Fabrication of Low-Porosity Indium Tin Oxide Ceramics in Air from Hydrothermally Prepared Powder

Compositionally homogeneous indium tin oxide (ITO) ceramics with low porosity were obtained successfully by sintering hydrothermally prepared powders. The fabrication technique began with the preparation of microcrystalline, homogeneously tin-doped (5 wt%) indium oxyhydroxide powder, under hydrothermal conditions. Low-temperature (∼500°C) calcination of the hydrothermally derived powder led to the formation of a substitutional-vacancy-type solid solution of In₂Sn_{1− x} O_{5− y} , and further heating of this phase at temperatures of >1000°C resulted in the formation of the tin-doped indium oxide phase, which had the C -type rare-earth-oxide structure. The sintering of uniformly packed, calcined powder compacts at 1450°C for 3 h in air resulted in low-porosity (∼0.7%) ITO ceramics.     

Synthesis and Colloidal Processing of Zirconia Nanopowder

Nanosized tetragonal 3 mol% Y₂O₃-doped ZrO₂ powder was produced by hydrothermal precipitation from metal chlorides and urea sol followed by a washing–drying treatment and calcination. The effects on powder properties of powder washing by water and ethanol with subsequent centrifuging, with possible deagglomeration using microtip ultrasonication, were experimentally shown. Ultrasonic irradiation induced pressure waves, which generated cavities that could violently collapse, producing intense stress. This induced stress was effective in minimizing secondary particle size, deagglomerating the powder, redispersing the ZrO₂ after all the washing–centrifuging cycles, and minimizing mean aggregate size after final calcination. A uniformly aggregated tetragonal ZrO₂ nanopowder with a mean secondary particle size of ∼45 nm and without hard agglomerates was prepared. The properties of the nanopowders produced by colloidal processing and CIP were studied. Determination of the best suspension parameters allowed for low-temperature sinterability, which resulted in a nanograined ∼95 nm ceramic.     

Preparation of Monodispersed Tin-Doped Indium Oxide Powders by Hydrothermal Method

This paper presents a hydrothermal method for synthesizing tin-doped indium oxide (ITO) powder. It shows that monodispersed ITO powders of diameter 70±10 nm can be readily prepared by a process using NaOH as a basic source and involving hydrothermal synthesis at 240°C in 12 h and post-calcination at 500°C. The powder size is decreased by increasing the initial indium and tin concentration ([In+Sn]_precursor) and the excess NaOH concentration ([NaOH]_excess) in the solution. The proposed technique has a significant potential for the commercial production of ITO nanopowders.     

Coprecipitation and Hydrothermal Synthesis of Ultrafine 5.5 mol% CeO2-2 mol% YO1.5ZrO2 Powders

Ultrafine 5.5 mol% CeO₂—2 mol% YO_1.5ZrO₂ powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m²/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.     

Hydrothermal Preparation of Ultrafme Monoclinic ZrO2 Powder

Ultrafine powder of single-phase manoclinic ZrO₂ was prepared by hydrothermal treatments of amorphous hydrated zirconia with 8 wt% KF solution under 100 MPa at 200° to 500°C for 24 h. The process yielded well-crystallized particles 16 nm and 22 nm in size at 200° and 500°C, respectively .     

Continuous Synthesis of Tin and Indium Oxide Nanoparticles in Sub- and Supercritical Water

Nanocrystalline tin and indium oxides (In₂O₃/SnO₂) were synthesized in sub- and supercritical water at 350°/380°C and 30 MPa in <73 s in a tubular flow reactor from an aqueous solution of {SnCl₄+InCl₃} (0.2 M ). The conversion rate for tin was 100%. Nanoparticles were analyzed by transmission electron microscopy (TEM), emitted X-rays, Raman, differential scanning calorimetry, and X-ray diffraction techniques. The bulk particles were composed of In, Sn, and O atoms, and made up of cubic In₂O₃ (10 nm) and tetragonal SnO₂ (5.5 nm) crystals. After calcination at 500°C for 2 h, little change occurred in the particle size and crystal phase. Traces of tin-doped indium oxide particles were also formed as confirmed by the TEM electron diffraction pattern. Using this one-step, high-temperature hydrothermal process, oxide nanoparticles can be continuously and conveniently produced in a well-controlled process.     

Hydrothermal Synthesis of Manganese Zinc Ferrites

Hydrothermal synthesis has been used to synthesize nano-sized manganese zinc ferrite powder. The results show that the pH value of the starting suspension has a decisive influence on the composition of the hydrothermally prepared manganese zinc ferrite powder. At a pH value of ∼8.5, stoichiometric amounts of manganese and zinc can be incorporated in the manganese zinc ferrite. The grain size of the powder increases with the temperature and time of hydrothermal treatment. The nanosized manganese zinc ferrite produced is superparamagnetic and becomes ferri-magnetic after the ferrite grains become larger than ∼100 nm. The nanosized ferrite grains are very prone to oxidation and disintegrate at 600°C in air. The surfaces of the synthesized ferrite grains are covered with a thin film of water.     

Synthesis of Nanocrystalline Lanthanum Strontium Manganite Powder by the Urea–Formaldehyde Polymer Gel Combustion Route

Nanocrystalline lanthanum strontium manganite (LSM) powder has been synthesized by combustion of a transparent gel obtained by the polymerization of methylol urea and urea in a solution containing La³⁺, Sr²⁺, and Mn²⁺ (LSM ions). Chemistry of the transparent urea–formaldehyde (UF) polymer gel formation and structure of the gel have been proposed such that the LSM ions act in between the growing UF polymer chains by interacting through NH, OH, and CO groups by co-ordination and prevent polymer self-assembly through inter-chain hydrogen bonding as evidenced from infrared spectrum. Thermally stable structures formed by the decomposition of UF polymer below 300°C undergo combustion in the presence of nitrate oxidant in a temperature range from 350°–450°C. A perovskite LSM phase has been formed by self-sustained combustion of the dried gel initiated with little kerosene. The powder obtained after deagglomeration and calcination at 600°C for 2 h has a D ₅₀ value of 0.19 μm, and the particles are aggregates of crystallites 10–25 nm in size.     

Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline cerium(IV) oxide (CeO₂) powders were prepared by heating solutions of cerium(IV) salts in the presence of urea under hydrothermal conditions at 120° to 180°C. The effects of the concentration of urea and hydrothermal treatment temperature on the morphology and crystallite size of the synthesized particles were investigated. The synthesized particles were angular, ultrafine CeO₂, with a cubic fluorite structure. Their crystallite size decreased from 20 to 10 nm with increasing urea concentration from 2 times to 8 times that of the Ce⁴⁺ ion. The size only slightly changed by calcining at temperatures below 600°C.     

New Method for the Preparation and Stabilization of Nanoparticulate t-ZrO2 by a Combined Sol–Gel and Solvothermal Process

Nonstabilized and silica-stabilized zirconia (ZrO₂) crystallites with sizes between 3 and 4 nm were synthesized by a novel combined sol–gel and solvothermal process. After adding zirconium n -propoxide to a solution of isobutanol, propionic acid, and water, a transparent nanoparticulate sol was synthesized by a sol–gel process. The average hydrodynamic diameter of the amorphous ZrO₂ nanoparticles was approximately 5 nm. A following solvothermal process led to a crystalline fraction of the powder consisting of 31 wt% tetragonal ( t ) phase ZrO₂. This fraction was doubled to 61 wt% by adding 10 mol% 3-methacryloxypropyl trimethoxysilane (MPTS) and increased to 96 wt% by a subsequent calcination at 800°C. The crystallite sizes were also confirmed by means of Brunauer–Emmett–Teller and high-resolution transmission electron microscopy.     

Synthesis of MgAl2O4 Spinel Powder by Combination of Sol–Gel and Precipitation Processes

MgAl₂O₄ spinel precursor was prepared by a novel method combining a sol–gel process with the "traditional" precipitation process. The thermal decomposition and phase development of the precursor were analyzed, and the degree of agglomeration of the calcined powder was assessed by determining its particle size and crystal size. The optimum calcination temperature was determined based on the variation of specific surface areas, crystal size, and particle size. Completely crystallized ultrafine spinel powder ( d ₅₀=600 nm, specific surface area=105 m²/g) was obtained after calcination at 900°C.     

Effects of Calcination on the Microstructures and Photocatalytic Properties of Nanosized Titanium Dioxide Powders Prepared by Vapor Hydrolysis

Ultrafine titanium dioxide powders are produced in an aerosol reactor using vapor hydrolysis of titanium tetraisopropoxide (TTIP) at 260°C and higher temperatures (600°, 700°, 800°, and 900°C). The effect of calcination on the microstructure characteristics and the photoactivity is studied. The powders are characterized using Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The photocatalytic activity of the powders is also studied using degradation of phenol in water as a test reaction. The powder produced at 260°C is calcined at 500° to 900°C while those produced at higher temperatures are calcined at 600°C for 3 h. Raw powder produced at 260°C is amorphous but becomes crystalline after calcination. As the calcination temperature increases, the surface area decreases but the rutile-to-anatase ratio and the anatase and rutile crystallite sizes increase. The photoactivity increases as calcination temperature increases to 900°C, when the powder becomes densified and the surface area drops significantly because of sintering. Powders produced at higher temperatures are predominantly anatase and are generally more photoactive. Calcination of the powders at 600°C for 3 h results in little loss of surface areas and enhances the photoactivity. Among the factors examined, large surface area and good dispersion of the powders in the reaction mixture are favorable to photoactivity. Conversely, prolonged calcination at high temperatures is detrimental to photoactivity. However, surface area, crystallite size, anatase-to-rutile ratio, and dispersity of the powders alone cannot account for the observed trend of photoactivity. The role of crystallinity needs to be investigated.     

Formation of Ultrafine Tetragonal ZrO2 Powder Under Hydrothermal Conditions

Ultrafine tetragonal ZrO₂ powder was prepared by hydrothermal treatment at 100 MPa of amorphous hydrous zirconia with distilled water and LiCl and KBr solutions. The resulting powder consisted of well-crystallized particles; at 200°C, the particle size was 16 nm and at 500°C, 30 nm. Under hydrothermal conditions tetragonal ZrO₂ appears to crystallize topotactically on nuclei in the amorphous hydrous zirconia.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia by the oleate complex route

Nanocrystalline 8 mol% yttria stabilized zirconia (YSZ) powder has been synthesized by the oleate complex route. Oleate complexes of zirconium and yttrium were formed in the hexane rich layer by the reaction of sodium oleate with zirconyl chloride and yttrium chloride at the interface of the two ternary solutions in water–ethanol–hexane system. The zirconyl oletae and yttrium oleate complexes on heating decomposed to oxide through the formation of carbonate intermediates. The powder obtained by calcination at 600 °C for 2 h was cubic YSZ with surface area of 42 m²/g. The YSZ powder contained primary particles of ∼300 nm size and the primary particles were aggregate of crystallites of 5–10 nm. The compacts prepared from the YSZ powder were sintered to ∼99% TD (theoretical density) at 1400 °C. The sintered YSZ had a low average grain size of 0.73 μm.     

Preparation and Characterization of Submicron Hafnium Oxide

Submicron hafnium oxide powder prepared by hydrolytic decomposition of alkoxides was studied. The particle size range of this powder was 10 to 50 Å. Emission spectrographic analysis of the powder after it was calcined at 250°C for 0.5 h indicated a purity of >99.995%. Up to 320°C, the powder showed no crystallinity by X-ray analysis. The amorphous HfO₂ was isothermally aged at 5° to 10°C intervals between 200° and 500°C. X-ray diffraction patterns indicate a sharp transition from an amorphous state to the monoclinic phase at 325°C. High-temperature X-ray studies and DTA suggest nucleation and growth of small crystallites at 420°C leading to conversion to monoclinic HfO₂ at 480°C. BET surface area measurements and TGA of the powders were also conducted. A powder which transformed at 325°C to the monoclinic phase was isothermally aged below 325°C for 150 h without change.     

Bismuth Titanate from Mechanical Activation of a Chemically Coprecipitated Precursor

Most of the chemistry-based preparation routes for bismuth titanate (BIT) involve calcination at elevated temperatures in order to realize precursor-to-ceramic conversion. In a completely different approach using an amorphous BIT hydroxide precursor, nanocrystalline particles of layered perovskite BIT are synthesized by mechanical activation, skipping the detrimental crystallite coarsening and particle aggregation encountered at high temperatures. Mechanical activation leads to nucleation and steady growth of BIT crystallites in the amorphous precursor matrix, while Bi₂O₃ is involved as an intermediate transitional phase. The activation-derived BIT particles demonstrate a rounded morphology of ∼50 nm in size. This is in contrast to the BIT derived from calcination of the coprecipitated precursor at 600°C that is dominated by coarsened platelike particles. The former is sintered to a density of >95% theoretical at 875°C for 2 h, leading to a dielectric constant of ∼1260 when measured at 1 MHz and the Curie temperature of 646°C.     

Annealing effects for calcination of tin oxide powder prepared via homogeneous precipitation

Tin oxide powders were prepared from a homogeneous precipitation using the aqueous solution of SnCl₄ with urea as a precipitator at 90 °C and followed by a calcination process. The calcination was performed using two different methods; conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The crystallization of the tin oxide finished at 600 °C regardless of the calcination method used. The crystallite size increased with as the calcination temperature increased due to the crystal growth and agglomeration. The tin oxide calcined using RTA has a relative smaller crystallite size than CFA at the same temperature. The tin oxide powders calcined with RTA showed higher specific surface areas than those that used CFA over a wide range of temperatures.     

Low-Temperature Synthesis and Growth of Hexagonal Boron-Nitride in a Lithium Bromide Melt

The synthesis and growth of hexagonal boron-nitride crystallites at low temperature were investigated by carrying out in a lithium bromide (LiBr) melt in 600°–700°C. Transmission electron microscopy study showed that the product obtained at 650°C was composed of two-dimensional plates having an average thickness of about 80 nm and widths from about 400 nm to several micrometers. The molten salt was found to have a strong effect on the crystal growth. The morphology of the crystallites changed from spherical particles to two-dimensional plates when LiBr was used as a crystallization medium. Products were also characterized by X-ray powder diffractions, Fourier Transformed infrared, FESEM/energy dispersive X-ray spectroscopy, and selected area electron diffraction. Possible formation mechanism was also discussed.     

Preparation of nanometer AlN powders by combining spray pyrolysis with carbothermal reduction and nitridation

Nanometer AlN powders were prepared by combining spray pyrolysis with carbothermal reduction and nitridation (CRN). The aluminum oxide/carbon composite powders prepared by spray pyrolysis from a sucrose spray solution were several microns in size, with hollow and porous structures. Precursor powder with 67 wt% carbon content was transformed into phase-pure AlN powder by CRN at temperatures above 1,400 °C. The hollow-structured AlN powder was ground to 20 nm mean size by simple milling.     

Characterization of Products Obtained during Formation of Barium Monoaluminate through Hydrothermal Precipitation–Calcination Route

Nanosized (10–30 nm particle size) hexagonal barium monoaluminate, having a high surface area of ∼30 m²/g, was synthesized by calcination of hydrothermally prepared precursors. The precursors were obtained by hydrolytic precipitation, using a mixed solution of barium and aluminum nitrates in the presence of aqueous urea at 180°C. Based on the results of XRD, FTIR, SEM/EDS, TEM, and TG-DTA studies, the most probable sequence of reactions leading to the formation of barium monoaluminate was (i) conversion of aqueous barium and aluminum nitrates in the presence of urea by hydrothermal precipitation to crystalline orthorhombic barium carbonate and boehmite, (ii) formation of an interim form consisting of amorphous fibrillar aluminum oxide(s) interlaced with crystalline barium carbonate in the calcination temperature range of ∼500°–800°C, (iii) initiation of formation of barium monoaluminate at 1000°C, and (iv) formation of a near monophase nanosized barium monoaluminate at 1200°C.