Sol-Gel Processed Strontium Titanate Ceramics

Ferroelectric strontium titanate (SrTiO₃) ceramics have been prepared by sol-gel processing technique. Strontium acetate [Sr(CH₃COO)₂] and titanium (IV) isopropoxide [Ti(OC₃H₇]₄ were used as the precursors for the fabrication of the samples. The stock solution for the preparation of strontium titanate powder was prepared by dissolving strontium acetate in ethanol and acetic acid solvents and mixing the resultant solution with titanium (IV) isopropoxide in equimolecular amounts. The as fired powder was found to be amorphous. Annealing the amorphous powder at 700°C for one hour in air yields cubic phase crystalline SrTiO₃ powder. Ceramic samples were prepared by pressing and sintering the crystalline powder. The effect of sintering temperature on the structural and dielectric properties were investigated and analyzed.     

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.     

Impedance Spectroscopy Study of Niobium-Doped Strontium Titanate Ceramics

Impedance spectroscopy was used to study the temperature dependence of the resistance and capacitance of niobium-doped strontium titanate ceramics and also to identify the main contributions determining potential applications. Nyquist plots ( Z " versus Z ') show a main contribution in the intermediate-frequency range, with a peak at ∼1 kHz at 500°C. The activation energy of this resistance contribution is close to 1.5 eV. Modulus representations (log M " versus log f ) show a second contribution in the high-frequency range, with a peak at ∼100 kHz at 500°C. This resistance contribution is much smaller, and its activation energy is also lower (close to 0.8 eV). The capacitance of the intermediate-frequency term increases from ∼0.1 μF/m at 700°C to ∼1 μF/m at 425°C. The capacitance values of the high-frequency contribution are also >0.1 μF/m, much higher than expected for the bulk. These contributions might be related to differences between the intergrain interfaces and outer grain shells, as suggested by SEM microstructures with core–shell formations. Representations of 1/ C against temperature suggest a Curie–Weiss law for the main contribution in the intermediate-frequency range, with a T _C of ∼400°C. However, the temperature dependence of the overall dielectric constant measurements (at constant frequency) fail to show a clear maximum, probably due to an additional low-frequency contribution ascribed to the Pt/SrTi_{1− x} Nb _x O_3−δ interface.     

Abnormal Grain Growth of Niobium-Doped Strontium Titanate Ceramics

We have investigated the influence of the Ti/Sr ratio and particle size on abnormal grain growth of Nb-doped SrTiO₃ ceramics. As the Ti/Sr ratio increases, the onset time and temperature of abnormal growth increase, for specimens sintered at temperatures above 1440°C. When the particle size is decreased by milling, the onset and completion temperatures of the abnormal growth and the final grain size are decreased. The abnormal grains growing during isothermal heating are nearly tetrakaidecahedral in shape, whereas those growing during rising temperature have the morphology of an edge-cut cube.     

Piezoresistance and Piezocapacitance Effects in Barium Strontium Titanate Ceramics

A large piezoresistance response to hydrostatic pressure is reported for a series of ceramic compositions in the system barium strontium lanthanum titanate (Ba, Sr, La)TiO₂. This property appears to be closely associated with the Curie region (cubic-tetragonal phase transition) and the phenomenon of a positive temperature coefficient of resistance for these same compositions. The piezoresistance coefficient, similar to the temperature coefficient of resistance, is positive in the Curie region, and is found to range up to the order of 700 X 10 ⁻¹² sq. cm. per dyne. Consistent with the observed piezoresistance effect, a positive piezocapacitive effect, ranging up to the order of 700 x 10⁻¹² sq. cm. per dyne, is observed in these compositions in the absence of the lanthanum impurity which produces semiconduction. It is suggested that these materials may be of interest in pressure-sensitive devices such as acoustic transducers and pressure-sensing elements and in techniques for measuring force and displacement.     

Synthesis of Strontium Barium Niobate Thin Films through Metal Alkoxide

Highly oriented Sr_0.5Ba_0.5Nb₂O₆ (SBN50) thin films have been prepared using a sol-gel method. A homogeneous and stable strontium barium niobate (Sr_1-xBa_xNb₂O₆, SBN) precursor solution could be prepared via the reaction control of metal alkoxides. The SBN precursor was stabilized by the coordination of the 2-ethoxyethoxy group to metals. SBN thin films on MgO(100) crystallized to a mixture of orthorhombic and tetragonal phase at 700°C and then transformed completely to the tetragonal phase of tungsten bronze at 1000°C. Two crystal lattice planes of SBN were intergrown at an orientation of 18.5° on MgO(100). SBN50 thin films on Pt(100)/MgO(100) substrates exhibited the P-E hysteresis.     

Preparation of Silica Nanostructured Spheres by Sol Spray Drying

Silica nanostructured spheres were obtained by spray drying of silica sol prepared in situ. Their morphologies were significantly affected by the aggregation of the primary particles in the sol. They had the mode of the pore sizes which was about the same order as Laser Particle Size Analyzer (LPA) diameter. Increasing the mixing ratio of the larger particles (20 nm) to smaller ones (7 nm) decreased the Brunauer, Emmett and Teller (BET) specific surface area, as expected. Pore volume decreased with the pore size distribution broadened and the mean pore diameter was not affected by the increase, due to the decrease in aggregation of the primary particles. Either the increase in the drying temperature and use of ethanol as a cosolvent made the agglomerates hollower, the larger of which with thinner shell transformed to doughnut type agglomerates, due to the structural and hydrodynamic instabilities.     

Preparation of Silica Nanostructured Spheres by Sol Spray Drying

Silica nanostructured spheres were obtained by spray drying of silica sol prepared in situ. Their morphologies were significantly affected by the aggregation of the primary particles in the sol. They had the mode of the pore sizes which was about the same order as Laser Particle Size Analyzer (LPA) diameter. Increasing the mixing ratio of the larger particles (20 nm) to smaller ones (7 nm) decreased the Brunauer, Emmett and Teller (BET) specific surface area, as expected. Pore volume decreased with the pore size distribution broadened and the mean pore diameter was not affected by the increase, due to the decrease in aggregation of the primary particles. Either the increase in the drying temperature and use of ethanol as a cosolvent made the agglomerates hollower, the larger of which with thinner shell transformed to doughnut type agglomerates, due to the structural and hydrodynamic instabilities.     

Strontium Zirconate and Strontium Titanate Ceramics for High-Voltage Applications: Synthesis, Processing, and Dielectric Properties

The synthesis, processing, and electrical properties of SrZrO₃ and SrTiO₃ materials have been examined. Phase-pure powders of SrZrO₃ and SrTiO₃ materials were synthesized using the Pechini method. Powder processing routes that used water and 2-propanol as carrier fluids were developed to achieve high green densities, which resulted in sintered densities of >99% of the theoretical density. The relative permittivity and average breakdown strength for carefully processed SrZrO₃ were 60 and 40 V/μm; the corresponding values for SrTiO₃ were 400 and 35 V/μm. The higher breakdown strengths suggest that these materials can be used in high-voltage capacitor applications.     

Phase Stability and Ferroelectric Properties of Lead Strontium Zirconate Titanate Ceramics

The effect of compositional modifications on the field-induced phase-transition behavior and dielectric properties of strontium-doped lead zirconate titanate (PZT) ceramics was studied. PZT compositions with different strontium and titanium contents, within the general formula Pb_{1– x} Sr _x (Zr_{1– y} Ti _y )O₃ and located in the tetragonal antiferroelectric (AFE) and rhombohedral ferroelectric (FE) phase fields were prepared by tape casting and sintering. X-ray diffraction and polarization measurements were used to locate compositions suitable for investigation of the field-induced AFE–FE phase transition. The results indicated that a higher Sr²⁺ content decreased the polarization and hysteresis and increased the switching field; a lower Ti⁴⁺ content decreased the polarization and increased the switching field and hysteresis. A high room-temperature dielectric constant was obtained for compositions near the phase boundary. These results suggest that a combination of both A -site and B -site modifications can be used to tailor ferroelectric properties, such as the switching field and hysteresis, of these strontium-doped PZTs displaying a field-induced AFE–FE phase transition.     

铌酸锶钡/钛酸锶钡复相陶瓷形成过程研究

应用粉末-溶胶工艺合成出铌酸锶钡/钛酸锶钡复相陶瓷(SBN/SBT)。采用X射线衍射技术研究了不同预烧温度的原始粉料、不同烧结温度和不同保温时间得到的铌酸锶钡/钛酸锶钡复相陶瓷。研究表明:钨青铜相和钙钛矿相共存于体系之中。复相陶瓷形成过程中形成了TiO2、BaNb2O6(BN)、SrNb2O6(SN)等中间相,增加烧结温度至1250℃,形成了铌酸锶钡/钛酸锶钡复相陶瓷。     

Effect of Additives on the Crystallization Kinetics of Barium Strontium Titanate Glass–Ceramics

Barium strontium titanate (BST) has been targeted as one potential ferroelectric glass–ceramic for high-energy density dielectric materials. Previous testing has shown that the dielectric constant of these materials was as high as 1000 and the dielectric breakdown strength up to 800 kV/cm. This did not, however, result in exceptional energy density (∼0.90 J/cc). In order to increase overall energy density refining agents can be added to the melt, but the nucleation and growth of the ceramic particles can also play a role. Therefore, in this study the crystallization kinetics were studied to more fully understand how BST phase forms so that the optimal energy density could be obtained. It was determined that the activation energy of the crystallization of BST 70/30 glass–ceramic is approximately 430 kJ/mol which is close to the dissociation energy of Si–O bonds. The Avrami parameter was found to be ∼3 meaning that three-dimensional growth is dominant and the mechanism of growth was interface controlled.     

钛酸铜锶钡陶瓷晶相结构及介电性能研究

采用溶胶-凝胶法制备(Ba0.5Sr0.5)1-xCuxTiO3高介电性能陶瓷,用X射线衍射和LCR分析仪对不同x取值和烧结温度制备陶瓷的晶相组分和介电性能进行分析对比,结果表明,x=0.1可以得到比较好的固溶体;随着Cu2比例上升,陶瓷中出现氧化铜杂相;并且,烧结温度升高,陶瓷中氧化铜相含量也增加.氧化铜相含量的增加使陶瓷的介电系数获得明显提升,可达104数量级,但与此同时,氧化铜相也使陶瓷的介电损耗有所增大.     

Raman Spectra of Strontium Titanate

At room-temperature the Raman spectrum was second-order, in agreement with, the selection rules for the cubic perovskite structure. The second-order spectrum originates largely from pairs of phonons with wave vectors near the Brillouin zone boundary. First-order Raman-active modes predicted in the tetragonal phase of SrTiO₃ are observed on cooling to 120 K, where a new impurity-related band is observed at 793 cm⁻¹. The presence of impurities raised the cubic to tetragonal phase transition temperature to 120 K.     

Preparation of Strontium Titanate Thin Film on Titanium Metal Substrate by Hydrothermal-Electrochemical Method

Polycrystalline SrTiO₃ thin films have been prepared on Timetal substrates by the hydrothermal-electrochemical method. The films were prepared at temperatures ranging from 100° to 200°C and under saturated vapor pressures in electrolytic solutions of 1 N to 5 N Sr(OH)₂. A galvanostat was used to apply a direct current of 5 to 25 mA/cm². The films had smooth and homogeneous surfaces without any visible pores or defects. The dielectric constant of the films was approximately 129 assuming 25 nm for film thickness.     

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.     

Electromechanical Properties of Lead Titanate Zirconate Ceramics with Lead Partially Replaced by Calcium or Strontium

By partially substituting Sr for Pb in ceramic lead titanate zirconate compositions near the morpho-tropic boundary, dielectric constants greater than 1300 can be obtained along with planar couplings of about 0.50. Calcium also raises the dielectric constant, but the planar coupling level is considerably lower than that obtained with Sr. Other effects noted with Sr are a lowering of the Curie temperature, a shift in the morphotropic boundary toward the rhombohedral side, and a decrease in the distortion from cubic symmetry. In addition, Sr was found to be effective as a fluxing agent.     

Sol–Gel Preparation of Lead Zirconate Titanate Powders and Ceramics: Effect of Alkoxide Stabilizers and Lead Precursors

Lead zirconate titanate (PZT) (52/48) powders were prepared by a sol–gel process, using different raw materials to introduce the lead component together with several solvents and chemical modifying agents. A study of the effect of these variables on crystallization behavior was conducted to determine the best conditions for preparing monophasic submicrometer-sized PZT perovskite powders in the morphotropic region. In the present work, well-crystallized, submicrometer-sized single-phase perovskite PZT powders were obtained after heat treatment at 600°C for 1 h. The dependence of this crystallization temperature on the preparation conditions was observed. The sol–gel-derived submicrometer-sized PZT powders were sintered to ∼96% of relative density after 2 h at 950°–970°C. The sintered ceramics exhibited a dielectric permittivity of 1000, a piezoelectric coefficient of 135 pC·N⁻¹, a remanent polarization of 20 μC·cm⁻², and a coercive field of 10.6 kV·cm⁻¹.     

Solution-Precipitation of Fine Powders of Barium Titanate and Strontium Titanate

Hydrolytic reactions of metal alkoxides offer a broad range of possibilities for their use in the preparation of ceramic powders. This paper reports a unique and novel process to prepare fine powders of BaTiO₃ and SrTiO₃ from stable precursor solutions by carefully controlling the pH and temperature. This simple route offers good control of stoichiometry and the powders are agglomerate-free with fine particles of size 0.06–0.1 µm and were well sintered at 1200° and 1350°C, respectively. The dielectric properties of the dense ceramics are also reported. The formation aspects of these perovskite phases are also discussed.     

Computer Simulation Studies of Strontium Titanate

We present the results of computer simulation studies of SrTiO₃. After deriving a reliable potential model, we concentrate on the properties of defects and dopants. Our calculations are used to propose a defect model that is consistent with experimental observations. For vacancy disorder, the calculations show that SrTiO₃ (Schottky) and SrO (Schottky-like) disorder have very similar energy, with a small tendency for the development of Sr deficiency at higher temperatures. All mono- and divalent cations prefer to substitute at strontium sites. For trivalent cations, three types of mechanism (Sr and Ti substitution and self-compensation) are proposed, whereas for tetravalent cations substitution on both A and B sites may occur, depending on the ionic radius.