Termokromik Özellik Gösteren Vanadyum Oksit Esaslı İnce Film Kaplamaların Sol - Jel Yöntemi İle Hazırlanması Ve Karakterizasyonu

Abstract

Dünyada tüketilen enerjinin önemli bir kısmı bina pencerelerinden kaybedilmektedir. Global düz cam pazarı günümüzde bu konu üzerinde yoğun çalışmalar yapmakta ve enerji tasarrufu sağlayan akıllı camlara yönelmektedir. Akıllı camlar, dinamik ve statik olmak üzere iki gruba ayrılmaktadır. Statik akıllı camlar düşük emisivite (Low-E) kaplamalı camlar olarak adlandırılır. Low-E camlar çeşitli saydam metal oksit kaplamalar ile ısı kontrolü sağlayabilen camlardır. Dinamik akıllı camlar ise çevre koşullarına göre optik özelliklerini değiştirebilen camlar olup; elektrokromik ve termokromik camlar olarak iki ana başlıkta sınıflandırılabilir. Elektrokromik camlar elektrik akımı değişimi ile optik özelliklerini değiştirerek ışık kontrolü sağlarken, termokromik camlar sıcaklık değişimi ile optik özelliklerinde değişim göstererek ısı ve ışık kontrolü sağlar.Vanadyum dioksit (VO2), yarı iletken (monoklinik) yapıdan metal (tetragonal) yapıya dönüştüğü 68 °C kritik termokromik dönüşüm sıcaklığı ile enerji tasarruflu pencerelerde kullanılma potansiyeli en yüksek olaninorganik esaslı termokromik malzemedir. Termokromik özellik gösteren VO2, kritik dönüşüm sıcaklığı üzerinde yakın kızıl ötesi ışın geçirgenliğini düşürmekte, görünür bölge geçirgenlik değerlerini ise büyük ölçüde korumaktadır. VO2 ince film eldesi için farklı oksidasyon derecelerindeki vanadyum oksit başlangıç malzemeleri kullanılabilmektedir. Başlangıç malzemeleri ile farklı formülasyonlarda hazırlanan malzemelerin cam yüzeyine kaplanmasıkimyasal buhar biriktirme (CVD), püskürtme (sputtering), lazer biriktirme, fiziksel buhar biriktirme (PVD) ve sol-jel gibi yöntemler ile gerçekleştirilmektedir. Daha sonra hedeflenen oksidasyon seviyesine ulaşmak için ise değişen sıcaklıklarda, atmosferlerde ve sürelerde ısıl işlem uygulanmaktadır. Sol-jel kaplama yönteminin diğer kaplama yöntemlerine göre avantajları, görünür bölgede yüksek geçirgenliğe sahip numunelerin elde edilebilmesi, ucuz olması, büyük ölçek ve seri üretime uyarlanmasının kolay olmasıdır. Bu tez çalışmasında, sol-jel yöntemi kullanılarak hazırlanan vanadyum oksit esaslı çözeltiler daldırarak kaplama yöntemi ile ince film halinde soda-kireç camlar üzerine kaplanmıştır. Hazırlanan çözeltiler farklı vanadyum oksit esaslı başlangıç malzemeleri (vanadyum pentaoksit, vanadyum triisopropoksit, vanadyum oksi asetilasetonat) ve formülasyonlar kullanılarak elde edilmiştir. Literatürde belirtildiği gibi cam altlıklardan ince film tabakasına olası sodyum atağını önlemek amacıyla seçilen bazı numunelere daldırarak kaplama yöntemi kullanılarak SiO2 ara katman kaplanmıştır. Daha sonra vanadyum oksit esaslı çözeltiler, ara katmanlı ve ara katmansız olarak hazırlanan soda-kireç camlarına, daldırarak kaplama yöntemi ile kaplanmıştır. Bu tez çalışması kapsamında, ara katmanlı ve ara katmansız olarak üretilen vanadyum oksit esaslı ince film kaplı camlara farklı gaz atmosferleri altında (N2 ve hacimce %5 hidrojen içeren argon-hidrojen karışımı, Ar/H2-%5), farklı sıcaklıklarda (500 °C ve 550 °C) ve farklı sürelerde (1 saat ve 2 saat) ısıl işlem uygulanmıştır. Uygulanan ısıl işlemler ile vanadyum oksit esaslı bileşiklerden, termokromik özellik gösteren, +4 değerlikli VO2 fazınıelde etmek amaçlanmıştır.Ayrıca hazırlanan vanadyum oksit esaslı çözeltiler, 24 saat 80 °C’de kurutularak toz formda numuneler elde edilmiştir. Toz numuneler 2 saatlik sürelerde, 500 ve 550 °C’de, N2 ve Ar/H2(%5) atmosferleri altında ısıl işleme tabi tutulmuştur. Elde edilen numunelerin ısıl özelliklerinin tespiti ve ısıl işlem sıcaklığının öngörülmesi amacıyla TG-DTA analizleri gerçekleştirilmiştir. İnce film ve toz halde elde edilen numunelerin faz analizleri için ince film ve toz XRD yöntemleri kullanılmıştır. İnce film kaplı numunelerin termokromik özelliklerin incelenmesi amacı ile özel olarak tasarlanmış dahili sıcaklık kontrol üniteli UV-Vis spektrofotometre analizleri ile numunelerin geçirgenlik değerleri tespit edilmiştir. Literatürde termokromik özellik gösteren VO2 esaslı ince film kaplamalı camlar hakkında çok sayıda çalışma olmasına rağmen; bu çalışmalar genel olarak tek bir başlangıç malzemesinden yola çıkarak VO2 esaslı termokromik ince film eldesine yöneliktir. Bu tez çalışmasında farklı başlangıç malzemeleri kullanarak, farklı formülasyonlar ve ısıl işlem parametrelerinin VO2 ince film eldesine ve numunelerin termokromik özelliklerine etkisinin incelenmesi hedeflenmiştir.

A considerable amount of energy is used for heating and cooling systems in buildings, while a large portion of this energy is lost through windows. In recent years there has been a lot of attention and research on energy efficient smart glasses which is now a requirement for today’s growing glass market. There is a wide range of smart glasses such as Low-E coatings, micro blinds, dielectric/metal/dielectric (D/M/D) films and switchable reflective devices like electrochromic, gasocromic, liquid crystal glazing and thermochromic windows. Smart glasses can be separated into two groups as dynamic and static smart glasses and static smart glasses are prepared by applying low emmisivity (Low-E) coatings. Low-E transparent oxide coatings provide energy savings by means of heat insulation. On the other hand, dynamic smart glasses have the ability to adjust their optical properties in accordance with the environmetal conditions. Thermochromic and electrochromic glasses are the two main classes of dynamic smart glasses. Electrochromic glasses change their optical properties with electric current difference and they provide light control. Thermochromic glasses on the other hand change their optical properties with changing temperature and provide control of both light and heat. Owing to their low cost preparation, simple structure, no wiring necessity and good ability to visible transmission, thermochromic thin film coated glasses showing spectrally selective properties have emerged as an innovative solution to the energy inefficiency problem in buildings and they become today’s cutting-edge products in smart windows. Vanadium dioxide (VO2) shows a semiconductor (monoclinic) to metal (rutile) transition at 68 °C which is defined as the critical thermochromic transition temperature. Due to its lower thermochromic transition temperature VO2 has a huge potential to be commercially used in smart windows. Above this critical temperature, thermochromic VO2 represents near infrared transmittance decrease and does not show a significant change of transmittance in visible the range. Therefore, VO2 is a good candidate for optical switching devices, caution sensors, optoelectronic applications and smart windows. For obtaining VO2 thin films, different vanadium precursors with different oxidation states can be used. These thin films synthesized with different precursors and formulations can be produced by chemical vapor deposition (CVD), physical vapor deposition (PVD), laser deposition, sputtering and sol-gel methods. Following the coating process an annealing process is applied at variable temperature, time and gas atmospheres. Sol-gel process has some advantages such as, cost effectiveness, high visible transmittance, ease of production and suitability for large scale production compared to other coating methods. Sol-gel process is based on hydrolysis and condensation of the precursors. Generally, it comprises a molecular precursor such as metal alkoxides, metal oxides, an acidic catalyst and a solvent. In this dissertation work vanadium oxide-based solutions were prepared with different precursors and formulations. Five valence precursors, vanadium pentoxide (Alfa Aesar, 99.9% purity) and vanadium triisopropoxide (Alfa Aesar, 96% purity); four valence precursor, vanadium oxyacetylacetone (Alfa Aesar) were used for the synthesis of sols. On the other hand a polymer (polyvinylpyrrolidone (PVP)(MW: 1300000, Sigma Aldrich)) was added to the solution preparedwith vanadium ocyacetylacetone precursor to facilitate the film formation and reduction process. Totally five different solutions were prepared: First solution (SOL-1) with the vanadium triisopropoxide precursor, second solution (SOL-2) with the vanadium ocyacetylacetone precursor, third solution (SOL-3) with the vanadium ocyacetylacetone precursor and PVP, fourth solution (SOL-4) with the vanadium pentoxide precursor and oxalic acid catalyst, fifth solution (SOL-5) with the vanadium pentoxide precursor and hydrogen peroxide catalyst. Solutions were coated on soda-lime glass slides (Corning® 2947) by dip coating method. As mentioned in the literature, to prevent possible alkali attack, some selectedglass slides were coated with SiO2 interlayer by dip coating method and dried at 80 °C for 20 minutes in an oven. Interlayer solution was prepared using tetraethyl orthosilicate (TEOS) (Sigma Aldrich) precursor. SiO2 coated slides were annealed at 500 °C for 1 hour to obtain better crystallization and surface adhesion. Afterwards, glass slides with and without interlayer were coated with vanadium solutions and dried at 80 °C for 20 minutes in an oven. Coating and drying steps were repeated 4 times for each sample. Vanadium oxide-based thin film coated glasses with and without interlayer were annealed under different gas atmospheres (N2 and 5% volume fraction of H2 in Ar-H2 mixture, Ar/H2-5%), different temperatures (500 °C and 550 °C) and different durations (1 hour and 2 hours). Annealing processes were applied to obtain thermochromic four valence VO2 phase from vanadium oxide-based compositions. To obtain powder samples, each vanadium oxide-based solution dried at 80 °C for 24 hours in an oven. Powder samples were annealed under N2 and Ar/H2-5% atmospheres at 500 °C and 550 °C for 2 hours. In the experimental studies, 48 thin film samples and 17 powder samples were investigated. Thermal characterizations of the samples were realized using thermogravimetric differential thermal analysis method (TG-DTA) to predict and confirm the reduction temperatures which were also confirmed with the literature. Phase characterizations of the thin film and powder samples were carried out by thin film and powder X-ray diffraction (XRD) analysis. For the investigation of thermochromic properties, a custom made in-situ temperature controlled UV-Vis spectrophotometer was used. Transmittance of each thin film sample was measured at 25 °C and 100 °C in the 360-1100 nm wavelength range. It was detected that before the annealing process there is no difference in the transmittance values of the samples. However, after annealing all thin film coated samples show transmittance decrease in the studied wavelength range. Samples which include PVP showed more pronounced transmittance decrease compared to other samples. Alkali attack was observed in SOL-1 coated thin film samples. Some thin film samples coated with SOL-1 with interlayer showed a significant transmittance decrease above 1000 nm wavelength at 100 °C compared to room temperature, which is an indication of the thermochromic change. Samples coated with SOL-2 showed a similar thermochromic transmittance decrease at all annealing conditions except for 500° C- 1 hr. - N2 condition. This transmittance decrease was also observed for SOL-3 coated thin film samples annealed under 550° C for 2 hrs. under Ar/H2-5%. From the XRD analysis it was found that the VO2 monoclinic phase crystallizes at 2θ = 28 ° for powder samples and at 2θ= 44 °peak for thin film samples (ICDD card number: 03-065-2358). This difference between powder and thin films may be caused by different orientation. When annealing temperature increased the peak intensities were also increased as expected. Pure monoclinic VO2 phase was observed in SOL-2 powder samples. SOL-1 powder samples annealed under Ar/H2-5% atmosphere, represent a monoclinic VO2 phase with V2O5 impurities. This result confirms the literature knowledge, which mentions five valence precursors need a reductive atmosphere for their reduction to four valence VO2. SOL-3 powder samples have amorphous character, which may be caused due to their polymer content. SOL-2 (Vanadium oxyacetylacetone precursor based) coated thin film samples showed featured results, when the results evaluated in terms of purity of VO2 phase in powder form samples, no observation of alkali attack on thin film layers, thermochromic properties and improvable visible transmittance of thin films. When annealing conditions were evaluated and thermochromic properties of the thin filmcoated glasses were investigated, it was concluded that annealing temperatures at 500 °C and 550 °C for 1 hour gave the best results. Although there exist numerous studies in the literatureon vanadium oxide-based thin film coated thermochromic glasses, most of these work have been focused on the preparation of thermochromic thin films using solely one type of precursors. However, in this dissertation work, effect of different precursors, formulations and annealing parameters on the formation of VO2 based thin films and their thermochromic properties were examined.