Thin Sohd Films, 90 (1982) 425-431

GENERAL FILM BEHAVIOUR 425

Pt-AI203 SELECTIVE CERMET COATINGS FOR HIGH TEMPERATURE PHOTOTHERMAL CONVERSION*

C. SELLA AND TRAN KHANH VIEN

Laboratmres de Bellevue, CNRS, 1 place A. Briand, 92190 Meudon Bellevue (France)

J. LAFAIT AND S. BERTHIER t

Laboratoire d'Optique des Solides, Equipe de Recherche associde au CNRS 462, Universitd Pierre et Matte Curie, 4 place Jussieu, 75230 Parts Cddex 05 (France)

(Received August 16, 1981, accepted September 24, 1981)

Preliminary results obtained with Pt-A1203 cermets prepared by cosputtering onto stainless steel substrates are presented. The reflectivity of the coatings is measured in the spectral range 0.35-15 lam for various platinum volume filling factors and thicknesses. From the variations in the reflectivity and in the electrical resistivity with the temperature of annealing in air the stability of the cermets up to 500 °C is found to be good. Very thin films (about 600 A) with a platinum volume filling factor q close to the percolation composition (q ~ 0.37) exhibit a selective profile well suited to photothermal conversion at high temperatures.

1. INTRODUC~ON

The absorber-reflector tandem is a good system for achieving selective surfaces for high temperature applications 1. For photothermal conversion the absorber has to be absorbing in the solar spectrum and transparent in the IR. It may be a single material with dielectric properties. In the form of a thin film, its dielectric constant and thickness fix the optical properties of the coating, i.e. the visible and IR reflectivity, and the position of the reflectivity threshold. Interest in composite media and especially in cermets arises mainly from the possibility of easily achieving selectivity by adjusting their dielectric constant through the volume filling factor q of metallic inclusions. A homogeneous thin film (where q is fixed) generally exhibits strong interference fringes in the visible range because of its high index of refraction. This effect can be eliminated by means of impedance matching. Considering the flexibility of the cermet dielectric constant, a good solution consists in producing an adequate graded composition-depth profile 2' a.

The cermets that are stable at high temperatures are composites of a refractory non-oxidizable metal and a refractory oxide. Among these systems Pt-AI203 films seem to be promising 4. We present preliminary results obtained with such composites deposited onto stainless steel substrates by cosputtering.

* Abstract of a paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea, Israel, September 21-25, 1981 t Also at: Laboratoire d'Energ6uque Solaire d'Ajaccm Universit6 de Corte, BP 24, 20250 Corte, France.

0040-6090/82/0000-0000/$02 75 © Elsevier Sequoia/Printed m The Netherlands

426 c. SELLA et al.

2. PREPARATION AND CHARACTERIZATION

Thin films (500--2500/~) of Pt-AI20 3 composites were deposited onto stainless steel by r.f. cosputtering. The target consists of an A120 3 disc (13 era in diameter) with circular holes of diameter 5 mm disposed in a hexagonal array. The holes can be filled with platinum pellets. The composition of the film can be varied over a wide range according to the number of platinum pellets used. The stainless steel substrates are located on a rotating sample holder. The films obtained by this rotating sample technique are homogeneous in composition and uniform in thickness. The rate of deposition is 50/~ min- 1 and the residual pressure of argon is 7 x 10 -3 Torr.

The thickness of the films was measured using a Talystep. Their structure and composition were studied using X-ray diffraction for films deposited onto stainless steel substrates and using transmission electron microscopy and electron diffraction on thinner films deposited onto carbon-coated copper grids. The optical properties were investigated by visible (with a Cary 17 spectrometer) and IR (with a Perkin- Elmer 580 B spectrometer) spectroscopy. The electrical resistivity was measured on films deposited onto A1203 substrates. The influence of annealing in air on the optical and electrical properties was also determined.

3. MORPHOLOGY

Pure platinum films prepared under the same conditions as the cermets exhibit a crystalline structure with crystallites of size 120 ~. Electron diffraction patterns and electron micrographs (Fig. 1) of the cermets show roughly spherical platinum grains of diameter 40-50 ~ and with a crystalline structure; the grains are embedded in an amorphous A1203 matrix. The size of the platinum grains seems to be strongly dependent on the temperature of the substrate.

Fig. 1. Electron diffracUon pattern and transmission electron mtcrograph of a Pt-AI20 a cermet film (platinum volume fiUmg factor q = 0.37).

4. ELECTRICAL PROPERTIES

The electrical resistivity Rr~ and its temperature coefficient (TCR) were studied

P t - A I 2 0 3 COATINGS FOR PHOTOTHERMAL CONVERSION 427

in detail according to the following rationale. The results obtained for other cerrnets 3, 4 and for black chrome 2 and the results given by the theories 5 indicate that cermet films with a metal concentration close to the critical filling factor (corresponding to percolation) exhibit adequate selectivity with sufficient thickness. Moreover, the electrical percolation threshold corresponds roughly to the zero of the TCR.

Consequently we studied the variations in the sheet resistance and the TCR with the composition of the cermet and the heat treatment (Table I and Fig. 2). For film thicknesses between 3000 and 5000/Yi, the TCR passes through zero for values of R m close to 104 f~. This value is characteristic of a cermet with a platinum volume filling factor close to 0.37, which is in good agreement with the critical filling factor (qc = 0.33) predicted by the effective medium theory of Bruggemann 4 for spherical inclusions.

The effect of annealing on R[] and the TCR is not significant for thin and thick films up to 600 °C. At higher temperatures, the effect is marked and depends on the

TABLE I PLATINUM VOLUME FILLING FACTOR, SHEET RESISTANCE AND TEMPERATURE COEFFICIENT OF RESISTIVITY OF

Pt-A1203 CERMET FILMS a

q R m (f~) TCR (K- 1)

0.22 (1-2) x 107 - (3 -4 ) x 10 -3 0.37 104 0 0.43 1-5 +(1-2.8) x 10 -3

a Film thickness, 3000-5000 A.

1,2

1

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- 1

--11,2

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~ t.,t.

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÷

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Fig. 2. TCR vs. Rra for Pt -AI20 3 films (thickness, 3000-5000J 0. For each point the change in temperature Is from 20 to 70 °C

428 c. SELLA et al.

thickness of the films. The sheet resistance of a thin film with a platinum volume filling factor of 0.37 remains at 10-20 kfl up to 650 °C. After annealing of the film for 1 h at 800 °C Rn = 3 Mf~ and after annealing for 1 h at 850 °C Rt~ = 200 Gfl. The increase in the resistivity can be accounted for by the increase in the platinum grain size due to recrystallization and the fact that the grains do not remain connected in a two-dimensional system. After annealing, the thin films are insulating. In contrast, R[] for thick films or films with a high platinum filling factor decreases during annealing because the platinum grains remain connected in a three-dimensional system. For instance, R[] ofa Pt-AI20 3 film with q = 0.51 decreases from 15 to 2 fl during annealing for 1 h at 900 °C.

5. OPTICAL PROPERTIES

The reflectivity of the films deposited onto stainless steel was measured in the spectral range 0.35-15 ttm using two spectrophotometers: a Cary 17 (0.35-2.5 lma) and a Perkin-Elmer 580 B (2.5-15 }tm). Since the substrates had been optically polished before deposition, the light scattered by the films can be neglected. Consequently only the specular reflectivity (under near normal incidence) was measured.

The two main parameters governing the spectral selectivity of these cermet films are the metal filling factor q and the thickness d. The filling factor q determines the effective complex dielectric constant ~ of the material. We can predict g using the theories of composite media. It should be noted that for q values close to qc (our conditions) theories should account not only for the optical percolation but also for the optical resonance occurring in the lower part of the visible spectrum (0.3- 0.5 ~tm). Values of q between 0.30 and 0.45 seem to be adequate (Fig. 3) to achieve spectral selectivity, i.e. high IR transparency and good visible absorption. For lower q, the cermet behaves like a dielectric with too low a visible absorptivity. At higher q, the cermet behaves like a metal with high visible reflectivity and low IR transmittivity. Since the intrinsic IR reflectivity of the cermet is too low, it is necessary to rely on the metallic substrate to provide the required high IR reflectivity. The intermediate values of q correspond to the percolation threshold range. In any case, the optical absorption of these cermets is too low in the visible and the cut-off generally occurs at wavelengths shorter than 2 lun (see experimental curves on Fig. 3).

Together with q (i.e. e') the thickness d of the thin film determines the main optical effects occurring in the solar spectrum: the interference phenomenon which strongly modulates the reflectivity of the film and determines the position of the cut- off wavelength (Fig. 4). If d is small compared with the visible wavelength (less than 1000 ,~), the reflectivity curves exhibit only one minimum in the solar spectrum. Considering the experimental curves of Fig. 4, it can be concluded that a film with d ~ 700/~ and q = 0.37 will show an interesting visible absorptivity (its minimum being centred close to the maximum of the solar spectrum) and a smooth reflectivity threshold at around 1.4 ttm. This profile is well suited for high temperature applications. If d is increased (1000-2500/~) two or three fringes appear in the spectrum and their reflectivity maximum reaches 20~o-50~. Moreover the reflectiv- ity threshold is steeper and is shifted towards longer wavelengths; it can be adjusted

Pt-AI203 COATINGS FOR PHOTOTHERMAL CONVERSION 429

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0

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430 c. SELLA et al.

between 2 and 3 lxrn. These thicker films cannot be used, as prepared, for solar applications because of their low solar absorptivity. However, the use of an antireflecting coating or, better, a graded composition-depth profile, may result in a good solar absorptivity for both films (d above or below 1000 ~).

6. THERMAL STABILITY

A Pt-A12Oa film with q = 0.37 and d = 2500 A deposited onto stainless steel was annealed in air in three steps. The variation in its spectral reflectivity is plotted in Fig. 5. Non-negligible changes are observed after annealing for 3 h at 500 °C. The interference is shifted towards longer wavelengths, while the reflectivity is somewhat decreased. At this temperature, the behaviour of the film seems to be stable and completely independent of the duration of the annealing (60 h). It should be noted that even below this temperature the stainless steel substrate is oxidized. After 16 h at 750 °C, the film is pitted and the optical properties are rapidly degraded.

6 0

R= FCM , / , '~ .t

~,/ !

I / I

o{~ I ! ! r/ / "~

, ~ J

d ~', I I I l /a%. ' ~ . t - -

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Y%- I g 1 ~ _

• :4 ,Txgi, ,., ,,

, I , l , , I , , , I , , , l l . . . . J, .a . , . s 1 s 1 o ~(p)

Fig. 5. Experimental spectral reflectiwty of a Pt-AlzO3 cermet film (q = 0.37; d = 2500 A) at various steps of annealing m air: O, as grown; n, 60 h at 500 °C; A, 16 h at 750 °C.

The optical properties of films deposited onto stainless steel seem to be less stable than the electrical properties of films deposited onto A1203 substrates (see Section 4). We conclude that, independently of migration in the film, the diffusion of certain elements of the substrate or the oxidization of its surface may contribute to the degradation of the optical properties of films deposited onto stainless steel. It

Pt-AI20 a COATINGS FOR PHOTOTHERMAL CONVERSION 431

should be noted that the behaviour of the thinner films presented before is qualitatively the same,

The production of a graded composit ion-depth profile with q = 0 (pure A1203) at the free surface of the film may also contribute to its stability. The deposition of a thin film of pure platinum onto the substrate before the deposition of the cermet may also be envisaged.

7. CONCLUSION

Pt -AI20 a cermet films deposited by cosputtering onto polished stainless steel substrates exhibit an interesting selectivity for platinum volume filling factors close to the critical value corresponding to percolation. Very thin films (about 600 A) present a selective profile well suited to photothermal conversion at high temperature. The selectivity of thicker films (exceeding 1000 ,~) should be improved by producing a graded composit ion-depth profile which will enhance the solar absorptivity by impedance matching. The optical properties of these coatings seem to be stable up to 500 °C in air.

REFERENCES

1 B. O Seraphm and A. B. Melnel, m B. O. Seraplun (ed.), Optical Propernes of Solids, New Developments, North-Holland, Amsterdam, 1976, p 927.

2 S. Berttuer and J. Lafait, J. Phys., 40 (1979) 1093. 3 D R. McKenzde, Appl. Phys. Lett.,34(1979)25. 4 H G. Craighead, R. Bartynski, R. A. Buhrman, L. Wojclk and A. J. Slevers, Sol Energy Mater., 1

(1979) 105. 5 D A. G. Bruggeman, Ann. Phys (Letpzig), 24 (1935) 636