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.     

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.     

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.     

Reaction of Zirconium Fluoride Monohydrate and Ammonium Bifluoride: Its Effect on Fluoride Glass Preparation and Quality

The products obtained from the roomtemperature reaction of ammonium bifluoride and zirconium fluoride monohydrate are ammonium heptafluorozirconate ((NH₄)₃ZrF₂), liquid water, and hydrogen fluoride. Ammonium bifluoride and zirconium fluoride monohydrate were reacted prior to glass batching, producing dry ammonium heptafluorozirconate which was used to prepare a high-quality ZBLAN fluoride glass.     

Mechanochemical Synthesis of Ammonium Hexafluorogallate

Mechanochemical reaction between ammonium fluoride (NH₄F) and gallium fluoride trihydrate (GaF₃·3H₂O) at a ratio of 3:1 is conducted by grinding the mixture at room temperature using a planetrary ball mill. A single phase of ammonium hexafluorogallate ((NH₄)₃GaF₆) is obtained as a product of the reaction. Other alkali hexafluorogallates (K₃GaF₆, Na₃GaF₆, and Li₃GaF₆) also can be synthesized by grinding constituent components (AF and GaF₃·3H₂O) under proper grinding conditions. A reaction mechanism related to the dissolution of alkali fluorides in water is suggested.     

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.     

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 Hollow BaTiO3 and Anatase Spheres by the Layer-by-Layer Colloidal Templating Method

Hollow BaTiO₃ and anatase spheres were prepared from multilayered colloidal titanate particles. An inorganic precursor, titanium (IV) bis(ammonium lactate) dihydroxide (TALH) (chemical formula: [CH₃CH(O–)CO₂–NH₄]₂Ti(OH)₂) was used. First, a layer-by-layer (LBL) colloid-templating method was employed using TALH to generate monodispersed hollow titanate spheres. These spheres were then treated in a Ba(OH)₂ solution or distilled water under hydrothermal conditions to transform them into hollow BaTiO₃ or anatase spheres, respectively.     

Effects of Ammonium Chloride on the Rheological Properties and Sedimentation Behavior of Aqueous Silica Suspensions

The influence of ammonium chloride (NH₄Cl) on the rheological properties and sedimentation behavior of aqueous silica (SiO₂) suspensions of varying solids volume fraction (φ_s) was studied. SiO₂ suspensions with low NH₄Cl concentration (≤0.05 M , pH 5.2) exhibited Newtonian behavior and a constant settling velocity ( U ). The volume fraction dependence was well described by the Richardson–Zaki form, U = U ₀(1 −φ_s) ⁿ , where n = 4.63 and U ₀= 1.0419 × 10⁻⁵ cm/s. At higher NH₄Cl concentrations (0.07–2.0 M , pH 5.2), suspensions exhibited shear thinning and more complicated sedimentation behavior due to their aggregated nature. For all suspensions studied, however, the apparent suspension viscosity, characteristic cluster size, and initial settling velocity were greatest at ∼0.5 M NH₄Cl and exhibited a similar dependence on salt concentration. Above 0.5 M NH₄Cl, considerable restabilization was observed. This behavior cannot be explained by traditional DLVO theory.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

Hot Isostatic Pressing of Reactive SnO2 Powder

Tin(IV) oxide (SnO₂) crystallizes at room temperature by adding hydrazine monohydrate ((NH₂)₂· H₂O) to a hydrochloric acid solution of tin, followed by washing and drying. Well-densified SnO₂ ceramics (99.8% of theoretical) with an average grain size of 0.9 μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are 14.4 GPa and 200 MPa, respectively. They exhibit an electrical conductivity of 2 × 10⁻³−9 × 10⁻³ S·cm⁻¹ at room temperature.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

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.     

Synthesis and Characterization of a High-Purity AlN Powder

High-purity AIN powder was synthesized by preparing high-purity (NH₄)₃ AIF₆, decomposing the (NH₄)₃ AIF₆ to AIF₃, and reacting the AIF, at 800° to 1000°C with high-purity NH₃. AIN powders could be synthesized with a cation purity of 99.99% and an oxygen content <0.2 wt%. The specific surface area depended on the process conditions and varied from 0.2 to 2.6 m²/g. The thermodynamics of the process are evaluated and the influence of various process parameters on the resulting powder characteristics are described.     

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.     

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.     

Preparation of Nanostructured Hexagonal Boron Nitride Powder

In this communication, we describe an inexpensive and feasible method for the preparation of hexagonal boron nitride (h–BN) nanorods in the absence of metal catalyst. Tertiary calcium phosphate (Ca₃(PO₄)₂) and ammonium biborate hydrate (NH₄HB₄O₇·3H₂O) were selected as starting materials where calcium phosphate was used as a diluting agent to prevent the formation of bulk B₂O₃ during the thermolysis of ammonium biborate hydrate. The mixture was nitrided at 900°C in the flowing ammonia and was transformed into h–BN nanorods after subsequent crystallization. After crystallization at 1650°C for 2 h, the unique microstructure of h–BN nanorods was observed.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

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.