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Materials engineering

Colored and sensitive

Films applied to windows and mirrors control changes in the luminosity of the environment

eduardo cesarBy the simple pressing of a button, the glass window of a house or office can become colored in hues of pink, blue, red or other colors. In an internal environment separated by windows its enough to go the  manual control in order not to be seen during a meeting, for example. In the case of external windows, the conditions of luminosity or of climate can also determine the necessary adjustments, without any intervention.  During a cloudy day, the glass would remain clearer, during a sunny day, darker, bringing about more thermal comfort and a reduction in the spending of energy with air conditioning and illumination systems.

The level of transparency is determined by fine films (coatings) with electro-chromatic properties, which change their optical properties with the application of an electric field and return to their initial state by the simple reversal of this field. It is these films that are being developed by researchers at the Physics Institute of the University of São Paulo (USP) with new properties in relation to the same materials used, even commercialized, abroad.

At USP, they are using ceramic materials, such as nickel oxide, which demonstrate the electro-chromatic effect. In the presence of an electrolyte, which is a conductor of electrons and ions (atoms that have lost electrons) and of a contra-electrode, responsible for the closure of the electronic circuit, these materials under the action of an electric field present reactions of electrical charge transference, responsible for the color change of the glasses. A photo-sensor, linked to a battery, controls the level of luminosity and the necessity to change the clarity to darker or vice-versa.

The fine films can also be applied to car rear mirrors in order to diminish the reflection of intense light hitting the mirror at night, such as a car’s head light. This type of mirror functions in conjunction with a photo-sensor that recognizes if the light is strong or weak. When the light source moves away from the car, the mirror returns to its normal condition of reflection.

The windows that change color are already being produced outside of Brazil and used in architecture projects. But there the fine films applied upon the glass are made with organic molecules, such as dyes, which suffer from degradation brought about by the sun’s ultraviolet radiation, and this therefore is a material that is unsuitable for a tropical climate.

“With the inorganic materials that we use, such as nickel oxide, the changing of the window can take as long as ten years”, says professor Márcia Carvalho de Abreu Fantini, from the Crystallography Laboratory of USP’s Physics Institute, one of the participants in the thematic project financed by FAPESP who is studying the preparation and development of ceramic or hybrid nanomaterials (with the approximate size of 1 millimeter divided by a million), coordinated by professor Celso Valentim Santilli, from the Chemistry Institute of the São Paulo State University (Unesp) in the town of Araraquara.

eduardo cesarGlass samples in water receive an electrical chargeeduardo cesar

Chemical routes
Obtaining the fine films is also the object of the study by the Unesp research group. Only that instead of physical processes such as sputtering, they are making use of chemical routes, such as the sol-gel method, for the preparation of nanoparticles. By this process, the researchers produce particles dispersed in a liquid (in the sol state), which are immobilized by a network formed by aggregation or controlled polymerization, resulting in a gel with properties similar to that of edible jelly. One of the studied application is protection against corrosion and the scratching of lenses used for example in video and photographic cameras, especially those for capturing images without visible light, but only with the emission of the heat (in infrared) from people, animals and objects.

While window glasses and traditional lenses absorb electromagnetic radiation both in the ultraviolet and infrared bands, the optical components of the ‘night’ photographic and video cameras have to possess special glass that is transparent and does not absorb the infrared rays emitted by bodies. “The glasses that exist commercially for this purpose function well in this region of the electromagnetic spectrum, but many of them are made of materials that have a high liking for humidity, as is the case of the glasses based on the fluoride of heavy metals, and for this reason they have to be protected”, says Santilli.

Another application for the films is for covering bathroom mirrors so that they will not become misted. In this case the material has as its main  property the fact that it is super-hydrophilic, or that is, it is a film that forms millions of water drops, smaller than those formed by water vapor, that radiate the light. In this manner, even with the shower switched on at hot, the mirror will not become misty. The same material can be used to protect plant house coverings in agriculture and the tiles of buildings. Even when covered with dust and other undesirable materials they become clean again with the rain, because the dirt and the fat don’t stick to the film.

To control and understand the change of state from sol to gel is the major interest of the Unesp group for obtaining materials at room temperature for this route, an innovative maternal was obtained, produced from molds formed by a periodic arrangement of aggregated cylinders of surfactant molecules, chemical substances that act like detergents and are composed of a long molecular chain. Chemical reactions carried out in the interior of these molds allow for the production, at room temperature, of long ceramic filaments. “Our process produces ceramic fibers of the same type that are manufactured today, but which are generally obtained by evaporation of the metal above 1,000º Celsius, making the product much more expensive”, says Santilli.

The Project
Ceramic and hybrid nanomaterials prepared by sol-gel process (nº 01/02258-8); Modality Thematic Project; Coordinator Celso Valentim Santilli – Unesp; Investment R$ 414,568.00 and US$ 311,544.36 (FAPESP)