Prof. Dr. Uğur YAHŞİ
Prof. Dr. Cumali TAV
Assoc. Prof. Dr. Ayşe YUMAK YAHŞİ
Recent Students at Laboratory
- Murat Yavuz YENER (PhD)
- Muhammed Adil UMER (MSc)
- Hümeyra KILIÇ (MSc)
- Murat Yavuz YENER (MSc) "Characteristic Properties Of Thermoplastic Polyurethane / Boron Nitride Composite Using Techniques Of Thermal, Electrical And Positron Annihilation Lifetime Spectroscopy" Physics MSc Thesis, Marmara University, Institute of Pure and Applied Sciences, İstanbul 2017.
- Saygın KÜZECİ (PhD) “On The Extraction of Free Volume Depth Profile Of Polymer Surface Using Simha-Somcynsky and Cahn Hilliard Theory,” Physics PhD Thesis, Marmara University, Institute of Pure and Applied Sciences, İstanbul 2011.
- Bilgehan ÇOŞKUN (MSc.) "An Investigation of Relationship Between Free Volume and Ionic Conductivity of Polymer-Salt Electrolytes By Using Positron Annihilatıon Lifetime Spectroscopy”, Physics MSc. Thesis, Marmara University, Institute of Pure and Applied Sciences, İstanbul 2009 (Turkish).
- Gonca ERDEMCİ (MSc.) "The Correlation Between Conductivity and Free Volume in Polymer Electrolytes Using Positron Techniques”, Physics MSc. Thesis, Marmara University, Institute of Pure and Applied Sciences, İstanbul 2009 (Turkish).
Prof. Dr. Abderrahim GUITTOUM (Senior Researcher, Maître de Recherche A /HDR, Mössbauer & Positron Laboratories
Nuclear Research Centre of Algiers, Nuclear Techniques Division, Algiers, Algeria. https://www.researchgate.net/profile/Abderrahim_Guittoum)
Dr. Rod GREAVES (First Point Scientific, Inc., ABD)
Prof. Dr. Y.C. Jerry JEAN (Curators' Professor of Chemistry and Physics, The University of Missouri-Kansas City)
Assoc. Prof. Dr. Renwu John ZHANG (Department of Chemistry and Biochemistry, California State University, San Bernardino)
Prof. Dr. Hüseyin DELİGÖZ (İstanbul University Department of Chemical Engineering-Chemical Technologies) Syntesis of Polymer ve Polymer-Salt Complexes
Assist. Prof. Dr. Kemal ULUTAŞ and Assoc. Prof. Dr. Deniz Değer (İstanbul University Physics Department) Electrical Characterization
Assoc. Prof. Dr. Fatih DUMLUDAĞ (Marmara University Physics Department) Electrical Characterization
Assoc. Prof. Dr. Dursun ÜSTÜNDAĞ (Faculty of Arts and Sciences Matematics Department) Applied Mathematics and Statistical Analysis; Bayes; Monte-Carlo Simulations
We have two main systems on Positron/Positronium. One is the fast-fast coincidence system to study defects such as free volume in bulk materials and the other is Slow Positron Beam System (SPBS) to study vacancy and chemical defects on surfaces and interfaces. We have Positronium Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadened Annihilation Radiation (DBAR) measurement systems in both systems. But we develope a PALS system on the SPBS for polymer works.
Fast-Fast Coincidence Systems
Positron Annihilation Lifetime Spectroscopy (PALS, PLTAS)
Doppler Broadened Annihilation Radiation (DBAR)
Slow Positron Beam System
Doppler Broadened Emission Spectroscopy (DBES)
Positron Annihilation Lifetime Spectroscopy (PALS) (under development)
- Investigation of Relationship Between Free Volume and Ionic Conductivity of Polymer Electrolytes
- Temperature Dependence of Free Volume in Polyurethane Using Positron Annihilation Lifetime Spectroscopy
- Temperature Dependence of Polyurethane Composites Using Positron Annihilation Techniques
- Bayesian Statistical Inference of Positron Lifetime Spectroscopy (Simulation)
- Positron Spectroscopy in Irradiated Materials: Effects of Irradiations
Free Volume Investigation of PAMAM Dendrimers Using Positron Annihilation Lifetime Spectroscopy
Uğur Yahşi, Ayşe Yumak Yahşi, Cumali Tav, Hümeyra Kılıç
For the different generation of PAMAM dendrimer macromolecule, the temperature-dependent change of structural free volumes will be investigated by Positron Annihilation Lifetime Spectroscopy (PALS) technique. In the PALS technique, the observed spectrum will be divided into three components, the longest lasting of these components is the ortho-positronium (o-Ps) formed by the free pick-off of the molecular structure cavities. By examining the relationship of ortho-positronium (o-Ps) with temperature, especially the change in glass transition temperature, it will be examined the difference in o-Ps annihilation parameters, heating and cooling processes. As a result, in the literature, it will be tried to establish a correlation between possible studies related to PAMAM and free volume.
Marmara University BAPKO FEN-A-110718-0395, 11.07.2018 - 11.07.2020 (64.109 ₺).
Positron Spectroscopy in Irradiated Materials: Effects of Irradiations
Goal: We report on positron studies (lifetime and Doppler broadening spectroscopy) in irradiated materials (with neutrons, alpha particles and gamma), especially, polymers. The positronium behavior of these materials is studied as a function of the irradiation fluence and etching time for CR39 (a kind of polymer)
Methods: Fast-Fast positron spectrometer, Doppler broadening spectrometer, Variable energy Doppler broadening spectroscopy, Variable energy positron lifetime spectroscopy
Polymer samples CR39 have been irradiated with alpha particles for different energies and fluences. Variable energy Doppler Broadening Spectroscopy VEDBS have been performed using positron beam accelerator. The evolution of S parameter (free volume) will be studied as a function of positron depth penetration. The effect of etching time on the evolution of S parameter will be also studied and related to the latent tracks in CR39 nuclear solid state detector.
Characteristic Properties of Thermoplastic Polyurethane / Boron Nitride Composite Using Techniques Of Thermal, Electrical And Positron Annihilation Lifetime Spectroscopy
Thermoplastic Polyurethane (TPU), dissolved in dimethylformamide at 80 ºC, were doped with different amounts of Boron Nitride (BN). TPU/BN composites, prepared by using a solvent casting technique, were obtained in the form of films. Optic characterization of TPU/BN composites was done by FT-IR in the 400-4000 cm-1 range; thermal by TGA under N2 atmosphere and 30 to 750°C range with 10°C.min-1 heating rate, DTGA and DSC technique - heating from 0 to 200°C, all at a heating rate of 5°C min-1, and cooled with the same rate. Finally, structural characterization done by SEM, and defects on the molecular level were studied using PALS.
Sample synthesis was observed to be successful by FT-IR. Thermal stability of the TPU/BN composite was observed to increase with increase in the amount of BN doped. SEM results showed that BN particles were dispersed homogeneously throughout the polymer matrix for lower amounts doped, but doping levels higher than 7% results in agglomerations, causing a decrease in the thermal stability of the material. PALS studies revealed that o-Ps lifetime (free volume) increases in all composites with increasing temperature. Free volume intensity (hole fraction) increased till 1% doping weight for all the samples, after which it decreased relatively gently.
Murat Yavuz YENER (MSc.) "Characteristic properties of thermoplastic polyurethane/boron nitride composite using thermal and positron annihilation lifetime spectroscopy techniques", Physics MSc. Thesis, Marmara University Institute of Pure and Applied Sciences, İstanbul 2017 (English).
“Effects of Boron Nitrite in Thermoplastic Polyurethane on Thermal, Electrical and Free Volume Properties”, F. Dumludag, M. Y. Yener, E. Basturk, S. Madakbas, V. Kahraman, M. A. Umer, U. Yahsi and C. Tav, Polymer Bulletin https://doi.org/10.1007/s00289-018-2560-2 Link to download
Investigation of Relationship Between Free Volume and Ionic Conductivity of Polymer Electrolytes
Gonca ERDEMCİ, Bilgehan ÇOŞKUN, Cumali TAV, Uğur YAHŞİ (Marmara Üniv. Fen Edebiyat Fak. Fizik Bölümü)
Hüseyin DELİGÖZ (İstanbul Üniversitesi Kimya Mühendisiliği Kimya Teknolojileri ABD)
Kemal ULUTAŞ, Deniz ULUTAŞ (İstanbul Üniversitesi Fizik Bölümü)
Polymer based ionic conducting materials have important interest because of their potential applications in polymer electrolytes and membranes used as fuel cell. Ionic conductivity mechanism is explained with the free volume model. Free volume notion in polymers is significant in order to explain viscosity, ionic conductivity, glass transition temperature and physical quantities such as mechanical and thermodynamic properties. In the study of Cohen and Turnbull and later Bamford et. al. the transport phenomena in polymers have stated in terms of free volume arising from the structural disorders in amorphous polymers. To interpret ionic conductivity mechanism, Yahsi et. al. have purposed a model in terms of temperature and pressure depended hole fraction as a measure of free volume. On the other hand, the size and the amount of free volume can also be measured by ortho-positronium (o-Ps) lifetime using the Positron Annihilation Lifetime Spectroscopy (PALS), a unique tool to probe the holes.
For this purpose, PVdF-co-HFP (Poliviniliden Florür-co-Hekzaflorur) was used as polymer matrix in this work because of high ionic conducting features and superior mechanic properties. Polymer electrolytes containing different types lithium salt (LiClO4, LiPF6, Li triflat) and various amounts of salt (1, 3, 5, 10, 15, 20% (weight) were prepared by solution casting method. The changes of the nanoscopic free volume and hole fraction were investigated with respect to temperatures using the PALS technique. Eventually, the hole fraction has an inflection point about 0.03 molar ratio. The free volume increases when it is less than 0.03. It is the fact that the formation of polymer and salt complications which occur between the fluorine groups in the polymer backbone and Li+ ion in salt retards the ordering of polymer crystalline and causes the formation of amorphous phases.
Key words: Positron Annihilation Lifetime Spectroscopy, Doppler Broadenned Annihilation Radiation, Free Volume, PVdF-co-HFP.
Bilgehan ÇOŞKUN (MSc.) "An Investigation of Relationship Between Free Volume and Ionic Conductivity of Polymer-Salt Electrolytes By Using Positron Annihilatıon Lifetime Spectroscopy”, Physics MSc. Thesis, Marmara University Institute of Pure and Applied Sciences, İstanbul 2009 (Turkish).
Gonca ERDEMCİ (MSc.) "The Correlation Between Conductivity and Free Volume in Polymer Electrolytes Using Positron Techniques”, Physics MSc. Thesis, Marmara University Institute of Pure and Applied Sciences, İstanbul 2009 (Turkish).
- "Polymer-Ionic Salt Electrolytes: (I) Investigation of Conductivity-Free Volume Relations by Positron Annihilation Techniques (PALS) and (II) Usage in Fuel Cells of PEM Type", U. Yahşi, C. Tav, H. Deligöz and K. Ulutaş, TÜBİTAK (107T133)- 1 July 2007-1 July 2010.
- "Investigation of Nanometric Defects in Materials by using Doppler Broadened Positron Annihilation Radiation with Double Dedectors," C. Tav and U. Yahşi, Marmara University Scientific Research Projects Units (BAPKO), June 2006-June 2008.
- " Investigation and Characterization of Nanometric Defects of Thin Films by using Slow Positron Beams," C.Tav and U. Yahşi, the State Planning Organisation, Research & Development Projects, January 2006-December 2009.
- "Investigation of Nanometric Defects in Materials by using Doppler Broadened Positron Annihilation Radiation (DBAR) ", C.Tav and U. Yahşi, TÜBİTAK (106T041), 3 May 2006-2 May 2009.
- “Dielectric properties and conductivity of PVdF-co-HFP/LiClO4 polymer electrolytes”, K. Ulutaş, U. Yahsi, H. Deligöz, C. Tav, S. Yılmaztürk, M. Yılmazoğlu, G. Erdemci, B. Coşkun, Ş. Yakut, and D. Değer, Can. J. Phys. 96, 786–791 (2018). (https://doi.org/10.1139/cjp-2017-0678) Link
- “Ionic Conductivity of PVdF-co-HFP with LiClO4: Free Volume Effects Probing by Positron Annihilation Lifetime Spectroscopy”, U. Yahsi, H. Deligöz, C. Tav, K. Ulutaş, D. Değer, S.Yılmaztürk, G. Erdemci, B. Coşkun, M.Yılmazoğlu, Ş. Yakut, Radiation Effects and Defects in Solids (soon to be published).
Temperature Dependence of Free Volume in Polyurethane Using Positron Annihilation Lifetime Spectroscopy
Uğur YAHŞİ and Cumali TAV, (Marmara Üniv. Fen Edebiyat Fak. Fizik Bölümü)
The dependence of free volume on the nanometric structure of polyurethane sample has been investigated for the broad range of temperature 18-373 K using positron annihilation lifetime spectrometer (PALS). Free volume and hole fraction deviations have been studied in five separate regions. Particularly the transitions of the glass state of polyurethane’s soft polyester-polyol and hard haxametylenediisocynate structures have been observed at Tg = 214.1 K and T'g = 339.4 K temperatures, respectively. The phase transition of hard part is in a good agreement with DSC measurement given in the literature. Yet the existence of the trapped oxygen and water molecules on synthesizing the sample has been observed and the effect of their phase transitions on the polyurethane structure has also been studied.
A plateau has occurred under Tg at the glassy-state transition. It’s also been observed that the existence of oxygen in the plateau regime has caused the structural collapse due to the melting-state transition of oxygen and remained the plateau regime invariable while constructing new free volume. For T > Tg, soft part of polyurethane has come off the glassy-state, therefore, the size and number of the free volume have increased. It’s been expected that the glassy portion of hard part of polyurethane has reached a plateau regime before . However, a structural collapse of hard part of polyurethane has occurred and restructuring free volume has arisen. The transition glassy-state to plastic-state has happened for T > T'g . Thermal energy at this state has caused the excitation and flexibility in structure of hydrogen bonds of the hard part and as a result there is a slow growth in free volume size and no difference in the hole numbers.
Key Words:Positronium Annihilation Lifetime Spectroscopy (PALS), Free Volume in Polymers, Phase Transformation, Polyurethane
- "Investigation of Nanometric Defects in Macromolecular Structures by using Positron Annihilation Spectroscopy (PAS)," C.Tav and U. Yahşi, TÜBİTAK, 1 september 2000-August 2004.
- "Nanometric Defects in Macromolecular Structures by using Positron Annihilation Lifetime Spectroscopy," C.Tav and U. Yahşi, Marmara University Scientific Research Projects Units (BAPKO), January 2003-July 2004.
- "Investigation of Viscose and Viscoelastic Properties in Macromolecular Structures by using Positron Annihilation Spectroscopy," Uğur Yahşi, the State Planning Organization, Research & Development Projects, January 1998-December 2001.
Temperature Dependence of Conducting Polyurethane Composites Using Positron Annihilation Techniques
Cumali TAV, (Marmara Üniv. Fen Edebiyat Fak. Fizik Bölümü)
Serpil YILMAZTURK (Istanbul University, Engineering Faculty, Chemical Engineering)
Polyvinylidene fluoride-co-hexafluoropropylene (PVdF-co-HFP) with conducting polyaniline (PAni) was studied to measure free-volume intensity (), ortho-positronium (o-Ps) lifetime (), and Doppler momentum distribution parameters (S and W) as a function of temperature varying from 273 to 373 K and PAni weigh percentage varying from 1%-20% using the techniques of Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadening Annihilation Radiation (DBAR). It has been observed that the free-volume intensity and the o-Ps lifetime increased sharply with PAni weight percentage up to 4% after which decreased slowly. DBAR momentum distribution parameters behaved similarly as well and it’s assumed that in the studied samples one-type defect was to exist. The thermal expansion coefficient also as a function of PAni weight was extracted from PALS data.
Keywords: Positron Annihilation Lifetime Spectroscopy, Free Volume, Conducting Polymers, PAni, PVdF-co-HFP, Thermal Expansion Coefficient.
“Temperature and Weight Dependence of Free Volumes in Conducting Composite Polymer Using Positron Annihilation Techniques”, C. Tav and S. Yılmazturk, High Temperature Materials and Processes, 2011, 0334-6455, 30, 1-2, 193-198. (https://doi.org/10.1515/htmp.2011.028) Link
Bayesian Statistical Inference of Positron Lifetime Spectroscopy
The most common way of analyzing positron annihilation lifetime spectroscopy (PALS) spectra involves fitting parameter dependent model to the experimental data but, requires local nonlinear optimization routines that depend on a reasonable guess for the search parameters. This, together with the fact that different sets of parameters may yield good fits for a given experimental spectrum, gives rise to ambiguities in data analysis in most but the simplest cases. In order to alleviate these difficulties, we develop a new procedure that incorporates a global nonlinear optimization routine based on Simulated Annealing algorithm (SA) with Reversible jump Markov Chain Monte-Carlo Bayesian Inference method (RJMCMC-BI) so that it provides a robust fitting tool and yields information on the reliability of the results. In this work, the methods used in the procedure are described. It is applied to the analysis of several lifetime spectra and compared with the results obtained from the well-established commercial programs.
Keywords: Bayes Logical Statistical Inference, Bayesian parameter estimation, Model selection, ill-posed inverse problems, Optimizations, MCMC techniques.
On the Positron/Positronium Measurement Systems:
Positron Annihilation Lifetime Spectroscopy (PALS)
A positron source on an aluminium foil (thickness of 5 micron)about 25x10-6 Ci obtained by the evaporation of aqueous 22NaCl solution was prepared and then sandwiched between two samples. The 22Na source emits positrons (99.93%) accompanied by a 1274 keV gamma-ray which serves as a starting signal for the timing. The emitted positron enters the material and instantly slows down to the thermal energies. The thermalized positron prefers to occupy any available vacancy in the material because of negative affinity of the surrounding molecules. In the vacancy the positron captures an electron to form either parapositronium (p-Ps) or orthopositronium (o-Ps) with a lifetime 125 ps and 132 ns in the vacuum, respectively. However, in the material positron annihilates with an electron from the surrounding molecules, called pick-off annihilation. Therefore, the lifetime of the o-Ps is dramatically reduced to a few ns in the polymers, but the lifetime of the p-Ps is not changed remarkably so it may be kept constant. The either annihilation is revealed by two gamma-ray, each of energy 511 keV, serving as a stopping signal for the timing. Conventionally, the positron lifetime is measured as a time difference between two gamma-ray. For the PALS, fast scintillation detectors were used: BaF2 scintillators were combined with Hamamatsu R2059 photomultiplier tubes (PMTs) based by 265 Ortec operated at negative 2200 Volts. As a time signal generator, two constant fraction differential discriminators (Ortec CFDD 583 B) were also used to filter one from the other signal within the selected energy range. A time to Amplitude Converter, TAC, was used to convert pulses of different heights to a time-to-pulse-height signal. The converted signals were fed to a multichannel analyzer (Ortec Model 919E Ethernim MCA). The spectroscopic data yielded from MCA was analyzed using the code of RESOLUTION and PATFIT to obtain the lifetime parameters revealing the information about the free volume.
Doppler Broadening Annihilation Radiation (DBAR)
We employed also the DBAR to measure Doppler shift in the annihilation radiation due to the longitudinal component of the momentum of an electron-positron pair. The DBAR generally consists of a high purity germanium (HPGe) detector and associated electronics to acquire the spectrum in the energy range around 511 keV. Under the high voltage of positive 4000 V applied, the signal of the high purity coaxial Ge detector (Canberra GC2519 HP Ge) with an energy resolution of 1.35 keV (FWHM) for 662 keV 137Cs was converted by a preamplifier (Model 20022CSI) into an electrical pulse. A spectroscopy amplifier (Model 7611 made by Silena) connected to the preamplifier provided the necessary pulses to a multichannel analyzer (Ortec Model 919E Ethernim MCA). Their amplitudes were measures of the photon energies. The spectrum was analyzed using SP-ver.1 program yielding S parameter (sharp) and W parameter (wing) to obtain momentum distribution of the annihilating electrons. The S and W parameters are sensitive to the changes in the momentum density of lower and higher-momentum electrons, respectively. In a larger size hole in which positronium is localized, a larger S parameter will occur due to smaller momentum uncertainty. Especially in polymers having large defects or voids, the S parameter is a qualitative measure of the defects size and defect concentration.
This page updated by Pozitron/Pozitronyum Araştırma ve Hizmet Laboratuvarı on 19.02.2019 18:51:42