Investigations of a Diol-based Sol-Gel Process for the Preparation of Lead Titanate Materials

Lead titanate sols were prepared by a sol-gel method from lead acetate trihydrate, Pb(OOCCH₃)₂3H₂O, and titanium di-isopropoxide bis-acetylacetonate, Ti (OC₃H₇)₂- (CH₃COCHCOCH₃)₂, in 1,3-propanediol, HO(CH₂)₃OH, as solvent. IR analysis and NMR spectroscopic techniques were used to study the reaction mechanisms and to identify possible polymer structures developed during synthesis of the solutions.     

Synthesis and Characterization of (Ce0.67Tb0.33)MnxMg1−xAl11O19 Phosphors Derived by Sol–Gel Processing

A (Ce_0.67Tb_0.33)Mn _x Mg_{1− x} Al₁₁O₁₉ phosphor powder was synthesized, using a simple sol–gel process, by mixing citric acid with CeO₂, Tb₄O₇, Al(NO₃)₃·9H₂O, Mg(OH)₂·4MgCO₃·6H₂O, and Mn(CH₃COO)₂. The phosphor crystallized completely at 1200°C, and the phosphor particle size was between 1 and 5 μm. The excitation spectrum was characteristic of Ce³⁺, while the emission spectrum was composed of lines from Tb³⁺ and Mn²⁺. The Mn²⁺ gave a green fluorescence band, and concentration quenching occurred when x > 0.10. The luminescent properties of the phosphor were explained by a configurational coordinate model.     

Hydrothermal Processing of PbTiO3 and PbTiO3/Polymer Thin Films from Titanium Metallo-organic Precursors

PbTiO₃ and PbTiO₃/polymer thin films were synthesized from metallo-organic precursors on metallized quartz substrates. Titanium dimethoxy dineodecanoate (TDD) was spin-cast onto the substrates and converted to polycrystalline TiO₂ via hydrolysis in deionized water for 5 h at 80°C. Polycrystalline PbTiO₃ films were then formed by reacting the TiO₂ films for 4 h at 200°C in aqueous solutions of KOH and Pb(CH₃COO)₂·3H₂O. Low KOH concentrations suppressed film coarsening, thereby facilitating the formation of fine-grain continuous PbTiO₃ films. PbTiO₃/polymer thin films were processed as above after first dissolving TDD and a polystyrene/polybutadiene block copolymer in p -xylene. PbTiO₃ and PbTiO₃/polymer films had relative permittivities of ≈56 and 34 and dielectric strengths of ≈250 and 850 kV/cm, respectively.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

Electroceramics from Source Materials via Molecular Intermediates: PbTiO3 from TiO2 via [Ti(catecholate)3]2-

The precursor [NH₄]₂[Ti(catecholate)₃] · 2H₂O is known to react with Ba(OH)₂· 8H₂O in an acid/base process that generates Ba[Ti(catecholate)₃] · 3H₂O, a compound which undergoes low-temperatue calcination to produce BaTiO₃ powder. Attempts to develop similar routes to PbTiO₃ have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb₆O(OH)₆⁴⁺, are both insufficiently basic to react with [NH₄]₂[Ti(catecholate)₃] · 2H₂O. Based on the large sizes of both the [Ti(catecholate)₃]^2- anion and the Pb²⁺ cation, a precipitation method has been developed in which lead nitrate and [NH₄]₂[Ti(catecholate)₃] · 2H₂O are added together in an aqueous medium causing precipitation and leaving only NH₄NO₃ in solution. The lead-titanium-catecholate complex that forms in this manner undergoes low-temperature pyrolysis to produce PbTiO₃. SEM indicates a submicrometer ultimate crystallite size.     

Electroceramics from Source Materials via Molecular Intermediates: BaTiO3 from TiO2 via (Ti(catecholate)3)2-

Rutile or anatase may be depolymerized and complexed by sequential treatment with (i) H₂SO₄/(NH₄)₂SO₄, (ii) H₂O, and (iii) catechol/NH₄OH to produce the intermediate (NH₄)₂(Ti(catecholate)₃) · 2H₂O. Treatment with Ba(OH)₂· 8H₂O leads to an acid-base reaction generating Ba(Ti(catecholate)₃) · 3H₂O, in which the Ba:Ti ratio is held at 1:1 at the molecular level. Calcination produces BaTiO₃ powder.     

Synthesis and Properties of Dielectric Ceramics from Metallo-Organic Precursors through the Solid-State Reaction of Mixed Gels

A perovskite structure of 0.4Pb(Mg_1/3Nb_2/3)O₃·0.3Pb(Mg_1/2W_1/2)O₃·0.3PbTiO₃ was prepared from metallo-organic precursors through the solid-state reaction of the mixed gels. Three types of mixed gels were crystallized to obtain PbTiO₃, MgNb₂O₆, and MgWO₄ powders. These powders were calcined at 900°C after mixing with a stoichiometric amount of Pb(CH₃COO)₂·3H₂O. The dielectric constant of the ceramic fired at 900°C was improved by adding an excess of 10 mol% Mg(OC₂H₅)₂, and the ceramic achieved X7T specification of the Electric Industries Association standard. The dielectric loss was reduced by adding an excess of 5 mol% Pb(CH₃COO)₂·3H₂O.     

Synthesis of Phase-Pure Pb(ZnxMg1–x)1/3Nb2/3O3 up to x= 0.7 from a Single Mixture via a Soft-Mechanochemical Route

Phase-pure perovskite Pb(Zn _x Mg_{1– x} )_1/3Nb_2/3O₃ solid solution (PZ _x M_{1– x} N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)₂, Nb₂O₅, and 2ZnCO₃·3Zn(OH)₂·H₂O at 1100°C for 1 h. Percent perovskite ( f _P) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f _P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×10⁴ and 1.7×10⁴ for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed.     

Solid-State Synthesis of LaCoO3 Perovskite Nanocrystals

A solid-state reaction process has been developed to synthesize perovskite-type LaCoO₃ nanocrystals with grain diameters of 15–40 nm. In the first step of the preparation, ∼5 nm composite hydroxide nanoparticles are synthesized by the solid-state reaction of La(NO₃)₃· n H₂O and Co(NO₃)₂·6H₂O with KOH at ambient temperature. A perovskite-type rhombohedral LaCoO₃ phase appears at 550°C, after the hydroxide has been calcined at various temperatures. The phase transformation process is complete at ∼800°C, yielding a single-phase binary oxide. The results indicate that the new process is convenient, inexpensive, and effective for obtaining LaCoO₃ nanocrystals with high yield.     

Influence of Citric Acid on Reactions in the System 3CaO·Al2O3-CaSO4·2H2O-CaO-H2O

The influence of citric acid on paste hydration of 3CaO· Al₂O₃ in the presence of CaSO₄·2H₂O and Ca(OH)₂ was studied using X-ray diffraction, scanning electron microscopy, and conduction calorimetry. The time at which the citric acid is added (either prior to or with the mixing water) determines how it affects the reactivity of the aluminate. Immediately after the paste is gaged citric acid promotes a more rapid reaction, but later reactions are retarded. Hexagonal calcium aluminate hydrates, ettringite, and monosulfate were all detected as early hydration products. The influence of citric acid on the hydration of 3CaO·Al₂O₃ slabs immersed in saturated CaSO₄·2H₂O solutions was also studied and a reaction scheme proposed.     

Nanosized Zinc-Oxide Particles Derived from Mechanical Activation of Zn5(NO3)2(OH)8·2H2O in Sodium Chloride

Nanosized ZnO particles are successfully synthesized via mechanical activation of a zinc nitrate hydroxide hydrate (Zn₅(NO₃)₂(OH)₈·2H₂O) precursor in NaCl matrix for 15 h. The ZnO particles obtained are in the nanosize range of ∼20 nm, with a well-established hexagonal morphology. They compare favorably with those derived from conventional calcination of the precursor. The decomposition of Zn₅(NO₃)₂(OH)₈·2H₂O precursor and formation of nanocrystalline ZnO cannot be completed by mechanical activation in the absence of NaCl, which acts as both an effective dispersing matrix and drying agent although it remains chemically inert during mechanical activation. The powder derived from calcination at 400°C does not possess powder characteristics comparable to that of the powder derived from the mechanical activation in NaCl, because of the extensive particle coarsening and aggregation at the calcination temperature.     

Ionic Conductivity Enhancement of La2Mo2−xWxO9 Nanocrystalline Films Deposited on Alumina Substrates by the Sol–Gel Method

Dense, crack-free, and uniform La₂Mo_{2− x} W _x O₉ ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly-alumina substrates via a modified sol–gel method, with inorganic salt of La(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, and (NH₄)₆H₂W₁₂O₂₄ as precursors. Pure La₂Mo₂O₉ phase was confirmed by X-ray diffractometer when the annealing temperature was >500°C. The average grain size of the La₂Mo_{2− x} W _x O₉ films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin-coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W-doping can suppress the phase transition that occurs at 580°C in pure La₂Mo₂O₉ and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La₂Mo₂O₉ films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain.     

Sol–Gel Processing and Microwave Characteristics of Ba(Mg⅓Ta⅔)O3 Dielectrics

The sol–gel method has been developed for the preparation of pure Ba(Mg_1/3Ta_2/3)O₃ ceramics. This involves the reaction of the heterometallic alkoxide Ta₂Mg(OEt)₁₂ with hydrated barium hydroxide Ba(OH)₂·8H₂O. Complete crystallization of the sol–gel-derived powder is achieved at 600°C, leading to a cubic perovskite type phase. After sintering at 1400°C (2–5 h), a trigonal cell arises from Mg–Ta ordering (the degree of order is greater than 0.9), and about 98.5% of the theoretical density is obtained. Preliminary microwave dielectric measurements show that the dielectric constant and the unloaded Q _u of the ceramics are 24.2 and 6750, respectively, at 7.7 GHz.     

Formation and Transformation of WO3 Prepared from Alkoxide

Amorphous WO₃ or WO₃ or H₂O is formed by hydrolysis of tungsten ethoxide. The temperature of hydrolysis influences the crystallization of WO₃·H₂O. Tungsten hydrate (WO₃·H₂O) has an orthorhombic unit cell with a=0.5235 nm, b = 1.0688 nm, and c=0.5123 nm. Orthorhombic WO₃ crystallizes at 350° to 500°Cfrom amorphous WO₃. Cubic WO₃ is formed at 200° to 310°C with dehydration of WO₃·H₂O. WO₃ transformations are examined by high-temperature X-ray diffraction. The kinetics of formation of the cubic modification have been studied by measuring the weight decrease with a thermobalcnce. Formation isotherms can be interpreted in terms of the first-order equation –In (1–f)=kt; activation energies are 110 and 80 kJ mol⁻¹ for initial and final stages, respectively.     

Preparation of Lead Zirconate Titanate (Pb(Zr0.52Ti0.48)O3) by Homogeneous Precipitation and Calcination

Preparation of phase-pure PZT (Pb(Zr_0.52Ti_0.48)O₃) powders was achieved, in the presence of urea (CH₄N₂O), by homogeneous precipitation. Aqueous solutions of PbCl₂, ZrOCl₂·8H₂O, and TiCl₄ were used as the starting materials in the synthesis of phase-pure PZT powders. Phase evolution behavior of precursor powders was studied by powder X-ray diffraction (XRD) in air, over the temperature range of 90° to 750°C. The morphology of the formed powders was studied by scanning electron microscopy (SEM). Semiquantitative chemical analyses of the samples were performed by energy-dispersive X-ray spectroscopy (EDXS).     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Formation of Calcium Phosphate Whiskers in Hydrogen Peroxide (H2O2) Solutions at 90°C

A novel, one-pot technique of synthesizing calcium phosphate whiskers was developed. Commercially available β-tricalcium phosphate (β-Ca₃(PO₄)₂) powders were aged in unstirred 30% H₂O₂ solutions at 90°C for 48 h in ordinary glass media bottles. Resultant samples consisted of whiskers (200 nm wide and 5 μm-long) of a biphasic mixture of octacalcium phosphate (OCP: Ca₈H₂(PO₄)₆·5H₂O) and carbonated apatitic (apatite-like) calcium phosphate (Ap-CaP). As-formed whiskers possessed a Ca/P molar ratio of 1.46 and a BET surface area of 8 m²/g. Upon soaking these whiskers in a Tris-HCl-buffered SBF solution of 27 mM HCO₃⁻ for 6 days, Ca/P molar ratio and surface area values were increased to 1.60 and 52 m²/g, respectively. The technique, owing to its simplicity, may prove useful in providing large amounts of biocompatible short whiskers for numerous technology sectors.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

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.     

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.