The characterization of the barium titanate ceramic powders prepared by the Pechini type reaction route and mechanically assisted synthesis

BaTiO₃ ceramic powders were prepared by a complex method based on the Pechini type reaction route and mechanically assisted synthesis. In both ways BaTiO₃ ceramics were sintered after 120 min on 1300 °C without pre-calcination steps. The crystal structure was investigated by the XRD, IR and Raman spectroscopy. The particle size and morphology of BaTiO₃ were examined by XRD and SEM. The XRD results of powders indicate the formation of cubic phase of BaTiO₃. It can be observed that in the case of Pechini process BaTiO₃ powder is well crystallized but in the case of mechanochemistry process, significant amount of amorphous phase was detected. The sintered BaTiO₃ ceramic sample prepared by Pechini process, shows the formation of tetragonal phase. However, IR and Raman spectrum showed a mixture of cubic and tetragonal for BaTiO₃ obtained by Pechini process and tetragonal for BaTiO₃ obtained by mechanically assisted synthesis.     

One-pot synthesis of monoclinic ZrO2 nanocrystals under subcritical hydrothermal conditions

This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO₂) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO₃)₂) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250 °C for 24 h to obtain pure monoclinic-structured ZrO₂ nanocrystals. The crystallization temperature of the ZrO₂ phase was also greater than 200 °C. However, the products of reactions conducted at 200 °C for 24 h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250 °C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12 h in order to obtain single-phase monoclinic ZrO₂ nanocrystals. The crystallite size of this product was approximately 20 nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface.     

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO₃) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5 µm range were formed at temperatures above 80 °C. Also, the cube-shaped morphology of the BaTiO₃ product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO₃ proceeded alongside secondary BaTi₂O₅ and Ba₂TiO₄ phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO₃ as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO₃ crystallization by the Johnson-Mehl-Avrami method showed that BaTiO₃ crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.     

Characteristics of Ag-doped BaTiO3 nanopowders prepared by spray pyrolysis

Pure and Ag-doped BaTiO₃ nanopowders were prepared by spray pyrolysis. Precursor powders, prepared from a spray solution with citric acid and ethylenediaminetetraacetic acid (EDTA) as chelating agents, had large, hollow particles irrespective of Ag doping. Both pure and Ag-doped powders had partially aggregated particles after post-treatment at 900 °C that could be easily milled to nanoparticles. The mean sizes of the pure and Ag-doped BaTiO₃ particles were 75 and 91 nm, respectively. The Ag-doped particles were mainly of cubic BaTiO₃ crystal structure, with small Ag phases observed. High-density BaTiO₃ pellets were formed by sintering the powders at the low temperature of 1000 °C. The silver was uniformly distributed in a tetragonal BaTiO₃ phase without phase separation in the doped pellet. The dielectric constants of the pellets formed from the pure and Ag-doped BaTiO₃ powders were 1826 and 2400, respectively.     

Template-free synthesis of neodymium hydroxide nanorods by microwave-assisted hydrothermal process,and of neodymium oxide nanorods by thermal decomposition

Based on the 150 °C and 1 h microwave-assisted hydrothermal reaction of Nd(NO₃)₃ dissolved in deionized water with pH 10 adjusted by concentrated NH₄OH solution, the phase and morphology of the product, characterized by XRD, SEM and TEM analyses, were specified as hexagonal Nd(OH)₃ nanorods 50 nm in diameter and 700 nm long, growing along the [0 0 1] direction. TGA analysis showed the evaporation of adsorbed water and dehydration of Nd(OH)₃ to synthesize the final pure Nd₂O₃ product. With 500 °C and 2 h calcination of the Nd(OH)₃ self-template precursor, Nd₂O₃ nanorods were synthesized, retaining both the morphology and growth direction of the precursor.     

Recovery of carbon fibres and production of high quality fuel gas from the chemical recycling of carbon fibre reinforced plastic wastes

A solvolysis process to depolymerize the resin fraction of carbon fibre reinforced plastic waste to recover carbon fibre, followed by hydrothermal gasification of the liquid residual product to produce fuel gas was investigated using batch reactors. The depolymerisation reactions were carried out in ethylene glycol and ethylene glycol/water mixtures at near-critical conditions of the two solvents. With ethylene glycol alone the highest resin removal of 92.1% was achieved at 400 °C. The addition of water to ethylene glycol led to higher resin removals compared to ethylene glycol alone. With an ethylene glycol/water ratio of 5, at 400 °C, resin removal was 97.6%, whereas it was 95.2% when this ratio was 3, at the same temperature. The mechanical properties of the recovered carbon fibre were tested and showed minimal difference in strength compared to the virgin carbon fibre. The product liquid, containing organic resin degradation products was then subjected to catalytic supercritical water gasification at 500 °C and 24 MPa in the presence of NaOH and Ru/Al₂O₃ as catalysts, respectively. Up to 60 mol.% of H₂ gas was produced with NaOH as catalyst, and 53.7 mol.% CH₄ gas was produced in the presence of Ru/Al₂O₃.     

Nano-sized Ag–BaTiO3 composite powders with various amount of Ag prepared by spray pyrolysis

The preparation of precursors of BaTiO₃ nanopowders with various amounts of Ag by spray pyrolysis is reported. The precursor powders obtained with hollow and thin-wall particles are composed of uniformly dispersed Ba, Ti, and Ag components. After post-treatment and a simple milling process, the precursor powders, irrespective of the amount of Ag, are transformed into Ag–BaTiO₃ composite nanoparticles. The mean particle size of the Ag (10 mol%)–BaTiO₃ powders is 142 nm. BaTiO₃ pellets containing Ag exhibit dense structures even at a low sintering temperature of 1000 °C. BaTiO₃ pellets with 10 mol% Ag show the highest dielectric constant of 2950, as opposed to the pure BaTiO₃ pellets (without Ag), whose dielectric constant is 1827.     

Structure and dielectric properties of barium titanate thin films for capacitor applications

BaTiO₃ is a typical ferroelectric material with high relative permittivity and has been used for various applications, such as multilayer ceramic capacitors (MLCCs). With the tendency of miniaturization of MLCCs, the thin films of BaTiO₃ have been required. In this work, BaTiO₃ thin films have been deposited on Pt-coated Si substrates by RF magnetron sputtering under different deposition conditions. The films deposited at the substrate temperature from 550 °C–750 °C show a pure tetragonal perovskite structure. The films deposited at 550 °C–625  °C exhibit (111) preferential orientation, and change to (110) preferential orientation when deposited above 650 °C. The film morphologies vary with working pressure and substrate temperature. The film deposited at 625 °C and 4.5 Pa has the relative permittivity of 630 and the loss tangent of 2% at 10 kHz.     

The effect of the hydrothermal synthesis variables on barium titanate powders

In this work, the influence of (a) Ba excess in the starting hydrothermal mixture with TiO₂, (b) hydrothermal reaction temperature, and (c) washing cycles on the hydrothermal synthesis of barium titanate (BaTiO₃) were investigated to assess their relative contributions to the final characteristics of the sintered oxide. BaTiO₃ cake was prepared by hydrothermal synthesis at 150 °C and 180 °C using BaOH₂·8H₂O and TiO₂·xH₂O as starting hydrothermal mixture with an excess of Barium (+1 Ba mol% and +2 Ba mol%). The obtained BaTiO₃ cake was washed several times from 0 to 14 (W_n<15) using simple de-ionized water and then sintered at 1120 °C for 3 h. All considered hydrothermal syntheses variables strongly contribute to the final characteristics of the sintered BaTiO₃ powders in terms of Ba²⁺/Ti⁴⁺ molar ratio, crystalline structure and mean particle size. In particular, it is clear from these experiments that the removal of the unfavorable barium salts from BaTiO₃ cake by long washing cycles before final calcination is a critical step in the hydrothermal synthesis of BaTiO₃.     

Structural and optical properties of solvothermally synthesized ZnS nano-materials using Na2S·9H2O and ZnSO4·7H2O precursors

Hexagonal wurtzite (HWZ) ZnS nanorods were formed in specimens with a S/Zn ratio of 1.3, synthesized at temperatures ≥200 °C in a solution containing 80 vol% water and 20 vol% of ethylenediamine (EN). In contrast, HWZ ZnS nanoparticles were formed in specimens synthesized at temperatures lower than 200 °C. Also, cubic zinc blende (CZB) ZnS nanoparticles were formed in specimen synthesized in water. The absorption peak for the HWZ nanorods and CZB ZnS nanoparticles was at wavelength of 325 nm and 339 nm, respectively, indicating that the band gap energy of the former is larger than that of the latter. Moreover, the HWZ ZnS exhibited two emission peaks at 474 nm and 580 nm. The peak at 474 nm is attributed to Zn vacancies but the origin of the peak at 580 nm remains undetermined. Since the intensity of the emission peak at 580 nm was significantly higher for the HWZ nanoparticles than for nanorods, this peak might be associated with defects in the HWZ ZnS nanoparticles.     

Comparison between microwave and conventional calcination techniques in regard to reactivity and morphology of co-precipitated BaTiO3 powder,and the electrical and energy storage properties of the sintered samples

Barium titanate (BaTiO₃) nanopowders were synthesized by an aqueous co-precipitation method followed by calcination. Either 2.45 GHz microwaves or conventional heating was used in order to investigate the impact of these techniques on the synthesis time, microstructure, and electrical properties of the materials. The heating temperatures ranged from 620 °C to 810 °C. X-ray diffraction (XRD) revealed pure BaTiO₃ formation by microwave heating in a noticeably shorter time (five minutes) compared to conventional heating (3 h). Field emission scanning electron microscopy (FESEM) results confirmed that the microwave process led to nanocube formation, whereas in the conventional procedure, the particles tended to form spherical shapes. To evaluate the electrical properties, the samples heated at 620 °C were conventionally sintered at 1280 °C, 1330 °C, and 1380 °C. Higher dielectric, piezoelectric, and ferroelectric properties and more energy-saving efficiency (ε_r=1012, tan δ=0.035 d₃₃=85 pC/N, p_r=6.2 µC/cm² and η=48% respectively) were achieved in the microwave-heated BaTiO₃ sintered at 1380 °C compared to the conventionally heated BaTiO₃ (ε_r=824, tan δ=0.030 d₃₃=75 pC/N, p_r=5 µC/cm² and η=27%) demonstrating that microwave calcination substantially affects the final electrical properties.     

Structural properties of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric ceramics

A systematic XRD investigation of poled and unpoled ceramics of the system (1 ? x) Bi_0.5Na_0.5TiO₃–x BaTiO₃ (0 ≤ x ≤ 0.2) (BNBT) was performed. The variation of the lattice parameters confirms the existence of a morphotropic phase boundary at 0.06 ≤ x ≤ 0.08; however, significant differences in unit cell parameters between poled and unpoled states appear. Lattice distortions of the rhombohedral and tetragonal phases are significantly increased in poled samples. Dramatic changes in peak intensities of the pseudo-cubic (2 0 0) reflections between poled and unpoled samples reveal a strong enhancement of the tetragonal volume fraction in the poled state. Temperature-dependent XRD studies confirm a transition into a cubic high-temperature phase. This transition is rather smooth in the unpoled state. In poled samples, the tetragonal distortion and the tetragonal volume fraction display a different temperature variation and tetragonal regions seem to persist into the cubic phase field.     

Low temperature synthesis of tetragonal BaTiO3 by a novel composite-hydroxide-mediated approach and its dielectric properties

High purity tetragonal BaTiO₃ powders were synthesized by a composite-hydroxide-mediated approach at low temperature using a novel hydrothermal reaction apparatus with a rolling system. The optimum synthesis conditions were explored, and the obtained samples were characterized by their XRD, TEM, TG-DTA and SEM. The powders with an average size of 150 nm in diameter were sintered to almost full theoretical density (ca. 99%) at 1200 °C for 5 h and the obtained ceramics presented a high dielectric constant (9500 at the Curie temperature).     

Dye sensitized solar cells based on hydrothermally grown TiO2 nanostars over nanorods

A rutile titanium dioxide nanostar over nanorods is synthesized by a simple and cost-effective hydrothermal deposition method onto conducting glass substrates. In order to study the effect of precursor concentrations on the growth of TiO₂, the amount of Ti precursor is varied from 0.1 mL to 0.5 mL at the interval of 0.1 mL. These TiO₂ thin films are characterized for their morphological, structural, optical and J–V properties using various characterization techniques. SEM images showed the formation of densely packed nanostars over nanorods for 0.3 mL titanium tetraisopropoxide (TTIP). XRD patterns show the formation of polycrystalline TiO₂ with tetragonal crystal structure possessing rutile phase. Further, the TiO₂ thin films are used for dye sensitized solar cells using N3-dye.The films were photoelectrochemically active and can be viewed as a promising application in DSSC with maximum current density of 1.459 mA/cm² with enhanced photovoltage of 696 mV for the sample prepared at 0.3 mL TTIP.     

Fabrication of piezoelectric particle-dispersed ceramic nanocomposite

The goal of this study is to fabricate perovskite type ferroelectric particles-dispersed ceramic nanocomposites though conventional hot-pressing or pulse electric current sintering (PECS). This type of nanocomposite is expected to show ferroelectricity or piezoelectricity with retaining mechanical properties. Magnesia (MgO) and barium titanate (BaTiO₃) were selected as a matrix and secondary phase dispersoid. From X-ray diffraction analysis, the BaTiO₃ was the phase compatible with the MgO matrix, and there were no reaction phases between the matrix and BaTiO₃. It was found that the BaTiO₃ enhanced the sinterability of the MgO ceramics. Relative density of pure MgO was lower than 80%, while dense MgO/10 vol% BaTiO₃ nanocomposites could be successfully prepared by sintering at 1200°C for 10 min through PECS method. Fine BaTiO₃ particles were homogeneously dispersed within the MgO matrix grain as well as at grain boundaries. Sintering behavior and microstructure development of the MgO/BaTiO₃ nanocomposites were discussed in terms of BaTiO₃ content and sintering temperatures.     

THz region dielectric properties of barium titanate fine particles using infrared reflection method

The THz region dielectric properties of barium titanate (BaTiO₃) fine particles were measured using the infrared (IR) reflection method. Prior to this measurement, to minimize the scattering light caused by surface roughness, the preparation of the dense 3D colloidal sphere arrays (colloidal crystals) with a flat surface was tried. First, the BaTiO₃ fine particles were well dispersed into diethylene glycol as an organic solvent, and this BaTiO₃ slurry was dried very slowly at 80 °C. Finally, the dense BaTiO₃ colloidal crystals were successfully prepared. The IR reflection method was performed using these dense BaTiO₃ colloidal crystals, and their IR reflection spectra were measured from 100 to 1200 cm⁻¹. As a result, by the use of the dense BaTiO₃ colloidal crystals, the high intensity reflection spectra of the BaTiO₃ fine particles were successfully obtained. The spectra were analyzed using a four-parameter semi-quantum (FPSQ) model, and finally, the THz region dielectric properties were estimated for the BaTiO₃ fine particles.     

Hydrothermal and electrochemical growth of complex oxide thin films for electronic devices

Thin film growth of complex oxides including BaTiO₃, SrTiO₃, BaZrO₃, SrZrO₃, KTaO₃, and KNbO₃ were studied by the hydrothermal and the hydrothermal–electrochemical methods. Hydrothermal–electrochemical growth of ATiO₃ (A = Ba, Sr) thin films was investigated at temperatures from 100 to 200 °C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6 to 3.0%. Tracer experiments revealed that the ATiO₃ film grows at the film/substrate interface. Thin films of AZrO₃ (A = Ba, Sr) were also prepared on Zr metal substrates by the hydrothermal–electrochemical method. By applying a potential above ca. +2 V versus Ag/AgCl to the Zr substrates, AZrO₃ thin films were formed uniformly. Thin films of KTaO₃ and KNbO₃ were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO₃ thin films were formed in 2.0 M KOH at 300 °C. Pyrochlore-type K₂Ta₂O₆ thin films were formed at lower temperatures and lower KOH concentrations.     

Ethanol sensing properties of tungsten oxide nanorods prepared by microwave hydrothermal method

Tungsten oxide nanorods have been prepared by a simple microwave hydrothermal (MH) method via Na₂SO₄ as structure-directing agent at 180 °C for 20 min. The structure and morphology of the products are characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The obtained nanorods are about 20–50 nm in diameter and several micrometers in length. The ethanol sensing property of as-prepared tungsten oxide nanorods is studied at ethanol concentration of 10–1000 ppm and working temperature of 370–500 °C. It was found that the sensitivity depended on the working temperatures and also ethanol concentration. The results show that the tungsten oxide nanorods can be used to fabricate high performance ethanol sensors.     

Fabrication of transparent yttria ceramics by alcoholic slip-casting

Y₂O₃ transparent ceramics were prepared from alcoholic slurries of Y₂O₃ nanopowders via a slip-casting method to avoid the hydrolysis issue. Polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG) and polyethylenimine (PEI) were used as dispersants to improve the rheological properties of the slurries. It was found that PEI is the most effective dispersant in ethanol. The adsorbed amount of PEI was evaluated by infrared absorption and rheology measurements. Y₂O₃ slurry with a solid loading of 20.8 vol% and a viscosity of <0.1 Pa s at the shear rate of 10 s⁻¹ was obtained using 1.5 wt% PEI. The slurry yielded a homogeneous green body, and finally resulted in a high-quality Y₂O₃ ceramic with the in-line transmittance of 80% at 800 nm.     

Trimethylamine sensing properties of nano-SnO2 prepared using microwave heating method

The precursor was obtained through the reaction between SnCl₄·5H₂O and NaOH in the presence of PEG400 (polyethylene glycol, M = 400). Tin oxide (SnO₂) nano-powders were prepared by heating the precursor with microwave method. SnO₂ thick film sensors were fabricated using SnO₂ nano-materials as sensing materials. The phase composition and morphology of the material particles were characterized through X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The average particle sizes of the samples obtained with 616 W microwave heating and 800 W microwave heating (20 min) are about 5 and 15 nm, respectively. The influence of the heating duration and heating power on the gas-sensing properties of sensors based on SnO₂ nano-materials were investigated. The sensitivities of the sensors based on SnO₂ nano-materials heated with 616 and 800 W for 20 min were higher than those of the sensors based on SnO₂ nano-materials heated with 136, 264 and 440 W for 20 min. When operating at 200–310 °C, the sensor based on SnO₂ heated with 616 W for 20 min exhibits highest sensitivities in all sensors based on SnO₂ heated with 616 W for different duration. The sensitivity to a few kinds of organic gases, such as (CH₃)₃N and (CH₃)₂CO were studied. It was found that the sensor based on SnO₂ nano-materials (with 616 W microwave heating for 20 min) exhibited good performance characterized by high sensitivity and short response time to dilute trimethylamine when operated at 255 °C. The sensitivity to 0.001 ppm (CH₃)₃N at 255 °C was 3. The response time and recovery time were about 30 and 100 s, respectively.