Key Engineering Materials Vols. 317-318

Abstract: Lanthanum strontium chromite (LSC) thin films were prepared from the solution of La, Sr, Cr nitrates, acetylacetone and 2-methoxyethanol. Dip-coating technique was used to decompose La0.8Sr0.2CrO3, LSC thin films. The obtained precursor solution was sufficiently spinable and dried gel films were crystallized after heat-treating above 500°C for 2 h. The oxidation at 800°C for 40 hr resulted in no Fe3O4 surface-insulating phase, which means that the LSC thin film would be an effective oxidation-protecting layer of SUS430 substrate. The carbon deposited on an uncoated SUS430 substrate was amorphous with spherical morphology. The obtained LSC thin film was found to be very effective on preventing carbon deposition under dry hydrocarbon atmosphere.

Abstract: Ceramics with zero thermal expansion coefficients at room temperature (293K) were investigated. We found the thermal expansion coefficient was controlled by a compounding ratio of lithium aluminum silicate (LAS) and silicon carbide (SiC), which have negative and positive thermal expansion coefficients respectively. Although it was difficult to densify the composite of the LAS and SiC (LAS/SiC) in the sintering process, an addition of nitride improved the sinterability of the LAS/SiC. In order to examine the effect of the nitride additive, at first, the melting point of the LAS with silicon nitride (Si3N4) or aluminum nitride was measured by TG-DTA. The melting point of the LAS decreased with existence of nitride. It is believed that the densification of the LAS/SiC was promoted by the nitride, because the nitride causes the LAS/SiC to form a liquid phase, thereby decreasing the melting point. Next, the lattice constant of the LAS with Si3N4 was measured by XRD and it was verified that the a-axis was longer and the c-axis was shorter than those of the LAS without additive. It is supposed that this phenomenon is due to the substitution of nitrogen for oxygen in the LAS lattice, and the decrease of the melting point of the LAS with nitride seems to be influenced by this substitution of nitrogen.

Abstract: The reduction behavior of Al2O3/CuO powder mixtures, prepared from Al2O3/CuO or Al2O3/Cu-nitrate, was investigated by using thermogravimetry and hygrometry. Also, the influence of powder characteristics on the microstructure and properties of hot-pressed composites is analyzed. It was found that the hydrogen reduction process of nano-sized oxide particles and properties of sintered composites strongly depends on their distribution and size in composite powder. The formation mechanisms of nano-sized Cu dispersions are discussed based on the powder characteristics and reduction kinetics. In addition, the dependence of the microstructure and mechanical properties of hot-pressed composites on powder characteristics is discussed in terms of the initial size and distribution of Cu particles.

Abstract: Silicon nitride - silicon carbide nanocomposite has been prepared by an in-situ method that utilizes formation of SiC nanograins by C+ SiO2 carbothermal reduction during the sintering process. The developed C/SiO2 derived nanocomposite consists of a silicon nitride matrix with an average Si3N4 matrix grain diameter of approximately 200 nm with inter- and intra- granular SiC inclusions with sizes of approximately 150 nm and 40 nm, respectively. The mean value of room temperature 4-point bending strength is 670 MPa with the Weibull modulus of 7.5 and indentation fracture toughness of 7.4 MPa.m1/2. The creep behaviour was investigated in bending at temperatures from 1200°C to 1450°C, under stresses ranking from 50 to 150 MPa in air. A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix. The inserts machined from this composite have three times longer life time compared to those available on the market.

Abstract: Si3N4-based composite powders have been synthesized by vapor phase and liquid phase processes. Nano-sized Si3N4-TiN composite particles were formed by the vapor phase method, in which TiN nanocrystallites were included in amorphous Si3N4 particles. The composite powder was also formed by the liquid phase method, where Si and Ti complex imide powders were prepared in an organic solvent and decomposed by heating. Si3N4-TiN nanocomposites were fabricated by hot pressing of the composite powders. The Si3N4-TiN composites were also fabricated by in-situ process from amorphous Si3N4 and Y2O3-TiO2-AlN additive. In all processes, rod-like Si3N4 grain growth was stimulated by TiN inclusion.

Abstract: Hydrothermal synthesis of nano ZrO2 powders were carried out at a temperature range of 100 to 200°C, using zirconium solution. Formation of ZrO2 formation was accelerated with temperature and precipitation of ZrO2 took place directly from the solution. XRD analysis revealed that monoclinic ZrO2 was formed at all conditions. Crystallite size was found to range from 2.6 to 4 nm. Mean particle size increased from 93 nm at 135°C to 125 nm at 200°C after reaction time of 2 h.

Abstract: Nanocrystalline α-Al2O3 powders have been prepared by the pyrolysis of a resin compound of aluminum with polyester by a two-step calcination process. A polymeric precursor was prepared using a complexing agent to keep the metal ions in homogeneous solution, which gives sufficient flexibility for the system to exist homogeneously throughout the reaction without undergoing precipitation. The metal-ion-polyester resin forms the precursor material on complete polymerization reaction of aluminum nitrate, citric acid and ethylene glycol. A single-phase α-Al2O3 powder resulted after calcinations above 1150°C, but during heat treatment α-Al2O3 particles grow very fast by coalescence. So 2-step calcination was used, where the first step was done in a reducing atmosphere at above 1150°C and second calcinations were done in oxidizing atmosphere at the relatively low temperature of 1000°C. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry, thermogravimetry , transmission electron microscopy(TEM), and BET surface area analysis. The nanocrystalline α-Al2O3 particles obtained by this 2-step calcinations method had an average specific surface area of >170m2/g, with an average particle size between 40 and 60nm.

Abstract: Magnetite (Fe3O4) particles were prepared for biomedical application through three different routes. Each route had the difference in the selection of pH controlling agents. Ammonia, Urea and combined use of ammonia and urea were tested in the synthesis through co-precipitation. With the combined use of urea and ammonia, the size of particles could be reduced as small as 10 nm while effectively suppressing the agglomeration among particles. The surface charge measured at physiological condition explained the good dispersion behavior of nano-magnetite particles. The superparamagnetism observed in nano magnetite particles was expected to be useful for biomedical application in the respect of magnetic localization and dispersion.

Abstract: Unagglomerated α-Al2O3 powders of 100 to 200nm were synthesized by combustion spray pyrolysis with droplet filtration. The ignition conditions of the oxidizer and fuel were well elucidated by the calculation of partial equilibrium species and thermal analysis. A metal screen filter with 500 mesh was employed to dilute the number concentration of the sprayed droplets. The Reynold’s number of the aerosol fluid was kept at the value of 1,200 to keep a short residence time and a laminar flow.

Abstract: Silicon carbide (SiC) nano-powders are successfully synthesized by a reaction between carbon nano-powders (carbon black) and SiO gas at 1300°C for 9 hrs in dynamic argon atmosphere (flow rate; 400 cm3 min-1), using the solid-vapor reaction method. The particle size of synthesized SiC nano-powders is below 40 nm and the shape is uniform. Unexpectedly, SiC nano-fibers are also coexisted in the SiC nano-powders. The quantitative and qualitative properties of the SiC nanopowders and nano-fibers are analyzed by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). Carbon residuals removed by heating over 700°C in air are estimated by thermogravimetry analysis (TGA). It is found that the SiC nanopowders are easily produced by direct synthesis via the solid-vapor reaction method. The morphological characteristics of the resulting SiC nano-powders are dependent upon the morphology of carbon black used as precursor.