Sintering behaviour of powder compacts with multiheterogeneities

The sintering behaviour of a powder compact containing uniformly distributed heterogeneities has been analysed. The results reveal a strong retardation of sintering in the presence of non-sinterable agglomerates, due to the development of a uniform hydrostatic tensile stress in the powder matrix. The variation in stresses and sintering rate depends strongly on the total volume fraction of heterogeneities especially when the volume fraction is small, but is insensitive to the actual number of heterogeneities. Specific results are calculated using densification and deformation laws pertinent to Al₂O₃.     

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.     

Sintering and properties of highly donor-doped barium titanate ceramics

The electrical properties of donor (La³⁺) doped BaTiO₃ samples with a donor concentration in the range from 0.3 to 1.5 mol.% of La were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly, the samples were annealed in air at 1100°C. Results show that with the use of a specific sintering profile PTCR ceramics containing an amount of donor dopant >0.3 mol.%, can be prepared. Heavily doped samples which do not exhibit anomalous grain growth show a core shell structure.     

Semiconducting ceramics produced using nanocrystalline barium titanate powder

Positive temperature coefficient of resistance ceramics of composition (Ba_0.89Ca_0.08Pb_0.03)TiO₃ + Y₂O₃ + MnO + SiO₂ have been produced using barium titanate powder with an average crystallite size of 125 nm prepared by calcining barium titanyl oxalate at 900°C. The effect of firing temperature on their microstructure and electrical properties has been studied. The results demonstrate that the ceramics possess semiconducting properties starting at a firing temperature of 1205–1215°C. The room-temperature resistivity of the ceramics has a minimum at t _firing ≈ 1245–1250°C. The samples sintered at 1250–1260°C have the largest positive temperature coefficient of resistance. The highest electric strength (360 V/mm at ρ_25°C = 290 Ω cm) is offered by the thermistor materials sintered at 1260°C, which is 60–70°C below the firing temperature of analogous ceramics produced by solid-state reaction.     

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.     

Barium ion leaching from barium titanate powder in water

A submicrometer-sized commercial BaTiO₃ (BT) powder was aged in water under three different conditions: pH, aging times, and pre-heat treatments of the powder. The amount of Ba²⁺ ions leaching from the BT particles was determined by the EDTA titration method. As predicted by thermodynamic calculation, the greater extent and the faster rate of Ba²⁺ leaching were found at the lower solution pH, leveling off at pH 8. The pre-heat treatments of the BT powder increased the amount of Ba²⁺ leaching when compared to the as-received one. This result was shown by the formation of soluble surface BaCO₃, which was detected using FT-IR spectroscopy. It was also shown that organic passivation agents were effective reducing the Ba²⁺ leaching but at high solution pH.     

微乳液-共沸蒸馏法制备钛酸钡纳米粉体

以水/正辛烷/正己醇/司班80油包水体系中的微乳液滴为纳米反应器,通过微乳液滴中增溶的醋酸钡和油相中草酸酰化的钛酸四丁脂反应生成钛酸钡前驱体,并通过共沸蒸馏去除前驱体中的水分,以减小煅烧过程纳米钛酸钡粉体的团聚.实验结果表明,此方法制备的前驱体只需在550℃煅烧就可以生成钛酸钡晶粒,只需在600℃煅烧10min就可以得到粒径只有40nm左右的高纯立方钛酸钡纳米粉体,而且粉体具有粒度分布窄、形貌规则、优越的钛钡摩尔比、低团聚等优点.     

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.     

Sintering of heterogeneous ceramic compacts

The sintering behaviour of alumina powder compacts containing inclusions has been studied. The densification rate is significantly retarded by the addition of coarse, dense, fused alumina particles. The influences of inclusion volume fraction, of inclusion size, of matrix density and of temperature on the reduction in densification rate are reported. A method is proposed for the evaluation of an effective back stress which opposes densification and which is generated by the presence of the inclusions.     

Sintering of heterogeneous ceramic compacts

Local differences in the density, grain size or chemical composition within a sintering compact can seriously limit the final densities which can be achieved. Experiments on zirconia-alumina composites have been conducted to test the influence of such variations. Experiments are described which measure the stresses produced in the sample as a consequence of local differences in the sintering rate, and which indicate variations in the accumulated density change occurring in the composites and in single-phase standards. An estimate is given for the acceptable degree of inhomogeneity if full density is to be achieved.     

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.     

Crystal orientation dependence of the out-of-plane dielectric properties for barium stannate titanate thin films

The orientation-dependent out-of-plane dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃ (BTS)) thin films prepared by sol–gel method were investigated. Films with (1 0 0), (1 1 0), and (1 1 1) orientation were grown on LaNiO₃-buffered (1 0 0), (1 1 0), and (1 1 1) LaAlO₃ (LAO) single-crystal substrates, respectively. The different temperature of the dielectric constant maximum (T_m) of the BTS thin films with different orientation was believed to be attributing to stress inside the films. Films with the (1 1 1) orientation had higher relative dielectric constant and tunability than (1 0 0)- and (1 1 0)-oriented films. This difference in dielectric properties in these three kinds of oriented BTS films may be attributed to change in the direction and magnitude of electric polarization in orientation engineered BTS films and stress in the films.     

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.     

Synthesis of nano (Ti,W)C powder with preferred orientation and twin boundary structure

(Ti,W)C is a novel additive for high performance cermets. In this study, (Ti_0.88W_0.12)C with the lowest formation energy is synthesized by carbothermal reduction-carbonization in Ar. The starting materials included WO_2.72 with one-dimensional nanostructure, TiO₂ and carbon black. The phase transition temperatures were established by thermal analysis. XRD analysis results disclose that once TiC is formed at a temperature over 1220 °C, W atoms begin to diffuse into the TiC lattices, which is independent of the existing form of tungsten. At a condition of 1500 °C for 180 min, W and C atoms from the decomposed W₂C and WC are fully dissolved in the TiC lattices. Under such a TiC-centered atomic reconfiguration environment, the as-synthesized powder is featured with a BET particle size of 76 nm and texture coefficients TC₍₁₁₁₎ of 1.53 and TC₍₂₀₀₎ of 1.33. Results from SEM and HRTEM reveal that the roughly equiaxed powder particles have characteristics of readily identified twin boundary structure and stacking faults. Microscopic inhomogeneity of W solution atoms is discussed. The revelation of the easily identified twin boundary structure and stacking faults is of great significance to the hard phase regulation for high performance Ti(C,N)-based cermets.     

Sintering and microstructure of ultrafine yttriazirconia compacts

Sintering behaviour of ultrafine yttria-zirconia solid solution compacts, with yttria contents ranging between 4 and 10 mol % have been investigated and the microstructural developments during sintering were followed. The 4 mol % yttria-zirconia compacts sinter at 1200 ° C to a high density with a relatively fine uniform microstructure. Firing at 1400 ° C produces no improvement in densification and discontinuous grain growth develops. The lowest density values are obtained with the 10 mol % yttria-zirconia compacts, with clustering in the grains and greater porosity in the microstructure. The activation energies were determined for both 4 and 10 mol % Y₂O₃ -containing bodies in the temperature range 800 to 1000 ° C.     

Microwave-hydrothermal synthesis of tetragonal barium titanate

Tetragonal barium titanate with c/a ratio of 1.0093 and average particle size of 240 nm was synthesized by the microwave-hydrothermal (MH) method at 240 °C in only 12 h. Temperatures above 200 °C were first introduced to the MH process in this study, since the temperature has a critical effect on the formation of tetragonal BaTiO₃, shown by the experiments here. Hydrous titanium oxide and Ba(OH)₂ were used as precursors, without halide anions and alkali-metal cations to avoid contamination. The kinetics of tetragonal phase formation in BaTiO₃ was considerably promoted in the MH processing, in comparison with the conventional hydrothermal (CH) route.     

Effects of barium dissolution on dispersing aqueous barium titanate suspensions

Dissolution of barium ion and its effect on dispersion behavior of aqueous barium titanate suspensions at various pH values have been investigated. The amount of leached barium ion decreases with increasing pH value. The dissolution of barium ion also causes an increase in pH value of suspension, but the change decreases with increasing initial pH value. The iso-electric point (IEP) of leached barium titanate powder increases with increasing leaching pH value and solid loading as well. The dissolution of barium ion enhances the colloidal stability of aqueous barium titanate suspension, in agreement with zeta potential measurement.     

Dielectric properties of lead zirconate titanate thin films seeded with barium strontium titanate nanoparticles

A low temperature synthetic method recently proposed by the authors was applied to the fabrication of lead zirconate titanate (PZT) thin films containing crystalline seeds of barium strontium titanate (BST) nanoparticles. PZT precursor and the BST particles were prepared with complex alkoxide methods. Precursor solution suspending the BST particles was spin-coated on Pt/Ti/SiO₂/Si substrate to film thickness of 500–800 nm at particle concentrations of 0–25.1 mol%, and annealed at various temperatures. Seeding of BST particles prevented the formation of pyrochlore phases, which appeared at temperatures above 400 °C in unseeded PZT films, and induced crystallization of PZT into perovskite structures at 420 °C, which was more than 100 °C below the crystallization temperature of the unseeded PZT films. Measurement of dielectric properties at 1 kHz showed that the 25.1 mol% BST-seeded PZT films annealed at 450 °C had a dielectric constant as high as 300 with a dissipation factor of 0.05. Leakage current density of the film was less than 1×10⁻⁶ A/cm² at applied electric field from 0 to 64 kV/cm.