Gold Black and Gold Cermet

Absorbing SurfacesDavid R. McKenzieSchool of Physics, University of Sydney, Australia

Gold black and gold cermet coatings present interesting properties for

radiation detection and solar energy conversion applications. The

characteristics required for Bach specific application can be obtained

by taking advantage of the critical dependence of the properties of

such coatings upon the conditions used for their preparation.

It has been known for some time that metalsmay be evaporated from a hot wire in gas at apressure of the order of 200 Pa to produce blackcoatings which consist of fine particles. Thecoatings produced using gold, gold blacks, arethe best known. The apparatus used by Pfund(1, 2), one of the first to produce metal blacks,is illustrated in Figure 1. The gold is placed inthe tungsten filament F which is electricallyheated. The gold vapour thus generated is chilledby the gas in the chamber and condenses toform tiny crystals which are carried by convectioncurrents to coat nearby objects. Coatings do notform equally heavily on all substrates; the crystalstend to avoid thick dielectric substrates presum-ably because of charge build-up.

Harris (3) found that gold blacks are extremelytenuous coatings with densities that may be aslow as 0.002 times that of bulk gold. A goodimpression of the morphology and tenuous natureof gold black coatings is given by Figure 2 wherecharacteristic crystals of gold, of roughly bipyra-midal shape, can be leen linked together to form anetwork. The size of the crystals depends on suchparameters as the rate of evaporation, the pressureand properties of the scattering gas, and the sourceto substrate distance (4). For instance, with heliumas the scattering gas, the crystallite size variesfrom 5 to 10 nm and the packing fraction of goldvaries from 0.03 to 0.003 as the pressure is in-creased from 60 to 1330 Pa (5).

The visible light reflected from a gold blackcoating is essentially all diffuse (3) and is of theorder of 1 per cent of the incident light for acoating of about 1.5 g/cm 2. When coated on tobright gold the total reflectance of the combinationvaries from about 1.6 per cent at 0.40 tm wave-length to about 3.1 per cent at 2.2 pm for acoating of about 1/gm 2 prepared in pure nitrogen(6). For the same coating, the reflectance becomes

more specular in the infrared but the absorptanceis greater than 97 per cent in the region up to15 pm wavelength.

Because of its high absorption properties andlow thermal mass, gold black is used as a coating

Fig. 1 The apparatus of Pfund (1, 2) for the pro-duction of metal blacks. F is a tungsten spiralinto which the metal to be evaporated is placed.The substrate film D is stretehed over a mercurysurface E

49

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0 2

10

WAVELENGTH (pm)

0.8

UJ

Z

W 0.6

0.4

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on radiation detectors of high sensitivity and fastresponse. Harris (3) has found that a coating of

0.75 g/cm 2 gives optimum performance in therange 1 to 15 pm for detectors in thermopiles.Heavier coatings, although increasing the absorp-tance, damage the response time because of theirthermal mass and relatively poor thermal conduc-tance. Bolometer receivers also have art optimumcoating thickness at about this value. Pyroelectricdetectors are not nearly as sensitive to the thermalmass of the black coating; a coating of 2 to 6 g/m 2

gives optimum performance for a detector usingTedlar polyvinyl fluoride film (7). Black paint,however, has a thermal mass two orders ofmagnitude higher than gold black and actuallylowers the performance of the coated detector atlow frequency, relative to the uncoated detector.

Fig. 2 Gold black viewed m the transmission electronmicroscope. This coating was prepared in 133 Pa of nitro-gen. The crystals are approximately 10 mn in diameter

Gold Smoke Deposits

If the gas in which the evaporation is made isnot completely free of oxygen, a coating is pro-duced which is superficially similar to normal goldblack but does not have the wide-band absorptionproperty. Coatings prepared, for example, in nitro-gen containing 20 per cent oxygen at 400 Pabecome almost transparent for wavelengths longerthan 2 µm (8, 9). These coatings are termed goldsmoke deposits to distinguish them from the nor-mal gold black.

Figure 3 provides a comparison of thereflectance of a copper surface coated with goldblacks prepared in various gases. Curves c and d arefor gold smokes and display the type of behaviourrequired from selective absorbers for the collectionof solar energy. The requirement for this appli-cation is a surface of high absorptance for wave-lengths less than about 2 µm and low emittancefor wavelengths longer than this value. Surfaces ofsolar absorptance 0.88 for normal incidence withemittance 0.10 for room temperature radiation canbe obtained using gold smokes (9). The dip in re-flectance at a wavelength of around 12 µm (seecurves c and d in Figure 3) is somewhat damagingto the low emittance property and has been shownto be due to the presence in the coatings, ofoxides of tungsten (3, 9) produced by oxidationof the evaporation filament. The tungsten oxideenters intimately into the chain structure of thegold crystallites as established (9) by imaging a

Fig. 3 Specular reflectanceof various gold black and goldsmoke coatings on a coppersubstrate (9) :a) gold black prepared inoxygen free nitrogen at 400Pa, mass thickness 0.81 g/m 2

b) gold black prepared innitrogen containing 5 per centoxygen at 400 Pa, mass thick-ness 0.90 g/m2

c) gold smoke prepared innitrogen containing 20 percent oxygen at 400 Pa, massthickness 0.98 g/m2

d) gold smoke prepared innitrogen containing 20 percent oxygen at 1300 Pa, massthickness 0.98 g/m 2

e) tungsten oxide depositprepared in pure oxygen at400 Pa, mass thickness 0.97

g/m2

kiel

sample of gold smoke in the electron microscopeusing one of the diffraction rings of tungsten oxide(dark field microscopy). The insulating crystals oftungsten oxide interrupt the conducting goldchains and cause a very large increase in theelectrical resistivity of the coatings, (3, 9) as wellas the change in optical properties.

There are, however, two disadvantages of goldsmokes as absorbing surfaces for Bolar collectors.The first is the very low resistance to abrasion:both gold black and gold smoke deposits areeasily damaged by mechanical contact as well ashumidity. The second is the tendency to sinterwhen held at temperatures above 69°C (3). In theprocess, the tungsten oxide crystals are excludedfrom their positions between gold crystals therebycausing the chains of crystals to become conductive.The optical and electrical properties of a goldsmoke sintered at 170°C for 3 hours and 180°Cfor 54 hours are almost indistinguishable fromthose of a normal gold black of similar arealdensity (3). Normal gold black deposits themselvessinter when held at temperatures above 100°C,first to a brownish coloured coating, then to ayellow gold film (3). The gold black sinteringprocess is somewhat less rapid if the coating isstabilized at 69°C for 24 hours. This stabilizationis recommended for gold blacks intended for usein radiometers.

Optical Theory of Gold Black and Gold

Smoke

The extreme blackness of gold black over a widespectral region is essentially caused by the widedistribution of effective particle sizes, ranging from

5 nm to about 8 pm (10). Light of wavelengths 7^ranging from the visible to about 50 µm findsconducting particles of dimension approximately) /2 it with which it interacts strongly and is largelyabsorbed or scattered. The smallest particles inthe coating are the gold crystallites themselvesand the larger particles are chains of crystallites.Zaeschmar and Nedoluha (11) have developed animpedance network analogy for gold black which,although neglecting scattering, simulates the be-haviour of gold black over a wide wavelengthband. For wavelengths longer than 50 pm thecoating appears as a smooth conducting film andhas appreciable reflectance

Variation of the oxygen content of the scatteringgas produces a range of coatings whose absorptionregion varies in width. The coatings made in thehighest oxygen content have the narrowest absorp-tion region. The gold crystals are then isolatedfrom each other by insulating material and havea size much less than the wavelength of light.The simple treatment of Maxwell-Garnett (12,13)is then applicable and gives reasonable agreementfor these coatings (9,14). The Maxwell-Garnettrelation for the relative permittivity e of a dilutesuspension in air of spheres of material of relativepermittivity s with volume fraction f is:

(s-1) ( —1)=f

(c+2) (É + 2)

The absorption minimum predicted by the theoryis, however, generally sharper than the measuredcurves. The reason for this may be due to oneor more of the following (15,16) :

Fig. 4 Three specimens ofselective gold-alumina eer-met films produced hy evapo-ration in high vacuum froman electron beam-heatedsource of alumina and a re-sistively-heated source ofgold

51

100

80

60

uu.

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20

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HEATED AT 400 °C

FOR 64 HOURS

0.93

0.10

0

0.2 0.5 1.0 2.0

i

15004Mg 0IAu

1000 A M ^-

STAINLESS

STEEL

5.0 10.0 20.0

Fig 5 Integrated speetral re-flectance of as-groen andheat-treated gold-magnesiacermet films deposited ontomolyhdenum-coated stainlesssteel. The coating method wasradio-frequency sputteringfrom a composite target. Thesolar absorptance and in-frared emittance are respec-tively a and B. After Fan andZavracky (18)

WAVELENGTH ( gum )

(1) A variation of the effective optical constantsof gold with particle size

(2) The effects of interaction between particles(3) The effect of an oxide coating on the particles

Coatings made in low oxygen-containing gasmixtures have broader absorption regions owingto the contribution of scattering (9). Some insightis gained by the application of the theory of Mie(17) for scattering and absorption by spheres. Theeffective particle sizes may be determined by fittingthe experimental curves.

Gold Cermets

A dispersion of gold particles is made morestable against sintering by using a ceramic materialas the dispersing medium rather than air orvacuum. Such dispersions may be produced byevaporation in high vacuum of a metal from onesource and the ceramic from another (Figure 4).An alternative method of production is radio-frequency sputtering from a composite target.These ceramic-metal alloys or cermets have pro-perties intermediate between metals and non-metals. Gold cermet coatings are generallyadherent to metal substrates and resistant toabrasion. When applied to a metal substrate inthe correct thickness and composition range, theyprovide a surface of high solar absorptance andlow thermal emittance (18,19).

Measurements have been made of the opticalconstants of cermets of gold with alumina (20),

magnesia (18), magnesium fluoride (21) and silica(22) with gold volume fractions less than 50 percent. The reason for the high absorptance propertyof cermet coatings on metal is simply that theoptical constants lie in the region where the inter-ference annulment for a quarter-wavelength filmis high over the solar spectrum (23, 24). Electronmicroscopy (19) of a typical cermet film of 25 percent gold volume fraction shows that the goldparticles are of the order of 10 nm in diameter.Scattering should therefore play no part in theoptical properties but the elementary Maxwell-Garnett theory or its extension to non-sphericalparticles by Galeener (25), do not explain thesignificant value of the absorption index in thenear infrared (19, 20) which is essential to the highsolar absorptance property.

This failure of the Maxwell-Garnett formula isunderstandable because of the failure of theassumption that the spheres are sufficiently separa-ted to be considered immersed in a uniform field.The effect of interaction between particles is tocause the induction of multipole moments on thegold particles (26, 27, 28). This leads to enhancedinfrared absorptance (20, 27). Additional effectssuch as the variation of the optical constants ofgold with particle size, the deviation from sphericalshape, the possiblity of electron tunnelling be-tween particles, and the retardation of the electro-magnetic field (29) may allo need to be takeninto account.

52

The absorptance of a single layer film may beincreased by grading the composition of the filmfrom the substrate to the surface in such a way thatthe optical constants vary from values charac-teristic of metals to values characteristic of trans-parent insulators (23). This may be done readilyin a gold cermet film by decreasing the volumefraction of gold continuously towards the surface.Gold cermet absorbers of absorptance 0.94 andemittance 0.03 have been made in this way (28).Furthermore, gold cermets appear to be stable inair at temperatures up to 400°C (18) and invacuum at temperatures up to 300°C (18, 28).This stability is in part due to the general re-sistance of gold to chemical reaction so that thetwo phases nature of the cermet is preserved.Figure 5 shows the reflectance wavelength profileof a gold cermet before and after ageing.

The propertjes of highly selective surfaces suchas gold cermet absorbers are best used in col-lectors which employ insulation by vacuumThe collector array in Figure 6, developed at theUniversity of Sydney (30), consists of tubularglass absorber tubes coated with a selective sur-face of absorptance 0.85 and emittance 0.03 (atroom temperature) and surrounded by anevacuated glass envelope. Collectors such as thisare capable of delivering heat at temperatures upto 300°C without the need for concentration ofsunlight or the need for steering. They are poten-tial suppliers of low and medium grade heat forindustrial processes as well as for domestic hotwater.

Acknowledgements

The author thanks Drs. Granqvist, Doland, O'Neill andIgnatiev for their assistance in preparing this review, Dr. BWindow for permission to use the photograph of the Universityof Sydney solar collector and Professor H. Messel and theScience Foundation for Physics for their support.

Referenees1 A. H. Pfund, Ree. Sci. Inst., 1930, 1, 3972 A. H. Pfund, .7. Opt. Soc. Am., 1933, 23, 3753 L. Harris, `The Optical Properties of Metal Blacks and Car-

bon Blacks', The Eppley Foundation for Research, Newport,Monograph Series No. 1, 1967

4 L. Harris, D. Jeffries and B. M. Siegel, J. Appl. Phys., 1948,19, 791

5 C. Doland, P. O'Neill and A. Ignatiev, J. Vac. Sci. Tech-nol., 1977, 14, 259

6 R. G. Johnston and R. P. Madden, J. Appl. Opt., 1965, 4,1574

7 W. R. Blevin and_ J. Geist, J. Appl. Opt., 1974, 13, 11718 L. Harris and J. K. Beasley, 1. Opt. Soc. Am., 1952, 42,

1349 D. R. McKenzie, _7. Opt. Soc. Am., 1976, 66; 249

10 L. Harris and K. F. Cuff, J. Opt. Soc. Am., 1956, 46, 16011 G. Zaeschmar and A. Nedoluha, 1. Opt. Soc. Am., 1972,

62, 34812 J. C. Maxwell-Garnett, Phil. Trans. R. Soc., (London),

1904, 203, 38513 J. C. Maxwell-Garnett, Phil. Trans. R. Soc., (London), 1906,

205, 237

Fig. 6 The prototype solar collector developed at theUniversity of Sydney. The ten collector tubes are vacuuminsulated and have a selective coating applied to them. Thecollector is shown producing steam at 100 C

14 C. G. Granqvist and O. Hunderi, Phys. Rev. B., 1977,(in press)

15 C. G. Granqvist and O. Hunderi, Solid State Commun.,1976, 19, 939

16 C. G. Granqvist, `Conference on Electrical, Transport andOptical Properties of Inhomogeneous Media', Ohio StateUniv., Columbus, Ohio, 7-9 September 1977, (to appearin A.I.P. Conf. Ser.)

17 G. Mie, Ann. Phys., 1908, 25, 37718 J. C. C. Fan and P. M. Zavracky, Appl. Phys. Lett., 1976,

29, 47819 J. L Gittleman, B. Abeles,•P. Zanzucchi and Y. Aris, Thin

Solid Films, 1977, 45, 920 D. R. McKenzie and R. C. McPhedran, `Conference on Elec-

trical, Transport and Optical Properties of InhomogeneousMedia', Ohio State Univ., Columbus, Ohio, 7-9 September1977, (to appear in A.I.P. Conf. Ser.) ;

21 P. H. Lissberger and R. G. Nelson, Thin Solid Films., 1977,21, 159

22 R. W. Cohen, G. D. Cody, M. D. Coutts and B. Abeles,Phys. Rev. B., 1973,8,3689

23 I. T. Ritchie and B. Window, Appl. Opt., 1977, 16, 143824 D. R. McKenzie, Appl. Opt., 1978, (in press)25 F. L. Galeener, Phys. Ree. Lett., 1971, 27, 42126 J. W. Rayleigh, Phil. Mag., 1892, 34, 48127 W. T. Doyle, `Conference on Electrical, Transport and

Optical Properties of Inhomogeneous Media', Ohio StateUniv., Columbus, Ohio, 7-9 September 1977, (to appear inA.I.P. Conf. Ser.)

28 D. R. McKenzie, unpublished work29 C. G. Granqvist and O. Hunderi, Phys. Rev. B, 1977, (in

press)30 Annual Report, Energy Research Centre, University of

Sydney, 1976

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