Solar induced chemical vapor deposition of carbon from ethanol
Highlights
► Solar chemical vapor deposition (S-CVD) is proposed. ► S-CVD may be applied to several kinds of material deposition. ► S-CVD of carbon from ethanol relies practically on solar radiation.
Introduction
Chemical Vapor Deposition (CVD) is a technique widely used in materials technology to produce coatings or bulk-solid deposits of a variety of materials on a variety of substrates. Basically in the CVD process the substrates are immersed in the vapor (or gas) phase of a compound that contains the elements that should be chemically deposited through solid/vapor reactions induced by external sources of energy. Using this technique materials of high or low melting points, semiconductors, conductors and insulators can be deposited in crystalline, polycrystalline and amorphous forms. Materials such as diamond, Si, W, SiO2, Si3N4, among others, are currently produced by the CVD industry. Different CVD techniques are named in accordance to the source of energy used to induce the CVD reactions, such as thermal-CVD, plasma-CVD, microwave-CVD, laser-CVD, hot-filament CVD, among others [1], [2], [3].
Solar energy received by the Earth is an immense source of renewable and clean energy. In this work, we report the synthesis of solid carbon samples by a Solar-CVD system using ethanol as the carbon precursor, highly diluted in hydrogen. Scanning electron microscopy (SEM), optical microscopy and Raman spectroscopy analyses of the samples are presented and discussed.
Section snippets
Experimental
Fig. 1 shows the Solar-CVD system schematics. A Fresnel lens of D = 40 cm diameter and f = 40 cm of focal length was mounted on a moving frame to track the position of the Sun in the sky and focus the light on the substrates that are immersed in the vapor, separated from the ambient atmosphere by a quartz window. The micro-sized carbon structures were synthesized on polished Si substrates (0.8 mm thickness) or silica substrates (1.0 mm thickness) using ethanol (C2H5OH) as the carbon source,
Results and discussion
The diameter (d) of the image of the Sun on the focus (f) is given by d = fTang(θS) were (θS) is the angle subtended by the Sun from an observer on the Earth. Assuming approximately θS = 0.52° we find d = 0.36 cm. The maximum brilliance or the maximum radiation flux (φ) on this image when illuminated by the Sun's constant flux of radiation (above the top of Earth's atmosphere E = 0.1366 W cm−2) is given by: φ = E(D/d)2 = 1.686 kWcm−2. In practice, about 30 percent of the solar energy striking
Conclusion
In conclusion, the Solar-CVD ability to produce carbon structures using almost only solar energy is demonstrated. Following our optical modeling there is theoretically sufficient solar radiation flux to promote thermo- or photo-dissociation of different precursors. We expect the use of Solar-CVD to deposit other materials such as nanotubes and diamond, or in other applications such as carbon sequestering.
Acknowledgments
The electron microscopy work was performed with the JSM-5900LV microscope of the LME/LNLS – Campinas. We wish to express sincere thanks to Dr. Kanad Mallik of University of Oxford, UK, for his technical assistance. We also gratefully acknowledge the Brazilian agencies FAPESP, CAPES and CNPq for financial support.
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