Synthesis of Oxynitride Powders via Fluidized-Bed Ammonolysis, Part I: Large, Porous, Silica Particles

Reaction of silica (SiO₂) with triethanolamine (TEA, N(CH₂CH₂OH)₃) and ethylene glycol (EG) under conditions (∼200°C) where byproduct water is removed resulted in the formation of the neutral silatrane glycolate complex, N(CH₂CH₂O)₃SiOCH₂CH₂OH (or TEASiOCH₂CH₂OH) in essentially quantitative yield. Solutions of this neutral precursor in EG, when rapidly pyrolyzed and then oxidized at 500°C, formed porous ceramic powders with high specific surface areas (>500 m²/g). These powders were nitrided via ammonolysis in a fluidized-bed reactor at temperatures of 700°-1000°C. The resulting nitrided powders were characterized by thermal and chemical analyses, diffuse reflectance infrared spectroscopy, gas sorption, and X-ray photoelectron spectroscopy. The apparent activation energy for the nitridation process was determined to be 54 kJ/mol. Following nitridation, the powders were amorphous and had nitrogen contents as high as 21 wt% with retained surface areas >300 m²/g at 1000°C. Under the nitridation conditions used, the extent of nitrogen incorporation correlated linearly with increases in material density. This linearity suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced divalent oxygen in the glass structure and nominally because of viscous flow. The linear density increase also suggested that pore trapping did not occur under these processing conditions. This work serves as a model for ongoing studies on the nitridation of high-surface-area ceramic powders produced by the rapid pyrolysis of mixed-metal TEA alkoxides.     

Growth of β-Alumina (Na2O · 11Al2O3) Single Crystals by the Flux Method

The crystal-growth process and growth conditions of β-alumina (Na₂O · Al₂O₃) were investigated using the Na₂B₄O₇-Na₃AlF₆ flux method. β-Alumina (electric fusion brick) was used as both nutrient and seed. Weight loss of the flux varied widely for various runs: ≅ 10 wt% of flux evaporated at 100 h, ≅ 17 wt% at 150 h, and 43 wt% at 600 h. When β-alumina crystal was grown, only 20 wt% Na₂B₄O₇ was added to the Na₃AlF₆ flux. The linear growth rates of the β-alumina single crystal grown by an Na₃AlF₆-20 wt% Na₂B₄O₇ flux method at 1040°C and Δ t = 18°C were ≅ 1.0 × 10⁻³ mm/h ( a face) and ≅0.3 × 10⁻³ mm/h ( c face). The β-alumina single crystals grown were bounded by only c [001] and a [100] and were colorless and transparent.     

High-Yield Synthesis of B4C/BN Ceramic Materials by Pyrolysis of Polymeric Lewis Base Adducts of Decaborane(14)

Polymers of type [B₁₀H₁₂˙diamine]x (diamine=H₂NCH₂CH₂NH₂, (CH₃)₂NCH₂CH₂NH₂, (CH₃)₂NCH₂CH₂N(CH₃)₂, etc.) have been found to be useful ceramic precursors. In a stream of argon, their pyrolysis gives B4C/BN; in a stream of ammonia, BN.     

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.     

Effect of Water Content on Transport in Na2O·3SiO2 Glass

Na₂O·₃SiO₂ glasses of high water content were prepared under high-pressure, hydrothermal conditions. The sodium diffusion coefficient, DNa , in these glasses measured at 100°C depended strongly on H₂O content. With increasing H₂O content, DNa . at 100°C decreased initially to a minimum at 3∼4 wt% H,O and then increased. This behavior of DNa . Was similar to that of dc conductivity.     

Low-Temperature Sintering and Piezoelectric Properties of CuO-Added 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

The sintering temperature of 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ (NKN–BT) ceramics needs to be decreased below 1000°C to prevent Na₂O evaporation, which can cause difficulties in poling and may eventually degrade their piezoelectric properties. NKN–BT ceramics containing CuO were well sintered at 950°C with grain growth. Poling was easy for all specimens. Densification and grain growth were explained by the formation of a liquid phase. The addition of CuO improved the piezoelectric properties by increasing the grain size and density. High piezoelectric properties of d ₃₃=230 pC/N, k _p=37%, and ɛ₃^T/ɛ₀=1150 were obtained from the specimen containing 1.0 mol% of CuO synthesized by the conventional solid-state method.     

Low-Temperature Sintering of Lead Zinc Niobate–Lead Zirconate Titanate Ceramics Using Nano-Sized Powder Prepared by the Stirred Media Milling Process

Lead zinc niobate–lead zirconate titanate (PZN–PZT) nano-sized powders with a diameter of ∼35 nm were fabricated by a high-energy stirred media mill using 50 μm diameter zirconia beads as the milling media at a rotation speed of 4000 rpm for 1 h. The sintering temperature of PZN–PZT was greatly reduced, and a fully densified bulk body was obtained at only 750°C when stirred media milled nanopowder was used. The control of evaporation of lead oxide was very important to obtain high electrical properties due to the increased surface area of nano-sized powders. The ferroelectric hysteresis, piezoelectric d ₃₃ coefficient, and dielectric properties of sintered ceramics using nanopowder were measured and compared with the values obtained from a sintered specimen using conventional milled powders. Remanent polarization, d ₃₃ coefficient, and relative dielectric constant of 750°C sintered stirred media milled powders containing 2% of excess PbO and 1% of 4PbO–B₂O₃ liquid phase were 10.3 μC/cm², 277 pC/N, and 1310, respectively.     

Phase Equilibria in the System Na2O – Nb2O5

Phase equilibria data, obtained both by differential thermal analysis and by quenching, are presented for the system Na₂O-Nb₂O₅. Five compounds corresponding to the formulas 3Na₂O.1Nb₂0₆, lNa₂O. 1Nb₂O₅, lNa₂O 4Nb₂O₆, lNa_zO.7Nb₂O₅, and lNa₂O. 10Nb₂O₆ have been found. The compound 3Na_z0.lNb₂O₅ melts congruently at 992°C. The compounds 1Na₂O. 4Nb₂O₆, lNa₂O.7Nb₂O, and 1Na₂O. 1Onb₂O₅ melt incongruently at 1265°, 1275°, and 1290°C., respectively. The well-known perovskite structure phase NaNbO₃ was found to melt congruently at 1412°C. The transition temperatures in NaNbO₅ were checked by thermal analysis and only the major structural changes at 368° and 640°C. could be detected. A new disordered form of NaNbO₃ could be preserved to room temperature by very rapid quenching.     

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.     

Conduction and Dielectric Loss Mechanisms in β-Alumina and Glass: A Discussion Based on the Paired Interstitialcy Model

A paired interstitialcy model is used as a basis for qualitative comparisons of conductivity and dielectric phenomena in β-alumina crystals and in glass. Thus, the increase in the conductivity of sodium silicate glasses with increasing Na₂O activity can be explained if the concentration of (Na₂*)²⁺ interstitial pairs increases with increased polarizability of O^2- ions, expressed in terms of the optical basicity parameter, Δ. Similarly, the occurrence of the pronounced minima in conductivity isotherms (the mixed-alkali effect in glass) is attributed to disappearance of mobile interstitial pairs, e.g. (Li₂*)²⁺ or (K₂*)²⁺, and the stabilization (by polarization interactions) of apparently immobile mixed-alkali pairs, (LiK*)²⁺. The phenomenon of coionic conduction in certain β-alumina crystals is an interesting departure from this general pattern. The orientation dependence of the electrical modulus spectrum of monocrys-talline β-alumina highlights the presence of a bimodal distribution of relaxation times, in which the low-frequency component ( v ₀=10¹¹ Hz) may arise from the rearrangement of interstitial pairs and the high-frequency component ( v ₀=2×10¹² Hz) may arise from less hindered ionic motions. It is suggested that the motions of interstitial pairs and surrounding cations are mutually catalytic and that some form of combined motion is responsible for both the electrical and mechanical relaxations in β-alumina and glass.     

Preparation of Potassium Tantalate Niobate by Sol–Gel Method

Perovskite potassium tantalate niobate (KTN) powders and thin films were synthesized from a metal alkoxide solution. Homogeneous KTN coating solutions were prepared from KOC₂H₅, Ta(OC₂H₅)₅, and Nb(OC₂H₅)₅ in absolute ethanol. The precursor crystallized to pyrochlore at ∼ 650°C, and then to perovskite at ∼ 750°C, depending upon Ta:Nb ratio. H₂O vapor during calcination was found to play a prominent role in the direct and predominant crystallization of perovskite films with preferred orientation. Highly oriented KTN films of perovskite structure were successfully prepared on MgO (100) substrates at 675°C.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Phase Formation in the System Na2O · Al2O3-CaO · A12O3-Al2O3 at 1200°C in Air

Phase relations in the system Na₂O_· Al₂O₃-CaO· Al₂O₃-Al₂O₃ at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na₂O · 3CaO · 5Al₂O₃, known from the literature, was reformulated as Na₂O · CaO · 2Al₂O₃. A new compound with the probable composition Na₂O · 3CaO · 8Al₂O₃ was found. Cell parameters of both compounds were determined. The compound Na₂O · CaO-2Al₂O₃ is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na₂O · Al₂O₃ up to 38 mol% Na₂O at 1200°C. The compound Na₂O · 3CaO · 8Al₂O₃ is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al₂O₃ is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between β and β "-Al₂O₃ make it very difficult to determine equilibrium phase relations in the high-Al₂O₃ part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram.     

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.     

Preparation of Small-Particle-Size, Semiconductor CdS-Doped Silica Glasses by the Sol–Gel Process

The sol–gel process has been applied successfully to the preparation of small-particle-size CdS-doped silica glasses with a significant quantum size effect. Gels prepared through the hydrolysis of a complex solution of Si(OC₂H₅)₄ and Cd(CH₃COO)₂·2H₂O were heated at 500°C, then reacted with H₂S gas to form fine, hexagonal, CdS-microcrystal-doped glasses. The optical absorption edge is blue shifted by ∼0.4 eV compared with the bulk absorption value of CdS crystal. This result is interpreted in terms of a quantum-confinement effect of small crystal size.     

Morphological Forms of α-Alumina Particles Synthesized in 1,4-Butanediol Solution

Phase-pure, monodispersed, hexagonal plates of single-crystal α-alumina (∼ 2 μm wide and ∼0.5 μm thick) have been prepared via precipitation by treating an aluminum hydrous oxide precursor in 1,4-butanediol at 300°C under autogenous vapor pressure. Present work shows that KOH is the only reagent that precipitates an aluminum hydrous oxide precursor suitable to synthesize α-alumina in 1,4-butanediol solution. In contrast, the use of NaOH or NH₄OH as the precipitating reagent for the precursor material does not yield the alpha phase. The solution pH at which the precursor materials are precipitated is also a critical factor for the formation of α-Al₂O₃. Phase-pure α-alumina powders were also only synthesized from the aluminum hydrous oxide precursors precipitated in the pH range from 10 to 10.5. The results of X-ray diffraction and scanning electron microscopy indicate that longer reaction times promote the phase transformation from the intermediate boehmite phase to α-alumina. The complete transformation from boehmite to α-alumina requires reaction times of about 12 h.     

Group II Tris(glycolato)silicates as Precursors to Silicate Glasses and Ceramics

Group II tris(glycolato)silicates, MSi(OCH₂CH₂O)₃ (where M = Ba, Ca, Mg), can be synthesized directly by reaction of silica with ethylene glycol and alkaline-earth (group II) oxides at 200°C. These hexa-alkoxy silicates serve as precursors to silicate glass and ceramic powders. They are readily modified by exchange with longer-chain diols into processable polymer precursors. These Theologically useful precursors may provide access to silicate or aluminosilicate powders, thin fllms, fibers, and coatings. Thus, we have examined the utility of hexacoordinate glycolatosilicates as model precursors. Pyrolysis of the compounds, MSi(OCH₂-CH₂O)₃, in air transforms them to their anticipated ceramic products, MO-SiO₂. The phase transformations and chemical changes that occur during pyrolysis were characterized using X-ray powder diffractometry (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and scanning electron microscopy (SEM). The hexacoordinate glycolatosilicates oxidatively decompose at ∼300°C to form amorphous materials. Moderate to significant quantities of the group II carbonates, MCO₃ (15–50 wt%), form coincidentally as the amorphous intermediates trap CO₂ generated by ligand oxidation. At ∼900°C, the amorphous materials crystallize into the expected, phase-pure, MO-SiO₂.     

Sodium Carbonate Flux Effects on the Luminescence Characteristics of (Y0.5Gd0.5)2O3:Eu Phosphor Particles Prepared by Spray Pyrolysis

Na₂CO₃ flux was introduced in the preparation of phosphor particles by spray pyrolysis to improve the photoluminescence (PL) characteristics of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles. The phosphor particles directly prepared by spray pyrolysis at 1300°C from solutions with 20 wt% Na₂CO₃ flux had the highest PL intensity, which corresponded to 130% of that of particles prepared from solution without flux. On the other hand, the maximum PL intensity of the annealed particles, which were as-prepared at 900°C and posttreated at 1200°C for 3 h, was obtained from a solution with 5 wt% Na₂CO₃ flux. The maximum PL intensity of particles directly prepared by spray pyrolysis without posttreatment was 86% of that of posttreated phosphor particles. Na₂CO₃ flux was also important in control of morphology of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles.     

Low-Temperature Sintering and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

(Na_0.5K_0.5)NbO₃ (NKN) ceramic with 1.5 mol% CuO added (NKNC) was well sintered even at a low temperature of 900°C with the addition of ZnO. Most of the ZnO reacted with the CuO and formed the liquid phase that assisted the densification of the specimens at 900°C. A few Zn²⁺ ions entered the matrix of the specimens and increased the coercive field ( E _c) and Q _m values of the specimens. High-piezoelectric properties of k _p=0.37, Q _m=755, and ɛ₃ ^T /ɛ₀=327 were obtained from the NKNC ceramics containing 1.0 mol% ZnO sintered at 900°C for 2 h.     

Synthesis and Structural Characterization of a Metastable Mullite-Like Alumina Phase

A solution combustion technique for the synthesis of different β-alumina compositions in the Na₂O· x Al₂O₃ system (where x =5, 6, or 7) is described along with the structural characterization of the materials prepared. The amorphous powder obtained after a combustion reaction between the nitrate salts of the cations and aminoacetic acid was calcined in air at different temperatures in the range from 600°C up to 1300°C. The phases were investigated by powder X-ray diffraction (XRD) and infrared spectroscopic measurements. A metastable mullite-like alumina phase was found to form as an intermediate at a minimum calcination temperature of 750°C and stable up to 1000°C, which transformed completely into β/β"-alumina phases beyond a temperature of 1100°C. The crystal structure of the mullite-like alumina phase was deduced by rietveld refinement of slow scan powder XRD data. A better understanding of the crystal structure of the mullite-like alumina was possible using other supplementary experimental evidences.