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.     

Zn掺杂BaTiO3微粉的微波水热合成

以钛酸丁酯、硝酸钡和醋酸锌为原料,采用微波水热法制备了掺锌的BaTiO3微粉。利用XRD和SEM对产物进行了表征。研究表明,掺杂后锌固溶到了钛酸钡的晶格中并取代钡位或钛位。微波水热合成锌掺杂钛酸钡微粉温度更低,同时反应速率明显提高。     

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.     

Superior Reliability Via Two‐Step Sintering: Barium Titanate Ceramics

Two‐step sintering (TSS) in a reducing atmosphere has been employed to obtain fine‐grain BaTiO₃ ceramics with a core‐shell microstructure, a more uniform grain‐size distribution, and superior reliability for multilayer ceramic capacitor applications. Compared to ceramics of the same composition conventionally sintered for about the same time, TSS ceramics feature a thinner shell thickness thus a stronger dopant localization, which leads to a lower concentration, higher internal resistance and more dopant‐ association. Improved reliability is manifest in a 50% higher breakdown strength at ambient temperature and a 400% longer endurance time to withstand DC stress at 185°C, in addition to a less field‐and‐temperature‐dependent capacitance. A scaling analysis of the redistribution and endurance dynamics identifies transmission across the shell‐grain‐boundary region as the critical element beneficially impacted by core‐shell structure and two‐step sintering.     

水热反应条件对合成BaTiO3纳米粉体影响的研究

通过正交实验法,对不同钡钛比、反应温度、保温时间进行了水热合成钛酸钡纳米粉体的实验研究,对所得粉体进行了DTA,XRD,TEM等分析表征,分析研究了不同水热反应条件对合成钛酸钡纳米粉体的影响。     

水热法制备钕掺杂BaTiO3粉体及其介电性能研究

以BaCl2·2H2O、TiCl4和NdCl3为原料,采用水热法制备了掺钕的BaTiO3纳米粉体,并经高温烧结后得到瓷体。利用DTA、XRD、SEM等测试手段,分析了钕掺杂对钛酸钡粉体及其陶瓷电性能的影响。研究表明,掺杂后,钕固溶到了钛酸钡的晶格中并取代钡位或钛位。钕的掺杂有助于获得细晶高致密的陶瓷,当W(NdCl3)%为0.6时,致密度最高,常温下相对介电常数高达5650,击穿场强达到3.5kV/mm。     

Grain-Size Effects on Diffuse Phase Transitions of Sol-Gel Prepared Barium Titanate Ceramics

Barium titanate ceramic samples, obtained by sol-gel methods, having different grain sizes have been analyzed by diffuse phase transition techniques. We performed a study of the influence of each method on the phase transition interval and on the Landau parameters of the ferroelectric phase transition of these materials. The results show the existence of a relationship between the diffuse character of the transition and the grain size of the sample. The method of fitting dielectric data presented here allows us to compare ceramics with single crystals and then analyze the sol-gel process used to obtain them.     

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.     

Particle Size Control of Barium Titanate Prepared from Barium Titanyl Oxalate

Barium titanates formed from decomposition of barium titanyl oxalates (BT-oxalates) exhibit the same powder state as that of the Bt-oxalates, so that controlling the particle size and shape of the starting BT-oxalates is necessary. In the present study, BT-oxalates were precipitated from aqueous solution under various aging conditions: aging fluids, temperature, and time. The particle size of the spherical BT-oxalates was controlled at 0.4 μm by aging at 25°C for 3 h, and the diameter of the barium titanate formed was similar to that of the BT-oxalates.     

Low-Temperature Sintering of Lead Zinc Niobate–Lead Zirconate Titanate Ceramics Using Nano-Sized Powder Prepared by the Stirred Media Milling Process

Lead zinc niobate–lead zirconate titanate (PZN–PZT) nano-sized powders with a diameter of ∼35 nm were fabricated by a high-energy stirred media mill using 50 μm diameter zirconia beads as the milling media at a rotation speed of 4000 rpm for 1 h. The sintering temperature of PZN–PZT was greatly reduced, and a fully densified bulk body was obtained at only 750°C when stirred media milled nanopowder was used. The control of evaporation of lead oxide was very important to obtain high electrical properties due to the increased surface area of nano-sized powders. The ferroelectric hysteresis, piezoelectric d ₃₃ coefficient, and dielectric properties of sintered ceramics using nanopowder were measured and compared with the values obtained from a sintered specimen using conventional milled powders. Remanent polarization, d ₃₃ coefficient, and relative dielectric constant of 750°C sintered stirred media milled powders containing 2% of excess PbO and 1% of 4PbO–B₂O₃ liquid phase were 10.3 μC/cm², 277 pC/N, and 1310, respectively.     

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.     

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.     

热压烧结制备高致密度B4C陶瓷及其致密化

以B4C粉末为原料,以酚醛树脂热解碳为烧结助剂,采用真空热压烧结工艺制备出高致密度的B4C陶瓷材料,探讨了酚醛树脂含量和烧结温度对材料致密化的影响,对比分析了纯B4C和掺碳B4C的热压烧结致密化过程.研究显示,烧结温度的升高有利于材料的致密化,酚醛树脂的添加则显著活化了B4C的烧结.实验条件下酚醛树脂的最优添加量为3％（质量分数）,烧结过程中热解碳固溶于B4C晶格,显著加速了其在1600℃至1800℃区间的致密化速率.     

固相法烧结温度对钛酸钡陶瓷介电性能的影响

研究了固相法烧结温度对钛酸钡陶瓷介电性能的影响.采用固相法在不同温度下烧结钛酸钡陶瓷.结果表明,不同烧结温度对钛酸钡陶瓷晶体结构、微观形貌、介电常数、介电损耗、居里温度等都会产生不同的作用效果.钛酸钡陶瓷只有在最佳烧结温度附近才具有最好的结构和性能,烧结温度过低会使烧结过程不充分,引起过多的点缺陷;而过高的烧结温度也会由于过烧现象的存在而使晶粒与晶界间相互作用出现异常,两种情况都会导致钛酸钡陶瓷介电性能的劣化.     

Colossal Permittivity in Microwave‐Sintered Barium Titanate and Effect of Annealing on Dielectric Properties

Colossal permittivity (ε′ = 301,484 at room temperature and 1 kHz) of barium titanate was induced in ceramics synthesized using the microwave sintering method. Three different sintering processes (conventional, spark plasma, and microwave) were performed to better understand colossal permittivity in sintered barium titanate. The dielectric permittivity measurements revealed that the appearance of colossal permittivity has strong dependence on the sintering temperature and atmosphere, and less on the grain size of the sintered ceramics. However, the as‐sintered barium titanate samples produced by microwave sintering show high dielectric loss (tanδ > 1) consistent with oxygen reduction during the microwave sintering process and consequent accumulation of oxygen vacancies and associated charge carriers at the grain boundary. Since the highly conductive state of as‐sintered ceramics precludes their use in dielectric applications, thermal annealing at different conditions was performed to recover insulating characteristics. Microwave‐sintered barium titanate with post annealing process (950°C for 12 h in air) showed low dielectric loss (tanδ = 0.045) at room temperature and 1 kHz, while still showing a much higher permittivity (ε′ = 36,055) than conventionally sintered barium titanate (ε′ = 3500).     

Effect of Crystal-Axis Orientation on Dielectric Properties of Ceramics Prepared from Fibrous Barium Titanate

Barium titanate (BaTiO₃) ceramics with high degrees of a- and c -axis orientation were fabricated by combining hydrothermal synthesis of fibrous particles, formation of green compacts with aligned extruded rods in the mold, and normal sintering. The orientation of crystal axes in such ceramics gives rise to high dielectric permittivity. Ceramics with an orientation degree f =76% showed values of ε/ε₀ which are 1.5 times (ε/ε₀= 2700) as large as those of a randomly oriented (commercial) ceramic (ε/ε₀= 1800).     

Vibration Assisted Hot‐Press Sintering of AlN Ceramics

AlN ceramics is very difficult to be sintered due to its strong covalent bonds and low self‐diffusion. In this work, a new hot‐press sintering system assisted by vibration was used to densify AlN ceramics. During hot‐pressing process an additional vibration caused by pressure fluctuation is imposed on the sample. The well‐densified AlN ceramics had been obtained at 1750°C under 15 MPa, or at 1800°C under 10 MPa which were much milder than traditional hot‐press conditions and would be beneficial to prolong working life of the graphite mold. The samples made by this method have a bending strength about 320 MPa. After annealing, it shows a thermal conductivity of 217 W/mK.     

Electric Fatigue in Ferroelectric Lead Zirconate Stannate Titanate Ceramics Prepared by Spark Plasma Sintering

Ferroelectric lead zirconate stannate titanate ceramics were prepared by spark plasma sintering (SPS). Compared with its counterpart densified by conventional sintering (CS), the SPS material shows a smaller remanent polarization and maximum strain as well as a higher coercive field. Electric fatigue in both materials was investigated. In contrast to CS samples, the SPS specimens show a lower resistance to bipolar electric cycling, characterized by a faster decrease in remanent polarization and maximum strain at cycle number below 10^6.5 and a subsequent slower reduction of the properties at high cycle numbers up to 10⁸.     

Electric Fatigue in Antiferroelectric Lead Zirconate Stannate Titanate Ceramics Prepared by Spark Plasma Sintering

The fatigue behavior of lead zirconate stannate titanate (PZST) ceramics prepared by spark plasma sintering (SPS) was investigated. Polarization and strain hysteresis loops were monitored. The material shows a high resistance to fatigue because of bipolar electric cycling. Both maximum strain and switchable polarization first show a fatigue stage 0 to 10⁵ cycles and then a fatigue-free period up to 10⁸ cycles. The maximum losses of maximum strain and switchable polarization are 18% and 10% of their initial values, respectively. The dominant fatigue mechanism is assigned to the pinning of domain walls by charged defects.