Novel 1-D and 2-D Carbon Nanostructures Based Absorbers for Photothermal Applications
Solar thermal energy is emerging as an important source of renewable energy for meeting the ever-increasing energy requirements of the world. Solar selective coatings are known to enhance the efficiency of the photo thermal energy conversion. An ideal solar selective coating has zero reflectance in the solar spectrum region (i.e., 0.3-2.5 µm) and 100% reflectance in the infrared (IR) region (i.e. 2.5-50 µm). In this thesis, novel carbon nanotubes (CNT) and graphene based absorbers have been developed for photo thermal applications. Carbon nanotubes have good optical properties (i.e., α and ε close to 1), high aspect ratios (> 150), high surface area (470 m2/g) and high thermal conductivity (> 3000 W/mK), which enable rapid heat transfer from the CNTs to the substrates. Similarly, graphene also exhibits high transmittance (97%), low reflectance, high thermal conductivity (5000 W/mK) and high oxidation resistance behaviour. The major drawback of using CNTs for photothermal applications is that it exhibits poor spectral selectivity (i.e., α/ε = 1). In other words, it acts as a blackbody absorber. On the other hand, graphene exhibits poor intrinsic absorption behaviour (α - 2.3%) in a broad wavelength range (UV-Near IR). The main objective of the present study is to develop CNT and graphene based absorbers for photothermal conversion applications. The growth of CNT and graphene was carried out using chemical vapour deposition and sputtering techniques. An absorber-reflector tandem concept was used to develop the CNT based tandem absorber (Ti/Al2O3/Co/CNT). The transition from blackbody absorber to solar selective absorber was achieved by varying the CNT thicknesses and by using a suitable underlying absorber (Ti/Al2O3). A simple multilayer heat mirror concept was used to develop the graphene based multilayer absorber (SiO2/graphene/Cu/graphene). The transition from high transmitance to high absorptance was achieved by varying the Cu thickness. The refractive indices and the extinction coefficients of Ti/Al2O3, AlTiO and graphene samples were determined by the phase-modulated spectroscopic ellipsometric technique. Finally, the optical properties (i.e., absorptance and the emittance) of the CNT and graphene based absorbers were investigated. Chapter 1 gives a brief introduction about solar thermal energy, spectrally selective coating and photothermal conversion. The different types of absorbers used to achieve the spectral selectivity have also been discussed shortly. A brief description about the carbon-based materials/allotropes and their properties are outlined. The properties of carbon nanotubes and graphene which are the 1-D and 2-D allotropes of carbon, respectively are tabulated. A detailed literature survey was carried out in order to identify the potential candidates for the photothermal conversion applications. The objectives and the scope of the thesis are also discussed in this chapter. Chapter 2 discusses the deposition and characterization techniques used for the growth and the study of 1-D and 2-D carbon nanostructures. Atmospheric pressure chemical vapour deposition (CVD) and hot filament CVD techniques were used to grow CNT and graphene, respectively. The magnetron sputtering technique was used for the growth of ‘Ti’, ‘Al2O3’ and Co layers which were needed to grow the CNT based tandem absorber on stainless steel (SS) substrates. The important characterization techniques used to examine various properties of the 1-D and 2-D carbon nanostructures include: X-ray diffraction, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), phase modulated ellipsometry, UV-VIS-NIR spectrophotometer, Fourier-infrared spectroscopy (FTIR), micro-Raman spectroscopy and solar spectrum reflectometer and emissometer. Chapter 3 describes the design and development of Ti/Al2O3 coating for the growth of CNT-based tandem absorber on SS substrates. The power densities of the aluminum and titanium targets and the oxygen flow rates were optimized to deposit the Ti/Al2O3 coatings. The optimized Ti/Al2O3 coating with a Co catalyst on top was used as an underlying substrate to grow the CNT-based tandem absorber at 800°C in Ar+H2 atmosphere (i.e., CNT/Co/Al2O3/Ti/SS). The formation of aluminum titanium oxide (AlTiO) was observed during the CNT growth process and this layer enhances the optical properties of the CNT based tandem absorber. The optical constants of Ti, Al2O3 and AlTiO coatings were measured using phase modulated spectroscopic ellipsometry in the wavelength range of 300-900 nm. The experimentally measured ellipsometric parameters have been fitted with the simulated spectra using the Tauc-Lorentz model for generating the dispersion of the optical constants of the Al2O3 and the AlTiO layers. The Ti and Al2O3 layer thicknesses play a major role in the design of the CNT based tandem absorber with good optical properties. Chapter 4 describes the synthesis and characterization of the CNT based tandem absorber (Ti/AlTiO/CoO/CNTs) deposited on SS substrates. CNTs at different thicknesses were grown on Ti/AlTiO/CoO coated SS substrates using atmospheric CVD at various growth durations. The transition from blackbody absorber to solar selective absorber was achieved by varying the thicknesses of the CNTs and by suitably designing the bottom tandem absorber. At thicknesses > 10 µm, the CNT forest acts as near-perfect blackbody absorber, whereas, at thicknesses ≤ 0.36 µm, the IR reflectance of the coating increases (i.e., ε = 0.20) with slight decrease in the absorptance (i.e., α = 0.95). A spectral selectivity (α/ε) of 4.75 has been achieved for the 0.36 µm-thick CNTs grown on SS/Ti/AlTiO/CoO tandem absorber. Chapter 5 discusses the growth of graphene on polycrystalline copper (Cu) foils (1 cm × 1 cm) using hot filament CVD. The roles of the process parameters such as gas flow rates (methane and hydrogen), growth temperatures (filament and substrate) and durations on the growth of graphene were studied. The process parameters were also optimized to grow monolayer, bilayer and multilayer graphene in a controlled manner and the growth mechanism was deduced from the experimental results. The presence of graphene on Cu foils was confirmed using XPS, micro-Raman spectroscopy, FESEM and TEM techniques. The FESEM data clearly confirmed that graphene starts nucleating as hexagonal islands which later evolves into dendritic lobe shaped islands with an increase in the supersaturation. The TEM data substantiated further the growth of monolayer, bilayer and multilayer graphene. The intensity of 2D and G peak ratio (i.e., I2D/IG = 2) confirmed the presence of the monolayer graphene and the absence of the ‘D’ peak in the Raman spectrum indicated the high purity of graphene grown on Cu foils. The results show that the polycrystalline morphology of the copper foil has negligible effect on the growth of monolayer graphene. In Chapter 6, the design and development of graphene/Cu/graphene multilayer absorber and the study of its optical properties are discussed. The multilayer graphene grown on Cu foils has been transferred on quartz and SiO2 substrates in order to fabricate the graphene/Cu/graphene multilayer absorber. The sputtering technique was used to deposit copper on top of graphene/quartz substrates. The uniformity of the transferred multilayer graphene films was confirmed using Raman mapping. A simple multilayer heat mirror concept was used to develop the graphene/Cu/graphene absorber on quartz substrates and the transition from high transmittance to high absorptance was achieved. In order to further enhance the absorption, the graphene/Cu/graphene multilayer coating was fabricated on SiO2 substrates. The thickness of the Cu layer plays a major role in creating destructive interference, which results in high absorptance and low emittance. A high specular absorptance of 0.91 and emittance of 0.22 was achieved for the SiO2 graphene/Cu/graphene multilayer absorber. The specular reflectance of the multilayer absorber coatings was measured using the universal reflectance accessory of the UV-VIS-NIR spectrophotometer. Chapter 7 summarizes the major findings of the present investigation and also suggests future aspects for experimentation and analysis. The results obtained from the present work clearly indicate that both CNT and graphene based absorbers can be used as potential candidates for photothermal applications. In particular, the CNT based tandem absorber can be used for high temperature solar thermal applications and the graphene based multilayer absorber finds applications in the area of photodetectors and optical broadband modulators.
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