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.     

Fabrication of NiO Nanoparticle-Coated Lead Zirconate Titanate Powders by the Heterogeneous Precipitation Method

NiO nanoparticle-coated lead zirconate titanate (PZT) powders are successfully fabricated by the heterogeneous precipitation method using PZT, Ni(NO₃)₂·6H₂O, and NH₄HCO₃ as the starting materials. The amorphous NiCO₃·2Ni(OH)₂·2H₂O are uniformly coated on the surface of PZT particles. XRD analysis and the selected-area diffraction (SAD) pattern indicate that the amorphous coating layer is crystallized to NiO after being calcined at 400°C for 2 h. TEM images show that the NiO particles of ∼8 nm are spherical and weakly agglomerated. The thickness of the nanocrystalline NiO coating layer on the surface of PZT particle is ∼30 nm.     

Effect of the Presence of Ammonium Peroxodisulfate on the Direct Precipitation of Ceria and Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

Direct precipitation of nanometer-sized particles of ceria–zirconia (CeO₂–ZrO₂) solid solutions with cubic and tetragonal structures was successfully attained from acidic aqueous solutions of cerium(III) nitrate (Ce(NO₃)₃) and zirconium oxychloride (ZrOCl₂) through the addition of ammonium peroxodisulfate ((NH₄)₂S₂O₈), because of promotion of the hydrolysis via the oxidation of Ce³⁺ ions, together with the simultaneous hydrolysis of ZrOCl₂ under hydrothermal conditions. Ultrafine CeO₂ particles also could be formed from relatively concentrated aqueous solutions of the same trivalent cerium salt in the presence of (NH₄)₂S₂O₈ via hydrolysis. The crystallite size and lattice strain of as-precipitated solid solutions varied, depending on the composition within the CeO₂–ZrO₂ system. Creation of a solid solution of ZrO₂ into a fluorite-type CeO₂ lattice clearly introduced lattice strain, as a consequence of the decreasing crystallite size. Both the direct precipitation process and the effectiveness of the presence of (NH₄)₂S₂O₈ for the synthesis of CeO₂–ZrO₂ solid solutions were discussed.     

Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods

Porous ceramics of Y₂O₃-stabilized ZrO₂ (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.     

Soft-Chemistry Synthesis and Characterization of Bismuth Oxyfluorides and Ammonium Bismuth Fluorides

Two bismuth oxyfluorides, BiOF and BiO_0.5F₂, have been prepared by soft chemistry. These two compounds are of interest for application as new type of positive electrode active materials in secondary lithium battery. BiOF was obtained directly from the fluorination of Bi₂O₃ by a saturated aqueous solution of NH₄F. BiO_0.5F₂ was obtained by the hydrolysis of β-NH₄BiF₄ at 70°C. The ammonium bismuth fluoride β-NH₄BiF₄, which is a new orthorhombic form of NH₄BiF₄, was obtained by reacting BiF₃ in a saturated aqueous solution of NH₄F. Two other ammonium bismuth fluorides, (NH₄)_0.5BiF_3.5 and NH₄Bi₃F₁₀, were also prepared by annealing β-NH₄BiF₄ at a relatively low temperature under inert atmosphere. The existence of the fluorite (NH₄)_0.5BiF_3.5 is reported for the first time in this paper.     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.     

Activity of Nickel(II) Oxide in Silicate Melts

Activity–composition relations of NiO have been determined at 1435°C in melts of the system CaO–NiO–SiO₂ and at 1400°C in melts of the systems CaO–MgO–NiO–SiO₂, CaO–MgO–NiO–Al₂O₃–SiO₂, and CaO–MgO–NiO–K₂O–SiO₂. In the CaO–NiO–SiO₂ and CaO–MgO–NiO–SiO₂ systems the activity coefficient of NiO (γ_NiO) decreases as the polymerization of the melt increases (basicity decrease). γ_NiO stays contant up to several weight percent NiO for melts with similar basicity, an indication of Henry's law behavior. Minima in the NiO activity coefficient are observed in melts of the CaO–MgO–NiO–Al₂O₃–SiO₂ and CaO–MgO–NiO–K₂O–SiO₂ systems at NBO/Si ratios between 1.0 and 1.5; i.e., γ_NiO decreases with decreasing basicity for NBO/Si ratios >1.5 and increases with decreasing basicity for melts with NBO/Si ratios <1.0 (NBO/Si; nonbridging oxygens per silicon). The addition of Al₂O₃ and K₂O to the CaO–MgO–NiO–SiO₂ system results in an increase in the activity coefficient of NiO.     

Optimization of a Composite Working Electrode for a New Family of Electrochemical Cell for NO Decomposition

The electrochemical properties of a composite (NiO) _x −(yttria-stabilized zirconia (YSZ))_{1– x} working electrode for a new type of electrochemical cell for NO decomposition in the presence of excess oxygen are investigated. It is shown that the dependence of the NO conversion on the value of the current passed through the electrochemical cell with a nanoporous (NiO) _x −(YSZ)_{1– x} working electrode is linear and that the value of current efficiency depends on the NO and O₂ gas concentrations only (η= [NO] /([NO] + 2[O₂]). The optimum NiO addition (35% by volume) to the YSZ resulted in a decrease of the cell operating voltage and, as a result, in a decrease in the electrical power required for NO decomposition. The observed high performance of the composite working electrode at this composition is consistent with the effective medium percolation theory, which predicts the ambipolar transport behavior of the composite mixed ionic−electronic (YSZ–NiO) conductors as a function of the volume fraction of each of the randomly distributed constituent phases.     

YSZ-Induced Crystallographic Reorientation of Ni Particles in Ni–YSZ Cermets

Metal–ceramic interfaces in Ni–YSZ (YSZ, yttria-stabilized zirconia)-textured porous cermets prepared by reduction of NiO–YSZ directionally solidified eutectics have been studied by transmission electron microscopy and X-ray pole figure experiments. Before reduction of NiO, the interfacial plane is but after reduction, the Ni phase does not maintain the same crystallographic orientation as the NiO parent phase. Ni undergoes an interface-induced crystallographic reorientation to form the lower energy (002)_Ni∥(002)_YSZ interface. This process has been studied as a function of the reduction temperature, and it seems to be more effective at ∼800°C. Metal–ceramic low-energy interfaces prevent Ni particle coarsening and impart long-term stability to the cermet.     

Anatase-Type TiO2 and ZrO2-Doped TiO2 Directly Formed from Titanium(III) Sulfate Solution by Thermal Hydrolysis: Effect of the Presence of Ammonium Peroxodisulfate on their Formation and Properties

Anatase-type TiO₂ powder containing sulfur with absorption in the visible region was directly formed as particles with crystallite in the range 15–88 nm by thermal hydrolysis of titanium(III) sulfate (Ti₂(SO₄)₃) solution at 100°–240°C. Because of the presence of ammonium peroxodisulfate ((NH₄)₂S₂O₈), the yield of anatase-type TiO₂ from Ti₂(SO₄)₃ solution was accelerated, and anatase with fine crystallite was formed. Anatase-type TiO₂ doped with ZrO₂ up to 9.8 mol% was directly precipitated as nanometer-sized particles from the acidic precursor solutions of Ti₂(SO₄)₃ and zirconium sulfate in the presence and the absence of (NH₄)₂S₂O₈ by simultaneous hydrolysis under hydrothermal conditions at 200°C. By doping ZrO₂ into TiO₂ and with increasing ZrO₂ content, the crystallite size of anatase was decreased, and the anatase-to-rutile phase transformation was retarded as much as 200°C. The anatase-type structure of ZrO₂-doped TiO₂ was maintained after heating at 1000°C for 1 h. The favorable effect of doping ZrO₂ to anatase-type TiO₂ on the photocatalytic activity was observed.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

Laser-Chemical Vapor Precipitation of Submierometer Silicon and Silicon Nitride Powders from Chlorinated Silanes

The laser-chemical vapor precipitation (L-CVP) of Si₃N₄ powders from miktures of SiH₂Cl₂ and NH₃ or SiC1,₄ and NH₃, was studied. The reactant gases were mixed in the laser beam, thus preventing low-temperature reactions. In a high-temperature electrostatic precipitator (ESP), Si₃N, was collected and separated from the waste product NH₄Cl. In the ESP, Si was collected at a lower temperature. A major problem in utilizing chlorinated silanes was their poor absorption of the 10.6-μm radiation. SF₆ was explored for use as an inert sensitizer. Si was prepared from SiH₂Cl₂. Particle diameters were typically 20 to 40 nm for Si3N₄ and 50 nm for Si.     

Vibrational Spectra of Water- and Ammonium-Chloride-Related Residues and Upconversion Fluorescence of Er3+ in ZnCl2-Based Glasses

Chloride glasses of the ZnCl₂ and 20KCl-20BaCl₂-60ZnCl₂-0.5ErCl₃ systems were prepared using NH₄Cl as a dehydrating and chlorinating agent, under the melt-quenching method. Water- and ammonium-chloride-related residues in ZnCl₂ glasses were investigated by infrared and near-infrared absorption spectra. Water, Zn—OH, ClO, ClO₂⁻, Zn²⁺-coordinated water, free NH₃, NH₄⁺, and Zn²⁺-coordinated NH₃ were identified in ZnCl₂ glasses. 20KCl-20BaCl₂-60ZnCl₂-0.5ErCl₃ glasses prepared by melting at 500°C for 20 min, under reduced pressure, contained the smallest amounts of water, Zn—OH, and Zn²⁺-coordinated NH₃ and showed strong Er³⁺ upconversion fluorescence.     

Synthesis of NiO-Deposited YSZ Composite Powders by Urea Hydrolysis

The urea hydrolysis method was used to prepare NiO-deposited YSZ composite powders. First, micrometer-sized YSZ particles were fabricated, and then the nanosized NiO particles were deposited on the surface of the YSZ particles. The microstructure of composite powders and the sintered bulk were further characterized with the aid of XRD, SEM, and TEM. The results indicated that the mesoporous and microsheet-like Ni(OH)₂· x H₂O ( x =0–1) crystals were deposited on the surface of YSZ particles. As the concentration of Ni²⁺ ion in the stock solution increased, the deposited NiO content and thickness of NiO layer on the YSZ particle surface also increased. In addition, the YSZ particle size showed significant influence on the microstructure and conductivity of Ni/YSZ cermet anode produced by NiO-deposited YSZ composite powders. Such NiO-deposited YSZ composite powders can be easily sintered to form a continuous NiO network.     

An Alternative Chemical Route for the Synthesis and Thermal Stability of Chemically Enriched Hydroxyapatite

Hydroxyapatite (HAp: Ca₁₀(PO₄)₆(OH)₂) was synthesized by aqueous precipitation using CaCl₂ and Na₃PO₄ with NaOH added to ensure completion of the reaction at room temperature. The HAp powder prepared using stoichiometric amounts of NaOH was stable even at 1200°C, but the HAp prepared with sub-stoichiometric amounts of NaOH resulted in its transformation into β-tricalcium phosphate at 600°C. The reaction pH, X-ray diffraction, thermal analysis, scanning electron microscopy, Fourier transform infrared analyses and inductively coupled plasma-optical emission spectroscopy were used to characterize the phase purity, thermal stability, morphology, and chemical composition of the synthesized HAp powder.     

A Chemical Route to Large-Scale Preparation of Spherical and Monodisperse Ni Powders

A facile chemical route to prepare a well-defined Ni powder on a mass-production scale without external heating was suggested. A highly concentrated Ni complex was formed between NiCl₂ and N₂H₄ solution, and subsequent addition of NaOH solution triggered serial reactions from the Ni complex decomposition via Ni(OH)₂ formation to the reduction of a Ni(OH)₂ gel into well-defined Ni spheres. The high temperature necessary for the reaction was attained not by external heating but by exothermic reactions such as complex formation and reduction. The preparation of monodisperse and spherical Ni powder was attributed to uniform distribution of the reducing agent (N₂H₄) within the Ni(OH)₂ gel.     

Sinterability of β-Calcium Orthophosphate Powder Prepared by Spray-Pyrolysis

Mixed solutions of Ca(NO₃)₂ and (NH₄)₂HPO₄ with Ca/P = 1.50 were spray-pyrolyzed at 600°C to produce β-calcium orthophosphate (β-Ca₃(PO₄)₂) powder; the spray-pyrolyzed powder was ground and then calcined at 600°C for 1 h. The best crystalline β-Ca₃(PO₄)₂ powder was obtained from the solution with 1.80 mol.L^–1 Ca(NO₃)₂, 1.20 mol.L^–1 (NH₄)₂HPO₄. The resulting powder was composed of primary particles with sizes of <0.5 μm. Dense β-Ca₃(PO₄)₂ ceramics with a relative density of 96.1% could be fabricated by firing this compressed powder at 1070°C for 5 h.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

A Novel Method for Fabrication of Y2O3-Stabilized ZrO2 Electrolyte Films

Gas-tight Y₂O₃-stabilized ZrO₂ (YSZ) films were prepared on NiO–YSZ and NiO–SDC (Sm_0.2Ce_0.8O_1.9) anode substrates by a novel method. A cell, Ni–YSZ/YSZ(10 μm)/LSM–YSZ, was tested with humidified hydrogen as fuel and ambient air as oxidant. The maximum power densities of 1.64, 1.40, 1.06, and 0.60 W/cm² were obtained at 850°, 800°, 750°, and 700°C, respectively. With methane as fuel, a cell of Ni–SDC/YSZ (12 μm)/LSM–YSZ exhibited the maximum power densities of 1.14, 0.82, 0.49, and 0.28 W/cm² at 850°, 800°, 750°, and 700°C, respectively. The impedance results showed that the performance of the cell was controlled by the electrode polarization rather than the resistance of YSZ electrolyte film.     

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.