Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-07-18
2002-06-04
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S267000, C359S272000, C359S274000
Reexamination Certificate
active
06400491
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is concerned with devices, such as mirrors and windows, having controllable reflectivity.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing “smart windows” with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. These approaches are only partially effective, since the window itself is heated and because these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable low voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications, e.g., energy efficient dimmers for displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
In prior art attempts to exploit reversible electrodeposition of a metal for light modulation, the deposits obtained on transparent substrates presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and exhibited poor reflectivity and high light absorbance, especially when thick. Such deposits have been investigated for display applications involving reflectance from the background, with white pigments often being added to improve contrast. Warszawski (U.S. Pat. No. 5,056,899), which is concerned with displays, teaches that reversible metal electrodeposition is most appropriate for display applications, since significant disadvantages for transmission devices were given (e.g., the possibility of metal deposition at the counter electrode). In general, the prior art literature teaches that an auxiliary counter electrode reaction is required for transmission-type devices to avoid metal electrodeposition at the counter electrode as metal electrodissolution occurs at the working electrode, which would produce no net change in transmission. Such teachings imply that the application of reversible metal deposition to smart windows must involve light absorption by the finely divided electrodeposited metal, which would result in heating of the device itself and thus the space inside. The low reflectance of this type of deposit would not be appropriate for adjustable mirror applications.
Electrolytes described in the prior art literature contain auxiliary redox species (e.g., bromide, iodide, or chloride) that are oxidized (e.g., to bromine, iodine, or chlorine) at the counter electrode during metal deposition under the high drive voltages used. This introduces chemistry-related instabilities during long term operation and leads to deposit self erasure on open circuit via chemical dissolution of the metal deposit, e.g., 2Ag
0
+Br
2
--->2
AgBr. In most cases, this auxiliary redox process hinders metal deposition at the counter electrode during erasure, introducing a threshold voltage that is desirable for display applications. This auxiliary redox process may represent a significant side reaction even when metal electrodeposition/dissolution occurs at the counter electrode and a threshold voltage is not observed. See, e.g., Warszawski, columns 3-4 (when copper or nickel were present in the counter electrode paste) and Duchene et al., Electrolytic Display, IEEE Transactions on Electron Devices, Volume ED-26, Number 8, Pages 1243-1245 (August 1979); French Patent No. 2,504,290 (Oct. 22, 1982). High switching voltages of at least 1 V were used for all the electrodeposition devices which have been found in the patent and literature prior art.
A paper by Ziegler et al. (Electrochem. Soc. Proc. Vol. 93-26, p. 353, 1993) describes an investigation for display applications of the reversible electrodeposition of bismuth in aqueous solutions containing a large molar concentration ratio of halide anions to the trivalent bismuth ion. Halide anion oxidation served as the counter electrode reaction with the 1.5 V write voltage used. The deposits obtained were dark in color and were shown to decrease the reflectance of the ITO surface. Subsequent reports by these authors (Electrochem. Soc. Proc. Vol. 94-31 (1994), p. 23; Solar Energy Mater. Solar Cells 39 (1995), p. 317) indicated that addition of copper ions to the electrolyte was necessary to attain complete deposit erasure. These authors also utilized a counter electrode reaction other than metal electrodeposition/dissolution, and also never obtained a mirror deposit. Thus, Ziegler et al. provide no teachings relevant to the effect of electrolyte composition on the deposition/dissolution rate and quality of mirror electrodeposits.
Warszawski teaches that the use of a grid counter electrode would give a less uniform deposit since deposition on the transparent working electrode is highly localized in the vicinity of the counter electrode grid lines (a consequence of the very thin film of gel electrolyte used). Warszawski also teaches the use of an aqueous gel electrolyte to minimize sensitivity to atmospheric contaminants and to avoid the necessity of having a leak tight seal. Such electrolytes, however, have much more limited temperature and voltage operating ranges compared with organic-based electrolytes with high boiling solvents.
One effort to improve the deposit quality of the electrolytic solution used in a reversible electrodeposition process, described in U.S. Pat. No. 5,764,401 to Udaka et al., requires the addition of organic additives to the solution. Unfortunately, such additives are typically destroyed during the electrodeposition process, greatly limiting cycle life. Furthermore, this approach fails to produce highly-reflectivemirror-like deposits that are required for adjustable mirror applications and provide the superior heat rejection needed for smart windows.
U.S. Pat. No. 5,880,872 to Udaka teaches that the “working” electrode of a reversible electrodeposition structure is degraded, and its working life thereby shortened, by the high voltage required to dissolve the metal film deposited upon it. Udaka states that this consequence can be avoided by adding an alkali metal halide to the device's electrolytic solution, preferably in an amount which provides an alkali metal halide to silver halide ratio of between 0.5 to 5. However, the described electrolytic formulation fails to provide the inherent stability, high quality deposits, good erasure and long cycle life needed for practical applications. Mirror deposits were never obtained.
Prior art literature teaches that the memory effect is temporary. This is a consequence of the occurrence of a counter electrode reaction other than metal electrodeposition/dissolution. The energetic oxidation products generated at the counter electrode can cause dissolution of the metal deposit on t
Rowell Petra V.
Tench D. Morgan
Warren, Jr. Leslie F.
Deinken John J.
Innovative Technology Licensing LLC
Lester Evelyn A.
Mack Ricky
Tench D. M.
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