G. KATUMBA1, G. MAKIWA1, L OLUMEKOR1 & A FORBES2

1Physics Department, University of Zimbabwe, Harare, Zimbabwe2CSIR-National Laser Centre, Pretoria 0001, South Africa

SAIP 2006

SOLAR SELECTIVE ABSORBER FUNCTIONALITY OF CARBON

NANOPARTICLES EMBEDDED IN SiO2, ZnO and NiO MATRICES

Concept and motivation

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200

400

600

800

1000

1200

0

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1

1 10

Pow

er d

ensi

ty (W

/m2 µµ µµm

)

Idea

l abs

orbe

r re

flec

tanc

e

Wavelength (µµµµm)

Solar (AM1.5)

Blackbody

Ideal selective absorber

100oC

200oC

300oC

Electromagnetic wave propagation

� Interaction of light with matter.� Complex refractive index.

� 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.

( ) ( )[ ]c/xntiexpc/xkexpEEm ωωω −−= 0

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.

�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: 2

1

1

N

NR

+−=

Tailoring materials

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:

2N=ε

( ) 02

12

=+−−+

+−

Brb

Brba

Bra

Braa ff

εεεε

εεεε

( )( )baaba

baababMG f

f

εεεεεεεεεε

−−+−++=

22

22

Theoretical optimisation

Vary fill factor, f.

EMAs – Brugeman andMaxwell-Garnett.

Variable fill factor, f.

MULTILAYER PROGRAM.Calculate theoretical reflectance, R.Variable fill factor, f and thickness, t.

Reflectance data treatment.Calculate theoretical solar absorptance, α

and thermal emittance, ε.

OPTIMUM?(qualitative)

END

Vary thickness, t.

Get n & k data.

No

Yes

No

Theoretical reflectance

�Bruggeman �Maxwell-Garnett

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tBR

=0.20 um

tBR

=0.50 um

tBR

=1.00 um

Wavelength (µµµµm)

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tMG

=0.20 um

tMG

=0.50 um

tMG

=1.00 um

R

Wavelength (µµµµm)

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 Kα, 35 keV, 30 mA

Crystal structure

Experimental results: SiO2 samples�Spin-coating speed�Carbon precursor

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2000 rpm (9:6)3000 rpm (9:6)4000 rpm (9:6)4000 rpm (9:12)5000 rpm (9:12)

R

Wavelength (µµµµm)

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6 g SUC9 g SUC11 g SUC12 g SUC

R

Wavelength (µµµµm)

Experimental results: IR spectrum

�FTIR reflectance spectrum

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5001000150020002500300035004000

R

Wavenumber (cm-1)

Si - O - Si

OHOH

Shoulder

Experimental results: defects

�Problem- cracked films

�Solutions to cracking

�MTES & Ac2O

Experimental results: defects

Experimental results: spectra

�Addition of MTES �Addition of Ac2O

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10 wt.% A c2O

15 wt.% A c2O

20 wt.% A c2O

15 wt.% A c2O (9:11)

0 wt.% A c2O

R

W avelength (µµµµm)

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10 wt.% MTES

20 wt.% MTES

30 wt.% MTES

0 wt.% MTES

R

Wavelength (µµµµm)

Experimental results: X-HRTEM� (a) TEOS only

� (b) TEOS + Ac2O

� (c) TEOS + MTES

� Uniform distribution

� Segregation7 nm

7 nm

a

bc

Experimental results: EELS� EELS mapping

� Carbon K peak

7 nm

7 nm

a

b

� (a) TEOS only

� (b) TEOS + Ac2O

� (c) TEOS + MTES

c

Experimental results: ZnO samples

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ZnO (A)

ZnO (B)

ZnO (C)

ZnO (D)

ZnO (F)

Ref

lect

ance

Wavelength (µµµµm)

Experimental results: NiO samples

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NiO (A)

NiO (B)NiO (C)NiO (D)

NiO (E)NiO (F)

NiO (G)NiO (H)

Ref

lect

ance

Wavelength (µµµµm)

XRD results: NiO and ZnO

Experimental results: Comparison

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NiO (E)ZnO (C)SOLKOTESiO

2

Ref

lect

ance

Wavelength (µµµµm)

Experimental results: XRD

2 0 3 0 4 0 5 0 6 0 7 0 8 0

2 θ ( ο )

Al

( 2 0 0 )( 1 0 4 )

ZnO

(A

) ( 1 1 0 )

( 1 1 0 )

ZnO

(B

)

( 1 1 0 )

ZnO

(C

)

( 1 1 0 )

NiO

(A

)

( 2 0 0 )

NiO

(B

)

Conclusions�∃ a possibility to produce low cost

selective solar absorbers with sol-gel technique.

�Addition of 15 wt.% Ac2O appeared to solve the problem of cracking in SiO2

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 nano-scale is very encouraging.

Conclusions

8.940.100.93C-NiO

6.290.140.89C-ZnO

2.900.310.90C-SiO2

α/εεαSample

Acknowledgement

�University of Zimbabwe.

�Uppsala University.

�NMMU.

�Defence Peace Safety & Security, CSIR.

�Rental pool.

�African Laser Centre.