Solar thermal absorbers employing oxides of Ni and Co

Solar Energy Materials 10 (1984) 55-67 North-Holland, Amsterdam

55

SOLAR THERMAL ABSORBERS EMPLOYING OXIDES OF Ni AND Co J.G. C O O K Division of Physics, National Research Council, Ottawa, Ontario K1A OR6, Canada

and F.P. K O F F Y B E R G Department of Physics, Brock University, St. Catharines, Ontario L2S 3,41, Canada

Received 28 June 1983; in revised form 30 November 1983 Despite much recent work on CoO and NiO solar selective thermal absorbers, the mechanism responsible for the selective absorption has not been identified. We take literature data for the optical properties of the oxides and a Ni substrate and find that, using rather simple arguments and calculations, the qualitative features of the published reflectancecurves of these absorbers may be explained. The main mechanism responsible for selective absorption of solar radiation by these oxides is crystal field splitting of the 3d electron states. We show that CoO is a much more promising absorber than NiO, because of its stronger crystal field absorption.

1. Introduction Transition metal compounds are used in selective solar absorbers in various ways [1,2]. Among these are absorbers in which transition metal oxides are employed to absorb visible radiation, hut to transmit the infrared. Two examples of such absorbers are NiO [3] and CoO [4]. CoO in particular has been described as a promising high temperature absorber and recently much work has been done to improve the performance of absorbing surfaces incorporating CoO. For both NiO and CoO, however, the selectively absorbing mechanism has not been clearly identified. In the present paper we select literature data for the optical properties of NiO, CoO and Ni, carry out some rather elementary calculations of the reflectance R ( k ) of thin oxide films on Ni and find that we can explain a number of the features of published R ( k ) curves. Furthermore, we are able to identify the principal mechanisms for selective absorption of sunlight in these structures. Below, we first briefly review the literature on NiO and CoO selective absorbers. We then review literature data for the optical properties of these oxides; we are forced to do so rather carefully, since some of the data are contradictory. Selected data are used to calculate the reflectance of oxide films o n Ni substrates. A discussion concludes the paper.

0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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2. Literature on NiO and CoO selective absorbers

Douglass and Pettit [3] have recently carried out an exploratory study of the selectively absorbing properties of some transition metal oxides. As part of this study, Ni films were heated in air or 02 to produce a NiO film on a reflective Ni substrate, which was subsequently found to be selectively absorbing. Values of the solar radiation integrated absorptance a s as high as 0.85 were reached, even though no attempts were made to improve the films using surface structure. The authors discuss two mechanisms which could conceivably explain their results for the oxides examined, a graded index of refraction and transitions across a band gap, but reject both possibilities. A coated absorbing foil has been marketed under the trade name Maxorb [5], which is prepared by a proprietary process and has an integrated absorptance of 0.97 and a low I R emittance. Mason ahd Brendel [5] describe it as a Ni surface "pile" which is later lightly oxidized and suggest it is selectively absorbing due to multiple reflections in the surface layer and to destructive interference in the oxide layer. Amblard et al. [6] have described an absorber which appears to be very similar to the Maxorb film. We do not discuss Ni oxide films which contain granular Ni particles [7], for resonant absorption falls outside the scope of this paper. Early work has identified films of Co oxide on a metal substrate as selective absorbers of high thermal stability [8,9]. The oxide was described as a semiconductor, but the mechanism responsible for the selective nature of the absorption was not identified. In the last few years, at least four groups [10;4,11;12;13-16] have carried out work on CoO or Co304 absorbers, because of their potential as a high temperature absorber and a replacement for black chrome. The latter, which is extensively used commercially, is known to degrade in the region 350 to 400°C; one reason for the strong interest in CoO is that some of the mechanisms responsible for the degradation of black chrome were considered to be less effective for CoO [11]. As a result of this renewed activity, absorbing films with an integrated absorptance a s as high as 0.98 have been prepared. CoO films on a Ni substrate are stable at temperatures as high as 420°C [4,11], while Co304 as part of a composite film employing noble metals for the reflective surface appears to be stable at 650°C [12]. Co oxides on an A1 substrate are said to be stable up to 600°C [13-16]. For some films the absorber has been clearly identified as CoO, with a thin surface layer consisting of Co304 and hydroxides of Co, while the films of McDonald are said to consist of C0304. The layers with the highest values of a s rely on surface structure, both to couple the incident light to the absorber and to act as an absorber on its own.

Smith et al. [4] and Kruidhof and Van der Leij [10] claim that CoO is a selective absorber of visible radiation, because it is a semiconductor with a band gap near 1 ~m. This suggestion has not played a role in the development of effective CoO absorbers; instead, improvements have been sought by preparing films in various ways and examining the end product. This work has nevertheless been very effective: Co oxides are obviously high temperature thermal absorbers of considerable promise.

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3. Optical properties of NiO and CoO The literature contains much information on the optical properties of Ni (and other) oxides that is of direct use in their exploitation as selective absorbers. This information may be obtained from two sources, which to some extent are separate. First, NiO, and, less so, CoO, have been the subject of much research in the fifties and sixties, since they were good examples of narrow-band conductors, which at that time were of considerable interest. The review of Adler and Feinleib [17] indicates that many careful studies of the electrical and optical properties of NiO especially have been made, in part with the intention of identifying the mechanism of electron transport in this oxide. A second general source of information is the large body of literature [18-20] on spectra of atoms which have been affected by the fields of other atoms or molecules in their environment. In many instances, atomic energy levels are split by such fields into groups of states, between which electron transitions are possible under the stimulus of visible radiation. Such absorption occurs in both NiO and CoO. From these two sources we conclude the following. Moderately pure NiO at temperatures of interest here is an insulator, with a band gap near 4 eV (see e.g. refs. [17,21-23]). Studies of the optical properties of single crystals indicate that their absorption spectrum a(~,) is qualitatively as shown in fig. 1 [24-26]. In the visible r e # o n a has a number of peaks, which are superimposed on a "background" which turns up to reach a plateau near 4 eV. Structure is observed in a at higher energies [22]. Each of the three features - peaks, background and plateau - of fig. 1 are of

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J.G. Cook, F.P. Koffyberg / Solar thermal absorbers

interest here and particularly so the peaks, for they are obviously positioned so as to make NiO, at least in principle, a selective absorber of visible radiation. It has been established that these peaks are due to electron transitions within the 3d energy levels of Ni 2+, which have been split by the ligand field into a doubly degenerate set and a higher triply degenerate set [26,27]. Such an identification can be made unambiguously, for the splitting of the Ni 2÷ states will depend on the symmetry of its environment, which in NiO is octahedral; it has been pointed out by Low [27] that the spectra of Ni 2÷ atoms in MgO, which also see an octahedral structure, are closely related to the NiO spectra. Similar comparisons can be made to Ni 2÷ ions in other octahedral environments and, in general, to transition metal ions in a variety of ligand fields [18-20]. The plateau of fig. 1 has been discussed in a number of publications and various explanations for it have been given. We must briefly mention these, for, obviously, the plateau is responsible in part for the absorption of short wavelength visible radiation and, as we discuss below, the transitions in question may have a bearing on the long term stability of an absorber. Adler and Feinleib [17] describe two mechanisms which could be responsible for absorption just above 4 eV. First, an incident photon can convert a 3d 8 Ni 2÷ ion to a 3d 7 Ni 3÷ ion, adding an electron to the Ni 4s band. Second, an electron can be excited from the 2p band of O into the Ni 4s band. This is a so-called charge-transfer transition, for an electron is transferred from one atom to another [22]. For these and other transitions, a later paper by Koiller and Falicov [21] should also be consulted. A number of other proposals to explain the absorption edge have been made (see ref. [25]), but to our knowledge its origin(s) has not been firmly identified. Less is known about the background, as is described in the literature, of fig. 1. It is of interest here that it is known to be sensitive to deviations from stoichiometry, for such deviations are likely to occur in commercially produced absorbers. Newman and Chrenko [26] have shown that the background of bulk NiO specimens increases with an excess of oxygen, while the peaks are little affected. As a result, stoichiometric NiO appears green, while NiO with an excess of O tends to be black. The studies of Austin et al. [24] on pure and on Li-doped NiO and CoO also indicate that upon Li-doping, the position o f the ligand field absorption peaks changes little, but the background increases. The work of Cherkashin and Vilesov [28] is also relevant in this regard. Generally, deviations from stoichiometry blacken a film for solar radiation, but also make it less selective. Although there is generally qualitative agreement regarding the optical properties of NiO, serious quantitative differences exist. Powell and Spicer [22] conclude from reflectance data that for most of the visible range, n ~ 2.3 and k << 0.1, n - ik being the index of refraction. Their values of n are supported by some literature data [22] and the limits on k agree with the results of refs. [24,26]. But LaFemina [25] has carried out an experiment and an analysis on bulk NiO very similar to that of Powell and Spicer and concluded that for visible radiation, n -- 1.5 and k ~ 0.5. The reason for these large differences is not made clear. Gupta et al. [29] and Blondeau et al. [30] have measured n - i k of thin Ni oxide films and found n to be well below that measured by Powell and Spicer for bulk NiO at the same wavelength, while k is

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59

much larger. It is tempting to conclude that the properties of thin films differ from those of bulk NiO, but Newman and Chrenko [26] have examined both thin films and bulk specimens of NiO and noted no difference. For the present paper we have used the NiO a data of Austin et al. [24] and the NiO n values of Powell and Spicer [22]. These publications are consistent and supported by a number of other sources [26,31]. It is not certain that the films of Gupta et al. [29] and of Blondeau et al. [30] were pure NiO; the former may have been inhomogeneous [29]. The k values of LaFemina [25] appear to be anomalously

high. Turning now to CoO, we note that CoO, like NiO, MnO and CrO, is known as an insulating transition metal oxide [21] and for the present purpose its properties are very similar to those of NiO [21,22,24]. Its band gap (O 2s to Co 4s) is near 6 eV [21,22,32] and not 0.5 or 1 eV as has been suggested recently [33]; see also refs. [4,10]. Fig. 2 gives the qualitative features of the absorption coefficient as measured for bulk specimens. As with NiO, there are three features: a series of peaks, a background on which these peaks are superimposed and, off the graph [22], a high energy plateau. As before, the peaks must be interpreted as ligand field spectra [24,34]. Dominant are two peaks, one at 0.5 ~m and another at 1.2 ~m. For wavelengths larger than 1.2 ~m, a rapidly drops until lattice absorption sets in [24], rendering CoO a selective absorber. It is well to note that the ligand field absorption in CoO is stronger than that of NiO by roughly an order of magnitude (depending on whose data one takes), making CoO a much more effective selective absorber. Furthermore, a ubiquitous f

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J.G. Cook, F P. Koffyberg / Solar thermal absorbers

but unexplained feature of reflectance curves for CoO absorbers, namely a hump near 1 ~m, may be attributed to the presence of a valley in ct at this wavelength. This m a y be seen from fig. 2, in which a typical reflectance curve [13] is also plotted. We are not certain why the ligand field spectra of CoO should be more intense than those of NiO; it may be due to the fact that in both cases the transitions are a priori forbidden, but that the selection rules are more easily circumvented in CoO, in which the d orbitals are less tightly bound [22]. Generally, the absorption spectra of CoO are not only more intense, but also more broad than those in NiO. A number of publications [24,28] discuss the effect of deviations from stoichiometry on a. When for instance an excess of O is introduced, and hence Co 3÷ ions, the background rapidly rises and the film may appear blacker, but it also becomes less selective for visible radiation, since the absorption in the near I R also increases. The presence of Co304 and Co hydroxides near the surface of many of the Co-based absorbers lead us to make the following comments. Co304 is a spinel, with Co 2÷ ions in a tetrahedral structure of the 0 2- ions and Co 3÷ ions in an octahedral structure of the anion sublattice. Its absorption spectra could therefore in principle be quite different from that of CoO. Cherkashin and Vilesov [28] find absorption peaks at 0.7 and 1.5 ~m. Their data suggest that a thin layer of Co304 on a CoO film would have little effect on the reflectance of the composite film and that Co304 by itself should also have a hump in R(?,) near 1 ~m. This is contrary to the conclusions of Smith et al. [4], who claim that a Co304 absorber would not suffer from the hump near 1 t~m. McDonald [12] does not give reflectance spectra for his absorbing films, which are said to consist of Co304, so that such spectra cannot be compared to the absorption spectra of Cherkashin and Vilesov. The absorbers of McDonald may well have consisted of CoO. Little appears to be known about the absorption by Co hydroxides, except that Co(OH)24 - is said to have two absorption peaks near 0.6 ~tm [18]. O H groups near the surface should therefore have little effect on the performance of a CoO absorber, and, in particular, should not affect the 1 ~tm hump. Smith et al. [4], however, claim that O H can cause absorption in the near I R and that it aids solar absorptance. These claims appear to be based on their observations that aging in air causes a large rise in the I R reflectance of their film and the appearance of a hump near 1 p.m, together with the removal of O H groups from the surface of the absorber. It is possible that these changes have little to do with the presence or absence of O H groups, but with changes in surface structure upon aging. Smith et al. do find that the initially fine surface structure tends to smooth out with aging. These matters, although perhaps somewhat tedious, are nevertheless important, for they relate to the difference between an excellent and an acceptable absorber. More information on the optical properties of Co304 and Co hydroxides will be required to sort these matters out.

4. Analysis We have calculated the reflectance R ( ~ ) of NiO and CoO films on a Ni substrate using the optical data discussed above, and some rather elementary theoretical i

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considerations, which nevertheless are quite revealing. We first discuss NiO on Ni, and then turn to CoO on Ni. For our first calculations of R(A), we have used the formula given by Hass and Bradley [35] (see also ref. [36]) for the reflectance at normal incidence of a smooth absorbing film on a smooth metallic substrate. Some typical results are shown in fig. 3. The metric thicknesses chosen correspond to some of the films examined by Douglass and Pettit (ref. [3], hereafter referred to as DP; we take the film thicknesses given by DP to be accurate). Strong interference occurs at these wavelengths and thicknesses. There is good agreement between the position of the maxima and minima above 0.7 ~m or so, and those of DP, fig. 2. If however we assume n = 1.4 [25,29,30] the oscillations in R shift to much shorter wavelengths and the agreement with DP is spoilt entirely. Furthermore, if we take an average value of 0.4 for k [25], the reflectance is reduced to the 5% level, well below that observed. The most likely explanation of all this is the simplest one: that the optical constants we have assumed well describe the NiO layers of DP, that the data of refs. [25,29,30] do not, and that the oscillations in R(~,) measured by DP are due to interference. Our calculated R(X) differs from those of DP in that below 0.7 ~m or so their R(A) is near 0.2 and shows no oscillations, whereas we find strong oscillations around an average value twice as high. Furthermore, between 0.7 and 2.5 ~m our average R(A) is near 50%, while theirs is half that. These differences are probably due to roughness of the air-NiO interface and, possibly, the N i O - N i interface. If 1.0

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J.G. Cook, F.P. Koffyberg / Solar thermal absorbers

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such roughness existed on the scale of i ~m or smaller, phase differences between short wavelength waves incident on and reflected from the Ni substrate would be made random and interference effects would disappear. It is apparent from for instance the work of Graham et al. [37] that the surface of thermally grown NiO layers on Ni may be quite rough on a 1 /~m scale, the oxide itself may be non-homogeneous and the metal-oxide interface may be irregular. These effects are quite sensitive to the grain size of the Ni, its surface preparation, and the temperature and pressure at which oxidation occurs. We have computed the reflectance of a NiO film on Ni that is rough on both its surfaces, so that all phasing effects disappear, by simply computing the intensities of the various reflected and transmitted beams using the formula [38] R=

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for the reflectance at normal incidence of a boundary between two thick regions characterized by indices n 1 - i k I and n 2 - i k 2. This formula is used at both NiO surfaces. Within the NiO, light absorption is taken account of. Interference effects are therefore ignored, as is the tendency of the beam to deviate from the normal. We find that, if the incident intensity is 10, for a 1 ~tm film and 0.6 ~tm radiation, the intensity of the first reflected beam is 0.16I 0 , while the second reaches 0.2210 because of the strong reflectance at the N i O - N i interface. Their simple sum is 0.38•0, which is well above the value ( --- 0.1510) observed by DP. (DP point out their measured value is close to that calculated for the air-NiO interface, but this is only part of the answer.) Higher order terms may be ignored and happily so; the roughness of the surfaces would probably cause the light to deviate appreciably from the normal. One of the main points of this paper is the simple observation that the calculated absorption within the NiO is very weak; for 0.6 ~m light and a 1 ~m thick film, the decrease in intensity is of the order of 10% per pass. Thus, even though ligand field absorption in NiO is a solar-selective process, it is weak, immediately implying that NiO makes a poor choice for smooth thin film absorbers on a metallic substrate. In retrospect, this could have been deduced immediately from for instance refs. [24, 26]. In fig. 4 the calculated R is given as a function for wavelength. For film thicknesses less than 1 ~m or so, the structure is a poor absorber and a weakly selective one at that. The use of thicker films will increase the absorptance, but will also increase the infrared emittance, as the data of DP show. This simple model has permitted us to average the peaks and valleys of the interference-affected R(X) curves of fig. 3. Still to be accounted for is the factor of two difference between our computed R ( ~ ) and the measured data of DP. One possible explanation for this discrepancy of course is that the value of a we assumed for NiO is too low. It is obvious from the above that a is not well known: it is not well known even for pure NiO and is sensitive to deviations from stoichiometry. Another cause for increased absorption is the roughness we have alluded to, for it will tend to soften the mismatch in n - i k at each boundary and decrease its

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reflectance. We have incorporated such grading in a very simple fashion by replacing each boundary between materials of index n ' - i k ' and n " - i k " , by a film of complex index ( n ' + n")/2- i ( k ' + k")/2 (see fig. 5). At 0.6 Fro, for instance, a reflectance of 0.14 at the air-NiO interface is replaced by Rlt = 0.06, R12 = 0.03; at the NiO-Ni interface, a reflectance of 0.4 is replaced by R21 = 0.13, R22 = 0.07. The absorption in the NiO and its top film is again weak, while the absorption in the lower (NiO-Ni) film is strong. In fig. 4 we also show the calculated total reflectance for a 1 Fm thick film of NiO, with 0.1 Fm thick layers at both top and bottom. R()~) is rather insensitive to the thickness of the various films; this is also true for the

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64

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measured data below 1 ~tm or so (DP). The calculated curve shown passes through the mean of the DP curves, if interference in the latter is averaged out; R = 0.15 for visible radiation, R ~ 0.3 at 2 I~m. Thus, some simple arguments based upon literature values for the intrinsic absorptance of NiO and qualitative effects of surface roughness are able to account for the data of DP. In addition, they imply that an absorber based on the instrinsic absorbing properties of NiO is not a promising one: the absorption coefficient of NiO is simply too small. One might attempt to make use of the fact that deviations from stoichiometry tend to fill in the valleys between the ligand field absorption peaks and hence blacken the sample, but such deviations also raise the long wavelength absorption. We have computed R ( ~ ) using the data of Newman and Chrenko [26] for oxygen-rich NiO, but the results are not promising. As Smith et al. have pointed out on qualitative grounds [4], CoO is a more attractive absorber and below we consider this oxide in more detail. We have made reference to the very effective absorbers of Mason and Brendel [5] and of Amblard et al. [6]. These authors suggest the principal mechanism for absorption of light is optical trapping by a Ni pile, but Lampert [39] has found the absorber to consist of a mixture of oxides of Ni and Cr, with some sulphide. The oxide coating grades in composition with depth to NiO. We are of the opinion that the oxidation this absorber is made to undergo in its manufacture does not improve its performance by destructive interference, as these authors [5,6] suggest, but that such oxidation serves to improve the grading of the surface [39]. Destructive interference by a thin NiO oxide layer would be difficult to achieve and the film could quickly detune by aging. This is not observed. We turn now to CoO. As with NiO, there is considerable uncertainty as to the optical properties of bulk samples; the data of Austin et al. [24] are well above those of Pratt and Coelho [40] and of Newman and Chrenko [41]. We have taken the Austin et al. a data as a working hypothesis, since their study appears to be comprehensive and careful and their NiO data appear to be valid for NiO films. For values of n of CoO and n - ik of Ni, we used ref. [22,35], respectively. The published reflectance curves for CoO absorbers indicate interference effects are at best very weak, suggesting that their surfaces are rough. This agrees with the conclusions on their topology reached by others (such as ref. [4]) on different grounds. We have therefore again used our intensity-summing technique and ignored interference. At 0.6 ~tm, we find the reflectances at the two CoO surfaces to be close to the NiO values, but the attenuation of the light intensity in the CoO is much larger: 30% per pass for a 0.3 ~tm film and 70% for a 1 ~tm film. The calculated total reflectance is shown in fig. 6 for these two film thickness. As expected, the 1 rtm hump is very obvious. These reflectance curves are generally higher than literature curves, which often indicate an integrated absorptance a s above 0.9. If we now introduce grading or impedance matching into our calculations as before, by assuming the existence of layers on the top and bottom surfaces of the CoO film with properties intermediate to those of CoO and air and CoO and Ni, respectively, we find, as expected, lower reflectapces R(h), as shown on fig. 6. The separate reflectances of the various

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5. Conclusions On the basis of the available data for the bulk optical properties of NiO, we conclude that NiO films on a smooth metallic substrate form poor selective absorbers, since the intrinsic absorption of NiO for visible radiation is too low. Much more promising are the absorbers formed by lightly oxidising a Ni film with suitable surface structure, which rely on strong and selective absorption by the rough N i - N i O surface and on the thin NiO surface layer to match the optical impedances of air and Ni. CoO on a metallic substrate is a much more promising selective absorber because of the larger intensity of its ligand field absorption, and its properties may be improved further by surface roughness, to avoid undesirable interference effects and to obtain the benefits of increased absorption, particularly near the 1 I~m hump. It is apparent from the above that more accurate information on the optical properties of smooth CoO and NiO films on metallic substrates is required. The effect of the method of film deposition and surface roughness should be studied. The

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optical properties of Co304 should be examined in more detail. The optical properties of irregular films should be calculated in a fashion more sophisticated than that used above. Following custom, the lifetime of CoO and NiO absorbers is usually determined by aging at elevated temperatures for a few months at best. Photon-stimulated degradation is not looked for. We have, however, noted that visible radiation will raise the Ni or Co atom to an excited (antibonding) state, or, if the wavelength is low enough, may produce charge-transfer transitions. The former process is unlikely to affect the stability of the absorber, but the latter are often related to increased chemical activity. It would be interesting to look for the degradation of the CoO absorbers or the Ni-pile absorbers under intense light at elevated temperatures. Until now, we have restricted our attention to NiO and CoO, but one could follow a similar tack with other coml~ounds. Obviously, one could determine if the available data for the optical properties of MnO and FeO, the other two insulating transition metal oxides which are often discussed together with NiO and CoO, indicate these would make good selective absorbers of sunlight. We have not done so in detail. Rather more interesting is the fact [20,24] that spectra due to ligand field splitting are very sensitive to the structure of which the transition metal ions are a part. Selection rules and reasons for which they may be circumvented (spin-orbit coupling, bond hybridisation, lattice vibrations, etc.) are very important in determining the position of the peaks, their height and their width. One could make use of the theory of such spectra to search for good candidates for solar thermal absorbers, or, alternatively, one could search the spectral data tables of for instance Jorgensen's book [18] to find such candidates.

Acknowledgements We are grateful to B. Hollebone for discussions on ligand field absorption, and to J.A. Dobrowolski and F.C. Ho for assistance with our reflectance calculations. Finally, we thank B.O. Seraphin for drawing our attention to the fact that transition metal oxide selective absorbers are poorly understood.

References [1] [2] [3] [4] 15] [6]

B.O. Seraphin, in: Topics in Applied Phys., vol. 31 (Springer, Berlin, in press). C.M. Lampert, Solar Energy Mater. 1 (1979) 319. D.L. Douglass and R.B. Pettit, Solar Energy Mater. 4 (1981) 383. G.B. Smith, A. Ignatiev and G. Zajac, J. Appi. Phys. 51 (1980) 4186. J.J. Mason and T.A. Brendel, Pro¢. SPIE 324 (1982) 139. J. Amblard, M. Froelicher, G. Blondcau, J. Lafait, J.M. Behaghei and R. Herrera, J. de Phys. 42 (suppl. 1) (1981) C1-147. [7] S.N. Kumar, L.K. Malhotra and K.L. Chopra, Solar Energy Mater. 3 (1980) 519. [8] P. Kokoropouios, E. Salam and F. Daniels, Solar Energy 3 (1959) 19. [9] P.J. Call, SERI/TR-31-103 (1979).

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