纳米钛酸钡粉体的制备和烧结动力学研究

以四氯化钛、八水合氢氧化钡和氨水为原料,首先制备出氢氧化钛凝胶,然后以乙醇为溶剂,采用两步沉淀法在低温常压下合成了立方相纳米钛酸钡粉体.研究了反应温度、反应物浓度和反应时间对粉体物相和粒径的影响,利用XRD和TEM对粉体进行了表征.实验结果表明,在低于80℃的反应温度下可以制备出粒径＜20nm、结晶性能良好且形貌规则的纳米钛酸钡粉体.此外,以合成的粉体为原料,以热膨胀仪为测试手段,对钛酸钡陶瓷的烧结动力学进行了研究.结果表明,陶瓷的烧结温度为1150～1250℃,在1250℃的烧结温度下保温2h,陶瓷坯体的相对密度约93%,室温介电常数约4020.     

Synthesis and formation mechanism of submicrometre spherical cordierite powders by ultrasonic spray pyrolysis

Cordierite powders containing very pure submicrometre spherical particles have been synthesized by the ultrasonic spray pyrolysis. Aqueous solutions of silicic acid, Al(NO3)3·9H2O and MgCl2·6H2O were used as precursors. Scanning electron micrographs have shown that particle surfaces were smooth and the mean particle diameter was 0.834 m. For the estimation of chemical and phase composition and phase transformation temperatures, differential thermal analysis, thermogravimetric analysis, X-ray diffraction, energy dispersive spectroscopy and infrared analysis have been applied. It was found that during spray pyrolysis, the condensation of silicic acid mostly occurred while aluminium and magnesium ion remained incorported between Si–O–Si chains. By subsequent heating to over 800°C, Si–O–M bonds (M=Al, Mg) were formed. The synthesis of cordierite occurred by the crystallization of -cordierite from the amorphous phase at 900°C followed by the phase transformation of - into -cordierite in the temperature range 1100–1200°C.     

Synthesis of mullite nanostructured spherical powder by ultrasonic spray pyrolysis

In this paper the synthesis of nanostructured spherical particles of mullite powders by ultrasonic spray pyrolysis is presented. The mullite crystallization and the nanostructure development during heating were examined by infrared spectroscopy, X-ray diffraction, scanning and transmission electron microscope analysis. Comparative analysis of experimentally determined and theoretically calculated particle size distribution, obtained on the basis of three-dimensional (3D) model of spherical/ellipsoidal waves generated by incident ultrasonic field, confirmed that the process of aerosol/powder particle synthesis can be regarded as deterministic process.     

Synthesis of mixed-phase titania films by low-temperature ultrasonic spray pyrolysis

Fully dense TiO₂ films with (1) mixed-phase rutile and anatase and (2) anatase (sole phase) were deposited on fused quartz substrates by ultrasonic spray pyrolysis at nominally 400 °C. The presence and absence of insulation around the entrainment pathway traversing 20 cm above the substrate/hot plate were investigated (174 °C vs 122 °C). The thick films were assessed in terms of mineralogies (qualitative and quantitative), microstructures (topography, thickness and grain size), and visible light transmission (optical and microstructural considerations). With insulation, opaque mixed anatase (∼ 30 vol.%; < 50 nm) and rutile (∼ 70 vol.%; ∼ 1 μm) were observed; without insulation, only transparent anatase (< 50 nm) was observed.     

Synthesis of nanosized spherical cobalt powder by ultrasonic spray pyrolysis

The ultrasonic spray pyrolysis (USP) method has been used to prepare nanosized powders of metallic, intermetallic compounds and ceramic materials. Spherical nanosized cobalt powders were obtained by USP of aqueous solutions of cobalt nitrate followed by thermal decomposition of generated aerosols in hydrogen atmosphere. Particle sizes of the produced cobalt powder can be controlled by the change of the concentration of an initial solution. Non-agglomerated spherical nanosized cobalt particles in the range of 158–1001 nm were obtained at 800 °C. A decrease of the concentration of cobalt nitrate decreases the mean particle diameter from 596 to 480 nm. The discrepancy between the experimentally and theoretically obtained values indicates that the partial coalescence of the droplets occur during the formation of aerosol.     

Preparation of barium titanate ultrafine powders from a monomeric metallo-organic precursor by combined solid-state polymerisation and pyrolysis

A recipe has been elaborated for preparing barium titanate (BaTiO₃) particles in a nucleation route which is mediated by thermal decomposition of polymeric barium titanium methacrylate. Adjustment of particle size d in the range from 10 nm to 1.5 m is easily done by choosing appropriate reaction temperatures and tempering atmospheres. In particular, doping with paramagnetic probe ions such as Mn²⁺, Gd³⁺ or Cr³⁺ can be readily accomplished by just adding the corresponding metal acetates to the monomeric precursor. In addition to well approved standard techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), the complementary spectroscopic methods electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), X-ray absorption near edge structure (XANES) and FT-Raman are applied to characterise the micro- and nanocrystalline BaTiO₃ powders prepared and to study the phase transition behaviour in dependence on the mean particle size. In contrast to expectation from literature, the dimensionality effect does not manifest itself in a temperature shift of the ferroelectric phase transition but, instead, the tetragonal-to-cubic phase transition is smeared out at reduced particle size and an increasing tetragonal-to-cubic phase admixture is detected.     

Synthesis and characterization of nanocrystalline barium strontium titanate powders prepared by citrate precursor method

Nanocrystalline and well dispersed barium strontium titanate (BST) powders were prepared by a novel and simple citrate precursor method. This method involved direct crystallization of a white precursor from a stable solution in the citric acid (CA)–ethylene glycol (EG)–tetrabutyl titanate–M²⁺ (M = Ba, Sr) system under a specific pH value range. Subsequent heat treatment of the precursor at 850 °C led to a pure phase BST powder. TG/DTA was used to examine the decomposition behaviour of the precursor. The crystalline phase and morphology of the BST powders were investigated by XRD and TEM. It was found that the BST powders synthesized by citrate precursor process were more homogeneous and uniform than that obtained by the citrate gel method.     

Synthesis of submicron carbon spheres by the ultrasonic spray pyrolysis method

Submicron carbon spherical particles were obtained by polycondensation of resorcinol and formaldehyde in a solution and subsequent ultrasonic spray pyrolysis of the prepared sol. Microscopic characterization indicates the regular spherical shape of the obtained particles and sphere diameters in 200-700 nm range. The carbon spheres are amorphous as confirmed by electron diffraction, EELS, XRD and HREM characterization. Activation procedure was performed with H₂O in a nitrogen flow for 15 and 30 min at 800 °C. The activation procedure preserved the initial spherical shapes of the particles while the particle porosity and specific surface area were increased. The amount of surface oxygen functionalities was also increased by activation procedure as indicated by FTIR analysis.     

High density sintering of pure barium titanate

The techniques of slow sintering, addition of a volatile flux, and grain growth inhibition have been used to produce high density sintering in pure barium titanate. The use of carbon as grain growth inhibitor gave a density of 99% theoretical at 0.7m grain size. This technique may be used for other suitable ceramics, when the proposed application requires improved physical properties. An extension of the procedure gives a range of electrically conducting ceramics.     

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.     

Preparation of spherical fine ZnO particles by the spray pyrolysis method using ultrasonic atomization techniques

An ultrasonic atomizer was used in the spray pyrolysis method to prepare fine, spherical and uniform ZnO particles. Almost spherical particles were obtained successfully which had a mean particle size of 0.15 m and had a very narrow particle size distribution. By using alcohol as the solvent, it was found that the particles do not have hollow shell layers which could usually be observed in the spray pyrolysis process by using water as the solvent. The morphology of the ZnO particles was strongly affected by the concentration of the starting solution.     

Synthesis of optical quality ZnO nanowires utilizing ultrasonic spray pyrolysis

ZnO nanowires were grown by an ultrasonic spray pyrolysis technique. The aspect ratio and size of the wire were dependent mainly on the pH value of a precursor solution and the growth temperature. By high-resolution transmission electron microscopic analysis and photoluminescence measurements, it was confirmed that the nanowires are monocrystalline with good optical quality.     

Preparation of spherical Pb(Zr,Ti)O3 powders by ultrasonic spray pyrolysis

Spherical Pb(Zr,Ti)O₃ powders were prepared from the aqueous acetate-base solution by ultrasonic spray pyrolysis. The raw materials were Pb(C₂H₃O₂)-3H₂O, ZrO(C₂H₃O₂)₂ and a mixture of Ti(C₃H₇O)₄ and C₅H₇O₂. A single-phase PZT was formed at 900 C in air and at 700 C in O₂ atmosphere. The PbTiO₃ phase, as an intermediate product, was observed in the formation of PZT and no PbZrO₃ phase was detected under our experimental conditions. The intensity of the PbTiO₃ peak decreased with increasing reaction temperature in air, while the reverse effect of reaction temperature was observed in O₂. Spherical and irregular-shaped particles coexisted in the powder containing the minor PbTiO₃ phase, while the particles of a single-phase PZT have spherical morphology with little evidence of irregular-shaped particle formation.     

Preparation of spherical TiO2/SnO2 powders by ultrasonic spray pyrolysis and its spinodal decomposition

Spherical TiO₂/SnO₂ powders were prepared from the TiCl₄-SnCl₄ aqueous solution by ultrasonic spray pyrolysis. The particle size, particle-size distribution and morphology of the powders were studied in relation to concentration of source solutions and reaction temperatures. The width of the compositional undulation induced by spinodal decomposition in the sintered bodies increased with increasing starting particle sizes, which resulted from micro-compositional heterogeneity in the form of a solid solution within a TiO₂/SnO₂ secondary sphere.     

Effect of pyrolysis temperature on structural,microstructural and optical properties of nanocrystalline ZnO powders synthesised by ultrasonic spray pyrolysis technique

In this article, we report on the effect of pyrolysis temperature on structural, microstructural and optical properties of nanocrystalline ZnO powder synthesised by ultrasonic spray pyrolysis (USP) technique. Powder samples P1, P2 and P3 were prepared at various pyrolysis temperatures (temperature of 2nd zone) of 973, 1073 and 1273?K, respectively. Phases were identified and crystallite sizes were calculated from X-ray diffraction (XRD) diagrams. The morphology and size of ZnO nanocrystallites associated with nanopowder were studied using transmission electron micrograph (TEM). It revealed that the powder consisted of crystallites ranging in size from 9 to 20?nm. These values were matching with the crystallite sizes calculated from XRD. Both XRD and TEM studies of ZnO nanopowders showed that the crystallite sizes increased with an increase in the pyrolysis temperature. The synthesised nanopowders exhibited direct band gap (E _g) in the range 3.37–3.40?eV.     

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.     

Estimation of formation mechanism of spherical fine ZrO2-SiO2 particles by ultrasonic spray pyrolysis

Spherical fine ZrO₂-SiO₂ (11) particles were synthesized from a hydrolysed mix-solution of Si(OC₂H₅)₄ and ZrOCI₂ · 8H₂O by ultrasonic spray pyrolysis, and the formation mechanism of the particles discussed. All of the resultant particles identified as t-ZrO₂ and amorphous SiO₂ with an atomic order dispersion were spherical, and mainly of diameter 0.8 to 1.0m. It was estimated that the three-dimensional ladder siloxane chains containing an equimolar Zr⁴⁺ homogeneously dispersed were formed by hydrolysis, and an atomized droplet itself converted into an isolated ZrO₂-SiO₂ (11) particle without aggregation.