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Publication Metadata only Effects of different milling conditions on the properties of lanthanum hexaboride nanoparticles and their sintered bodies(Elsevier, 2019) Ağaoğulları, Duygu; Akçamlı, Nazlı; Duman, İsmail; Oveçoğlu, M. Lutfi; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531Publication Metadata only Effects of milling parameters on the microstructural and thermal properties of nanocrystalline lanthanum hexaboride powders(Australian Ceramic Society, 2018) Agaogullari, Duygu; Ovecoglu, M. Lutfi; Duman, Ismail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531This study reports the effects of some milling parameters (duration, ball-to-powder weight ratio (BPR), process control agent (PCA)) on the fabrication of nanocrystalline LaB6 powders via a combined process of mechanochemical synthesis (MCS) and acid leaching, starting from La2O3-B2O3-Mg powder blends. MCS was carried out in a high-energy ball mill (SPEX (TM) 8000D Mixer/Mill, at 1200 rpm) using hardened steel vial and balls under Ar atmosphere. As-synthesized products were leached out with 4 M HCl from unwanted MgO phase and probable Fe impurities. When a BPR of 18:1 was used, 5 h of MCS and leaching yielded LaB6 powders with an average particle size of 100 nm, containing a very small amount of FeB49 phase. Pure LaB6 powders were obtained with an average particle size of 53 nm after 12 h of MCS of the 0.5 wt% PCA added La2O3-B2O3-Mg powder blends with a BPR of 10:1 and after leaching.Publication Metadata only Effects of solvent on TEOS hydrolysis kinetics and silica particle size under basic conditions(Springer, 2013) Malay, Ozge; Menceloglu, Yusuf Z.; Department of Chemistry; Yılgör, İskender; Faculty Member; Department of Chemistry; College of Sciences; 24181In-situ liquid-state Si-29 nuclear magnetic resonance was used to investigate the temporal concentration changes during ammonia-catalyzed initial hydrolysis of tetraethyl orthosilicate in different solvents (methanol, ethanol, n-propanol, iso-propanol and n-butanol). Dynamic light scattering was employed to monitor simultaneous changes in the average diameter of silica particles and atomic force microscopy was used to image the particles within this time frame. Solvent effects on initial hydrolysis kinetics, size and polydispersity of silica particles were discussed in terms of polarity and hydrogen-bonding characteristics of the solvents. Initial hydrolysis rate and average particle size increased with molecular weight of the primary alcohols. In comparison, lower hydrolysis rate and larger particle size were obtained in the secondary alcohol. Exceptionally, reactions in methanol exhibited the highest hydrolysis rate and the smallest particle size. Ultimately, our investigation elaborated, and hence confirmed, the influences of chemical structure and nature of the solvent on the formation and growth of the silica particles under applied conditions.Publication Metadata only First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: microstructural, mechanical and thermal characterization(Elsevier B.V., 2023) Kavak, S.; Bayrak, K. G.; Mansoor, M.; Kaba, M.; Ayas, E.; Derin, B.; Öveçoğlu, M.L.; Ağaoğulları, D.; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531First principles calculations were conducted on (HfTiWZr)B2 high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B2 HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB2 structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10−4 mm3, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C. © 2022 Elsevier LtdPublication Metadata only Mechanical activation-assisted autoclave processing and sintering of HfB2-HfO2 ceramic powders(Elsevier Sci Ltd, 2016) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531This study reports on the synthesis and consolidation of HfB2-HfO2 ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl4, B2O3 and Mg starting powders were mechanically activated for 5 min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500 degrees C for 6 or 12 h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl2 by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190 nm comprised the HfB2 phase with an amount of 79.6 wt% in addition to the HfO2 phase and a very small amount of Mg2Hf5O12 phase after mechanical activation for 5 min and autoclave processing for 12 h. They were consolidated at 1700 degrees C both by PS for 6 h and SPS for 15 min. The Mg2Hf5O12 phase decomposed during sintering and bulk samples only had the HfB2 and HfO2 phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB2-HfO2 ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (similar to 91%), microhardness (similar to 16 GPa) and relative wear resistance (2.5) values of the HfB2-HfO2 ceramics consolidated by SPS. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Publication Metadata only Preparation of yttria-stabilized zirconia by the reverse microemulsion method and the effect of sc and ce doping on microstructure and ıonic conductivity for solid oxide fuel cell applications(Wiley-Blackwell Publishing, Inc, 2011) Öksüzömer, Faruk; Vatansever, Sibel; Koç, Serkan Naci; Deligöz, Hüseyin; Gürkaynak, Mehmet A.; Department of Chemistry; Somer, Mehmet Suat; Faculty Member; Department of Chemistry; College of Sciences; 178882In this study, 8 mol% yttria-stabilized zirconia (8YSZ), ceria-, and scandia-doped YSZ powders were synthesized by the reverse microemulsion method. The powders were calcined at 1000 degrees C and sintered at 1450 degrees C. The crystalline properties and microstructure of the samples were characterized by X-ray diffraction and SEM methods, respectively. Oxygen ionic conductivities of the samples were measured by electrochemical impedance spectroscopy in the temperature range 250-375 degrees C. Grain interior and grain boundary resistivities were also calculated. Particle sizes of YSZ and Sc-doped YSZ were found to be < 100 nm, while that of Ce-doped YSZ was 120-150 nm. Approximately 1 mu m grains were observed after sintering of YSZ at 1450 degrees C. Particularly, the grain boundary and also the grain interior resistivities decreased with 3% scandia-doped YSZ. It was observed that the total conductivity of 3YSZ was higher than that of 3% ceria-doped YSZ.Publication Metadata only Reduction of viscosity of alumina nanopowder aqueous suspensions by the addition of polyalcohols and saccharides(Wiley, 2010) Akinc, Mufit; N/A; Department of Chemistry; Department of Chemistry; Department of Chemistry; Yar, Yasemin; Acar, Havva Funda Yağcı; Yurtsever, İsmail Ersin; PhD Student; Faculty Member; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; 178902; 7129Flow behavior of nanosize alumina suspensions in the presence of polyalcohols was investigated. Viscosity of high solids content (35 vol%) alumina nanopowders was measured at room temperature. It was shown that after the first advancing shear rate branch, viscosity measurements show excellent reproducibility. All the suspensions showed the formation of large clusters, which are broken down as shear rate increases indicating reversible cluster breaking and reforming. Viscosity of suspensions decreases dramatically with up to 10 wt% polyalcohol addition. Only ethylene glycol showed an increase in viscosity indicating no interaction with the particle surface. The addition of polyalcohols is believed to prevent the formation of large and/or strong clusters by modifying the alumina particle surface. The exact nature and quantitative description of viscosity reduction by polyalcohol addition have yet to be established.Publication Metadata only Room-temperature mechanochemical synthesis and consolidation of nanocrystalline HfB2-HfO2 composite powders(Goller Verlag Gmbh, 2018) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lutfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531This study reports on the in-situ preparation of HfB2-HfO2 composite powders at room temperature by means of mechanochemical synthesis (MCS) from HfCl4-B2O3-Mg powder blends. The effects of milling duration and excess amounts of B(2)O3 and Mg reactants (20 and 30 wt%) on the HfB2 formation mechanism were investigated. After MCS and purification, HfB2, HfO2 and Mg2Hf5O12 phases were obtained. The Mg2Hf5O12 phase decomposed during the annealing treatment conducted at 1000 degrees C under Ar flow. The as-synthesized, purified, annealed and subsequently leached powders were characterized with an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM) and particle size analyzer (PSA). The HfB2-Hf-O2 composite powders with an average particle size of 140 nm and predominantly rounded morphology were consolidated with cold pressing/pressureless sintering (PS) and spark plasma sintering (SPS) techniques. The relative density values of the HfB2-Hf-O2 composites obtained by means of PS (with 2 wt% Co) and SPS techniques were 91.82 % and 93.79 %, respectively. A relatively high densification rate for the HfB2-Hf(O2 )ceramic was achieved by means of Co addition, which was considered a promising sintering aid for HfB2-based ceramics. The HfB2-HfO2 composite sample consolidated with SPS exhibited hardness, wear volume loss amount and friction coefficient values of 18.45 GPa, 4.30 mm(3) and 0.60, respectively.Publication Metadata only Size effects in Fe3+-doped PbTiO3 nanocrystals-Formation and orientation of (Fe-ti '-V-O(center dot center dot))(center dot) defect-dipoles(Elsevier, 2010) Erdem, Emre; Eichel, Ruediger-A.; N/A; Department of Chemistry; Kiraz, Kamil; Somer, Mehmet Suat; Researcher; Faculty Member; Department of Chemistry; N/A; College of Sciences; N/A; 178882A set of Fe3+-modified PbTiO3 nanopowders has been synthesized according to the combined polymerization and pyrolysis (CPP) route of metallorganic precursors [Erdem E, Bottcher R, Semmelhack H-C, Glasel H-J, Hartmann E, Hirsch D. Preparation of lead titanate ultrafine powders from combined polymerisation and pyrolysis route. J Mater Sci. 2003;38:3211-7] with subsequent calcination at various temperatures. X-ray diffraction verifies the formation of a PbTiO3 perovskitic phase and Raman-spectroscopy proves the existence of ferroelectricity. Furthermore, the prevailing defect structure has been investigated by means of electron paramagnetic resonance (EPR) spectroscopy. The EPR results clearly indicate marked size effects by approaching to the critical grain size (d(crit) 12 nm) at which a size-driven tetragonal-to-cubic phase transition is observed at room temperature. As a function of mean grain size, either (Fe-Ti'-V-O(center dot center dot))(center dot) defect dipoles or 'isolated' defects (Fe-Ti', V-O(center dot center dot)) are formed. These results are analyzed in terms of a core-shell model. Accordingly, the obtained Fe3+-modified PbTiO3 nanoparticles consist of a ferroelectric core, a distorted interface region, and a cubic dead layer which is paraelectric.Publication Metadata only Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends(Elsevier, 2016) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, Mustafa Lutfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531Samarium borate (SmBO3) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm2O3) and boron oxide (B2O3) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B2O3 phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700-1250 degrees C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV-vis) techniques. Fine-grained and pure SmBO3 powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO3 powders were synthesized after mechanical activation for 1 h and annealing at 700 degrees C for 2 h. The polymorphic transformation temperature of SmBO3 powders from triclinic to hexagonal is about 1080 degrees C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO3 phase and its transformation to hexagonal form were found to take place at similar to 50-100 degrees C lower temperatures than those reported in other methods. Mainly hexagonal SmBO3 powders were obtained after annealing at 1150 degrees C in the presence of a very small amount of triclinic SmBO3. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO3 phase) and 0.68% (hexagonal SmBO3 phase) depending on their crystal structures. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.