LEE- Malzeme Mühendisliği-Yüksek Lisans
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ÖgeDevelopment and characterization of high entropy (HfTiZrMn/Cr)B2 based ceramics(Graduate School, 2022-12-26) Süzer, İlayda ; Ağaoğulları, Duygu ; 506201403 ; Materials EngineeringMaterials are divided into four groups: metals/alloys, ceramics, polymers and composites. Materials science includes the study of the physical, mechanical, thermal, chemical and many other properties of materials and the development of new materials. Advanced ceramic materials, including transition metal borides, carbides and nitrides, have attracted attention in recent years compared to traditional ceramics. Transition metal borides are characterized by a high melting point, high strength, high hardness, high wear and corrosion resistance, good thermal shock resistance, high chemical and thermal stability and high transmission stability. Thanks to all these superior properties, transition metal borides can be used as catalysts, refractory parts, sensors in high resolution detectors, decorative coatings, abrasive materials, coatings on cathodes, neutron absorption materials, sanding and polishing processes and in the aerospace industry, defense industry and nuclear technology. Various methods have been used to synthesize transition metal borides until today. The thermal plasma method, self-propagating high-temperature synthesis, metallothermic or carbothermic/borothermic reduction, autoclave synthesis, molten salt electrolysis and solid-state synthesis methods are the main ones. Another method used in the synthesis of metal borides, different from the methods mentioned, is the mechanochemical synthesis process, which has been used for other material groups for the last 20 years and is still being developed. Mechanochemical synthesis is a powder metallurgy production method that allows for the production of composite metal powders with small crystal grains and controlled microstructures at room temperature, using a cold welding-fracturing-rewelding mechanism and starting from easily accessible raw materials, as opposed to high reaction temperature production methods. In recent years, it has been necessary to develop new materials to meet the needs of many sectors, such as medicine, biomedicine, energy, aerospace technologies, automotive and electronics. High-entropy alloys (HEA) are one of the materials developed to meet these needs. Traditional alloying includes combining two or more elements. In high-entropy alloys, four or more elements are combined in equimolar ratios. Contrary to expectations, solid-solutions are formed instead of intermetallic compounds. In this way, HEAs have a single-phase structure even though they contain more than one element. Although there are many elements in the structure, high-entropy alloys mostly have body-centered cubic or face-centered cubic crystal structures. Recent studies have shown that such alloys may also have a hexagonal close-packed structure. Along with solid-solutions, high-entropy alloys also show four different core effects. The high-entropy effect explains its relationship with thermodynamic properties. The sluggish diffusion effect explains the kinetic state. The severe-lattice distortion effect represents both the crystal structure and the formation of mechanical properties. The effect of all the elements added to the alloy is examined under the cocktail effect. High-entropy alloys have high thermal and chemical resistance, good wear, oxidation and corrosion resistance, and mechanical properties such as high hardness, fracture toughness, and strength due to the elements in the alloy, the solid-solutions formed, and the four core effects. Thanks to its superior properties, it is used in the nuclear industry, shipping, the production of refractory materials, the aerospace industry, and cutting tool tips. Many methods are preferred in the production of high-entropy alloys, but arc melting, mechanical alloying, pressureless sintering and pressure sintering are the most common ones. The production of high-entropy ceramics, which is a new class based on high-entropy alloys, is a subject that has been studied in recent years. High-entropy ceramics include oxides, borides, carbides, nitrides and silicides. The idea of producing high-entropy metal borides, which is considered a new type of high-entropy materials and a new class of ultra-high-temperature ceramics, also has been emerged in 2016. High-entropy diboride ceramics have a P6/mmm space group and a hexagonal close-packed structure. In this structure, there are metal-boron, boron-boron and metal-metal bonds. It is characterized by superior properties as it contains metallic, ionic and covalent bonds together. High-entropy metal borides have the combination of superior properties of ceramics, such as low density, excellent high temperature strength, high hardness and strength, high wear and corrosion resistances and specific physical (optical, electrical and magnetic) properties. Due to these superior properties, it can be used in aviation, the solar and nuclear energy sectors, cutting edges and microelectronic systems. Many methods are used in the synthesis of high-entropy metal boride ceramics, a material group that has attracted attention recently due to its high thermal stability, improved mechanical properties, high oxidation resistance, and radiation damage tolerance. Mechanical alloying, boro/carbothermal reduction, self-propagating high-temperature synthesis, pressureless sintering, pressure sintering like spark plasma sintering or hot pressing are the main ones. In cases where a single-phase high-entropy diboride structure cannot be obtained, two consequent methods can be used. Mechanical alloying is a powder metallurgical production method and has the advantages of being carried out at room temperature, using cheap starting materials and inexpensive equipment. In the spark plasma sintering method, single-phase structure can be obtained with high temperature and high pressure. Within the scope of this study, HfB2, TiB2, ZrB2, TaB, Mn boride, Cr boride, Mo boride and W boride powders were synthesized by a mechanochemical route and purified by leaching in the lab-scale using the optimum conditions. Boride powders synthesized without any by-products were synthesized from optimum ones. The reproduced powders were blended in an equimolar ratio of consisting three to eight components. The three-component (Hf0.33Ti0.33Zr0.33)B2 medium-entropy alloy was chosen as the main alloy. The selected composition was first synthesized in a planetary ball mill for 30 h, 60 h or 100 h at ball-to-powder weight ratios of 10:1, 20:1 and 30:1. Then, the same composition was milled in a high-energy ball mill at a ball-to-powder weight ratio of 10:1 for 6 h, 10 h, 15 h and 20 h. In the high-energy ball mill, a ball-to-powder weight ratio of 10:1 and a milling time of 6 h were chosen as the optimum conditions. All prepared compositions were synthesized under optimum situation. For the characterization of powder samples, X-ray diffractometry, particle size measurement and density measurement with pycnometer were performed. Single-phase high-entropy diboride could not be obtained after mechanical alloying. The highest density was observed at 7.1379 ± 0.0057 g/cm3 (Hf0.142Ti0.142Zr0.142Mn0.142Cr0.142W0.142 Ta0.142)B2 composition, while the lowest density was observed in the (Ti0.25Zr0.25Mn0.25Cr0.25)B2 compositions at 4.9708 ± 0.005 g/cm3. A single phase high-entropy structure was synthesized by spark plasma sintering after milling. In addition, low intensity (Hf, Zr) oxide phases were observed. Again, secondary phases with low intensity were formed in five different compositions. X-ray diffractometer, scanning electron microscope/energy dispersive spectrometer, hardness measurement with the Vickers method, dry-sliding wear test and density measurement with the Archimedes method were used for characterization of sintered samples. The composition (Hf0.125Ti0.125Zr0.125Mn0.125Cr0.125Mo0.125W0.125 Ta0.125)B2 has the highest density value of 7.4794 ± 0.0065 g/cm3, while the composition (Ti0.25Zr0.25Mn0.25Cr0.25)B2 has the lowest density value of 4.7517 ± 0.0015 g/cm3. When all samples were examined, the hardness values ranged from 17.08 ± 2.32 GPa to 26.74 ± 1.85 GPa. The average hardness value of all samples was calculated at about 24 GPa. (Hf0.125Ti0.125Zr0.125Mn0.125Cr0.125Mo0.125W0.125Ta0.125)B2 has the lowest wear resistance and (Hf0.166Ti0.166Zr0.166Mn0.166Cr0.166Mo0.166)B2 has the highest wear resistance.
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ÖgeFabrication and characterization of a carbon fiber/ epoxy composite reflector antenna(Graduate School, 2024-07-03) Özcan Yazan, Sümmeya ; Ağaoğulları, Duygu ; 506191449 ; Materials EngineeringAntennas are crucial elements in communication technology, serving as the interface between transmission lines and free space to facilitate signal transmission and reception. Various antenna types are deployed across different applications, each tailored to specific needs such as frequency, bandwidth, radiation pattern, and polarization. Reflector antennas, in particular, are highly valued for their capacity to focus electromagnetic waves, thereby enhancing signal strength and directivity. These antennas are extensively utilized in satellite communications, radio telescopes, and radar systems where high gain and precise radiation pattern control are essential. Reflector antennas generally feature a parabolic-shaped reflector that channels incoming or outgoing electromagnetic waves to or from a focal point. Their primary advantage lies in their ability to achieve high directivity and gain, making them ideal for long-distance communication and high-resolution imaging. Designing a reflector antenna involves meticulous consideration of factors like the reflector's shape and size, the materials used, and the feed mechanism's integration. The material choice for reflector antennas is pivotal in determining their performance, durability, and weight. Traditional materials such as aluminum and steel are commonly used due to their excellent electrical conductivity, ease of fabrication, and relatively low cost. However, these materials have limitations, including a higher weight and vulnerability to deformation under mechanical and thermal stresses. As the need for high-performance, lightweight, and durable antennas grows, attention has shifted to advanced composite materials, notably carbon fiber-reinforced polymers (CFRP). Carbon fiber-reinforced polymer matrix polymer (CFRP) has remarkable mechanical qualities, including low density, stiffness, and high tensile strength. By binding the fibers together, the polymer matrix gives them resilience and form. This combination makes CFRP an ideal material for applications requiring weight reduction and high structural performance. For reflector antennas, CFRP offers numerous advantages over conventional materials. A key benefit of using CFRP in reflector antennas is the significant weight reduction. This is especially critical in satellite and aerospace applications, where reducing weight translates to substantial cost savings and enhanced performance. CFRP's low density enables the construction of large reflector antennas without the weight burden associated with metals like aluminum or steel. This weight reduction also simplifies structural design and assembly requirements, enhancing overall system efficiency. Another crucial advantage of CFRP is its high strength-to-weight ratio. The carbon fibers impart high tensile and compressive strength to the composite, allowing the reflector to maintain its shape and structural integrity under various loads and environmental conditions. This thesis explores the fabrication and characterization of a carbon fiber/epoxy composite reflector antenna, detailing the comprehensive process from material selection to final performance evaluation. The study starts with the material selection, with a focus on carbon fiber-reinforced polymer (CFRP) because of its excellent strength-to-weight ratio, lightweight design, and exceptional mechanical qualities. To identify the optimal configuration, carbon fibers were oriented at various angles, and ANSYS simulations were conducted for three different orientations to assess their performance under anticipated load conditions. To fabricate the CFRP specimens, the prepreg sheets were stacked in a predetermined order and orientation. A reflector model was created, with a total thickness of 24 millimeters and a thickness of 0.2 millimeters for each layer. ANSYS was used to analyze the stacking sequence at various angles, including 0°/90°, 45°/0°, and 30°/0°/60°/0°/90°/0°/0°/0° to ascertain the impact of fiber orientation on the material's structural performance. Because of its balanced strength and structural stability, the 0°/90° orientation was found to be the most appropriate, and it was chosen for final fabrication. In this research, both CFRP and aluminum alloy 6061-T6 were employed to examine the structural integrity and performance of an antenna reflector under different loading conditions. CFRP was specifically chosen for its excellent strength-to-weight ratio and remarkable mechanical properties, utilizing the VTP H 300 CFA 210 3KT RC42 HS carbon fiber prepreg. This material weighs 210 g/m^2 and contains 42% epoxy content. The manufacturing process involved layering eight carbon fiber layers, followed by a foam core, and then another eight carbon fiber layers. This sandwich structure was selected to enhance mechanical properties while minimizing weight. The layers were cured in an autoclave, a high-pressure oven that applies heat and pressure to eliminate voids and ensure proper adhesion between the layers, enabling the composite to achieve the desired strength and durability. Following production, the CFRP reflector sample underwent a comprehensive characterization process to verify its performance against design requirements. The characterization included various mechanical and electromagnetic tests. Tensile and shear tests were conducted to evaluate the mechanical properties of CFRP and confirm its high strength and stiffness. The integrity of the epoxy matrix was confirmed, and its chemical composition was examined using Fourier transform infrared spectroscopy (FTIR). To evaluate the antenna's performance in terms of signal transmission and reflection, radio frequency (RF) tests were done. These RF measurements were crucial in evaluating the reflector's performance in its intended application and were conducted in an anechoic chamber to prevent external electromagnetic interference. The performance of the CFRP reflector was compared to that of a conventional aluminum reflector. The CFRP reflector demonstrated superior RF performance, with better signal strength and clarity, attributed to its precise shape and material properties that reduce signal loss and distortion. Additionally, the weight comparison underscored one of the significant advantages of using CFRP. The CFRP reflector weighs approximately 650 kg, significantly lighter than its aluminum counterpart, which weighs around 1.2 tonnes due to its frame and structural components. This substantial weight reduction translates to easier handling, lower launch costs for satellite applications, and reduced structural stress during deployment and operation. These results show that the use of CFRP for reflector antennas not only reduces the overall weight but also improves the antenna performance, making it a more advantageous choice compared to conventional aluminium structures. The detailed analysis and successful implementation of a 7.3 m CFRP reflector antenna in this study is a testament to the material's capabilities and potential in the field of communications technology. The comprehensive approach, ranging from material selection and simulation to fabrication and characterization, provides valuable insights and lays a strong foundation for future developments in the use of advanced composites in high-performance engineering applications. In conclusion, this thesis concludes that the application of carbon fibre composites in reflector antennas represents a significant advance in antenna technology and offers a lightweight, high-strength alternative that meets the stringent demands of modern communication systems.
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ÖgeSynthesis, characterization and biocompatibility tests of magnetic nanoparticles(Graduate School, 2023-08-10) Azmoudeh, Aysa ; Ağaoğulları, Duygu ; 506191434 ; Material EngineeringNanotechnology advancements have surged recently in numerous fields, particularly in energy, environmental, electronic, and biological applications. Magnetic nanoparticles (MNPs) are one of the improvements that nanotechnology has brought to these application fields. MNPs are employed in biological, electrical, soil, or water filtration and catalytic applications. MNPs are one of them and are essential for the diagnosis and treatment of cancer. In research such as magnetic resonance imaging (MRI) for cancer diagnosis, like hyperthermia, magnetic nanoparticles are used as contrast agents. Particle sizes, shape, surface characteristics, biocompatibility, magnetic properties, and thermal and chemical stabilities are crucial for using nanomaterials in biomedical applications. Using magnetic nanoparticles will improve the surface area on which the medicine is carried and released because surface area increases as particle size decreases. Additionally, unless encased in protective layers, several iron oxide nanoparticles lose their chemical stability in bodily fluids. The efficiency of imaging applications could suffer from them becoming oxidized and having lower magnetization values. The concept of coating nanoparticles with protective layers has arisen to stop the degradation of magnetic nanoparticles in body fluids and to stop them from losing their magnetic capabilities. These types of materials are referred to as core/shell materials. Different inert materials are coated on the magnetic core to ensure its stability in biological settings. There have been studies on how to surround a metal with its noble metal or oxide to form a passivation layer. Silica and carbon-based (graphene, graphene oxide, etc.) shell materials are frequently chosen coating materials. MNPs and their encapsulations have been created using a variety of production techniques. Solvothermal synthesis could be used to create magnetic nanoparticles like Fe3O4. On the other hand, biocompatible surfaces can be created by encapsulating magnetic nanoparticles in various substances, including graphene. Although too many techniques were explored to encapsulate MNPs in graphene, chemical vapor deposition is one of the more effective ones. In this research, Fe3O4 and Fe3O4@rGO nanoparticles are synthesized by the solvothermal method, and for having high crystallinity and removal of organic compounds, the calcination process is applied by argon gases. Furthermore, encapsulation studies were carried out by feeding these substrates to the chemical vapor deposition (CVD) system and using methane (CH4) and hydrogen (H2) gases. The temperature (950°C), holding times (1 h), system pressures (50 mbar), and gas flow rates (100 mL/min) were investigated as variables. Leaching steps using HF and HCl acid solutions ensure the synthesis of pure powders free of uncoated Fe3O4 and demonstrate the chemical stability of synthesized nanoparticles. According to the magnetic saturation and coercivity values obtained from VSM tests, synthesized Fe3O4@rGO@graphene nanoparticles have soft ferromagnetic properties that demonstrate potential for biomedical and environmental applications. Magnetic saturation and coercivity values of Fe3O4@rGO@graphene were determined as approximately 139 emu/g and 402 Oe. Second, they are functionalized by coating Fe3O4@rGO@graphene core-shell nanoparticles with PMA-POEGMA polymer via Atom Transfer Radical Polymerization (ATRP). Cytotoxicity tests were carried out to demonstrate the usage of these nanoparticles in biomedicine. These nanoparticles were tested for biocompatibility (biocompatibility on MCF7 cancer cells for up to 48 h). In conclusion, Fe3O4 and Fe3O4@rGO nanoparticles were made via solvothermal synthesis. The calcination procedure is carried out using argon gases to achieve high crystallinity and the elimination of organic contaminants. These substrates were fed into the chemical vapor deposition (CVD) system together with methane (CH4) and hydrogen (H2) gases to conduct encapsulation tests. After leaching by HF and HCl acid solutions, Fe3O4@rGO@graphene nanoparticles are coated with PMA-POEGMA polymer. Then biocompatibilities were carried out to evaluate the materials' potential for biomedical applications (biocompatible up to 48 h on MCF7 cancer cells). The graphene encapsulation investigations are another thesis goal that optimization experiments on multilayer graphene-encapsulated magnetic nanoparticles (Fe3O4@rGO) in the CVD system. This work offers a novel contribution to the literature in terms of analyzing the biocompatibility of the encapsulated products made possible by optimizing the chemical vapor deposition technique. These magnetic nanomaterials, created in core/shell structures under optimal conditions and whose biocompatibility has been demonstrated by cytotoxicity testing, are candidates for biomedical applications.
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ÖgeYüksek entropi (HfTiZrTa/Cr)B2 esaslı seramiklerin farklı yöntemler kullanılarak sinterlenmesi ve karakterizasyonu(Lisansüstü Eğitim Enstitüsü, 2023-06-01) Aysel, Esin ; Ağaoğulları, Duygu ; 506191411 ; Malzeme MühendisliğiSon on yıldır, yüksek entropi alaşımları (YEA) adı verilen yeni malzemeler yaratmak üzere, yüksek konsantrasyonlarda çoklu ana elementlerin kombinasyonunu içeren yeni bir alaşım stratejisi ortaya çıkmıştır. Yapılan bazı çalışmalarda, yüksek entropi alaşımlarının, geleneksel alaşımlara göre daha üstün özelliklere sahip olduğu görülmüştür. Yüksek sertlik, yüksek mukavemet ve yüksek aşınma direnci gibi bazı özellikler, YEA'ları bazı kullanım alanları için (termoelektrik, manyetokalorik, süper iletken ve kataliz malzemeleri) çekici kılmaktadır. Ayrıca, YEA'lar, özel ekipmanlara ihtiyaç duyulmadan üç farklı konvansiyonel yol ile üretilebilmektedir. Birincil yöntem, ark ergitme, elektrik dirençli eriyik katılaştırma ve lazerle tasarlanmış net şekillendirmeyi içeren sıvı hal sentezi; İkincil yöntem, mekanik alaşımlama, yüksek enerjili bilyalı öğütme ve spark plazma sinterleme dahil olmak üzere katı hal sentezi; son olarak da plazma püskürtme işlemi, termal püskürtme ve magnetron püskürtme dahil olmak üzere gaz fazdan sentezdir. Yüksek entropi alaşımlarını sentezlemek popüler olmasına rağmen, çok yüksek işlem sıcaklığı gerektirir. Ayrıca, bu üretim esas olarak homojen hale getirmek için kapsamlı ısıl işlem gerektiren dendrit mikroyapısına yol açar. Yüksek entropi alaşımlar, bulk halinde oksitler, borürler, karbürler, nitrürler, silisitler ve florürler olarak sentezlenebilmektedir. Yüksek entropi metal borürleri, seramiklerin düşük yoğunluk, mükemmel yüksek sıcaklık mukavemeti, yüksek aşınma ve korozyon direnci ve spesifik fiziksel (optik, elektriksel ve manyetik) özellikler gibi üstün özelliklerinin kombinasyonuna sahip olabilmektedir ve bu özellikler yüksek entropi borürlerine geniş bir yelpazede kullanım potansiyeli (havacılık, güneş enerjisi sektörü, nükleer reaktörler, kesici uçlar, metalurji sektörü, mikroelektronik, vs.) sağlamaktadır. Yayınlanmış literatürde, yüksek entropi borür tozları, çoğunlukla parçacık boyutunu düşürmek ve yüksek entropi borürlü seramiklerin düşük sıcaklıkta yoğunlaşmasını hedeflemek için yüksek enerjili bilyalı öğütme (HEBM) yoluyla mekanik olarak alaşımlandırılarak elde edilmiştir. Bu çalışmada, (HfTiZrTa/Cr)B2 bazlı yüksek entropi borür seramiklerinin elde edilmesi için, ilk olarak hibritleştilecek olan borür tozları ucuz oksit hammaddelerinden hareketle yerli bor oksit ve magnezyum redüktan varlığında, oda sıcaklığında mekanokimyasal sentezleme ve takibindeki liç işlemi ile saflaştırma yöntemleri kullanılarak HfB2, ZrB2, TiB2, TaB-TaB2, CrB-CrB2-Cr3B4 tozlarının sentezi gerçekleştirilmiştir. Bu temel kompozisyona ilaveten, W-B, Mo-B, Mn-B tozları da optimum koşullarda aynı metot kullanılarak üretilmiştir. Tozların sentezlenmesinde çelik kap ve bilyalar kullanılmış, bilya/toz ağırlık oranı 10/1 oranı olarak seçilmiştir. Liç sonrası elde edilen tozların faz ve morfoloji karakterizasyonu X-ışınları difraktometresi (XRD) ve partikül boyut analizi ile gerçekleştirilmiştir. Ayrıca, sentezlenen tozların yoğunluk ölçümleri de piknometre ile yapılmıştır. Elde edilen bu tozlar daha sonra eş molar olacak şekilde harmanlanıp tungsten karbür kap ve bilyalar kullanılarak yine bilya-toz ağırlık oranı 10:1 seçilip, 6 sa bouyunca öğütülerek hibrit hale getirilmiş, basınçsız sinterleme ve spark plazma sinterleme (SPS) yöntemleri ile sinterlenerek bulk hale getirilmiştir. Elde edilen sinter bünyelerde, faz karakterizasyonu için X-ışınları difraktometresi, taramalı elektron mikroskobu/enerji dağılımlı spektroskobu (SEM/EDS) kullanılmıştır. İlaveten Vickers sertlik testi, aşınma testi ile profilometre ölçümleri, ve Arşimet yöntemi kullanılarak yoğunluk ölçümleri gerçekleştirilmiştir. Mekanokimyasal yöntemle hibrit hale getirilen (HfTiZrTa/Cr)B2 esaslı tozların partikül boyutları lazer kırınım teknikleri ile analiz edilmiş ve öğütme işlemi nedeniyle tozların partikül boyutunun azaldığı görülmüştür. Toz morfolojisi, elementel kompozisyon ve dağılım, taramalı elektron mikroskobu / enerji dağılımlı spektrometre (SEM / EDS) ile karakterize edilmiştir. Son olarak, öğütülmüş tozların yoğunluğu He gaz piknometresi ile ölçülmüştür. Bu hibrit tozlar daha sonra 1650 ̊C de atmosfer kontrollü fırınında basınçsız olarak sinterlenmiş, X-ışınları difraktometresi karakterizasyonu yapıldığında tek faz olacak şekilde elde edilemediği görülmüştür. Farklı bir yöntem kullanılarak tek faz olacak şekilde sentezlenmek istenen yüksek entropi borürleri, SPS yöntemi ile 2000 ̊C'de sinterlenerek bulk halde elde edilebilmiştir. Bu bulk numunelere taramalı elektron mikroskobu / enerji dağılımlı spektrometre (SEM / EDS) analizi yapılmıştır. Daha sonra Arşimet prensibi ile yoğunluk ölçümleri gerçekleştirilmiş, SPS ile üretilmiş numunelerde yoğunluğu en yüksek numunenin 8,4788 g/cm3 yoğunluk değerine sahip olduğu görülmüştür. Bu numunelere Vickers sertlik analizi yapılmıştır ve en yüksek sertlik değerine sahip olan SPS numunesinin 22,25 ± 1,31 GPa değerinde olduğu bulunmuştur. Bu değerlerin literatür ile karşılaştırıldığında uyumlu olduğu görülmüştür. En son olarak da sinter numunelerin aşınma karakterlerini incelemek için WC bilyalar ile bilya-disk tipi aşınma testi uygulanmıştır. Aşınma testi sonucunda aşınma özellikleri yüzey profilometresi yardımıyla ölçülen aşınma hacmi kaybından hesaplanmıştır ve aşınma izi optik mikroskop cihazı ile incelenmiştir.