Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 27 TUESDAY MORNING
JS1-TuM-INV.6 SMART OPTICAL WINDOWS OF VARIABLE TRANSPARENCY. E.
Matveeva, Universidad Politécnica de Valencia (UPV), Centro Materiales y Tecnologías de Micro
Fabricación (MTM), Unidad Asociada al Instituto de Ciencias de Materiales de Madrid, Cami de
Vera s/n, E-46022 Valencia, España
The rare earth metals as well as its alloys with magnesium, or proper magnesium alloyed with iron,
nickel, manganese, etc. while being put into the contact with hydrogen form metal hydrides which
are wide-band semiconductors transparent in visible range of electromagnetic spectrum. Reversibility
of hydrogenation reaction allows for fabrication of the so-called Smart Optical Windows (SOW) that
change their reflectivity in function of external impact (application of electric field, change of intensity
of external light, change of temperature, etc.). The two approaches presently existed are hydrogenation
of alloys in gas phase and in liquid phase. The first process is performed in pure H 2 and allows
for achieving a long life-time, mere contrast of reflecting/transparent states and fair reproducibility
of results. However, it would need pumping the hydrogen in and out to the chamber where hydrogenating
metal is contained. Second approach is related with hydrogenation of alloys in liquid
phase under the cathodic bias. Alkaline solutions are mainly used as electrolytes for hydrogenation
(typically 1-6M KOH). This is due to the fact that those metals which possess hydrogenation ability
are stable at high pH values, while neutral or acid electrolytes provoke their fast oxidation. Examples
of electrochemical behavior of different hydrogenated materials will be demonstrated during the talk
together with the optical transformations accompanied the hydrogenation/ dehydrogenation processes.
The generalized design of the SOW with an electrical control consists of the two thin film electrodes
and a conducting media between them. The whole system must be sealed to assure functional stability
and integrity of the device similarly to devices used liquid crystals. One of the electrodes (principal)
is hydrogenated and another is the transparent auxiliary electrode that mainly serves as a means
to apply the potential. Due to limitations toward the electrolytes demonstrated by the principal electrode
(basic conductive media) the second auxiliary electrode must work at the same conditions: immersed
in 1-6M KOH. Indium-tin-oxide (ITO) gave firstly the impression to be a suitable candidate
for an auxiliary electrode in SOW. Nevertheless, its proper electrochemistry in basic electrolytes
governed by the mobility of the structural oxygen ions, deep reduction and changes in composition
and conductivity make the SOWs fabricated with ITO as an auxiliary electrode not reliable. Other
approaches to make conductive a transparent surface (glass/polymer) and exploit possibilities to fabricate
a sandwich-type device are analyzed.
In more advanced SOW technology the use of the polymeric conductive media has been demonstrated.
Nowadays, the polymeric electrolytes (or conductive ionic membranes) have extended applications
in hydrogen production, separation technologies and electrochemical synthesis. The key point
in the conductive polymers (membranes) is the formation of the highly extended and penetrated network
of the conductive nano-metric ionic passes formed from the overlapped solvated (hydrated)
shells of the functional (acid) groups attached to the polymer chains. The dual character of the resulted
material (consistency of polymer and conductivity of electrolyte) offers new options in SOW
fabrication. The device implemented the Nafion membrane soaked in water and other solvents
showed an acceptable working voltage, good transition to the transparent state and satisfactory lifetime
under cycling.
The principal sources for instability and device destruction are discussed based on the experience acquired
in the MTM Center of the UPV.
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