Browsing by Subject "Composite"
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Item Open Access Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity(American Chemical Society, 2012-09-14) Otto, T.; Mueller, M.; Mundra, P.; Lesnyak, V.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.The incorporation of colloidal quantum dots (QDs) into ionic crystals of various salts (NaCl, KCl, KBr, etc.) is demonstrated. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Moreover, the ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment and as a result providing them with extraordinary high photo- and chemical stability. A prototype of a white light-emitting diode (WLED) with a color conversion layer consisting of this kind of mixed crystals is demonstrated. These materials may also find applications in nonlinear optics and as luminescence standards.Item Open Access A conducting composite of polypyrrole II. As a gas sensor(Elsevier, 1995) Selampinar, F.; Toppare, L.; Akbulut, U.; Yalçin, T.; Süzer, Ş.Pure polypyrrole (PPy) and polypyrrole-polyamide (PPy-PA) composite films were synthesized electrochemically. The gas-sensing ability was investigated for both pure PPy and PPy-PA films. The composite films' response to several gases are better defined and reproducible compared to pristine conducting polymer. Electrochemical behaviour of PPy and PPy-PA electrodes in the presence of pyrrole and pyrrole-free medium is investigated via cyclic voltammetry. Mass spectrometry studies strictly reveal that the composite is completely different to a mechanical mixture. This phenomenon is discussed in comparison to polyaniline-polycarbonate composite. © 1995.Item Restricted Epsilon (Kompozit Teknoloji Ve Savunma Sanayii A.Ş.) kuruluşu, gelişimi ve Türkiye’ye katkıları(Bilkent University, 2020) Işlak, Buse Selin; Korkmaz, Cansu; Uğur, Defne; Bakırkaya, Eda; Erdoğan, AnılBu makale, 1999 yılında kurulmuş, günümüzde hem Türkiye'de hem Dünya'da havacılık alanında önemli işlere imza atan Epsilon firması hakkındadır. Makalede firmanın tarihi, kompozit üretimindeki rolü, önemli ortaklıkları ve Türkiye'nin savunma sektörüne yaptığı katkılardan bahsedilmektedir. Bununla birlikte Türkiye savunma sanayisinin kısa tarihinden, sektördeki açıklıklardan ve kompozit malzemelerinin kullanım sebeplerinden bahsedilmiştir. Faaliyetine tek bir çalışanla model uçak üreterek başlayan Epsilon, günümüzde bazı Dünya firmalarının kritik parçalarının tek üreticisi konumuna gelmiştir ve bu durum ülke ekonomisine de katkı sağlamıştır. Bu makale yazılırken kullanmaya karar verilen yazılı kaynaklar şirket yetkilileriyle yapılan röportajlardan edinilen bilgilerle desteklenmiştir.Item Open Access Fabrication of AlN/BN bishell hollow nanofibers by electrospinning and atomic layer deposition(AIP Publishing, 2014-09-08) Haider A.; Ozgit Akgun, C.; Kayaci, F.; Okyay, Ali Kemal; Uyar, Tamer; Bıyıklı, NecmiAluminum nitride (AlN)/boron nitride (BN) bishell hollow nanofibers (HNFs) have been fabricated by successive atomic layer deposition (ALD) of AlN and sequential chemical vapor deposition (CVD) of BN on electrospun polymeric nanofibrous template. A four-step fabrication process was utilized: (i) fabrication of polymeric (nylon 6,6) nanofibers via electrospinning, (ii) hollow cathode plasma-assisted ALD of AlN at 100 degrees C onto electrospun polymeric nanofibers, (iii) calcination at 500 degrees C for 2 h in order to remove the polymeric template, and (iv) sequential CVD growth of BN at 450 degrees C. AlN/BN HNFs have been characterized for their chemical composition, surface morphology, crystal structure, and internal nanostructure using X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction. Measurements confirmed the presence of crystalline hexagonal BN and AlN within the three dimensional (3D) network of bishell HNFs with relatively low impurity content. In contrast to the smooth surface of the inner AlN layer, outer BN coating showed a highly rough 3D morphology in the form of BN nano-needle crystallites. It is shown that the combination of electrospinning and plasma-assisted low-temperature ALD/CVD can produce highly controlled multi-layered bishell nitride ceramic hollow nanostructures. While electrospinning enables easy fabrication of nanofibrous template, self-limiting reactions of plasma-assisted ALD and sequential CVD provide control over the wall thicknesses of AlN and BN layers with sub-nanometer accuracy. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.Item Open Access Impact of Li2O/metal mole ratio on lithium-ion battery anode performance(Electrochemical Science Group, 2018) Kunduraci, M.; Ghobadi, T. G. U.; Yilmaz, E.In this study the electrochemical impact of Li2O/metal mole ratio on the cycle life of lithium-ion battery anode materials is demonstrated. For this purpose, nanostructured layered LiNi1/3Mn1/3Co1/3O2 (LiNMC) and spinel LiMn1.5Ni0.5O4 (LiMNO) materials, traditionally known as cathode materials, are evaluated as anode materials and compared against their lithium-free versions NMC (Ni:Mn:Co=1:1:1) and MNO (Mn:Ni=3:1). The Li2O/metal ratio in fully lithiated states are 2.0 for lithium containing (LiNMC and LiMNO) and 1.3 for lithium-free (NMC and MNO) samples. Battery tests show that capacity fading of lithium containing samples is 3 to 4 times larger than lithium-free samples. The differences in the electronic conductivities and voltages profiles of lithium containing and lithium-free anode materials are suggested to be the origin of such electrochemical disparity.Item Open Access Milling force modelling of multidirectional carbon fiber reinforced polymer laminates(Elsevier, 2012) Karpat, Yiğit; Bahtiyar, O.; Deger, B.Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using a polycrystalline diamond cutter. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates and capable of investigating stability of machining. © 2012 The Authors.Item Open Access Performance evaluation of fiber-based ballistic composites against laser threats(Elsevier, 2019) Candan, C.; Seymen, A. Aytaç; Karatutlu, Ali; Tiken, M.; Midilli, Yakup; Orhan, E.; Berberoğlu, H.; Ortaç, BülendThe interaction between a laser and a composite material has been an intense subject within the past decade and become an emerging field for the defense and manufacturing industry since high-power lasers were initiated to be utilized for the directed-beam applications. In this study, a specially developed composite material for the ballistic applications was shined to a continuous wave (CW) laser beam at 915 nm. The ballistic material was composed of 77 layers of the single sheet of the SR-3136 by Spectra Shield® from Honeywell consisting of ultra-high molecular weight polyethylene (UHMW-PE) fibers reinforced with low-density PE (LD-PE) fibers and a polyurethane-based thermoplastic resin. At the instant of the exposure, the region of interest was completely evaporated and punctured with a slight swelling around the hole where the temperature was over 450 °C. The composite material was drilled completely upon exceeding 20 kJ of laser energy. The chemical and physical changes on the composite material after the laser exposure were extensively studied by a combination of techniques including High-Resolution Scanning Electron Microscopy (HR-SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). The physical properties of a single layer of the SR-3136 were also studied using HR-SEM, UV-VIS-NIR Absorption Spectroscopy, Thermogravimetric analysis (TGA), Differential Scanning Calorimetry (DSC) and XPS. The research presented here reveals the first study on the effects of the high-power laser beam irradiance on the fiber-reinforced composite materials utilized for the ballistic protection.Item Open Access Processing and environmental effects on composite repairs(2012) Elaldı, Faruk; Elaldı, PelinThe scarf-joint technique is one of the latest techniques used for repairing composite aircraft structures. But this technique is mostly used at depot level repairing activities since it requires autoclave and other equipments. This article focuses on scarf joint comprised of vacuum and autoclave precured and co-cured composite patches bonded to autoclave and vacuum precured parent laminates. Autoclave and vacuum cured parent laminates and scarf joints were prepared and exposed to the same temperature and moisture environment for comparison. All specimens were loaded in tension at three temperatures. Interlaminar shear strength (ILSS) tests were also carried out for the parent materials. As noted, the tensile strength and ILSS decrease when the material has been exposed to moisture and tested at elevated temperature. But, no significant difference was reported for either tensile strength or ILSS between autoclave and vacuum cured materials. The room temperature repair efficiencies are reported for single scarf repairs comprised of vacuum co-cured and precured patches. These vacuum cured repair efficiencies were found to be similar to the efficiency of the autoclave precured patch repair. This result supports the feasibility of scarf joint repairs with precured or co-cured patches under vacuum curing conditions in field level facilities. Therefore, repairs with vacuum precured or vacuum co-cured patches requiring less equipment seems to be a serious potential alternative to the composite patch repair requiring autoclave conditions which might be only available at depot level maintenance centers.Item Open Access A study on curing processes and environmental effects for rapid composite repair(2011) Elaldı, Faruk; Elaldı, PelinThis article focuses on scarf joint comprised of vacuum-precured, vacuum-cocured, autoclave-procured, and autoclave-cocured composite patches bonded to autoclave- and vacuum-precured parent laminates. Autoclave- and vacuum-cured parent laminates and scarf joints were prepared and exposed to the same temperature and moisture environment for comparison. All specimens were loaded in tension at three temperatures. Interlaminar shear strength (ILSS) tests were also carried out for the parent materials. As noted, the tensile strength and ILSS decrease when the material has been exposed to moisture and tested at elevated temperature. But, no significant difference was reported for either tensile strength or ILSS between autoclave- and vacuum-cured materials. The room temperature repair efficiencies are reported for single-scarf repairs comprised of vacuum-cocured and vacuum-precured patches. These vacuum-cured repair efficiencies were found to be similar to the efficiency of the autoclave-precured patch repair. This result supports the feasibility of scarf joint repairs with precured or cocured patches under vacuum curing conditions in field-level facilities. Therefore, repairs with vacuum-precured or vacuum-cocured patches requiring less equipments seem to be a serious potential alternative to the composite patch repair requiring autoclave conditions which might be only available at depot-level maintenance centers.Item Open Access Synthesis of a hexafluoropropylidene-bis(phthalic anhydride)-based polyimide and its conducting polymer composites with polypyrrole(John Wiley & Sons, Inc., 1997) Selampinar, F.; Akbulut, U.; Yilmaz, T.; Gungor, A.; Toppare, L.A new electrically conducting composite film from polypyrrole and 4,4′(hexafluoroisopropylidene)-bis(phthalic anhydride)-based polyimide was prepared. Pyrrole and the dopant ion can easily penetrate through the polyimide substrate and electropolymerize on the platinum (Pt) electrode due to the swelling of the polyimide on the metal electrode. The electrochemical properties of polypyrrole-polyimide (PPy/PI ) composite films have been investigated by using cyclic voltammetry. The PPy/PI composite film is suitable for use as the electroactive material owing to its stable and controllable electrochemical properties. The electrical conductivity of composites falls in the range 0.0035-15 S/cm. Scanning electron micrograph, FTIR, and thermal studies indicate that PPy and PI form a homogeneous material rather than a simple mixture. © 1997 John Wiley & Sons, Inc.