Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

Characterization of Ca2SiO4:Eu2+ Phosphors Synthesized by Polymeric Precursor Process

The green emitting Ca₂SiO₄:Eu²⁺ (C₂S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C₂S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu²⁺ by annealing in 5% H₂/N₂ atmosphere. The obtained C₂S:Eu²⁺ phosphors exhibited a strong emission at 504 nm under the excitation of λ_exc=350 nm. The highest photoluminescence (PL) intensity was observed in the C₂S:Eu²⁺ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.     

Fabrication and Luminescent Properties of Nd3+-Doped Lu2O3 Transparent Ceramics by Pressureless Sintering

The fabrication of transparent Nd³⁺ ion-doped Lu₂O₃ ceramics is investigated by pressureless sintering under a flowing H₂ atmosphere. The starting Nd-doped Lu₂O₃ nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH₄OH+NH₄HCO₃ mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000°C for 2 h, monodispersed Nd³⁺:Lu₂O₃ powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m²/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd³⁺:Lu₂O₃ ceramics are obtained after being sintered under a dry H₂ atmosphere at 1880°C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu₂O₃ ceramic at 807 nm reached 14 cm⁻¹, while the emission cross section at 1079 nm was 6.5 × 10⁻²⁰ cm².     

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.     

Nano α-Al2O3 Powder Preparation by Calcining an Emulsion Precursor

An experimental study has been conducted to evaluate the formation of nano α-Al₂O₃ under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al₂O₃ powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al₂O₃, indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al₂O₃ was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

Synthesis of V2O3 Powder by Evaporative Decomposition of Solutions and H2 Reduction

The evaporative decomposition of solutions method was used to form V₂O₅. Spraying above the congruent melting temperature of V₂O₅ (690°C) resulted in dense spherical particles with a smooth surface. Spraying below the V₂O₅ melting temperature yielded porous V₂O₅ powder with a rough surface. Reduction of the V₂O₅ to V₂O₃ was done in a H₂ atmosphere. Spherical V₂O₃ powder was attained when the reduction temperature was low enough to reduce the V₂O₅ surface before partial sintering (necking) between V₂O₅ particles occurred. The resulting V₂O₃ particle size was smaller than the precursor V₂O₅ powder as expected by the differences in densities between V₂O₅ ( p = 3.36 g/cm³) and V₂O₃ ( p = 4.87 g/cm³).     

Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.     

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.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

CaO Powders from Oyster Shells for Efficient CO2 Capture in Multiple Carbonation Cycles

The carbonation/calcination loop of CaO/CaCO₃ is an efficient process for CO₂ capture. This study investigated the CO₂ capture capability of CaO powders derived from oyster shells and reagent-grade CaCO₃. The oyster shell-derived CaO powder had an oxide impurity content as high as 9 wt%, and a larger CaO crystal grain size and smaller specific surface area than the CaO derived from the reagent-grade CaCO₃. In cyclic CO₂ capture tests, the cyclability and CO₂ capacity of the oyster shell-derived CaO was significantly improved by inserting an intermediate cooling step between carbonation and calcination. At a carbonation temperature of 740°C, the overall performance of the oyster shell-derived CaO in cyclic carbonation was superior to that of the CaO from the reagent-grade CaCO₃. On the basis of X-ray diffraction analysis, it was suggested that the impurities contained in the oyster shell-derived CaO may have constituted transition zone on the CaO crystal grain-boundary to suppress crystal growth in calcination as well as to ease up lattice expansion in CO₂ fixation. The intermediate cooling enlarged the transition zone to mitigate lattice dislocations resulting from CO₂ fixation and thus, the decay in CO₂ capacity.     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel Powder

MgAl₂O₄ (MA) spinel powder was synthesized by heating an equimolar composition of MgO and Al₂O₃ in LiCl, KCl, or NaCl. The synthesis temperature can be decreased from >1300°C (required by the conventional solid–solid reaction process) to ∼1100°C in LiCl, or to ∼1150°C in KCl or NaCl. The molten salt synthesized MA powder was pseudomorphic and retained, to a large extent, the size and morphology of the original Al₂O₃ raw material, indicating that a "template formation mechanism" plays an important role in the synthesis process.     

Single-Calcination Synthesis Mechanism of Pure-Perovskite Pb(Ni1/3Nb2/3)O3-PbTiO3 Using a Coating Method

Pure-perovskite 0.64Pb(Ni_1/3Nb_2/3)O₃–0.36PbTiO₃ (PNN–PT) powder has been successfully synthesized by only one-step calcination using a coating method. SEM photograph shows that PNN–PT powder with the size of 2–4 μm is cubic and well dispersed. Based on X-ray diffraction analysis, solid-state reactions in the process of calcination in PbO–Nb₂O₅–TiO₂–NiO system are investigated. In comparison with conventional solid state method, the single-calcination synthesis mechanism of pyrochlore-free lead-based niobate ferroelectrics using a coating method is tentatively proposed. A typical coating structure of Ni precipitate-coated Nb₂O₅ powder facilitates the formation of perovskite PNN–PT phase at a relatively low calcination temperature, resulting in the successful synthesis of pyrochlore-free PNN–PT powder in one-step calcination at 900°C.     

ZrO2 Nanopowders Prepared by Low-Temperature Vapor-Phase Hydrolysis

ZrO₂ powder is prepared by low-temperature vapor-phase hydrolysis of ZrCl₄. TG-DTA, XRD, Raman, BET, and TEM methods are used to investigate the particle size, phase composition, and agglomeration before and after heat treatment. The results show that the as-prepared ZrO₂ powder is characterized by large surface area (150 m²/g), fine grain size (5.8 nm), and weak agglomeration. Additionally, the as-prepared ZrO₂ powder shows predominantly tetragonal phase attributed to a grain size effect. This route is free of powder drying and calcination processes that are essential for wet chemical preparation, contributing to less agglomeration.     

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

The formation process of barium hexaaluminate (BaO 6Al₂O₃) from BaCO₃/γ-Al₂O₃ powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO₃ and Al₂O₃ via formation of BaO·Al₂O₃. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al₂O₃ proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m²/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al₂O₃.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Synthesis and Characterization of MgAl2O4 Spinel Precursor from a Heterogeneous Alkoxide Solution Containing Fine MgO Powder

A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl₂O₄ spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg₄Al₂(OH)₁₄· 3H₂O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al₂O₃ at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C.     

Influence of Precipitating Atmosphere on Sintering of ZrO2+12 Mol% Y2O3

Solid solutions of ZrO₂ and Y₂O₃ prepared by decomposing coprecipitated hydroxides exhibited differences in sinterability which did not correlate with variations in particle size, surface area, or purity. These differences were traced to the fact that some of the materials had been exposed to CO₂ during preparation, whereas others had not. The presence of CO₂ during precipitation stabilized the amorphous phase such that energy release and the onset of nucleation and crystal growth occurred at a higher temperature than in Ar-prepared material. It is suggested that during preparation in CO₂ a carbonate complex is formed, which decomposes on calcination, liberating CO₂, some of which is trapped in voids formed by microsintering. The pressure of CO₂ in the voids limits their removal.     

Microstructure of Varistors Prepared with Zinc Oxide Nanoparticles Coated with Bi2O3

Zinc oxide (ZnO) nanoparticles coated with 1–5 wt% Bi₂O₃ were prepared by precipitating a Bi(NO₃)₃ solution onto a ZnO precursor. Transmission electron microscopy showed that a homogeneous Bi₂O₃ layer coated the surface of the ZnO nanoparticles and that the ZnO particle size was ∼30–50 nm. Scanning electron microscopy showed that ZnO grains sintered at 1150°C were homogeneous in size and surrounded by a uniform Bi₂O₃ layer. When the ZnO grains were surrounded fully by Bi₂O₃ liquid phases, further increases in the ZnO grain size were not affected by the Bi₂O₃ content. This predesigned ZnO nanoparticle structure was shown to promote homogeneous ZnO grains with perfect crystal growth.