Thin Films on Float Glass: The Extraordinary ... - ACS Publications

Beginning with the late 1960s, new thin film technologies were applied to change the surface properties of float glass, and the article started to eme...
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Ind. Eng. Chem. Res. 2001, 40, 26-32

Thin Films on Float Glass: The Extraordinary Possibilities Charles B. Greenberg Glass Technology Center, PPG Industries, Inc., Pittsburgh, Pennsylvania 15238

Beginning in the late 1950s, and up to present time, the continuous float process has become, worldwide, the preferred method of flat glass manufacture. For almost all architectural and vehicular uses, the glass ribbon is made within a narrow range of soda-lime-silica composition. For centuries up to the 20th, flat glass had been made commercially by batch processes. Thereafter, prefloat continuous processes were introduced either to draw fire-polished sheets upward in ambient air or to draw glass horizontally through rollers, followed by grinding and polishing of both rough surfaces. Respectively, these were sheet and polished plate glass. However, despite the significant glassmaking changes, the flat glass article itself remained an essentially undifferentiated window to the untrained user. There were, of course, improvements in bulk quality and distortion over time, as well as cost gains and new body tints. Beginning with the late 1960s, new thin film technologies were applied to change the surface properties of float glass, and the article started to emerge as a more complex and extraordinary material. This memorial review is about two technologies that drove this and two others that are evolving. Introduction To write now about the technologies that have changed, and may yet change, this most commonplace soda-lime-silica float glass is to remember good friend Jerry Seiner. During my most memorable hours with him, often while traveling on business, we intently shared our views about our respective and very different technologies. We used each other to sound out new possibilities. It was I who learned so much about extraordinary possibilities from him. He spoke often from the point of view of relatively thick polymer coatings on any substrate, and I spoke often from the point of view of very thin inorganic films on glass, both optically passive and optically switchable. The review of “my technology” that follows is really a bringing together of what has been contributed by many PPG colleagues. It is given in the spirit of coalescing and sharing information again, perhaps to crystallize extraordinary thoughts for the speaker and listener both. Passive Thin Films for Solar Control The computer calculations that are now routinely done, to characterize solar performance for windows, have their origin in standard solar radiation curves proposed about 60 years ago.1 Glass was yet to be floated on molten tin commercially,2 and application of thin films to modify surface properties was also ahead. Parry Moon, nevertheless, was assimilating various data and characterizing the light source of primary interest, direct sunlight at the surface of the earth. Moon’s engineering equation for the spectral atmospheric transmission factor after scattering, τλ, is

τλ ) [(τRλ)p/760(τωλ)ω/20(τδλ)δ/800]m

(1)

where λ ) wavelength, τRλ is the spectral factor for scattering by a dry atmosphere at 760 mm pressure, sun at zenith, τωλ is the spectral factor associated with scattering by 20 mm of precipitable water directly overhead, and τδλ is the factor associated with 800 particles/cm3 of dust at the observer level, sun at zenith;

p ) barometric pressure, ω ) depth of precipitable water, δ ) number of particles/cm3, m ) sec θ ) air mass (m ) 1 for sun at zenith), and θ ) zenith angle of the sun. Figure 1 gives a result for air mass ) 2 at sea level with the illuminated surface perpendicular to the sun’s rays; it is assumed that ω ) 20 mm, δ ) 300 particles/cm3, and ozone depth ) 2.8 mm. Starting from this basis, the extraordinary glass can be compared to the ordinary, baseline glass. A nominal baseline clear float glass composition is given in Table 1.3 Various methods of thin film deposition have been used successfully on float glass to modify solar transmittance and reflectance, including electroless chemistry, physical deposition under vacuum, and chemical vapor deposition.4 The last, commonly called CVD, includes both the case of transporting a metal-organic vapor to a hot substrate, with vaporization having occurred at a remote site, and delivery of the metalorganic constituent in a liquid by spray. In both cases the metal-organic constituent is pyrolyzed from the vapor phase on the surface of the hot glass, resulting in growth of a thin film. Pyrolytic spray has been used for a long time, more than 30 years, and one extraordinary, solar-reflective oxide film in particular has proven to have glass-like durability for normal handling, storage, and exposure to the weather.5 The X-ray diffraction pattern for this 40 nm thick (Co1-y-zFeyCrz)3O4 spinel film, where y ) 0.18 and z ) 0.13, is given again in Figure 2.4 Oxide films with a high cobalt content are highly absorbing and reflecting in the visible and near-infrared (NIR) regions of the solar spectrum. For this film, with complex index of refraction ns - iks at 550 nm, ns ) 2.8 and ks ) 0.5. The film has been applied to several different tinted float glass substrates in practice, with the intent to combine the desirable visible/NIR film properties with solar-absorbing substrate properties. It is well-known to make uncoated glass absorbing in different parts of the spectrum by adding small amounts of transition-metal oxides. A very effective film/glass combination is the one given in Figure 3, which includes a substrate designed specifically for high solar infrared attenuation.

10.1021/ie990839r CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2000

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 27

Figure 4. Schematic view of a float line from the side. Not to scale. The horizontal melt line is shown within the furnace. The parallel horizontal lines within the float/forming chamber indicate the formed glass ribbon as it flows on top of a molten tin bath (gray field). The solid circles thereafter are the rolls on which solidified glass travels. The top and bottom brackets between the forming chamber and annealing lehr denote the spray coating area schematically.

Figure 1. Solar energy distribution at sea level, air mass ) 2.1

Figure 5. Pseudoplastic behavior of an aqueous suspension of good stability.11 Data were obtained with a Brookfield viscometer, model LVTD, and spindle no. 1. The time at each spindle speed was 2-5 min; high and low viscosities were averaged. Suspension volume ≈ 100 mL. Figure 2. X-ray diffraction pattern (Cu KR source) of the 40 nm thick, lattice-shifted (Co1-y-zFeyCrz)3O4 film on an annealed, pristine, tinted float glass surface.4

Figure 3. Normal solar transmittance and near-normal film-air reflectance of a (Co1-y-zFeyCrz)3O4 spinel film on 5.5 mm thick IRabsorbing float glass. For reference, the uncoated substrate (AZURLITE glass, a trademark product of PPG Industries, Inc.) has a luminous transmittance ) 0.67 and reflectance ≈ 0.04 at each surface over the whole wavelength range. Table 1. Nominal Composition of a Clear Float Glass3 component

wt %

component

wt %

component

wt %

SiO2 Na2O CaO

73.0 13.8 8.8

MgO Al2O3

3.8 0.17

Fe2O3 SO3

0.12 0.30

Because all float glasses reflect only about 4% at each substrate-air surface, over the spectral range of interest, the effect of the film is dramatic irrespective of the glass composition. What makes the optical result really exceptional, however, relative to what is possible gener-

ally with metals, other oxides, and non-oxides, are the additional film properties: (1) ease of growth over a large area and (2) the ability to withstand normal outdoor ambient exposures. Generally, for thin films to exhibit comprehensive chemical and physical durability, the state of the art requires deposition at elevated temperature. It is useful to do that directly on the hot, clean upper float ribbon surface, in the spray area shown schematically in Figure 4. For this spinel film, easily sublimed, finely ground, trivalent transition-metal acetylacetonates6-8 are delivered in an aqueous suspension to spray guns.9 The concentration of solids is the same as that given earlier for a chlorinated hydrocarbon solution,10 which has also been used.5 The suspension overcomes constraints imposed by the limited solubility of acetylacetonates generally and in an environmentally friendly liquid particularly. In the case of Co(C5H7O2)3, the solubility in water is just 0.2 wt % at 24 °C. It is necessary, however, that the suspension have low viscosity under high spray shear rate and relatively higher viscosity at low shear rate, to aid in stability. That the suspension of the indicated composition is pseudoplastic, at least over some range, is shown by the plot in Figure 5.11 The shear rate associated with pneumatic spraying is at least 103 s-1,12 which is well in excess of the maximum used for Figure 5, but pseudoplasticity is assumed. The goal of film uniformity over a large area limits an oxide’s upper thickness to a value corresponding to the onset of destructive interference.13 It is difficult to control optical path-dependent interference colors over large areas. The target appearance, therefore, is optical neutrality in ambient lighting, accompanied by high

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Figure 6. CIE x-y chromaticity plot for reflected illuminant C, using a Pacific Scientific Spectrogard color system with 2° standard observer. The spinel films were grown from suspension at 595 °C (b) or 650 °C (O) on 6 mm thick clear float glass.11 The normal emissivity given in the text, which was used for the determination of surface temperature, was based on eq 2, using reflectance data from a Perkin-Elmer IR spectrometer, model 621. Application to the higher temperatures is a reasonable approximation.15 The plotted lines in the figure result from including data known for other, larger sample populations prepared at approximately these same preheating temperatures; these data are denoted by + and ×, respectively. Target tristimulus Y (CIE C illuminant) ) 0.36 ( 0.01.

reflectance for the best solar attenuation. The first peak of the reflectance-thickness channel spectrum marks the target for any wavelength, in this instance at about 40 nm thickness for visible light. The CIE x-y chromaticity plot14 in Figure 6 shows the neutral range attained in the target area for two different temperatures of film deposition, for experiments that simulate float ribbon conditions.11 The preheating temperatures, 595 and 650 °C, bracket the lift-off temperature from the tin bath for newly drawn float glass, just ahead of the coater. In the laboratory simulation they were determined as the precut glass, 0.2 m2 in area, exited a preheater at constant velocity and passed under an infrared pyrometer (Raytek Raynger II instrument). The pyrometer was referenced for normal glass emissivity ) 0.86 for its 8-14 µm spectral window. The necessities of the target conditions for reflection predetermine the maximum film thickness, only 40 nm, and the thinness of the film is what makes its actual high level of physical and chemical durability so essential. Hardness, scratch resistance, acid resistance, and alkali resistance are good enough to prevent objectionable film thinning or selective site removal in normal use over many years, which would be easily distinguished optically. Optical changes anticipated from atypical degradation have been considered quantitatively by the following reasoning. Float glass is both acid-resistant, except to HF, and hard (Mohs scale ≈ 5.5). It is, however, much less resistant to alkali attack generally. This is well-known, and care is usually taken to buffer alkali buildup between glass surfaces in storage. In this case the source of alkali is sodium leached from the glass surface itself by condensed water. The spinel film is grown densely enough, at 40 nm, to limit access of alkaline solvents to the underlying glass at the film-glass interface. Debonding is possible, however, in extreme and atypical storage or use as a result of alkali intrusion. A debonding model, which we have used, is shown in Figure 7.11 It illustrates the extreme case of forming an interfacial reacted layer that can be treated for purposes of calculation as a thin film. The model assumes ingress of glass solvent to the film-glass interface by way of open grain boundaries, without thinning of the film.

Figure 7. Debonding model for the extreme case of attack by an aggressive glass solvent.11 The n’s and k’s are the real index of refraction and extinction coefficient, respectively, at 550 nm. Subscripts are indicated parenthetically for spinel film (s), glass (g), ambient air (a), and interfacial reacted zone (r).

This leads, in the model, to a reacted, porous glass structure in the interface, from which debonding is facilitated. In Figure 7, values for complex indices of refraction are assigned. Computations13 have been made successfully this way, in particular for the case of debonding in HF. These show an increasing reflectance in the visible region as debonding progresses, and the result is distinguishable from the decreasing reflectance that accompanies simple film thinning. The model was used during research with the spinel film to help understand that which might actually occur in the extremes. The dependence on similar thin film calculations has been even greater and more commonplace for the manylayered, high-transmittance/low- film designs to be described next. High-Transmittance/Low-E Films The hemispherical emissivity, h, of uncoated float glass over the spectral range 5-40 µm is 0.84. Because glass is opaque over this range and energy is conserved, the emissivity is determined from FTIR near-normal reflectance, R, data (Mattson Galaxy 5030 spectrometer). The calculation is done using reflectance values at 1 µm increments and weighted ordinates for room temperature.16,17 The standardized method is now widely accepted, and this h value for glass is commonly applied, although the actual radiation envelope extends from about 3 to 50 µm. Thus, the value of h is based on a conversion from ⊥, the normal wavelength-dependent emissivity, which is determined from N

⊥ )

∑ i)1

N

(1 - Rλ(i))Ebλ(i)∆λ(i)/

Ebλ(i)∆λ(i) ∑ i)1

(2)

where Ebλ ) radiation emitted by a blackbody at wavelength λ. With new, vacuum-deposited, thin film stacks developed over 20 years ago,18,19 h for coated glass has, remarkably, been reduced to as low as 0.04. This has been accomplished even while maintaining the luminous transmittance above 0.70, a marketing requirement in some cases and a legislated code in others. The details of this technology have been extensively reviewed recently20-22 and will not be repeated here, except to illustrate, briefly, key points about the concept. Films made by CVD, which impart good but less dramatic reductions in emissivity, will not be discussed at all; the growth of fluorine-doped tin oxide films on float glass, as well as the growth of other CVD films, has been reviewed.23 The reflectance of metallic silver, even in thin film form, is high, ideally so in the context of eq 2, and is

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 29

Figure 8. High-transmittance/low- double-stack configuration, with thickness ranges for silver (M1 and M2) and antireflecting dielectric (D1, D2, and D3) layers.21 Not to scale.

Figure 10. Four solid-state electrochromic configurations: three laminated substrates (a) without a counter electrode, (b) with a grid counter electrode,26 and (c) with complementary films; (d) thin film stack. Not to scale. The conductive glass is made conductive with a SnO2-F or indium-tin oxide (ITO) film, not shown here. The conductive film contacts another film or the electrolyte.

examples of the whole class of passive thin films. They, like many others in the class, have attained commercialization, while that is not so for the films to be introduced in the following two sections. Optically Switchable Films

Figure 9. Normal transmittance and near-normal solar reflectance (film-air side) for a coated, monolithic glass of double-stack configuration, from Figure 8.21,22 The substrate is a 3 mm thick clear float glass.

high as well off the normal incidence. So, the two silver layers in Figure 8 account for the accomplished low h when the vacuum-deposited, double-stack configuration of Figure 8 is applied to the glass surface. However, without antireflection in the visible region, these layers would impart undesirably high reflectance to the human eye and low transmittance. The three dielectric layers shown have moderately high nD and near-zero kD over the visible wavelengths (real and imaginary components, respectively, of a D-layer complex index) and are antireflecting in the visible. The secondary layers are present either as primers or as a protective overlayer. The overlayer is helpful, because the stack is grown near room temperature by magnetron sputtering in a vacuum, and does not have the permanence of a CVD film. The high-transmittance/low- film requires incorporation in a sealed and desiccated insulating glass unit or in a laminated environment. The spectral result in the solar range is shown in Figure 9. It is interesting that the retained high solar infrared reflectance has made the film stack very useful for solar attenuation as well as thermal insulation.20,22 This high-transmittance/low-/solar-control film and the spinel solar-control film represent just two of many that have been described for changing the optical properties of glass and the thermal environment in enclosed spaces.4 They are probably the most interesting

There are numerous chromogenic materials that have been prepared as optically switching films, including those exhibiting the following reversible chromisms: (1) electrochromism, driven by charge insertion/extraction under applied voltage, with alternating polarity; (2) photochromism, which usually means darkening under ultraviolet illumination, bleaching thermally; (3) thermochromism, deriving from thermally driven structural or magnetic transitions; (4) piezochromism, deriving from applied pressure or stress. The number of original papers and reviews is vast. This author and others have written reviews that can be used to find original sources.24-28 Out of all of this work, perhaps the most has been written about electrochromism with respect to potential application for flat glass, and there is one thin film that has been most often studied. It is electrochromic R-WO3‚ xH2O, which has been known for about 30 years.29-32 The film is R to indicate that it is amorphous to X-ray diffraction for the highest coloration efficiency, CE, in the visible region. It is hydrated because that, as well as porosity, seems to be associated with chromogenically active electrochromic films.33 CE is expressed as

CE(λ) ) ∆OD(λ)/q

(3)

where ∆OD(λ) is the wavelength-dependent change in single-pass optical density due to transfer of charge q (C‚cm-2). The frequently given reversible ion insertion/ extraction reaction is

WO3‚xH2O + ye- + yJ+ T JyWO3‚xH2O

(4)

for which the film darkens to visible light cathodically, 0 < y < 0.5. JyWO3‚xH2O is a tungsten bronze material, with J ) H or Li most commonly. Unlike the case of the passive films, or even the other types of chromogenic films, the electrochromically changing film only functions within a system. Figure 10 gives some examples of configurations. Each of four examples in the figure shows two transparent, conductive electrodes to carry electrons, as well as an electrolyte,

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reasons, among many others, and despite a large effort at various different research institutions, electrochromism has not been reduced to practice for largearea transparencies. This chromogenism and the three others mentioned here remain, at best, delayed dreams for now for windows. There is very recent testimony to support this viewpoint for electrochromism in a testing report issued by scientists working under the sponsorship of the Department of Energy.41 The DOE has been a longtime supporter of research on chromogenism. Self-Cleaning Films

Figure 11. Electrochromic switching of normal transmittance for R-WO3‚xH2O in configuration a of Figure 10: bleached (s) and darkened (- - -).4,26 The resistively evaporated WO3‚xH2O thickness ) 370 ( 50 nm. The conductive glass is 3 mm thick clear float with a SnO2-F film; sheet resistance ) 25 Ω/sq. The cover glass is 3 mm thick, and the solid polymer electrolyte is PAMPS with ion conductivity ≈ 10-3 Ω-1 cm-1.32,34 Data are for a fresh sample.

which, like the chromogenic film, also must be hydrated, whether polymeric or inorganic. Some hydration, probably even with a Li+ conductor, is necessary to achieve suitably high ion conductivity at room temperature. Poly[2-(acrylamido)-2-methylpropanesulfonic acid] (PAMPS) is an example of a polymeric electrolyte used with lamination (Figure 10a-c); tantalum oxide is a frequently used inorganic electrolyte for all-film stacks (Figure 10d). Both are commonly used hydrogen ion conductors. The tungsten oxide film is grown on one conductive electrode of what is essentially meant to be a stable, charge-balanced, transparent battery, although the configuration in Figure 10a is an exception. It lacks a designed-in counter-reaction to eq 4 for completeness of the cell. It, therefore, lacks stability under repeated cycling, in the sense that it depends on unwanted, oxidation-reduction side reactions within the electrolyte to balance the reversible cathodic coloring of eq 4. One side reaction is

2H2O(polymer) f O2(g) + 4H+ + 4e-

(5)

Generally, this side reaction becomes evident eventually as gas bubbles or delamination, but until it does, it is possible to obtain optical data for the chromogenism of R-WO3‚xH2O. Such pre-bubble-cycling is what has been done to obtain the data in Figure 11. This figure shows the relatively unobstructed, excellent optical switching of the R-WO3‚xH2O film and why it has been of such high interest in the scientific literature. The cell of Figure 10b is balanced by the grid counter reaction

Cu T Cu + + e-

(6)

while there are various well-known possibilities for thinfilm counter electrodes for the configurations of Figure 10c,d. Known counter films include Prussian blue,35 nickel oxide, and iridium oxide. However, chemical reactions are not always so clearly defined, as has been discussed, for example, for Prussian blue,36-38 and maintaining the charge balance is no easy task.39 Furthermore, there may even be instability when a cell is only at rest at room temperature, as demonstrated for the R-WO3‚xH2O/Prussian blue couple.40 For these

Earlier work about decomposing water to oxygen and hydrogen, which led to more recent interest in selfcleaning, traces from a discussion on the electrochemical photolysis of water while using a titania electrode.42,43 An n-type, semiconducting, single-crystal, rutile titania electrode was illuminated at ultraviolet energies more energetic than the 3.0 eV band gap while connected with a Pt black electrode. With generation of an electron, e-, and hole, p+, pair, oxygen evolved at the titania, while hydrogen evolved at the Pt black electrode.

hν (at TiO2) f e- + p+

(7)

H2O + 2p+ f 1/2O2 + 2H+ (at TiO2)

(8)

2H+ + 2e- f H2 (at Pt)

(9)

Hydrogen generation in this way drew high interest subsequently, as a potential bulk source of fuel, but sufficiently high conversion efficiency of solar energy has never been demonstrated for that goal. In part, the limited opportunity is a result of having the active material’s band gap in the ultraviolet. Also, the use of titania, especially in its more active anatase form, has been widely discussed for the purpose of decomposing organic matter, particularly organic pollutants. Anatase titania has a less favorable band gap, 3.2 eV, than does rutile. However, it may generally grow with higher porosity and surface area for the freeradical chemistry that has been described for photocatalysis.44 Both liquid and gaseous pollutants have been targeted, and a compilation of the vast body of literature has been produced and periodically updated.45-48 It was an extension of such work that led to the understanding that similar chemistry could be applied to air purification with thin sol-gel films49,50 and to photocatalytically self-cleaning windows.51-53 The latter work of Heller, Paz, and associates is obviously of particular interest in the context of the present review. The initial reactions of the photogenerated electronhole pair, leading to a free-radical and oxidative chain of events, were given by Heller51 as

H2O + p+ f H+ + •OH

(10)

H+ + O2 + e- f •OOH

(11)

Stearic acid was used experimentally as the organic model for studying free-radical stripping by UV-irradiated anatase on soda-lime-silica glass and fused quartz.52,53 The preparations used to grow anatase films by sol-gel chemistry in the open literature are many. Two were used by Heller’s group to form clear, well-adhering films on 2.5 cm × 2.5 cm substrates. The

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Figure 12. FTIR data54 showing photocatalytic stripping of a spin-cast stearic acid film, at 23 °C and approximately 50% relative humidity, with increasing time of UVA 340 exposure at 2.6 ( 0.2 mW cm-2 (calibrated with a Black Ray UV meter, model J-221). The casting solution contained 1.5 × 10-3 M stearic acid and was spun at 50 rpm. The anatase surface was precleaned with 50/50 2-propanol/water, followed by a 30 min germicidal UV exposure (Voltarc lamp G10T5-L). Mattson Infinity FTIR with a MCT detector. The active, sol-gel anatase film thickness ≈ 110 nm (Tencor P-1 profiler; HCl-etched step); silica barrier thickness ≈ 400 nm.55

titanium compounds were either poly(titanyl acetylacetonate) or Ti[OCH(CH3)2]4, dissolved in n-propanol or water/n-propanol/acetylacetone, respectively. Sols were spin-coated, followed by calcining films at 400 or 500 °C. The soda-lime-silica glass was either used as received or boiled with sulfuric acid to exchange H+ for Na+ at the surface. The latter was done to avoid a loss in photocatalytic activity associated with sodium diffusion into the anatase film.50-53 Photoactivity, while illuminating with a 254 nm Hg line or wide-band 365 nm UV irradiation, was determined for the thin film of stearic acid cast on the anatase film. The rate of decrease of integrated stearic acid absorbance was measured by FTIR spectroscopy for the C-H stretching vibrations between 2700 and 3000 cm-1. It is the promising, quantitative result of this work in particular that suggests the relatively new extraordinary possibility of self-cleaning float glass windows. The possibility has not yet been developed to the extent of the first two films discussed in preceding sections. Data of our own are shown in Figure 1254 for a clear and adherent anatase film on 3 mm thick clear float glass, precoated, as is well-known, with silica to inhibit Na+ diffusion.55 The substrate was not preboiled in acid, and the coated area ≈ 5 cm × 4 cm. The initial, hydrolyzed titania film was prepared at ambient conditions by extracting the dipped substrate, at 10 cm/min, from a sol prepared with 0.3 M Ti(OC3H7)4, a 0.006 M HNO3 catalyst, and 0.3 M H2O2 in 2-propanol. The sol-gel was less than 1 month old at use. The film was crystallized in air in a Thermolyne F6020 muffle furnace preheated to 500 °C. The sample was inserted into the hot furnace, held for 15 min, removed with the supporting block, and cooled with the hot block to room temperature. The ethanol sol for growing the silica barrier film was prepared with 2.6 M tetraethyl orthosilicate, 4.2 M H2O, and 0.03 M HCl. It was reacted at room temperature for 2 h and then at 70 °C to thicken. When used, it was less than 1 week old. The hydrated silica film was dipped/extracted and calcined in the same manner as the titania, before forming the titania overlayer. Anatase is the only confirmed crystalline form of titania in the film, based on X-ray diffraction data

(Philips X’Pert diffractometer, thin film goniometer, 1° grazing angle). All five experimental peaks are accountable, matching up with the five strongest anatase lines or line clusters (JCPDS card 21-1272). Based on an absent diffraction pattern for silica, it is, at best, poorly crystallized. The photocatalytic data in Figure 12 are confirming of what Heller’s group has observed for stripping of stearic acid by a well-formed sol-gel anatase film. Very possibly, self-cleaning will become the next jump step associated with coating technologies applied to float glass, after solar-attenuating and thermally insulating high-transmittance/low- films. Concluding with selfcleaning here, I have tried to briefly review these three, as well as switchable films, so as to give continuity to some remarkable and ongoing changes for otherwise common float glass. Concluding Remark Although research related to the making and coating of flat glass was not Jerry Seiner’s specialty, transitions in glass-related technology over a century have in them much that Jerry understood and taught us about the impact of science and engineering. First there was a process revolution in glassmaking, to the float process itself ultimately, then the application of thin films to alter how electromagnetic energy is passed through glass, and now dreams of doing two new revolutionary things, controlling spectral properties in a dynamic way and making the glass self-cleaning. These are dramatic changes with which I have meant to deal in this paper. Jerry understood the significance of dramatic change in technology, even for such a commodity as glass, or in his case paint and polymers. He sought to lead the way and, in so doing, left me with the certainty that exciting possibilities remain, always. Hopefully, I have helped convey that spirit for him. Acknowledgment Research and publications of several of Jerry Seiner’s and my Distinguished Colleagues on the PPG Collegium have contributed to this paper. The Colleagues are John C. Crano, James J. Finley, and F. Howard Gillery. I thank them all. I also thank Robert Heithoff and staff for providing me with updated data in electronic format for Figures 1 and 3 and for a better understanding about how to calculate emissivities. My thanks to James Finley, Robert Heithoff and Mehran Arbab for taking the time to review this paper; all made valuable inputs. Thanks to Harold E. Donley for all of his early leadership with the spinel film, to Ja´nos Szanyi for providing the expertise for Figure 12, and to Adam Heller for sharing his enthusiasm for self-cleaning. Literature Cited (1) Moon, P. Proposed Standard Solar-Radiation Curves for Engineering Use. J. Franklin Inst. 1940, 230, 583. (2) Pilkington, L. A. B. The Float Glass Process. Proc. R. Soc. London, Ser. A 1969, 314, 1. (3) Swarts, E. L.; Grimm, R. E. Bubble Defects in Flat Glass from Large Tanks. Am. Ceram. Soc. Bull. 1976, 55, 705. (4) Greenberg, C. B. Enabling Thin Films for Solar Control Transparencies: A Review. J. Electrochem. Soc. 1993, 140, 3332. (5) Donley, H. E.; Cathers, W. P. Apparatus for Coating an Advancing Substrate. U.S. Patent 4,111,150, 1978. (6) Sievers, R. E.; Sadlowski, J. E. Volatile Metal Complexes. Science 1978, 201, 217.

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Received for review November 17, 1999 IE990839R