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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6
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Publication Open Access Effect of reaction solvent on hydroxyapatite synthesis in sol-gel process(The Royal Society, 2017) Department of Chemistry; Nazeer, Muhammad Anwaar; Yılgör, Emel; Yağcı, Mustafa Barış; Ünal, Uğur; Yılgör, İskender; PhD Student; Researcher; Researcher; Faculty Member; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; Graduate School of Sciences and Engineering; N/A; 40527; N/A; N/A; 24181Synthesis of hydroxyapatite (HA) through sol-gel process in different solvent systems is reported. Calcium nitrate tetrahydrate (CNTH) and diammonium hydrogen phosphate (DAHP) were used as calcium and phosphorus precursors, respectively. Three different synthesis reactions were carried out by changing the solvent media, while keeping all other process parameters constant. A measure of 0.5 M aqueous DAHP solution was used in all reactions while CNTH was dissolved in distilled water, tetrahydrofuran (THF) and N, N-dimethylformamide (DMF) at a concentration of 0.5 M. Ammonia solution (28-30%) was used to maintain the pH of the reaction mixtures in the 10-12 range. All reactions were carried out at 40 +/- 2 degrees C for 4 h. Upon completion of the reactions, products were filtered, washed and calcined at 500 degrees C for 2 h. It was clearly demonstrated through various techniques that the dielectric constant and polarity of the solvent mixture strongly influence the chemical structure and morphological properties of calcium phosphate synthesized. Water-based reaction medium, with highest dielectric constant, mainly produced beta-calcium pyrophosphate (beta-CPF) with a minor amount of HA. DMF/water system yielded HA as the major phase with a very minor amount of beta-CPF. THF/water solvent system with the lowest dielectric constant resulted in the formation of pure HA.Publication Open Access Improvement in the transport critical current density and microstructure of isotopic (MgB2)-B-11 monofilament wires by optimizing the sintering temperature(Nature Publishing Group (NPG), 2016) Qiu, Wenbin; Jie, Hyunseock; Patel, Dipak; Lu, Yao; Luzin, Vladimir; Devred, Arnaud; Shahabuddin, Mohammed; Kim, Jung Ho; Ma, Zongqing; Dou, Shi Xue; Al Hossain, Md. Shahriar; Department of Chemistry; Somer, Mehmet Suat; Faculty Member; Department of Chemistry; College of Sciences; 178882Superconducting wires are widely used in fabricating magnetic coils in fusion reactors. In consideration of the stability of B-11 against neutron irradiation and lower induced radio-activation properties, MgB2 superconductor with B-11 serving as boron source is an alternative candidate to be used in fusion reactor with severe irradiation environment. In present work, a batch of monofilament isotopic (MgB2)-B-11 wires with amorphous B-11 powder as precursor were fabricated using powder-in-tube (PIT) process at different sintering temperature, and the evolution of their microstructure and corresponding superconducting properties was systemically investigated. Accordingly, the best transport critical current density (J(c)) = 2 x 10(4) A/cm(2) was obtained at 4.2 K and 5 T, which is even comparable to multi-filament (MgB2)-B-11 isotope wires reported in other work. Surprisingly, transport Jc vanished in our wire which was heat-treated at excessively high temperature (800 degrees C). Combined with microstructure observation, it was found that lots of big interconnected microcracks and voids that can isolate the MgB2 grains formed in this whole sample, resulting in significant deterioration in inter-grain connectivity. The results can be a constructive guide in fabricating (MgB2)-B-11 wires to be used as magnet coils in fusion reactor systems such as ITER-type tokamak magnet.Publication Open Access High thermoelectric performance enabled by convergence of nested conduction bands in Pb7Bi4Se13 with low thermal conductivity(Nature Publishing Group (NPG), 2021) Hu, Lei; Fang, Yue Wen; Qin, Feiyu; Cao, Xun; Zhao, Xiaoxu; Luo, Yubo; Repaka, Durga Venkata Maheswar; Luo, Wenbo; Suwardi, Ady; Soldi, Thomas; Huang, Yizhong; Liu, Zheng; Hippalgaonkar, Kedar; Snyder, G. Jeffrey; Xu, Jianwei; Yan, Qingyu; Department of Chemistry; Aydemir, Umut; 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); College of Sciences; 58403Thermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450-800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moire fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.Publication Open Access Nanoparticle and gelation stabilized functional composites of an ionic salt in a hydrophobic polymer matrix(Public Library of Science, 2014) Department of Chemistry; Demirel, Adem Levent; Kanyas, Selin; Aydın, Derya; Kızılel, Rıza; Kızılel, Seda; Faculty Member; Researcher; Researcher; Faculty Member; Department of Chemistry; The Center for Computational Biology and Bioinformatics (CCBB); College of Engineering; College of Sciences; 6568; N/A; N/A; N/A; 28376Polymer composites consisted of small hydrophilic pockets homogeneously dispersed in a hydrophobic polymer matrix are important in many applications where controlled release of the functional agent from the hydrophilic phase is needed. As an example, a release of biomolecules or drugs from therapeutic formulations or release of salt in anti-icing application can be mentioned. Here, we report a method for preparation of such a composite material consisted of small KCOOH salt pockets distributed in the styrene-butadiene-styrene (SBS) polymer matrix and demonstrate its effectiveness in anti-icing coatings. The mixtures of the aqueous KCOOH and SBS-cyclohexane solutions were firstly stabilized by adding silica nanoparticles to the emulsions and, even more, by gelation of the aqueous phase by agarose. The emulsions were observed in optical microscope to check its stability in time and characterized by rheological measurements. The dry composite materials were obtained via casting the emulsions onto the glass substrates and evaporations of the organic solvent. Composite polymer films were characterized by water contact angle (WCA) measurements. The release of KCOOH salt into water and the freezing delay experiments of water droplets on dry composite films demonstrated their anti-icing properties. It has been concluded that hydrophobic and thermoplastic SBS polymer allows incorporation of the hydrophilic pockets/phases through our technique that opens the possibility for controlled delivering of anti-icing agents from the composite.