İki boyutlu, heksagonal yapılarda fonon ortalama serbest yolu.

Two-dimensional materials such as graphene and few layer hexagonal Boron Nitride (h-BN) have been the center of attention in the last decade. These materials provide anisotropic and exclusive properties making them ideal candidates for the modern electronic and optoelectronic applications. With the enhancements in fabrication techniques and the ability to separate thin layers their popularity is continuously increasing. Understanding the thermal properties of these materials is necessary to make better devices. Phonon mean free path (MFP) is one of the most thermal properties that determines the limits of ballistic-diffusive thermal transport in micro and nanoscale domains. In this study thermal properties, specifically the phonon MFP behavior, of few layer graphene, h-BN, and composite graphene/h-BN structures were studied after obtaining the phonon dispersion of each material. After, finding the thermal properties of discrete phonon modes, a plot of accumulated thermal conductivity with respect to phonon MFP of phonons is obtained to understand the ballistic-diffusive limits of in such structures. Bulk structures of graphene and h-BN were also analyzed for comparison purposes and it was observed that single layer structures will experience ballistic effects more since they have considerably higher MFP than their bulk counterparts. M.S. - Master of Scienc

Atomistic Simulation of 2D Materials and Ionic Liquids for Energy Storage and Conversion

Energy is a crucial part of our everyday life, but traditional generation is unsustainable, necessitating alternative sources. Storing energy, especially from renewable types of energy like solar and wind, ensures supply during disruptions and enables transportation to areas in need. To meet increasing demands and minimize environmental impact, advancing energy storage and conversion technologies is essential. The focus of this dissertation is investigating energy storage and conversion at microscopic level through 2-dimensional materials and battery electrolytes. Recently, two-dimensional (2D) transition metal carbides and nitrides (MXenes) have become prominent in electronics and electrochemical energy conversion and storage systems, especially since heat production greatly affects these devices' safety and efficiency. Within chapter 2, the effect of surface termination (bare, fluorine and oxygen) on the lattice thermal conductivity of Ti3C2Tz has been investigated using density functional theory and linearized solution of Boltzmann transport equation. It was found that thermal conductivity of fluorine-terminated Ti3C2Tz (108 W/m.K) is approximately one order of magnitude higher than its oxygen-terminated counterpart (11 W/m.K). Phonon dispersions, group velocities, specific heats and scattering rates were studied to shed light on difference in thermal conductivities for different surface terminations. Ternary mixtures of organic solvents, ionic liquids (ILs) and Li salts have shown outstanding performance in Li-ion and Li-oxygen batteries, outperforming both binary mixtures of organic solvents or ILs with Li salts electrolytes. In chapters 3, 4 and 5, the phase behavior of ternary mixtures of ionic liquid, organic solvent and lithium salt is investigated using molecular dynamics simulations. It is shown that at room temperature, the electrolyte separates into distinct phases with specific compositions; an ion-rich domain that contains a fraction of solvent molecules and a second domain of pure solvent. Volume fraction of IL/solvent as well as the temperature were shown to be the important factors for separation of mixtures into two domains. The phase separation is shown to be entropy driven and is independent of lithium salt concentration. In chapter 4, using large-scale classical molecular dynamics simulations, we looked into ten different ternary electrolyte mixtures using combinations of [EMIM]+, [BMIM]+ and [OMIM]+ cations with [NO3]-, [BF4]-, [PF6]-, [ClO4]-, [TFO]- and [NTf2]- anions, tetraglyme and Li salt to study the effect of ionic liquid composition on the phase behavior of ternary electrolyte mixtures. We uncovered that in these electrolytes, phase separation is mainly a function of pairwise binding energy of constituents of the mixture. To corroborate this theory, many simulations were performed at various temperatures ranging from 260K to 500K for each mixture, followed by calculating the binding energy of ionic liquid pairs using density functional theory. In chapter 5, the formation of multiple stable ionic domain within a system was investigated. To this extend, first to remove the potential finite size effect, multiple systems larger than previously simulated systems were constructed and simulated. Second, the effect of presence of both [NO3]- and [NTF2]- anions, corresponding to the anions that make the strongest and weakest binding with cation, was investigated. Finally, we looked into a way to engineer to target or avoid formation of ionic domains by adding appropriate anions to the mixture

Phonon Mean Free Path in Few Layer Graphene, Hexagonal Boron Nitride, and Composite Bilayer h-BN/Graphene

In this study, ab-initio calculations were performed to obtain the phonon dispersions of seven different structures: single layer graphene, bilayer graphene, graphite, single layer h-BN, bilayer h-BN, bulk h-BN, and finally composite bilayer h-BN/graphene. Using these dispersions specific heat, group velocity, and single mode relaxation times of phonons were obtained to calculate their thermal conductivities, and mean free paths at room temperature. Calculated variables were used to understand the effects of additional layers to thermophysical properties, phonon mode contributions to thermal conductivity, and the limits for ballistic-diffusive heat transfer of all seven structures. Finally, based on the obtained data, thermal properties of the composite bilayer h-BN/graphene were investigated and compared to those of few layer graphene and h-BN structures

Nanoscale Thermal Transport in Single, Bilayer Graphene, and Graphite

In this study, ab-initio calculations were performed to obtain phonon dispersions of single, bilayer graphene, and graphite structures. Using these dispersions single mode relaxation times, thermal conductivities, and mean free paths (MFPs) have been calculated. Finally, calculated variables were used to understand the effects of additional layers to thermophysical properties, phonon mode contributions to thermal conductivity, and the limits for ballistic-diffusive heat transfer of single, bilayer graphene, and graphite structures

AlGaN/GaN HEMT Aygıtların Isıl Performansında Radyasyonun Önemi

AlGaN/GaN HEMT aygıtlarının kullanım alanları gittikçe artmakta ve melez (ing. hybrid) otomotivlerden, askeri radar uygulamalarına kadar farklılık gösteren yüksek güç ve frekans uygulaması gerektiren uygulamalarda kullanılmaktadır. Bu aygıtların yüksek güç uygulamalarında kullanılması ve çok ince filmlerden oluşan yapısı nedeniyle ısınma problemleri gözlemlenmiştir. Isınma problemlerini daha iyi anlamak, aygıtların sıcaklıklarını azaltmak ve böylece daha iyi elektriksel performanslar elde etmek için ısıl benzetimler gereklidir ve araştırmacılar tarafından yapılmaktadır. Yapılan benzetimlerin doğruluğunu arttırmak üzere benzetimlerde mümkün olduğu kadar gerçek fiziksel koşullar yaratılmalıdır. Bu çalışma aygıtlarda yüksek ısının görüldüğü aktif alanlarda radyasyonla ısı iletiminin önemini araştırarak benzetimlerde dâhil edilmesine yöneliktir

Phase Separation and Ion Diffusion in Ionic Liquid, Organic Solvent, and Lithium Salt Electrolyte Mixtures

The highly desirable characteristics of ternary mixtures of ionic liquids, organic solvents, and metal salts make them a promising candidate for use in various electrothermal energy storage and conversion systems. In this study, using large-scale classical molecular dynamics simulations, we looked into 10 different ternary electrolyte mixtures using combinations of [EMIM]+, [BMIM]+, and [OMIM]+ cations with [NO3]−, [BF4]−, [PF6]−, [ClO4]−, [TFO]−, and [NTf2]− anions, tetraglyme, and Li salt to study the effect of ionic liquid composition on the phase behavior of ternary electrolyte mixtures. We uncovered that in these electrolytes, phase separation is mainly a function of pairwise binding energy of the constituents of the mixture. To corroborate this theory, several simulations are performed at various temperatures ranging from 260 to 500 K for each mixture, followed by calculating the binding energy of ionic liquid pairs using density functional theory. Our results verify that the transition temperature for the phase separation of each system is indeed a function of the pairwise binding energy of its ionic liquid pairs. It is also found that in some cases, the diffusion coefficient of the Li+ ions decreased even with the increase in the temperature, an effect that is attributed to the presence of condensed ionic domains in the electrolyte. This study provides a new insight for the design of multicomponent electrolyte mixtures for a wide range of energy applications

Speciation of Cr(III) and Cr(VI) in environmental samples after solid phase extraction on amberlite XAD-2000

Soylak, Mustafa/0000-0002-1017-0244WOS: 000248925000010A method for speciation of Cr(III) and Cr(VI) in real samples has been developed. Cr(VI) has been separated from Cr(III) and preconcentrated as its pyrrolidinedithiocarbarnate (APDC) complex by using a column containing Amberlite XAD-2000 resin and determined by FAAS. Total chromium has also been determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with KMnO4. Cr(III) has been calculated by subtracting Cr(VI) from the total. The effect of pH, flow-rate, adsorption and batch capacity and effect of various metal cations and salt anions on the sorption onto the resin were investigated. The adsorption is quantitative in the pH range of 1.5-2.5, and Cr(VI) ion was desorbed by using H2SO4 in acetone. The recovery of Cr(VI) was 97 4 at a 95% confidence level. The highest preconcentration factor was 80 for a 200 mL sample volume. The adsorption and batch capacity of sorbent were 7.4 and 8.0 mg g(-1) Cr(VI), respectively, and loading half time was 5.0 min. The detection limit of Cr(VI) is 0.6 mu g/L. The procedure has been applied to the determination and speciation of chromium in stream water, tap water, mineral spring water and spring water. Also, the proposed method was applied to total chromium preconcentration in microwave digested moss and rock samples with satisfactory results. The developed method was validated with CRM-TMDW-500 (Certified Reference Material Trace Metals in Drinking Water) and BCR-CRM 144R s (Certified Reference Material Sewage Sludge, Domestic Origin) and the results obtained were in good agreement with the certified values. The relative standard deviations were below 6%