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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 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 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 Competitive hydrogen bonding in aspirin-aspirin and aspirin-leucine interactions(Scientific Technical Research Council Turkey-Tubitak, 2012) Department of Chemistry; Department of Chemical and Biological Engineering; Department of Chemistry; Yurtsever, Zeynep; Erman, Burak; Yurtsever, İsmail Ersin; Undergraduate Student; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Chemistry; College of Sciences; College of Engineering; College of Sciences; N/A; 179997; 7129Aspirin-aspirin and aspirin-leucine interactions are studied by the density functional theory (DFT) and high level ab initio calculations with second order Moller-Plesset perturbation theory (MP2). The rotational isomers of aspirin are identified by their relative stability both in gaseous phase and in water using the polarizable continuum method (PCM). Local minima of aspirin monomers in water are found to be all highly populated compared to the gas phase behavior. Homodimers of aspirin form hydrogen bonds with bond energies of 10 kcal/mol. Weak hydrogen bonds utilizing phenyl and methyl groups are also found. The interaction between aspirin and leucine is stronger with relatively short bond lengths compared to homodimeric aspirin interactions. The potential energy surface has several minima with comparable stability. This study shows the significance of diverse bonding schemes, which are important for understanding complete interaction mechanisms of aspirin.Publication Open Access Dimerization of pyrrole(TÜBİTAK, 1998) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Accurate ab-inito quantum mechanical calculations of pyrrole dimers are reported. The thermodynamical stabilities of dimers with alpha - alpha, alpha -beta, and beta - beta type linkages are compared in order to predict the possibilities of branching in polypyrroles. Calculations employing large basis sets and including electron correlation effects predict the alpha - alpha dimers as the most stable form. However, an alpha - beta type bonding requires only 1.5-2.0 kcal/mol, and the energy necessary to introduce a beta - beta type bond is 3.6-4.0 kcal/mol. These values show that a high degree of branching is possible even at room temperatures.Publication Metadata only Dissociation dynamics of small carbon clusters(Scientific Technical Research Council Turkey, 1997) Elmacı, Nuran; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The dissociation dynamics of small carbon clusters is studied by classical trajectory analysis. A large number of initial conditions are chosen to analyze the effects of the energy, angular momentum and the initial geometry of the cluster. The dissociation times, decay rate constants, kinetic energy of the dissociating atoms and the geometrical structure of the remaining part of the clusters are computed from microcanonical sampling of the phase space. Distributions of these properties as well as the possibilities of various fragmentation channels are presented.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 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 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.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.