Selective black nickel coatings on zinc surfaces by chemical conversion

Solar Energy Yol. 23, pp. ,105--408 © Pergamon Press Ltd.. 1979. Printed in Great Britain

0038-092X179/I 1014)405f502.0010

SELECTIVE BLACK NICKEL COATINGS ON ZINC SURFACES BY CHEMICAL CONVERSION P. K. GOGNAand K. L. CHOPRA Department of Physics, Indian Institute of Technology,Delhi, New Delhi-110029,India (Received 21 July 1978; revision accepted 24 May 1979) Abstract--An electrochemicalconversion technique has been developed to deposit selective black nickel coatings of solar absorptance 0.90-0.94 and thermal emittance (at 100°C)0.08--0.15 on galvanizediron, zincated, and zinc electroplated aluminium surfaces. The effect of electrochemical conversion parameters on the microstructure, optical and thermal properties and durabilityof the coatings has been established.

INTRODUCTION

In flat plate solar collector, a selective coating is used to convert solar radiation into thermal energy that is transferred from the absorber to the working fluid. The selectivity/l/is evaluated in terms of solar absorptance and thermal emittance. The spectral selectivity is possible because the solar spectrum and the spectral distribution of the thermal reradiation emitted from the coating are distinct as illustrated graphically elsewhere[2]. To have maximum solar thermal conversion efficiency, the coating should have the maximum possible absorptance over the solar spectrum while having the minimum possible emittance in the infrared region. Selective coating is a composite (metal particles embedded in a dielectric of metal oxide or sulfide) prepared by coating a polished metal having high infrared, reflectivity (i.e. low |Remittance, by Kirchoff's law) with a thin film (
zincated aluminium substrates. In this paper, we report the microstructure, optical and thermal properties of black nickel deposited on these zinc surfaces. The effect of various deposition parameters such as solution temperature pH, and dipping time on the optical and thermal performance of the coatings have been studied. Durability tests of thermal cycling, humidity tests and exposure to sun-light (out door field test) have been carried out to evaluate the coatings. EXPERIMENTAL DETMLS

A typical chemical bath consisting of salts of nickel and ions of thiocyanate and ammonium was prepared to deposite selective black nickel coatings on galvanized iron, zincated and zinc electroplated aluminium surfaces. The solution pH was buffered with sodium acetate/acetic acid to desired value. Thermostat was used to control the solution temperature within _+I°C. Aluminium samples were zincated and zinc electroplated after a treatment cycle of cleaning, deoxidizing, etching and desmutting. Zinc was electroplated on aluminium from a cyanide bath to get a bright deposit. Finally, water and alcohol rinsed substrates were employed for chemical conversion. Microstructure of the coating was studied with Cambridge $4-10 scanning electron microscope. Spectral reflectance of samples (7 cm × 7 cm) was measured with Beckman DK2 integrating sphere reflectometer. Freshly prepared MgO sample was used as a standard for total reflectance measurement in the spectral region 0.32.5/zm. IR reflectance in the range 2.5-25p.m was measured with Grubb Parson's SP/MK III spectrophotometer. Solar absorptance and thermal emittance was calculated by weighting with the solar spectrum at AM2 condition and black body spectrum at the desired temperature, respectively. Emittance of the sample as a function of temperature was measured by radiometric method/10, l l/ in which Kipp and Zonen compensated thermopile radiation detector was used. Stagnation temperature of the sample (25 cm x 25 cm) was measured under solar radiation using two glass covers and back thermal insulation of 10cm thick glass wool. Outdoor exposure test was conducted on these samples for six 405

P. K. GOGNAand K. L. CHOPRA

406

months. A mini collector with two glass covers was constructed to simulate a flat plate collector. Thermal cycling test from 100 to 200°C was carried out in an oven for 50 hr to determine the thermal stability of the coatings. Accelerated humidity test (MIL-STD-810B, Method 507, Procedure l) was conducted to determine the relative durability of the coatings. RESULTS AND DISCUSSION

Figure I shows the microstructure of black nickel coating deposited on a zinc surface. It was observed that during the initial stage of chemical conversion, the coatings follow the substrate topography. With increasing dipping time, the topography is governed by the conditions of the solution. Typical reflectance spectra of black nickel coatings deposited on galvanized iron, zincated and zinc electroplated aluminium are shown in Fig. 2. The reflectance spectra of the coatings depend strongly on the chemical nature of the zinc surface. Zinc

layer on galvanized iron is different from that of zincated or zinc electroplated on aluminium. The results of absorptance and thermal emittance of black nickel formed on galvanized iron as a function of dipping time for different values of bath temperature and solution pH values are shown in Figs. 3 and 4, respectively. At the start of dipping, both nickel and zinc deposit on the substrate. With increase in dipping time and consequently the increase in the black nickel thickness, there is first a rapid increase in absorptance without a significant increase in thermal emittance until a nearly maximum value of absorptance is reached after which the emittance rapidly increases with a little decrease in absorptance. The marked effect of solution temperature on the optical and thermal properties of the deposit is clear from Fig. 3. Over the temperature range (25-45°C), there is a steady increase in the emittance with dipping time, while absorptance in the beginning increases slowly, then starts decreasing and the coating turns grey from dark blue colour. The chemical conversion rate is greatly affected by the pH of the solution (Fig. 4). High solution pH value results in a fast deposition of black nickel and hence increase in both absorptance and thermal emittance of the coatings.

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Fig. 4. Dependence of abso~tance and emittance on solution pH values of black nickel deposited on galvanized iron as a function of dipping time.

Selective black nickel coatings on zinc surfaces by chemical conversion Figure 5 shows the results of absorptance and thermal emittance of black nickel deposited on zinc electroplated aluminium substrates for different values of bath temperature. The conversion of zinc electroplated aluminium surface to black nickel is similar to galvanized iron, as is clear from Fig. 3. The optical and thermal properties of black nickel deposited on zincated aluminium substrates for different zincating times (i.e. thicknesses) are shown in Fig. 6. Results of out door exposure test are given in Table 1. After 6 months of exposure, there was no effect on coating deposited on galvanized iron and zinc electroplated aluminium substrate. Solar absorptance decreased from 0.90 to 0.87 in case of coatings deposited on zincated aluminium substrate after a period of 6 months. No condensation was observed during the test period. Stagnation temperature of these coatings under no fluid condition was obtained between 140 and 150°C with two glass covers. Under the thermal cycling test of 200°C for 50hr there was no change in the value of solar absorptance of the coatings deposited on galvanized iron and zinc electroplated aluminium. But an increase in thermal emittance (0.2) was observed. After 24hr of thermal cycling, a large decrease in solar absorptance

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Table 1. Outdoor exposure test on black nickel coating deposited by chemical conversion

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(from 0.90 to 0.70) was observed in coatings deposited on zincated aluminium. Results of accelerated humidity tests are given in Table 2. Degradation in optical and thermal properties has been observed after two cycles. CONCLUSIONS

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Stable, cheap and high (10--12) absorptance to emittance ratio black nickel coatings have been developed by chemical conversion technique on galvanized iron and zinc electroplated aluminium.

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Fig. 6. Dependence of absorptance and emittance on zincating aluminium as a function of dipping time.

REFERENCES

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P.K. GOGNAand K. L. CHOPRA

9. R. S. Silo and P. A. Mladinik, Appl. Solar Energy 5(S), 18-12 (1%9). 10. W. P. Fussell, J. J. Trido and J. H. Henninger, Measurements of Thermal Radiation Properties of Solids (Edited by J. C. Richmond) NASA-SP 31 p. 83 (1%3).

11. Y. S. Touloubian and D. P. Dewitt, (editors) Thermal Radiative Properties-Metallic Elements and Alloys. Plenum Press, New York (1970).