Surface temperatures under high solar flux illumination

Solar Energy Materials 11 (1984) 353-359 North-Holland, Amsterdam

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SURFACE TEMPERATURES U N D E R HIGH SOLAR FLUX ILLUMINATION A. M E S A R W I and A. I G N A T I E V Department of Physics, University of Houston, Houston, TX 77004, USA Received 4 September 1984 Aluminum samples were heated in air in the bulk temperature range of 350-520°C using two different methods; infrared oven and solar flux heating. The oxidized samples were inserted in a UHV chamber and elemental composition of the surface was performed using Auger Electron Spectroscopy(AES). The oxide films also were depth profiled using inert gas ion beam sputtering. The results of such analyses for infrared oven heated and solar high and low flux exposed samples were compared. Such results indicate that, at equivalent bulk temperatures, the solar flux heated samples have significantly higher surface temperatures than those of the oven heated samples. Also, Photon Stimulated Desorption (PSD) of CO2 from the top surface region of the solar exposed samples was observed, thus changing the surface composition of the oxide and the temperature differential in the sample.

1. Introduction The oxidation of aluminum at elevated temperatures has long been of much interest and therefore widely investigated. In most of such investigations, oxidation was accomplished by means of heating with infrared radiation [1-4]. For solar applications as well as in optical instrumentation, aluminum is often used as a reflector for the full solar spectrum from the infrared to the ultraviolet. In such and other applications high solar fluxes (500 to 5000 Suns) may be encountered. Exposure of aluminum to such high levels of solar flux and the resultant oxidation is the focal point of this investigation. This paper describes and explains differences observed between high solar flux heating, low solar flux heating and infrared oven heating of aluminum in air with respect to surface temperature, oxide thickness and growth using Auger electron spectroscopy.

2. Experimental Polycrystalline aluminum samples were cut into 1 / 1 6 " thick disks from a high purity 1 / 2 " diameter rod (99,95% Al). The samples were mechanically flattened and polished to obtain optically smooth surfaces by using successively decreasing grit size Buehler polishing pastes (5, 1, 0.3 and 0.05 ~Lm alumina). A sample thus polished 0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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A. Mesarwi, A. lgnatiev / Surface temperatures under high flux illumination

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in air served as a control sample ("as prepared" sample) for comparison purposes. For each experimental run, a set of three "as prepared" samples were utilized. One sample was infrared oven heated at a given temperature while the other two were exposed to solar simulated radiation (one sample to high flux ( > 1 M W / m 2)) and the other to low flux ( = 300 kW/m2)) at the same bulk temperature. The solar simulated flux was obtained using a 5000 W xenon lamp source which simulates satisfactorily the solar radiation spectrum. Using this source together with a set of optical UV grade quartz lenses, peak flux levels of 2.1 M W / m 2 could be achieved by focusing and concentrating the simulator beam to achieve a flux distribution of = 4 mm half height peak width on the sample surface (fig. 1). For low flux irradiation (200-600 k W / m 2) the beam was defocused on the sample. The sample holder was designed to allow sample cooling through various thermal resistances so as to obtain specific sample temperatures at different flux levels. The sample temperature was measured using a chromel-alumel thermocouple spot welded to the sample just below the surface. Auger Electron Spectroscopy (AES) and inert gas sputter-depth profiling of the sample surfaces were done in an Ultra High Vacuum (UHV) stainless steel chamber utilizing a PHI model 550 E S C A / S A M system. Typical base pressure for the system was = 5 × 10 -1° Torr. The system was equipped with a double pass Cylindrical Mirror Analyzer (CMA) with coaxial electron gun. Auger spectra were obtained in the derivative (dN(E)/dE) mode using a primary beam energy of 5 keV and 4 V peak to peak modulation. Depth profiling was accomplished by filling the chamber with argon gas to a pressure of 5 × 10-7 Torr and sputtering the sample surface at ion beam energies of 2 to 2.5 keV and current densities of = 60 ~ A / c m 2. These conditions corresponded to a sputtering rate of = 90 . ~ / m i n for A1203 . 3. Results The main Auger peaks which were found in AES surface scans of all samples were :

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metallic aluminum. oxidized aluminum (A1203). carbon. oxygen.

The atomic concentrations of the species present at the surface region were found by normalization of the respective Auger peak-to-peak heights in the AES spectra using the published relative sensitivity factors [5]. The atomic concentrations are

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356

A. Mesarwi, A. Ignatiev / Surface temperatures under high flux illumination

plotted as a function of sputter time (depth from the surface) to obtain the depth profile for individual samples. The results of such depth profiles on oven (infrared) and solar flux heated samples in the temperature range 350 to 520°C indicate three distinct temperature regimes with respect to the oxide growth and thickness on aluminum: i) up to 350°C, fi) in the temperature range 3 5 0 - 4 5 0 ° C ; and iii) above 500°C. :00 90

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A. Mesarwi, A. Ignatiev / Surface temperatures under high flux illumination

357

Up to a bulk temperature of 350°C, no differences are observed between the solar heated and the infrared oven heated samples. Also, no differences are found between samples exposed to high and low solar flux levels. Fig. 2 gives typical depth profiles in this regime for solar high flux, solar low flux and infrared oven heated samples at a bulk temperature of 350°C. The oxide produced at this temperature on all samples is of the same thickness and is relatively thin (150-200 A), as indicated by the sputter time to 90% of the bulk aluminum AES signal. In the temperature range 350 to 450°C, the solar flux heated samples showed thicker surface oxides as compared to samples exposed to similar conditions but . ~ ' ~

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A. Mesarwi, A. lgnatiev / Surface temperatures under high flux illumination

using infrared radiation. Fig. 3 illustrates typical depth profiles of samples heated at 430°C. Both the high solar flux and the low solar flux heated samples (figs. 3a and b) showed oxide thicknesses of the order of ~ 350 A as compared with --- 250 A oxide thickness for the infrared oven heated sample (fig. 3c). Above 500°C, the infrared oven heated samples showed thicker oxides than samples exposed to high solar flux under the same conditions of time and temperature. Fig. 4 gives the depth profiles for such samples heated at a bulk temperature of 520°C. In addition to the smaller oxide thickness, the solar exposed samples show an increase in the amount of oxygen with depth from the surface for the first minute of sputtering (top few tens of Angstroms of the surface region), before it decreases (figs. 3a, b and 4a, b). This reduction of oxygen in the top surface region is especially significant for samples exposed to high levels of solar flux (>/1 M W / m 2).

4. Discussions and conclusions

The major differences observed between the solar simulator heated and infrared heated samples are different oxide thicknesses. These differences can be well accounted for by recalling the oxidation kinetics of aluminum [1,2,4,6] in the 3 noted temperature regimes and by realizing that the absorption of solar radiation in aluminum (as compared to infrared radiation) is localized to the surface of the samples since the skin depth for 5000 A wavelength optical radiation is much smaller and thus more surface localized ( - 5 0 A) than for 10 ~tm wavelength infrared radiation (500 A) [7]. Such surface localized absorption can cause severe temperature gradients between the surface of the sample and the bulk. Model calculations of the surface temperature under solar irradiation in a thermal transport model have not yielded realistic results due to the complexity of the system used, however, indirect estimates of heat loss put the temperature differential between surface and bulk at 50-150°C depending on sample conditions of temperature, surface composition and the wavelength dependence of the optical skin depth. In the low temperature regime (T < 350°C) the oxidation rate for aluminum is small and essentially temperature independent [1]. The surface localized absorption of solar radiation could lead to an increased surface temperature in this regime (---50 to 100°C greater than bulk), however the weak temperature dependence of oxidation rate in this regime would result in observation of little or no difference in oxide thickness for solar (low or high flux) heated and oven heated samples. This is as observed experimentally. In the temperature range 350 to 450°C the rate of oxide growth increases considerably with increasing temperature. An increased surface temperature under high flux solar heating would result in an increased rate of oxidation and hence a thicker oxide for the high solar flux heated sample as compared to the oven heated sample. This is observed experimentally with a 30% thicker oxide film obtained under 1.4 M W / m 2 solar flux irradiation at 430°C bulk temperature as compared to oven heated at 430°C. The thicker oxide under high flux irradiation is consistant with a surface temperature some 60 to 100°C greater than the bulk temperature. The

A. Mesarwi, A. lgnatiev / Surface temperatures under high flux illumination

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low solar flux irradiation (400 k W / m 2) at 430°C should result in a surface temperature lower by a factor of about 4 and hence an oxidation rate similar to that obtained under infrared (oven) heating as is observed. In the temperature regime greater than 500°C, the oxidation kinetics of aluminum take an abrupt turn with rates above 550°C being much less than rates near 500°C [1,3,4]. This would result in high solar flux heated samples (with resultant higher surface than bulk temperatures) having thinner oxides than oven heated or low flux heated samples. This is observed in the data taken for a bulk temperature of = 520°C. The decrease in oxide thickness for the high flux heated sample over the infrared heated sample correlates to a surface temperature of some 100°C greater than the bulk (--- 520°C) temperature. A final point to reiterate is that there is observed a reduction of oxygen near the top surface of the oxidized aluminum under solar simulated irradiation. This reduction in oxygen gives a non stoichiometric aluminum oxide at the surface and is the result of photon stimulated desorption of oxygen bearing species from the surface. Additional experiments [8] have shown the photo-desorption of CO 2 from aluminum oxide species under concentrated solar irradiation. Such desorption enriches the surface in aluminum and causes enhanced absorption of solar radiation in the top atomic layers of the sample thereby incrasing temperature gradients in the sample.

AcknowledgementPartial support for this work was provided by the Department of Energy.

References [1] [2] [3] [4] [5]

M.W. Smeltzer, J. Electrochem. Soc. 103 (1956) 209. M.S. Hunter and P. Fowle, J. Elecrochem. Soc. 103 (1956) 482. P.E. Doherty and R.S. Davis, J. Appl. Phys. 34 (1963) 619. P.E. Blackburn and E.A. Gulbransen, J. Electrochem. Soc. 107 (1960) 944. L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy, PHI Physical Electronics (1979). [6] N.F. Mott, Trans. Faraday Soc. 43 (1947) 429. [7] J.D. Jackson, Classical Electrodynamics(1974). [8] A. Mesarwi and A. Ignatiev, to be published.