Synthesis of (K1/2Bi1/2)xBa1−x TiO3 powder with high aspect ratio

In order to synthesize high aspect ratio (K_1/2 Bi _1/2)_xBa_1−x TiO₃ powders, fibrous potassium titanates was used as template materials. In the fibrous potassium titanate family, there are several molecular formulas depending on potassium and titanium ratios. The molecular compounds selected in this study were K₂O⋅6TiO₂ and K₂O⋅8TiO₂ for synthesizing (K_1/2Bi_1/2)_2/3Ba_1/3TiO₃ and (K_1/2Bi_1/2)_1/2Ba_1/2TiO₃, respectively because they were derived from only adding with bismuth oxide and barium oxide. Making use of the simple reaction formulas, it was expected that the fibrous powder brought high aspect ratio powders. The synthesized powders showed high aspect ratios, inherited the fibrous potassium titanates though the diameter was thicker than the each original. By XRD, the crystal structures of obtained powders were identified as tetragonal perovskite. Furthermore, it is found that the crystal c-axis is likely to be perpendicular to the powder axis.     

Electromechanical Properties and Morphotropic Phase Boundary of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3-BaTiO3 Lead-free Piezoelectric Ceramics

The crystal structure and electromechanical properties of two ternary ceramic Na_0.5Bi_0.5TiO₃- K_0.5Bi_0.5TiO₃-BaTiO₃ (NBT-KBT-BT) systems were investigated. A gradual change in crystalline structure and microstructure with the increase of KBT and BT concentrations were observed. It was ascertained that the rhombohedral-tetragonal morphtropic phase boundary (MPB) lies in the range of 0.024 ≰ x ≰ 0.030 for (1–5x) NBT-4x KBT-x BT system and 0.025 ≰ y ≰ 0.035 for (1 − 3y) NBT—2y KBT—y BT system at room temperature. The piezoelectric constant d₃₃ and electromechanical coupling factor k_p of the ceramics attain a maximum value of 150 pC/N and 0.298, respectively. The MPB phase diagram of NBT-KBT-BT ternary system was determined by phase analysis of XRD patterns from calcined specimens. The ferroelectric properties of the (1 − 5x) NBT—4x KBT—x BT system have been characterized. The ternary system ceramics have relatively high Curie temperature T_c.     

Structure and ferroelectric property of Nb-doped SrBi4Ti4O15 ceramics

Nb-doped SrBi₄Ti₄O₁₅ (SBT) was produced by conventional method. Structural and ferroelectric properties of SBT were examined as a function of niobium composition. Analyzing the structure futures of SBT by XRD, XPS and Raman spectrum, Nb⁵⁺ substituted Ti⁴⁺ to form NbO₆ octahedron and did not change the structure of SBT. The XRD patterns indicated the formation of the single phase of SBT for x = 0.01and 0.03 and secondary phase of Sr₃Ti₂O₇ appeared when x > 0.1. To compare the effect of Nb doping, the ferroelectric properties (hysteresis loop, piezoelectric coefficient) of Nb-doped SBT were measured. The SBT doped with x = 0.15 was found to exhibit higher remanent polarization with d ₃₃ = 17 pC/N.     

Dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

Ultra fine rutile powders (below 50 nm) were prepared via the sol-gel process and bulk type TiO₂ specimens were fabricated using spark plasma sintering (SPS). The TiO₂ specimen sintered at a low temperature (720^∘C) exhibited a highly relative density (97%) and a nano-sized grain structure (200 nm). Dielectric properties of spark plasma sintered TiO₂ specimens including dielectric constants (k) and losses (tan δ) were measured. The TiO₂ specimen, obtained by SPS, showed a high dielectric constant (∼780) and a low tan δ (∼0.005), and a relaxation behavior at 1 MHz. After the subsequent annealing process of the TiO₂ specimen in O₂ flow, the dielectric constants remarkably decreased (k = 100s). These dielectric properties of nanocrystalline TiO₂ specimens prepared by SPS were discussed in terms of space charges produced by the reduction of Ti⁴⁺ ions and crystallographic orientations of grains.     

Sintering behavior, structures and microwave dielectric properties of a rutile solid solution system: (AxNb2x)Ti1–3xO2 (A=Cu, Ni)

The sintering behavior, structures and microwave dielectric properties in a rutile solid solution system—(A_xNb_2x)Ti_1–3xO₂ (A=Cu, Ni)—were investigated and the samples were prepared by conventional solid state reaction method. Single phase of tetragonal rutile structure has been obtained through the entire range of compositions (0.02 ≤ x ≤ 0.20). The sintering temperature was lowered to 900°C by (Cu _{x /3}Nb_2x/3)⁴⁺ substituting for Ti⁴⁺ in the solid solution. Comparing with that of rutile TiO₂ (465 ppm/°C), the temperature coefficient of resonant frequency (TCF) of the rutile solid solution is much lower (about 250 ppm/°C), and the dielectric constant and the quality factor (Qf value) of the solid solution are about 70~80 and 7,000G Hz. The substitution of (Cu _{x /3}Nb_2x/3)⁴⁺or (Ni _{x /3}Nb_2x/3)⁴⁺ for Ti⁴⁺ in the solid solution improved the microwave dielectric properties of the rutile TiO₂ ceramics.     

Synthesis of Pb(Zr, Ti)O3 Nanopowders by Milling Coprecipitation Method

Nano-size powders of lead zirconate titanate (PZT) were fabricated by a new milling coprecipitation method (MCP) improved from the conventional wet ball milling and precipitation. This method consists of slurry preparation from nanoparticles of TiO₂ with aqueous solution of ZrO(NO₃)₂ and Pb(NO₃)₂ with zirconia ball mill media, followed by precipitation with NH₄OH as precipitant. Milling media (1mm and 3mm balls) improves the precipitation homogeneity during processing. Single-phase perovskite structure of PZT was formed at a calcination temperature of 500C and powders of 50 nm particle size were obtained. Powders were characterized using TG-DTA, SEM and XRD methods. Sintering ability of powders and piezoelectric properties of the ceramics were also investigated.     

Synthesis of fine Ca-doped BaTiO3 powders by solid-state reaction method—Part I: Mechanical activation of starting materials

Fine (Ba_0.98Ca_0.02)_1.002TiO₃ powders for high capacitance multilayer ceramic capacitors (MLCCs) application were synthesized by solid state reaction method. The effects of mechanochemical activation using high energy milling and the starting materials properties on the reaction temperature and on the final powder properties were investigated. Previous heavy milling of BaCO₃ and the adoption of fine, anatase-rich TiO₂ phase were effective in decreasing the reaction temperature and in increasing the tetragonality (=c/a). BaCaTiO₃ powders with a tetragonality of 1.0097, an average particle size of 213 ± 43 nm and a specific surface area of 6.30 m²/g were acquired after heat treatment at 985 °C for 2 h. MLCCs utilizing this developed powder showed superior dielectric and temperature characteristics to those with conventional, Ca-free BaTiO₃ powder.     

Synthesis and Characterization of Zn-Doped TiO2 Nanoparticles via Sonochemical Method

Zn-doped TiO₂ nanoparticles were successfully fabricated using sonochemical method accompanying post calcination process. Titanium isopropoxide (Ti[OC₃H₇]₄) and Zinc chloride (ZnCl₂) were used as starting precursors for Ti and Zn sources, respectively. The homogeneous mixing solution of different Zn (0–1 mol%) and Ti ratio were irradiated in high intensity ultrasound sonometer (750 W 20 kHz) for 30 min at room temperature to obtain as-synthesized Zn-doped TiO₂ nanoparticles followed by calcination at 400–700°C. To evaluate the structure and phase identification of prepared powders, the X-ray diffraction (XRD) and Raman spectroscopy were employed. The results reveal that the as-synthesized Zn-doped TiO₂ nanoparticles are in anatase phase and their crystallinity increases with increasing calcined temperature. The morphology of as-synthesized powders was investigated by transmission electron microscope (TEM). The effect of Zn content and calcinations temperature on TiO₂ properties was also discussed.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Influence of synthesis method on the properties of mixed Aurivillius phase Bi5TiNbWO15

The ceramic material Bi₅TiNbWO₁₅ (BTNW) was obtained and is characterized by a layered, Aurivillus type structure (MBLPO—mixed bismuth oxide layered perovskites) with m = 1.5. In order to optimize the production, BTNW was synthesized by two methods: synthesis of a mixture of simple oxides Bi₂O₃, Nb₂O₅, WO₃, TiO₂ and synthesis of a mixture of appropriate, layered Aurivillus type structures Bi₂WO₆ (m = 1) and Bi₃TiNbO₉ (m = 2). Synthesized solid solutions were consolidated with a conventional method. The crystal structure of Bi₂WO₆ and Bi₃TiNbO₉ Bi₅TiNbWO₁₅ was examined at room temperature with an X-ray diffraction method. Microstructure and chemical composition were analyzed. The authors analyzed the temperature—permittivity ɛ relationship for the ceramic with m = 1,5, which was obtained with two methods, as well as for m = 1 and m = 2.     

Low operating temperature CO sensor prepared using SnO<Subscript>2</Subscript> nanoparticles

A low operating temperature CO (carbon monoxide) sensor was fabricated from a nanometer-scale SnO₂ (tin oxide) powder. The SnO₂ nanoparticles in a size range 10–20 nm were synthesized as a function of surfactant (tri-n-octylamine, TOA) addition (0–1.5 mol%) via a simple thermal decomposition method. The resulting SnO₂ nanoparticles were first screen-printed onto an electrode patterned substrate to be a thick film. Subsequently, the composite film was heat-treated to be a device for sensing CO gas. The thermal decomposed powders were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD), and surface area measurements (BET). The CO-sensing performance of all the sensors was investigated. The experimental results showed that the TOA addition significantly decreased the particle size of the resulting SnO₂ nanoparticle. However, the structure of the powder coating was crucial to their sensing performance. After heat-treatment, the smaller particle tended to cause the formation of agglomeration, resulting in the decline of surface area and reducing the reaction site during sensing. However, the paths for the sensed gas entering between the agglomerated structure may influence the sensing performance. As a CO sensing material, the SnO₂ nanoparticle (~12 nm in diameter) prepared with 1.25 mol% TOA addition exhibited most stable electrical performance. The SnO₂ coating with TOA addition >0.75 mol% exhibited sensor response at a relatively low temperature of <50°C.     

Developing novel gas sensors for NO2 detection based on Ce(1-x)MXO2, {M?=?Ru, In} solid solutions

CeO₂, Ce_(1-x)M_XO₂, {M = Ru, In} compounds with sensing properties were fabricated using the sol–gel route. The main purpose was to compare the efficiency of CeO₂ vs. Ce_(1-x)M_XO₂ doped compounds as gas sensors for NO₂ detection. Characterization was performed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area determination (BET). Measurements of electrical resistance under different conditions of time, concentration and temperature in the presence of NO₂ were carried out. Ruthenium inclusion increased the CeO₂ sensor response in a great extent, gas response (S) = 1.8 for CeO₂ vs. gas response (S) = 350 for Ce_0.95Ru_0.05O₂ and gas response (S) = 35 for Ce_0.95In_0.05O₂. This behavior is reported by the first time. Our results demonstrate that ruthenium or indium inclusion has been beneficial for CeO₂. Conclusively the materials herein described could be applied as NO₂ gas sensors.     

Conductivity and expansion at high temperature in Sr0.7La0.3TiO3−α prepared under reducing atmosphere

Sr_0.7La_0.3TiO_3−α specimens were prepared in reducing atmosphere, and the structural and electrical properties were studied. The lattice parameter of Sr_0.7La_0.3TiO_3−α at room temperature was larger than that expected from Vegard’s law between SrTiO₃ and LaTiO₃ due to the reductive expansion. The conductivity of this specimen was 100 S cm⁻¹ at 1000°C, p_O2 = 10⁻¹³ Pa. However, the conductivity was not preserved after an oxidation-reduction cycle. Over p_O2 = 10² Pa, the conductivity drastically dropped with increasing p_O2. The thermal expansion coefficient of Sr_0.7La_0.3TiO_3−α was 11.8 × 10⁻⁶ K⁻¹ in 9% H₂/N₂ (room temperature – 1000°C). In this Sr_0.7La_0.3TiO_3−α, the chemical expansion on oxidation reached Δl/l_o = 0.51%, when changing p_O2 from 10⁻¹¹ Pa to 2 × 10⁴ Pa (air) at 1000°C.     

Microstructure and dielectric properties of ferroelectric barium strontium titanate ceramics prepared by hydrothermal method

Ba _x Sr_1-x TiO₃, nanoparticles with different Ba compositions were synthesized by a hydrothermal method. The mechanism of hydrothermal reactions was discussed based on DTA/TG, XRD and TEM characterizations. The result showed that perovskite structure was developed through the mutual diffusion between the intermediate phases and TiO₂ phase. The grain size of the Ba_0.77Sr_0.23TiO₃ (BST77) powders was about 20–40 nm. BST ceramics were made from the hydrothermal-derived BST powders and the dielectric properties of the BST ceramics were measured. Due to the small grain size and active surface energy of the BST powders prepared by hydrothermal method, the BST ceramics showed low sintering temperature. It was found that the BST77 ceramics sintered at 1280 °C showed dielectric constant peak dispersion which was believed to be caused by dimension domino effect.     

Electrical and optical characterization of Ag2V4O11 and Ag4V2O6F2

The two silver vanadium oxide phases—Ag₂V₄O₁₁ and Ag₄V₂O₆F₂—were prepared by hydrothermal synthesis. The electrical conductivity of both silver vanadate powders was determined by the powder-solution-composite (PSC) method. The conductivities obtained were 0.0085 ± 0.0005 and 0.0005 ± 0.00015 S/cm for the Ag₂V₄O₁₁ and Ag₄V₂O₆F₂, respectively, the first such report for the Ag₄V₂O₆F₂ phase. The optical gap and the transmission where studied by diffuse reflectance. Both were larger for Ag₄V₂O₆F₂ than Ag₂V₄O₁₁, concomitant with a decrease in carrier content.     

Photocatalytic property of SiO2/TiO2 nanoparticles prepared by sol-hydrothermal process

Silica-embedded nanocrystalline TiO₂ powders were synthesized by sol-hydrothermal process. The influence of the composition of the solvent and the embedded silica content on the phase transition, grain growth and subsequently, on the photoactivity of TiO₂ were investigated. The volume ratio of ethanol to water for the solvent composition was varied from 0.125 to 8 and the mole fraction of silica content was changed from 0 to 0.4, while the condition for hydrothermal reaction was fixed at 250^∘C for 2 hour. With an increase in ethanol content in solvent composition, the crystallite sizes of pure TiO₂ particles decreased from 15 nm to 6 nm and crystal phase changed from rutile/anatase mixed phase to pure anatase phase. Addition of silica to TiO₂ brought about an increase in the photocatalytic activity by suppressing the phase transition from anatase to rutile. Judging from the result of the decomposition of 1, 4-dichlorobenzene, the most efficient catalyst was found to be 0.2 mole fraction SiO₂ embedded TiO₂ prepared with ethanol-rich solvent (the volume ratio of ethanol to water is 4).     

Effect of excess TiO2 on the phase evolution and densification of sol-gel derived (Ba,Sr)TiO3 powders

Nanoscaled (Ba_2/3Sr_1/3)Ti_{1 + x} O₃ powders have been prepared by sol-gel technique. Their phase evolution and densification behaviors have been studied by differential thermal analysis (DTA) and high temperature dilatometer, respectively. It is found the addition of 2 mol%-excess amount of TiO₂ lowers the activation energy required for the formation of the perovskite phase by about 130 kJ/mol and thus lowers the crystallization temperature of BST powders. However, the excess amount of TiO₂ makes the nano-powder difficult to sinter. Transmission electron microscopy reveals that a metastable nano-porous layer has formed on the surface of TiO₂-excess nanopowder and this may account for the slow densification rate.     

Effect of attrition milling on the piezoelectric properties of Bi0.5Na0.5TiO3-based ceramics

Bismith sodium titanate (BNT)-based powders were prepared by conventionally mixed-oxide method using Bi₂O₃, Na₂CO₃ and TiO₂. The La₂O₃ was added as the modifier to the BNT composition for easily poling and reducing an abnormal dielectric loss at high temperatures. In this study, the investigated compositions were Bi_0.5Na_0.5TiO₃ and Bi_0.5Na_0.485La_0.005TiO₃. The powders were calcined at 900 °C for 2 h by slow heating rate at 100 °C/h. The calcined BNT-based powders were then attrition-milled for 3 h with a high speed at 350 rpm. After drying, the fine powders were uniaxially pressed and then cold-isostatically pressed (CIP) at 240 MPa for 10 min. All pressed pellets were sintered at 1000–1100 °C for 2 h in air atmosphere. The microstructure of sintered pellets was investigated by SEM. Results of dielectric and piezoelectric property measurement were also reported.     

Lead-free piezoelectric ceramics based on perovskite structures

The piezoelectric properties of a solid solution based on three components of bismuth sodium titanate (Bi_1/2Na_1/2)TiO₃ (BNT), bismuth potassium titanate, (Bi_1/2K_1/2)TiO₃ (BKT), and barium titanate, BaTiO₃ (BT), that is x(Bi_1/2Na_1/2)TiO₃−y(Bi_1/2K_1/2)TiO₃−zBaTiO₃, [, abbreviated as BNBKy:z(x)] and potassium niobate, KNbO₃ (KN) based ceramics, that is KN+MnCO₃ x wt.%, [abbreviated as KN−Mn x], were studied as a lead-free piezoelectric material. In the case of BNBK2:1 system, high piezoelectric properties were obtained near the MPB composition, and the highest electromechanical coupling factor, k ₃₃ and piezoelectric constant, d ₃₃, were 0.58 for BNBK2:1(0.89) and 181 pC/N for BNBK2:1(0.88). Nevertheless, the depolarization temperature, T _d , shifts to lower temperature around the MPB compositions, and the T _d ’s of BNBK2:1(0.88–0.90) are only about 100 °C. On the tetragonal side, the T _d shifts to higher temperature with increasing the lattice anisotropy, c/a. As T _d higher than 200 °C was obtained in the range of x < 0.78, with a k ₃₃ and d ₃₃ for BNBK2:1(0.78) being 0.45 and 128 pC/N, respectively. In the case of Mn doped KN ceramics, dense and non deliquescence KN ceramic were successfully obtained via ordinary firing technique in air by optimizing the fabrication process. Mn doping for KN ceramics was effective to obtain full poling state easily under poling conditions of high temperature and high electric field. As a result, we obtained the excellent piezoelectric properties of k ₃₃ = 0.507 for KN−Mn0.2.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.