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: 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: 15Citation - Scopus: 17Preparation of Particulate/Polymeric Sol-Gel Derived Microporous Silica Membranes and Determination of Their Gas Permeation Properties(Elsevier Ltd., 2010) Topuz, Berna; Çiftçioğlu, MuhsinMonodisperse silica sols with well-defined spherical particles ranging in size from 5 to 310 nm were prepared through Stober process. Both particulate and polymeric sol-gel routes were employed for the preparation of stable silica sols. The use of polymeric species in combination with particulate silica spheres may allow the design of predefined membrane pore structures with high thermal stability by cubic/random/close packing of monodisperse spherical particles incorporated into the polymeric network. The size and volume content of spheres were varied in order to modify the consolidation behaviour of 2-structural silica membranes which would enhance the thermal stability. The low shrinkage level for sphere loaded 2-structural systems compared to the pure polymeric counterparts might be explained by the decrease in the structural free energy of the polymeric/particulate 2-structural system. The thermal stability of the microporous membranes may thus be improved by incorporating particulates into the polymeric network through the formation of a lower extent of thermally induced microcrack formation. The N2 permeation through 90 nm silica sphere added silica membranes remained constant when they were heat treated in the 250-400 °C range indicating the stability of the pore network. © 2009 Elsevier B.V. All rights reserved.Article Citation - WoS: 36Citation - Scopus: 42Sol-Gel Derived Mesoporous and Microporous Alumina Membranes(Springer Verlag, 2010) Topuz, Berna; Çiftçioğlu, MuhsinStable polymeric and colloidal boehmite sols were prepared by sol-gel process through controlled hydrolysis/condensation reactions. The particle sizes of the colloidal sols were in the 12-25 nm range depending on the process parameters and about 2 nm for polymeric sols. The presence of a significant increase in the microporosity content of the heat treated polymeric membranes relative to the mesoporous colloidal membranes might make the design of thermally stable microporous alumina membranes with controlled pore structures possible. The phase structure evolution in the 600-800 °C range had shown that the crystallization of the gamma alumina in the amorphous matrix starts at about 800 °C. This indicated that the pore structure stability may be enhanced through processing up to this relatively high temperature in polymeric alumina derived unsupported membranes. The permeance values of the two and three layered colloidal alumina membranes were observed to be independent of pressure which implies that the dominant gas transport mechanism is Knudsen diffusion in these structures. This was also supported by the 2.8 nm BJH pore sizes of the colloidal membranes. The Knudsen diffusion equation derived permeances of the polymeric alumina membranes with thicknesses of about 300 nm were determined to be very close to the experimentally determined permeance values. © 2010 Springer Science+Business Media, LLC.Article Citation - WoS: 8Citation - Scopus: 11Permeation of Pure Gases Through Silica Membranes With Controlled Pore Structures(Elsevier Ltd., 2006) Topuz, Berna; Çiftçioğlu, MuhsinThe superior thermal/chemical/mechanical stability and the ability of ceramic membranes in affecting the transport rates of chemical species through their processing controllable pore structures make them very attractive for many separation problems. Highly selective microporous silica membranes with high fluxes could be prepared by sol–gel dip coating processes [1]. The structure of the thin silica layer mainly depends on the size and the shape of the silicalite polymers and their packing behavior during drying and heat treatment. Design of the pore networks has a great importance to decide the transport properties through the membrane since permeation and the permselectivity are mainly determined by the microstructure of the membrane such as pore size and distribution, porosity as well as the interaction of permeating species with pore walls.Conference Object Citation - WoS: 2Citation - Scopus: 2Investigation of the Permeability of Pure Gases in Sol-Gel Derived Al 2o 3 Membrane(Trans Tech Publications, 2004) Topuz, Berna; Çiftçioğlu, Muhsin; Özkan, FehimeThe preparation, characterization and pure gas permeation of sol-gel derived alumina membranes were investigated in this work. The effects of acid concentration/type and water content on the particle size in the sols and pore size distributions of the unsupported membrane were investigated by N 2 adsorption/desorption isotherms and Dynamic Light Scattering. Increasing the H +/Al 3+ mole ratio from 0.1 to 0.25 caused the hydrodynamic sol particle size and BJH pore size to decrease from 65 to 30 nm and 3.6 to 2.9 nm, respectively. The pore size increased from 2.8 nm to 3 nm upon increasing the calcination temperature from 500 to 600°C. Unsupported membranes were heat treated in the 200 to 1200 °C range for the characterization of the phase structure. Pinhole and crack free alumina membranes about 3 μm (2-layer) in thickness was observed from the SEM pictures with insignificant infiltration. The CO 2 permeability through the double layer γ-Al 2O 3 membrane calcined at 600 °C was 2.25*10 -7 mol/m 2.s.Pa, and had a slight pressure dependence indicating Knudsen Diffusion and Laminar Flow being the effective transport mechanisms. Upon the calcination of a similar 2-layer alumina membrane at 500°C, the CO 2 permeability decreased to 1.51 *10 -8 mol/m 2.s.Pa. without pressure dependence.Conference Object Citation - Scopus: 1Effects of Processing on the Properties and Permeability of Pure Gases Through Sol-Gel Silica Membranes(Trans Tech Publications, 2004) Topuz, Berna; Çiftçioğlu, Muhsin; Özkan, FehimeN2, O2 and CO2 pure gas permeation through sol-gel derived silica membranes were determined and the effects of processing parameters on the microstructure of the membrane was investigated. Silica sols were prepared in an alcoholic solution by hydrolysis and condensation of TEOS as a function of acid content. The thickness of the silica membranes was determined to be about 2μm and significant infiltration into the support was observed from the SEM pictures. The supported membranes were heat treated in the 50-400°C. The N2 permeabilities of silica membranes varied in the 2.2*10-10-2.7*10-8 mol/m2.s.Pa range for single layer membranes dipped for 10s. in the sol. The CO2 permeability of these membranes varied in the 1.2*10-9-6.95 *10-8 mol/m2.s.Pa range. The sols became viscous and gelled at 50°C in 16 hours. The O2 permeability increased with aging time. The optimum dipping time during processing was determined to be 10 seconds. The increase in the acid content of the sols were observed to increase permeabilities of the membranes significantly.