Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Article Non-Equilibrium Molecular Dynamics for Calculating the Thermal Conductivity of Graphene-Coated Aluminum(2020) Toprak, Kasım; Yılmaz, Ahmet BerkNon-equilibrium Molecular Dynamics (NEMD) simulations have been created in C++ using Message Passing Interface (MPI) library to calculate the phonon thermal conductivity of bare graphene, aluminum, and graphene-coated aluminum. This study focuses on how graphene can alter the thermal conductivity of graphene-coated aluminum. The effect of length, graphene, and the number of graphene layers are analyzed. Even though electrons are dominant on thermal conductivity of aluminum, the effect of graphene coating can be seen in the results. The results show that the thermal conductivity of aluminum increases by up to 149% by graphene coating. When the number of layers increases to two layers, the thermal conductivity increases by up to 261%. Moreover, the results increase with the length of all models.Article Citation - WoS: 17Citation - Scopus: 18Electric Field Controlled Heat Transfer Through Silicon and Nano-Confined Water(Taylor & Francis, 2019) Yenigün, Onur; Barışık, MuratNanoscale heat transfer between two parallel silicon slabs filled with deionized water was studied under varying electric field in heat transfer direction. Two oppositely charged electrodes were embedded into the silicon walls to create a uniform electric field perpendicular to the surface, similar to electrowetting-on-dielectric technologies. Through the electrostatic interactions, (i) surface charge altered the silicon/water interface energy and (ii) electric field created orientation polarization of water by aligning dipoles to the direction of the electric field. We found that the first mechanism can manipulate the interface thermal resistance and the later can change the thermal conductivity of water. By increasing electric field, Kapitza length substantially decreased to 1/5 of its original value due to enhanced water layering, but also the water thermal conductivity lessened slightly since water dynamics were restricted; in this range of electric field, heat transfer was doubled. With a further increase of the electric field, electro-freezing (EF) developed as the aligned water dipoles formed a crystalline structure. During EF (0.53 V/nm), water thermal conductivity increased to 1.5 times of its thermodynamic value while Kapitza did not change; but once the EF is formed, both Kapitza and conductivity remained constant with increasing electric field. Overall, the heat transfer rate increased 2.25 times at 0.53 V/nm after which it remains constant with further increase of the electric field.Article Citation - WoS: 21Citation - Scopus: 24Effect of Nano-Film Thickness on Thermal Resistance at Water/Silicon Interface(Elsevier Ltd., 2019) Yenigün, Onur; Barışık, MuratParallel to the developments in micro/nano manufacturing techniques, component sizes in micro/nano electro mechanical systems have been decreasing to nanometer scales. Decrease in lengths in heat transfer direction below the heat carrier phonon length scales reduces thermal conduction in semiconductors. This study shows that such altered phonon spectrums with the decrease of size also reduce the heat transfer at the solid/liquid interfaces and can be correlated with the thermal conductivity of the slab. Using Molecular Dynamics (MD), we measured heat transfer between water and silicon of different thickness between 5 nm and 60 nm. Silicon slabs exhibit a linear temperature profile through the bulk where thermal conductivities measured based on Fourier law decreased by the decreasing slab thickness. We applied a semi-theoretical formulism on variation of conductivity by slab thickness. At the interface of these slabs and water, heat passage is disturbed due to the phonon mismatch of dissimilar materials, which is mostly considered as solid/liquid couple interface properties by the earlier literature. Resistance for phonon passage characterized as Kapitza length (L-K) is measured for different slab thicknesses at different surface wetting conditions varying between hydrophilic to hydrophobic. Increasing surface wetting decreases the L-K while at a certain wetting, decreasing the slab thickness increases the L-K. Once the L-K of different size slabs normalized by its bulk value (assumed to be the L-K of the thickest slab at the corresponding wetting), L-K variation by silicon thickness shows a universal behavior independent of surface wetting. A mathematical model describing the exponential increase of L-K by decreasing thickness was developed and validated by an earlier model. We further developed a correlation between the corresponding changes of L-K and conductivity with respective to their bulk values by analytically combining two models as (L-K/L-K-(Bulk)) = exp (3.94(k(Bulk) - k)/(k x k(Bulk))), using which L-K can be predicted from available thermal conductivities of a certain material. Results are crucial for thermal management of current and future electronics. (C) 2019 Elsevier Ltd. All rights reserved.Article Citation - WoS: 18Citation - Scopus: 17Wetting of Single Crystalline and Amorphous Silicon Surfaces: Effective Range of Intermolecular Forces for Wetting(Taylor and Francis Ltd., 2020) Özçelik, Hüseyin Gökberk; Özdemir, Abdullah Cihan; Kim, Bohung; Barışık, MuratWetting at nanoscale is a property of a three-dimensional region with a finite length into the solid domain from the surface. Understanding the extent of the solid region effective on wetting is important for recent coating applications as well as for both crystalline and amorphous solids of different atomic ordering. For such a case, we studied the wetting behaviour of silicon surfaces at various crystalline and amorphous states. Molecular distributions of amorphous systems were varied by changing the amorphisation conditions of silicon. Semi-cylindrical water droplets were formed on the surfaces to be large enough to remain independent of line tension and Tolman length effects. Contact angles showed up to 38% variation by the change in the atomic orientation of silicon. Instead of a homogeneous solid density definition, we calculated different solid densities for a given surface measured inside different extents from the interface. We correlated the observed wetting variation with each of these different solid densities to determine which extent governs the wetting variation. We observed that the variation of solid density measured inside a 0.13 nm extent from the surface reflected the variation of wetting angle better for both single crystalline and amorphous silicon surfaces.Article Citation - WoS: 15Citation - Scopus: 16Parametrizing Nonbonded Interactions Between Silica and Water From First Principles(Elsevier, 2020) Özçelik, H. Gökberk; Sözen, Yiğit; Şahin, Hasan; Barışık, MuratSilica has been used in a vast number of micro/nano-fluidic technologies where interactions of water with silica at the molecular level play a key role. In such small systems, an understanding of mass and heat transport or surface wetting relies on accurate calculations of the water-silica interface coupling through atomic interactions. Molecular dynamics (MD) is a convenient tool for such use, but force field parameters for nonbonded interactions are required as an input, which are very limited in literature. These interaction parameters can be predicted by density functional theory, but dispersion forces are not calculated in standard models for electron correlations that additional correction models have been proposed at different levels of sophistications, and still under development. Accordingly, this work employs state of the art quantum chemistry to compute the binding energies. Force field parameters for silica/water van der Waals interactions were calculated, and later tested in MD simulations of water droplet on silica surface. While the standard dispersion corrections overestimated the binding energy, Becke-Johnson model yielded interactions parameters recovering experimentally measured wetting behavior of silica with a water contact angle of approximately 12.4 degrees on the flat and clean silica surface. Results will be useful for the current molecular modelling attempts by providing transferable parameters for simple silica/water van der Waals interactions as an alternative to existing complex surface interaction models.Conference Object Molecular Dynamics Study of the Thermal Conductivity of Graphene Coated Copper(Avestia Publishing, 2019) Toprak, Kasım; Ersavaş, GizemIn this study, the thermal conductivity of various size of pure copper, pure graphene and, different number of layer graphene coated copper models are studied using non-equilibrium molecular dynamics (NEMD) simulations. Our findings show that the thermal conductivity of graphene coated copper is higher than the uncoated ones. Furthermore, results also indicate that single layer graphene (SLG) model has the highest thermal conductivity as compared to the other model. Even though multiple layer graphene (MLG) has lower thermal conductivity value compare to SLG, this study shows that the thermal conductivity of MLG coated copper has higher thermal conductivity than SLG coated one. The most important finding in this study suggests that the thermal conductivity of copper can be improved using high thermal conductivity materials like graphene. © 2019, Avestia Publishing.Article Citation - WoS: 37Citation - Scopus: 38Wetting of Chemically Heterogeneous Striped Surfaces: Molecular Dynamics Simulations(American Institute of Physics, 2018) Nguyen, Chinh Thanh; Barışık, Murat; Kim, BoHungUsing molecular dynamics simulations, we thoroughly investigated the wetting behaviors of a chemically heterogeneous striped substrate patterned with two different wetting materials, face-centered cubic gold and face-centered cubic silver. We analyzed the density distributions, normal stress distributions, surface tensions, and contact angles of a water droplet placed on the substrates at different heterogeneities. We found that the density and stress profile of the water droplet near the substrate-water interface were noticeably affected by altering the gold and silver contents in the substrate. Specifically, a greater portion of gold (more wetting) or smaller portion of silver (less wetting) in the substrate composition induced higher densities and higher normal stresses in the vicinity of the substrate surface. Also, it was observed that the surface tensions at liquid-vapor interface and solid-vapor interface were not largely impacted by the change of the substrate composition while the solid-liquid surface tension decreased exponentially with increasing fraction of gold. Most importantly, we found that contact angle of a nanometer-sized water droplet resting on the chemically heterogeneous striped substrate does not show linear dependence on corresponding surface fractions like that predicted by Cassie-Baxter model at the macro-scale. Consequently, we proposed a method for successfully predicting the contact angle by including the critical effects of the substrate heterogeneity on both surface tensions and line tension at the three-phase contact line of the water droplet and the chemically striped substrate.Article Silika Yüzeylerin Islanma Hareketlerinin Moleküler Dinamik ile Modellenmesi(Gazi Üniversitesi, 2018) Barışık, MuratYeni üretim tekniklerine paralel olarak nano-boyutlu teknolojiler çok geniş bir uygulama alanında kullanılmaya başlanmakta ve yeni uygulamalar geliştirmek için keşfedilmesi ve anlaşılması gereken konular süratle artmaktadır. Bu doğrultuda, yeni uygulamalarda sıkça yer bulan silikon ve silikon-dioksitin mikro/nano boyutlardaki malzeme özelliklerinin anlaşılmasına büyük ihtiyaç oluşmaktadır. Özellikle bu yüzeylerin ıslanma hareketlerinin anlaşılabilmesi ve hatta kullanılacak uygulamaya göre ayarlanabilmesi sayısız uygulama için önem arz etmektedir. Bu nedenlerle, nano-teknolojide sıkça kullanılan silikon-dioksit malzemesinin ve su moleküllerinin nano-ölçeklerde moleküler olarak modellenmesi bu çalışmada gerçekleştirildi. Modelleme molekuler dinamik hesaplamaları ile yapıldı. Silikon-dioksit yüzey üzerinde nano su damlacıkları oluşturup, denge halinde oluşan ıslatma açısı ölçümleri yapıldı. Literatürde işlem yükünü azaltmak için sıklıkla uygulanan, katı yüzey termal titreşimlerinin ıslatmaya olan etkisinin ihmal edilmesi ve modellenmemesinin ıslatma açısına olan etkisi incelendi. Katı moleküllerin termal titreşimlerinin ıslatma modellenen ıslatma fiziğine baskın bir etkisi olduğu görüldü. Geçtiğimiz yıllarda doğa taklidi olarak bilinen çalışma çevreleri tarafından hayata geçirilmeye çalışılan Lotus yaprağı etkisi temelli yüzey ıslatma kontrolu moleküler seviyede uygulandı. Yüzey üzerinde oluşturulan nano boyutlardaki yüzey yapılarının ıslanma açısını değiştirebildiği gösterildi. Temiz (0 0 1) silika yüzeyinde nano ölçek çizgi gerilimi etkisi altında ölçülen ıslanma açısının deneysel silika ıslanma açısı aralığında olduğu bulundu.Article Citation - WoS: 44Citation - Scopus: 47Electric Field Controlled Transport of Water in Graphene Nano-Channels(American Institute of Physics, 2017) Çelebi, Alper Tunga; Barışık, Murat; Beşkök, AliMotivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.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.