Solar Energy Materials and Solar Cells 31 (1993) 109-117 North-Holland

Solar Energy Materials and Solar Cells

Cobalt oxide selective coatings for all glass evacuated collectors

Sushama Pethkar , M.G. Takwale, Chitra Agashe and V.G. Bhide

School of Energy Studies, Department of Physics, Unic'ersity of Poona, Poona 411 007, India

Received 18 September 1992; in revised form 31 March 1993

Black cobalt deposits were formed on glass substrates for use in all glass evacuated collectors. Glass was made conducting by depositing fluorine doped tin oxide films using spray pyrolysis. The reflectivity of this glass was further improved by electroplating a thin nickel layer (0.35 ~m) on it. Cobalt oxide thin films were deposited on these nickel coated conducting glass substrates using spray pyrolysis.

The effect of cobalt acetate concentration in the precursor solution on the performance of these multilayer coatings was studied. Solar absorptances as high as 0.93-0.94 and thermal emittances as low as 0.09 were obtained for films deposited with lower concentrations. For films deposited with higher concentrations the selectivity significantly reduces from ~ 33 to ~ 3.0. Structural analysis confirms the presence of Co304.

1. Introduction

Evacuated tubular collectors have been developed since the mid-1970s for medium temperature (100-130°C) applications such as industrial process heat and low pressure steam generation [1-3]. "All Glass" collectors with glass absorbers were found to be economically viable because of the elimination of the glass to metal seal. These collectors possess significantly high efficiencies (60-70%) at higher operating temperatures (> 100°C) mainly because of the selectively coated absorbers and high vacuum maintained in the gap between the absorber and the enclosure tube. In all glass evacuated collectors selective surfaces are developed on glass. These surfaces should have high absorptance for solar radiation (0.3-2.0 Ixm) and low emittance for thermal radiation (A > 2.0 Ixm). Additionally, they should have excellent adhesion on glass and mechanical as well as optical stability at elevated temperatures.

It is necessary for glass to be conducting if it is to be electroplated. Garg et al. [4] have reviewed methods of metallization of glass as well as coatings which can be developed on metallized glass by electroplating. Metal carbide and metal silicide layers on metallized glass deposited by DC sputtering have been reported to have excellent performance as selective surfaces, having solar absorptances of 0.90-0.95 and thermal emittances at room temperature of 0.03-0.04 [5]. Although having

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110 S. Pethkar et al. / Cobalt oxide selective coatings

excellent properties and thermal stability, the manufacturing processes of these coatings are quite expensive.

The two forms of cobalt oxide, viz. CoO and CO304, are found to be stable upto temperatures of 500°C. Thus cobalt oxide can serve as a suitable absorbing material for high performance of the collectors [6-12]. Thin films of cobalt oxide have been deposited by spray pyrolysis [8,9], electrodeposition [6,7,12] and thermal or chemical oxidation of metallic cobalt films [7,11].

In the present work, we have developed cobalt oxide films as an absorber coating using spray pyrolysis. Spray pyrolysis is one of the cheapest techniques to deposit thin films and can be used for large areas. Fluorine doped tin oxide (FTO) films were deposited by the spray pyrolysis technique to make the glass conducting. Black cobalt oxide was deposited by spraying cobalt acetate solution on FTO glass electroplated with nickel. The selectivity of these coatings with concentration of cobalt acetate solution was studied.

2. Experimental

Corning 7059 glass was used as a substrate. FTO films were deposited by spray pyrolysis on these glass substrates to make the glass conducting. The films were deposited at 400°C. A 0.16 M solution of stannic chloride in water and methanol with volume ratio 1 :9 was sprayed on the heated glass. Fluorine doping was achieved by adding N H a F with a F / S n ratio of 243 at% in the solution. The conductivity of the films obtained with this concentration was 949 12-1 c m - 1. The FTO coated glass will hereafter be referred to as the conducting glass.

To improve the IR reflectance of the conducting glass, it was overcoated with a thin layer of nickel by electroplating. Nickel was chosen since it is easy to form a Ni layer at room tempera ture and Ni has a good adhesion to FTO films. The bath concentration for Ni deposition was 105 g nickel sulphate, 18 g nickel chloride and 15 g boric acid dissolved in one litre water. The pH of the electrolyte and the current density were kept constant at 4 and 5 m A cm -z, respectively. Peeling of nickel layers beyond 0.5 Ixm was observed. Thin films of nickel ( < 0.3 Ixm) were found to be partially t ransparent and could thus not offer sufficient IR reflectance to the tandem. For the present work a nickel film of 0.35 Ixm thickness was grown on the conducting glass, as no appreciable improvement in IR reflectance was observed for nickel film thicknesses beyond 0.35 Ixm.

Deposits of cobalt oxide were formed by spraying a solution of cobalt acetate on the Ni coated conducting glass at 300°C ( + 5°C). Nickel films were observed to deteriorate above 300°C. Other process parameters which influence the properties of the coatings like nozzle diameter, substrate to nozzle distance and solution flow rate were kept constant at 0.3 mm, 30 cm and 1.25 ml /min , respectively. The effect of solution concentration on the properties of the films was studied. The concentration of the solution was varied by dissolving different amounts of cobalt acetate in a mixture of water and methanol (volume ratio 1 : 1). The concentration of the spraying solution was varied from 0.047 M to 0.14 M.

s. Pethkar et aL / Cobalt oxide selective coatings 111

The solar absorptance of samples was measured by a model A1 alphatometer manufactured by Devices and Services, USA. The near normal specular spectral reflectance in the range of 0.2-2.6 txm was measured using a Hitachi 330 double beam spectrophotometer. Thermal emittance values were calculated from the IR spectral reflectance data (with reference to an aluminium mirror) obtained with a Perk in-Elmer 783 IR spectrophotometer in the range 2.5 ixm to 40 t~m. Thermal emittance values were obtained at 100°C by integrating spectral reflectance.

Structural data were collected using a Rigaku Ru 200 B thin film goniometer for different grazing angle (ag).

To study the decomposition of these films at higher temperatures, thermogravi- metric analysis was carried out using a Perkin-Elmer T G A / S C Delta Series (model 3700).

The thermal stability of the samples was checked by annealing the samples in air at 400°C.

3. Results and discussion

In the present work black cobalt with a high IR reflecting base was prepared on glass substrates. FTO films due to their high conductivity possess high IR re- flectance. The IR reflectance of FTO films coated on a glass substrate is shown in fig. 1. This IR reflectance of FTO films is insufficient to form a low emittance base for selective coating. The IR reflectance of this conducting glass was improved by overcoating it with thin nickel as shown in the same fig. 1.

The properties of the absorber-ref lector tandem depend mainly upon film thickness, i.e. mass per unit area. It has been observed by Pillai et al. [9], that solar absorptance and thermal emittance of cobalt oxide coatings deposited by spray pyrolysis depend on mass of deposits per unit area. The mass per unit area deposited can be controlled by (1) changing the concentration of the solution and spraying equal amounts of the solution or (2) changing the amount of the spray solution for a fixed concentration. In the present work the mass per unit area (and hence the film thickness) of the cobalt oxide layer was varied by varying the concentration of the cobalt acetate solution. The volume sprayed was kept con- stant. This changed the film growth rate as a function of solution concentration as shown in fig. 2. At lower concentrations (upto 0.047 M) the rate of impinging flux is very low, which results in lower growth rates. As a result the cobalt oxide films formed are very thin. In such a case the thermal stability of the tandem ( g l a s s / F T O / N i / b l a c k cobalt) will depend upon the base Ni layer. As the deterio- ration of Ni layers was observed above 300°C, these surfaces have been found to be thermally unstable above 300°C.

To check whether the film properties are governed predominantly by the growth rate or mass per unit area, films of equal thickness were grown using different growth rates (cobalt acetate concentration). Films of equal thickness, but grown at different growth rates were found to be similar. This implies that the performance

112 S. Pethkar et al. / Cobalt oxide selectit,,e coatings

100

90

BO

70

6O

~ 50 CK

40

3O

2C

IC

- - S n 0 9 : F f~

. ~ " - . . . . SnO 2 : F/Ni

0 I I I I 2-0 4 . 0 6 . 0 1 0 . 0 2 0 . 0

Fig. 1. Specular reflectance in the range 2/zm-40/zm of FrO coated glass ( glass overcoated with nickel ( . . . . . . ).

200.0

) and FTO coated

of the cobalt oxide coatings is mainly dependent on the mass deposited per unit area.

Fig. 3 shows the reflectance spectra of samples with different cobalt acetate concentrations and same thickness of Ni and FTO films. Increase in solar absorp- tance is observed for the films grown with a higher concentration of Co acetate solution. The sharpness of the transition edge is found to increase for films grown with lower concentrations. The shape of the curve and the position of the transition edge is dependent on the thickness as well as the roughness of the absorber layer. The films coated with lower concentration were found to be smoother and thus, a sharp transition in IR reflectance is observed for thin films. The shift in transition wavelength towards lower wavelengths as the thickness decreases proves the lower absorptance and emittance values for thinner films (fig. 4). A hump around 1 ~m is significant in the films prepared with lower concentra- tion. This hump is a characteristic of CoO but also expected for C o 3 0 4 [13,14]. The

S. Pethkar et al. / Cobalt oxide selective coatings 113

r ' r "

100'

90

80

70

60

50

l O.O~ r-

E

E 0.06

¢ ,

0.04

o~0.02 t..9

OI 0.025

/ o

i i i I i 0.050 0.075 0.100 0.125 0.150

- - Mola r i ty of c o b a l t a c e t a t e s o l u t i o n --J--

Fig. 2. Var i a t ion of growth ra te wi th the molar i ty of the cobal t ace ta te solut ion.

i . . ' "

[ i ...... i / " k ./ i

' : , l / / i ...,- ,1

/ ..."" I ,

! /

/ I "'l i I-"/ ~ivj" J t" , b ~

2C

1C

i / i l

i , i!/,, I \ j l

: i ,"i } i t , /

0 1 r ~ i i i i I i [ [ i 1 l i i i i i ~ i i i i i i i I 0.2 0.3 0.5 0-7 1.0 2-0 3-0 5-0 I0-0 2 -0 40.0 I00-0

Gum ) ~,~ Fig. 3. Ref lec t ion spec t ra of the samples for d i f fe ren t cobal t ace t a t e concen t ra t ion : 0.047 M ( . . . . . ); 0.056 M ( . . . . . . ); 0.09 M ( - - - - - - ) ; 0.11 M ( . . . . . . ); 0.14 M ( ). Fo r ,~ < 2 . 5 p~m spec t ra were

t a k e n at n e a r no rma l inc idence and for A > 2.5 ~ m at 30 ° angle of incidence.

114 S. Pethkar et al. / Cobalt oxide selectit,e coatings

t d

v

rq Q_

Ln c'~

<

I

1.00

0.90

0.80

0 . 7 0 I 0 0.025

E

I I I I I 0.050 0.075 0.100 0.125 0.150

--Molarity of cobalt acetate sotution

3.30 l

3.20 0J u ¢o

0.10 E hl

I

Fig. 4. Solar absorptance and thermal emittance values as a function of cobalt acetate concentration.

presence of CO304 in the film is confirmed from the X-ray diffraction patterns as shown in fig. 5.

The formation of C0304 in the film can be explained from the following series of reactions:

CO(CH3COO)2 + H 2 0 + MeOH(Solut ion) 3°°°c CoO" H 2 0

O O II II

+ CH 3 C - 0 - C - C H 3,

1 2CoO • H 2 0 + ~ O 2 ) C 0 2 0 3 • H 2 0 .

C o 2 0 3 being unstable at 300°C further reacts with CoO to form C0304 along with some occluded water,

CoO • H 2 0 + C 0 2 0 3 " H : O > C 0 3 0 4 - H 2 0 .

The loss of weight obtained in T G A (fig. 6) proves the presence of occluded water in the film. Since the drop in weight is slow, incorporation of other volatile species formed during the reaction cannot be denied.

The vibration frequencies of C % O 4 [15] are clearly seen in IR region, which are prominent for thicker films (t > 0.3 p~m). This is also one of the causes for the high emittance values in thicker films. The reflectance curves also show an absorption dip around 6.5 ~m, which is present in all films grown with different concentra- tions. Its presence is characteristic for all cases but its reason is not clear.

As a consequences of these changes in the reflection spectra, one observes that emittance continuously increases with film thickness (solution concentration), as shown in fig. 4. However, the solar absorptance was thickness dependent only upto 0.17 ~m (solution concentration 0.056 M). The increase in solar absorptance was not significant and saturated for thicker films. As a result, the selectivity was

S. Pethkar et al. / Cobalt oxide selective coatings 115

I-.-

Z W

Z

0"51{I .~ cL =02" CPS I ,,, o .4

o 8 o ~ j -, ; e, %

.° z

2.OK CPS o LL

o i . -

20.0

o 0 t . = 1

z

at. = 10"

z

40.0 60.0 80.0 90.0

- - 2 O (DEGREE)

Fig. 5. X-ray diffraction pattern of the best film for three different grazing angles.

maximum at ~ 33 for thinner films (grown using lower concentrations) and continuously decreased to ~ 3 for thicker films.

Fig. 7 shows the reflection spectra of the best sample before and after heating in air at 400°C. Due to removal of water from the film, absorption in IR is increased which changed the thermal emittance of the film after heating. The shift in transition edge to higher wavelength and increase in absorption in IR clearly indicates the increase in thermal emittance.

Further studies to achieve better stability and improved selectivity are in progress.

4. Conclusions

It is proposed that cobalt oxide coatings formed by spray pyrolysis on nickel coated glass can serve as a good selective coating in all glass collectors. Better

116 S. Pethkar et al. / Cobalt oxide selectiue coatings

10(:

95

9C AGB

85 5 J 0 t l I I i t t i t • 0 100.0 150.0 200 .0 2 5 0 . 0 300 .0 350 .0 400.0 450 .0 500 .0

T e m p e r a t u r e (C )

Fig. 6. Thermogravimetric analysis of the film material (0.056 M cobalt acetate concentration).

100

90

80

7O

5O rr"

40

30

20

10

O! 0.1

BEFORE HEATING

J I

I I

i I

I I

i

i I

/ t

i i ~.. . i

AFTER HEATING IN AIR

AT 400"C

I i J |

0.2 o.~ o:~ ~ o'W,'.o 2.0 ~ , j u m

4.'o 'do'go',b.o 2'0 3b 4;;o 3=_

Fig. 7. Reflection spectra of the best sample (a = 0.93, elo o = 0.09) before and after heating.

S. Pethkar et aL / Cobalt oxide selective coatings 117

properties (a = 0.93, E~00 = 0.09) were obtained for films prepared with 0.056 M cobalt acetate solution. The films mainly consist of Co304 and some percent of occluded water.

The emittance of these films is thickness dependent since absorption of Co304 in IR becomes significant for thicker films. The changes in the properties of the films after heating are due to removal of water content from the films. Further studies to improve thermal stability of these films are under progress.

References

[1] G.L. Harding, B. Window and R. Grammon, Int. J. Ambient Energy 3 (1982) 171. [2] B. Window and G.L. Harding, Sol. Energy 32 (1984) 609. [3] S.P. Chaw, G.L. Harding and R.E. Collins, Sol. Energy Mater. 12 (1985) 1. [4] H.P. Garg, A.R. Shukla, R.C. Agnihotri and S. Chakravertty, Appl. Energy 13 (1983) 295. [5] I.T. Ritchie and G.L.Harding, Thin Solid Films 57 (1979) 315. [6] G. McDonald, Thin Solid Films 72 (1980) 83. [7] G.B. Smith, A. Ignatiev and G. Zajac, J. Appl. Phys. 51 (1980) 4186. [8] C. Chaudhury and H.K. Sehgal, Sol. Energy 28 (1982) 25. [9] P.K.C. Pillai and R.C. Agarwal, Energy Conversion Manage. 22 (1982) 111.

[10] K.J. Cathro, Sol. Energy Mater. 9 (1984) 433. [11] M.G. Hutchins, P.J. Wright and P.D. Grebenik, Sol. Energy Mater. 16 (1987) 113. [12] B. Vitt, Sol. Energy Mater. 19 (1989) 131. [13] D.E. Cherkashin and F.I. Vilesov, Sov. Phys. Solid State 11 (1969) 1068. [14] J.G. Cook and F.P. Koffyberg, Sol. Energy Mater. 10 (1984) 55. [15] J.R. Ferraro, Low Frequency Vibrations of Inorganic and Coordinate Compounds (Plenum Press,

New York, 1971).