Synthesis of Nanosized Spherical Rhabdophane Particles

Spherical 10 nm rhabdophane (LaPO₄·H₂O) particles were made by controlled precipitation in water using lanthanum citrate chelate and phosphoric acid (La-Cit⁺/H₃PO₄) at a temperature of 30°C. Rod-shaped 10 nm × 100 nm rhabdophane particles were made by direct precipitation from lanthanum nitrate and phosphoric acid (La(NO₃)₃/H₃PO₄). The lanthanum nitrate to phosphoric acid molar ratios (La:P) were fixed at 1:1 and 1:5 for both methods to study their effect on particle size and shape. The particles were characterized with DTA/TGA, XRD, TEM, SEM, Fourier Transform infrared spectroscopy, and ζ potential measurements. The spherical particle surfaces had absorbed citric acid. The point of zero charge of the rod-shaped and spherical particles was pH 5.4 and 4.3, respectively. Formation mechanisms for the spherical particles are discussed.     

Spherical Rhabdophane Sols. II: Fiber Coating

A rhabdophane (LaPO₄·nH₂O) sol with fine spherical particles was used to coat Nextel™ 720 fiber tows continuously with monazite (LaPO₄). The coatings are compared with those made previously from rod-shaped particles. The coated fibers were heat-treated at 1000°–1300°C for 1, 10, and 100 h. The effect of heat treatment temperature and time on coating microstructure was characterized by scanning electron microscopy and transmission electron microscopy, and the strengths of the coated fibers were measured after coating and heat treatment. Grain shapes and grain growth rates were measured. Coating thickness uniformity was quantified by a fit to a truncated extreme-value distribution. Coating hermeticity was evaluated by analysis of grain growth rates. The spherical particles promote more rapid coating densification and local hermeticity, but introduce problems with sintering shrinkage cracking that are not present in coatings derived from rod-shaped particles.     

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 Al(OH)3 Addition on β-LiAlO2 Crystal Growth

Crystal growth of rod-shaped β-LiAlO₂ was previously reported by us, and the rod-shaped β-LiAlO₂ crystals were 1.5 μ in diameter and 10 to 15 μm long. In the present study needle-shaped β-LiAlO₂ crystals which were thinner and had larger aspect ratios (length/diameter) than the rodshaped β-LiAlO₂ crystals were grown by using LiOH–Al₂O₃–Al(OH)₃–NaOH as the raw material. These crystals were 0.7 to 1 μm in diameter, 9 to 13 μm long, and had aspect ratios of about 10 to 13.     

Magnesium Aluminate (MgAl2O4) Spinel Powders from Water-Based Sols

Magnesium aluminate (MgAl₂O₄) spinel powders of irregular and spherical morphologies were obtained from the bi-component water-based sols following the sol–gel and sol–emulsion–gel methods, respectively. For the synthesis of the oxide microspheres, the surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined powders were investigated by using thermogravimetry analysis, differential thermal analysis, X-ray diffraction (XRD), optical and scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy, and particle size analysis. XRD results indicated crystallization of the only phase MgAl₂O₄ spinel from 200° to 1000°C. Formation of hollow microspheres with a single cavity was identified by SEM.     

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.     

Spherical Yttrium Aluminum Garnet Embedded in a Glass Matrix

Containerless processing was used to investigate the glass-forming behavior of Al₂O₃–Y₂O₃ glass. The amorphous bulk samples were obtained at compositions with 25–37.5 mol% yttria when the melt was cooled at a cooling rate of ∼250 K/s. Although small spherical particles (∼10 μm) with the same composition of the matrix were detected in the amorphous samples with 32.5–37.5 mol% yttria, the microfocus X-ray diffraction result indicated that the small spherical particles were crystalline Y₃Al₅O₁₂ garnet (YAG), rather than being amorphous. This observation suggested that small YAG particles could not grow larger after their nucleation, because of the high viscosity at high undercooling and the high cooling rate, which would graze the nose of the continuous cooling temperature diagram of YAG.     

Influence of Solid Fraction on the Optimum Molecular Weight of Polymer Dispersants in Aqueous TiO2 Nanoparticle Suspensions

The effects of solid fraction and molecular weight of a polymer dispersant—sodium polyacrylate—on the apparent viscosity of an aqueous TiO₂ nanoparticle suspension were investigated by varying the particle solid fractions from 15 to 37 vol% and the molecular weight of the dispersant from 1200 to 30 000. When the solid fraction was increased, the molecular weight of the dispersant was decreased to obtain the lowest suspension viscosity. The effect of the molecular weight of the dispersant on the suspension viscosity was discussed based on the surface interaction between TiO₂ particles characterized by a colloid probe atomic force microscope. The polymer dispersant adsorbed on a particle surface enhanced the repulsive force between the TiO₂ particles and reduced adhesion by electrosteric interaction, which reduced the suspension viscosity. The size of the dispersant in the aqueous solution determined by a dynamic light scattering method was compared with the estimated particle surface distance. As the solid fraction increased, the average surface distance and the size of the dispersant attainable to the particle surface reduced; therefore, the optimum molecular weight of the dispersant for a dense suspension is also reduced.     

Synthesis of High Aspect Ratio Platelet SrTiO3

A method for synthesis of high aspect ratio platelet seeds by growth of SrTiO₃ on Sr₃Ti₂O₇ core particles is reported. The aim of this study was to identify and control the morphology and size of SrTiO₃ particles via molten salt synthesis. Platelet and tabular morphologies with rectangular faces were obtained using rutile and anatase, respectively. Platelet SrTiO₃ particles with an edge length of 10–40 μm and a thickness of 1–4 μm were obtained. High aspect ratios (edge length to thickness) of 7–10 were measured for platelet particles as opposed to lower aspect ratios of 2–4 for tabular particles. Highly anisotropic platelets are suitable template candidates to achieve textured ceramics.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

AI2O3 Coatings as Diffusion Barriers Deposited from Particulate-Containing Sol-Gel Solutions

A procedure for the formation of A1₂O₃ coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi₂ and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A1₂O₃ particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A1₂O₃ particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A1₂O₃ and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially.     

Abnormal Grain Growth During Rapid Annealing of Sol–Gel Alumina Thin Film Deposited on Ni-Based Superalloy

A ∼50 nm thick alumina layer was deposited on an Ni-based superalloy substrate by a sol–gel method. α-AlOOH particles presented in the layer after drying at 140°C transformed mostly to α-Al₂O₃ grains within ∼1 min at 1100°C under a low oxygen partial pressure annealing environment. During the same time period, the α-Al₂O₃ grains grew significantly in the lateral direction, resulting in the aspect ratio of grain diameter to thickness of ∼20. The presence of a preferred orientation in the α-Al₂O₃ layer suggested that the mechanism for the lateral growth was abnormal. The lateral growth mechanism appeared to become very slow when a critical thickness (∼100 nm) was reached.     

Synthesis of Rare-Earth Gallium Garnets by the Glycothermal Method

Single-phase rare-earth gallium garnets were obtained by the reaction of stoichiometric mixtures of gallium acetylacetonate and rare-earth (Nd-Lu) acetates in 1,4-butanediol at 300°C (glycothermal reaction). Particles of gadolinium gallium garnet (GGG) and other gallium garnets with rare-earth elements larger than Gd were spherical with diameters of 0.5-2 μm, while particles of garnets with smaller rare-earth ions (Tb-Lu and Y) were much smaller (100-300 nm) with particle size distributed in a quite narrow range. TEM observation revealed that each particle was essentially a single crystal grown from one nucleus, but that defects frequently occurred during the crystal growth. Cerium and praseodymium gallium garnets were also formed when the reaction was carried out in the presence of GGG seed crystals. Hydrothermal reactions of the same starting materials under identical conditions yielded mixtures of gamma-Ga₂O₃ and the garnet phase.     

Homogeneous Distribution of Sintering Additives in Liquid-Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A1₂O₃ and Y₂O₃ sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both A1₂O₃ and Y₂O₃ sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-SiC) with 2 wt% A1₂O₃ and 1 wt% Y₂O₃ were prepared by freeze granulation/ pressing and sintering at 1880^deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y₂O₃ and A1₂O₃ added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.     

Sol–Gel Synthesis and Microstructure Analysis of Amino-Modified Hybrid Silica Nanoparticles from Aminopropyltriethoxysilane and Tetraethoxysilane

Agglomerated amino-modified silica nanoparticles were prepared from a novel Stöber-like precursor system consisting of aminopropyltriethoxysilane (APTES), tetraethoxysilane (TEOS), ethanol, and water where the molar ratio APTES/TEOS was 0, 0.1, 1.0, and 2.0, and the molar ratio H₂O/-SiOC₂H₅ was about 20 to 60, or great excess amounts of H₂O were employed. APTES catalyzed the hydrolysis and condensation of both silanes. ²⁹Si magic angle spinning nuclear magnetic resonance spectra confirmed that the particles consisted of Qⁿ species (Si(OSi) _n (OH)_{4− n} ; n =2, 3, 4) and Tⁿ species (NH₂(CH₂)₃–Si(OSi) _n (OH)_{3− n} ; n =2, 3). The APTES content in the precursor solutions controlled the agglomerating spherical particle size and morphology: 0.1 in the ratio APTES/TEOS led to almost independent spheres of 300–400 nm, while the larger ratios 1 and 2 led to ∼250 and ∼150 nm spheres, respectively, that were largely agglomerated and some were fused to look like peanut-shells. When soaked in Kokubo's simulated body fluid, those amino-modified particles deposited apatite. The mechanisms of particle formation and apatite deposition were discussed in terms of an intraparticle hydrated layer.     

An experimental study of the flow of nonspherical grains in a rotating cylinder

The effect of particle aspect ratio on the rheology of the flow of granular materials is studied experimentally in a quasi–two‐dimensional rotating cylinder, using two varieties of prolate spheroidal grains with different aspect ratios. Image analysis of high speed videos is used to obtain the flow profiles near the centre of the cylinder. The dynamic angle of repose and apparent viscosity in the medium show significant increase with increasing aspect ratio. The mean velocity, root mean square velocity and shear rate profiles are qualitatively similar for nonspherical and spherical particles, however, their magnitudes increase with increasing aspect ratio. A simple scaling is shown to predict the maximum thickness of the flowing layer for all the particles. The predictions of a model for the flow match with the measured mean velocity profiles and layer thickness. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4307–4315, 2017     

Morphology of Oxide Particles Made by the Emulsion Combustion Method

Various oxide powders were prepared by the emulsion combustion method (ECM) using metal precursors, kerosene, and a surfactant. The product particles were characterized by transmission electron microscopy (TEM), nitrogen adsorption, and X-ray diffraction. Hollow γ-Al₂O₃ particles were produced from aluminum nitrate or chloride precursors dispersed in air, whereas dispersion of the precursor emulsion in oxygen resulted in solid α-Al₂O₃ particles. Hollow spheres were obtained also for TiO₂, ZrO₂, and Y₂O₃ by ECM of TiCl₄, zirconium oxynitrate, and yttrium nitrate in aqueous solution. A simple method was developed to predict the thickness and diameter of hollow particles using the nitrogen adsorption data and initial droplet concentration of the ECM spray. The TEM diameter and shell thickness of hollow particles were consistent with those predicted. In contrast, solid particles were formed by ECM for ZnO, Fe₂O₃, CeO₂, and MgO from aqueous solutions of their corresponding nitrates.     

Nanosized PbZrO3 Powder from Oxalate Precursor: Microwave-Aided Synthesis and Thermal Characterization

Nanosized lead zirconate (PbZrO₃) powder was synthesized from its oxalate precursor, namely lead zirconyl oxalate (LZO). LZO heated in a microwave heating system for 1 h yielded the PbZrO₃ at 600°C. The same precursor (LZO), when heated in a resistance-heated furnace at 850°C for 3 h, does not give a pure product. Thermogravimetry, differential thermal analysis, and X-ray diffraction techniques were used to characterize the precursor and optimize the conditions for microwave processing. The particle size of PbZrO₃ powder prepared at 600°C using microwave heating was measured using transmission electron microscopy (TEM). The TEM images show that the particles of PbZrO₃ are spherical in shape and that the particle size varies between 20 and 22 nm.     

Hydrothermal Synthesis and Formation Mechanisms of Lanthanum Tin Pyrochlore Oxide

Well-defined La₂Sn₂O₇ with a phase-pure pyrochlore structure was produced by hydrothermal synthesis at temperatures as low as 200°C. Production of phase-pure La₂Sn₂O₇ requires a pH above 10, and higher pH accelerates the crystallization process. The synthesis produced spherical particles of average particle size ∼0.59 μm (±0.12) and surface area ∼14.1 m²/g. SEM and TEM observation for morphologic evolution and kinetic analysis during crystallization indicated that La₂Sn₂O₇ formation probably proceeds via a two-step reaction. First a transient dissolution–precipitation occurs. Then the primary crystallites aggregate because of their colloidal instability, and heterocoagulation with the lanthanum hydrous oxide precursor particles also occurs. The sluggish reaction rate at the later stage of reaction is characterized by an in situ transformation, where the soluble tin species is diffused through the porous La₂Sn₂O₇ aggregates to react with entrapped lanthanum precursors.     

Low-Temperature Atomic Layer-Deposited TiO2 Films with Low Photoactivity

Atomic layer deposition (ALD) has been successfully utilized for the conformal and uniform deposition of ultrathin titanium dioxide (TiO₂) films on high-density polyethylene (HDPE) particles. The deposition was carried out by alternating reactions of titanium tetraisopropoxide and H₂O₂ (50 wt% in H₂O) at 77°C in a fluidized bed reactor. X-ray photoelectron spectroscopy confirmed the deposition of TiO₂ and scanning transmission electron microscopy showed the conformal TiO₂ films deposited on polymer particle surfaces. The TiO₂ ALD process yielded a growth rate of 0.15 nm/cycle at 77°C. The results of inductively coupled plasma atomic emission spectroscopy suggested that there was a nucleation period, which showed the reaction mechanism of TiO₂ ALD on HDPE particles without chemical functional groups. TiO₂ ALD films deposited at such a low temperature had an amorphous structure and showed a much weaker photoactivity intensity than common pigment-grade anatase TiO₂ particles.