SOLAR SELECTIVE ABSORBER FUNCTIONALITY OF CARBON NANOPARTICLES EMBEDDED IN SiO 2, ZnO and NiO MATRICES G. KATUMBA 1, G. MAKIWA 1, L OLUMEKOR 1 & A FORBES 2 1 Physics Department, University of Zimbabwe, Harare, Zimbabwe 2 CSIR-National Laser Centre, Pretoria 0001, South Africa SAIP 2006
Concept and motivation Power density (W/m 2 µm) 1200 1000 800 600 400 200 Solar (AM1.5) Ideal selective absorber 300 o C Blackbody 200 o C 100 o C 1 0.8 0.6 0.4 0.2 Ideal absorber reflectance 0 1 10 Wavelength (µm) 0
Electromagnetic wave propagation Interaction of light with matter. Complex refractive index. E m = E 0 exp ( kωx / c) exp[ i( ωt nωx / c) ] Magnetic field is not altered in optical and IR. Logarithmic reduction in intensity for a film thickness d is ln (I/I 0 ) = -αd = -4πkd/λ.
Tailoring materials Factor kd/λ determines the efficiency of a solar absorber surface. Proper combination of k and d. Homogeneous films can only vary d. Composite films change k! Practically constant for most materials used. Can also tune n by changing porosity.
Tailoring materials Therefore can only tune d to place the crossover at an appropriate wavelength. Reflectance and efficiency of absorber surface depends on k and d. Near-normal reflectance of bulk material is: 1 N 2 R = 1 + N
Device structure Many designs are possible. A tandem device: a composite layer deposited on a metallic substrate. Composite layer Aluminium substrate
Effective medium approximations Bruggeman: Maxwell-Garnett: 2 = N ( ) 0 2 1 2 = + + + Br b Br b a Br a Br a f a f ( ) ( ) b a a b a b a a b a b MG f f + + + = 2 2 2 2
Theoretical optimisation Get n & k data. EMAs Brugeman andmaxwell- Garnett. Variable fill factor, f. Vary fill factor, f. MULTILAYER PROGRAM. Calculate theoretical reflectance, R. Variable fill factor, f and thickness, t. Vary thickness, t. No OPTIMUM? (qualitative) No Yes Reflectance data treatment. Calculate theoretical solar absorptance, α and thermal emittance,. END
Theoretical reflectance Bruggeman Maxwell-Garnett 1 1 0.8 t BR =0.20 um t BR =0.50 um 0.8 t MG =0.20 um t MG =0.50 um 0.6 t BR =1.00 um 0.6 t MG =1.00 um R 0.4 0.4 0.2 0.2 0 1 10 0 1 10 Wavelength (µm) Wavelength (µm)
Experimental procedure Sol production Tube furnace
Experimental procedure Carbonization Filtration of nitrogen gas
Optical characterisation Varian Cary 500 UV-VIS-NIR Bomem DA8 NIR-IR
Non-optical characterisation SEM: Philips XL30 Surface mophology XRD: Philips PW1840 Diffractometer Cu K α, 35 kev, 30 ma Crystal structure
Experimental results: SiO 2 samples Spin-coating speed Carbon precursor 1 0.8 1 0.8 6 g SUC 9 g SUC 11 g SUC 12 g SUC 0.6 0.6 R 0.4 0.2 0 2000 rpm (9:6) 3000 rpm (9:6) 4000 rpm (9:6) 4000 rpm (9:12) 5000 rpm (9:12) 1 10 Wavelength (µm) R 0.4 0.2 0 1 10 Wavelength (µm)
Experimental results: IR spectrum FTIR reflectance spectrum 1 0.8 R 0.6 OH OH 0.4 0.2 Shoulder 0 4000 3500 3000 2500 2000 1500 Wavenumber (cm -1 ) Si - O - Si 1000 500
Experimental results: defects Problem- cracked films Solutions to cracking MTES & Ac 2 O
Experimental results: defects
Experimental results: spectra 1 Addition of MTES Addition of Ac 2 O 1 0.8 0.8 0.6 0.6 R R 0.4 0.4 0.2 0 10 wt.% MTES 20 wt.% MTES 30 wt.% MTES 0 wt.% MTES 1 10 Wavelength (µm) 0.2 0 10 wt.% A c O 2 15 wt.% A c O 2 20 wt.% A c O 2 15 wt.% A c O (9:11) 2 0 wt.% A c O 2 1 10 W avelength (µm)
Experimental results: X-HRTEM c (a) TEOS only (b) TEOS + Ac2O (c) TEOS + MTES Uniform distribution Segregation a b 7 nm 7 nm
Experimental results: EELS EELS mapping Carbon K peak (a) TEOS only (b) TEOS + Ac 2 O (c) TEOS + MTES a 7 nm c b 7 nm
Experimental results: ZnO samples Reflectance 1 0.8 0.6 0.4 ZnO (A) ZnO (B) ZnO (C) ZnO (D) ZnO (F) 0.2 0 1 10 Wavelength (µm)
Experimental results: NiO samples 1 Reflectance 0.8 0.6 0.4 NiO (A) NiO (B) NiO (C) NiO (D) NiO (E) NiO (F) NiO (G) NiO (H) 0.2 0 1 10 Wavelength (µm)
XRD results: NiO and ZnO
Experimental results: Comparison 1 0.8 NiO (E) ZnO (C) SO LKOTE SiO 2 Reflectance 0.6 0.4 0.2 0 1 10 Wavelength (µm)
Experimental results: XRD NiO (B) ( 2 0 0 ) NiO (A) ZnO (C) ZnO (B) ( 1 1 0 ) ( 1 1 0 ) ( 1 1 0 ) ZnO (A) ( 1 1 0 ) Al ( 1 0 4 ) ( 2 0 0 ) 2 0 3 0 4 0 5 0 6 0 7 0 8 0 2 θ ( ο )
Conclusions a possibility to produce low cost selective solar absorbers with sol-gel technique. Addition of 15 wt.% Ac 2 O appeared to solve the problem of cracking in SiO 2 samples better than 20 wt.% MTES.
Conclusions New and interesting microstructure of the sol-gel derived samples have been revealed: A short chain-like structure of both a silica matrix and carbon nanoparticles is quite evident. Homogeneity of the coatings at nanoscale is very encouraging.
Conclusions Sample α α/ C-SiO 2 0.90 0.31 2.90 C-ZnO 0.89 0.14 6.29 C-NiO 0.93 0.10 8.94
Acknowledgement University of Zimbabwe. Uppsala University. NMMU. Defence Peace Safety & Security, CSIR. Rental pool. African Laser Centre.