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Publication Metadata only Influence of soft segment structure, hydrogen bonding, and diisocyanate symmetry on morphology and properties of segmented thermoplastic polyurethanes and polyureas(Tubitak Scientific & Technological Research Council Turkey, 2023) Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Department of Chemistry; College of SciencesA comprehensive review of the structure-morphology-property relations in segmented thermoplastic polyurethanes and polyureas (TPU) is provided. Special emphasis is given to the influence of the soft segment structure, polarity, and molecular weight, diisocyanate symmetry and the nature, extent, and strength of hydrogen bonding on the morphology and thermal and mechanical properties of TPUs. Experimental results obtained on composition-dependent TPU morphology and properties by various techniques were also compared by the morphology profiles generated by computational methods such as quantum mechanical calculations and molecular dynamics simulations.Publication Metadata only Effect of ambient and cryogenic milling on the microstructure and properties of tungsten matrix composites fabricated by activated sintering(Sivas Cumhuriyet Üniversitesi, 2019) Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Department of Chemistry; Department of Chemistry; Balcı, Özge; Somer, Mehmet Suat; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 295531; 178882Tungsten matrix composites reinforced with TiB2 and Y2O3 particles were fabricated by milling under ambient/cryogenic conditions and Ni activated sintering. Powder blends constituting the W - 1 wt. % Ni - 2 wt. % TiB2 - 1 wt. % Y2O3 composition were mechanically milled for 12 h under ambient condition or cryomilled for 10 min or sequentially milled under ambient and cryogenic conditions. Milling was carried out in a high-energy ball mill under ambient condition whereas cryogenic milling was conducted in externally circulated liquid nitrogen. Milled powders were compacted using a hydraulic press and the pellets were sintered at 1400°C for 1 h under Ar / H2 gas flowing conditions. The effects of different milling types on the microstructural and mechanical properties of the sintered composites were investigated. After sintering, in addition to dominant W phase, small amounts of WB and NiW phases were detected in all sintered samples. The application of cryomilling after milling at ambient condition provided the disappearance of the clustered TiB2 and Y2O3 particles in the sintered sample: They were located at the grain boundaries of W1Ni matrix and homogeneously distributed through the microstructure. Sequentially milled and sintered composite had the highest relative density (95.77 %) and the highest microhardness (7.23 GPa) values among the samples. Nanoindentation tests showed that there was an improvement in the hardness and elastic modulus of W matrix phase, which yielded the values of 8.9 and 373.7 GPa, respectively. / Özet: Bu çalışmada, Ni ile aktive edilerek sinterlenen tungsten esaslı matrisin TiB2 ve Y2O3 partikülleri ile takviye edilmesiyle, tungsten esaslı kompozit malzeme üretimi gerçekleştirilmiştir. W - % 1 ağ. Ni - % 2 ağ. TiB2 - % 1 ağ. Y2O3 kompozisyonundan oluşan toz harmanları, normal koşullarda 12 sa mekanik olarak öğütülerek ya da 10 dk kriyojenik ortamda öğütülerek ya da normal ve kriyojenik şartlarda ardışık olarak öğütülerek hazırlanmıştır. Normal şartlarda öğütme yüksek enerjili bir değirmende uygulanırken; kriyojenik şartlarda öğütme sıvı azot ile dışarıdan çevrelenen bir sistemde yapılmıştır. Öğütülmüş tozlar hidrolik pres kullanılarak preslenmiş ve pekiştirilen bünyeler Ar/H2 gazaltı şartlarında 1400°C’de 1 sa sinterlenmiştir. Farklı öğütme koşullarının sinterlenen kompozit malzemelerin mikroyapısı ve özellikleri üzerindeki etkisi incelenmiştir. Sinterlenme sonrasında, baskın W fazına ek olarak düşük mikarda WB ve NiW fazları oluşumu gözlemlenmiştir. Normal şartlarda öğütme sonrası uygulanan kriyojenik öğütme, sinter malzemelerin mikroyapısındaki TiB2 ve Y2O3 partiküllerin topaklanmasının yok olmasına neden olmuştur: Partiküllerin W1Ni matrisinin tane sınırlarında ve homojen olarak mikroyapıda dağılması sağlanmıştır. Ardışık olarak öğütülmüş ve sinterlenmiş kompozitler, numuneler arasında en yüksek rölatif yoğunluk (% 95,77) ve mikrosertlik değerlerini (7,23 GPa) sunmuştur. Nano-indentasyon testleri sayesinde, W matris fazının sertlik ve elastisite modül değerlerinin sırasıyla 8,9 ve 373,7 GPa değerlerine kadar iyileştirildiği kanıtlanmıştır.Publication Metadata only A coarse graining approach in molecular simulations: fuzzy potentials(Istanbul Technical University, 2003) Department of Chemistry; Department of Chemistry; Yurtsever, İsmail Ersin; Eşsiz, Şebnem; Faculty Member; Undergraduated Student; Department of Chemistry; College of Sciences; College of Sciences; 7129; 191615A new representation for interaction potential functions is presented. Unlike the orthodox approaches, the potential function is not a fixed function in terms of internuclear coordinates but a probabilistic one which contains information over a wide range of angular degrees of freedom. It is shown that such approaches can provide practical solutions for bulk systems of high density.Publication Metadata only Autoclave processing and sintering of ZrB2 -ZrO2 powders and investigation of microstructural and some mechanical properties of bulk products(TENMAK Bor Araştırma Enstitüsü, 2017) Akçamlı, Nazlı; Ağaoğulları, Duygu; Öveçoğlu, M. Lütfi; Duman, İsmail; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531In this study, the production of ZrB2 -ZrO2 powders and their sintered bodies was aimed by utilizing mechanical activation assisted autoclave process and pressureless sintering (PS) or spark plasma sintering (SPS) techniques. ZrCl4 , B2 O3 and Mg powders were used as starting materials and the powder mixtures were mechanically activated for 5 min by using high energy ball milling system. Mechanically activated powders were heated in an autoclave at 450°C for 12 h and subsequently purified by HCl leaching. Pure powders were consolidated by PS (1700°C, 6 h) and SPS (1700°C, 60 MPa, 15 min) to obtain the bulk samples. Characterization studies of the powders and sintered products involve the XRD, SEM, SM, OM techniques, particle size and Archimedes density measurements, microhardness and wear tests. Synthesized powders comprise of ZrB2 and ZrO2 phases with 78.5 % wt. ZrB2 content and have an average particle size of 203 nm. The bulk samples produced by PS and SPS techniques have relative densities of 92.8 % and 99.3 %, respectively and the microhardness reaches to the value of 12.6 GPa. The bulk sample fabricated by SPS technique having a wear volume loss of 8,52x10-3 mm3 reveals a smooth wear track instead of micro-grooves along the sliding direction. / Bu çalışmada, ZrB2 -ZrO2 tozlarının ve sinter ürünlerinin, mekanik aktivasyon destekli otoklavda sentezleme ve basınçsız sinterleme (PS) veya spark plazma sinterleme (SPS) teknikleri kullanılarak üretimi amaçlanmıştır. Hammadde olarak ZrCl4 , B2 O3 ve Mg kullanılmış olup, toz karışımları yüksek enerjili öğütme sisteminde 5 dk süre ile mekanik olarak aktive edilmiştir. Mekanik olarak aktive edilmiş toz karışımları, 450°C’de 12 sa süre ile otoklavda reaksiyona tabi tutulmuş ve ardından HCl çözeltisi ile liç işlemi uygulanarak saflaştırılmıştır. Saf tozlar, basınçsız sinterleme (1700°C, 6 sa) ve spark plazma sinterleme (1700°C, 60 MPa, 15 dk) yöntemleriyle yığın yapıya getirilmiştir. Elde edilen toz ve sinter ürünlerin karakterizasyonu, XRD, SEM, SM, OM, partikül boyutu, Arşimet yoğunluk ölçümü, mikrosertlik ve aşınma testi çalışmalarını kapsamaktadır. Elde edilen tozlar, ağırlıkça % 78,5 ZrB2 fazı ihtiva eden ZrB2 -ZrO2 tozları olup hiçbir empürite içermemektir ve 203 nm ortalama partikül boyutuna sahiptir. PS ve SPS metotları ile elde edilen yığın yapıdaki ürünler, sırasıyla % 92,8 ve % 99,3 rölatif yoğunluk değerlerine sahiptir ve mikrosertlik ortalama 12,6 GPa değerine ulaşmaktadır. SPS ile elde edilen numunedeki aşınma hacimsel kayıp 8,52x10-3 mm3 olup; aşınma izi, derin oyuklar yerine malzemenin yüzeysel olarak aşındığını göstermektedir.Publication Metadata only A two-dimensional Monte Carlo polymerization of 5-membered rings(Scientific Technical Research Council Turkey, 1997) Esentürk, O.; Pamuk, H. A.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129A modification of the kinetic growth model in two dimensions for the polymerization of 5-membered rings is presented. The preliminary results reveal the validity of the modified model.Publication Metadata only Lanthanide doped bismuth tungstates: an investigation for led and photocatalysis applications(Eskişehir Teknik Üniversitesi, 2018) Department of Chemistry; Department of Chemistry; Akkaya, Ceren Yılmaz; Ünal, Uğur; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; N/A; 42079In this work, we studied optical, photocatalytic and photoelectrochemical properties of a series of lanthanide-doped double layered Aurivillius type bismuth oxides. The visible-light harvesting photocatalysts doped with Eu, Pr, Nd, Tm, Ho and Ce ions were synthesized by solid-state synthesis method. Doping amount was varied between 0.2-to-10 percent. X-ray diffraction (XRD) analysis revealed that lanthanide atoms were incorporated into the Bi2W2O9 lattice successfully up to 5%. Higher dopant ratios resulted in formation of secondary phases. Particles crystallized in micro-platelet morphology. Furthermore, luminescence of Pr3+ and Eu3+ ion doped catalysts are evaluated and the study showed that red emission intensity of Pr3+ doped powder is 29 times higher than that of Eu3+ doped ceramics. Lanthanide doped powders absorb in the visible regions. Especially, doping double layered bismuth oxide with Ce significantly narrowed down the band gap from ~3.3 eV to ~2.1 eV. Photoelectrochemical behaviour of the powders was also investigated and rare-earth dependent enhanced photocurrent production is demonstrated. Best photoelectrochemical response is achieved with Tm3+ ion doped sample.Publication Metadata only The synthesis of binary and ternary cobalt based metal borides by inorganic molten salt technique(TENMAK Bor Araştırma Enstitüsü, 2020) N/A; N/A; Department of Chemistry; Department of Chemistry; Altıntaş, Zerrin; Khoshsima, Sina; Somer, Mehmet Suat; Balcı, Özge; Researcher; Researcher; Faculty Member; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); N/A; N/A; College of Sciences; College of Sciences; N/A; N/A; 178882; 295531Crystalline metal boride powders were synthesized via low temperature method in inorganic molten salt medium, and binary and ternary metal boride composite powders were investigated using anhydrous metal chlorides and sodium borohydride powder mixtures. The reactions were carried out in an aluminum crucible placed in a silica tube under argon which was put in a vertical tube furnace. At the end of the reaction, the resulting powder mixture was leached with hot water to remove any undesirable chloride phases. In order to improve crystalline properties, some of pure powders were selected and annealed at 1100°C. Characterization of synthesized and annealed powders was carried out using X-ray diffractometer (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM / EDX) and dynamic light scattering technique (DLS). The results showed the positive effect of inorganic molten salt technique (LiCl/ KCl eutectic mixture) on the formation of phases during the reaction between CoCl2 , NiCl2 and NaBH4 powder mixtures. Following the reactions at between 750-950 °C, the binary and ternary metal boride powders consisting of CoB-Ni2 BCoBx , CoB-Ni4 B3 ve CoB-NiB-Ni2 Co0.67B0.33 phases were obtained. The measured particle size of the final particles had an average of 60 nm. / Öz: Kristalin metal borür tozları inorganik ergimiş tuz ortamında düşük sıcaklık yöntemi ile sentezlenmiş, ikili ve üçlü metal borür kompozit tozlarının eldesi susuz metal klorürler ve sodyum borhidrür toz karışımları kullanılarak incelenmiştir. Reaksiyonlar, argon altında dikey bir tüp fırında silika tüp içine yerleştirilmiş alüminyum pota içinde gerçekleştirilmiştir. Reaksiyon sonunda elde edilen toz karışımına sıcak su ile liç işlemi yapılarak istenmeyen klorür fazları giderilmiştir. Seçilen bazı saf tozlar, kristalin özelliği geliştirmek amacıyla 1100°C’de tavlama işlemine tabi tutulmuştur. Sentezlenen ve tavlanmış tozların karakterizasyonu, X-ışını difraktometresi (XRD), X-ışını floresans spektrometresi (XRF), taramalı elektron mikroskobu (SEM/EDX) ve dinamik ışık saçma tekniği (DLS) kullanılarak analiz edilmiştir. Sonuçlar, inorganik ergimiş tuz tekniğinin (LiCl/KCl ötektik karışımı) CoCl2 , NiCl2 ve NaBH4 toz karışımları arasındaki reaksiyon sırasında oluşan fazlar üzerindeki olumlu etkisini ortaya koymuştur. 750-950°C sıcaklıkları arasında gerçekleşen reaksiyonlarda CoB-Ni2 B-CoBx , CoB-Ni4 B3 ve CoB-NiB-Ni2 Co0.67B0.33 fazlarını içeren ikili ve üçlü metal borür tozları nano boyutta elde edilmiştir. Sentezlenen tozların partikül boyutu ortalama 60 nm civarında hesaplanmıştır.Publication Metadata only Effects of TiB2 and La2O3 particle reinforcements on the tungsten matrix composites fabricated by mechanical alloying and activated sintering(Akademik Perspektif Derneği, 2019) Ağaoğulları, Duygu; Gökçe, Hasan; Ovalı, Didem; Öveçoğlu, M. Lütfi; Department of Chemistry; Balcı, Özge; Researcher; Department of Chemistry; College of Sciences; 295531In this study, 1 wt.% Ni activated tungsten matrix composites (W1Ni) reinforced with different amounts of TiB2 and La2O3 particles were fabricated using mechanical alloying, cold pressing and pressureless sintering methods. In this scope, different amounts of TiB2 (3 and 4 wt.%) and La2O3 (0.5 and 1 wt.%) particles were incorporated into W1Ni matrix and mechanical alloying process was conducted for different durations (6 and 12 h). As-blended and mechanically alloyed powders were cold pressed using a uniaxial hydraulic press and green bodies were sintered under Ar/H2 gas flowing condition. The physical, microstructural and hardness properties of cold pressed and sintered W1Ni composites reinforced with different amounts of TiB2 and La2O3 particles were investigated. The addition of TiB2 and La2O3 reinforcements led an increase in the density and/or microhardness values of the composites. It was observed that mechanical alloying time had a significant effect on the properties and microstructure of the sintered products. The combined usage of 3 or 4 wt.% TiB2 with 1 wt.% La2O3 reinforcement resulted in the formation of W2B phase in addition to the dominant W phase after sintering. The highest relative density and microhardness values were obtained as % 92.14 and 6.27±1.54 GPa, respectively, for the W1Ni-3 wt.% TiB2-1 wt.% La2O3 composite after mechanical alloying for 6 h, cold pressing and sintering. / Bu çalışmada, farklı miktarlarda TiB2 ve La2O3 partikülleri ile takviye edilmiş ve ağırlıkça % 1 Ni ile aktifleştirilmiş volfram matrisli (W1Ni) kompozitler mekanik alaşımlama, soğuk presleme ve basınçsız sinterleme yöntemleri kullanılarak üretilmiştir. Bu kapsamda, farklı miktarlardaki TiB2 (ağ.% 3 ve 4) ve La2O3 (ağ.% 0,5 ve 1) partikülleri W1Ni matris içerisine katkılandırılmış ve farklı sürelerde (6 ve 12 sa) mekanik alaşımlama prosesi uygulanmıştır. Karıştırılmış ve mekanik alaşımlanmış tozlar tek eksenli hidrolik pres kullanılarak soğuk preslenmiş ve pekiştirilen bünyeler Ar/H2 gaz akış şartlarında sinterlenmiştir. Farklı miktarlardaki TiB2 ve La2O3 partikülleri ile takviye edilmiş, preslenmiş ve sinterlenmiş W1Ni matrisli kompozitlerin fiziksel, mikroyapısal ve mikrosertlik özellikleri incelenmiştir. TiB2 veya La2O3 katkılarının, numunelerin yoğunluk ve/veya mikrosertlik değerlerinde artışa neden olduğu saptanmıştır. Mekanik alaşımlama süresinin, sinter ürün özelliklerinde ve mikroyapıda önemli bir etkisinin olduğu gözlemlenmiştir. Ağ.% 3 veya 4 miktarındaki TiB2 ve ağ.% 1 miktarındaki La2O3 katkılarının birlikte kullanımı, sinterlenme sonrasında, baskın W fazına ek olarak W2B faz oluşumuna neden olmuştur. En yüksek rölatif yoğunluk ve mikrosertlik değerleri sırasıyla % 92,14 ve 6,27±1,54 GPa olup, 6 sa mekanik alaşımlama uygulanmış, preslenmiş ve sinterlenmiş olan W1Ni-ağ.% 3 TiB2-ağ.%1 La2O3 kompozisyonu için elde edilmiştir.Publication Metadata only The effect of different carbon sources on the phase formation and microstructure of boron carbide powders(TENMAK Bor Araştırma Enstitüsü, 2019) N/A; N/A; Department of Chemistry; N/A; Department of Chemistry; Kiraz, Kamil; Balcı, Özge; Çoşut, Özge; Somer, Mehmet Suat; Researcher; Researcher; Researcher; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); N/A; College of Sciences; N/A; College of Sciences; N/A; 295531; N/A; 178882The objective of this study is to synthesize boron carbide powders starting with boron oxide (B2 O3 ) and varying carbon sources such as graphite and activated carbon by using carbothermal method. Powder mixtures are mechanically activated with ball mill, pelletized in cold hydraulic press and reacted under Argon atmosphere at 1450°C. Milling speed and time; reaction duration; varying carbon source and the ratio of the starting materials are experimented to observe the results upon the phase structure and the yield of the B4 C. The product characterization is carried out via XRD, SEM, particle size analyzer and organic elemental analysis methods. Mechanical pretreatment significantly reduces the amount of the unreacted carbon and lowers the theoretical reaction temperature down to 1360°C. Under the optimum conditions, the yield contains 4 % remaining carbon and approximately 10 µm boron carbide particles. / Bu çalışmada, bor oksit ve farklı karbon kaynaklarından hareketle karbotermik yöntemle bor karbür tozu üretimi amaçlanmıştır. Hammadde olarak, B2 O3 ve grafit ya da aktif karbon kullanılmış olup, toz karışımları bilyalı değirmende mekanik olarak aktive edilmiştir. Mekanik olarak aktive edilen tozlar, hidrolik preste peletler haline getirilmiş ve Ar atmosferi altında 1450°C’de reaksiyona tabi tutulmuştur. Öğütme süresi/hızı, reaksiyon süresi, başlangıç toz miktarları ve farklı karbon kaynaklarının son ürün verimi ve faz yapısı üzerindeki etkisi incelenmiştir. Elde edilen toz ürünlerin karakterizasyonu, XRD, SEM ve organik element analizi yöntemleri ile gerçekleştirilmiştir. Öğütme ve presleme işlemleri sayesinde, son ürün yapısındaki kalıntı karbon miktarı önemli ölçüde azaltılmış ve teorik oluşum sıcaklığı 1360°C’ye kadar düşürülmüştür. Optimum koşullarda elde edilen tozlar, yapısında en fazla % 4 kalıntı karbon içeren ve ortalama 10 µm partikül boyutuna sahip bor karbür tozlarıdır.Publication Metadata only Graphene oxide: a promising material for the germination of melon seeds under salinity stress(Scientific and Technological Research Council Turkey, 2022) Kaymak, Haluk Caglar; Sevim, Melike; Department of Chemistry; Metin, Önder; Faculty Member; Department of Chemistry; College of Sciences; 46962It is well-known that salinity is an important adverse environmental factor (abiotic stresses) that limits plant growth, development, and productivity all over the world. Herein, we report for the first time the use of graphene oxide (GO) as a promising material against the salinity stress using melon (Cucumis melo L.) seeds as the model crop. GO as a priming material possesses various advantages such as being highly water-soluble, nontoxic, producible on a large scale from graphite, and low-cost over the existing materials. In this study, we identified the effect of different GO doses (0, 0.25, 0.50, 0.75, and 1.0 mg/mL) on seed germination of melon (Cucumis melo L. cvs. 'Hasanbey', 'Kirkagac-589', and 'Kirkagac-637') seeds under the salinity stress. The effect of GO as a priming material showed variance depending on the GO dose, melon cultivars, and level of salinity stress. It was determined that the germination percentage and the length of the radicle were higher in GO primed seeds compared to the control in all the melon cultivars tested. All cultivars did not germinate in the control at the highest salinity stress (26.20 dS m-1). The germination percentage of the GO-treated seeds changed in the range of 12.25% (0.50 mg mL-1 GO) -16.63% (0.25 mg mL-1 GO) at the highest salinity stress in 'Kirkagac-589'. Radicle lengths of 'Hasanbey', 'Kirkagac-589', and 'Kirkagac-637' varied from 0.47, 0.57, 1.67 cm to 7.80, 9.47, and 8.80 cm, respectively, as the salinity stress elevated to 15 g L-1 (0 to 26.20 dS m-1). We believe that this study will pave the way for the application of GO as a priming material for both melon and different crops for eliminating problems of salinity stress.