Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Citation - WoS: 47Citation - Scopus: 46Surface Charge-Dependent Transport of Water in Graphene Nano-Channels(Springer Verlag, 2018) Çelebi, Alper Tunga; Barışık, Murat; Beşkök, AliDeionized water flow through positively charged graphene nano-channels is investigated using molecular dynamics simulations as a function of the surface charge density. Due to the net electric charge, Ewald summation algorithm cannot be used for modeling long-range Coulomb interactions. Instead, the cutoff distance used for Coulomb forces is systematically increased until the density distribution and orientation of water atoms converged to a unified profile. Liquid density near the walls increases with increased surface charge density, and the water molecules reorient their dipoles with oxygen atoms facing the positively charged surfaces. This effect weakens away from the charged surfaces. Force-driven water flows in graphene nano-channels exhibit slip lengths over 60 nm, which result in plug-like velocity profiles in sufficiently small nano-channels. With increased surface charge density, the slip length decreases and the apparent viscosity of water increases, leading to parabolic velocity profiles and decreased flow rates. Results of this study are relevant for water desalination applications, where optimization of the surface charge for ion removal with maximum flow rate is desired.Article Citation - WoS: 2Citation - Scopus: 2Determination of Solid/Liquid Fraction of Three Aluminium Binary Alloys Using a New Single-Pan Scanning Calorimete(Springer Verlag, 2017) Savaş, Sinan; Dong, HongbiaoThere is an increased demand for data with higher precision for the enthalpy changes and the fraction of solid/liquid temperatures of materials. Therefore, continuous efforts are often devoted to design calorimeters that can accurately measure materials’ thermophysical properties. In this study, a new single-pan scanning calorimeter was used to measure the transition temperature and enthalpy change of three aluminium binary alloys. Measured results also were compared with the calculated results using thermodynamic software. The measured high accuracy enthalpy data were used to determine transient temperature. It is concluded that the new instrument is a promising device that can achieve reliable and reproducible materials’ thermophysical data.Article Citation - WoS: 33Citation - Scopus: 39Molecular Free Paths in Nanoscale Gas Flows(Springer Verlag, 2015) Barışık, Murat; Beşkök, AliAverage distance traveled by gas molecules between intermolecular collisions, known as the mean free path (MFP), is a key parameter for characterizing gas flows in the entire Knudsen regime. Recent literature presents variations in MFP as a function of the surface confinement, which is in disagreement with the kinetic theory and leads to wrong physical interpretations of nanoscale gas flows. This controversy occurs due to erroneous definition and calculation practices, such as consideration of gas wall collisions, using local bins smaller than a MFP, and utilizing time frames shorter than a mean collision time in the MFP calculations. This study reports proper molecular MFP calculations in nanoscale confinements by using realistic molecular surfaces. We utilize molecular dynamics (MD) simulations to calculate gas MFP in three-dimensional periodic systems of various sizes and for force-driven gas flows confined in nano-channels. Studies performed in the transition flow regime in various size nano-channels and under a range of gas–surface interaction strengths have shown isotropic mean travelled distance and MFP values in agreement with the kinetic theory regardless of the surface forces and surface adsorption effects. Comparison of the velocity profiles obtained in MD simulations with the linearized Boltzmann solutions at predicted Knudsen values shows good agreement in the bulk of the channels, while deviations in the near wall region due to the influence of surface forces are reported.Article Citation - WoS: 17Citation - Scopus: 26Thixoforming of A356/Sic and A356/Tib2 Nanocomposites Fabricated by a Combination of Green Compact Nanoparticle Incorporation and Ultrasonic Treatment of the Melted Compact(Springer Verlag, 2014) Kandemir, Sinan; Atkinson, Helen V.; Weston, David P.; Hainsworth, Sarah V.Thixoforming is a type of semi-solid processing which is based on forming metals in the semi-solid state rather than fully liquid or solid state. There have been no reports of the thixoforming of nanocomposites in the literature. The incorporation of ceramic nanoparticles into liquid metals is a challenging task for the fabrication of metal matrix nanocomposites due to their large surface-to-volume ratio and poor wettability. Previous research work by a number of workers has highlighted the challenges with the incorporation of nanoparticles into liquid aluminum alloy. In the present study, SiC and TiB2 nanoparticles with an average diameter between 20 and 30 nm were firstly incorporated into green compacts by a powder forming route, and then the compacts were melted and treated ultrasonically. The microstructural studies reveal that the engulfment and relatively effective distribution of the nanoparticles into the melt were achieved. The hardness was considerably improved with only 0.8 wt pct addition of the nanoparticles. The nanocomposites were successfully thixoformed at a solid fraction between 0.65 and 0.70. The microstructures, hardness, and tensile mechanical properties of the thixoformed nanocomposites were investigated and compared with those of the as-received A356 and thixoformed A356 alloys. The tensile properties of the thixoformed nanocomposites were significantly enhanced compared to thixoformed A356 alloy without reinforcement, indicating the strengthening effects of the nanoparticles.Article Citation - WoS: 23Citation - Scopus: 25Microcalorimetric Investigation of Water Vapor Adsorption on Silica Gel(Springer Verlag, 2011) Demir, Hasan; Mobedi, Moghtada; Ülkü, SemraWater vapor adsorption on silica gel was investigated using Tian-Calvet-type microcalorimetry. Differential heat of adsorption data was obtained. The setup of microcalorimetry was used volumetric system to determine adsorption isotherms of water vapor-silica gel. The Langmuir model was used in the interpretation of the adsorption data. The Clausius-Clapeyron diagram was also given. Effective mass diffusivity of water vapor in the silica gel particle as a function of temperature was also determined. The silica gel, which was degassed under vacuum at 10 -7 mbar and 120 °C for 18 h, was found to adsorb 0.6, 0.98, 1.1, 1.4, 2, 3.5, 11, 13, and 14 wt% water vapor at 120, 110, 100, 90, 75, 60, 40, 35, and 30 °C, respectively. The diffusivities of water vapor inside the silica gel for short- and long-range periods were described using kinetics data as a function of temperature in the Arrhenius form.Article Citation - WoS: 29Citation - Scopus: 32Antimicrobial Activity of Tio2-Coated Orthodontic Ceramic Brackets Against Streptococcus Mutans and Candida Albicans(Springer Verlag, 2010) Özyıldız, Figen; Güden, Mustafa; Uzel, Ataç; Karaboz, İsmail; Akil, Övünç; Bulut, H.Polycrystalline alumina ceramic orthodontic brackets were coated with anatase TiO2 film via a sol-gel dip-coating method. The surface structure morphology and composition of the films were evaluated via scanning electron microscopy, atomic force microscopy, and X-ray diffraction. The antimicrobial activity of the ceramic brackets was assessed against two oral pathogens, S. mutans and C. albicans. The results demonstrated that TiO 2-coated brackets exposed to low energy UV-A illumination efficiently reduced the populations of test microorganisms relative to the uncoated brackets. The reduction efficiencies were 98% for S. mutans ATCC 10449 and 93% for C. albicans ATCC 60193. © The Korean Society for Biotechnology and Bioengineering and Springer 2010.Article Citation - WoS: 29Citation - Scopus: 31Cure Kinetics of Epoxy Resin-Natural Zeolite Composites(Springer Verlag, 2008) Cansever Erdoğan, Beyhan; Seyhan, Abdullah Tuğrul; Ocak, Yılmaz; Tanoğlu, Metin; Balköse, Devrim; Ülkü, SemraThe cure kinetics of epoxy resin and epoxy resin containing 10 mass% of natural zeolite were investigated using differential scanning calorimetry (DSC). The conformity of the cure kinetic data of epoxy and epoxy-zeolite system was checked with the auto-catalytic cure rate model. The results indicated that the hydroxyl group on the zeolite surface played a significant role in the autocatalytic reaction mechanism. This group was able to form a new transition state between anhydride hardener and epoxide group. The natural zeolite particles acted as catalyst for the epoxy system by promoting its curing rate.Conference Object Citation - WoS: 39Citation - Scopus: 48Temperature Dependence of Electrical Conductivity in Double-Wall and Multi-Wall Carbon Nanotube/Polyester Nanocomposites(Springer Verlag, 2007) Şimşek, Yılmaz; Özyüzer, Lütfi; Seyhan, Abdullah Tuğrul; Tanoğlu, Metin; Schulte, KarlThe aim of this study is to investigate temperature dependence of electrical conductivity of carbon nanotube (CNT)/polyester nanocomposites from room temperature to 77 K using four-point probe test method. To produce nanocomposites, various types and amounts of CNTs (0.1, 0.3 and 0.5 wt.%) were dispersed via 3-roll mill technique within a specially formulized resin blend of thermoset polyesters. CNTs used in the study include multi walled carbon nanotubes (MWCNT) and double-walled carbon nanotubes (DWCNT) with and without amine functional groups (-NH2). It was observed that the incorporation of carbon nanotubes into resin blend yields electrically percolating networks and electrical conductivity of the resulting nanocomposites increases with increasing amount of nanotubes. However, nanocomposites containing amino functionalized carbon nanotubes exhibit relatively lower electrical conductivity compared to those with non-functionalized carbon nanotubes. To get better interpretation of the mechanism leading to conductive network via CNTs with and without amine functional groups, the experimental results were fitted to fluctuation-induced tunneling through the barriers between the metallic regions model. It was found that the results are in good agreement with prediction of proposed model.Article Citation - WoS: 3Citation - Scopus: 4Strain Gauges of Gasb-Fega1.3 Eutectic Composites(Springer Verlag, 2004) Aliyev, M.I.; Khalilova, A. A.; Arasly, D. H.; Rahimov, R. N.; Tanoğlu, Metin; Özyüzer, LütfiA needle-shaped metallic FeGa1.3 phase oriented in a specific direction and uniformly distributed within a GaSb matrix was grown by a vertical Bridgman method. Strain-gauge characteristics, such as strain-sensitivity coefficient (S), temperature coefficient of strain sensitivity (TCS) and temperature coefficient of resistance, of GaSb and GaSb-FeGa1.3 eutectic alloy have been investigated in the range of 200 to 400 K under deformation up to strains of 1.3 × 10-3. The value of S of the GaSb-FeGa1.3 composition is measured to be 40 ± 5 and its TCS is about 0.2% deg-1 when the current is perpendicular to the needles and the needles are parallel to the plane of the gauge substrate. The strain-sensitivity characteristics are linear and hysteresis free in the investigated temperature range in the aforementioned direction. It was found that GaSb-FeGa1.3-based strain gauges possess better deformation characteristics than GaSb-based gauges.Article Citation - WoS: 54Citation - Scopus: 62Sic-Particulate Aluminum Composite Foams Produced by Powder Compacts: Foaming and Compression Behavior(Springer Verlag, 2003) Elbir, Semih; Yılmaz, Selahattin; Toksoy, Ahmet Kaan; Güden, Mustafa; Hall, Ian W.The foaming behavior of SiC-particulate (8.6% by volume) aluminum composite powder compacts contained Titanium Hydride blowing agent was investigated by heating above the melting temperature (750°C) in a pre-heated furnace. Aluminum powder compacts were also prepared and foamed using similar compaction and foaming parameters in order to determine the effect of SiC-particulate addition on foaming and compression behavior. The linear expansions of the compacts at various furnace holding times were ex situ determined. Optical and scanning electron microscopy techniques were used to characterize prepared and deformed foams microstructures. The SiC-particulate addition was found to increase the linear expansion and reduce the extent of the liquid metal drainage and cell coarsening of the aluminum compacts. The composite foam samples also showed higher compressive stresses, but a more brittle behavior as compared with aluminum foams.