This invention relates to a process for the manufacture of polycrystalline thin films of Mmo.2Tbo.sC02..
An object of this invention is to manufacture polycrystalline thin films of Mnio.2Tbo.BC02 capped with layered palladium by high temperature electron beam evaporation technique on clean glass substrates.
It is also an object of the present invention to manufacture a new material, namely, thin films of Mmo.2Tbo.sC02 with optical switching properties when interstitially loaded with hydrogen by electrochemical route. A layered palladium capping is achieved in situ over Mmo.2Tbo.8C02 thin film acting as protective and catalytic during electrochemical optical switching.
Accordingly, this invention describes a simple process 10 acnieve
polycrystalline Mnio.2Tbo.8C02 thin films of thickness l00 nm by high temperature electron beam evaporation technique. Film is grown over Ar+ plasma cleaned glass substrates with precise thickness monitoring using quartz crystal thickness monitor. An in situ layered capping of palladium is achieved which prevents oxidation of active alloy layer and in addition

acting as catalytic material for hydrogen dissociation and absorption during the electrochemical loading. Structural and morphological
characterization using glanzing angle X-ray diffraction and scanning electron microscopy demonstrate the phase formation and smooth morphology of the film respectively. The compositional analysis gives details regarding the presence and concentration of the constituent elements. Comparing the EDX of the alloy thin film and Pd capped thin film reveals a good capping since peaks only due to Pd are seen in the Pd capped alloy thin film. Contrary to the observation of island like growth of Pd thin film over rare earth thin film base, in the present study it has been observed a closed top layer of Pd. This could be due to cubic crystal structure of Pd matching with the cubic structure of the alloy. Pure rare earth film crystallizes with hexagonal crystal structure and hence favor island like growth of Pd over them.
Electrochemical hydrogen loading in thin film has the advantage of precisely being able to control the hydrogen concentration by careful selection of the electrical current. Electrochemical charging and discharging of interstitial hydrogen form 1 M KOH solution demonstrate the optical switching in the thin film. The optical transmittance is close to zero during the initial stages of loading till a hydrogen concentration of ~ 2.4 hydrogen atoms per formula unit. Transparency of ~ 60-70 % is

achieved on higher hydrogen concentration (> 3 hydrogen atoms per formula unit). Charging of the working electrode results in accumulation of charges on the electrode/electrolyte interface and hence a sharp fall in the electrode potential by -0.5 V. Thereafter hydrogen atoms diffuse into the thin film matrix to form hydride with accompanied change in electronic and optical properties. The metallic dark color of the film changes to bright yellowish color. When a potential of-1.2 V
vs Ag/AgCl is applied, the film reaches the high-hydrogen state and becomes transparent. Note that the maximum transmission of a sample is limited to 60% by the reflection and absorption in the Pd topcoat, the discharged state shows a minimum of 40% transtnittance. Hence we see a switching action from the dihydride to trihydride state with a transmittance variation of 20-30 %. The switching time (defined as the time required to reach 90% of the maximum transmittance) is about 50 s for a current density of 1 niA/cm2. The cyclic durability of the films has been studied and are comparable with that of rare earth based switchable mirror films. Preferential interstitial site occupancy for the hydrogen atoms has been analyzed based on the curent voltage characteristics of alloy thin film.

Wide industrial applications in areas like hydrogen sensor, switchable mirror, smart controllable widows, domestic and commercial decorative applications are highly promising.

We Claim:
1. A process for the manufacture of polycrystalline thin films of Mmo.2Tbo.8C02 capped with layered palladium comprising the steps of growing a thin film of Mmo.2Tbo.sC02; on clean glass substrates by high temperature electron beam evaporation method; providing a layered protective and catalytic palladium capping on the said film by the said method; and electrolytic hydrogen loading of the said film.
2. A process as claimed in Claim 1 wherein the film is grown over
Ar+ plasma cleaned glass substrates.
3. A process as claimed in any one of the preceding Claims wherein
the film is grown with its thickness monitored by quartz crystal
thickness monitor.
4. A process as claimed in any one of the preceding Claims wherein
the thickness of the film is 100 nm.
5. A process for the manufacture of polycrystalline thin films of Mmo.2Tbo.8C02 capped with layered palladium substantially as herein described.
6. Polycrystalline thin films of Mmo.2Tbo.8C02 capped with layered palladium whenever manufactured by a process as claimed in any one of the preceding Claims.