Fabrication of crystal-oriented barium-bismuth titanate ceramics in high magnetic field and subsequent reaction sintering

Abstract

High magnetic field was applied to fabricate novel lead-free piezoelectric ceramics with a textured structure. A compact of crystallographically oriented grains was prepared by dry forming in a high magnetic field from a mixed slurry of bismuth titanate and barium titanate powders. Bismuth titanate particles with a size of about 1 μ m were used as the host material. In the forming process, the slurry was poured into a mold and set in a magnetic field of 10 T until completely dried. Bismuth titanate particles were highly oriented in the slurry under the magnetic field. The dried powder compact consisted of highly oriented bismuth titanate particles and randomly oriented barium titanate particles. Barium bismuth titanate ceramics with a- and b-axis orientations were successfully produced from the dried compact by sintering at temperatures above 1100 ° C.     

Grain-oriented calcium hydroxyapatite ceramic and film prepared by magnetic alignment

Grain-oriented calcium hydroxyapatite (HA) ceramics are potential to enhance or select their bioactive properties. In the current work, highly a-axis oriented calcium HA bulk and thick film samples were prepared by a novel process combining magnetic alignment and polymerization techniques. The slurry containing 40 vol.% commercial HA powder, monomer, initiator and catalyst were poured into a rubber mold for bulk samples or drop-coated on Al₂O₃ substrate for films at first, and then moved them into a vertical 10 T magnetic field. The HA particles in the low viscosity slurry were aligned by magnetic force and then in situ locked via polymerization of the slurry. After sintering, the grain-oriented HA ceramic bulk and thick film samples with a-axis parallel to the magnetic field direction were obtained. Theoretically, the grain-orientation can be easily controlled by adjusting the magnetic field direction.     

无铅压电陶瓷与义齿基托树脂结合强度的研究

制备了3种不同类型的无铅压电陶瓷,包括钛酸钡(BT)、钛酸铋钠(BNT)、铌酸钾钠(KNN),用XRD对无铅压电陶瓷进行物相分析;将这3种无铅压电陶瓷与义齿基托树脂聚甲基丙烯酸甲酯(PMMA)粘接,通过粘接剪切强度测试评价了无铅压电陶瓷与义齿基托树脂间的粘接强度.实验结果表明,3种不同的无铅压电陶瓷与义齿基托树脂之间的粘接剪切强度值有统计学的差异(P<0.05),由高到低依次为KNN>BNT>BT;所采用的无铅压电陶瓷与义齿基托树脂之间有良好的粘接性能;粘接破坏形式主要为内聚性破坏.研究结果为无铅压电陶瓷与义齿基托树脂将来在口腔临床治疗中的应用提供了重要的实验依据.     

Processing of chemical solution-deposited BaTiO<Subscript>3</Subscript>-based thin films on Ni foils

BaTiO₃ films on base metal foils are of interest for capacitor applications, but the processing requires reducing atmospheres that influence the film defect chemistry and density. In this study, powders dried from barium titanate solutions and barium titanate films were studied by X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis, infrared spectroscopy, and spectroscopic ellipsometry at various points in the processing. It was found that atmospheres designed to minimize Ni oxidation delay decomposition of organics, leading to retained carbonate phases. Thus, crystallization of the barium titanate occurs via decomposition of a barium carbonate phase. Retained organics that are present during high temperature processing can cause porosity in the films. On annealing at 1000 °C, there is slightly increase in the refractive index of the film due to further crystallization and densification. The final refractive index is comparable to that of 95% dense barium titanate ceramics. Re-oxidation did not change the refractive index of the film over the wavelength range from 350 to 650 nm.     

高Tc铋层状压电陶瓷结构与性能

综述了铋层状压电陶瓷的结构特点及性能研究．铋层状压电陶瓷的结构由（Ｂｉ_２Ｏ_２）^２＋层和钙钛矿层（Ａ_ｍ－１Ｂ_ｍＯ_３ｍ＋１）^２－按一定规则共生排列而成．此处 Ａ为适合干 １２配位的离子;Ｂ为适合于八面体配位的离子,ｍ为一整数,其值一般为１～５．与钛酸钡（ＢａＴｉＯ_３）或锆钛酸铅（ＰＺＴ）陶瓷相比,铋层状压电陶瓷具有以下特点：低介电常数、高Ｔ_ｃ、机电耦合系数各向异性明显、低老化率、高电阻率等． 先前研究证明,居里温度不仅与极化原子位移、自发极化强度、Ａ位Ｂｉ含量有关,而且还与取代离子的特性诸如离子半径、电负性、核外电子排布有关．压电活性低是铋层状陶瓷的本质缺点,通常发展该材料的途径为化学取代或晶粒取向技术．研究材料结构与性能之间的关系有助于发展铋层状压电陶瓷材料．     

Characterization of dielectric barium titanate powders prepared by homogeneous precipitation chemical reaction for embedded capacitor applications

Various articles have reported that a highly pure and uniform form of barium titanate can be prepared by homogeneous precipitation. However, most of these works emphasize the mechanism of thermal decomposition of barium titanyl oxalate tetrahydrate, and only a few have discussed morphology or particle size. The morphology and particles size of barium titanyl oxalate tetrahydrate are governed by reaction temperature, pH value and solvent ratio; the barium titanate structure can be obtained by calcinating barium titanyl oxalate tetrahydrate above 600 °C or hydrothermally in a basic solution at 200 °C. The final morphology of barium titanate in this investigation was similar to that of barium titanyl oxalate tetrahydrate and the particle size of barium titanate increased with the calcination temperature. Using this barium titanate in a polymer/ceramic composite provided better dielectric characteristics than commercial ceramic powders use in embedded capacitor applications.     

Effect of seed particles content on texture formation of Si_3N_4 ceramics by gel-casting in a strong magnetic field

In this paper,the textured Si_3N_4 ceramics were prepared by adding seed particles during gel-casting in the magnetic field of 6 T,followed by pressureless sintering.The effect of pH on the stability and dispersibility of Si_3N_4 slurry and the effect of seed particles content on texture formation of Si_3N_4 ceramics were both studied.Those results showed that the slurry with good stability and dispersibility was obtained when pH was about 11.6.The a or b-axis of Si_3N_4particles or crystals was aligned parallel to the direction of the magnetic field in the magnetic field of 6 T.The degree of texture of Si_3N_4 ceramics further increased during sintering.With the increasing of additional β-Si_3N_4 particles in the magnetic field of 6 T,the degree of texture increased from0.19 without seed particles to 0.76 with 9%(mass fraction)seed particles.The increase of seed particles content promoted the texture formation of Si_3N_4 ceramics.     

Preparation of a–b plane oriented Nb-doped Bi4Ti3O12 ceramics by magnetic alignment via gelcasting

Highly a–b plane grain-oriented Nb-doped Bi₄Ti₃O₁₂ ceramics (Bi₄Ti_2.96Nb_0.04O₁₂, BINT) were successfully prepared by magnetic alignment (MA) via gelcasting technique using only conventional solid-state-synthesized starting powder. The micro-size BINT particles with irregular shape were aligned in slurry by strong magnetic force and then in situ locked by polymerization via gelcasting technique in 30 min in a 10 T magnetic field. Highly a–b plane orientation parallel to the magnetic field direction (//B) was obviously observed in the green compact (f_(200)/(020) = 0.41) and sintered sample (f_(200)/(020) = 0.67). The sintered sample contained plate-like grains and reached 97% theoretical density. Compared to the controlled sample without magnetic alignment, the magnetically aligned sample shows enhanced dielectric constant in //B direction (160 versus 120 at room temperature and 350 versus 250 at 580 °C). This method, using typical gelcasting technique in a strong magnetic field, readily applicable to prepare other ceramics and is expected to facilitate the mass preparation of large and dense grain-oriented ceramic components.     

Effect of BaTiO3 additive on the electrical properties of Na0.50Bi0.50TiO3 lead free ceramics

The near morphotropic phase boundary (MPB) compositions of lead-free piezoelectric ceramics based on sodium bismuth titanate (Na_0.50Bi_0.50TiO₃: NBT) and barium titanate (BaTiO₃: BT) were carefully investigated by conventional high temperature mixed-oxide method. All the ceramics exhibit single phase rhombohedral symmetry. The frequency (100 Hz to 1 MHz) and temperature (Room temperature–500 °C) dependence of impedance spectroscopy of (1 − x)Na_0.50Bi_0.50TiO3–xBaTiO₃ (x = 0.0, 0.06, 0.07 and 0.08) ceramics were investigated by impedance analyzer. The frequency explicit plots of Z″ versus frequency at various temperatures show peaks in the higher temperature range (>400 °C). The compounds show dielectric relaxation, which is found to be of non-Debye type and the relaxation frequency shifted to higher side with increase in temperature. The activation energy values obtained for different BT content suggest that the electrical conduction in NBT is mainly due to the mobility of the ionized oxygen defects.     

Preparation of monodispersed spherical barium titanate particles

Spherical barium titanate particles with cubic phase were synthesized by a low-temperature hydrothermal reaction. Firstly, The method of hydrolysis of titanium tetrachloride was used for producing spherical TiO₂ particles (0.45–1.5 μ m) with various concentrations of TiCl₄(0.05–0.2 M) and volume ratios of acetone to water solutions (RH = 0–4). These TiO₂ particles were converted to barium titanate by a hydrothermal conversion in a barium hydroxide solution. The size and morphology of the TiO₂ particles was controlled by the volume ratio of acetone to water (RH ratio) in the mixed solvent. At the RH ratio of 3, the morphology of TiO₂ particles was very uniform and discrete. These TiO₂ particles were in the anatase phase and were converted to the rutile phase when the calcination temperature increased to 700∘C and above. Uniform and spherical barium titanate particles were successfully synthesized from the as-prepared TiO₂ particles by using a hydrothermal reaction in a barium hydroxide solution. The Ba/Ti ratios, reaction temperature, and reaction time did not influence the size and morphology of BaTiO₃ particles, but increased the concentration of unfavorable salts such as Ba(OH)₂ and BaCO₃. The high purity BaTiO₃ particles could be obtained by washing with formic acid to remove the unfavorable salts. The size and morphology of the BaTiO₃ particles remained the same as those of the TiO₂ particles, confirming the in-situ transformation mechanism for the conversion of TiO₂ to BaTiO₃. The as-synthesized particles were cubic phase and transformed to tetragonal phase after calcinations at 1150∘C for 1 h. The mean density of the pellets sintered at 1300∘C for 2 h was 5.86 g/cm³ and accounted for 97.34% of the theoretical density.     

Sol-gel processed barium titanate ceramics and thin films

Ferroelectric barium titanate (BaTiO3) ceramics and thin films have been prepared from barium acetate (Ba(CH3COO)2) and titanium (IV) isopropoxied (Ti((CH3)2CHO)4) precursors by a sol–gel technique. The as-grown powder and thin films were found to be amorphous, which crystallized to the tetragonal phase after annealing at 700°C in air for 1 h. Both the ceramics and thin films showed well-saturated polarization–field (P–E) hysteresis loops at room temperature. The value of the spontaneous polarization, PS, remnant polarization, Pr, and coercive field, Ec, of the ceramics and thin films determined from the P–E hysteresis loop were found to be 19.0 and 12.6; 14.0 and 3.2 G cm–2, and 30 and 53 kV cm–1, respectively. The coercive field of the film determined from the capacitance–voltage, C–V, characteristics is slightly lower than that determined from the P–E hysteresis loop (43 kV cm–1). The room-temperature dielectric constant, , of the ceramics and films was found to be 1135 and 370, respectively. Both the films and ceramics showed dielectric anomaly peaks at 125 °C, showing ferroelectric to paraelectric phase transition. © 1998 Kluwer Academic Publishers     

Effect of the crystallinity of BaTiO<Subscript>3</Subscript> powders prepared using the merker method on the properties of PTCR ceramics

Barium titanate powders differing in particle size (110–740 nm) were prepared by calcining barium titanyl oxalate precipitated by the Merker method. The powders were sintered to produce PTCR ceramics with the composition 100(Ba_0.89Ca_0.08Pb_0.03)TiO₃ + ^0.8TiO₂ + ^0.7Y + ^0.1Mn + ^2.5SiO₂ and electrical properties of the ceramics were studied. The results demonstrate that improving the crystallinity of the barium titanate powder suppresses recrystallization of the ceramics and has a significant effect on their resistance ratio and electric strength. We found the optimal range of calcination temperatures (950–1000°C) for barium titanyl oxalate which ensures the highest electric strength of thermistors with a resistance of 31 Ω. The average crystallite size of the parent barium titanate powder is ∼250–320 nm.     

The effects of CuO-doping on dielectric and piezoelectric properties of Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–Ba(Zr,Ti)O<Subscript>3</Subscript> lead-free ceramics

CuO-doped lead-free ceramics based on bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) and barium zirconate titanate (Ba(Zr_0.07Ti_0.93)O₃, BZT) were prepared via a multi-step solid-state reaction process. The BNT–BZT with CuO dopant ceramics sintered at 1150–1180 °C for 2 h in air showed a pure perovskite structure. SEM images reveal that a small amount of CuO (<2 mol%) play a significant role on the microstructure to improve its sintering attributes, while it will degrade when the dopant is added beyond 2 mol%. The dielectric and piezoelectric properties of CuO-doped BNT–BZT ceramics were evaluated. At room temperature, the sample doped with 2 mol% CuO shows quite good properties such as a high piezoelectric constant (d ₃₃ ∼156.5 pC/N) and a high electromechanical coupling factor (k _t ∼52%). The depolarization temperature increased dramatically and the maximum permittivity temperature decreased slightly.     

Synthesis and structural characterization of Ce-doped bismuth titanate

Ce-modified bismuth titanate nanopowders Bi_4−xCe_xTi₃O₁₂ (x ≤ 1) have been synthesized using a coprecipitation method. DTA/TG, FTIR, XRD, SEM/EDS and BET methods were used in order to investigate the effect of Ce-substitution on the structure, morphology and sinterability of the obtained powders. The phase structure investigation revealed that after calcinations at 600 °C powder without Ce addition exhibited pure bismuth titanate phase; however, powders with Ce (x = 0.25, 0.5 and 0.75) had bismuth titanate pyrochlore phase as the second phase. The strongest effect of Ce addition on the structure was noted for the powder with the highest amount of Ce (x = 1) having a cubic pyrochlore structure. The presence of pure pyrochlore phase was explained by its stabilization due to the incorporation of cerium ions in titanate structure. Ce-modified bismuth titanate ceramic had a density over 95% of theoretical density and the fracture in transgranular manner most probably due to preferable distribution of Ce in boundary region.     

铅系弛豫体基与钛酸钡基独石电容器介电特性的比较

对比研究了典型铅系钙钛矿型弛豫铁电陶瓷独石电容器及钛酸钡基独石电容器的介电特性随温度、电场等条件下的变化规律。结果表明,尽管在弱电场下弛豫体陶瓷与钛酸钡陶瓷的介电行为存在较大的差别,但在强电场下这种差别会减小。从应用的角度看,两种电容器的介电性能在强电场下的相似性是弛豫体电容器取代传统钛酸钡电容器进入大规模应用的重要保证。同时,弛豫铁电陶瓷具有较高的介电常数,这一优势可为电容器设计中提高元件综合性     

Preparation of monolithic crystalline lead titanate aerogels

Lead titanate gels were prepared via sol–gel synthesis and dried either in supercritical carbon dioxide or isopropanol. Both types of aerogels were characterized by means of thermal analysis and transmission electron microscopy. Helium pycnometry and nitrogen adsorption measurements were performed on samples that had been annealed at temperatures up to 600°C. Phase transformations and the crystallization of tetragonal PbTiO3 were monitored by X-ray diffraction. While aerogels supercritically dried in carbon dioxide disintegrate upon heat treatment, samples dried in isopropanol can be transformed to the crystalline state as monoliths. With an envelope density of 0.75 g cm-3 these ceramics exhibit a porosity of 90%. © 1998 Chapman & Hall     

Problems of YAG nanopowders compaction for laser ceramics

Slip casting and colloidal slip casting at high pressure of yttrium aluminum garnet powders were investigated. It was found that the presence of residual pores in laser oxide ceramics was determined mainly by big size pores in the compact. The size of pore in compact is critical when it is greater than the mean size of initial particles. It was shown that formation of pores’ structure in compact was controlled by appearance of quasi-particles in heavy loaded slurry. Pores concentration is critical for ceramics optical transmittance.     

Selective laser sintering of barium titanate–polymer composite films

A selective laser sintering process has been used to consolidate electro-ceramic thin films on silicon substrates. Methods of forming pre-positioned layers of barium titanate were investigated by spin-coating the feedstock powder mixed with a commercial polymer photo-resist. The ceramic–polymer composite was deposited directly onto a nickel film which was evaporated onto a silicon substrate, pre-oxidised to form an electrically insulating layer. A range of laser processing parameters was identified in which consolidated barium titanate layers could be formed. The laser power was found to be more influential in forming sintered microstructures than laser exposure time. The microstructure of barium titanate films is sensitive to the SLS laser-processing conditions, with the optimum laser powers for the processing of the BaTiO₃–polymer found to be in the range 17–20 W. This article highlights the possibility of using ‘direct write’ techniques to produce piezoelectric materials upon silicon substrates.     

Modeling of local thermal fields in semiconducting barium titanate ceramics

A mathematical model for describing local temperature fields in barium titanate ceramics with a positive temperature coefficient of resistance under the action of electric current has been constructed. The space-time temperature distribution within the grains of semiconducting barium titanate with varying microstructure and specific resistance has been studied. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 4, pp. 114–117.     

Improving the electromechanical performance of dielectric elastomers using silicone rubber and dopamine coated barium titanate

In this work, a new soft dielectric elastomer (DE) was fabricated from dopamine coated barium titanate particles and silicone rubber (SR). The results showed that the barium titanate (BaTiO₃, BT) was coated by dopamine and the coated particles were highly compatible with SR. In order to achieve a maximum voltage-induced deformation, the minimum secant moduli of DEs were obtained in experimentation at a stretch ratio of approximately 1.6 by applying equi-biaxial tensile strain using the bubble inflation method. Additionally, it was found that the addition of DP-BT into SR led to an increased dielectric constant and decreased dielectric loss tangent for the matrix by comparison with SR/BT composites. Furthermore, the electromechanical properties of the SR/DP-BT composites were greatly improved in terms of voltage-induced deformation (s_a), electromechanical energy density (e) and coupling efficiency (K²). A maximum actuated area strain of approximately 78%, which was 30% larger than that of the SR/BT composites, was achieved for the sample having a DP-BT content of 20 wt.%. This strain corresponded to a low dielectric strength of around 53 V/μm, the composite exhibited a maximum energy density of 0.07 MJ/m³ and coupling efficiency of 0.68.