tesa SE (Hamburg, DE) researchers Klaus Keite-Telgenbuscher, Bernd Luhmann and Alexander Prenzel are developing transparent carbon nanotube heating elements for use in plastic and ceramic window panes that may be used in motor vehicles, locomotives, or aircraft. According to U.S. Patent 20100059494, the heating element includes a carbon nanotube current conductor through which electric power is conducted and electricity is then converted into heat by a voltage drop across an ohmic resistor.
The heating element is a planar or a strip-shaped structure and is provided with at least one support layer and an adhesive layer. The current conductor is designed as an additional, current-conducting layer which is arranged between the support layer and the adhesive layer. The support layer, the current-conducting layer, and the adhesive layer are transparent. The current-conducting layer is designed such that at least 90% of the current flowing overall through the heating element flows through the current-conducting layer. The current-conducting layer is comprised on carbon nanotubes and although single wall carbon nanotubes can be used, multiwall carbon nanotubes are used because of the lower costs.
The current-conducting layer is comprised of carbon nanotubes (CNT). These materials are enormously conductive and, as a result of their fibrous structure, are also readily able to develop a conductive network, with the consequence that, by this means, a conductivity which is sufficient for heat generation is achieved even in the case of a very low fraction in the current-conducting layer. This makes it possible, in a particularly simple way, to achieve the desired transparency of the current-conducting layer. In order to achieve sufficient conductivity, the carbon nanotubes ought to be used as a filler in an amount of at least 0.01% by weight.
The carbon nanotube eating elements can be employed in particular for heatable panes, whether of mineral glass or of plastic glass such as Plexiglas in a motor vehicle, including, in particular, for exterior rearview mirrors, or for an aircraft. Further fields of use of such panes of glass are helmet visors or eyewear glass, for ski goggles, for example. In these and many other fields of application it is advantageous to place a limit on the transparency of the heating element, since it is then able to act simultaneously as a dazzle prevention means.
FIG. 1 shows in a schematic view a heating element of the invention configured as a planar structure. The planar structure has a backing layer 1, a current-conducting layer 2, and an adhesive layer 3. The current-conducting layer 2 is disposed between backing layer 1 and adhesive layer 3, so as to be largely protected from effects of weathering.
Also visible in FIG. 1 is electrical contacting 4 for the current-conducting layer 2. For this purpose, in two surface regions which in this case, and preferably, are located at the edge of the heating element, there is no adhesive layer 3. Instead, at those points the current-conducting layer 2 is covered with a different kind of electrically conductive layer with a greater electrical conductivity 4. This different kind of electrically conductive layer allows current to be fed into the current-conducting layer 2.
Advantageous for the heating element is the use of carbon nanotubes having an average outer diameter of less than 40 nm. With carbon nanotubes, a decreasing outer diameter is accompanied by an increase in mobility, thereby making it easier for a network to form and consequently meaning that fewer carbon nanotubes are used for sufficient conductivity. By reducing the amount of carbon nanotubes used it is possible to increase the transparency of the heating element. Furthermore, as the outer diameter goes down, there is a fall in the scattering of light by the carbon nanotubes themselves, with the consequence again of an increase in transparency.
Modified carbon nanotubes for use in the heating elements are obtainable for example from the companies like FutureCarbon, Bayreuth, and Zyvex, Richardson (Texas, USA), under the trade name NanoSolve.RTM. Moreover, from a market standpoint, there are already carbon-nanotube dispersions in organic solvents and water available (e.g., from the companies Eikos, Boston, under the trade name Invisicon.TM.; Zyvex, Richardson (Texas, USA), under the trade name NanoSolve.RTM., and FutureCarbon GmbH, Bayreuth) that can easily be dispersed into the binder systems used to form the heating system.
tesa Se is an adhesive manufacturer that produces more than 6500 products.