Person: TAV, CUMALİ
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TAV
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CUMALİ
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Publication Metadata only Viscous Behavior of PS, PP, and ABS in Terms of Temperature and Pressure-Dependent Hole Fraction(JOHN WILEY & SONS INC, 2010) TAV, CUMALİ; Akdeniz, Guelsuem; Yahsi, Ugur; Tav, CumaliWe have developed a zero-shear viscous model in terms of temperature- and pressure-dependent hole fraction computed from Simha-Somcynsky Hole Theory. This model successfully interprets the viscosity data of PS, PP, and ABS as a function of hole fraction for a broad range of temperature and pressure. We have also introduced and discussed a new term: Viscoholibility; the derivative of logarithm of viscosity with respect to hole fraction. When the hole fraction takes highest available value, the viscoholibility approaches asymptotically to a constant value by which the viscosity changes linearly with the hole fraction. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 110-113,2010Publication Metadata only Relaxation time of polypropylene glycol and polypropylene glycol dimethylether-like polymers in terms of fluid-phase temperature and pressure dependent hole fraction(PERGAMON-ELSEVIER SCIENCE LTD, 2015) YUMAK YAHŞİ, AYŞE; Yahsi, Ugur; Coskun, Bilgehan; Yumak, Ayse; Boubaker, Karem; Tav, CumaliWe present a model explaining relaxation time of PPG (polypropylene glycol) and PPGM (polypropylene glycol dimethylether) as representative compounds, with different molecular weights, PPG2 (M-w = 2 kg mol(-1)), PPG3 (M-w = 3 kg mol(-1)), PPGM1 (M-w = 1 kg mol(-1)), and PPGM2 (M-w = 2 kg mol(-1)), as a function of hole fraction at various fluid phase temperatures and high pressures. Proposed model based on the structural relaxation behavior of chain molecules given by thermo-occupancy function in terms of temperature and tempetature-pressure dependent hole fraction is discussed and compared with the free volume model. The former is the most universal behavior than the latter in terms of temperature and pressure. We discuss activation energy parameter and transmission coefficient for the species in terms of chain length and end groups such as dimethylether. Scaling relaxation time parameter in the proposed model has similar trend with relaxation time parameter of Adam-Gibbs theory. (C) 2015 Elsevier Ltd. All rights reserved.Publication Metadata only The Effect of Hole Fraction on Viscosity in Atactic and Syndiotactic Polystyrenes(SPRINGER/PLENUM PUBLISHERS, 2015) TAV, CUMALİ; Sahin-Dinc, Fatma; Sorrentino, Andrea; Tav, Cumali; Yahsi, UgurThe non-Newtonian viscosity behavior of atactic and syndiotactic polystyrenes (aPS and sPS), at temperatures from to and pressures up to 30 MPa, was studied using the recently proposed Yahsi-Dinc-Tav model, in particular, the Cross-like model. Viscosity data with shear rates of to were taken from literature at ambient pressure and were reproduced from the Cross-Vogel model at elevated pressures. The viscosities were predicted using the Yahsi-Dinc-Tav model with average absolute deviations of 4.37 % at ambient pressure and 9.63 % at high pressures for aPS and 2.54 % at ambient pressure and 4.79 % at high pressures for sPS. We extended this work to relate the zero shear and the constant shear rate viscosities in terms of the temperature- and pressure-dependent hole fraction using the thermo-occupancy function. The hole fraction (h) was computed from Simha-Somcynsky hole theory. Analytical relationships of specific density and hole fraction in terms of scaled temperature and pressure were constructed for practical purposes. The effects of tacticity of polystyrenes were discussed in terms of the thermo-occupancy function in detail. In particular, the derivative of the logarithm of the viscosities (viscoholibility) was found to decrease with an increasing hole fraction.Publication Metadata only Effects of boron nitrite in thermoplastic polyurethane on thermal, electrical and free volume properties(SPRINGER, 2019) TAV, CUMALİ; Dumludag, Fatih; Yener, Murat Y.; Basturk, Emre; Madakbas, Seyfullah; Kahraman, Vezir; Umer, Muhammad A.; Yahsi, Ugur; Tav, CumaliThe effects of various filler amounts of boron nitride (BN) on surface morphology, DC and AC conductivity and free volume properties of thermoplastic polyurethane (TPU) were investigated using FTIR, TGA, DSC, SEM, PALS and conductivity (DC and AC) measurements. The free volume properties, electrical measurements and surface morphology of TPU/BN were found to be dependent on temperature and weight percentages of BN content. When one weight percentage of BN was added in TPU, the glass transition temperature jumped to 69.42 degrees C from -1 degrees C. Increasing BN weight percentages caused the glass transition temperature of the TPU/BN composites to decrease slightly. Morphological measurements indicated that agglomerations occurring throughout the TPU/BN composites did not cause remarkable effects on their thermal stabilities. Orthopositronium lifetime (or free volume) and intensity increased significantly as a function of temperature, but with respect to BN weight percentages in TPU, they showed a peak with a highest value at about 2% BN content. Similarly, DC and AC conductivity measurements were carried out on TPU with and without BN content at temperatures ranging from 23 to 105 degrees C in dark under vacuum and in the frequency range of 40Hz-100kHz. The DC and AC conductivity values with increasing BN weight percentages showed the similar behavior as free volume fraction that had a peak at about 2% BN content.Publication Metadata only Linking the viscous and vacancy behavior of mixtures of high-molecular-weight hydrocarbons(SPRINGER/PLENUM PUBLISHERS, 2006) TAV, CUMALİ; Sahin, F.; Tav, C.; Yahsi, U.A zero-shear viscosity model has now been extended to mixtures of three-branch alkanes with and without cyclopentyl or cyclohexyl attachments. The model links the equilibrium theory of Simha-Somcynsky (SS) statistical thermodynamics of liquids and the Eyring significant structure (ESS) transport theory. The predicted quantity of the hole fraction as a measure of the free volume, computed from the SS equation of state, plays a primary role in the approach to combine both theories. Experimental PVT eta data from 310 to 408 K and 0.1 to 344.5 MPa have been used. The proportionality constant of the activation energy and the transmission coefficient have been calculated for each species separately, and they are in the expected trend with the fraction of mixtures. Satisfactory agreement in the prediction of viscosity from the hole fraction has been obtained. The volumetric and viscous data of physical mixtures are predicted from and compared with their chemical correspondence.Publication Metadata only Structural and spectroscopic properties of Nd3+:Y2Si2O7 phosphors(ELSEVIER SCI LTD, 2013) TAV, CUMALİ; Erdem, Murat; Ozen, Gonul; Tav, Cumali; Di Bartolo, BaldassarePhosphors of alpha-Y2Si2O7 doped with Nd3+ ions were prepared using the sol gel technique. Nano-sized crystalline phosphor powders were obtained by annealing the dried gels at 960 degrees C. The crystallization properties of the phosphor powders were determined from their XRD patterns. The alpha-Y2Si2O7 phase was the only phase observed in all compositions. As the amount of amorphous SiO2 in the composition was increased, the crystalline sizes and the widths of the size distribution curves were found to decrease from 17.8 nm to 10.6 nm and from 15.6 nm to 12.2 nm, respectively. The spectroscopic properties of the powders were studied by measuring the luminescence and the decay patterns of the F-4(3/2) -> I-4(9/2) and F-4(3/2) -> I-4(11/2) transitions between 50 K and 310 K. No appreciable effect of the crystallite sizes on the average lifetime of the F-4(3/2) level was observed at temperatures below 100 K. The effect of temperature, however, becomes relevant above 100 K as the size of alpha-Y2Si2O7 nano-crystal becomes smaller. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Publication Metadata only Investigation of Nd3+:Y2Si2O7Phosphors Using Photoluminescence and Positron Annihilation Lifetime Spectroscopy(2023-08-01) TAV, CUMALİ; YAHŞİ, UĞUR; Akay L. N., Kuzeci S., Akti N., Erdem M., TAV C., YAHŞİ U., Eryürek G., Di Bartolo B.Neodymium-doped Y2Si2O7 nanopowders were synthesized by sol-gel and annealed under varying annealing temperature conditions. The phase transformation of Y2Si2O7 and the crystalline size change were observed with annealing temperature. Photoluminescence and the decay patterns of the 4F3/2→ 4I9/2,4I11/2, and 4I13/2 transitions were examined to reveal the spectroscopic characteristics of the powders. The spectral outputs of Nd3+:Y2Si2O7 phosphors remain strongly connected to the phase properties of Y2Si2O7. The positron lifetimes (τ 2, τ 3, and τ 4) and intensities (I 2, I 3, and I 4) obtained by positron annihilation lifetime spectroscopy were discussed for neodymium-doped Y2Si2O7 phosphors to relate the structural and boundary changes in terms of the phases and molar contents of the Nd3+ ions. The τ 2 and I 2 components were related to the structural open space in the grain, the τ 3 and I 3 components were correlated with vacancy-like defects in the grain boundaries, and the τ 4 and I 4 components were associated with vacancy defects in the grain, together with the effect of the increasing content of the Nd3+ ions.Publication Metadata only Ionic conductivity of PVdF-co-HFP/LiClO4 in terms of free volume defects probed by positron annihilation lifetime spectroscopy(TAYLOR & FRANCIS LTD, 2019) TAV, CUMALİ; Yahsi, Ugur; Deligoz, Huseyin; Tav, Cumali; Ulutas, Kemal; Deger, Deniz; Yilmazturk, Serpil; Erdemci, Gonca; Coskun, Bilgehan; Yilmazoglu, Mesut; Yakut, SahinPolymers based on ionic conducting materials have important interest because of their potential applications in polymer electrolytes and membranes for fuel cell application. PVdF-co-HFP poly(viniliden-co-hexafluoropropylene) was chosen as a polymer matrix because of its high ionic conductivity and better mechanical properties. Polymer matrix composites were prepared with various amounts of LiClO4 salt by a solution casting method. The sample-ionic conductivity measurements were recorded by AC impedance analyzer at different frequencies from 0.1 Hz to 20 MHz and at different temperatures from 273 to 373 K. The changes of nanoscopic free volume and free volume fraction in these materials were investigated in terms of temperature from 273 to 373 K using Positron Annihilation Lifetime Spectroscopy (PALS) and Simha-Somcynsky (SS) Hole Theory. The free volume had a bump at about 3% in weight percentage of the salt and there is a slight increase after 10%. The effects of weight percentages of LiClO4 and temperature were investigated. The mechanism of the ac ionic conductivity was presented in terms of the free volume models, however thermo-occupancy function justifies the best accurate representation of the data.Publication Metadata only FİZİK, Fen ve Mühendislik için Teknoloji Güncellemeli, Serway, Jewett,, CİLT 2, Elektrik ve Manyetizma, Optik(Palme Yayın Dağıtım, 2022-07-01) KESKİN, SIDDIK SİNAN; DUMLUDAĞ, FATİH; TAV, CUMALİ; YUMAK YAHŞİ, AYŞE; YAHŞİ, UĞUR; Yahşi U., Saçlı Ö. A., Keskin S. S., Tav C., Dumludağ F., Altındal A., Yumak Yahşi A., Avgın İ., Atav Ü., Ulutaş D., et al.Publication Metadata only Crystallization behaviour of neodymium doped yttrium silicate nanophosphors(ELSEVIER SCI LTD, 2011) TAV, CUMALİ; Erdem, Murat; Ozen, Gonul; Tav, CumaliSol-gel technique was used to prepare yttrium silicate powders doped with 0.01Nd(3+). The crystallite sizes were determined to be 23 +/- 0.5 nm from the XRD patterns of the powders annealed at 960 degrees C. The Y(4.67)(SiO(4))(3)O, Y(2)Si(2)O(7) and Y(2)SiO(5) crystalline phases were observed upon heat treatment at 960 degrees C which is much lower than 1500-1650 degrees C are reported before. (C) 2011 Elsevier Ltd. All rights reserved.
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