1980-06-18
1982-08-10
Davie, James W.
G02F 117
Patent
active
043435378
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION OF INVENTION
The present invention relates to an electro-optical cell, more particularly for display purposes, comprising a working electrode having electrically controllable optical properties, a counter-electrode, an ion conductor disposed between the two electrodes, and means for applying an electric field or electric current. The cell may be provided with a reference electrode.
In electro-optical cells known as "electrochromic" cells, the electrochromic effect is as follows: When an electric field or electric current is applied, the absorption of transmitted light by an electrochromic layer is so altered that the layer can for example be switched from a state where it transmits light to a state where it is completely or partly opaque. Absorption by the electrochromic material is usually frequency-selective, so that coloured displays can be obtained. Electro-optical cells of the aforementioned kind, based on the electrochromic effect, have been described, for example by M. Green et al in "Solid Films", 38 (1976), 89-100; J. F. Chang et al, "Journal of the Electrochemical Society", 122 No. 7 (1975), 955-962 and numerous other publications and in a number of patent specifications, for example German Offenlegungsschrift No. 2 603 200 (corresponding to U.S. Pat. No. 4,231,641). The previously-mentioned references contain numerous references to the other publications not mentioned here.
At high switching frequencies, electro-optical devices based on the aforementioned known electrochromic effect require relatively high power, for example about 100 times the normal value for liquid-crystal field effects at 1 Hz.
An object of the present invention is to produce an electro-optical cell which is based on an effect different from the known electrochromic effect, having the advantages without the disadvantages of the known electrochromic effect.
According to the present invention there is provided an electro-optical cell, containing an electrode which comprises an intercalatable layer structure.
In a preferred embodiment, the working electrode is made of graphite. The term "graphite" is meant to cover all known forms of graphite, for example highly-orientated pyrolytic graphite (HOPG), evaporated graphite, vitreous graphite and graphite foil (Grafoil, Sigraflex).
A layer structure is called "intercalatable" if foreign atoms or molecules can be inserted between the layers of starting material, generally by a chemical or electro chemical reaction. It is known, for example, that intercalated graphite compounds of the aforementioned kind can be used to obtain metal conductivities comparable with those of copper and aluminium. The optical properties of graphite are also greatly changed by intercalation. Intercalation is often brought about by gas-phase reactions (A. Herold, Bull. Soc. Chim. fr. 999 (1955)). Intercalation can also be brought about electrochemically (M. J. Bottomley et al, J. Chem. Soc. (1963) 5674).
In principle, there are two types of graphite intercalation compounds, namely donor and acceptor compounds. A donor compound is present if the intercalated substance releases electrons to the graphite; we are dealing with an acceptor substance if the intercalated compound receives electrons from the graphite. Examples of donor compounds are alkali, alkaline earth and rare-earth graphite intercalation compounds. Examples of acceptor compounds are transition metal halides, halogen and acid graphite intercalation compounds.
The invention is based on the fact that the optical reflection of a layer structure can be electrically controlled by electrochemical intercalation of foreign atoms. To this end, the layer structure can be partly intercalated in a preliminary operation. The colour effects can be varied, depending on the layer material, the foreign atoms used and the amount thereof. The optical change in the surface of the layer structure, which can be used for display purposes, consists in differences in the colour of an intercalated compound at different concentrations of intercalated foreign a
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Knowles, "Optical Regeneration of Aged WO.sub.3 Electrochromic Cells", App. Phys. Lett., 31(12), Dec. 15, 1977, pp. 817-818.
Green, M., Thin Solid Films, vol. 50, No. 1, 145-150, (1978).
Boersma; M., Cat. Rev.-Sci. Eng., 10(2), 243-280, particularly 245-255, (1974).
Billaud et al., C. R. Acad. Sc. Paris, Series C-305, t. 281, (1975).
Guntherodt Hans-Joachim
Kunzi Hans-Ulrich
Pfluger Peter
Davie James W.
Hoffmann-La Roche Inc.
Johnston George W.
Leon Bernard S.
Saxe Jon S.
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