Chemical Engineering / Kimya Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/14
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Article Citation - WoS: 20Citation - Scopus: 24Preparation of Monodisperse Silica Spheres and Determination of Their Densification Behaviour(Elsevier Ltd., 2014) Topuz, Berna; Şimşek, Deniz; Çiftçioğlu, MuhsinMonodisperse silica spheres in the 50-520 nm size range were prepared by using the Stober process. Diffusive growth has been determined from Nielsen chronomal analysis for the 520 and 310 nm monodisperse silica spheres. The densification behaviour and evolution of the microstructure of the sphere compacts indicated an inverse dependence of shrinkage rate on the sphere size due to viscous sintering. The increase in sphere size from 50 to 500 nm shifted the densification temperature from ∼ 1120 °C to 1240 °C. The amorphous nature of the spheres was conserved up to 1200 °C where cristobalite crystal nucleation started and complete transformation to cristobalite phase has been observed upon heat treatment at 1300 °C. The activation energies for viscous sintering according to the Frenkel and Mackenzie/Shuttleworth models were calculated as 125 and 335 kJ/mol, respectively. These substantially low activation energies can be attributed to the presence of a significant level of silanol groups.Article Citation - WoS: 21Citation - Scopus: 23Monetite Promoting Effect of Nacl on Brushite Cement Setting Kinetics(Royal Society of Chemistry, 2013) Şahin, Erdem; Çiftçioglu, MuhsinBrushite forming calcium phosphate cements (CPCs) have received growing interest for scaffold applications due to their high surface area and high bioresorbability. The dehydrated form of brushite, monetite, has a finer microstructure with higher surface area, higher strength and bioresorbability comparable to brushite, making it a viable alternative phase in CPCs. The increase in monetite content of the β-tricalcium phosphate (β-TCP)-monocalcium phosphate monohydrate (MCPM) cement system due to the reduction in its supersaturation upon addition of NaCl to excess setting liquid was investigated kinetically. The relaxation period was monitored by pH-stat titration of the cement solution by 0.1 M NaOH. Monetite growth was achieved in shorter periods at higher NaCl concentrations where the supersaturation gap between brushite and monetite is thought to be narrowed due to high ionic strength in accord with Pitzer's ion interaction model. The brushite/monetite ratio decreased consistently with increasing NaCl concentration in the 3-6 M range.Article Citation - WoS: 2Citation - Scopus: 3Preparation and Characterization of Diphasic Sol-Gel Derived Unsupported Mullite Membranes(Springer Verlag, 2011) Topuz, Berna; Çiftçioglu, MuhsinDiphasic gels prepared by mixing freshly prepared polymeric silica and polymeric boehmite sols through a modified Al-alkoxide route in mullite compositions led to the crystallization of mullite upon heat treatment at 775°C. Mullite formation was observed at a 100°C higher temperature when diphasic gels were formed by mixing aged polymeric sols containing about 2 nm in diameter boehmite species. These relatively low mullite formation temperatures were attributed to the nanoscale sizes of the polymeric species of the two amorphous phases present in the diphasic gels.Article Citation - WoS: 10Citation - Scopus: 11The Effects of Sb Substitution on Structural Properties in Yba 2cu 3o 7 Superconductors(Springer Verlag, 2011) Akyüz, Gönül B.; Kocabaş, Kemal; Yıldız, Aylin; Özyüzer, Lütfi; Çiftçioglu, MuhsinIn this study, the effects of partial Sb 3+ ion substitutions for the Y sites and the Cu sites on the superconducting properties of YBa 2Cu 3O y (Y123) ceramic superconductors were investigated. The samples were prepared by the conventional solid-state reaction method and the properties of the samples were investigated by means of X-ray diffraction, AC magnetic susceptibility measurements, scanning electron microscope, and energy dispersive X-ray analysis. The critical temperatures were determined to be in the range of 80-92 K for both Systems I and II. It was found that Sb-addition leads to the formation of the non-superconducting YBa 2SbO 6 phase, which has a negative effect on the critical temperature, since the highest critical temperature was measured for pure Y123. However, the increasing substitution level has a negligible effect on the X-ray diffraction analysis peak intensities of the superconducting phases. In addition, SEM images showed that Sb substitution decreases the grain size and modifies the microstructure development, which makes the samples denser.