Pencil and paper create thermoelectric generator

Pencil and paper create thermoelectric generator

Technology News |
By Nick Flaherty

The thermoelectric effect was discovered by Thomas J. Seebeck almost 200 years ago: If two different metals are brought together, an electrical voltage can arise when one metal is warmer than the other. This effect converts waste heat into electrical energy. Waste heat is a by-product of almost all technical and natural processes. It is one of the world’s largest unused energy sources, but it mostly vanishes.

A technical use of the Seebeck effect is so far prevented by the fact that this effect is extremely small in normal metals. Metals not only have high conductivity for electricity but also for heat, so that differences in temperature disappear immediately. In order to use this effect, materials must therefore have low thermal conductivity despite their high electrical conductivity. In technology, thermoelectrics made of inorganic semiconductor materials such as bismuth telluride are already being used in places. However, such material systems are expensive, and their use only pays off only in very few cases. In addition, flexible, non-toxic organic materials based on nanostructures made of carbon are also being investigated for use in the human body.

A team around professor Norbert Nickel at the HZB has now shown that it thermoelectric generators can be created in a much simpler way: With a normal pencil they drew a small area on ordinary copy paper. As a second material, they applied a transparent, conductive plastic coating.

Specifically, the pencil samples (graphite) deliver a voltage of about 0.875 millivolts at a temperature difference of 50°C. This result is comparable to other, much more expensive nanocomposites currently used for flexible thermoelectric elements. The value could be increased by a factor of ten by adding indium selenide to the graphite.

Under the scanning electron microscope and using spectroscopic methods (Raman scattering) at the HZB, the researchers investigated graphite and plastic coating films, “The results were very surprising for us,” says Nickel. “But we have now found an explanation why this works so well: The pencil abrasion forms on the paper a surface of unordered graphite flakes, some graphene and clay. While this reduces the electrical conductivity only slightly, heat can be transported much less effectively.”

These simple ingredients could be used in the future to print thermoelectric components on paper that are extremely inexpensive, environmentally friendly and non-toxic. Such tiny and flexible components could also be used directly on the body and could use body heat to operate small devices or sensors, the researchers believe.

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