TECH SPECIAL.... Paper electronics

  Paper electronics 

Researchers have created a process that transforms paper into a conductive graphite sheet, using an inkjet printer first and then applying heat. The paper could be used for use-and-throw electronics

    Cost-efficient and flexible microchips are opening up applications in the electronics sector for which silicon chips are too expensive or difficult to make, and for which RFID chips, now available on a widespread basis, simply do not suffice: clothes, for instance, that monitor bodily functions, flexible screens, or labels that give more information about a product then can be printed on the packaging.
    Although many scientists around the world are successfully developing flexible chips, they have been forced to almost always rely on plastics as the carrier and, in some cases, use polymers and other organic molecules as conductive components.
    These materials may meet many requirements; however, they are all, without exception, sensitive to heat.
    “Their processing cannot be integrated into the usual production of electronics, because temperatures in production can reach over 400 degrees Celsius,” says Cristina Giordano, who leads a working group at the Max Planck Institute of Colloids and Interfaces and as now come up with an alternative solution.
    They have created conductive structures on paper using a method that is quite simple: with a conventional inkjet printer, they printed a catalyst on a sheet of paper andt hen heated it. The printed areas on the paper converted into conductive graphite. Being an inexpensive, light and flexible raw material, paper is highly suitable for electronic components in everyday objects.
LOW-COST ELECTRONICS
Carbon electronics, which Giordano and her colleagues create from paper, can withstand temperatures of around 800 degrees Celsius during production in an oxygen-free environment, and would not have a negative impact on established processes.
    And that is not the only trump card of the paper-based electronics. The light and inexpensive material can also be processed very easily, even into three-dimensional conductive structures.
    The researchers first convert the cellulose of the paper into graphite with iron nitrate serving as the catalyst.
    “Using a commercial inkjet printer, we print a solution of the catalyst in a fine pattern on a sheet of paper,” says Stefan Glatzel, who wrote a thesis on the project.
    The treated sheets are then heated to 800 degrees Celsius in a nitrogen atmosphere, the cellulose will continue to release water until all that remains is pure carbon.
    Whereas an electrically conducting mixture of regularly structured carbon sheets of graphite and iron carbide forms in the printed areas, the non-printed areas are left behind as carbon without a regular structure, and they are less conductive.
    In another experiment, the team demonstrated how three-dimensional, conductive structures can be created using their method. For this experiment, the team folded a sheet of paper into an origami crane. This was then immersed in the catalyst and baked into graphite.
    “The three-dimensional form was completely retained, but consisted entirely of conductive carbon after the process,” says Stefan Glatzel.
    He demonstrated this again by electrolytically coating the origami bird with copper. The entire crane subsequently had a copper sheen.
    The researchers plan to use the process to create sheets of graphene in the future.

(A) First, an inkjet printer prints a catalyst in a certain pattern on a sheet of paper. (B) Heat then converts the printed areas into conductive graphite in a nitrogen atmosphere and the non-printed areas are converted into amorphous carbon. (C) The fact that only the printed areas are conductive is demonstrated by the subsequent process of electrolysis, whereby the conductive structure serves as a cathode and only the pattern treated with a catalyst is coated with copper.

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