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Thermochromic effect with electrochromic materials in domestic appliances Disclosure Number: IPCOM000012548D
Publication Date: 2003-May-14
Document File: 2 page(s) / 38K

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Thermochromic effect with electrochromic materials in domestic appliances

Many domestic appliances are equipped with hot plates to supply heat, e.g. coffeemakers and irons. These hot plates are generally open and exposed to human touch (i.e. the consumer's touch), which leads to obvious problems if the plate is still hot even when the main power supply is switched off. It is desirable to have an indicator to tell a consumer when such a hot plate is too hot to touch, mainly for safety purposes. To this end, it is desirable to provide the hot plate with a thermochromic coating (i.e. a coating that changes colour reversibly when at a higher temperature).

� � � � � � � � � � � � � � � Sol-gel coatings are used in steadily more appliances, for functional (e.g. abrasion resistance or gliding in irons) or decorative effects (colours in irons or shavers). Hence sol-gel technology/chemistry is a well-developed field as far as Philips goes. In many cases, fillers are used to tune the functionality (scratch-resistance) or decorative value (colour) of such coatings. One way to combine a decorative effect in sol-gel coatings, along with the thermochromic functionality desired above, would be to incorporate fillers into the sol gel coating which are thermochromic (materials which change their colour in response to a temperature change). However, it is not a trivial matter to find suitable materials that can withstand high temperatures (thermally stable), and are stable to repeated colour change.

� � � � � � � � � � � � � � � This invention describes an effective, indirect method to obtain a thermochromic effect in hot plates. Electrochromic materials change their colour when an electric field is applied across them. However, if the electric field is coupled to a thermostat that is activated at the desired temperature, a thermochromic effect can be achieved, provided that the electrochromic change is bistable .

Electrochromism relates to a change of the transparency of a given material upon the application of a small voltage. In principle an electrochromic device consists of a� two transparent conductive glass (1) , an electrolyte (3), an electrochromic material (4),� and a counter electrode (2). In general the reaction leading to darkening and bleaching of the glass is a result of electron transfer accompanied by ion insertion or extraction at the counter electrode layer. In the cases of tungsten oxide the reaction can be summarized as follows:

Colourless� � � � WO3 + xLi+� � � � � � � � � � � � � � � � LixWO3� � � � � � � coloured

Here colourless hexavalent tungsten oxide is reversibly reduced to a compound of mixed valence, partly having pentavalent tungsten in the coloured state film.

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The most common way of deposition of electrochromic films on ITO (Indium Tin Oxide) used today, is sputtering or evaporation of tungsten oxide. This process has to be performed in vacuum and/or high temperatures and is quite expensive and energy consuming. Electrochrom...