Synthesis of Spherical β-Silicon Carbide Particles by Ultrasonic Spray Pyrolysis

Fine agglomerate-free spherical β-SiC powder was synthesized from a dispersion of colloidal silica, saccharose, and boric acid, by means of an ultrasonic spray pyrolysis method. Droplets of 2.2 μm were formed with an aerosol generator, operated at 2.5 MHz, and carried into a reaction furnace at 900°C with argon. Spherical X-ray amorphous gel particles of 1.1 μm were obtained. β-SiC particles with a mean diameter of 0.79 μm and spherical shape resulted when the SiC gel precursor particles were heated at 1500°C in argon.     

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.     

A Layered Alumina Composite Tested at High Temperature in Air

An oxidation-resistant interphase for layered alumina composites was prepared by aerosol spray deposition of submicrometer alumina powder. A model composite specimen was made by placing the interphase between thin layers of monolithic alumina. The composite sandwich was hot-pressed to control the interphase fracture resistance for successful crack deflection. Specimens were tested under four-point bending in air at two crosshead speeds at ambient temperature, 1000°C, and 1200°C. The fracture behavior was temperature dependent, with a higher work of fracture at higher temperatures. Interphase delamination and composite toughening behavior were very pronounced at all temperatures. At the highest temperature, the transition to multiple widely distributed cracks and increased crack deflection may be related to inelastic deformation in the alumina.     

Preparation of Highly Dense PZN–PZT Thick Films by the Aerosol Deposition Method Using Excess-PbO Powder

Lead zinc niobate–lead zirconate titanate thick films with a thickness of 50–100 μm were deposited on silicon and alumina substrates using the aerosol deposition method. The effects of excess lead oxide (PbO) on stress relaxation during postannealing were studied. Excess PbO content was varied from 0 to 5 mol%. The as-deposited film had a fairly dense microstructure with nanosized grains. The films deposited on silicon were annealed at temperatures of 700°C, and the films deposited on sapphire were annealed at 900°C in an electrical furnace. The annealed film was detached and cracks were generated due to the high residual compressive stress and thermal stress induced by thermal expansion coefficient mismatch. However, the film deposited using powder containing 2% of excess PbO showed no cracking or detachment from the substrate after the postannealing process. The PbO evaporation at elevated temperature during the postannealing process seemed to have reduced the residual compressive stress. The remanent polarization and relative dielectric constant of the 50 μm thick films annealed at 900°C were 43.1 μC/cm² and 1400, respectively, which were comparable with the values of a bulk specimen prepared by a powder sintering process.     

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.     

Chemical Vapor Deposition of YBa2Cu3Ox in a Cold Plasma Reactor sing an Aerosol Vaporization Technique

A plasma-enhanced chemical vapor deposition technique, utilizing an aerosol decomposition/vaporization process in a cold plasma reactor, was used to form YBa₂Cu₃O, (YBCO) thin films on single-crystal MgO substrates. Aerosol droplets of the precursors were generated by an ultrasonic nebulizer operating at 1.63 MHz, while a 50 kW rf generator, operating at 2.87 MHz, was used to create the plasma and heat a stainless steel susceptor. Nitrate, acetylacetonate, and tetramethylheptanedionate compounds were used as precursors, and distilled water, ethyl alcohol, and an aqueous benzoic acid solution were investigated as solvents for the aerosol solution. The effects of the solubility and decomposition temperature of the chemical precursors, and the vapor pressure of the solvents, on the microstructure and phase assemblage of the deposits were determined. Specific combinations of substrate temperature, in the range of 800°-940°C, and oxygen partial pressure, in the range of 0.3–2.7 kPa, were found to produce in situ , crystalline, stoichiometric YBCO films.     

Decomposition-Crystallization of Polymer-Derived Si-C-N Ceramics

Monolithic polymer-derived Si-C-N ceramics were processed by blending an oligomeric Si-C-N precursor (liquid polysilazane) with 70 vol% of crosslinked or pyrolyzed Si-C-N powder particles, which were obtained from the same liquid precursor preheated at 300° or 1000°C, respectively. Powder compacts subsequently were annealed at 300°C to crosslink the liquid precursor acting as a binder between the powder particles, thus yielding monolithic green bodies. Heat treatment at 1540°C was performed to initiate crystallization in the various samples. Microstructure development and, in particular, crystallization behavior were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and preliminary nuclear magnetic resonance (NMR) spectroscopy. The material containing 300°C polymer powder (with oligomeric binder, also crosslinked at 300°C) revealed a homogeneous amorphous microstructure after exposure to temperatures of 1540°C. In contrast, the specimen containing powder particles preheated at 1000°C exhibited a high volume fraction of SiC crystallites within regions that were previously filled by the binder; however, the Si-C-N powder particles themselves remained amorphous. SEM observations as well as XRD studies showed the formation of idiomorphic SiC and Si₃N₄ crystallites on specimen surfaces as well as along internal crack walls. This finding suggested that vapor-phase reactions at the surface were involved in the formation of crystalline phases at temperatures >1250°C. Moreover, NMR spectroscopy data indicated a phase separation process, implying structural rearrangement prior to crystallization.     

Thermal Oxidation of Aluminum Nitride Powder

The oxidation kinetics, morphology, and crystallinity of aluminum nitride (AlN) powder thermally oxidized in flowing oxygen were determined from 800° to 1150°C. At 800°C the oxidation became detectable with weight change. AlN powder was almost completely oxidized at 1050°C after only 0.5 h. Amorphous aluminum oxide formed at relatively low temperatures (800°–1000°C), with a linear oxidation rate governed by the oxygen–nitride interfacial reaction. Transmission electron microscopy displayed individual aluminum oxide grains which formed a discontinuous oxide layer at this temperature range. The aluminum oxide was crystalline at higher temperatures (>1000°C), as detected by X-ray diffraction, and the density of oxide grains increased with temperature.     

Sol–Gel Synthesis and Characterization of Alumina–Calcium Hexaaluminate Composites

Boehmite sol was prepared by hot water hydrolysis of aluminum iso-propoxide using nitric acid as the catalyst. Calcium nitrate to yield 0–20 vol% calcia was added to the boehmite sol. The boehmite with additives was calcined at 600°C for 3 h. The calcined powder was milled at 230 rpm for 6 h and particle size was measured using Laser particle size analyzer. The powder samples were calcined at 1600°C for 3 h and the formation of calcium hexaaluminate was discussed using phase diagram, transmission electron microscope, energy dispersive spectra, and X-ray diffraction spectra. The powder samples calcined at 600°C for 3 h were compacted into cylindrical pellets and sintered at temperatures ranging from 1400° to 1600°C for 6 h and the formation of hexaaluminate (platelike) grains were confirmed using Scanning electron microscope and optical microscopy.     

Production of Nanosized YAG Powders with Spherical Morphology and Nonaggregation via a Solvothermal Method

Monodispersed nanosized yttrium aluminum garnet (YAG) powder was synthesized via a mixed-solvent thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. Pure-phase YAG crystalline powder was obtained at low temperature (290°C) and low pressure (10 MPa). The resulting products were characterized by X-ray powder diffraction (XRD), infrared, and transmission electron microscopy (TEM). XRD results showed that single-phase YAG could be formed directly from an amorphous precursor at 280°C and become fully developed at 290°C. TEM images showed that the YAG powder particles in the study were basically spherical in shape and well-dispersed with a mean grain size of about 60 nm.     

Pyrolytic Conversion of Spherical Organo-silica Powder to Silicon Nitride under Nitrogen

Monodisperse, spherical Si₃N₄ powder composed of fine particulates was synthesized by pyrolyzing spherical organo-silica powder under nitrogen. The organo-silica powder was prepared by hydrolyzing a mixture of phenyltrimethoxysilane (PTMS) and tetraethoxysilane (TEOS) in a methanol solution of water and ammonia. The organo-silica powder consisted of 81.3 at.% silicon units derived from PTMS and 18.7 at.% silicon units derived from TEOS. During the pyrolysis under nitrogen, the organo-silica powder decomposed to a mixture of free carbon and silica, with an increase of the surface area, at 500°-600°C, followed by the formation of alpha-Si₃N₄, with ß-Si₃N₄ as a minor phase, at 1450° and 1500°C and ß-SiC at 1550°C. The pyrolyzed powders, which retained the spherical shape and monodispersity of the organo-silica powders, with a reduction in mean particle diameter, were composed of fine particulates that were ~40 nm in size.     

Dense Nanostructured Hydroxyapatite Coating on Titanium by Aerosol Deposition

In order to improve biocompatibility of Ti metal substrates, 1-μm-thick nanostructured hydroxyapatite (HAp) coatings were deposited on the substrates through aerosol deposition, which sprays HAp powder with an average particle size of 3.2 μm at room temperature in vacuum. The original HAp particles were fractured into nanoscale fragments to form highly dense coating during the deposition process. Density of the HAp coating was 98.5% theoretical density (TD). Transmission electron microscopy observation revealed that the as-deposited coating consisted of HAp crystallites with average grain size of 16.2 nm and amorphous phase. Tensile adhesion strength between the coating and the substrate was 30.5±1.2 MPa. Annealing up to 500°C in air increased crystallinity and grain size in the coating without any delamination or crack according to X-ray diffraction analysis and electron microscopy. MTS assay and alkaline phosphatase activity measurements with MC3T3-E1 preosteoblast cell revealed that the biocompatibility was greatly improved by postdeposition heat treatment at 400°C in air due to well-crystallized HAp with average grain size of 29.3 nm. However, further heat treatment at 500°C deteriorated biocompatibility due to rapid growth of HAp grains to average size of 99 nm. Cross section of the coating on a commercially available Ti dental implant revealed full coverage of the surface with HAp.     

Kinetics and Mechanisms for Nitridation of Floating Aluminum Powder

Aluminum powder entrained by ammonia-containing nitrogen gas was reacted at various temperatures and time to form aluminum nitride powder. The kinetics of nitride formation were determined by a quantitative X-ray analysis and compared with those determined by a nitrogen analysis of the product. The conversion to aluminum nitride increased with the reaction time following a sigmoidal rate law. The reaction time for full conversion decreased as the temperature increased in the range 1050°–1300°C. The reaction rate constant at a given temperature was evaluated using the Avrami equation. The activation energy for the reaction was 1054 ± 91 kJ/mol in the temperature range of 1050°–1200°C, and decreased to 322 ± 70 kJ/mol above 1200°C. Comparative analysis of powders formed below and above 1200°C suggested strongly that the rate-controlling step changed from chemical reaction to mass transport above 1200°C.     

Nanometer-Sized ZrO2 Particles Prepared by a Sol–Emulsion–Gel Method

ZrO₂ powder was prepared by a sol–emulsion–gel method at temperatures below 140°C from ZrO(NO₃)₂· n H₂O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600°C. The metastable tetragonal powder (600°C) was comprised of ultrafine 4- to 6-nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100°C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100°C and with a grain size of 0.25 μ.     

Synthesis of Ultrafine Grain Ferrites

Methoxides of nickel(II) and iron(III) were synthesized, mixed in the ratio to form NiFe₂O₄, aerosolized, hydrated, and then fired in a streamline tubular reactor. Submicrometersized spherical particles were obtained from pyrolysis at temperatures up to 570°. At 425°, the sudden loss of volatiles shattered the particles to much smaller spheres (with 30% less than 0.1 μm) giving a remarkable increase in surface area. Pyrolysis up to 570° caused a gradual increase in the powder density toward that of NiFe₂O₄. Heating the aerosol product up to just 600° with a heating rate of 5°/min yielded pure crystalline nickel ferrite.     

Reaction of YBa2Cu3O7-δ Powder with NOx Gas in an Aerosol Reactor

Thermodynamic calculations predict, and experiments verify, that YBa₂Cu₃O₇-₈ (123) powder is unstable in the presence of NO_x-containing aerosol reactor exhaust gases at temperatures below about 600°C. Powders collected above the stability temperature are single-phase 123, while powders collected at lower temperature contain Ba(NO₃)₂ formed by reaction of the powder with NO_x, after exit from the hot zone.     

Nanostructured Dense ZrO2 Thin Films from Nanoparticles Obtained by Emulsion Precipitation

Nonagglomerated spherical ZrO₂ particles of 5–8 nm size were made by emulsion precipitation. Their crystallization and film-forming characteristics were investigated and compared with nanosized ZrO₂ powders obtained by sol–gel precipitation. High-temperature X-ray diffraction indicated that the emulsion-derived particles are amorphous and crystallize at 500°C into tetragonal zirconia, which is stable up to 1000°C. Crystallite growth from 5–20 nm occurred between 500°–900°C. Films of 6–75 nm thickness were made by spreading, spin coating, and controlled deposition techniques and annealed at 500°–600°C. The occurrence of t -ZrO₂ in the emulsion-precipitated powder is explained by the low degree of agglomeration and the corresponding low coarsening on heating to 500°–800°C, whereas the agglomerated state of the sol–gel precipitate powder favors the occurrence of the monoclinic form of zirconia under similar conditions.     

Reaction Processes and Characterization of ZrB2 Powder Prepared by Boro/Carbothermal Reduction of ZrO2 in Vacuum

The present work was concentrated mainly on the reaction processes of boro/carbothermal reduction (BCTR) of ZrO₂ with B₄C and carbon in vacuum, and characterization of morphology and sinterability of the obtained ZrB₂ powder. Combining the thermodynamic calculations, X-ray diffraction results, and the trend of furnace pressure with temperature during synthesis, a detailed explanation of the reaction processes of BCTR was developed. Most of the ZrB₂ particles obtained at 1650°C presented a nearly spherical morphology, whereas those synthesized at 1750°C showed a nearly columnar morphology with an increased size. Compared with the powder synthesized at 1750°C as well as the commercially additive-free powder used in the reported work, the ZrB₂ powder synthesized at 1650°C showed a better sinterability due to its smaller particle size and lower oxygen content.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

Hierarchically Porous Ceramics from Diatomite Powders by Pulsed Current Processing

Hierarchically porous ceramic monoliths have been fabricated by pulsed current processing (PCP) of diatomite powders. The partial sintering behavior of the porous diatomite powders during PCP or spark plasma sintering was evaluated at temperatures between 600° and 850°C. Scanning electron microscopy and mercury porosimetry measurements showed that the PCP method was able to bond the diatomite powder together into relatively strong monoliths without significantly destroying the internal pores of the diatomite powder at a temperature range of 700°–750°C. Little fusion at the particle contact points occurred at temperatures below 650°C while the powder showed partial melting and collapse of both the interparticle pores and the internal structure at temperatures above 800°C.