Sol-jel yöntemi ile süperhidrofobik kaplama üretimi
Production of superhydrophobic coating by sol-gel method
Yazar: ATAKAN BARKINAY
Danışman: DOÇ. DR. YILDIZ YARALI ÖZBEK
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Hidrofobik kaplamalar = Hydrophobic coatings ; Sol-jel yöntemi = Sol-gel method ; Titanyum dioksit = Titanium dioxide

Özet:
Süperhidrofobik kaplamalar günümüzde güneş panellerinde, korozyon korumasında, buz fobiklik, buğu önleyici vb. alanlarda kullanılmaktadır. Yüzey pürüzlülüğün artması sonucu sıvı damlacıkları yüzeyin üzerinde kayar ve tozları yüzeyden zahmetsizce temizler. Bunun en büyük örneği güneş panelleridir. Süperhidrofobik yüzey sayesinde panellerin üzerindeki tozlar su yardımıyla temizlenir. Bir yüzeyin süperhidrofobik olması için 150 °'nin üzerinde bir temas açısına gerek duyulmaktadır. Normal bir yüzey ile süperhidrofobik bir yüzey arasındaki fark, süperhidrofobik yüzeyde hava cepleri olmasıdır ve damlaların bu ceplerin üzerinde kayması sonucu oluşur. 

Sol-jel yöntemi; metal kaynağının uygun bir alkol içinde çözünmesi ve daha sonra su ve asit ilave ederek önce sıvı ve akışkan 'sol', daha sonra hidroliz ve kondenzasyon reaksiyonları ile yaşlanarak kıvamlı 'jel' yapısı oluşturmasına verilen isimdir. Sol-jel yöntemi yedi adımdan oluşur. İlk aşama karışma aşamasıdır. Bu aşama da uygun pH ortamında malzemeler karıştırılır. İkinci adım dökümdür. Bu aşamada sıvı sol bir kalıba dökülür. Üçüncü adım jelleşmedir. Sol içinde çapraz bağlanmalar meydana gelir ve buda solün viskozitesinin artmasına sebep olur. Dördüncü adım yaşlanmadır. Bu aşamada polikondenzasyon reaksiyonları devam eder ve jelin dayanıklılığı artar. Beşinci adım kurutmadır. Bu aşamada sıvı gözeneklerden uzaklaştırılır. Eğer dikkatli olunmazsa jelde çatlaklar olabilir. Altıncı adım dehidrasyon. Bu aşamada jelin içindeki -OH moekülleri uzaklaştırılır. Yedinci ve son adım ise yoğunlaşmadır. Bu aşamada yüksek sıcaklıklara çıkılır ve yüzey yapısı değiştirilir. Sol-jel yüzeylere iki şekilde uygulanır. İlk yöntem daldırma kaplama yöntemidir. Bu yöntemde kaplanacak malzeme jelin içine daldırılır ve sonra yavaşça çekilir. Çekilme esnasında üst tarafta bulunan jel kütlesi yerçekimi etkisiyle süzülerek aşağı düşer. Daha sonra ise kurutma işlemi yapılarak bu süreç bitirilir. İkinci adım ise döndürme kaplamadır. Bu yöntemde malzeme döndürülmek üzere bir cihaza konulur. Malzeme yüksek hızlarda dönerken üstüne sol-jel dökülür. Yüksek hızda döndürüldüğü için üstündeki artık jel ayrılır ve malzeme yüzeyinde ince bir film olarak kalır. Daha sonra kurutma işlemine yollanarak süreç tamamlanır. Sol-jel bir çok alanda kullanılır. Kaplama olarak güneş panellerinin yüzeyinde yansıma önleyici olarak ve fotokromik gözlüklerde kullanılır. Elektrokromik olarak akıllı camlarda kullanılır. Ayrıca korozyon korumasında ve toz olarak da kullanılır.

Bu çalışmada süperhidrofobik yüzey üretimi yapılmıştır. Üretim yöntemi olarak sol-jel yöntemi kullanılmıştır. Güneş panelini temsil etmesi için soda-kireç camı kullanılmıştır. Ana malzeme ise titanyum kullanılmıştır. Süperhidrofobik malzeme olarak stearik asit kullanılmıştır. Örneklerin yarısı önce sol-jel kaplanarak, daha sonra stearik asit kaplanmıştır. Diğer yarısı ise sadece stearik asit ile kaplanmıştır. Analiz olarak yüzey açısı ölçümü, yüzey pürüzlülüğü, UV ölçümü, Raman, XRD, FTIR, SEM ve EDS analizleri uygulanmıştır.

Abstract:
Superhydrophobic coatings are currently used in solar panels, corrosion protection, ice phobicity, anti-fog, etc. As a result of increased surface roughness, liquid droplets slide on the surface and effortlessly remove dust from the surface. The biggest example of this is solar panels. Thanks to the superhydrophobic surface, the dust on the panels is cleaned with the help of water. For a surface to be superhydrophobic, a contact angle of over 150° is required. The difference between a normal surface and a superhydrophobic surface is that there are air pockets on the superhydrophobic surface and the drops slide on these pockets.

The ability of a solid surface to get wet is called the water contact angle (wca). This angle is called hydrophobic if it is greater than 90° and hydrophilic if it is less than 90°.  A superhydrophobic surface is used when the contact angle is greater than 150°.  In nature, animals and plants have superhydrophobic surfaces. This is to protect them from contamination and prevent water accumulation. In plants, it is mostly used to clean leaf surfaces. In animals, it is used for various purposes. For example, insects have this feature to prevent their wings from getting wet, while geckos have it for climbing on flat surfaces. The contact angle is defined as the angle formed by the intersection of the liquid-solid interface and the liquid-vapor interface. In 1805 Thomas Young described the equilibrium conditions on an ideal smooth surface. Young also defined hydrophilicity, hydrophobicity and superhydrophobicity with respect to the contact angle. In 1936, Wenzel related surface roughness and surface energy to the contact angle. Wenzel calculated only homogeneous surfaces. In 1944 Cassie and Baxter created another model for heterogeneous surfaces. In Cassie and Baxter's model, it is assumed that the liquid is represented only by roughness surfaces with the solid and that air pockets are trapped under the liquid. In this case, the air is considered to be trapped and part of the surface is not wetted by the liquid. Superhydrophobic surfaces are used for corrosion resistance, anti-fogging, anti-icing and in solar panels. As corrosion protection, the corrosion resistance mechanism of a superhydrophobic surface coating is mostly contributed to by the presence of air pockets between the substrate and the solution, providing an effective barrier against the movement of corrosive ions. as an anti-fog, it is known that fogging is prevented when the contact angle is less than 40°. Ice phobia refers to the low adhesion force between ice and a solid surface. The surfaces of solar panels often become covered with dust over time. Thanks to superhydrophobic surfaces, dust cannot adhere to the surface and the surface of the panel remains clean.

The sol-gel method is to use a compound containing a chemical component with high activity as a precursor, homogeneously mix these raw materials in the liquid phase, and apply hydrolysis and condensation chemical reactions to make a stable sol. The sol-gel process refers to the formation of a liquid suspension called 'sol', a stable colloid solution obtained by hydrolysis and partial condensation of precursors to form metal-oxygen-metal bonds (M-O-M). At the same time, upon condensation, the sol particles form an inorganic 3D network called a 'gel'. Inorganic sols and gels are obtained by various techniques and are generally synthesized directly from chemical reactants dissolved in a liquid medium. A chemical reactant containing M cations is called a precursor. When the reaction starts, solvents may be required to homogenize the reaction mixture of alkoxide systems. The solvent's polority, dipole moment, viscosity and protic and non-protic behavior change the reaction rates and indirectly the structure of the sol-gel coating. Acid and base catalysts can affect hydrolysis and condensation rates and change their structure. The basic definition of gelation is the growth of aggregates as a result of condensation or fragmentation of polymers. The clusters must then form and join together to be called a gel. The aging process is the time taken to wait for the chemicals in the gel to transform the gel into a stable structure. Sintering is a densification process driven by interfacial energy. The dip coating process consists of four stages: dipping the substrate into the solution, drawing the substrate, drainage during and after drawing, and evaporation and drying during and after drawing the substrate. Spin coating consists of four steps: deposition, spinning, separation and evaporation.

In this study, superhydrophobic surfaces were produced. Sol-gel method was used as the production method. Soda-lime glass was used to represent the solar panel. Titanium was used as the main material. Stearic acid was used as superhydrophobic material. Half of the samples were first coated with sol-gel and then with stearic acid. The other half was coated with stearic acid only. Surface angle measurement, surface roughness, UV measurement, Raman, XRD, FTIR, SEM and EDS analyses were performed.

Titanium sol-gel coated on soda-lime glass, stearic acid coated on top and finally stearic acid + TiO2 nanoparticle coatings were made. The sol-gel samples were sintered at 500°C, while only one sample was sintered at 600°C. About half of the samples were uncoated with titanium sol-gel and these uncoated samples were coated with stearic acid and stearic acid + TiO2 coatings. The other samples were first coated with titanium sol-gel and then top coated with the same treatments. Stearic acid was used for the hydrophobic coating. 0.1 g, 0.2 g, 0.3 g and 0.4 g of stearic acid were used. TiO2 nanoparticles were used at a constant 0.4 g.

Contact angle measurement is the measurement of the angle formed by dropping water drops on surfaces. According to contact angle measurements, normal sol-gel coatings showed hydrophilic properties. The contact angle of the sample sintered at 600 °C is 34.8°, while the contact angle of the sample sintered at 500 °C is 55°. The contact angles of the hydrophobic contents were observed between 98°-100°. The combination of stearic acid + TiO2 made the coating superhydrophobic. Sol-gel coated 0.1 g stearic acid was 138.5°, while 0.1 g stearic acid without sol-gel was 135.7°. Sol-gel coated 0.4 g stearic acid was 143.9°. This shows that the contact angle increases when the amount of stearic acid increases. XRD is an analysis technique in which the structure of materials is learned using x-rays. Looking at the XRD results, the ordinary glass sample gave more or less the same value as the other samples (except TiO2 nanoparticles) and sol-gel coatings. Titanium sol-gel was sintered at 500°C. At this temperature titanium is in the anatase phase and is given in the peaks. The samples with nanoparticles gave sharper and clearer peaks. SEM analysis is used to visualize surface images at high magnification using an electron beam. SEM analysis showed that sol-gel coatings have cracks. The coating is clearly visible in cross-sectional images. In samples with nanoparticles, the grains come together and this causes the formation of a superhydrophobic surface. Raman analysis measures the vibration of molecules at different wavelengths and shows the reactions that occur. Raman results are in parallel with XRD results. Sol-gel coatings give the same peak as each other. The stearic acid coating had no effect on this. The samples with nanoparticles showed sharper and clearer peaks. In FTIR analysis, infrared rays are directed at the bonds in the material and data is generated based on the absorption of the molecular bonds. In FTIR analysis, O-H, Ti-OH, Ti-O and Ti-O-Ti bands are seen respectively. There is only Ti-O-Ti band in samples with nanoparticles. This may be due to the nanoparticles. UV absorption analysis is the measurement of the reduction of the beam of light transmitted through the material. In the UV absorption test, the values of the samples sintered at 500° and 600° temperatures are the same. When we look at the hydrophobic coatings, the permeability decreases when the stearic acid content increases. Non-sol-gel coated samples are slightly better in permeability than sol-gel coated samples. Surface roughness analysis is the measurement of the three-dimensional topography of the surface. The surface roughness test shows that sol-gel coatings are smoother. The roughness parameter is 0.371 µm. Therefore, the contact angle of these samples is low. It is about 0.224 µm and finally in the case of superhydrophobic coatings the roughness increases much more, measuring 3.7 µm.