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  • Öge
    Investigation of the electrochemical co2 reduction mechanism on tin electrodes
    (Fen Bilimleri Enstitüsü, 2020) Yılmaz, Tuğçe ; Ürgen, Mustafa Kamil ; 633383 ; Malzeme Bilimi ve Mühendisliği
    In today's world, one of the biggest concern is climate change. Even though fossil fuels are started to replaced by renewable energy sources in recent years, this solution turned out to be not sufficient to decrease the accumulated CO2 in the atmosphere Increased human population and energy demand escalated the rate of CO2 emission with a higher rate than environmentally friendly energy sources. The released CO2 gas to the atmosphere by human activities is the main factor causing climate change. The increased amount of CO2 in the atmosphere causes a greenhouse effect that led to an increment in temperature. Thus, the studies concentrate on the CO2 conversion and storage methods to get rid of the excess CO2 in the atmosphere. The conversion of CO2 is not only useful to eliminate the CO2 gas in the atmosphere but also the products of this conversion is used as a raw material to produce valuable materials. There are various methods for the conversion of CO2 such as chemical, thermochemical, electrochemical, biochemical, photochemical, and etc. Among these methods electrochemical CO2 reduction method has numerous advantages such as no need for heating or pressure, harmless reactants, the possibility to have a carbon- neutral chemical production by supplying energy from renewable energy sources, and convenience to scale-up. Electrochemical CO2 reduction is a method in which a CO2 is reduced on an electrode surface acts as a catalyst. The typical setup for this process involves a cell divided by a proton exchange membrane. The counter electrode and working electrode are located on different sides of the cell. While CO2 is reduced to various chemicals by taking + electrons and H , besides the hydrogen evolution reaction on the working electrode; water-splitting reaction occurs on the counter electrode. There are many products that can be produced by electrochemical CO2 reduction and their formation potentials are close to each other. In addition, CO2 is a highly stable molecule due to its linear molecular structures. These two conditions make the use of a catalyst inevitable. The metal catalysts capable of reducing CO2 are grouped according to their selectivity to specific products. These metals and the products selectively formed on them are Pb, Hg, Tl, In, Sn, Bi, Cd − formate, Au, Ag, Zn, Pd, Ga – CO, Cu − alcohols and hydrocarbons. The optimal binding energy between the catalyst surface and the key intermediate to produce a certain product led to high selectivity. The aim of this study is to produce selectively formate because of its high energy density and economical value, and its nontoxic nature. Among the metals that are selectively produced formate Sn stands out because of its relatively low price, high availability, innocuousness, and most importantly high selectivity towards formate. Even there are numerous studies published regarding design highly selective Sn-based catalyst to produce formate, a few of them discourse the mechanism providing the high selectivity. However, to design a highly selective and efficient catalyst, one should understand the factors favoring formate production. Thus, understanding the mechanism will be a breakthrough in electrochemical CO2 reduction. In this study, the mechanism of electrochemical CO2 reduction to formate on the Sn electrode is investigated. At first, the reliable setup was built-up to have replicable and trustworthy results. To achieve the reproducibility, adjustment of the position of the reference electrode and the working electrode with respect to the membrane, stabilization of temperature, the distance between the counter and working electrode, and anode area were done. The working electrode was masked to have an area of 4 2 cm . However, the results were not reproducible. Following this, the annealing at 150, 180, and 200oC and anode area studies were done. As a result, the annealing has no significant effect on faradaic efficiency in this study. The faradaic efficiencies obtained on 4 cm2 were not reproducible, but when the electrode area was decreased to 2 cm2 results were reproducible. The increased counter electrode to working electrode area ratio and the more similar sizes of the electrode and membrane, ease the charge transfer and resulted in uniform charge distribution. Once the reliable setup is achieved, a cyclic voltammetry analysis was done on the pure Sn electrode in CO2 saturated 0.1 M KHCO3 electrolyte with pH 6.8. As a result, the reduction peak of tin oxide appeared at 1.0 V vs. Ag/AgCl, and in the literature, the electrochemical CO2 reduction experiments were done at more cathodic potentials. However, there are many studies indicating that the oxide layer on the tin surface is the key factor governing the high selectivity for formate. To clear up this contradiction, the longtime electrochemical CO2 reduction experiment was designed to understand the effect of tin oxide on selectivity towards formate. In this experiment, the formate production rates are detected for every ten minutes. The results revealed that in the first ten minutes the produced formate amount was almost 4 times higher than the remaining time intervals. This could be concluded as after the initial reduction of tin oxide the formate production rate decreased dramatically. Thereafter, 6 different polarization experiments were conducted on pure Sn electrodes to have a better understanding of the relationship between the tin oxide and CO2 reduction reaction. In the beginning, the pure Sn electrodes were polarized from open circuit potential to -2 V vs. Ag/AgCl repeatedly until the polarization curves no longer changed. Firstly, this process was done in Ar saturated 0.1 M KHCO3 electrolyte with pH 8.5. The resulting curve preserved its oxide peak, however, it was slightly reduced. When the experiment was repeated in CO2 saturated 0.1 M KHCO3 with pH 6.8, the oxide was greatly reduced. This contrast between two experiments might be originating from the differences in CO2 presence or pH of the electrolytes. Thus, the final experiment was done in H2SO4 added 0.1 M KOH which has a pH of 6.8 but there was no CO2 or even HCO3- in the electrolyte. The eventuated curve preserved its oxide peak, and that was more similar to polarization curves obtained in Ar saturated 0.1 M KHCO3 than the CO2 saturated 0.1 M KHCO3. However, it is worth noting that after the first cycle the oxide is reduced even less than the Ar saturated 0.1 M KHCO3. The second set of polarization experiments was designed to reduce the tin oxide electrochemically before the polarization. At first, the pure Sn electrodes were reduced at -1.8 V vs. Ag/AgCl for 15 minutes to give enough time to have a full reduction and simulate the real experimental conditions. Then, the polarizations were started at -1 V rather than to open circuit potential to prevent the re-oxidation of the Sn surface, and it continued until the -2 V vs. Ag/AgCl. This process was repeated similar to the previous set of experiments until the polarization curves did not change anymore. The results were consistent with the experiments that do not involve prior electroreduction. The polarization curves obtained in Ar saturated 0.1 M KHCO3 with pH 8.5 and H2SO4 added 0.1 M KOH with pH 6.8 were similar, but the reduction of the oxide was less in the H2SO4 added 0.1 M KOH. In the polarization curve obtained in CO2 saturated 0.1 M KHCO3 with 6.8 pH, no oxide peak has appeared. Thus, it can be suggested that the oxide is more prone to be reduced in the presence of CO2 or HCO3-. This experiment proves that the electrochemical CO2 reduction proceeds with not only CO2 gas but also HCO3- ions in the electrolyte. Additionally, at more cathodic potentials than -1.6 V vs. Ag/AgCl, it was clear that both hydrogen evolution reaction and electrochemical CO2 reduction reactions become kinetically limited because the slope of the curves become steeper. The steepness of the curves at more negative potentials than -1.6 V vs. Ag/AgCl can be listed as higher to lower, CO2 saturated 0.1 M KHCO3, Ar saturated 0.1 M KHCO3, and H2SO4 added 0.1 M KOH. Since, as the CO2 and HCO3- amount increases in the electrolyte the reaction becomes more limited, it can be said that the electrochemical CO2 reduction reaction becomes more limited than the hydrogen evolution reaction. In conclusion, the two main outcomes of this study is that the formate production rate was decreased with the reduction of the tin oxide, and total reduction of tin oxide on the surface is only achieved by the presence of CO2 in the solution indicating a synergic interaction between tin oxide and CO2. Thus, this study reveals the strong relationship between the tin oxide reduction reaction and ECR. In addition, it reveals the missing point in the literature about how tin oxide is affected by the electrochemical CO2 reduction reaction.
  • Öge
    Production of copper - silicon carbide composites for thermal management applications by electroforming process
    (Institute of Science And Technology, 2019-06-11) Evren, Burak ; Ürgen, Mustafa Kamil ; 521171005 ; Malzeme Bilimi ve Mühendisliği ; Material Science and Engineering
    Technology of an era is limited with advancement in materials science. Validity of this phrase is much more sensible in todays world where science and technology are developing with acceleration. This statement is also perceptible in electronics. Size of the electronic elements is getting smaller every day in conjunction with an increment in number. Thus, components reach higher surface area values. As a result, released heat exceeds critical points and leads to deficiency in material performance and may even lead to material failure. For this reason, thermal management applications require a further attention. In this study, copper–silicon carbide composites are fabricated by electrochemical deposition method to serve as a thermal management material. Electrochemical deposition is conducted by using a modiefied version of Sediment-Codeposition (SCD) technique. A particular electrode system is designed for obtaining a self standing deposit. Acid copper sulfate electrolyte is selected as the electroforming bath.Silicon carbide particles are initially wetted in a solution containing the electroforming bath, afterwards they are manually settled on substrate surface of designed system. Different sets of experiments are conducted for achieving a smooth copper deposit between silicon carbide particles by 6 hours of electroforming. Operating conditions are benchmarked through obtaining cathodic polarization curve. The cathode potential corresponding to the optimized current density (1 A/dm2) is determined. Composite electroforming process is conducted potential controlled for keeping the current density stable because of continous area change of the cathode with the advancement of the coating process.After achieving the desired copper morphology for reinforcing silicon carbide particles, deposition time is extended to 30 hours. A set of samples are produced by using SiC particle with sizes of 75 µm and 1 mm. Scanning Electron Microscopy (SEM) investigations revealed that under these conditions it became possible to produce composite with 500 µm average thickness and over 65% reinforcing particle volume. At the matrix-particle interfaces some void formations but strong particle incorporation are observed.In order to enhance matrix-particle interface, as deposited samples are treated by annealing at 650 °C for 2 hours, cold isostatic pressing under 1500 kN for 10 minutes and cold isostatic pressing prior to annealing. SEM images of cold isostatic pressed samples reveal that copper is compressed from both surfaces and smooth copper morphology is converted to a needle-shaped, cornered form, while voids at the interfaces are diminished. For investigating the structural changes, x-ray diffraction patterns of samples are plotted. Copper peaks of as deposited sample shift to smaller angles, showing that compression stress forms during deposition. Annealing revokes compression stresses induced by deposition. Silicon carbide peaks are observed in diffraction pattern of cold isostatic pressed sample since copper gets compressed through harder silicon carbide particles, bringing these particles closer to composite surface. Wide peak broadening is seen for pressed samples as a result of induced compression strength and dislocations. Peaks are sharpened after annealing.X-ray diffraction patterns also show that silicon carbide particles do not chemically react with copper. Composite can be annealed at 650 °C without any significant structural change.Formability of the as deposited and annealed samples are measured by three point flexural test. Flexural strength of as deposited sample is 11.72 MPa while flexural strength of annealed sample is 4.67 MPa. Ductility is enhanced to composite by annealing.The viability of process for desired application is evaluated and concluded through experimental findings. Copper – silicon carbide composite is produced in the desired form and desired reinforcement concentration by electroforming. Depending on the usage area, the fabricated material is suitable for directly using as product right after deposition, or it can be cold isostatic pressed and annealed before operation.
  • Öge
    Effect of preparation methods on morphology and rheological properties of PLA/CNC nanocomposites
    (Institute of Science And Technology, 2020-06-12) Özdemir, Burcu ; Nofar, M Reza ; 521171006 ; Material Science and Engineering ; Malzeme Bilimi ve Mühendisliği
    Bio-based and biodegradable polymers are produced from biomass which is renewable by cultivation. The biopolymers degrade in natural environments decomposing into non-toxic substances. Moreover, during the cultivation process of raw material, biomass, of biopolymers CO2 is consumed and the amount of CO2 emission from the life cycle of biopolymers is much lower compared to that of petroleum-based polymers. Poly(lactic acid) or polylactide, PLA, is both Bio-based and biodegradable polyester-based polymer. It has comparable mechanical and physical properties with commonly used petroleum-based polymers. Thus, it is considered as a candidate to be replaced with widely used petroleum-based polymers. However, PLA has some drawbacks which limits is processability and applications. The shortcomings of PLA are slow crystallization kinetics and poor rheological properties. They result in slow crystallization rate, low thermal properties, low melt strength hence limited processability. Studies have shown that making nanocomposites of PLA by incorporation of nanoparticles into the PLA matrix could enhance its crystallization kinetics and viscoelastic properties. Cellulose nanocrystals (CNC) as being a biodegradable and Bio-based polymer has outstanding mechanical and thermal properties. Studies have shown that making PLA/CNC nanocomposites improve the thermal, crystallization, and rheological properties of PLA by preserving its biodegradable and Bio-based characteristics. However, the hydrophilic character of CNCs challenges its homogeneous dispersion in the hydrophobic PLA matrix which is crucial to improve PLAs properties, especially at low percolation concentrations. Studies have shown that without the use of any modification, the melt mixing process is not successful to disperse CNCs since the shear force applied during the process is not enough to break the strong hydrogen bonds between CNCs. Although it is not industrially feasible as melt mixing, it is possible to deagglomerate CNCs and to obtain good dispersion by solution casting method. The solution casting method involves the dispersion of CNC and PLA in proper solvents by using a water bath sonicator, tip sonicator, or magnetic stirrer followed by casting the mixture and followed by evaporation of the solvent. Studies have shown that the application of solution casting greatly differs in the instrument, the sequence of application, duration, PLA type, CNC type, and solvent type. This study aims to investigate the effect of preparation method, solvent, and CNC type on CNC dispersion thus the rheological and thermal properties of PLA/CNC nanocomposites. Low molecular weight and highly crystallizable PLA and two types of CNC were used to make nanocomposites. CNCs differ in their drying processes: spray-dried CNC (SCNC) in powder form and freeze-dried CNC (FCNC) in flake form. The rheological analysis was used as a characterization method to analyze the CNC dispersion. As the first step of the study, four different solvents were used to prepare nanocomposites of PLA/SCNC and PLA/FCNC at a fixed amount of CNC (3 wt.%). The solvents are tetrahydrofuran (THF), chloroform (CHL), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). The preparation method is followed as first dispersing CNC in the solvent using a bath sonicator for 2 hours at room temperature. Then PLA was added to the CNC solvent mixture and mixed with a magnetic stirrer for 2.5 hours (4 hours for DMSO). The temperature during magnetic stirring was selected according to the type of solvent. It was 55oC for THF and CHL since PLA is soluble in those solvents. PLA is not soluble in DMF and DMSO in room temperature thus, the temperature was increased above the glass transition temperature of PLA and it was 75oC for DMF and 95oC for DMSO (gradually increased). After the mixing process, the solvent containing PLA and CNC were casted into petri dishes and left to dry for 2 days under the fume hood followed by vacuum drying at 85oC for 2 days. After two days of vacuum drying, the films were ground with a coffee grinder and powders were further dried for 2 days under vacuum at 85oC. Nanocomposites prepared with DMSO revealed the highest improvement in rheological properties followed by DMF. THF resulted in a slight increase in total compared to neat PLA due to its much lower dielectric constant. However, nonpolar CHL couldn't deagglomerate CNC thus nanocomposites prepared with CHL revealed much lower viscoelastic properties than neat PLA. PLA/SCNC nanocomposites revealed slightly higher improvement in DMF and DMSO however, the difference was insignificant in THF and CHL. To investigate the effect of the preparation method, DMSO was chosen. Both PLA/SCNC and PLA/FCNC nanocomposites were prepared with varying water sonication times and sequences. In total four different methods were compared: 0WS-4MS, 2WS-4MS, 4WS-4MS, and 2WS-4MS-2WS. As an example, 2WS-4MS means 2 hours of water sonication followed by 4 hours of magnetic stirring. The highest improvement achieved with PLA/SCNC by the 4WS-4MS method. The increased water sonication time increased the dispersion of CNC however, the other methods displayed similar improvements. Moreover, since the CNC was in powder form only four hours of vigorous stirring at elevated temperature resulted in the second best improvement. It could be due to the DMSO's higher ability to disperse CNCs even without a bath sonicator. In the case of PLA/FCNC, 0WS-4MS revealed the highest improvement. Since the FCNC is in flake form the bath sonication at room temperature was not enough to disperse them. The method 2WS-4MS-2WS revealed the second-best result and better than the 2WS-4MS method. In the first method after 4 hours of magnetic stirring the PLA/FCNC and solvent mixture is placed in the bath sonicator when it was still hot. Thus, the high temperature of the solvent could aid the further dispersion of FCNC. To see the effect of CNC content (1, 2, 3, and 5 wt.%) it was decided to continue with SCNC since the results were more consistent. PLA/SCNC nanocomposites with DMSO by using 2WS-4MS method with varying CNC contents were prepared. To compare the results, nanocomposites with DMF was also prepared. PLA/SCNC nanocomposites with 5 wt.% revealed the highest improvement in rheological properties. In nanocomposites prepared with DMF, except PLA/SCNC1, nanocomposites revealed increased rheological properties. For the NCs prepared with DMSO, this increase was more pronounced. The rheological percolation threshold of NCs prepared with DMF and DMSO was calculated by employing the empirical power-law equation and found to be 1.59 and 0.44 wt.% respectively. The percolation concentration again revealed that DMSO is much more effective to obtain high quality dispersion of CNC. The apparent yield stress of NCs were calculated by employing Modified Herschel-Bulkley equation. NCs prepared with DMSO revealed much higher apparent yield stress values compared to ones prepared with DMF. Both the modified cole-cole plot analysis and Herschel-Bulkley plots of NCs revealed solid like behaviour started at percolation concentration. The isothermal DSC analysis of NCs revealed that due to better dispersion of CNC with DMSO, the NCs prepared with DMSO exhibit faster crystallisation kinetics. However, no significant change was observed between FCNC and SCNC. The neat PLA processes with DMSO revealed faster crystallisation compared to one with DMF. This could be due to higher plasticizing effect of DMSO since it is expected that higher amount of DMSO is remained in samples. Similarly, NCs prepared with different CNC content revealed that crystallisation time is close to that of neat PLAs. Those, results shows the effect of remaining solvent on NCs crystallisation.
  • Öge
    Investigation of infrared phosphorescence properties of chromium doped lanthanum gallogermanate phosphors sythesized by sol-gel method
    (Institute of Science And Technology, 2020-02-04) Can, Burcu ; Solak, Nuri ; 521161015 ; Material Science and Engineering ; Malzeme Bilimi ve Mühendisliği
    The need to illuminate in which they have lived since the ancient times is one of the most important needs of people in order to facilitate daily life. As a result of the discoveries made over the years and the improvements made based on these, lighting technologies have reached the present level. The process started with the discovery of fire; candles, gas lamps and the invention of the bulb in the end has made human life much easier. Following these pioneering discoveries, technology has advanced significantly and fluorescent lamps have been used as lighting tools. These technological developments have contributed to the improvements of many value-added products such as, light emitting diodes (LEDs), plasma display panels and photoluminescent materials, as well as lighting. The mechanism of photoluminescence can be explained by two different sub-types , fluorescence and phosphorescence. The working principles of these two mechanisms are separated according to the paths followed by electrons during the transition between energy levels. In fluorescence radiation, electrons complete the excitation and emission steps between singlet energy levels, while in the phosphorescence mechanism these transition occurs between singlet and triplet energy levels. The traps locating between the singlet and the triplet levels capture electrons and ensure long-lasting radiation, which is the main logic of the phosphorescence phenomenon. In the case of photoluminescence, the photons are the source that stimulate electrons to radiate and so this mechanism called as "photo"luminescence. There are also different luminescence mechanisms which are called according to different radiation sources. For instance; heat in "thermo"luminescence, electrical charge in "electro"luminescence, cathode rays in "cathado"luminescence and energy resulting from chemical reactions in "chemi"luminescence direct the electrons to an upper state. The first appearance of phosphorescence materials took place in Italy in the early 17th century. A volcanic stone found by an Italian alchemist was exposed to high temparatures to obtain a metal. However, the heated stone has become a glowing material in the dark, rather than becoming a metal. The name "Bolognian stone" which takes its name from the city where the alchemist lives was the first material in which the self-glare feature in the dark was discoved. Subsequent studies have shown that this stone was in fact BaS2, ie, barite, which undergoes heat treatment into Ba(SO4). After the illumination mechanism of barium sufide, studies on phosphorescence materials gained importance. ZnS, CaS, SrS are among the most widely stuied phosphors. As a result of the studies, it has been proved that doping with different ions gives positive results for improving the intensity and duration of radiation. Transition metals such as Cu, Co, Cr as well as rare earth elements such as Eu, Dy, Er have been used as doping ions. In phosphors synthesized by co-doping method of these elements to the main matrix, it has been proved that the duration and intensity of radiation are prolonged. However, due to chemically unstable nature of the sulfide-based materials, it is possible to produce H2S gas in contact with moisture in the air. Therefore, in the following studies, alkaline earth aluminate based phosphorescence materials have been focused. The most common of these type of phosphors are SrAl2O4, CaAl2O4, BaAl2O4 and many studies have been carried out to improve their afterglow properties by adding rare earth elements to the structure. Owing to their positive features like being chemically stable, having long-term and high intensity radiation in visible emission wavelength (450-530 nm) with bright colours, they have several industrial usage area such as lighting equipments, warning signs, functional paints, etc. In addition to the phosphors emitting in the visible region, many detailed studies have been conducted on the types of phosphorescence material that emit in the near-infrared (NIR) region. Near-infrared radiating phosphors have started to be used in night vision camera systems with advancing defense technologies or in systems that provide biological imaging of cells and tissues with advances in biomedical field. These materials are generally obtained by adding Cr3+ ions to spinal gallate and aluminate or gallogermanate matrices. Also in some studies, radiation intensity and duration were improved by addition of rare earth elements. Due to the ability of Cr3+ ion to be replaced by Ga or Al in the matrix, structural defects and electron traps occur in the crystal system. As a result of excitation with the UV source, emission occurs in 700-1300 nm wavelengths. One of the phosphors which emit in the infrared region of the spectrum and have been studied in detail is the materials having the lanthanum gallogermanate matrix. The optimum doping rate of Cr3+ and reaction conditions of the main phase structure of lanthanum gallogermanate, which is expressed by the formula La3Ga5GeO14, are investigated. Highly detailed studies on the effects of different secondary doping elements on the crystal structure and radiation properties are also being conducted. The most widely used powder synthesis method for the production of phosphorescence materials is the solid state reaction method. In the solid state reaction, which is accepted as the conventional method, oxide forms of the elements to be used as starting materials are used. These powdered materials are subjected to heat treatment after being weighed based on the stoichiometric ratios and grinded. Although it is the most widely used method, it is challenging and difficult to obtain homogeneous grain structure since the solid state reaction involves many milling steps. Sol-gel method, which is another powder synthesis method is much easier, faster and gives better results than the solid state method. In the sol-gel method, the reaction begins with the aqeous solution of nitrate salts selected as starting materials. Then homogeneneous solution loses its water content with heating and transforms into gel form. When the obtained gel is put in an oven at 200 °C, production of powder material is occured. As a result of heat treatment steps such as calcination and sintering, a very small particle size and homogeneous structure is obtained. In the majority of studies on La3Ga5GeO14 phosphors, solid state reaction was chosen as the synthesis method. With this study; the luminescence duration, intensity and the crystal structure of samples synthesized using sol-gel method were investigated. Cr3+ doping ratio and Nd3+, Gd3+, Pr3+, Dy3+ co-doped situations were chosen as the main parameters to be investigated for their effects on phosphorescence properties. Additionally, the impacts of substitution of La3+ with Sr2+ ion at two different rates and also Ga3+ substitution with Al3+ and Mg2+ ions on afterglow properties were investigated in the following steps of the experimental study. In these studies, firstly, phosphor synthesis was carried out with a Cr3+ addition ratio of 0.5%, 1% and 3%. Subsequently, 1% Cr3+ - 1% Nd3+/1% Gd3+/1% Pr3+/1% Dy3+ co-doped materials was produced to examine the effect of secondary doping element. After determining the optimum doping rate and co-dopant, Sr2+ added to the structure for the substitution of La3+, stoichiometrically 0.1 and 0.2. In the last step of the study, Al3+ and Mg2+ were added to final decided sturucture by substitution of Ga3+ at the stoichiometric rate of 1. For the phosphor synthesis, nitrate and acetate compounds of the elements were converted into aqueous solution to form gel at ~100 °C. After obtaining the powder in at 200 °C, calcination was carried out for 6 hours at 800 °C, then sintering was completed for 4 hours at 1300 °C. Before sintering, powder samples were filled into silicone molds and shaped under isostatic press under the 400 MPa pressure. The heat treated samples were characterized by XRD and fluorescence spectrometer and the results were compared. As a result of the experimental studies and all the characterizations, it was determined that the optimum Cr3+ dopant ratio was %1 and the best co-dopant was Nd3+ in terms of the emission intensity and decay time. In the second stage of the experimental study, the effects of Sr2+ ion, which was added to the structure by substituting a certain amount of La3+ ion, were examined and it was found that the amount of substitution with the best afterglow performance was 0.2. In the Sr2+ substituted structure, the effects of co-dopants were examined and it was concluded that Nd3+ and Gd3+ additives had positive effects on the afterglow intensity and decay time. In the last stage, when Al3+ and Mg2+ elements are added to the structure as a substitute for Ga3+ ion, how the phosphorescent properties change was examined. While both of these additives reduced the emisson intensity of the phosphor, the Al3+ additive extended the decay time, unlike Mg2+. While the emission wavelength of the initial structure, in which the synthesized phosphorus has only Cr3+ and co-dopants, was 719 nm, this value changed to 709 nm with the addition of Sr2+ and 726 nm with the addition of Mg2+.
  • Öge
    Baryum magnezyum tantalat seramiklerinin dielektrik özelliklerine nadir toprak elementlerinin etkileri
    (Fen Bilimleri Enstitüsü, 2020-03-13) Şener, Berhan ; Solak, Nuri ; 521171003 ; Malzeme Bilimi ve Mühendisliği ; Material Science and Engineering
    Son yıllarda haberleşme teknolojisinin gelişmesi ile mikrodalga dielektrik seramiklere talep artmıştır. Mikrodalgalar radyo dalgalarından daha yüksek frekansta salınım yaptığı için bilgi taşıma kapasitesi radyo dalgalarına kıyasla yüksektir. Mikrodalgalar 300MHz ile 30GHz bandında yer almaktadırlar ve bu bant içerisinde çok çeşitli uygulamalarda kullanılmaktadırlar.Cep telefonları, kablosuz internet, askeri arama radarları ve uydu iletişimi gibi sistemler mikrodalga frekanslarda çalışmaktadırlar. Mikrodalga seramikler bu sistemlerde mikrodalgaların çift oluşturulmasında ve filtrelenmesi amacıyla kullanılmaktadırlar. Geleneksel radyo frekanslarında kullanılan sistemler mikrodalga dalgaların işlenmesi için kullanılamamaktadırlar. Yüksek frekansta salınım yapan mikrodalgalar geleneksel transistör parçalarında kullanıldığında oluşan direnç sebebiyle istenilen konuma ulaşmadan kaybolmaktadır. Mikrodalga uygulamalarında kullanılacak olan seramik malzemelerden belirli özellikler beklenmektedir. Uygulamalarında tamamında düşük dielektrik sıcaklık katsayısı istenmektedir. Bu değer kullanılacak malzemenin özelliklerinin sıcaklığa bağlı değişimini göstermektedir ve idealde sıfır olması istenmektedir. Yine uygulamaların tamamında düşük dielektrik kayıp değerinin olması istenmektedir. Dielektrik kayıplar hem malzemenin kristal yapısındaki hatalardan hem de çalışma koşullarından dolayı ortaya çıkmaktadırlar. Dielektrik sabiti değeri ise kullanılacak uygulama alanına göre belirlenmektedir. Dielektrik sabiti 50 ve üzerinde olan dielektrik seramikler sinyal minyatürizasyonunun istendiği cep telefonu iletişimi gibi alanlarda yer bulmaktadır. Dielektrik sabiti 25 den küçük olan seramikler yüksek frekansta çalışan anten devre elemanlarında kullanılmaktadırlar ve genellikle iyonik karakteri yüksek olan alüminat yapısına sahiptirler. Dielektrik sabiti 25 ile 50 arasında olan seramikler ise uydu haberleşme uygulamalarında yer almaktadırlar. Mikrodalga devre uygulamalarında kullanılan bir çok dielektrik seramik mevcuttur ve bunların büyük bir kısmı perovskit kristal yapısına sahiptir. En sık kullanılan dielektrik seramik bileşimleri baryum çinko tantalat (BZT) ve baryum magnezyum tantalat (BMT)dır. Her iki malzeme de çok iyi mikrodalga dielektrik özelliklere sahip olmasına karşın üretim masrafları yüksek olduğu için literatürde bu malzemelerin üretim maliyetini azaltarak dielektrik özelliklerini geliştirmek için çeşitli araştırmalar yapılmaktadır. BMT üretimi sırasında karşılaşılan zorluklar yüksek sinter sıcaklığı ve uzun süren sinter süreleridir. Yüksek sinter sıcaklıkları ve süresi sebebiyle baryum magnezyum tantalat seramiklerinde üretim sırasında magnezyum oksidin uçuculuğundan kaynaklanan magnezyum içermeyen ikincil fazlar oluşmaktadır. Oluşan bu ikincil fazlar BMT tanelerinin büyümesine sebep olduğu için dielektrik özelliklerini olumsuz olarak etkilemektedir. Bu fazların oluşmasını engellemek ve yapının sinterlenebilirliğini arttırmak için yapılan çalışmalarda literatürde daha önce uygulanmamış üç aşamalı ısıl işlem içeren bir üretim yöntemi kullanılmıştır. Kullanılan yöntemi iki aşamalı kalsinasyon ve takibinde sinterleme işlemi içermektedir. BaCO3 812 °C derecede erimekte ve 1360 °C derecede kaynamaktadır. Üretilen yapı içerisinde bu faz değişikliklerinin gerçekleşmesi istenmediği için girdi malzemesi olarak BaCO3'ın parçalanarak BaO'e dönüşmesi gerekmektedir. Bu sebeple kalsinasyonun ilk aşaması 775 °C derecede yapıya katılan BaCO3'ın parçalanarak BaO'e dönüştürülmesini içermektedir. BaO elde edildikten sonra bileşimler daha yüksek bir sıcaklıkta belirli bir süre beklenerek ön reaksiyonların tamamlanması ve kısmi olarak faz dönüşümlerinin oluşması sağlanmıştır. İki aşamalı kalsinasyon işleminden sonra hazırlanan bileşimler bilyalı değirmende öğütülmüş ve yapının homojenizasyonu sağlanmıştır. Kalsinasyon sonrasında soğuk izostatik pres ile şekillendirilen numuneler sinterleme işlemine tabi tutulmuştur. Literatürde daha önce kullanılmayan bu metotla beraber yapılan bu çalışmada BMT yapısının içine La ve Y dopantları eklenerek sinter süresinin optimize edilmesi ve dielektrik özelliklerin geliştirilmesi amaçlanmıştır. Bunun için çeşitli oranlarda La ve Y yapıya katılarak baryum miktarı azaltılarak dopantların etkisiyle beraber stokiyometri dışına çıkılarak yapının dielektrik özellikleri incelenmiştir. Üretilen numunelerin faz tayinleri X-Işını Saçılması (XRD) tekniği ile tetkik edilmiştir. Numunelerin dielektrik sabitleri ve dielektrik kayıp faktörleri ise broadband dielektrik spektroskopisi ile belirlenmiştir.