Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Now showing 1 - 5 of 5
  • Master Thesis
    Grawth and Electrical Characterization of High Purity Carbon Nanotubes
    (Izmir Institute of Technology, 2009) Kır, Serap; Selamet, Yusuf
    This thesis work is focused on growing high purity vertically aligned Carbon nanotubes by ethylene gas thermal chemical vapor deposition method on very thin Cobalt and Iron catalyst thin films deposited on to Si/SiO2/Al2O3 substrates by DC magnetron sputtering. In this study, the effective parameters were changed to grow aligned CNTs. Hence, the vertically aligned CNTs were performed and also the ideal parameters were determined for this kind of growth mechanism. The effect of support layer was examined for three different hydrocarbon gas ratios. SiO2 and Al2O3 were used as support layers between metal catalyst thin films and Si substrate. Two kinds of samples were compared; one of them had only Al2O3 and the other one includes both Al2O3 and SiO2 layers. As a result, the sample that had both oxide layers gave better results for density of CNTs on the surface of samples. Moreover, types of catalyst material effect also were examined on growth of CNTs for three different temperatures. For this aim, the performance of Fe and Co catalyst thin films was compared. According to our results, Fe was more reactive with ethylene gas than Co catalysts and also, the density of CNTs was increased by using Fe as a catalyst material. Hydrogen pretreatment time was performed for another significant effect. Seven different time parameter which were 0, 5, 10, 15, 20, 25, 30 minutes, were examined. The density and diameters of catalysts particles were compared for these different treatment times. Finally, the electrical characterization of CNTs was performed. The resistance of CNTs was measured by using two point contact technique. Moreover, the interaction between resistance of CNTs and humidity was examined.
  • Master Thesis
    Growth and Characterization of Carbon Nanotubes by Thernal Chemical Vapor Deposition Method
    (Izmir Institute of Technology, 2008) Aksak, Meral; Selamet, Yusuf
    This thesis work is focused on producing carbon nanotubes (CNTs) by methane gas thermal chemical vapor deposition method on very thin Cobalt, Iron, and Nickel catalyst thin films deposited onto SiO2/Si substrates by DC magnetron sputtering. This thesis is also devoted to understanding some parameters affecting the growth of CNTs; such as catalyst material, temperature, and catalyst layer thickness effects In this study, CNT growth was performed on directly Si substrates, which was observed that the growth was too difficult and requiring very high temperatures. Hence, very thin catalyst films were deposited on SiO2/Si substrates, and the CNT growth was observed. The temperature effect was also examined. When the growth temperature was increased, the average diameters of the CNTs were decreased up to a critical temperature, but after this point the average diameter of CNTs were increased. This effect was studied systematically by utilizing Fe and Co catalyst thin films and with the help of Raman spectroscopy and Scanning Electron Microscopy results.Catalyst thickness effect was also examined. For this aim, Ni catalyst thin films with three different thicknesses; 0.7 nm, 1.4 nm, and 6 nm, were utilized. It was observed that CNTs were grown well on 0.7 and 1.4 nm thick Ni films, while there was a little growth on 6 nm thick Ni film. The roughness analysis of 0.7 nm and 1.4 nm thick Ni films were also done. Some of as-grown CNTs were also examined by X-ray diffraction method, and the results were compared one another.
  • Master Thesis
    The Effects of Oxidizers on the Diameters of the Carbon Naotubes Grown by Chemical Vapor Deposition Method
    (Izmir Institute of Technology, 2012) Bülbül, Gülay; Selamet, Yusuf
    This thesis was focused on growing high quality and small-diameter carbon (C) nanotube (NT) on Fe/Al2O3/SiO2/Si thin film catalyst by ethylene decomposition thermal chemical vapor deposition (CVD) method in the presence of a weak oxidizer (CO2). Moreover, the importance and functional properties of the oxidizer in pretreatment and CNT growth were determined. At first, it was worked in different growth conditions to examine the effects of CO2 in CNT growth. The main parameters were pretreatment time, CO2 gas flow rates during pretreatment and growth, growth temperature. Pretreatment and growth times were kept at 15 min. each and CO2 introduced 5 and 10 min. during pretreatment stage prior to the CNT growth with the flow rates 8:2, 10:2, 10:8 and 10:10 sccm, respectively. Additionally, three different growth temperatures; 750 oC, 760 oC and 770 oC were studied. Secondly, pretreatment time was kept at 15 min. The effects of CO2, which was sent in the system at 8:2, 10:2, 10:8 and 10:10 sccm ratios, on density and height of the catalyst particles were investigated. At last, all catalyst particles and CNTs obtained from the experiments were analyzed by several characterization techniques such as AFM, EDX, SEM, STEM and Raman Spectroscopy, respectively. The optimal values of amount and introduction time of CO2, the ratio of CO2 in growth to that in pretreatment were identified. Moreover, the relation between currently obtained catalyst particles and previously being grown CNTs on them were determined. It was observed that using the appropriate amount of CO2 in pretreatment and growth process positively affected the catalyst sizes and CNT diameter distributions.
  • Master Thesis
    Optimization of Carbon Nanotube Properties by Controlled Amount Oxidizers
    (Izmir Institute of Technology, 2011) Söylev, Deniz; Selamet, Yusuf
    Finally, all data were subjected through a statistical analysis. From this study we have seen that the majority of the samples had Log-Normal diameter distributions by adjusting oxidizer amount and pretreatment timing. We were able to control CNT diameters within very narrow diameter ranges.This thesis work focuses on growing high quality carbon nanotubes using ethylene as hydrocarbon gas in thermal chemical vapor deposition method in presence of weak oxidizers. We carried out experiments to study the ratio of amount of CO2, O2 and H2O in pretreatment to growth and the timing of the metal-oxide to metal conversion in presence of the oxidizers.Firstly, the effects of CO2 addition were investigated on a variety of growth conditions to study the controlled diameter growth of CNTs. The main growth parameters controlled in this work in the CNT growth were CO2 level, pretreatment time and temperature. Pretreatment times of 15, 10, 5, 2 minutes were studied. CO2 flow rates were send in system during both pretreatment time and growth time at 10:1, 10:2, 5:1, 5:2, 2:1 and 2:2 sccm ratios, respectively. We also studied two different growth temperatures at 740 oC and 760 oC.Secondly, the influence of O2 for effective CNT growth was investigated by utilizing different pretreatment times and O2 levels. Pretreatment times of 15, 10, 5, 2 minutes were studied. O2 gas flow rates during pretreatment time and growth time were studied as 5:1, 5:2, 2:2, 2:1, 2:0.5, respectively. As a result, we obtained small diameters with the pretreatment time of 15 min. in the presence of O2.Thirdly, the effects of H2O vapor for effective CNT growth on Fe/Al2O3/SiO2 were examined. During the experiments, pretreatment time was kept at a constant value of 15 min. and H2O vapor content in Ar was varied. The results of examined analyses show that the best results were obtained at the growth 60 o C (A) / 60 o C (A).
  • Master Thesis
    Growth and Characterization of Carbon Nanotubes Over Co-mo/Mgo Catalysts
    (Izmir Institute of Technology, 2010) İnce Yardımcı, Atike; Selamet, Yusuf
    This thesis work is focused on synthesis of high quality and high yield Carbon nanotubes by methane decomposition catalytic chemical vapor deposition method on Co-Mo/MgO catalyst prepared by gel-combustion method. Catalyst contained weight %1.06 Co and weight %0.86 Mo having a molar ratio of Co:Mo:MgO;0.5:0.25:10. CNTs were grown in a quartz tube. In this study, three different growth conditions were examined. Argon, hydrogen or mixture of two gases were added to methane during growth. And for all of the three growth conditions, four different pretreatment processes were investigated. Pretreatment lasted for one hour at 850 oC. Firstly hydrogen effect was examined with 200 sccm flow rate, then argon effect was examined with again 200 sccm flow rate. And the third pretreatment included both argon and hydrogen gases flow together. For these three pretreatment conditions cooling and heating processes also took place with the same gas rates. However, the last pretreatment condition was carried out with hydrogen gas at 850 oC for just one hour, and for heating and cooling processes argon was used. The highest quality CNTs were sythesized under pure hydrogen atmosphere for both pretreatment and growth processes. Then, three different H2 flow rates were investigated; 100, 150 and 200 sccm. High hydrogen flow rate during growth was better for CNT growth in terms of quality. Growth temperature was performed as another important parameter. Four different temperatures were investigeted; 850 oC, 900 oC, 950 oC and 1000 oC. With increasing growth temperatures, structural quality increased and tangled CNTs formation decreased. It was found that 950 oC was the optimum growth tempertaure to obtain high yield of CNT. Finally, the growth time effect on CNT growth was examined for four different growth times; 10, 20, 30 and 40 minutes and the results showed that amount of CNTs increased with increasing time and CNTs became longer and graphitization was higher at longer growth times. Disorder also decreases with increasing time.