Phase formation kinetics of nanoparticle-seeded strontium bismuth tantalate powder

Strontium bismuth tantalate (SBT) having composition of Sr_0.7Bi_2.3Ta₂O₉ has been prepared through sol-gel method using their corresponding metal alkoxides as precursors. Seeded SBT powder was prepared by the addition of 5 wt% of nanometer-sized SBT particles to the sol followed by pyrolysis. Differential thermal analyses (DTA) were performed on the unseeded and seeded powder and Aurivillius phase formation temperatures were found to be reduced by 60°C in the seeded ones. Non-isothermal kinetic analyses were applied to the DTA results to obtain activation energy and Avrami exponent values for the Aurivillius phase formation in unseeded and seeded samples. The activation energy for the Aurivillius phase formation was found to be 268 kJ/mol for the seeded ones, while 375 kJ/mol for the unseeded ones, which plays a major role for the enhanced kinetics in the seeded ones. The Avrami exponent values for the Aurivillius phase formation in unseeded and seeded ones were determined as 2.80 and 1.15, respectively, revealing different nucleation and crystal growth mechanisms.     

Optical and electric properties of strontium bismuth tantalate thin films

High quality SrBi₂Ta₂O₉ ferroelectric thin films were fabricated on platinized silicon using pulsed laser deposition. The optical properties of the thin films were studied by spectroscopic ellipsometry from the ultraviolet to the infrared region. Optical constants, n ∼ 0.25 in the infrared region and n ∼ 2.18 in the visible spectral region, were determined through multilayer analyses on their respective pseudodielectric functions. The band-gap energy is estimated to be 4.14 eV. It was found that the leakage current mechanism of the film was from bulk limited Poole-Frenkle emission to interface-controlled Schottky emission with applied field increasing, and that the breakdown field of the film had a negative linear variation with the logarithm of the electrode area.     

Effect of vanadium doping on sintering and dielectric properties of strontium barium niobate ceramics

Strontium barium niobate, Sr_0.5Ba_0.5Nb₂O₆ (SBN50) ceramics doping up to 3 wt% V₂O₅ were fabricated by solid state reaction route, starting from raw materials (oxides and carbonates) of analytical grade. The phase composition, microstructure and dielectric properties were investigated by X-ray diffraction, scanning electric microscope and impedance analyzer. The results show that the addition of V₂O₅ improves sintering densification of SBN ceramic samples. The relative density of the samples firstly increases and then slightly decreases with increasing amounts of V₂O₅ and sintering temperature. With the help of the additive of 1 wt% V₂O₅, the relative density of the sample sintered at 1,280 °C for 3 h can reached 97.2%. Only single tetragonal tungsten bronze phase SBN exists in all the doped samples. With increase in V₂O₅ content, the dielectric constant of SBN ceramics at both room temperature and in the vicinity of the phase transition temperature increases significantly and the Curie temperatures (Tc) obviously shifts to low temperature as well as the dielectric loss remains below 0.06. The diffuseness in the phase transition is found to increase with increase in vanadium doping level. The addition of V₂O₅ results in an increased grain size associated with rod-like grain growth.     

Forming processes of strontium barium niobate ceramics

In forming strontium barium niobate (SBN) ceramics, two methods (pressure filtration and slip casting) were employed to investigate the consolidation behaviour. The zeta potentials were measured to understand the interparticle forces of SBN powders. It was found that the zeta potentials of SBN powders were negative above pH 2.2. Several experiments have been conducted to investigate the effect of pH on the rheological behaviour of SBN slurries with 20 vol% solids loading. The rheological behaviour of the slurries SBN with 20 vol% solids loading at pH 11.5 is shear thinning. It is suggested that the increase of the flow rate of the fluid might have the advantages to enhance the packing density and prevent fine particles from clogging in pressure filtration and slip casting. Two different moulds i.e. plaster and alumina have been used to investigate the effect of pore morphology of the moulds on the cake microstructures. A uniform microstructure of cast cake was formed for using an alumina mould and significant contamination was observed in using a plaster mould.     

Influence of samarium doping on structural and dielectric properties of strontium bismuth tantalate ceramics derived by molten salt synthesis route

Layered Sr(Bi_1?xSm_x)₂Ta₂O₉ ceramics with x ranging from 0 to 0.10 (10 mol%) were fabricated by the low temperature molten salt synthesis route. X-ray powder diffraction studies revealed that the single phase orthorhombic layered perovskite structure is retained in all these compositions. Scanning electron microscopic studies on these ceramics confirmed the presence of well packed equiaxed plate shaped grains. The dielectric and electrical conductivity properties were studied in the 100 Hz–1 MHz frequency range at 300 K. Interestingly, the 10 mol% samarium doped SrBi₂Ta₂O₉ ceramics exhibited high dielectric constant (ε_r = 155) and low dielectric loss (0.00298) compared to those of other compositions. The electrical conductivity of undoped and samarium doped ceramics increased linearly with increase in frequency.     

Sintering behavior and dielectric properties of Ce doped strontium barium niobate ceramics with silica sintering additive

Sr_0.5Ba_0.5Nb₂O₆ ceramics with and without CeO₂ dopant were prepared by a partial co-precipitation method and a liquid phase sintering process. The cooperative effects of Ce doping and silica sintering additive on the sintering behaviors and the dielectric properties of Sr_0.5Ba_0.5Nb₂O₆ ceramics was investigated. It was observed that the lattice parameters of a-axis and c-axis of Sr_0.5Ba_0.5Nb₂O₆ ceramics decrease with the increase of Ce dopant, namely contraction of crystal cell volume occurs. Amorphous silica used for sintering additive can effectively restrain abnormal grain growth and prevent the rise of sintering activation energies caused by Ce doping, but Ce doping has more effect on the average size of the grains and the dielectric properties than the silica sintering additive when the Ce dopant and the silica sintering additive are introduced. Both the Curie temperatures and the maximum of dielectric constant at Tc decrease as the Ce³⁺ concentration increases.     

Transitions of barium strontium titanate ferroelectric ceramics for different strontium content

Ceramic based barium strontium titanate (BST) solid solutions with the formula Ba_1 − xSr_xTiO₃ are very important candidates for a wide range of device applications. Several doped (Mn and Mg) and undoped samples were prepared by standard solid-state reaction. Special emphasis was put on compositions with x = 0.35 and 0.60, with high potential for applications. The samples were sintered at temperatures in the 1200 ÷ 1260 °C range. Structural X-ray diffraction analysis preformed confirms the perovskite structure. The dielectric parameters were investigated in a wide temperature range between − 150 and 150 °C. The temperature was cyclically changed in both directions, up and down, at a rate of less than 2 °C/min. Both permittivity and dielectric loss were measured at low frequencies, 1 kHz. The peak values of the permittivity are increasing from 2000 to 4000 with the sintering temperature increase. Moreover the dielectric parameters were measured at room temperature in microwave domain (1 ÷ 2 GHz). The Curie temperature of BST samples with x = 0.35 and x = 0.6 is in agreement with the Curie point dependence on Sr content, as we have previously reported.     

Aging effects on dielectric properties of barium neodymium titanium tantalate ceramics

The aging effects on crystal structures and dielectric properties were investigated in Ba_pNd_6−pTi_8−pTa_2+pO₃₀ (p = 3, 4 and 5) ceramics with tungsten bronze structure. From XRD analyses on fresh and aged specimens, the slight change of crystal constant was concluded in Ba_pNd_6−pTi_8−pTa_2+pO₃₀ ceramics. Strong aging effects on the dielectric properties were observed in the present ceramics. After aging, a slightly decreased dielectric constant was observed combined with a significantly reduced dielectric loss in all the three compositions. These results had been discussed in term of the disorder-order state of ions at A- and B-sites. From tolerance factor and electronegativity difference, the stability was discussed for the filled type tungsten bronze phase.     

Relaxation characteristics of strontium barium niobate ferroelectric ceramics

The complex dielectric response of tungsten bronze Sr1-xBaxNb2O6 (SBN (1–x)/x, x=0.40, 0.50, and 0.60) relaxor ferroelectric ceramics has been carefully studied as a function of temperature between –180 and 450°C. Three distinct relaxation features were observed in all the compositions. The thermal hysteresis of dielectric maximum was considered to be associated with "lock in" of the incommensurate phase. It was confirmed by investigation of the Curie–Weiss behaviour in the higher temperature range. In addition, it was found that the weakness of the dielectric dispersion below the ferroelectric–paraelectric phase transition temperature can be induced by ageing and poling. These results are discussed in terms of the polarization fluctuation and metastability of polar microregions. © 1998 Chapman & Hall     

Structure and electrical properties of strontium barium niobate ceramics

SBN, x=0.25, 0.35, 0.50, 0.60 and 0.75 series of ceramics prepared by traditional sintering method have been studied systematically. The impact of composition and sintering temperature on structures, microstructures, and electrical properties of SBN ceramics was characterized of X-ray diffraction, scanning electron microscopy, and electrical measurements. It is found that the composition and temperature play an important role on the fabrication of single phase tetragonal TTB SBN ceramics. At x=0.5, TTB SBN ceramics can be obtained at 1200°C. For x<0.5, it consists of a mixture of TTB structure SBN and orthorhombic phase BaNb₂O₆ even at 1300°C; TTB structure SBN and orthorhombic phase SrNb₂O₆ for x>0.5. The complete TTB phase is produced at 1350°C. With Sr content increasing, the electrical performances show a regular change, strongly conforming to the reducing of the Curie temperature. SBN with the Sr composition of x=0.60-0.75 is a promising candidate for electro-optics device applications.     

钙钛矿型钛酸锶的制备、结构与性能

钛酸锶是一种具有重要电性质的陶瓷材料 ,因而在工业上得到了广泛的应用。本文介绍了钛酸锶的几种常见的合成方法 ,并简要的介绍了钛酸锶的结构与性能     

铬掺杂钛酸锶钡陶瓷的介电调谐性能

研究了Cr掺杂对钛酸锶钡(Ba0.6Sr0.4TiO3,BST)陶瓷的介电及其可调性能的影响.结果表明,少量的Cr可进入BST品格形成固溶体,并促进晶粒生长.当Cr掺杂量(摩尔分数)低于1.0%时,陶瓷的介电损耗急剧降低,调谐率明显提高,综合性能显著改善,其中Cr掺杂0.6%的BST陶瓷具有最佳的综合性能,其在1MHz下的介电损耗为0.0005,品质因子(FoM)达到500,而未掺杂样品的FoM值仅为60.Cr掺杂陶瓷损耗的急剧降低可归因于Cr3 离子的还原和Cr3 、Cr2 受主行为中和了氧空位的施主行为.     

铌酸锶钡陶瓷材料的制备及介电性能研究

以BaCO3，SrCO3和Nb205作为原料，采用高能球磨工艺制备SBN50陶瓷粉体。球磨后的粉体不经煅烧，直接压片成型，在1250～1350℃下保温1．5～12h可制备出SBN50陶瓷材料，并对此进行了X射线衍射分析、扫描电镜观察和性能测试。结果表明：球磨30h的粉体在1100℃时合成SBN50单相；随着烧结温度的升高和保温时间的延长，SBN50陶瓷的介电常数先增大后减小，晶粒大小呈有规律的变化。1300℃下保温3h制得的陶瓷样品介电常数最高（εmax=1447），居里温度（L）为130℃。     

Lead-free electrostrictive bismuth perovskite ceramics with thermally stable field-induced strains

Electric field-induced strain (EFIS) properties of Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ (BNKT) ceramics modified with Sr(K_1/4Nb_3/4)O₃ (SKN) have been investigated as functions of composition and temperature. BNKT ceramics near a phase boundary revealed the coexistence of ferroelectric rhombohedral and tetragonal phases, resulting in a typical ferroelectric butterfly-shaped bipolar S–E loop at room temperature, whose normalized strain (S_max/E_max) showed a significant temperature coefficient of 0.38 pm/V/K. However, 5 mol% SKN-modified BNKT ceramics revealed a typical electrostrictive behaviour with a thermally stable electrostrictive coefficient, Q₃₃ = 0.021 m⁴/C², which is almost comparable to that of Pb(Mg_1/3Nb_2/3)O₃ (PMN) ceramics that have been dominantly used as Pb-based electrostrictive materials over the last decades.     

改进的sol-gel工艺制备细晶Ba0.6Sr0.4TiO3陶瓷及其介电性能的研究

用改进的sol-gel工艺制备了细晶钛酸锶钡(Ba0.6Sr0.4TiO3,BST)陶瓷块体,研究了BST陶瓷的结晶与介电性能.在这种改进sol-gel的工艺中,用传统的固相反应煅烧形成BST粉体,经高能球磨制备BST纳米陶瓷粉体,再将一定质量的纳米粉体加入到相同化学组成的BST的溶胶液中,经普球球磨12h后,制备成悬浮性好,分散均匀的浆料.浆料可用来制备BST陶瓷,并在1200℃保温2h烧结成瓷,结果显示,BST陶瓷块体结构致密,晶粒尺寸在0.15～2μm之间.分析了样品的介电性能和晶粒尺寸对材料介电性能的影响.介电温谱显示,在0℃,100kHz时,相对介电常数为2500,介电损耗为0.02;并且存在明显的弥散相变.     

Lamination of magnesium oxide spacers to barium strontium zirconium titanate ceramics

We propose an innovative idea to bond the dielectric barium strontium zirconium titanate (BSTZO) plates with magnesium oxide (MgO) as the spacers to achieve a hermetic module without any air gaps between the dielectric and the spacer. The gold metallization can be applied across the whole assembly to create an integrated electrode. The gold metallization also eliminates pressure contact by external copper plates assemblies, which are required to achieve good contacts between the copper plates and the metallized surfaces of the BSTZO. The MgO spacers are processed using a dry-pressing and pressureless-sintering method. The thermal expansion coefficient (CTE) of BSTZO and MgO spacer was measured. In addition to matching the CTE between BSTZO dielectric and the MgO spacer, it is also critical to develop a good bonding material with CTE matching to BSTZO and MgO spacer. The effect of CTE for various bonding compositions on the dielectric properties was thoroughly studied and reported. The mechanism explaining the high and low dielectric constants for the laminates is proposed and discussed based on the CTE results and their effect on microstructural development.     

Ferroelectric properties of barium calcium titanate ceramics doped with bismuth oxide

Ferroelectric properties of Bi doped (Ba_1 − xCa_x)_0.925Bi_0.05TiO₃ (Bi-BCT, 0.10 < x < 0.30) ceramic prepared by the solid-state technique have been studied. An interesting double-like hysteresis (P-E) loop at room temperature (300 K) with the remarkable linear dielectric response was observed over a certain electric field range. Room temperature P-E loops before and after de-aging, as well as those at different values of field cycling, could exclude the possibility that the double P-E loops might result from the antiferroelectric components and the electric field induced paraelectric-ferroelectric (PE-FE) transition near Curie temperature (T_c) in Bi-BCT, and verify that there exists a diffusional aging effect in Bi-BCT. A symmetry-conforming short-range ordering (SC-SRO) of point defects is suggested to be responsible for the observations of the interesting double-like P-E loop in Bi-BCT.     

Non-isovalent metal substitution for bismuth,strontium, calcium,and copper in bi-based 2212 superconducting ceramics

The aim of this paper is to investigate the influence of non-isovalent alkali (Li, Na, K, and Cs) substitution in Bi₂Sr₂CaCu₂O₈, in particular on its synthesis conditions and on their resistive and thermopower electrical properties. Liquid phase development during the synthesis process leads to the formation of sintered materials at a lower temperature compared to the undoped 2212 compound. The final composition is discussed as a function of which crystallographic site is partially substituted. The presence of alkali ions in the initial chemical composition leads to improved electrical properties. Such data are correlated with the microstructure and chemical composition of the end-product.