrifugally sintered film are thought to be attributed to the enhancement of particles' rearrangement at an elevated temperature by a centrifugal force.     

Novel Doping Mechanism for Very-High-Permittivity Barium Titanate Ceramics

Barium titanate (BaTiO₃) can be doped with La³⁺ ions via partial substitution for Ba²⁺ ions; charge balance is maintained by the creation of Ti⁴⁺ vacancies. Samples processed in an atmosphere of 1 bar O₂ and a temperature of 1350°C are insulating and free from electronic defects associated with either O₂ loss or reduction of Ti⁴⁺ to Ti³⁺. The Curie temperature ( T _c) decreases approximately linearly as the lanthanum content increases and, at the same time, an increase in the permittivity (ɛ') maximum at T _c occurs. For the composition Ba_{1- x} La _x Ti_{1- x /4}O₃, where x = 0.05, ɛ' has a maximum value of 19000 at 18°C, compared with a typical value of 10000 at 130°C in undoped BaTiO₃ ceramics. This value is the highest value reported for A-site-doped BaTiO₃ and is linked to the mechanism of combined A-site doping and Ti-vacancy creation.     

Spark Plasma Sintering of Bismuth Titanate Ceramics

Spark plasma sintering (SPS) was used to fabricate bismuth titanate (Bi₄Ti₃O₁₂) ceramics. The densification, microstructure development and dielectric properties were investigated. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 200°C lower and the microstructure was much finer than that of the pressureless sintered ceramics, and dense compacts with a high density of over 99% were obtained at a wide temperature range of 800°–1100°C. Dielectric property measurement indicated that the volatilization of Bi³⁺ was greatly restrained during SPS, resulting in an unprecedented low dielectric loss for pure Bi₄Ti₃O₁₂ ceramics.