Alkoxide-Hydroxide Route to Syntheltize BaTiO3-Based Powders

When preparing homogeneous, fine barium titanate powders, the major difficulty is to avoid the spontaneous self-condensation between the Ti-OH groups. In the usual way of preparing fine barium titanate powders, chelating agents (citrate, oxalate) or simply unidentate ligands (alkoxy or carboxyl groups) are used to complex titanium atoms. Another way is to mix barium and titanium precursors in a strongly basic medium. The condensation between the Ti(OH)^2-₆Ba²⁺ species directly gives the perovskite compound. Using an alkoxide-hydroxide route, a homogeneous Ba-Ti solution was prepared that completely advanced by condensation between the Ti(OH)^2-₆Ba²⁺ species and led to a controlled-stoichiometry powder. Concerning pure barium titanate, dried powders exhibited the cubic perovskite structure, and a direct sintering at 1150°C, without calcination, led to highly dense BaTiO₃ bodies with fine-grained uniform microstructure (1 μm) that exhibited a high permittivity value at room temperature ( K = 5400). The alkoxide-hydroxide method was also used to prepare dense alkaline-earth perovskite ceramics with complex compositions.     

Effect of pH on the Chemistry of the Barium Titanium Citrate Gel and Its Thermal Decomposition Behavior

In this investigation, X-ray diffractometry and ¹³C NMR spectroscopy were used to characterize the effect of pH on the formation of gel and its thermal decomposition by the Pechini process. It was found that the major effect of pH was to destroy the esterification between citric acid (CA) and ethylene glycol, which in turn could influence Ba species and the formation of a mixed-metal CA complex. It was also found that barium carbonate was derived from the Ba species and was not related to the instability of Ba₂Ti₂O₅CO₃, but Ba₂Ti₂O₅CO₃ was formed by the thermal decomposition of a mixed-metal CA complex. Moreover, BaTiO₃ formed via two routes, the reaction of BaCO₃ with Ti species and thermal decomposition of Ba₂Ti₂O₅CO₃.     

GC-MS and 13C NMR Investigation of Lead Zirconate Titanate Precursor Sols for Fiber Preparation

Different macroscopic properties of PZT fibers have been obtained when using acetic acid and methacrylic acid to modify the PZT precursor. In order to clarify the role of the acids the molecular structure of the acidified PZT precursors was investigated and compared by gas chromatography-mass spectrometry, Fourier transform infrared, ¹³C nuclear magnetic resonance (NMR) spectroscopy (solution and solid state ¹³C NMR) and the reason for obtaining long PZT fibers is discussed. The results indicate that when methacrylic acid was used, long gel and ceramic fibers have been obtained because strongly co-ordinating carboxylate groups of methacrylic acid were formed. Linear chains, like those of methacrylic acid propyl ester and methacrylic acetate, have been formed in the PZT precursor sols. In addition, after heat treatment the polymer decomposed quickly so that pure perovskite could be obtained at low temperature in the PZT fibers. When acetic acid was used short fibers were obtained. Acetic acid may act as chelate agent to form oxo acetate in the precursors; this oxo acetate nature also resulted in PZT fibers drawing. However, the longest gel and ceramic fibers have been prepared from precursors with methacrylic acid.     

Polymerized Complex Route to Synthesis of Pure Y2Ti2O7 at 750°C Using Yttrium-Titanium Mixed-Metal Citric Acid Complex

A polymerized complex technique was used to prepare high-purity pyrochlore Y₂Ti₂O₇ powders at 750°C. Heating of a mixed solution of citric acid (CA), ethylene glycol (EG), and yttrium and titanium ions, with a molar ratio of CA:EG:Y:Ti = 5:20:1:1, at 130°C produced a yellowish transparent polymeric gel without any precipitation; this material was used subsequently as a precursor for Y₂Ti₂O₇. Based on the results of ¹³C-NMR spectroscopy, it was suggested that a mixed-metal (Y,Ti)-CA₃ chelate complex formed in a starting CA/EG solution. The formation of pure pyrochlore Y₂Ti₂O₇ occurred when the precursor was heat-treated in a furnace set at 750°C in static air for 4 h.     

Effects of Molar Ratio of Citric Acid to Cations and of pH Value on the Formation and Thermal-Decomposition Behavior of Barium Titanium Citrate

Several experiments were conducted to examine the effects of the molar ratio of citric acid to cations and of the pH value on the chemistry of a solution prepared by the Pechini method. Two types of precipitates, barium titanium citrate (BTC) and barium citrate, developed, depending on the experimental conditions. Characterization of BTC by FT-IR spectroscopy and solid-state ¹³C-NMR spectroscopy indicated that barium and titanium ions were chelated simultaneously by the central deprotonated alcoholic ligands and by dissociated carboxylic acid groups with a unidentate type of three citric acid molecules. The possible coordinated structure of BTC was proposed. The thermal decomposition behavior of BTC was investigated by powder X-ray diffractometry, FT-IR spectroscopy, and solid-state ¹³C-NMR spectroscopy. During decomposition, the nature of bonding between carboxylate groups and cations changed in the order unidentate → bridging → ionic, and carbonate species were detected at 500°C. A small amount of BaCO₃ and the intermediate oxycarbonate coexisted at 550°-600°C.     

Structural Investigations of Prehydrolyzed Precursors Used in the Sol-Gel Processing of Lead Titanate

A variety of titanate gels were prepared from lead acetate and titanium isopropoxide in methoxyethanol. In the present study, ¹H and ¹³C Fourier transform-nuclear magnetic resonance and mass spectroscopic techniques were used to identify the structure of the lead and titanium precursors formed from lead acetate and titanium isopropoxide, respectively, when refluxed in methoxyethanol. The lead and titanium precursors were determined to be Pb(OOCCH₃) (OC₂H₄OCH₃). x H₂O, where x < 0.5, and Ti₂(OC₂H₄OCH₃)₈, respectively. Similar spectroscopic procedures were applied to identify the lead titanium complex formed from the reaction between lead and titanium precursors.     

Preparation of BaTi4O9 from Oxalates

A barium titanate precursor with a barium:titanium ratio of 1:4 was prepared by controlled coprecipitation of mixed barium and titanium species with an ammonium oxalate aqueous solution at pH 7. The results of thermal analysis and IR measurement show that the obtained precursor is a mixture of BaC₂O₄·0.5H₂O and TiO(OH)₂·1.5H₂O in a molar ratio of 1:4. Crystallized BaTi₄O₉ was obtained by the thermal decomposition of a precipitate precursor at 1300°C for 2 h in air. The dimensions of the powder calcined at 1000°C are between 100 and 300 nm. The grain dimensions of the sintered sample for 2 h at 1300°C are of the order of 10 to 30 μm. Dielectric properties of disk-shaped sintered specimens in the microwave frequency region were measured using the TE₀₁₁ mode. Excellent microwave characteristics for BaTi₄O₉—ɛ= 38 ± 0.5, Q = 3800–4000 at 6–7 GHz and τ _f = 11 ± 0.7 ppm/°C—were found.     

Crystallinity and Stoichiometry of Nano-Structured Sol-Gel-Derived BaTiO3 Monolithic Gels

The crystallization behavior and stoichiometric changes of barium titanium alkoxide-derived monolithic gels prepared by the sol-gel process using a high-concentration Ba₂Ti precursor solution (0.8 mol/L) were investigated during aging at room temperature. Crystallization of the gels (which were amorphous, per X-ray diffraction analysis immediately after gelation) into the BaTiO₃ perovskite phase increased during aging and was associated with significant shrinkage of the gels. Crystallization reached a value of ∼82% by the final stage of shrinkage, assuming the degree of crystallization of a gel treated at 600°C to be 100%. The stoichiometry of the gels (Ba/Ti molar ratio) also changed considerably during aging, as estimated by the concentrations of Ba and Ti that remained in the expelled liquid resulting from syneresis at any time during the aging process. Deviation in the Ba/Ti ratio of the precursor solution ranged from 0.015 at the initial stage of shrinkage to 0.003 at the final stage, a value determined by inductively coupled plasma atomic emission spectroscopy. The present study demonstrates the great advantage of using high-concentration precursor solutions of barium titanium alkoxides, rather than low-concentration solutions, to obtain BaTiO₃ gel monoliths with high density and crystallinity and little stoichiometric deviation, by sol-gel processing at room temperature.     

Synthesis of Highly Oriented K(Ta,Nb)O3 (Ta:Nb = 65:35) Film Using Metal Alkoxides

Highly oriented K(Ta,Nb)O₃ (Ta:Nb = 65:35) (KTN) thin films of perovskite structure were synthesized successfully on Pt(100)/MgO(100) substrates from a metal alkoxide solution through reaction control. Homogeneous KTN coating solutions prepared from KOC₂H₅, Ta(OC₂H₅)₅, and Nb(OC₂H₅)₅ in ethanol were analyzed by ¹H, ¹³C, and ⁹³Nb NMR spectroscopy. The KTN precursor included a molecular-level mixture of K[M(OC₂H₅)₆] (M = Ta, Nb) units interacting in ethanol solution. X-ray pole figure measurement showed that perovskite KTN films crystallized on Pt(100)/MgO(100) substrates had not only a (100) orientation but also a three-dimensional regularity of grains. The remanent polarization and coercive field of the KTN film (thickness, 1.0 μm) crystallized at 700°C were 1.5 μC/cm² and 8.7 kV/cm, respectively, at 225 K.     

Stoichiometry Control and Phase Selection in Hydrothermally Derived BaxSr1–xTiO3 Powders

Ba _x Sr_{1- x} TiO₃ (BST) powders were processed at temperatures <100°C by reacting nanosized TiO₂ powders in alkaline, aqueous solutions of BaCl₂, SrCl₂, and NaOH. The effects of processing variables (NaOH concentration, time, temperature, and the ratios of barium, strontium, and titanium initially in solution) on the resultant BST powder stoichiometry and solid solubility were examined. In all cases, strontium was more readily incorporated into the BST powders than barium, and the extent varied systematically with the processing variables. BST powders that were processed in solutions with a large initial excess of barium and strontium, relative to titanium, consisted of a single-phase solid solution. In contrast, BST powders that were processed in solutions with a small initial excess of barium and strontium, relative to titanium, contained a biphasic solid solution which corresponded to separate barium-rich and strontium-rich phases.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

Synthesis and Characterization of Silicon Oxycarbide Glasses

It has been found that X-ray amorphous silica glasses containing up to 18% carbon can be synthesized using a sol/gel process. In this study, the sols were prepared using four different organometallic precursors—methyl-, ethyl-, propyl-, and phenyltrimethoxysilanes. ¹H NMR,¹³C NMR, and TGA revealed that the methoxy groups are hydrolyzed in the solutions and, therefore, are absent in the gels. But the alkyl groups are retained in the dry gels. The ²⁹Si NMR data verified that the Si—C bonds associated with these alkyl groups are intact in the dry gels. Most importantly, however, Si—C bonds are found in the glasses obtained after heat-treating the gels at high temperature in an inert atmosphere; i.e., the synthesis does, in fact, create an oxycarbide network structure. TGA showed that the dense oxycarbide glasses are stable to 1000°C in argon and in air.     

Hydrothermal Synthesis of Bismuth Titanate Powders

Bismuth titanate was synthesized under hydrothermal conditions from an amorphous bismuth–titanium precursor gel. The gel was formed by mixing a bismuth acetate complex with titanium butoxide and then adding the solution dropwise into 6 M NaOH. The resulting gel suspension was reacted under hydrothermal conditions at temperatures ranging from 160° to 200°C to form crystalline bismuth titanate. The gel crystallization kinetics increased with temperature, which resulted in 100% crystalline bismuth titanate in 5 h at 200°C. Wavelength-dispersive spectroscopy data indicated that sodium was incorporated into bismuth titanate during processing, and X-ray diffractometry suggested that the powder was composed of the Bi₅Ti₄O₁₅ phase. Transmission electron microscopy micrographs showed that the gel particles decomposed to 100–200 nm crystalline bismuth titanate particles during hydrothermal processing.     

Alternative Route for Synthesis of Barium Titanyl Oxalate: Molecular Precursor for Macrocrystalline Barium Titanate Powders

An important molecular precursor to barium titanate, namely, barium titanyl oxalate [BaTiO(C₂O₄)₂.4H₂O], has been synthesized by an alternative route. An alcoholic solution containing 1 mol of butyl titanate monomer [(C₄H₉O)₄Ti] is reacted with alcoholic solution containing 2 mol of oxalic acid (H₂C₂O₄:2H₂O) to form an intermediate soluble oxalotitanic acid [H₂TiO(C₂O₄)₂.nH₂O]. The oxalotitanic acid in alcoholic medium is subjected to cation exchange reaction with aqueous solution containing equimolar barium acetate to form an insoluble barium titanyl oxalate (BTO) in yields of 80–85% at room temperature. The pyrolysis of BTO in air at T .750°C/5 h produced barium titanate (BT) powders.     

Synthesis of a Mullite Precursor from Aluminum Nitrate and Tetraethoxysilane via Aqueous Homogeneous Precipitation: An 27Al and 29Si Liquid- and Solid-State NMR Spectroscopic Study

A simple aqueous process is described for the preparation of aluminosilicate colloids and chemically homogeneous mullite precursor gel. Starting from a solution of aluminum nitrate and silicic acid, aluminum is slowly hydrolyzed at 80–100°C by in situ generation of ammonia. A silica gel is rapidly made, probably by a catalytic effect of urea, the base generator. This gel is then slowly digested by partially hydrolyzed aluminum species which break the Si-O-Si bonds and link to the gel by Si-O-Al bonds. Progressively a clear colloidal sol is obtained and the colloidal particle size continues to decrease toward aluminosilicate species where the silicon atoms are in a single environment and may be linked to three hexacoordinated aluminum atoms and a hydroxyl group, by reference to natural imogolite. When the hydrolysis of aluminum is nearly complete, these particles are cross-linked and a final gel precursor of mullite is obtained. This gel is chemically very homogeneous and crystallizes to mullite at 980°C. The structural evolution, from the first gel to the ceramic, has been followed by ²⁷Al and ²⁹Si liquid- and solid-state MAS NMR spectroscopy.     

Chemical Solution Deposition of (Pb1−xCax)TiO3 Thin Films with x∼0.5 as New Dielectrics for Tunable Components and Dynamic Random Access Memories

Calcium lead titanate ((Pb,Ca)TiO₃) thin films, with calcium contents of ∼50 at.%, have been prepared by chemical solution deposition (CSD). Different synthetic sol–gel methods have been used for the preparation of the precursor solutions. 1,3-propanediol, OH(CH₂)₃OH, and water, H₂O, were used as solvents. Lead (II) acetate trihydrate, Pb(OCOCH₃)₂·3H₂O, and titanium di-isopropoxide bis(acetylacetonate), Ti(OC₃H₇)₂(CH₃COCHCOCH₃)₂, were used as reagents of lead and titanium, respectively. Calcium was incorporated into the solutions as calcium acetate hydrate, Ca(OCOCH₃)₂· x H₂O, or as calcium acetylacetonate hydrate, Ca(CH₃COCHCOCH₃)₂· x H₂O. Only the use of calcium acetate led to precipitate-free solutions. Pb(II)–Ti(IV)–Ca(II) sols were obtained when calcium acetate was refluxed with the lead and titanium reagents in a diol–water solvent. These sols led to films with a homogeneous compositional profile. Solutions obtained by mixing a water solution of calcium acetate with a Pb(II)–Ti(IV) sol led to films with a heterogeneous compositional profile in which an interface between the film and the Pt bottom electrode is formed. The films derived from the Pb(II)–Ti(IV)–Ca(II) sols have values of dielectric constant at room temperature of ∼500, which, together with their low leakage currents, low dielectric losses, and tunability, make these films promising for dynamic random access memories and tunable devices.     

Crystallization of Fluorite-Type Solid Solution from Alkoxides in the System Y2O3-TiO2

Fluorite-type solid solutions crystallize directly at lower temperatures from amorphous materials prepared by the simultaneous hydrolysis of yttrium and titanium alkoxides. The lattice constant of the solid solution changes continuously as a function of composition. The kinetics of crystallization of the fluorite phase have been studied by X-ray measurements. The initial stage at each temperature proceeds rapidly in a short time. The final stage can be expressed in terms of the contracting cube equation 1—(1 —f)¹¹³= kt, the activation energy being 184 mol⁻¹. A pyrochlore-type solid solution corresponding to the composition Y₂Ti₂O₇ crystallizes at 8l5° to 900°C.     

Sol—Gel Fabrication of Pb(Zr0.52 Ti0.48)O3 Thin Films Using Lead Acetylacetonate as the Lead Source

Lead zirconium titanate (PZT) thin films of the morphotropic phase boundary composition [Pb(Zr_0.52Ti_0.43)O₃] were deposited on platinum-coated silicon by a modified sol-gel process using lead acetylacetonate as the lead source. The precursor solution for spin coating was prepared from lead acetylacetonate, zirconium n -butoxide, and titanium isopropoxide. The use of lead acetylacetonate instead of the widely used lead acetate trihydrate provided more stability to the PZT precursor solution. Films annealed at 700°C for 12 min formed well-crystallized perovskite phase of Pb(Zr_0.52Ti_0.48)O₃. Microstructures of these films indicated the presence of submicrometer grains (0.1 to 0.2 μm). The dielectric constant and loss values of these films measured at 10 kHz were approximately 1200 and 0.04, respectively, while the remanent polarization and coercive field were ∼ 13 μC/cm² and ∼ 35 kV/cm. Aging of the solution had almost no effect on the dielectric and ferroelectric properties of these films.     

A Simple System for the Preparation of Submicrometer Lead Titanate Powders by the Sol-Gel Method

This paper reports a new system for the preparation of lead titanate powders by the sol-gel method. In this system basic lead acetate, (CH₃COO)₂Pb.Pb(OH)₂, is used as the lead precursor instead of the widely used Pb(OOCCH₃)₂.3H₂O and titanium tetrabutoxide monomer. This new system simplifies the chemical processing of precursor solutions of lead titanate, increases their stability in air, and offers good control of Pb:Ti stoichiometry. The xerogel, obtained from the precursor solution by aging at room temperature, is found to have a higher inorganic content. Gel-to-ceramic conversion is achieved by calcining the xerogel at 600°C. The phase purity, particle size, morphology, and compositional homogeneity of the gel-derived powders are examined by XRD, TEM, and ICP-OES.     

Sol–Gel Processing of Lead Titanate in 2-Methoxyethanol: Investigations into the Nature of the Prehydrolyzed Solutions

A previously developed system for the sol–gel processing of lead titanate powders and films has been analyzed by a variety of spectroscopic, chromatographic, and wet-chemical means. The results indicate that refluxing Pb(OAc)₂˙3H₂O in 2-methoxyethanol is accompanied by the volatilization of 0.5 equiv of organic acetate and results in solutions containing what is believed to be anhydrous basic lead acetate, 3Pb(OAc)₂˙PbO. This has been isolated as its monohydrate, which has been fully characterized by elemental (C, H, Pb), infrared, ¹H NMR, and XRD analyses. The anhydrous salt generated in situ reacts further with Ti(OR)₄ under reflux conditions to liberate more organic acetates. The final stoichiometries (one volatilized acetate per lead) tend to preclude the possibility that significant amounts of polymeric intermediates are generated. Analyses of the organic volatiles also indicate that, at least under our conditions, significant trans-alcoholysis of the precursor Ti(OR)₄ (R=Et, i-Pr, n-Pr) occurs (80% to 90% replacement). The conclusions and consequences of these and related observations are presented.