Silisyum katkılı Al-Ti-N solar soğurucu kaplamaların üretimi ve karakterizasyonu
Characterization and production of silicon doped Al-Ti-N solar absorber coatings
Yazar: SERDAR SONAY ÖZBAY
Danışman: PROF. DR. MUHAMMET KÜRŞAT KAZMANLI
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences
Dizin: Alüminyum nitrür = Aluminum nitride ; Fiziksel buhar biriktirme = Physical vapour deposition ; Titanyum nitrür = Titanium nitride

Özet:
Günümüz dünyasında artan enerji ihtiyacını karşılamak ve çevreyi korumak amacıyla yenilenebilir enerji kaynaklarına yönelim her geçen gün artmaktadır. Bu kaynaklar arasında önemli bir yere sahip olan güneş enerjisi, güneş kolektörleri yardımıyla ısı enerjisine dönüştürülebilmektedir. Diğer enerji kaynakları ile karşılaştırıldığında ekonomik olarak yetersiz kalan güneş kolektörlerinin verimi, kullanılan soğurucu malzemenin özelliklerine büyük ölçüde bağlılık göstermektedir.
Solar soğurucu yüzeyler, güneş kolektörlerinde verimin arttırılması amacıyla yüksek soğurma ve düşük emitans değerine sahip malzemelerden seçilmektedir. Bu özelliklerle beraber, malzemenin yüksek sıcaklık oksitlenme direncine sahip olması ve optik özelliklerini yıllar boyu koruması beklenmektedir. 
AlTiN kaplamalar, TiN kaplamalara alternatif olarak birçok uygulamada kullanılan ve üstün mekanik özelliklere sahip malzemelerdir. Yüksek sıcaklık oksitlenme direnci ve aşınma dayanımı gerektiren uygulamalarda başarıyla kullanılan AlTiN kaplamalar, içerisindeki yalıtkan AlN ve metalik TiN fazları sayesinde solar soğurma özelliği de göstermektedir. Bir çeşit metal-yalıtkan kompozit malzeme olan AlTiN, solar soğurucu kaplama olarak literatürde bakır, paslanmaz çelik, alüminyum gibi metal altlıklara birçok çalışmada uygulanmıştır. Yapılan çalışmalar sonucunda, yüksek sıcaklık uygulamalarında kullanıma uygun yüksek soğurma, düşük emitans özelliği gösteren soğurucu yüzeyler üretilmiştir. 
Bu çalışmada, bakır altlık üzerine manyetik alanda sıçratma yöntemi ile AlTiN kaplama uygulanarak %95'in üzerinde solar soğurma, 0,07'den düşük emitans değerine sahip solar soğurucu kaplama üretimi amaçlanmıştır. Kaplamalar, yüksek metal oranına sahip(YMOS), düşük metal oranına sahip(DMOS) ve yansıma önleyici özelliğe sahip katmanlar kullanılarak çok katmanlı yapıda üretilmiştir. YMOS katman, düşük N2 ortamında metalik karakter, DMOS katmanlar ise yüksek N2 ortamında yalıtkan karakter gösterecek şekilde kaplanmıştır. Kaplamalara, optik özelliklerin geliştirilmesi amacıyla yüksek AlN içeriğine sahip yansıma önleyici katman uygulanmıştır. Yapılan çalışmalar sonucunda %97,33 solar soğurma, 0,06 emitans değerine sahip çok katmanlı AlTiN kaplamalar üretilmiştir. 
Solar soğurucu kaplamaların yüksek sıcaklık oksitlenme direncinin geliştirilmesi amacıyla AlTiN kaplamalar, Ti/Si alaşımı hedefler kullanılarak silisyum ile katkılandırılmıştır. Üretilen AlTiN ve AlTiSiN numunelere hızlandırılmış ömür testi uygulanmış ve optik özellik değişimleri incelenmiştir. Testin ilk aşamasında 250°C'de 200 saat boyunca fırında tutulan numuneler, %0,5'ten daha düşük optik özellik değişimi göstermiş ve AlTiN ve AlTiSiN solar soğurucu kaplamaların en az 25 yıl kullanım ömrüne sahip oldukları belirlenmiştir. 
Hızlandırılmış ömür testinin ilk aşaması sonucunda numunelerin optik özelliklerinde meydana gelen değişimler çok az miktarda olduğu için silisyum katkılandırmasının etkileri net olarak gözlenememiştir. Bu sebeple, testin ikinci aşaması AlTiN ve AlTiSiN numunelere uygulanmıştır. 300°C sıcaklıkta 75 saat boyunca uygulanan ısıl işlem sonucu numunelerde meydana gelen optik özellik değişimleri belirlenmiş ve AlTiSiN numunelerin optik özelliklerinde meydana gelen kayıp miktarının AlTiN numunelere göre daha yüksek seviyede olduğu görülmüştür. Yapılan çalışmalar sonucunda, silisyum katkılandırmasının AlTiN kaplamaların morfolojilerini değiştirerek taban malzemeye oksijen difüzyonunu hızlandırdığı belirlenmiştir.

Abstract:
In recent years, due to increased environmental problems the need for renewable sources for energy production has been increased. Among others, such as wind, wave and water, sun is an essential source for energy production, because solar power can be converted to thermal energy through solar collector panels. However, solar collectors are not the best choice due to their low productivity. The properties of material, which is chosen in solar collectors, determine the productivity of photo-thermal conversion systems. Most solar collectors have a spectrally selective absorber surface which has high absorptance at solar spectrum range (300-2500 nm) and transmittance at infrared spectrum range on the highly reflective metal substrate such as copper and aluminum. These properties of solar selective surface are the most important parameters for identifying absorbing performance of solar collectors. In addition, high temperature oxidation behavior of these materials has crucial impact on the stability of optical properties and durability of collector system.
Spectrally selective solar absorber surfaces have become an important research area in solar energy applications since the 1950's. Nowadays, solar selective surfaces can be categorized as semiconductor coatings, multilayer absorbers, metal-dielectric composite coatings, textured surfaces and blackbody-like absorbers. In addition to these, several approaches were used in order to produce solar selective surfaces such as physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemistry, spraying and sol-gel techniques etc. 
Semiconductor coatings consist of semiconductor materials with low band gap such as silicon or germanium, which absorb more energetic photons than its band gap energy. However, these coatings need an anti-reflection coating as a top layer to minimize reflectance losses because of high refractive index of the semiconductor material. Multilayer absorbers can be prepared by stacking of metal and dielectric layers. Optical properties of these absorbers depend on the interference effects of reflected light and incident light between metal and dielectric interfaces. Textured surfaces are usually produced by creating surface texture on metal substrate. These surfaces act as rough surfaces for short wavelength radiation and traps the light by geometric effects. For long wavelength radiation, surface acts as a mirror and reflects the incident light to possess low emissivity. Blackbody-like absorbers mostly have high absorptance at solar region and infrared region. Therefore, these absorbers should have infrared reflector coating on the top to achieve high absorptance and low emittance values. Lastly, metal-dielectic composite coatings are thin film structures, which consist of metallic phase in a dielectric matrix. These types of absorbers are transparent in high wavelength infrared region and they are highly absorbing in solar energy region. In general, single layer metal-dielectric composite film doesn't show high solar absorptance values. However, complex structures such as thin film with gradually changing metal-dielectric content ratio or two layers of a low metal volume fraction layer (LMVF) on a high metal volume (HMVF) fraction layer can make it possible to achieve higher solar absorbing performance. Accordingly, the flexibility of metal-dielectric composite coatings has made them an attractive research area for various solar collector applications in recent years.
In this study, metal-dielectric composite coatings were chosen as solar absorber type and as a composite material, aluminum titanium nitride (AlTiN) which consist aluminum nitride (AlN) dielectric matrix and titanium nitride (TiN) metallic phase was selected because of its excellent mechanical and oxidation properties. 
AlTiN composite coatings were applied on rotating copper substrates by a direct-current reactive magnetron sputtering technique with using two sputtering cathodes. Multilayer metal-dielectric coating design approach was followed, such that coatings were deposited in different Ar/N2 partial pressures and Al/Ti ratios. The design contained, from bottom to top, high metal volume fraction (HMVF), low metal volume fraction (LMVF) and anti-reflection (AR) layers. HMVF layer mainly consisted of Al, Ti and TiN metallic phases and small amount of AlN dielectric phase. However, LMVF layer contained high amount of AlN phase and little amount of TiN phase to achieve high transmittance at infrared region and high absorptance at solar spectrum. In addition to those layers, on the top, anti-reflection layer was formed nearly pure AlN phase.
Initially, HMVF layer was deposited and optimized to obtain high absorptance and low emittance values. Samples were prepared in three different N2 flow rates (10, 15, 20 sccm) and three different thicknesses (40, 60, 80 nm). After characterizing the optical properties of these nine samples, coating, which was prepared in 15 sccm N2 flow rate and had 60 nm thickness, showed optimal optical properties and also a metallic character. It reached 87.50% solar absorptance and 0.03 thermal emittance values, hence selected as high metal volume fraction (HMVF) layer of the designed multilayer coating.
Secondly, low metal volume fraction (LMVF) layer was deposited on HMVF layer to achieve highest solar absorptance and low emittance values. In order to obtain low metal volume fraction layer, N2 flow rate was risen to 30 sccm and formation of dielectric AlN phase was promoted. With the addition of LMVF layer, which had 60 nm thickness, solar absorptance value increased from 87.50% to 96.60%, and thermal emittance value of 0.06 was obtained. 
After optimization of double layer metal-dielectric composite coating, AlN anti-reflection layer was added as a top layer by increasing aluminum content bottom to the top in order to enhance solar absorbing properties. Finally, a solar absorptance of 97.33% and 0.06 thermal emittance values have been achieved for the optimal multilayer solar selective surfaces. 
To improve oxidation behavior of AlTiN solar selective coatings, silicon was doped in the coating by using Ti/Si alloy targets with different silicon contents. After the deposition of AlTiSiN coatings, accelerated life testing was applied to AlTiN and AlTiSiN samples in order to check durability of solar absorber coatings for at least 25 years. In the first stage of life testing, samples were backed at 250°C for 200 hours and optical properties were analyzed. A slight change(less than %0.5) in solar absorptance and as well in emittance values was identified. Consequently, samples were successful on the test, which was an outstanding initial achievement for our solar absorbers. However, the results of the first stage of accelerated life testing were not sufficient to observe silicon effect on the oxidation behavior difference between AlTiN and AlTiSiN samples.
Thereby, in order to investigate effect of silicon doping effect, second stage of accelerated life testing was applied to AlTiN and AlTiSiN samples at 300°C for 75 hours. The results of the test showed that samples with silicon addition, AlTiSiN, exhibit higher solar absorptance and emittance losses than AlTiN samples. The reasons for this situation could be the oxidation of AlTiSiN coating or copper substrate. Therefore, to eliminate substrate material's effect on oxidation behavior, 304 stainless steel was used as a substrate material instead copper, and samples were oxidized at 800°C for 3 hours. Visual inspections showed that silicon doping protected samples against high temperature oxidation by forming oxide films on the surface. As a result, it is concluded that, silicon doping of AlTiN coatings accelerated oxidation of copper substrate by changing coating morphology and allowing to oxygen reach to the substrate material.