Mufit Akinc

Preparation of spherical, monosized Y2O3 precursor particles

Abstract Spherical, monosized yttria precursor particles were obtained by homogeneous precipitation in aqueous solutions by reaction with the thermal decomposition products of urea. Increasing [Y 3+ above 0.05 M resulted in a deviation from spherical morphology and caused agglomeration of particles. Over the concentration range studied, excess urea did not affect particle morphology, but increased the yield. Increasing aging time appeared to increase particle size as well as to improve yield, as long as the urea was not depleted. The approximate chemical composition of the precipitate was YOHCO 3 . The YOHCO 3 particles formed were amorphous to X-rays, and underwent a two-stage thermal decomposition, first forming Y 2 O 2 CO 3 near 180°C, and then cubic Y 2 O 3 above 610°C. Generation of CO 3 2− appeared to be crucial to the formation of the solid phase. Heating the aqueous yttrium solution with trichloroacetic acid (CCl 3 COOH) instead of urea as the precipitating agent produced a solid phase, while heating the same solution with formamide (HCONH 2 ) substituted for urea formed no precipitate.

Synthesis and high-temperature stability of Ti3SiC2

Abstract Titanium silicon carbide (Ti 3 SiC 2 ) was synthesized by reactive sintering of elemental reactants. By using an off-stoichiometric ratio (1.2 moles of silicon), Ti 3 SiC 2 with a purity of better than 98 vol.% was synthesized. The synthesized materials were annealed at 1600°C and 1800°C for up to 10 h to verify its chemical stability at high temperatures. The ternary compound was found to be stable at temperatures as high as 1800°C under an argon atmosphere. Dense compacts of Ti 3 SiC 2 were obtained by hot pressing of reactively sintered compacts. The oxidation behavior of the ternary compound was investigated in air at 1000°C and found to be paralinear.

Oxide scale formation and isothermal oxidation behavior of Mo–Si–B intermetallics at 600–1000°C

Initial scale formation in the range 600–1000°C and isothermal oxidation behavior at 1000°C was investigated for Mo–Si–B intermetallics containing 81–88 wt% molybdenum. All compositions exhibited an initial transient oxidation period consisting of a mass gain due to MoO3 and SiO2 formation, followed by a rapid mass loss starting at 750°C due to MoO3 volatilization. After the initial transient oxidation period, oxidation proceeded at a much slower rate. During isothermal oxidation at 1000°C the oxidation rate was found to vary inversely with the ratio of B/Si in the intermetallic, indicating that viscous flow of the scale was an important factor in determining the isothermal oxidation rate at 1000°C.

Synthesis of yttrium aluminum garnet precursor powders by homogeneous precipitation

Abstract A study was performed to characterize the homogeneous precipitation of yttrium aluminum garnet (YAG) precursor particles by the thermal decomposition of urea in aqueous solutions. Cation concentrations were varied from 0·005 m to 0·30 m . Observation of powder morphology together with chemical analysis suggests a sequential precipitation process with aluminum ions forming a solid phase first. Fine-grained, reactive powders were obtained that crystallized to single-phase YAG upon heating to 850°C. The precipitate was a hydrated basic carbonate.

Mie Scattering Effects from Monodispersed ZnS Nanospheres

Transmission spectra of monodispersed particles exhibit pronounced resonance features from which particle properties can be deduced. ZnS powders were used as model materials since they can be synth ...

Synthesis of nickel hydroxide powders by urea decomposition

Abstract Synthesis of nickel hydroxide from aqueous solutions by decomposition of urea was investigated. Spherical agglomerates of nanocrystalline particles were obtained. Precipitated powders show turbostratic α-phase with significant carbonate intercalated into the structure. Addition of dispersant HPMC increases the specific surface area and reduces the median agglomerate size, whereas increase in aging time increases the precipitation yield considerably.

The effect of calcination temperature on some of the adsorptive properties of fine alumina powders obtained by emulsion evaporation technique

Abstract The thermal decomposition of an alumina precursor, obtained by the emulsion evaporation technique, was investigated by thermogravimetry (TG) and differential thermal analysis (DTA). The thermal decomposition was completed in four steps between 100°C and 550°C. For each step, the activation energy was determined from the TG data by the Coats–Redfern procedure and the decomposition was discussed. The X-ray diffraction (XRD) data revealed that diaspore (β-AlOOH) was formed during the emulsion evaporation. The resulting pure but porous α-Al 2 O 3 powders were then calcined for 2 h at various temperatures between 600°C and 1600°C. The adsorption and desorption of N 2 on the calcined specimens at approximately 77 K was investigated. The variation of the total micro and mesopore volumes, the micropore volumes, the surface area and heat of adsorption as calculated from experimental data were discussed as a function of the calcination temperature. It was observed that the above mentioned properties reached a maximum at 800°C, and then decreased rapidly by increasing temperature. It was concluded that it was possible to prepare fine α-Al 2 O 3 powders whose specific surface areas varied between 20 and 90 m 2 g −1 by changing the calcination temperature between 600°C and 1600°C.

Lattice trends in Ti5Si3Zx (Z=B,C,N,O and 0<x<1)

Abstract Several Ti5Si3Zx compositions (Z=boron, carbon, nitrogen and oxygen) were synthesized by arc-melting. Powder X-ray diffraction indicates the materials maintain the Mn5Si3-type structure of the binary compound Ti5Si3. Calculated cell constants were correlated to ternary composition based on chemical analysis. Nitrogen and oxygen additions to Ti5Si3 promote a cell volume decrease, while boron additions promote a cell volume increase. Carbon additions cause a decrease in the a-cell constant and an increase in the c-cell constant with a concomitant cell volume increase. Room temperature X-ray single crystal structural analysis was performed on one composition for each ternary addition. Each analyzed composition has the P63/mcm space group (No. 193) with the Mn5Si3-type structure. The ternary addition occupies the normally vacant interstitial site at the center of the trigonal antiprisms of titanium (Ti2) atoms (Ti6Z polyhedra), located in chains at the corners of the hexagonal unit cell. Bonding between the interstitial atoms and the titanium (Ti2) atoms in the Ti6Z polyhedra is indicated by a decrease in the Ti2–Ti2 and Ti2–Z atomic separations.

Morphology of lanthanum carbonate particles prepared by homogeneous precipitation

The synthesis of lanthanide oxide precursor particles through urea decomposition has received considerable attention over the past few years. It was previously reported by several different workers that the synthesis of spherical, monodispersed light lanthanide ceramic precursor particles was consideably more difficult than the precipitation of spherical particles from the heavy lanthanide elements. These difficulties have been overcome to a large extent by proper choice of processing conditions. This paper describes the synthesis conditions for which dispersed spherical particles and plates may be formed of lanthanum, the lightest of the lanthanide elements. The experimental conditions for the synthesis of these particles is given in the form of morphological diagrams and the chemistry and yield of the process is also reported. The process was also analyzed as a function of time in order to observe the supersaturation and growth regimes. The effect of calcining on the morphology of the particles is also discussed.

Solubility of boron in Mo5+ySi3−y

Abstract Using X-ray analysis, optical microscopy, chemical analysis, and electron probe microanalysis (EPMA) it has been determined that the level of boron solubility in Mo5+ySi3−y at 1800°C reaches a maximum value of approximately 2 at% in a narrow region within the homogeneity range of −0.08⩽y⩽0.04. Both the B doped and undoped Mo5+ySi3−y (T1) have the tetragonal W5Si3 crystal structure, which corresponds to the I4/mcm space group. Within the single phase Mo5+ySi3−yBx (T1) region there is a contraction in the lattice volume with increasing boron concentration while there is an increase in cell volume with an increase in Mo:Si ratio.