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ÖgeInvestigation of thermal conduction in microcontacts created by indentation(Graduate School, 2022)Thermal contact conduction has been investigated on different scales for many practical and scientific motivations in the literature. Demands for engineering the interfaces are increasing for accurately managing the contact mechanics and heat transfer with miniaturization of the electronics devices. In this study, microcontacts, that are created by indentation, have been investigated with experimental, simulation, and analytical works. The spreading resistance perspective of the disc constriction case has been extended for the studied highly plastic microcontacts of indentation. Creating the microcontacts and investigating the conductance through them had been realized by indentation of metallic surfaces by specially prepared diamond micro-particles/indenters. Thermal measurements had been realized by mounting thin thermocouples on diamond tips. The experimental setup is home-built with commercial piezo, motor, DAQ utilities, and other miscellaneous devices. PC User Interface and Intercessor Microcontroller Unit had been programmed to properly manage to conduct experiments. Furthermore, to measure the resistance, we employed an oscillatory experimental procedure and lumped analysis of transient heat transfer. The application of oscillations at different indentation depths has enabled us to extract the RC behavior of the microcontacts created by high plastic deformation. Therefore, the time constant of the contacts can be obtained. Additionally, we could find an effective measure of the thermal diffusivity of the contact through the diamond tip by fitting the change of time constant to depth with the proposed modified constriction models. Moreover, to analyze and predict the change of the time constant with respect to depth and load, several simulations and calculation work had been pursued. The increase in the contact area by indenter penetration into the sample has been concerned to be suppressed by gradient occurrence along the tip-sample contact. Moreover, with help of the simulations, we deduced the effect of plasticity such as pile-up on the improvement of the indentation contact for the heat transfer can be effective. Consequently, for the first time, we conducted the periodic contact procedure for the thermal contact of single micro asperity of indentation. The periodic experimental procedure and fin efficiency application to spreading cases for single microcontact are unique parts of this work. Results with the diamond tip on three different metallic samples showed that the gradient occurrence along the indentation contact can be analyzed with the fin solutions of the literature. Experimental results were fitted properly to a unified function of conic fin and spreading resistance functions. In addition, parameters of the fits can be deduced for the conductivity and interface conductance. However, state of the results are not sufficient to exactly determine the contact and material parameters due to need for exact parameters for transient analysis and, uncertainties in the properties of the tip and samples. With help of more precise thermal measurements and indenter systems, this experimental procedure may provide further advances and ease in the investigation of the thermal contacts of many different materials and scales. In addition, for the solid-state thermal interface materials solutions, we deduce that investigation of the geometry optimization for pressure and heat transfer as indicated in this thesis would provide insights into the bottlenecks of the contact heat transfer. Specifically, the gradient occurrence and its effectivity on the overall contact heat transfer should be taken into account for the indentation contacts while improving the contact by plasticity.
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ÖgeNewtonian perturbation theory in cosmology: From inflation to large-scale structure(Graduate School, 2025-01-28)Cosmology is the scientific study of the physical characteristics of the universe, its beginning, development and organization, based on observational outcomes and theoretical foundations. The Lambda-CDM model is currently one of the most popular theories in cosmology. This model of the universe outlines the behavior of the cosmos through the use of dark matter and energy. The cosmological constant (dark energy) is an energy density used to describe the acceleration of the expansion of the universe. From this model, it can be seen that cold dark matter and dark energy contribute greatly to the total mass-energy density of the universe. While dark matter affects the dynamics of galaxies and large-scale structures, dark energy drives the accelerated expansion of the universe. However, ongoing problems led to the formulation of "inflation theory." Inflation theory is a convincing paradigm that solves fundamental questions like the flatness problem and the horizon problem, which ask why the universe appears nearly flat and why distant parts show similar properties. Inflation hypothesis argues that the universe had a rapid expansion during its formative period, which mitigated initial anomalies and established the foundational conditions for the world we observe today. Numerous mathematical models have been introduced to advance inflation theory, including scalar field inflation, Starobinsky inflation, and Higgs inflation, which explain the dynamics of early expansion and the transformation of primordial perturbations into extensive cosmic structures. We also need observational evidence from the early cosmos to prove these theoretical hypotheses. The cosmic microwave background (CMB) and large-scale structure (LSS) are two of the most critical. CMB is described as the conditions immediately after the Big Bang and gives us a perspective on what the early universe was like, while Large Scale Structure (LSS) refers to the general arrangement of galaxies and matter throughout cosmic history. To form these structures one has to consider both the observation of them and the processes by which they are formed. The growth of cosmic structures is mainly due to gravitational collapse, which amplifies small density perturbations in the early universe. This process is also understood by using Newtonian perturbation theory, which is a useful approach to describing how early anisotropies evolve into the large scale structures we see today. The concepts of Jeans length, growth function, transfer function and power spectrum are useful tools to study the evolution of structures and distribution of matter and to generate theoretical data to compare with experimental data. However, the examination of nonlinear evolution show that the creation of xxi structures has a more complex background. Different theoretical instruments have been used to analyze this complicated structure. The spherical collapse model elucidates the evolution of overdense regions into stable entities like galaxies and galaxy clusters, whereas the idea of virialization delineates the equilibrium state of these structures, especially dark matter halos. Moreover, the Press-Schechter theory offers a statistical framework for elucidating the creation of cosmic formations. This theory provides an analytical approach to assess the mass distribution of collapsed entities. The mass function forecasts the probability of structure formation across various masses, whereas biasing delineates the correlation between observable galaxies and the fundamental density field. Comprehending the genesis and evolution of the universe necessitates a comprehensive methodology that integrates theoretical, observational, and statistical analyses. Newtonian perturbation theory is a crucial instrument for examining large-scale structures, with its validity corroborated by empirical evidence and simulations.
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ÖgeTransition dynamic in the LSCDM model: Implications for bound cosmic structures(Graduate School, 2024-06-27)We explore the predictions of $\Lambda_{\rm s}$CDM, a novel framework suggesting a rapid anti-de Sitter (AdS) to de Sitter (dS) vacua transition in the late Universe, on bound cosmic structures. In its simplest version, $\Lambda_{\rm s}$ abruptly switches sign from negative to positive, attaining its present-day value at a redshift of ${z_\dagger\sim 2}$ i.e., $\Lambda_{\rm s} \equiv \Lambda{\rm sgn}(z_{\dagger}-z)$. We will show that in the case of an abrupt sign-switching cosmological constant, there occurs a type II (sudden) singularity at the transition redshift, $z_{\dagger}$, where the total pressure of the universe diverges to infinity and the total energy density remains constant and finite. To avoid type II singularity, one can ``smooth-out'' the sudden sign-switch and describe it by using sigmoid functions (e.g., $\tanh$, logistic). However, since this correction would introduce an additional parameter ($\sigma$) to the model, we decided to examine the scenario in which the sign change of the cosmological constant is abrupt. This will also allow us to study the behavior of structure formation in the most extreme case without adding an extra parameter to our analysis. We will start our analysis by studying the spherical collapse model for a universe that contains dust (consisting of cold dark matter and baryons) and cosmological constant ($\Lambda$). For this universe, we will derive the equations describing the dynamics of the overdensity as a function of the background universe. Due to the shell crossing---and consequently the breakdown of the homogeneity and isotropy after the turnaround---, one cannot use the Friedmann equations (i.e., spherical collapse model) to describe the dynamics of the overdensity. Thus, we must refer to the semi-Newtonian approach and use the virialization condition to describe its dynamics. In the next step, we will extend our analysis of the spherical collapse model to include $\Lambda_{\rm s}$CDM, by incorporating the sign-switching cosmological constant ($\Lambda_{\rm s}$) into our calculations. To understand this process more clearly, we will separate our discussion into three parts. In the first part, we will study the evolution of the overdensity, if it enters turnaround under the effect of the positive cosmological constant (i.e., $\Lambda_{\rm s} \equiv +\Lambda$). In the second part, we will discuss the dynamics of the overdensity, if it enters turnaround under the effect of the negative cosmological constant (i.e., $\Lambda_{\rm s} \equiv -\Lambda$). In the third and final part, we will discuss the halos that completely virializes before the AdS-dS transition, and study the effect of the type II singularity on the bounded cosmic structures. At a first glance, it's clear that depending on the time of the transition, the overdensity will be effected differently. In summary, we can identify three primary influences which effects the structure formation in the $\Lambda_{\rm s}$CDM model: (i) the negative cosmological constant (AdS) phase for $z > z_\dagger$, (ii) the abrupt transition marked by a type II (sudden) singularity, leading to a sudden increase in the universe's expansion rate at $z=z_\dagger$, and (iii) an increased expansion rate in the late universe under a positive cosmological constant for $z < z_\dagger$, compared to $\Lambda$CDM. We find that the virialization process of cosmic structures, and consequently their matter overdensity, varies depending on whether the AdS-dS transition precedes or follows the turnaround. Specifically, structures virialize with either increased or reduced matter overdensity compared to the Planck/$\Lambda$CDM model, contingent on the timing of the transition. Despite its profound nature, the singularity exerts only relatively weak effects on such systems, thereby reinforcing the model's viability in this context.
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ÖgeMEMS ile entegre mikro ısıtıcı ve IDE mikro sistemlerin fabrikasyonu ve nano kompozit yarı iletken gaz sensör uygulaması(Lisansüstü Eğitim Enstitüsü, 2024-08-05)Günümüzde Mikro Elektro Mekanik Sistemler (MEMS) teknolojisi ile mikro ısıtıcı sistemlerin inter dijital elektrotlar (IDE) ile entegrasyonunun geliştirilmesine yönelik ihtiyaç gün be gün artmaktadır. MEMS teknolojisi, mikroskopik ölçekte mekanik ve elektronik bileşenlerin entegrasyonunu içerir. Mikrosistem mühendisliği, elektronik, kimya, biyoloji ve fizik gibi birçok farklı disiplini birleştirir. Bu entegrasyon, daha karmaşık sistemlerin ve uygulamaların geliştirilmesini mümkün kılar. Örneğin, kimyasal algılama için kullanılan sensörler, biyolojik materyallerle birleştirilerek hastalık tespitinde kullanılabilir. Bu tür çok disiplinli çalışmalar, araştırma ve geliştirme süreçlerini zenginleştirir ve bilim ile mühendislik arasındaki sınırları aşarak yenilikçi çözümler üretir. Bu teknoloji, yüksek hassasiyet ve düşük maliyet avantajları ile öne çıkarak, biyomedikal uygulamalar, tüketici elektroniği gibi bir çok alanda kullanılmaktadır. MEMS teknolojisinde kullanılan gaz sensörleri, endüstriyel süreçlerin kontrolü, hava kalitesinin izlenmesi, çevresel güvenlik ve tıbbi teshişler için hayati öneme sahip alanlarda kritik rolller üstlenmektedir. Özellikle, endüstriyel ve çevresel uygulamalarda zararlı gazların tespit edilmesi, halk sağlığı ve güvenliği açısından büyük öneme sahiptir. Bu nedenle, düşük maliyetli, yüksek duyarlılık ve hızlı yanıt süresine sahip gaz sensörlerine duyulan ihtiyaç büyüktür. Bu sensörler biyomedikal uygulamalarda, solunum yolu hastalılarının teşhisinde öneme sahip uçucu organik biyo belirteçlerin (VOC) tespitiden kanser tipine kadar geniş bir kullanım alanına sahiptir. Bu çalışmada tek bir silikon yonganın üzerine ince film biriktirme yöntemlerinde kullanılan; çok katlı foto-litografi, PVD (e-beam), PECVD, elektrokimyasal yöntemler, üst üste entegrasyon, ICP-RIE kuru aşındırma, metalizasyon gibi yöntemler kullanılarak bir çok uygulama alanında kullanılabilir platformlar üretilmiştir. Üretilen platformun çalışıp çalışmadığının kontrolü için gaz sensör uygulaması seçilmiştir. İlgili malzemelerin sentez, katkılama ve platform üzerine kaplanması için hidro termal ve damlatma metodları ile gaz sensörleri üretimi başarılı bir şekilde gerçekleştirilmiştir. Çalışma sonucunda 2.2 mm en ve 4.8 mm boy oranlarına sahip, 300 µm Si-yonga üzerine çok katlı (2 µm SiO2 / 30 nm Ti / 30 nm Au / 600 nm Pt ) mikro ısıtıcı sistemleler üretilerek, 1 dakikada max 417℃ sıcaklığa yükselen platin mikro ısıtıcılar üretilmiştir. Platin mikro ısıtıcıların sıcaklık karakterizasyonları için hem kendi oluşturduğumuz devre hem de termal kamera ile ölçümler yapılmıştır. Ölçüm sonuçlarından, sıcaklık değişimine karşı direnç değişim grafiğinden platin metali için α sabiti 0.00345 ℃-1 olarak hesaplanmıştır. Üst üste biriktirme teknolojisi sayesinde 250 nm kalınlığında Si3N4 pasivasyon malzemesi kullanılarak ve üretilen mikro ısıtıcıların 200℃ ve 400℃ de 2 saat tavlama işlemi gerçekleştirilmiştir. Çok katmanlı xxvi (100 nm Ti / 100 nm Au) IDE'lerin üretimi ve Si3N4 ara katman üzerine entegrasyonu gerçekleştirilmiştir. Bu platform için 4 çıkışlı 2 si mikro ısıtıcı, 2 si IDE sistem çıkışlı bakır PCB'ler üzerine ilk olarak mikro ısıtıcı sistemlerin ısı kaybını önlemek için 2 mm en ve 2 mm boya sahip 300 µm kalınlığında Si-yonga (wafer) takoz kesimi gerçekleştirilip üst üste yapıştırılmıştır, ardından tel bağlama (wire bonder) tekniği ile 25 µm Au teller ile bond işlemleri gerçekleştirilmişitir. Gaz sensör uygulaması için elektro aktif polimer ve metal oksitler kullanılmıştır. PANI, SnO2 malzelemelerin sentez kısımları gerçekleştirilmiş (PANI için emeraldin baz yalıtkan formu HCl ile muamele edilerek iletken hale getirilmiştir) ve ticari olarak satılan ZnO malzemesi ile 1:1 mg ve 1:5 mg gibi farklı oranlarında PANI, PANI / SnO2, PANI / ZnO nano kompozit metal oksit 3 tip gaz sensörleri üretilmiştir. Bu gaz sensörleri ile gaz sensör uygulamasının; endüstriyel süreçlerin kontrolü, hava kalitesinin izlenmesi, çevresel güvenlik için öneme sahip NO2 gazı ve solunum yolu hastalıkları için öneme sahip olan aseton, etanol, nem ve kloroform gazlarının akım-zaman yanıt grafikleri MATLAB kodu geliştirilerek analiz edilmiştir. Tüm sensörlerin saf gazlara karşı ve bu saf gazların %30, %50, %70 neme maruz bırakılmış konsantrasyonları için, oda sıcaklığında ve 55℃ sıcaklıkta ölçümler alınarak bar grafikleri elde edilmiştir. Platin metali için α sabiti 0.00345 ℃-1 olarak hesaplanması çok katmanlı mikro ısıtıcı sistemlerin doğru bir biçimde geliştirildiği, 2 mm – 2 mm (en-boy) oranlarındaki takozların sisteme yapıştırılması ısı kaybını önlemiştir ve max 417℃ sıcaklık elde edilmiştir. Üretilen 3 sensör tipinin çalışır durumda olduğu ölçüm sisteminden alınan verilerin MATLAB analizi ile çalışır durumda olduğu tespit edilmiştir.
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ÖgeNon-relativistic gravity theories and their relations to multi-metric theories(Graduate School, 2022)Lie algebra expansion is an exciting method to obtain higher dimensional algebras and using this method one can write some interesting non-relativistic gravitational theories beginning from the Poincaré algebra. This method was first developed by Hatsuda and Sakaguchi (2003) and has been used in many other studies. In this work, we will first give a brief introduction to the gauge theories, which are seminal for understanding gravitational theories in depth, especially the algebraic structure of gravitational theories. Note that this is crucial for many gravity theories, such as supergravity. After that, we will study the general aspects of differential geometry shortly. This will give us the main mathematical framework to study gravity as a gauge theory. Thirdly, we will try to understand the theories of gravity, especially general relativity, as a gauge theory. After a simple introduction to the second-order formalism of GR, we will define the first-order formalism and its action. In the last part of this section, we will obtain GR beginning from the Poincaré algebra and by gauging this algebra. At last, we will give the definition of Newton-Cartan theory, its conditions, and its action. We will first show that this theory can be obtained from an algebraic point of view, i.e. by using Lie Algebra Expansion. We will also give the method, which is based on Ekiz et al. (2022), to obtain the same results by contraction of a multi-metric theory.
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