Gama ışınlarına maruz kalmış ZrO2 ince filmlerin karakterizasyonu
The characterization of ZrO2 thin films irradiated with gamma radiation
Yazar: DEFNE ABAYLI
Danışman: PROF. DR. NİLGÜN BAYDOĞAN
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Nükleer Mühendislik = Nuclear Engineering

Özet:
Zirkonyum oksit veya zirkonya (ZrO2) ince film, yüksek termal ve mekanik direnç, yüksek korozyon direnci, yüksek dielektrik sabiti, yüksek kırılma indisi ve mor ötesi-görünür-kızıl ötesi bölge olmak üzere geniş aralıkta optik saydamlık gibi eşsiz özelliklerinden dolayı son zamanlarda dikkat çeken zehirli olmayan inorganik bir malzemedir. Bu nedenle, ZrO2 ince film kaplamalar, teknolojik uygulamalar ve sanayide geniş kullanım alanı bulmaktadır. Ayrıca, ZrO2 ince filmlerin sahip olduğu bu üstün özelliklerinin daha da geliştirilmesi için araştırmalar devam etmektedir.  
ZrO2 ince filmler sahip olduğu bu optik, elektrik ve kimyasal özelliklerinden dolayı, güneş pillerinde yansıtıcı ve yansıtıcı olmayan optik kaplamalar, optik yükseltici, fotonik aletler gibi optik uygulamalarda, yakıt hücreleri, jet ve dizel motorlar gibi yüksek sıcaklık uygulamalarının söz konusu olduğu termal bariyer kaplamalarda, oksijen hassasiyetinden dolayı sensör teknolojilerinde, yüksek dielektrik sabiti, düşük kaçak akım gibi gibi elektrik özelliklerinden dolayı yarı iletken teknolojisinde tercih edilen bir malzemedir. Ayrıca, hem termal kararlılığı hem de radyasyona karşı direnci sayesinde de nükleer rekatör yakıt hücrelerinde de kullanılmaktadır.
Söz konusu yüksek lisans tez çalışmasında, ZrO2 ince filmler, sol-jel daldırma yöntemi ile soda kireç silika cam taşıyıcılar üzerinde kaplanmıştır. Daha sonra Co-60 radyoizotop kullanılarak gama ışınlarına maruz bırakılmıştır. Gama radyasyonu ile ışınlama ZrO2 ince filmlerin yapısal özelliklerini etkilediğinden, bu etkileşimi belirleyebilmek için dört farklı soğurulma doz seviyesi uygulanmıştır. 
Gama ışınları ile ışınlama işleminden sonra ZrO2 ince filmlerin yapısal özelliklerindeki değişimler taramalı elektron mikroskobu ile belirlenmiştir. Doz artışına bağlı olarak değişimler detaylı olarak incelenmiştir. ZrO2 ince filmlerin yapısal özelliklerindeki değişim optik özellikleri de etkilemiştir.  Bu nedenle, ZrO2 ince filmlerin optik özelliklerindeki değişimler 190–1100 nm dalga boyu aralığında 
PG Instruments T80 UV-Vis-NIR spektrofotometre ile belirlenmiştir. ZrO2 ince filmlerin soğurulan doza bağlı olarak geçirgenlik, yansıtıcılık ve soğurma eğrilerindeki ve optik sabitleri (kırılma indisi, söndürme ve soğurma katsayısı)'ndeki değişimler ayrıntılı olarak analiz edilmiştir. Ayrıca, optik yoğunluk, elektronik geçiş özellikleri ve optik bant aralıkları belirlenmiştir. ZrO2 ince filmlerin yüzey özelliklerinin ışınlamaya bağlı olarak değişimi belirlemek adına da temas açısı ölçümleri alınmıştır. Soğurulan doza bağlı olarak hidrofilik yapısındaki değişim değerlendirilmiştir. 
Ayrıca ZrO2 ince filmlerin, soğurulan doza bağlı olarak gama ve beta ışınlarını geçirme özellikleri incelenmiştir. ZrO2 ince filmlerin gama ve beta ışınlarını zayıflatma katsayıları soğurulan doz artışına bağlı olarak detaylı bir şekilde belirlenmiştir. ZrO2 ince filmlerin morfolojik yapısını, optiksel özelliklerini etkileyen doz değeri ayrıntılı bir şekilde tespit edilmiştir.

Abstract:
Zirconium oxide or zirconia thin film (ZrO2) have attracted special attention due to advantageous properties like good mechanical, chemical, thermal, optical and electrical properties for future thin layers technology. Additionally, it has a high refraction index, a very good transparency in the visible and near infrared region, large optical band gap, high dielectric constant, low leakage current level, a great chemical stability, high corrosion resistance and short wavelength photoluminescence (PL) emission property.
Because of all these properties, ZrO2 thin film is widely used as essential material in several applications such as broadband interference filters, high- reflectivity mirrors, active electro-optical devices, optical coatings, photonic devices, solar energy converters, in semiconducter and super-conducter technology, fuel cells, radiation shielding, biomaterial for prostheses, gas sensor, scintillators and luminescent oxygen-sensors etc. 
The optical properties of films are closely correlated with the microstructure of the films. Because of the link between them has been widely investigated. Nowadays, a lot of work still investigate the correlation between the structural character and the optical properties of ﬁlms. ZrO2 thin films with different morphologies are prepared by several methods such as chemical vapor deposition (CVD), spray pyrolysis, reactive RF sputtering, polyol route, hydrothermal method, bio template method, ultra-sound assisted precipitation method, room temperature precipitation method, solution combustion method and sol–gel method. 
Among these techniques, the sol–gel method can be used in the produce of pure or doped oxide films for electronics and optics as an environment-protective method. The sol–gel method arouses interest because of several advantages such as its simplicity, low processing temperature, stoichiometry control and its ability to produce uniform, chemically homogenous films and simultaneous coating of two faces and possibility of forming multi and mixed layers. The sol–gel process involves the preparation of 
a 'sol' (mostly colloidal), the transformation of the sol into a solid 'gel' phase and crystallization by heating at increased temperatures (calcination). 
In the present thesis, ZrO2 thin film samples were produced by sol-gel dip coating method and irradiated by gamma radiation. A Co-60 radioisotope was used as a gamma ray source emitting the photons with two different energies (1.17 MeV and 1.33 MeV) and it was provided by assuming a monochromatic gamma rays beam at 
~ 1.25 MeV. The used Co-60 radioisotope had an activity level of 0.018021 Ci. Four different absorbed dose levels (0.430 Gy, 0.672 Gy, 1.195 Gy and 2.690 Gy) were applied to the ZrO2 thin films. The irradiation process was performed in room temperature. The thicknesses of irradiated ZrO2 thin film samples were determined in the range ~ 80 nm by using a surface profilometer. 
The application of the ionizing radiation is key parameter to control the optical properties and microstructures of the ZrO2 thin film. The ionizing radiation like gamma radiation can change the microstructure and surface morphology of films, so their important characteristics may be controlled for the use in technological applications containing these thin films. It was investigated the changes in optical and structural properties of ZrO2 thin films with the rise of the absorbed dose. Hence, it was possible to examine the details on the behaviours of the optical properties with the evaluation of the structural characterization of the irradiated ZrO2 thin film samples.
The structural changes of the irradiated ZrO2 thin films were evaluated by scanning electron microscope (SEM) according to increase of absorbed dose. The changes of the optical properties of the irradiated films supported the gathering of the grains at the surface of the film with the rise of the absorbed dose according to the SEM images.
The irradiation treatment by gamma radiation affected the surface morphology and optical properties of the ZrO2 thin films. According to the SEM images, the dimension of grains in which induced by gamma irradiation at 0.672 Gy was the highest one with respect to the dimension of grains in other ones. The properties of the surface started to change obviously at 0.672 Gy. The absorbed dose of 0.672 Gy was important to investigate the optical properties of the thin film. It was interesting to note that when the absorbed dose of the specimen reached to 1.195 Gy, the dimension of the grains decreased again. Moreover, the gathering of the grains in the induced film took place more obviously at the certain places on the surface of the films But it was determined that there was the combination of grains with each other on the surfaces of irradiated specimens. The changes of the thin film grains can be explained with the formation of new electronic order in defect centers while the gathering of small grains according to the absorbed dose.
The radiation shielding properties of a material are stated with the term linear attenuation coefficients, as the probability of radiation interacting with a material per unit path length. The magnitude of linear attenuation coefficients can vary with the incident radiation energy, the atomic number and the density of the shielding materials. The linear attenuation coefficient of a material can be measured experimentally using the application of Lambert–Beer's law with standard transmission method.
The changes in the linear attenuation coefficient for gamma and beta radiation of irradiated ZrO2 thin film samples were determined using transmission method. The details in characteristic properties were assessed with respect to two different activity levels (respectively, 2558-0.0513 µCi) of Sr-90 radioisotope for beta transmission method and Co-60 and Cs-137 radioistope for gamma transmission method. 
The changes of gamma and beta transmission of the ZrO2 thin films supported the variation of the grain size. It was determined that the linear attenuation coefficient for gamma and beta rays has increased linearly up to 0.672 Gy and then decreased to 2.690 Gy after the investigation of the gamma and beta transmittance method. 
The optical properties of ZrO2 thin films were determined by using optical transmittance and the reflectance measurements in the range between 190 – 1100 nm obtained from PG Instruments T80 UV-Vis spectrophotometer. The changes in the optical constants such as the extinction coefficient, absorption coefficient and refractive index, optical band gap and optical density determined with the rise of the absorbed dose.
The changes in optical transmittance, reflectance and absorbance of the irradiated ZrO2 thin film samples were examined with the rise of the absorbed dose. The rise of the grain size according to the irradiation resulted with the decrease of the optical transmittance and reflectance and the increase of the optical absorbance of the irradiated thin film at 0.672 Gy. 
The optical constants of ZrO2 thin films were affected by the gamma irradiation. It was possible to determine the changes in optical constants such as the refractive index (n), extinction coefficient (k) and absorption coefficient (α) with the rise of the grain size in this study. There were the increases in n, k and α values of the irradiated ZrO2 thin film as the result of the increase in grain size. The increase of the optical constants of the irradiated films was related with the gathering of the grains at the surface of the film with the controlling of the absorbed dose according to the SEM images. Photoluminescence studies provide information about the electronic band transitions, structure, defects and chemical composition of the optical materials. For ZrO2 thin film, the photoluminescence is of much interest for both theoretical and experimental investigations. A broad photoluminescence emission is observed at ~480 nm for ZrO2 structures after the samples are induced by UV irradiation in the literature. 
In this study, it was determined that there were the dramatic changes in optical constants at ~ 480 nm. The photoluminescence effect at ~480 nm supported the rise of the optical transmittance and decrease of the optical absorbance slightly depending on the increase of the scattered photons. Besides, the decrease of the optical absorbance has led the decrease of the optical constants over ~ 480 nm. It was thought that the improvement of the grain sizes in irradiated ZrO2 thin film at 0.672 Gy indicated the enhancement of the photoluminescence effect dominantly due to the coarse grains.
Similarly, the maximum change in optical density was determined at the thin film irradiated at 0.672 Gy which had coarse grains. Because of the photoluminesans effect over ~480 nm in the film structure, there was a characteristic change in optical density of the irradiated thin films especially at 0.672 Gy. It was determined that irradiated ZrO2 thin films had direct band transition and the optical band gap was decreased with the rise of the grain size.
In addition, the contact angle measurement was applied to the irradiated ZrO2 thin films and the changes of contact angle were determined according to the increase absorbed dose. The contact angle values of the ZrO2 thin film decreased slightly and a minimum value of around 24.62° was obtained with the increase of the grain size when the absorbed dose was set to 0.672 Gy. The rise of the grain size led to increase the surface roughness and the decrease of contact angle values of the thin film resulted with the improvement of the hydrophilic properties. Hence, the contact angle was decreased by irradiation and wettability of hydrophilic nanostructured ZrO2 surface was modified slightly with increasing surface roughness. The nanoscale roughness having coarse grains (~87 nm) has influenced the surface wettability slightly at
0.672 Gy. The ZrO2 thin film derived on soda-lime glasses has presented a relation between the refractive index and hydrophilic properties as the result of the change in the size of the grains.
In the present thesis, the optical properties of ZrO2 thin film irradiated at 0.672 Gy was enhanced as more anti-reflective and hydrophilic. The optical and surface properties of coatings with ZrO2 thin film can be improved by irradiation with ionizing radiation. The applications including ZrO2 coating irradiated by ionizing radiation can use in many industrial field such as optic, semiconducter, optoelectronics, automotive, etc.