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Öğe A comparison of the electrical characteristics of TiO2/p-Si/Ag, GNR-TiO2/p-Si/Ag and MWCNT-TiO2/p-Si/Ag photodiodes(SPRINGER, 2019) Erdal, Mehmet Okan; Yildirim, Murat; Kocyigit, AdemThe TiO2/p-Si/Ag, graphene nanoparticles doped (GNR) TiO2/p-Si/Ag and multi-walled carbon nanotube (MWCNT) doped TiO2/p-Si/Ag photodiodes were fabricated by electro-spinning technique at the same experimental conditions, and their structural, morphological and electrical properties were compared for photodiode applications. XRD measurements were confirmed undoped, GNR and MWCNT doped TiO2 structures, and brookite phase of (121) preferred orientation TiO2 has been observed from XRD patterns. SEM images of the heterojunctions showed that undoped and doped TiO2 layer have homogenous surfaces. I-V measurements were performed for electrical characterization of the TiO2/p-Si/Ag, GNR-TiO2/p-Si/Ag and MWCNT-TiO2/p-Si/Ag photodiodes under dark and light illumination conditions at room temperatures. The results imparted that all heterojunctions have good rectifying and photodiode properties. Some heterojunction parameters such as ideality factor, barrier height, series resistance were calculated and discussed in details according to thermionic emission theory, Cheung and Norde techniques. The determined ideality factor values are 8.55, 9.70 and 8.99, and barrier height values are 0.75 eV, 0.74 eV and 0.73 eV for the TiO2/p-Si/Ag, GNR-TiO2/p-Si/Ag and MWCNT-TiO2/p-Si/Ag photodiodes, respectively. These heterojunctions can be considered and improved as photodiodes in industrial applications.Öğe The effect of nonwoven electrospun PAN nanofiber mat on mechanical and thermal properties of epoxy composites(2018) Erdal, Mehmet Okan; Yazman, Şakir; Gemi, Lokman; Yapıcı, AhmetIn this study mechanical and thermal properties of epoxy resin reinforced withdifferent numbers of nanofiber layers which produced with electrospinning method wasinvestigated. Solution of 10 wt% of polyacrylonitrile (PAN) in N,N-dimethylformamide(DMF) was used for electrospinning. The diameters of the obtained nanofibers were in therange of 380-420 nm. The average thickness of the produced nanofiber layer was about200 ?m. The special molds were prepared to produce the laminated composite plates. Thetensile tests show that the using of nanofiber PAN layers increase the tensile force 34.54%and decrease the elongation 8.87% in comparison with neat epoxy. The fracture surfaces ofthe specimens were inspected by using optical and scanning electron microscopy (SEM).The thermal properties of the nanofiber layered composites were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. It wasobserved that the glass transition temperature increased parallel to this as the number ofPAN layers increased and rose up to 86?C, while the thermal stability did not show mucheffect of PAN layers.Öğe The effect of synthesis technique on thermoelectric properties of nanocrystalline NaCo2O4 ceramics(SPRINGER, 2014) Erdal, Mehmet Okan; Koyuncu, Mustafa; Uslu, IbrahimSodium cobalt oxide (NaCo2O4) nanocrystalline thermoelectric ceramic powders were synthesized using conventional polymerized complex sol-gel and electrospinning techniques and then consolidated into bulk ceramics. We have investigated the differences in the microstructure and thermoelectric properties of the samples prepared using electrospinning and sol-gel methods. The crystal structures of the powders and nanofibers were characterized using X-ray diffraction and scanning electron microscopy before and after calcination at different temperatures. Nanofibers prepared by electrospinning have a diameter of approximately 300 nm, while the diameter of the grains of sol-gel powders ranges from 1 to 5 mu m. Thermoelectric properties of the bulk ceramics were measured by physical properties measurement system in the temperature range of 15-300 K. The calculated values of dimensionless figure of merit at 300 K are 6 x 10(-7) and 4 x 10(-5) for sintered powders from sol-gel and nanofibers, respectively. While electrospinning production technique increase thermoelectric power approximately 15 % and it fold electrical conductivity average 20 times, it decreased thermal conductivity by 90 %.Öğe Elektrospin yöntemiyle termoelektrik nano yapılar üretimi ve karakterizasyonu(Selçuk Üniversitesi Fen Bilimleri Enstitüsü, 2013-12-04) Erdal, Mehmet Okan; Koyuncu, MustafaBu çalışmada, oksit termoelektrik madde olarak bilinen sodyum kobaltit ( NaxCo2O4 ) ve onun nikel ve bor katkılı türevleri elektro-eğirme metoduyla üretilerek, termoelektrik ve diğer fiziksel özellikleri incelenmiştir. Bu amaçla 5 farklı polimer çözelti hazırlanmış olup bu çözeltilerden 4 tanesi elektro-eğirme yöntemi ile nanolif haline dönüştürülmüştür. Diğer örnek ise polimerik sol-jel metodu ile üretilmiştir. Her iki yöntemle üretilen numuneler kalsinasyon işlemine tabi tutularak toz halinde termoelektrik nanokompozit seramik malzemeler elde edilmiştir. Nanoliflerin kalsinasyon işlemi öncesi ve sonrasındaki morfolojileri taramalı elektron mikroskobu ile incelenmiştir. Numunelerin yapısal analizi XRD ve kızıl ötesi spektroskopisi (FTIR) ile karakterize edilmiştir. Aynı zamanda elektro-eğirme yöntemi ile üretilen numunelerin termal özellikleri, DSC, TGA ve DTA ölçümleri ile belirlenmiştir. Daha sonra, toz halindeki malzemeler soğuk pres yöntemiyle pelet haline getirilmiş ve uygun sıcaklıkta sinterleme işlemine tabi tutulmuştur. Sinterleme sonrası bütün numunelerin termoelektrik özellikleri (Seebeck katsayısı, elektriksel iletkenlik, ısıl iletkenlik, termoelektrik kalite faktörü) 10-300 K sıcaklıkları arasında Lot ? Oriel PPMS sistemi ile ölçülmüştür. Sonuç olarak, üretim yönteminin ve Ni, B ilavesinin sodyum kobaltitin termoelektrik özellikleri üzerindeki etkileri ayrıntılı olarak incelenmiştir.Öğe Investigation of Electrical Conductivity of Polyacrylonitrile (PAN) Nanofibers/Nano Particul (Ag, Cu, CNT and GNR)(TRANS TECH PUBLICATIONS LTD, 2017) Onal, Gurol; Erdal, Mehmet Okan; Dincer, KevserIn this study 1% Ag (silver), Cu (copper), CNT (carbon nanotube) and graphene nanoribbon (GNR) nanoparticle reinforced PAN fibers were prepared and the effects of nanoparticle reinforcements upon electrical conductivity were investigated. In experimental study, graphene nanoribbon powders were produced from multiwalled carbon nanotube (MWCNT) through using the chemical approach of Hummers method. Fiber layer was dissolved at room temperature in magnetic mixer with Polyacrylonitrile(PAN) and Dimethil Formamide (DMF) which was at the rate of 10 % by mass. Thus, a viscou gel solution was obtained then nanoparticles were added to the PAN/DMF solution and the solution was vigorously stirred for one hour at room temperature. After stirring that solution was continued for 15 m in ultrasonic bath. The polymeric solution was first transferred to a 5 mL syringe, which was connected to a capillary needle with an inside diameter of 0,8 mm. A copper electrode was attached to the needle, a DC power supply produces 25 kV against a grounded collector screen distant 15cm. With the syringe pump set at 2 mL/h, the electric force overcomes the surface tension of the solution at the capillary tip, and a jet emerges. Produced fibers were collected on the rotary collector which spins at 250 rpm. Nanofiber was dried at 60 degrees C for 12 h in vacuum oven. Eventually, nanofiber of polyacrylonitrile (PAN) reinforced by metallic nanoparticles and graphene nanoribbon (GNR) were prepared by electro spinning process. Electrical conductivity of the obtained nanofiber were studied by measuring the electrical resistance thanks to home-made plate electrodes.Öğe Investigation of Optoelectronic Properties of Organic Semiconductor Tetracyaoquinodimethane Based Heterostructures(Selçuk Üniversitesi Fen Fakültesi, 2021) Avcı, Burcu; Hussaini, Ali Akbar; Erdal, Mehmet Okan; Yıldırım, MuratRecently, interfacial layer such as metal oxide, insulator and polymer have been used by scientists between the metal and semiconductor to increase the stability of the metal-semiconductor heterojunctions. These materials have been varied according to their usage aims. In this study, graphene nanoribbons (GNR) and 7,7,8,8 Tetracyanoquinodimethane (TCNQ, C12H4N4) layer has been used as interfacial layer between the metal and semiconductor for photodiode applications. The TCNQ layer collects and extracts more electrons in the interface of the device and is used as electron acceptor material for organic solar cells. Herein, we fabricated Al/p-Si/Al, Al/p-Si/TCNQ/Al and Al/p-Si/TCNQ:GNR/Al heterojunctions by physical vapor deposition technique. I-V measurements has been employed under dark and various light illumination conditions to show dielectric properties of the fabricated heterojunctions. From current-voltage characteristics, we calculated the electronic parameters such as ideality factor, barrier heights, series resistances and rise times. It can be concluded from overall results that TCNQ and TCNQ:GNR layers had a major impact on quality and can be considered as quite proper materials for optoelectronic applications.Öğe Thermoelectric Properties of Nickel and Boron Co-Substituted NaCo2O4 Prepared by Electrospinning Technique(TRANS TECH PUBLICATIONS LTD, 2018) Erdal, Mehmet Okan; Koyuncu, Mustafa; Aksu, Mehmet Levent; Uslu, Ibrahim; Koçyiğit, SerhatIn this study nickel and boron doped sodium cobalt oxide NaCo2-xNixByO4 (0 <= x <= 0.3, 0 <= y <= 0.1) nanocrystalline thermoelectric ceramic powders were synthesized using electrospinning techniques and then consolidated into bulk ceramics. The differences in the microstructure and thermoelectric properties of the samples as a result of doping effect have been investigated. The crystalline structures of the powders and nanofibers were characterized using Xray diffraction and scanning electron microscopy and BET Analysis before and after the calcination process at different temperatures. Nanofibers prepared by the use of electrospinning technique, have a diameter of approximately 300 nm, and the diameter of the grains of calcined powders was observed to range between 150 to 500 nanometers. Thermoelectric properties of the bulk ceramics were measured by physical properties measurement system (Lot-Oriel PPMS) in a temperature range of 15-300 K. The calculated values of dimensionless figure of merit at 300 K are 4.25 x 10(-5), 5.3 x 10(-6), 8.6 x 10(-5) and 9 x 10(-6) for sintered powders from undoped, Ni and B doped powders, respectively.
