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    A numerical approach to the heat transfer in monolithic and SiC reinforced HfB2, ZrB2 and TiB2 ceramic cutting tools
    (Elsevier Sci Ltd, 2019) Moghanlou, Farhad Sadegh; Vajdi, Mohammad; Sha, Jianjun; Shokouhimehr, Mohammadreza; Asl, Mehdi Shahedi; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    Cutting tools are widely used in industry and must be hard enough for machining processes, which should work appropriately at low temperatures to improve cutting speed and productivity. In this research, a numerical method was employed to calculate the temperature distribution in the cutting tools made of different diborides. Monolithic and SiC reinforced HfB2, ZrB2 and TiB2 ceramics were selected for investigation and comparison studies. In this regard, 3-dimensional heat conduction equation was solved in a cutting tool with radiative, convective and heat flux boundary conditions by finite element method using COMSOL Multiphysics. This study clarifies that the maximum temperature in the tools made of ZrB2 and TiB2 among the monolithic ceramics is lower than that of HfB2. Moreover, the temperature variation slope versus time is the highest in HfB2. All composite materials reinforced with SiC showed lower maximum temperature than the monolithic ones. The thermal performance of TiB2-SiC and ZrB2-SiC composites was acquired to be better than that of the other investigated materials. The dominant heat transfer mechanism in the cutting tools was conduction.
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    Effect of yttrium doping on structural and electrical properties of Bi2Sr1.9Ca0.1-xYxCu2O7+delta (Bi-2202) cuprate ceramics
    (Elsevier, 2016) Boudjadja, Yazid; Amira, Abderrezak; Saoudel, Abdelmalek; Mahamdioua, Nabil; Varilci, Ahmet; Terzioğlu, Cabir; N/A; Polat-Altıntaş, Sevgi; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; 150017
    In this work, we report on the effect of Y3+ doping on structural, mechanical and electrical properties of Bi-2202 phase. Samples of Bi2Sr1.9Ca0.1-xYxCu2O7+delta with x=0, 0.025, 0.05, 0.075 and 0.10 are elaborated in air by conventional solid state reaction and characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) combined with EDS spectroscopy, density, Vickers microhardness and resistivity measurements. A good correlation between the variations of the bulk density and the Vickers microhardness with doping is obtained. The SEM photograph shows that the samples are composed of grains with a flat shape that characterizes the Bi-based cuprates. Quantitative EDS analysis confirms the reduction of Ca content and the increase of Y content when xis increased. The variation of resistivity with temperature shows that only samples with x=0, 0.025 and 0.05 present an onset transition to the superconducting state. The higher onset transition temperature is obtained for x= 0.025 and is about 93.62 K. The transition is wide and is realized in two steps confirming then the presence of the low T-c Bi-2201 phase in the samples. For x=0.075 and 0.10, a transition to a semiconducting state is seen at low temperatures. Some physical parameters are extracted from these curves and discussed.
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    Effects of ceramic-based CrN, TiN, and AlCrN interlayers on wear and friction behaviors of AlTiSiN+TiSiN PVD coatings
    (Elsevier Sci Ltd, 2021) Ozkan, Dogus; Yilmaz, M. Alper; Szala, Miroslaw; Turkuz, Cenk; Chocyk, Dariusz; Tunc, Cihan; Goz, Onur; Walczak, Mariusz; Pasierbiewicz, Kamil; N/A; Yağcı, Mustafa Barış; Researcher;  Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    In this study, the wear and friction behavior of cathodic arc physical vapor deposited AlTiSiN+TiSiN coatings on H13 tool steels were investigated by using CrN, TiN and AlCrN interlayers with tribometer tests both under unlubricated and boundary lubricated conditions. 6 mm alumina balls were used as counter surfaces to test ceramic hard coatings. Surface coatings were characterized through nanoindentation, scanning electron microscopy coupled with an energy-dispersive X-ray spectrometer (SEM/EDXS), optical profilometry, and atomic force microscopy (AFM) techniques. The results showed that especially AlTiSiN+TiSiN coating with TiN interlayer resulted in a much more enhanced tribological performance of the tool steels at both unlubricated and the boundary lubricated conditions even at elevated contact pressures.
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    Effects of different milling conditions on the properties of lanthanum hexaboride nanoparticles and their sintered bodies
    (Elsevier, 2019) Ağaoğulları, Duygu; Akçamlı, Nazlı; Duman, İsmail; Oveçoğlu, M. Lutfi; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
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    Effects of graphite nano-flakes on thermal and microstructural properties of TiB2–SiC composites
    (Elsevier Sci Ltd, 2020) Moghanlou, Farhad Sadegh; Nekahi, Sahar; Vajdi, Mohammad; Ahmadi, Zohre; Shokouhimehr, Ali; Shokouhimehr, Mohammadreza; Jafargholinejad, Shapour; Asl, Mehdi Shahedi; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    In the last decades, the production of ultra-high temperature composites with improved thermo-mechanical properties has attracted much attention. This study focuses on the effect of graphite nano-flakes addition on the microstructure, densification, and thermal characteristics of TiB2-25 vol% SiC composite. The samples were manufactured through spark plasma sintering process under the sintering conditions of 1800 degrees C/7 min/40 MPa. Scanning electron microscopy images demonstrated a homogenous dispersion of graphite flakes within the TiB2-SiC composite causing a betterment in the densification process. The thermal diffusivity of the specimens was gained via the laser flash technique. The addition of graphite nano-flakes as a dopant in TiB2-SiC did not change the thermal diffusivity. Consequently, the remarkable thermal conductivity of TiB2-SiC remained intact. It seems that the finer grains and more interfaces obstruct the heat flow in TiB2-SiC-graphite composites. Adding a small amount of graphite nano-flakes enhances the densification of the mentioned composite by preventing the grain growth.
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    Fabrication and gas sensing properties of C-doped and un-doped TiO2 nanotubes
    (Elsevier, 2014) Sennik, Erdem; Işık, Müge; Ahsen, Ali Şems; Öztürk, Osman; Öztürk, Zafer Ziya; Department of Electrical and Electronics Engineering; Kılınç, Necmettin; Researcher; Department of Electrical and Electronics Engineering; College of Engineering; 59959
    In this work, un-doped and carbon (C) doped TiO2 nanotubes were fabricated and their hydrogen sensing properties were investigated. A Ti foil was anodized in an aqueous hydrofluoric acid (H:F) electrolyte (0.5 wt%) at room temperature to form TiO2 nanotube arrays. C-doped TiO2 nanotubes were obtained through two methods; a chemical process and thermal acetylene (C2H2) treatment. In the chemical method, a Ti foil was anodized 'in-situ' in aqueous solution of 0.5 wt% polyvinyl alcohol (PVA)+0.5 wt% HF. In the heat treatment method, a Ti foil was anodized in an aqueous (HF) electrolyte (0.5 wt%) to obtain TiO2 nanotubes, and then C-doped TiO2 nanotubes were obtained by heating as-prepared nanotubes at 500 degrees C in a quartz tube under a continuous N-2 and C2H2 flux (1:1). The obtained un-doped and C-doped TiO2 nanotubes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The H-2 sensing properties of the nanotubes exposed to 5000 ppm H-2 were investigated at 100 degrees C. C-doped TiO2 nanotubes showed a lower response to H-2 than the undoped TiO2 nanotubes.
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    First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: microstructural, mechanical and thermal characterization
    (Elsevier B.V., 2023) Kavak, S.; Bayrak, K. G.; Mansoor, M.; Kaba, M.; Ayas, E.; Derin, B.; Öveçoğlu, M.L.; Ağaoğulları, D.; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    First principles calculations were conducted on (HfTiWZr)B2 high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B2 HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB2 structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10−4 mm3, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C. © 2022 Elsevier Ltd
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    Halloysite clay nanotube in regenerative medicine for tissue and wound healing
    (Elsevier, 2022) Same, Saeideh; Samee, Golshan; Navidi, Golnaz; Jahanbani, Yalda; Davaran, Soodabeh; N/A; Nakhjavani, Sattar Akbar; Researcher; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); N/A; N/A
    The vital necessity of effective treatment at damaged tissue or wound site has resulted in emerging tissue en-gineering and regenerative medicine. Tissue engineering has been introduced as an alternative approach for common available therapeutic strategies in the terms of restoring deformed tissue structure and its functionality via the developing of new bio-scaffold. Designed three-dimensional (3D) scaffolds, alone or in combination with bioactive agents, should be able to stimulate and accelerate the development of engineered tissues and provide proper mechanical support during in-vivo implantation and later regeneration process. To cover it up, a series of new bio-structures with higher mechanical strength were designed through the combination of halloysite nanotubes (HNTs) into 3D bio-polymeric networks. HNTs clay mineral with its unique rod-like structure and distinctive chemical surface features, exhibits excellent biocompatibility and biosafety for doping into regen-erative scaffolds to enhance their mechanical stiffness and biological performance. In this paper, the ongoing procedures of bone/cartilage tissue engineering and wound healing strategies focusing on the designing of 3D-HNTs bio-composites and their multi-cellular interactions in-vitro and in-vivo preclinical studies are reviewed. Furthermore, the challenges and prospects of 3D-HNTs and HNTs-based functional bio-devices for regenerative medicine are also discussed.
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    Heat transfer, thermal stress and failure analyses in a TiB2 gas turbine stator blade
    (Elsevier Sci Ltd, 2019) Vaferi, Kourosh; Nekahi, Sahar; Vajdi, Mohammad; Moghanlou, Farhad Sadegh; Shokouhimehr, Mohammadreza; Sha, Jianjun; Asl, Mehdi Shahedi; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    Gas turbine stator blades do not experience centrifugal force contrary to the rotor blades; but they are exposed to high-temperature combustion gases causing thermal stresses. In the present work, a series of numerical simulations were carried out to clarify the feasibility of TiB2 utilization as an appropriate material for gas turbine stator blades. The governing equations of heat transfer and solid mechanics were discretized by the finite element method and solved using Comsol Multiphysics software. The boundary conditions were applied, and temperature, displacement and maximum principle stress were obtained. The results showed that using ceramics such as TiB2 instead of conventional alloys can enhance the maximum displacement. Temperature distribution in the blade is more uniform than that of alloys, and consequently, the thermal stresses are reduced. The TiB2 can withstand the applied stresses according to the Coulomb-Mohr theory with a safety factor of 2.4.
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    Mechanical activation-assisted autoclave processing and sintering of HfB2-HfO2 ceramic powders
    (Elsevier Sci Ltd, 2016) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531
    This study reports on the synthesis and consolidation of HfB2-HfO2 ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl4, B2O3 and Mg starting powders were mechanically activated for 5 min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500 degrees C for 6 or 12 h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl2 by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190 nm comprised the HfB2 phase with an amount of 79.6 wt% in addition to the HfO2 phase and a very small amount of Mg2Hf5O12 phase after mechanical activation for 5 min and autoclave processing for 12 h. They were consolidated at 1700 degrees C both by PS for 6 h and SPS for 15 min. The Mg2Hf5O12 phase decomposed during sintering and bulk samples only had the HfB2 and HfO2 phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB2-HfO2 ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (similar to 91%), microhardness (similar to 16 GPa) and relative wear resistance (2.5) values of the HfB2-HfO2 ceramics consolidated by SPS. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.