Mehmet Ali Gülgün

The effect of yttrium on densification and grain growth in α-alumina

The grain growth and densification have been investigated in very high-purity α-alumina doped with varying amounts of yttrium (0 to 3000 wt ppm of yttria) and sintered in air at 1450, 1550 and 1650 °C. Yttrium doping inhibited densification and coarsening at 1450 °C, but had very little effect at 1550 °C and no effect at 1650 °C. The change in densification behaviour is suggested to be related to the transition with increasing temperature from grain boundary diffusion to lattice diffusion controlled densification. The coarsening rate increases faster with temperature than the densification rate. This was correlated with a higher measured activation energy for grain growth than for the diffusion processes, which control the densification.

Cation Segregation in an Oxide Ceramic with Low Solubility: Yttrium Doped α-Alumina

The segregation behaviour of a cation (yttrium) with a low solubility in the polycrystalline oxide host (α-Al2O3) has been investigated at temperatures between 1450 and 1650°C using analytical scanning transmission electron microscopy. Three distinct segregation regimes were identified. In the first, the yttrium adsorbs to all grain boundaries with a high partitioning coefficient, and this can be modelled using a simple McLean-Langmuir type absorption isotherm. In the second, a noticeable deviation from this isotherm is observed and the grain boundary excess reaches a maximum of 9 Y-cat/nm2 and precipitates of a second phase (yttrium aluminate garnet, YAG) start to form. In the third regime, the grain boundary excess of the cation settles down to a value of 6–7 Y-cat/nm2 that is in equilibrium with the YAG precipitates. In a material (accidentally) co-doped with Zr, the Zr seems to behave in a similar way to the Y and the Y + Zr grain boundary excess behaves in the same way as the Y grain boundary excess in the pure Y-doped system. In this latter system, Y-stabilised cubic zirconia is precipitated in addition to YAG at higher Y + Zr concentrations.

Crystallization of hafnia and zirconia during the pyrolysis of acetate gels

Hafnia and zirconia gels were prepared by drying hafnyl or zirconyl acetate solutions. Hafnia and zirconia gels contain both hydroxyl group and bidentate acetates which are directly bonded to the metal ions. Thermal decomposition and crystallization behavior of the gels were investigated through XRD, FT-IR and TEM. Hafnium-containing gels crystallized directly into stable monoclinic hafnia around 500{endash}540{degree}C, while zirconium containing gels first formed metastable tetragonal zirconia around 450{degree}C. The dissimilar crystallization behavior of the gels into metastable, tetragonal zirconia or into stable, monoclinic hafnia can be explained through the difference in free-energy changes of the tetragonal-to-monoclinic phase transformation. {copyright} {ital 1997 Materials Research Society.}

Electron states of YAG probed by energy-loss near-edge spectrometry and ab initio calculations

Abstract Al K, Al L2,3, O K, Y L2,3, and Y M2,3 energy-loss near-edge structures (ELNESs) of Y3Al5O12 (YAG) were measured using a dedicated scanning transmission electron microscope equipped with a parallel electron energy-loss spectrometer system. An attempt was made to interpret the experimental edges with the help of calculated near-edge structures. An ab initio orthogonalized linear combination of atomic orbitals method, within the local density approximation (LDA), was used to calculate the electronic structure of YAG. Site-decomposed and symmetry projected local densities of states (LDOS) and photo-absorption cross-sections (PACS) were compared to experimental ELNES spectra. There is reasonable agreement between the five different, measured spectra and calculated near-edge structures. PACS calculations correlated better than LDOS with the experimental edges in terms of relative intensities of the peaks. Through a comparison with site-decomposed LDOS calculations, several features of the experimental...

Residual Stress Relaxation and Microstructure in ZnO Thin Films

Stability under normal environmental conditions over a long period of time is crucial for sustainable thin-film device performance. Pure ZnO films with thicknesses in the 140 - 450 nm range were deposited on amorphous glass microscope slides and (100)-oriented single crystal silicon wafers by radio frequency magnetron sputtering. The depositions were performed at a starting temperature of 200 oC. ZnO films had a columnar microstructure strongly textured along the <0002> direction. XRD peak-shift analysis revealed that the films were under residual, compressive, in-plane stress of -5.46 GPa for the glass substrate and -6.69 GPa for the Si substrate. These residual stresses could be completely relaxed by thermal annealing in air. When left under normal environmental condition over an extended period of time the films failed under buckling leading to extensive cracking of the films. The XRD and SEM results indicated different mechanisms of stress relaxation that were favored in the ZnO thin films depending on the energy provided. Although thermal annealing eliminated residual stresses, serious micro-structural damage upon annealing was observed. Thermal annealing also led to preferential growth of some ZnO crystals in the films. This kind of behavior is believed to be indicative of stress-induced directional diffusion of ZnO. It appears that for the extended stability of the films, the stresses have to be eliminated during deposition.

Self-induced Crystallinity in RF Magnetron Sputtered ZnO Thin Films

ZnO films were coated on the order of micrometer thickness on various substrates using RF magnetron sputtering. Glass, mica and Si were used as amorphous and crystalline substrates to study film growth. X-ray diffraction measurements revealed a self-induced, (002) oriented texture on all substrates. Effects of residual stresses on growth behavior and possible mechanisms of textured crystallization on crystalline and amorphous substrates are discussed.

Crystallization of CaAl4O7 and CaAl12O19 powders

Calcium is always present in alumina systems as an unintentional (or intentional) dopant, and yet the fundamental effect of its incorporation into the aluminas is not well understood, and is further complicated by the presence of Si. The synthesis of powders of two calcium aluminate phases (CaAl4O7, which is also known as CaO · 2Al2O3 or CA2, and CaAl12O19, which is also known as CaO · 6Al2O3 or CA6) has been investigated using low-temperature chemical-processing techniques. The crystallization of these powders from the amorphous precursor has been examined using various characterization techniques. The precursors for the powders were prepared by mixing stoichiometric proportions of the nitrate salts into a 5 wt% aqueous solution of poly(vinyl alcohol). Conversion of the amorphous precursors to crystalline powders and the subsequent phase transitions were monitored using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and powder X-ray diffractometry (XRD). While powders with CA2 stoichiometry crystallized directly at 883°C, amorphous powders with CA6 stoichiometry first crystallized into an intermediate structure without partitioning and then transformed into CA6 at 1175°C. Fully and partially crystallized powders were analyzed using transmission electron microscopy and electron energy-loss spectroscopy (EELS). Measured near-edge structures (Al–L2,3, Ca–L2,3 and O–K) are presented for the CA2, γ-Al2O3 and CA6 phases. The intermediate phase, identified as γ-Al2O3, was found to accommodate a significant concentration of Ca.

X-RAY PHOTOELECTRON SPECTROSCOPY STUDIES OF BOND STRUCTURE BETWEEN POLYVINYL ALCOHOL AND A TITANATE CROSS-COUPLING AGENT

Chemical interactions between polyvinyl alcohol (PVA) and triethanol amine titanate chelate were studied using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The titanate chelate cross coupled the PVA solution and produced a viscous gel. The gel had a three- dimensional network structure containing --C{sub PVA}--O--Ti--O--C{sub PVA}--organic complexes. A new C(1{ital s}) signature at 285.7 eV and an O(1{ital s}) signature at 531.25 eV were associated with the formation of these complexes. The water of the PVA solution was physically retained in the gelled structure and was readily available for chemical reactions. The removal of this entrapped water was irreversible and lead to a collapsed film of Ti-cross-linked PVA.

Investigation of change in surface morphology of heated surfaces upon pool boiling of magnetic fluids under magnetic actuation

Nanofluids are becoming a significant candidate for new generation coolants to be used in industrial applications. In order to reduce clustering and sedimentation of nanoparticles and improve the heat transfer performance simultaneously, magnetic fluids prepared with magnetic Fe3O4 nanoparticles dispersed in water, which were placed in a pool and were exposed to varying magnetic fields to actuate nanoparticles in the system. The effect of magnetic actuation on boiling heat transfer characteristics and on the surface morphology of the pool was examined. An average enhancement of 29% in boiling heat transfer was achieved via magnetic actuation with rather low magnetic field (magnetic flux densities up to 11 mT) densities. Furthermore, it was observed that magnetic actuation significantly prevented the deposition and sedimentation of the nanoparticles in the pool. Otherwise, significant destabilization of nanoparticles causing aggregation and heavy sedimentation was present as a result of the performed surface analysis. Even though magnetic actuation reduced the sedimentation on the macroscale, the deposition of a thick and porous film occurred onto the pool floor, increasing the surface roughness.

Discovery of Superior Cu-GaOx-HoOy Catalysts for the Reduction of Carbon Dioxide to Methanol at Atmospheric Pressure

Catalytic conversion of carbon dioxide to liquid fuels and basic chemicals by using solar‐derived hydrogen at, or near, ambient pressure is a highly desirable goal in heterogeneous catalysis. If realized, this technology could lead to a more sustainable society together with decentralized power generation. A novel class of holmium‐containing multi‐metal oxide Cu catalysts, discovered through the application of high‐throughput methods, is reported. In particular, ternary systems of Cu‐GaOx‐HoOy>Cu‐CeOx‐HoOy∼Cu‐LaOx‐HoOy supported on γ‐Al2O3 exhibited superior methanol production (10×) with less CO formation than previously reported catalysts at 1 atm pressure. Holmium was shown to be highly dispersed as few‐atom clusters, suggesting that the formation of trimetallic sites could be the key for the promotion of methanol synthesis by Ho.