Browsing by Author "Yavuz, Koray"

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Open Access
Ablation-cooled material removal at high speed with femtosecond pulse bursts
(OSA, 2015) Kerse, Can; Kalaycıoğlu, Hamit; Elahi, Parviz; Akçaalan, Önder; Yavaş, S.; Aşık, M. D.; Kesim, Deniz Koray; Yavuz, Koray; Çetin, Barbaros; İlday, Fatih Ömer
We report exploitation of ablation cooling, a concept well-known in rocket design, to remove materials, including metals, silicon, hard and soft tissue. Exciting possibilities include ablation using sub-microjoule pulses with efficiencies of 100-mJ pulses.
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Bir müze otel olarak Pera Palas
(Bilkent University, 2013) Yazıcı, Elifnur; Kondel, Gülizar; Yavuz, Koray; Özdemir, Ömer; Yıldızhan, Yağmur
Open Access
Crystal growth and investigations on the effects of hydrogen doping of VO2
(2019-03) Yavuz, Koray
Vanadium Dioxide(VO2) has been studied extensively for its interesting electronic structure that allows it to go through Metal-Insulator Transition(MIT) at 65 C. The nature of this phenomena is not entirely clear and more research is needed to firmly establish the science behind it and to realize possible applications; such as ultra-fast electrical and optical switching, sensor devices and Mott-Field Effect Transistors. One of the important experiments to understand the electronic structure of a material is Hall-effect measurements but due to acicular (needle like) nature of VO2 crystals, this subject is only studied either on millimeter sized samples which are not suitable for many device applications or on poly crystalline thin films that are under non-uniform stress due to the substrate effects which gives unsatisfactory results when performing experiments. This thesis suggest a new method of chemical vapour deposition(CVD) growth for low aspect ratio VO2 crystals that have lengths between 50-100 m and thicknesses between 40- 170 nm. These crystals can be mechanically removed from the substrate and transferred to use in different applications such as Hall-effect measurements or Transmission Electron Microscope(TEM) studies. Additionaly this work shows some aspects of the surface chemistry of the widely used Silica, Si, quartz and Sapphire substrates; relating with the control of oxygen saturation on the surface. Another VO2 growth method for c-plane sapphire that leads to considerably more crystal yield is shown. Hydrogenation of the VO2 crystals suppresses the MIT so understanding this phenomena might help us better understand the effects lying behind the transition. To study this phenomena a crystal is doped only from half by blocking the passage of hydrogen to other half so the interplay between the insulating phase and hydrogenated conductive phase can be observed. As the analysis tool, TEM is used on this sample. Using a two-terminal device of a VO2 crystal, the effects of hydrogenation on the electronic properties have also been studied. Overall this thesis introduces a new method for CVD growth of VO2 which is used in various applications such as Hall-effect experiments, two terminal devices and TEM studies. To control the growth process the interplay between oxygen and surface chemistry of sapphire, silica, Si and quartz substrates have been investigated. With these studies a better understanding of the mechanics of growth is intended.
Open Access
Effect of spider's weight on signal transmittance in vertical orb webs
(The Royal Society Publishing, 2024-10-02) Yavuz, Koray; Soler, A. Mahy; Zaera, Ramon; Jahangirov, Seymur
Spider orb web is a sophisticated structure that needs to fulfil multiple roles, such as trapping prey and transmitting web-borne signals. When building their web, heavier spiders tend to increase the pretension on the web, which seems counterintuitive since a tighter web would decrease the chances of stopping and retaining prey. In this article, we claim that heavier orb-weaving spiders increase tension on the web in order to reduce the attenuation of the vibratory signal coming from the bottom part of the web. We support our claim by first building a detailed spider web model, which is tuned by a tension-adjusting algorithm to fit the experimentally observed profiles. Then, the effects of the spider weight and the web tension on the signal transmittance properties are investigated using state-of-the-art finite element analysis tools.
Embargo
Spider orb web, investigating structural features and using biomimicry for lattice design
(2024-12) Yavuz, Koray
Spider orb web has evolved to fulfil multiple roles, such as catching prey and acting as a sensing tool. For orb-weavers, the web must stop and retain prey, which can fly into it with considerable momentum. Considering orb weaver spiders are mostly blind, the web must also transmit accurate information about the prey’s location. There are many web features aiding these roles; some are known, and some are waiting to be shed light on. Considering these two cases, there are also two parts in this thesis, the first part is about a particular web-building behaviour of spiders and how it affects the signal transmittance, and the other is about creating a new lattice design for energy absorption utilizing some of the known structural characteristics of the spider web. The first part of this study is about designing a new lattice (SW) for energy absorption inspired by the structure of spider webs. Spider orb web comprises four structural elements: anchor, frame, radial, and spiral threads. The first three are the main components that provide structural integrity. These components have a hierarchical nature; the anchors bind to the environment and are generally thickest, while radii form the innermost part with the thinnest threads. The frames make up the connection between the anchor and radii; thus, there is no direct connection between them, and they generally have a thickness value between the radii and anchor threads. These features help the spider orb web to be a resilient, efficient structure for energy absorption, so using the same properties, a 3D lattice was designed for energy absorption. This design is then optimized for improved Energy Absorption Efficiency(EAE) and Energy Absorption(EA) value. The second part is about web-building behaviour that seems counterintuitive, heavier spiders increase the pretension of the threads as they get heavier, which diminishes the ability of the web to stop and retain prey. To investigate this behaviour, a spider web model with controllable pretension is needed; thus, a pretension-adjusting algorithm has been developed. A realistic spider web model was created using non-linear material properties to describe the mechanical behaviour of the spider silk and web pretension values seen in nature. Using this model, different scenarios with changing spider weight and web pretension were simulated using a numerical method based on Solid Mechanics. Our results show that this behaviour is likely related to the signal transmittance on the spider web. Spider web evolved to withstand damaging environmental factors such as wind and rainfall while preserving its functionality for trapping prey. Understanding spider web structure could lead us to improve engineering designs by implementing similar resiliency. This thesis presents a study investigating spider webs and a biomimicking application inspired by spider web structure. So, while the two areas are different in the sense that one is closer to biology while the other is to mechanical engineering, they serve the same purpose: understanding how this structure, spider orb-web, functions and how we can take ideas from it.
Open Access
Ultrafast micromachining of Cu and Si at ultra-high repetition rates with pulse bursts
(IEEE, 2016) Kerse, Can; Kalaycıoğlu, Hamit; Elahi, Parviz; Yavuz, Koray; Mirza I.; Bulgakova N.M.; İlday, F. Ömer
We report a novel ultrafast burst mode fiber laser system, which can deliver pulses at ultra-high repetition rates in order to systematically investigate micromachining efficiency on copper and silicon samples. © 2015 IEEE.