Optical switching device

Details Optical switching device Optical switching device

2000-11-13

2001-10-30

Phan, James (Department: 2872)

Optical: systems and elements

Light interference

Produced by coating or lamina

C385S018000, C385S016000

Reexamination Certificate

active

06310725

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an optical switching device comprising a substrate and a switching film which comprises a hydride of a trivalent metal, which hydride can be reversibly switched between a low-hydrogen, mirror-like composition and a high-hydrogen transparent composition by an exchange of hydrogen. The invention additionally relates to the use of such a switching device.
In international patent application WO 98/10329 (PHN 15969) filed by Applicants, a switching device is disclosed in which a film of a hydride of a trivalent metal, such as gadolinium hydride GdH
x
, can be reversibly switched between a low-hydrogen (x<≈2) composition and a high-hydrogen (x>≈2) composition by an exchange of hydrogen. Both compositions have different optical properties. At a low hydrogen content (x<≈2), the film has a metallic mirror-like or reflective character and is non-transparent. At a high hydrogen content (x>≈2), the film is semiconductive, transparent and yellow in transmission. When the switching film in the low-hydrogen state is exposed to hydrogen, the mirror-like state is converted to the hydrogen-rich transparent state. The transparent film can be converted. back to the mirror-like state by heating and/or evacuation of hydrogen. The switching operation can be carried out at room temperature. Said patent application also discloses the use of a hydride of an alloy of the trivalent metal and magnesium. The presence of magnesium in the alloy increases the transmission of the switching film in the transparent state and decreases; the transmission in the non-transparent state, resulting in an improvement of the contrast, which is the ratio between the transmission in of the transparent and the non-transparent state of the switching film. The addition of magnesium makes the switching film colourless in the transparent state. Moreover, the switching speed from the mirror-like state to the transparent state is increased.
The reflectivity of a GdH
x
switching film in the mirror-like (low-hydrogen) state is about 15 to 20% in the visible wavelength range. The reflectivity of a (Gd
0.3
Mg
0.7
)H
x
switching film is about 50 to 70% in the same state. For some applications it is desirable that the reflectivity is increased to a level which, preferably, is comparable to that of a metal reflector like aluminium, which has a reflectivity >90%.
SUMMARY OF THE INVENTION
It is an object of the present invention to increase the reflectivity of an optical switching device in the low-hydrogen state, without adversely affecting the transmission in the high-hydrogen state.
This object is achieved in an optical switching device as specified in the opening paragraph, which is characterized in that a thin magnesium layer is arranged between the substrate and the switching film. Surprisingly, it has been found that the insertion of a thin magnesium layer between the substrate and the switching film remarkably increases the reflectivity of the switching device when the switching film is in the reflective state, without diminishing the transmission of the device in the transparent state. For example in a switching device having a gadolinium hydride film in the low-hydrogen state (composition about GdH
2
), the reflectivity is increased from about 15% to about 70%. When the gadolinium hydride film is in the high-hydrogen state (composition about GdH
3
), the transmission of the device is practically not changed by the magnesium film.
A thin magnesium layer is to be taken to means a layer having a thickness which is about 0.001 to 0.1 of the thickness of the switching film. In a preferential embodiment, the thickness of the magnesium layer is between 0.1 and 25 nm. Below 0.1 nm the effect is less remarkable, whereas above 25 nm the effect is not further improved. Moreover, greater thicknesses of the magnesium film will cause the transmission of the switching device in the transparent state to be be reduced.
Apart from Gd, other trivalent transition and rare earth metals, and alloys of these metals, exhibit similar phenomena. Amongst these metals are e.g. erbium (Er), samarium (Sm), lutetium (Lu), yttrium (Y) and lanthanum (La).
Instead of the above described alloy of the trivalent metal and magnesium, a multilayer stack of very thin (1-2 nm) alternating layers of a trivalent metal and Mg may be used, e.g. a multilayer stack of 50 Mg|Gd pairs. Such a multilayer has the additional advantage that it leads to an increase of the switching speed between the optical states.
Switching of the switching film takes place with an exchange of hydrogen. The transmission of the switching film is governed by the hydrogen content: the transmission increases as the hydrogen content increases. If molecular hydrogen gas is supplied to the switching film, the transmission increases as the hydrogen pressure increases. The hydrogen must be dissociated to atomic H. The rate of dissociation can be increased by providing the surface of the switching film with a thin layer of palladium having a thickness, for example, of 5 nm. At said thickness, the palladium layer is discontinuous. The layer thickness is not critical and is chosen to be in the range between 2 and 25 nm. Thin layers of 2 to 10 nm are preferred, however, because the thickness of the palladium layer determines the maximum transmission of the switching device. In addition, the palladium layer protects the underlying switching film against oxidation.
Apart from palladium, other catalytically active metals which promote hydrogen dissociation, such as platinum, nickel and cobalt, or alloys with these metals, or a niobium-palladium bilayer, can be provided on the switching film.
The molecular hydrogen can be passed in a simple manner from a hydrogen gas cylinder to the switching film at room temperature. A low-hydrogen, mirror-like switching film then changes to a transparent hydrogen-rich state. This conversion is reversible: the transparent film is converted to a mirror-like state by heating and/or evacuation of hydrogen. Said reversible conversion can take place at a temperature close to room temperature, or at higher temperatures. Switching can also be carried out by heating or cooling the switching film in a hydrogen atmosphere.
Atomic hydrogen can also be obtained in other ways, such as by electrolytic reduction of water at the switching film in accordance with the following reaction:
H
2
O+e
−→H+OH
−
Atomic hydrogen can additionally be generated from a hydrogen plasma. In this case, a catalytically active layer, for example, of palladium is not necessary. Atomic hydrogen can also originate from another metal hydride, such as metal alloys for hydrogen storage, which are known per se.
The switching film in accordance with the invention is thin, i.e. its film thickness is less than 2 &mgr;m. The film thickness of the switching film preferably ranges between 100 and 1,000 nm. As hydrogen must diffuse in the switching film, the film thickness determines the rate of full conversion from the mirror-like to the transparent state, and conversely.
Substrates onto which the layers of the switching device may be provided are transparent materials, such as glass, quartz, diamond, aluminium oxide or foil of a (flexible) synthetic resin. The substrate may be flat or curved.
The substrate may be provided with a thin layer of a transparent electroconductive oxide, such as ITO or ATO. Such an oxide layer serves as a transparent electrode in electrochromic devices, such as described in the international patent application WO 98/08139. The use of a thin Mg layer between the electroconductive oxide layer and the switching film according to the invention has the additional advantage of an improved adhesion between the switching film and the electroconductive oxide layer. From a viewpoint of adhesion the thickness of the magnesium layer is preferably between 0.1 and 10 nm.
A preferential embodiment of the device according to the invention is characterized in that the switching film comprises a hydride

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