The computed values of (i) ASTM and (ii) IOS grain size numbers (Gn and Gm), (iii) average grain

Area (iv) average grain diameter d, (vi) grain size distribution (or uniformity) (Gd) and porosity factor (PT) for the films (Table 1) show the followings for the different films.

Snl2 and MnBr2 Films: NA = Gn and Gm values are higher for MnBr2 than for Snl2 indicating that

MnBr2 contain more grains than Snl2, as explained in the section above. A and dvalues are larger for Snl2 than for MnBr2, indicating that Snl2 films have larger grains. L value is larger for MnBr2 than for Snl2 showing that MnBr2 films have more oblong grains, while Snl2 grains tend to be more roundish or circular. Gd for MnBr2 is larger than for Snl2 showing that Snl2 have more uniform grains than MnBr2. PT for Snl2 is larger than that for MnBr2 indicating that Snl2 is more porous than MnBr2. These also explain why MnBr2 tends to have is higher absorption of UV radiation that Snl2. This is because when a film with tinny grains is porous, there is little or no absorption of UV — VIS radiation by the grains, hence the radiation finds it easier to pass through as in Snl2. But when a tinny grained film is dense, UV absorption tends to be higher with a substantial NIR reflection as well. The only wavelength of transmission for such films would therefore be the visible radiation as shown in fig 1.

PbBr2 and Pbl2 films: Gn and Gm are about the same for both films although they are a little bit

higher for Pbl2 than for PbBr2 indicating somewhat more grains for Pbl2. A and d are a bit larger for PbBr2 than Pbl2 showing somewhat larger grains for PbBr2. Lv is higher for PbI2 then for PbBr2 indicating that PbI2 have more oblong grains than PbBr2. Gd PbBr2 is larger than Pbl2 showing that Pbl2 grains are more evenly distributed than PbBr2. PT is higher for PbBr2 than for Pbl2 showing that PbBr2 is more porous than Pbl2. Generally, these films are highly porous, hence they will transmit highly in the UV but have improved late visible and infrared reflection from about a cut off of 450 to 550mn.

Ag2S and PbS films:Gn and Gm are higher for Ag22 than PbS, hence Ag22 have more grains than

PbS. A and d for PbS are higher than those for Ag22, indicating that PbS grains are larger than Ag2s grains. Lv for PbS is higher indicating that their grains are more oblong than those of Ag2S Gd for PbS is higher showing that PbS is less evenly distributed. RT is higher for Ag2S showing that it is more porous. The very large nature of the grains of these films combines with moderate porosity and high void make them highly absorbing in the UV and Visible but highly transmitting in the Near Infrared. High voids and vacancies through out the film would allow infrared radiation to go through easily whereas the visible ultraviolet radiations would be easily intercepted (absorbed) by the large grains.

3. Conclusion

From the above, film microstructures (i. e. the geometric exterior and grain size parameters) were observed to be related to the spectral characteristics of films [2]. Films with smaller grains, which tend towards continuous, show high UV absorption, NIR reflection and Visible transmission as in Snl2 and MnBr2 and Pbl2 films. Films with moderate grains, which are more widely and uniformly spaced show high UV-VIS transmission and improved NIR reflection like the PbBr2 and Pbl2 films. On the other hand, films with larger grains which tend towards continuous absorb UV-VIS radiation completely but transmit in the infrared region as in the Ag2S and PbS films [2].

The Snl2 and MnBr2 films can be used as window coatings for warm climates to prevent UV and IR radiation from entering buildings but allow only visible radiations for “day lighting” [2, 15-16]. Hence natural air conditioning can be created by using these films.

Therefore Snl2 and MnBr2 films can be used as visible transparent window coating. (VTWC) otherwise known as “Cooling Windows” to block and prevent UV (high energy) AND IR (high heat) radiations from entering but allow visible radiation only into the buildings if the glass doors and windows of such buildings are coated with these films. This can create natural air-conditioning (cooling) of the building interior [12, 15, 16].

Pbl2, PbBr2, PbS and Ag2S films can be used to split the solar spectrum into two broad bands, which can be used for different applications. The lead halide films, which favour UV-VIS transmission and IR reflection can be used to reflect infrared radiation and prevent it from getting to the surface of photovoltaic cells if the films are used to coat solar cell surfaces [2, 15-16]. This is because solar cells work better in the absence of infrared radiation. Such films are called Visibly transmitting spectral splitters (VTSS) and behave like “Cold Mirror”. On the other hand, PbS and Ag2S films can be used to coat glasses, which can be used to absorb UV-VIS radiation, while NIR radiation is transmitted. These glass systems called infrared transmitting spectral splitters (IRTSS) and behave like “Heat or Dark Mirrors” they can be used to regulate solar and thermal energy for Conventional Green House Agriculture (CGHA) and for Control Environmental Agriculture

(CEA), in which various parameters that affect yield and quality of crops are optimized and

regulated [15, 18-20].

References

Reather, H., 1976. Structure of Thin Films. In: Physics of Non-Metallic Thin Films, Eds C. H. S. Dupuy., A Cachard, Plenum Press, New York. 123-140.

Okujagu, C. U., 1992. Growth and Characterization of Thin Film Selective Surfaces and their Applications. Ph. D Thesis, Department of Physics and Astronomy, University of Nigeria, Nsukka.

Sirimane, P. M., Tributsch, H., 2007. Generation of Inhomogeneous Photo Current in Solid State Ti O2/Dye/CuI Cells and Effect of Ligands Attached to Surfactant on Morphology of CuI Films. Solar Energy 81 (4) 535-539.

Vander Voort, G. F., 1984. Grain Size Measurement. In: Practice and Appl. of Quantitative Metallography, Symposium Proc. ASTM, STPB 39, Philadelpha, Eds J. L. Mc Call., J. H. Steel, Jr. 85-131.

ASTM 1984. Committee E-4 on Metallography. Standard Methods for Determining Average Grain Size Designation: E 112-84 (Pub. August), 116-149.

Papini, M., 1986. Chemical, Structural and Optical Characterization of Ni-P Chemical Conversion Coatings for Photo-Thermal Absorption of Solar Energy. Solar Energy Materials 13, 233-265.

Hillard, J. E 1962. Grain Size Estimation. Report No 62 Rl-1133, General Electric Laboratory, Schenectady,

N. Y; December.

ASTM 1982 Standard Methods for Estimating the average Grain Size of Metals (E 112-82). Annual Books of Standards, Part 11, American Society for Testing and Materials.

Okujagu, C. U., Okeke, C. E., 1997. Growth Characteristics of Chemically Deposited Halide Thin Films. Nigerian Journal of Renewable Energy 5(1&2) 125-130.

Okujagu, C. U., Okeke, C. E., 1998. Growth Characterization of some Spectrally Selective Halide and Chalcogenide Thin Films, Nigerian Journal of Renewable Energy. 6 (1&2) 56-61.

Kokate, A. V., Suryavanski, U. B. Bhosale, C. H., 2006. Structural Compositional and Optical Properties of Electrochromically Depsited Stoichiometric Cdse Films from Non-Acqieous Bath. Solar Energy 80 (2) 156-160.

Razykov, T. M., Kouchkarov, K. M., 2006. Influence of Growth Rate on the Nanocruystallinity of II-VI Films in Chemical Vapour Deposition. Solar Energy 80 (2) 182-184.

Okujagu, C. U., 1999. Thin Film Technology for Energy Supply and Regulation in Nigeria. Journal of Applied Science and Environmental Management 3, 5-9.

Murali, K. R., 2008. Properties of Brush Plated Cdx ZnJ-X Te Thin Films. Solar Energy 82 (3) 220-225.

Okujagu, C. U., Okeke, C. E., 2000. Prospects of Spectral Splitting Thin Films in the Development of Photothermal, Photovoltaic and Green House Technologies. Nigerian Journal of Renewable Energy 8 (1&2) 24-31.

Okujagu, C. U., Okeke, C. E., 1997. Effects of Materials Properties on the Transmission of Selective Transmitting Thin Films. Nigerian Journal of Physics, 9, 50-66.

Sethi, V. P., Sherme, S. K., 2007. Thermal Modeling of Green House Integrated to an Acquifer Coupled cavity Flow Heat Exchanger. Solar Energy 8 (6) 723-741.

Sethi, V. P., Sharma, S. K., 2007. Survey of Cooling Technologies for World Wide Green House Applications. Solar Energy 81 (12) 1447-1459.

Davies, P. A., 2005. A Solar Cooling System for Green House Food Production in Hot Climate. Solar Energy 79, (6) 661-668.

Kumar, A., Twari, G. N., 2006. Thermal Modeling of a Natural Convection Green House Drying System for Jaggrey: An Experimental Validation. Solar Energy 80 (9) 1135-1144.