水热合成高纯四方相钛酸钡纳米粉末研究

以廉价易得氯化钡,四氯化钛为原料,在240℃下仅用12h水热合成得到四方相钛酸钡纳米粉末,从而为纳米粉末的产生化提供了简便条件。水热产物经XRD,SEM,XRF等手段特征,发现提高氢氧化钠过量的浓度,降低反应物四氯化钛浓度有利于四方相钛酸钡纳米粉末的生成,同时在水热产物中没有发现碳酸根和氯离子等残余的杂质表明产物具有较高的纯度。产物粒度小于100nm,比表面积为12m^2/g.     

Molten-salt synthesis of tetragonal micron-sized barium titanate from a peroxo-hydroxide precursor

Barium titanate (BaTiO₃) was produced from an barium-titanate-peroxo-hydroxide precursor material in NaCl-KCl and Na₂SO₄-K₂SO₄ salt mixtures or fluxes at temperatures up to 1080 °C via the molten-salt synthesis (MSS) method. Beside the different salt mixtures several other parameters were studied to determine the effect on the particle morphology. A relatively fast heating rate and a relatively high salt to precursor ratio resulted in a highly faceted morphology. The effect of the salt flux was clearly seen in the morphology of BaTiO₃ obtained by MSS at 1080 °C. Cube-shaped particles were observed for the chloride-flux-grown BaTiO₃ while hexagonal-shaped flat particles were observed for the sulfate-flux-grown BaTiO₃. BaTiO₃ particles produced in the chloride-flux were either {1 0 1} or {0 0 1} faceted. The hexagonal surface of the sulfate-flux-grown BaTiO₃ was {1 1 1} faceted.     

Microwave synthesis of nano-sized barium titanate

Nano-sized barium titanate powders have been synthesized by microwave processing at 2.45 GHz. Using barium titanyl oxalate (BTO) as a precursor, microwave processing was carried out by heating the precursor to a temperature between 600 °C and 750 °C with different heating rates from 10 °C/min to 20 °C/min without isothermal holding. X-ray diffraction analysis indicates that the decomposed product at 680 °C was pure cubic BaTiO₃. The BET specific surface area of barium titanate powder, after microwave heating to 680 °C, was 14.2 ± 0.5 m²/g, corresponding to an average particle size of 70 nm. This particle size was confirmed by the scanning electron microscopy (SEM). Parallel study shows that the conventional heating in a regular resistance furnace using a similar heating schedule did not result in complete conversion of BTO. This study shows that the microwave processing significantly accelerated the decomposition of barium titanyl oxalate and reduced the temperature of barium titanate nano-powder formation, resulting in nano-sized pure cubic barium titanate powder.     

Hydrothermal synthesis of lead doped barium titanate

Lead doped barium titanate was synthesized hydrothermally at 363 K for 140 h. A molar formula of Ba_(1–x)Pb _x TiO₃ was used, where x ranged between 0.025 and 0.75. The crystal structure, phase purity, and particle morphology was investigated by x-ray diffraction, Raman spectroscopy and electron microscopy. Under the synthesis conditions used, lead (Pb²⁺) was shown to incorporate into the perovskite structure when the dopant was kept below 20%. Above 20% Pb, other phases appeared and at 75% Pb no reaction to the perovskite structure took place. Unexpectedly, barium titanate containing from 2.5% Pb to 10% Pb appeared to be of orthorhombic symmetry. This was concluded by total pattern fitting of x-ray diffraction profiles and from splitting of the 222 reflection. The factors controlling the tendency for these materials to adopt orthorhombic symmetry as opposed to the more commonly observed tetragonal or cubic symmetries are briefly discussed.     

微弧氧化法生成四方相BaTiO3薄膜工艺研究

针对微弧氧化法生成四方相BaTiO3薄膜的工艺参数影响规律进行研究.将Ti板置于含Ba2+的电解液中在0.15～0.55mol/L和1.0～5.5A的浓度、电流区域内进行微弧氧化.实验发现微弧只在一定的浓度-电流区域中发生,可用平面图直观表示;在不同的浓度-电流区域内,薄膜具有不同的形貌,据此将浓度-电流区域图进一步划...     

Low-temperature synthesis and analysis of barium titanate nanoparticles with excess barium

BaTiO₃ nanoparticles with excess barium are directly synthesized from solution at 65 °C. It is noted that a small amount of BaCO₃ exists in the as-prepared samples. A series of standard mixtures with different proportion of BaTiO₃ and BaCO₃ are measured by XRD analysis to quantify the amount of BaCO₃ in our synthesized samples. After a deduction of the amount of BaCO₃, the concentration of excess barium in the lattice of BaTiO₃ nanoparticles can be determined and the results show that the excess barium can reach a considerable concentration. The influences of defects and surfaces on BaTiO₃ nanoparticles with excess barium are analyzed and two important reasons for the high concentration are also given.     

Sol-gel synthesis and characterization of barium titanate powders

A simple aqueous sol-gel route has been refined to prepare yttrium-doped barium titanate powders. Thermal decomposition of the fabricated gel was studied by means of DTA/TGA measurements. The precursor gel was intermediate calcined at 600 °C, and the final synthesis products were characterized by IR spectroscopy, X-ray powder diffraction and scanning electron microscopy. The barium titanate obtained from the calcination of the gel at 1300 °C was a tetragonal phase, and showed a positive temperature coefficient of resistivity (PTCR) effect which depends on sample cooling rate during the final heat treatment.     

Microwave assisted semi-solvothermal synthesis of nanocrystalline barium titanate

In an endeavor to synthesize tetragonal nanocrystallites of BaTiO3 at much reduced reaction time, we explored the possibility of performing microwave assisted semi-solvothermal reaction by using Ba(OH)2 . 8H2O and amorphous titanium hydrous gel as precursors and 1,4-butanediol and water as solvent. Typically, such a microwave assisted reaction was accomplished within 2 hrs at 220 degrees C as against 12 hrs required in conventional approach. The crystallized BaTiO3 powders (microwave assisted as well as conventionally processed for reference) were characterized by X-ray diffraction, thermal analysis, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. We have detected metastable cubic phase by XRD while locally symmetric tetragonal phase by Raman spectroscopy in case of conventional semi-solvothermal processing. On the contrary, we could detect co-existence of tetragonal and cubic phases by XRD and only tetragonal phase by Raman spectroscopy in case of microwave assisted semi-solvothermal processing. The TEM analysis indicates typical particle size distribution in the range of approximately 20 to 80 nm for conventionally processed powder while that in the range of approximately 20 to 50 nm for microwave processed powder. HRTEM images evince the distortion from an ideal cubic structure in case of microwave processed powder which can be correlated with anisotropic lattice contraction during the microwave induced heating. AFM analysis exhibited relatively less aggregation of nanoparticles for microwave assisted process.     

Microwave assisted synthesis of barium zirconium titanate nanopowders

The paper reports the synthesis, structural and high frequency dielectric properties of Ba(Zr _x Ti_1−x )O_3,BZT, nanopowders where x = 0, 0.1, 0.2, 0.3. These powders were synthesized using both microwave assisted and conventional heating, with the former requiring lower temperature and shorter times compared to the latter, viz., 700 °C for 30 min versus 900 °C for 5 h. The synthesized nanopowders were characterized using X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy, BET surface area analysis, differential scanning calorimetry and high frequency dielectric measurements. All the microwave synthesized BZT compositions were found to have well crystallized, finer nanoparticles with less agglomeration and higher dielectric permittivity compared to the conventionally prepared powders. The rapidity and less demanding processing conditions associated with the microwave assisted method augers well for the general applicability of the technique for the production of nanocrystalline powders.     

A modified method for barium titanate nanoparticles synthesis

In this research, a modified, cost effective sol-gel procedure applied to synthesize BaTiO₃ nanoparticles. XRD and electron microscopy (SEM and TEM) applied for microstructural characterization of powders. The obtained results showed that the type of precursors, their ratio and the hydrolysis conditions had a great effect on time, temperature and therefore the costs of the synthesis process. By selection, utilization of optimized precursor's type, hydrolysis conditions, fine cubic BaTiO₃ nanoparticles were synthesized at low temperature and in short time span (1 h calcination at 800 °C). The proposed procedure seems to be more preferable for mass production.The result indicated that the polymorphic transformation to tetragonal (ferroelectric characteristic) occurred at 900 °C, which might be an indication of being nanosized.     

Semiconducting properties of barium titanate and iron substituted barium titanate

Single crystals of barium titanate and members of the series BaTi_1?xFe_xO_3?xF_x have been grown from a potassium fluoride flux. These crystals were made conductive by heating in a sealed evacuated silica tube in the presence of freshly ground titanium. Electrical, optical and magnetic measurements were performed in order to characterize these crystals. Optical measurements indicated a decrease in the band edge from 3.2(1) eV for barium titanate to 2.8(1) eV for Bati_1?xFe_xO_3?xF_x(x = 0.02, 0.05, 0.07, 0.10). The photoelectrolytic behavior of reduced barium titanate was strongly quenched upon the substitution of iron and fluorine.     

Rate-controlled synthesis and sintering of nanocrystalline barium titanate powder

The densification and grain size during sintering as well as particle size, of the barium titanate powder during Pechini decomposition-synthesis depends on the heating rate and this dependence originates from the kinetic competition between elementary transformation mechanisms, inherent in the two processes. The given competition is the main reason for development of the optimum mode of the mentioned thermal activated processes. As developed at the NCSU (USA) the Rate-Controlled Sintering (RCS) is known to allow the obtaining of dense and finegrained ceramics with improved properties. As developed at the IPMS (Ukraine) the Rate-Controlled Synthesis is directed to prepare nanocrystalline unagglomerated ceramic powders suitable for RCS. The extreme behavior of the BaTiO₃ particle size as a function of heating rate is established and the optimal temperature-time path is calculated and verified to obtain the best powder of 20–25 nm particles. The change in microstructure and phase composition with heating rate is considered in details. The advantages of the RCS to achieve density of 99.9% and grain size around 100–150 nm are presented in comparison with the linear heating rate regime.     

Semiconducting barium titanate

Barium titanate, which is well known as a basic ferroelectric material, is also of interest when doped because of the interaction between semiconductivity and ferroelectricity. The resistance of blocking layers at surfaces and grain-boundaries is governed mainly by the ferroelectric properties, so that a resistance jump of four decades is observed on heating above the Curie temperature. A survey of the chemical and physical properties of such blocking layers both at surfaces and interfaces is presented. Doped titanates have been used as the basis of two new types of material:

1. Semiconductors with high positive temperature-coefficient of resistivity in special temperature ranges suitable for temperature-sensors and stabilisers.
2. Dielectrics with extremely high dielectric constant, used in so-called barrier-layer condensers.

     

Doped barium titanate nanoparticles

We have synthesized nickel (Ni) and iron (Fe) ion doped BaTiO₃ nanoparticles through a chemical route using polyvinyl alcohol (PVA). The concentration of dopant varies from 0 to 2 mole% in the specimens. The results from X-ray diffractograms and transmission electron micrographs show that the particle diameters in the specimen lie in the range 24–40 nm. It is seen that the dielectric permittivity in doped specimens is enhanced by an order of magnitude compared to undoped barium titanate ceramics. The dielectric permittivity shows maxima at 0.3 mole% doping of Fe ion and 0.6 mole% of Ni ion. The unusual dielectric behaviour of the specimens is explained in terms of the change in crystalline structure of the specimens.     

Hydrothermal synthesis of ferroelectric barium and strontium titanate extremely fine particles

BaTiO₃, SrTiO₃ and (Ba_{1– x}Sr_x)TiO₃ fine particles were synthesized from amorphous TiO₂ fine particles by a hydrothermal method. The relationship between the specific dielectric constant of the sintered disk and the mean size of composing particles has been investigated. The specific dielectric constants of the sintered disks of BaTiO₃ and SrTiO₃ particles attain values exceeding 6000 in the range of the particle size of 0.10–0.17 m. The specific dielectric constant for the sintered disk of (Ba_{1– x}Sr_x)TiO₃ particles synthesized at various atomic fractions of Sr (x) attains maximum values at both atomic fractions of 0.08 and 0.95. It has been suggested that the size of particles composing the disk should considerably affect the specific dielectric constant of the sintered disk.     

Preparation of powder barium titanate

Phase-pure BaTiO₃ powder (free of Ba₂TiO₄, BaCO₃, Ba(NO₃)₂, and OH impurities) with an average particle size of about 100 nm is prepared by solid-state reaction between titanium oxyhydroxide and barium hydroxide ground and mixed by sonication in an inert organic liquid.