Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Crystalline BaTiO₃ powders were precipitated by reacting fine TiO₂ particles with a strongly alkaline solution of Ba(OH)₂ under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO₃ increased from 50 nm at 90°C to 100 nm at 240°C. At synthesis temperatures below 150°C, the BaTiO₃ particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO₃ powder is discussed.     

Effects of doping with La and Mn on the crystallite growth and phase transition of BaTiO3 powders

The effects of La and Mn dopants on the crystallite growth and the phase transformation of BaTiO₃ powders were studied. The barium titanate powders were obtained by calcining barium titanyl oxalate tetrahydrate in the temperature range 800 to 1200 °C. Crystallite growth of BaTiO₃ powders was promoted by the use of Mn dopant due to the increase of oxygen vacancies. The dissolution of La dopant into BaTiO₃ structure may decrease the oxygen vacancies so that the growth of BaTiO₃ crystallites is inhibited at high temperature ( 900 °C). When the crystallite size is small, the barium titanate can exist as a cubic phase due to the manifestation of the surface energy. Undoped cubic BaTiO₃ powders can be stable at a size < 30 nm. Doping with La and Mn would bring the crystallite size for the cubic-to-tetragonal phase transformation to 100 nm, resulting from the presence of cation or oxygen vacancies.     

Barium titanate fabricated from fur-fibres

Long-fibrous barium titanate (BaTiO₃) particles were prepared by a hydrothermal reaction of potassium titanate hydrate (2K₂O·11TiO₂·3H₂O) and barium hydroxide (Ba(OH)₂). Effects of preparation conditions on crystal structure and powder morphology were examined. Fur-fibres of K₂O·4TiO₂, 1–10 mm long and 1–100 m in diameter, were obtained by heating a mixture of K₂CO₃ and TiO₂ powders at 1000 C for 100 h. Keeping the fur-fibres in ion-exchanged water for 4 days gave fur-fibres of 2K₂O·11TiO₂·3H₂O). Long-fibrous BaTiO₃, with fibres 100m–1mm long and 1–10 m in diameter, was obtained by a hydrothermal reaction of the hydrate and Ba(OH)₂ (Ba/Ti ratio of 1) at 150 C for 24 h. As-prepared long-fibrous BaTiO₃ was composed of fine crystallites (average size about 270 nm) of cubic phase. The cubic phase and morphology of fur-fibres were maintained up to 1250 C, but heat treatment at 1300 C brought about a growth of crystallites to a few micrometers and a phase transformation to tetragonal phase. It was found that the hydrothermal reaction was effective in producing crystalline BaTiO₃ powder at a low temperature of 150 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.     

Synthesis, characterization and dielectric properties of europium-doped barium titanate nanopowders

Europium-doped cubic barium titanate (BT) nanocrystals with % [Eu/Ti] mol ratio varying from 0.05 to 0.25 were prepared through hydrothermal route. The nano nature of these powders was confirmed by XRD and TEM studies. Pellets were prepared after calcining the powders at 1000 °C for 2 h. These pellets were annealed at 200, 500, 700 and 1000 °C for 2 h at each temperature and used for dielectric measurements. Raman spectra of two typical pellets with %[Eu/Ti] Eu/Ti mol ratios of 0.15 and 0.25 showed all the peaks characteristic of tetragonal BaTiO₃. Pure BT showed a low dielectric constant (DC) with a value of 398. Doping with small amounts of Eu resulted in many fold increase of DC values. A maximum value of 10576 at 1 KHz frequency was observed for the sample with % [Eu/Ti] mol ratio of 0.15. Lowering of Curie temperature T_c (95 to 110 °C) was observed for pure as well as Eu-doped barium titanate.     

Characterization of tetragonal BaTiO3 nanopowders prepared with a new soft chemistry route

Preparation of BaTiO₃ nanopowders (37-42 nm) is carried out by a controlled reconstructive thermal decomposition and crystallization from an amorphous polymeric precursor with polyvinyl alcohol (PVA) and sucrose at 400-600 °C in air. The Rietveld refinement of the XRD profile, processed at 600 °C in 2 h, infers the P4mm tetragonal crystal structure (95% of tetragonality) of the as prepared BaTiO₃ nanopowders, with a = 0.3994 nm and c = 0.4024 nm. A cubic symmetry (Pm3m) of 5% in amount with a = 4.0057 is also detected in addition with tetragonal symmetry. The characteristic tetragonal splitting of 002/200 XRD peaks also supports the tetragonal symmetry (c / a = 1.0075) of the as prepared BaTiO₃ nanopowders. The average particle size (D) of the BaTiO₃ powders, estimated with the help of the specific surface area, measured by BET method, is 39.91 nm. Average D value, calculated by Δ2θ_1 / 2 in the XRD peaks with the Debye Scherrer relation is ∼ 40 nm. TEM study measures the particle size of the BaTiO₃ powders with an average diameter of 37 to 42 nm.     

Preparation and characterization of Ce-doped BaTiO3 thin films by pulsed laser deposition

Thin films of BaTiO₃ doped with 5.5 mol%CeO₂ have been deposited on Pt/Si substrate by pulsed laser deposition. These films crystallize on the tetragonal BaTiO₃ structure without any preferential orientation. Ce-doped BaTiO₃ films deposited by PLD at 675°C in 30 Pa ambient oxygen exhibits a smooth surface: mean surface roughness (Rms) of 48 nm, mean size of grain of 40 Å, average size of aggregates of 315 nm. Thin films as prepared presented good dielectric characteristics: dielectric constant and dielectrics loss (tan ) at a frequency of 1 KHz were 220 and 0.2, respectively. The temperature dependence of dielectric constant exhibited a diffuse ferroelectric to paraelectric phase transition at about 0–10°C. The ferroelectric nature of Ce-doped BaTiO₃ film was confirmed by the hysteresis of the C-V curves.     

Sintering and dielectric properties of BaTiO3 prepared by a composite-hydroxide-mediated approach

High purity tetragonal BaTiO₃ powders consisted of uniform particles of ca. 150 nm in diameter were synthesized by a composite-hydroxide-mediated approach at 240 °C using a novel hydrothermal reaction apparatus with a rolling system. The product showed sinterability superior to the commercial powder, i.e., it could be sintered to full theoretical density at 1200 and 1100 °C without an additive and with 0.3 wt% of Li₂CO₃-0.04 wt% of V₂O₅ mixed sintering aid, respectively. The sintered body of the product also showed piezoelectric properties superior to the commercial one.     

Synthesis and characteristics of near-stoichiometric barium titanate powder by low temperature hydrothermal reaction using titanium tetra(methoxyethoxide)

Near-stoichiometric BaTiO₃ powders with ultrafine particle size and high crystallinity were prepared by low temperature hydrothermal reaction of Ba(OAc)₂ and Ti(OCH₂CH₂OCH₃)₄. BaTiO₃ particles were synthesized in the spherical, metastable cubic crystalline grains with size distribution between 60–90 nm in diameter. Ultrafine particle size was resulted from the control of the hydration rate and the decrease of Ti-O-Ti cross-linking extent of titanium precursor, Ti(OCH₂CH₂OCH₃)₄, which gives electronic, steric, and weakly chelating effect to titanium ion. Increasing the Ba/Ti mole ratio in reactant could not overcome the notorious Ba-deficiency but, improved stoichiometry and produced finer and less agglomerated particles. Interestingly, adding a slight pressure to autogeneous hydrothermal condition (total 4–10 atm) has yielded near-stoichiometric, highly crystalline, and less agglomerated BaTiO₃ particles. These particles, which were in metastable cubic form as synthesized, initiated phasetransition to tetragonal form by calcination at below 400 °C.     

Stability of tetragonal ZrO2 phase in Al2O3 prepared from Zr-Al organometallic compounds

Zr-Al organometallic compounds have been spray-dried and heated at temperatures 600 to 1400°C to prepare ZrO₂-Al₂O₃ composite powders. The powders consist of balloon-like particles 0.5 to 2 m in diameter with homogeneously dispersed tetragonal ZrO₂ grains 0.1 to 0.2 m in diameter. The tetragonal fraction of ZrO₂ in the composite powders is higher than that in the powders prepared from sols of Zr(OBuⁿ)₄ and Al[OCH(CH₃)₂]₃. The fraction is affected by the organofunctional group in the Zr-Al compounds.Zr(OBuⁿ)₄ = Zr(OC₄Hgⁿ)₄; Al[OCH(CH₃)₂]₃ = Al(OPrⁱ)₃.     

Hydrothermal stability of yttria- and ceria-doped tetragonal zirconia-alumina composites

Changes in the crystalline phase and microstructure resulting from hydrothermal ageing of Y-TZP, (Y,Ce)-TZP, Y-TZP-Al₂O₃ composites and (Y,Ce)-TZP-Al₂O₃ composites were investigated under hydrothermal conditions at 180 °C and 1 MPa. Although (Y,Ce)-TZP showed no tetragonal-to-monoclinic (tm) phase transformation during low-temperature ageing in air as compared with 3Y-TZP, the tetragonal phase of (Y,Ce)-TZP easily transformed to monoclinic phase by ageing under hydrothermal condition. This tm phase transformation invaded the inside of the body accompanied by microcracks. (Y,Ce)-TZP-Al₂O₃ composites were resistant to phase transformation during hydrothermal ageing.     

The influence of anions on the products of BaTiO3 under hydrothermal conditions

Tetragonal BaTiO₃ has been prepared from a reactive titanium source, tetrabutyl titanate, with barium acetate in alkaline aqueous solution at temperatures as low as 200–240 °C. Various methods, such as X-ray diffraction, differential scanning calorimetry and scanning electron microscopy have been used to investigate the effects of alkalinity, anions, reactivity of the titanium source and degrees of fill, on the particle sizes and morphologies of BaTiO₃ generated hydrothermally. The most appropriate KOH concentration is about 1.0 mol l^–1 in which BaTiO₃ is the stablest phase in the BaO-TiO₂ system. Compared with chloride and nitrate ions, the acetate ion accelerates the formation of large-grained particles of BaTiO₃ more prominently. A more reactive titanium source and a higher fill can promote the formation of tetragonal BaTiO₃.     

The effect of copper oxide on sintering, microstructure, mechanical properties and hydrothermal ageing of coated 2.5Y-TZP ceramics

Copper oxide dopants in amounts up to 1 wt% were added to 2.5 mol% yttria-coated zirconia powders in studies of sintering, microstructure, mechanical properties and hydrothermal ageing behaviour. High densities (>6 Mgm^–3), high tetragonal phase content (>95%), and phenomenal fracture toughness values (>17 MPam^1/2), were obtained for lower dopant levels. Grain sizes of 0.13 to 0.25 m were measured for all samples sintered at 1300°C. Rounded pores in some doped samples indicated that a liquid phase was involved during sintering. Copper oxide additions aid low temperature sintering and offer potential for property enhancement with a particularly high toughness being measured as well as improving resistance to structural degradation in 180°C hydrothermal ageing.     

Effects of mono-substituting chelating agents on BaTiO3 prepared by the sol-gel process

BaTiO₃ of various grain size was prepared by the sol-gel process from Ti (OR)₄ (R = isoC₃H₇ or C₄H₉)+Ba(CH₃COO)₂+chelating agent CH₃COCH₂COR(R = CH₃ or OC₂H₅) in a composition of equal molar ratio. Fourier transform infrared and fast atom bombardment mass spectrometry analyses suggested that the chelating agent substituted for one of the OR groups in Ti (OR)₄ to form Ti (OR)₃ (CH₃COCHCOR). The gelation time varied from 3 to 5 months and diminished with increasing steric hindrance. The amorphous gel was crystallized into cubic phase BaTiO₃ upon heating above 650°C. The tetragonal phase was obtained after heating for 1 h at 1350°C with the theoretical Ba/Ti ratio and 1.0096 c/a value. The measured dielectric constants diminished with increasing grain size. The results illustrated the merits of altering the chemistry of the precursors to control the properties of the BaTiO₃.     

Influence of stoichiometry and impurity on the sintering behaviour of barium titanate ceramics

The influence of stoichiometry, i.e. Ba/Ti ratio, and impurity on the densification of BaTiO₃ were investigated. The BaTiO₃ powders were prepared by conventional calcination of BaCO₃ and TiO₂. The stoichiometric ratios (Ba/Ti) were in the range 0.99 to 1.005. Impurity effects on the sintering behaviour were investigated with different purities of raw powders. The sintering behaviour of BaTiO₃ has been studied extensively but an understanding of stoichiometric effects on densification is still incomplete. An excess of TiO₂ lowered the onset temperature of sintering (initial state of sintering — 3% shrinkage). These results indicate that stoichiometric variation of BaTiO₃ affects the initial state of sintering. The rate of densification for a Ti-rich sample was considerably faster than that for a Ba-rich sample. It was a so-called activated sintering. The TiO₂ excess reacts with BaTiO₃ to form Ba₆Ti₁₇O₄₀, which forms with BaTiO₃ a eutectic melt at 1320 °C. The liquid phase, however, enhanced grain growth, not densification.     

Coating of hydroxyapatite on various substrates via hydrothermal reactions of Ca(edta)2- and phosphate

Hydroxyapatite was coated on various substrates such as 12 mol % ceria-doped tetragonal zirconia (12Ce-TZP), 3 mol % yttria-doped tetragonal zirconia (3Y-TZP), alumina, monetite coated titanium (Ti/CaHPO₄) and calcium titanate coated titanium (Ti/CaTiO₃) via hydrothermal reactions of Ca(edta)^2- and 0.05 M NaH₂PO₄ at initial pH 6 and 160–200 °C for 0.5–6 h. Rod-like particles of hydroxyapatite precipitated to form film on the substrates above 160 °C. The morphology of the film changed significantly depending on the characteristics of substrate, i.e. hydroxyapatite entirely coated the surfaces of 12Ce-TZP, Ti/CaHPO₄ and Ti/CaTiO₃ plates, but sparsely deposited on 3Y-TZP and Al₂O₃ plates. Film thickness increased with time (ca. 20 and 90 m on 12Ce-TZP plates for 0.5 and 6 h, respectively, at pH 6 and 200 °C). The adhesive strength of the film for the substrate was in the order, 12Ce-TZP/hydroxyapatite(28 MPa) > Ti/CaTiO₃/hydroxyapatite (22 MPa) > Ti/CaHPO₄/hydroxyapatite (9 MPa). © 2001 Kluwer Academic Publishers     

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.     

Preparation via gelling of porous Li2ZrO3 for fusion reactor blanket material

Crystalline powders of Li₂ZrO₃ were prepared by gelling ZrCl₄ and CH₃COOLi with NH₄OH at 50 °C, at different pH values (5, 7 and 8) and for different times (3, 12 and 24 h), then drying and calcining. Compacts from these powders were found sinter to higher densities than a commercial Li₂ZrO₃; their porosity at 1200°C was close to 3 m and was suitable for blanket material applications. Tritium release as HTO begins at 300 °C and reaches 73% at 400 °C, a temperature much lower than that required for Li₂O.     

Precipitation of ultrafine powders of zirconia polymorphs and their conversion to MZrO3 (M = Ba,Sr, Ca) by the hydrothermal method

Ultrafine powders of ZrO₂ with a high degree of crystallinity and chemical purity are hydrothermally precipitated from aqueous solutions of impure zirconyl oxychloride or the acid extract of zircon (ZrSiO₄)-frit at 2 to 8 MPa and 180 to 230 ° C. Monoclinic ZrO₂ is produced from aqueous hydrochloric acid, whereas tetragonal ZrO₂ is formed from the same medium when sulphate ions are present with [SO ₄ ^2– ]/[Cl–] 0.08. If cation impurities such as Y³⁺ or Ce³⁺ are incorporated, the stability range of the tetragonal phase is extended to higher temperatures. Ultrafine powders are characterized by X-ray broadening methods, TEM and thermally as well as mechanically induced transformation characteristics. The tetragonal ZrO₂ powder is constituted of polydomain crystallites with higher hydroxyl ion content than the monoclinic phase. Both series of powders convert to BaZrO₃, SrZrO₃ or CaZrO₃ perovskites when suspended in the corresponding hydroxide solution at 190 to 480 °C and 2 to 100 M Pa.     

Characteristics of powder and sintered bodies of hydrothermally synthesized Mn-Zn ferrites

To improve the sinterability of powders fabricated by the conventional mixed-oxides method, ultrafine Mn-Zn ferrite powders were hydrothermally synthesized from metal nitrates solution using ammonia as a precipitant. The R value (alkalinity) was introduced to adjust the amount of added OH^– in the reaction suspension. The characteristics of the powders synthesized at different hydrothermal conditions and the properties of the sintered bodies were investigated. The results show that the R value and hydrothermal time have a great effect on the compositions and phases of hydrothermally synthesized Mn-Zn ferrite powders. Powders synthesized from a starting suspension with a higher content of Zn ions (or lower content of Mn²⁺) may approach to a stable spinel structure with a lower Mn/Zn ratio as the hydrothermal time is longer. Factors affecting the position of the diffraction angle (2) of the spinel Mn-Zn ferrite (311) of powders may include both the compositions of spinel ferrite structure and crystallite sizes (or particle sizes) of powders. Some possible reasons were suggested to explain the dependence of composition and phase of hydrothermally synthesized Mn-Zn ferrite powders on the R value and hydrothermal time. The temperature that the green compact begins to shrink at increases with increasing R value, and ranges from 510°C (R = 2) to 650°C (R = 6). After being sintered at 950°C for 2 h in N₂ atmosphere, the relative sintered density of each specimen reaches a value of 94.5–99.8%.