A type of paper that has the thermal properties of glass but is flexible, foldable and optically transparent has been fabricated by scientists in Japan (Adv. Mater. 21, 1595–1598; 2009). The key to the paper's enticing properties is that it is made from cellulose pulp fibres that are orders of magnitude smaller (left image) than the microscale structure found in ordinary paper (right image).
Conventional paper (centre image: the white disk) is not transparent because the size of its fibres and associated voids means that they are very efficient at scattering visible light. In contrast, the nanofibre paper (centre image: the clear disk) made by Masaya Nogi and co-workers from Kyoto University is chemically the same as conventional paper but with a miniaturized physical structure. Because the fibres are smaller than the wavelength of light and densely packed, the nanofibre paper behaves effectively as a homogeneous medium with little scatter and appears transparent.
The nanofibre paper is made from wood flour of the North American variety of the Douglas-fir. It is prepared in a water slurry and then turned into sheets, which are dried and polished with a fine sandpaper. The chemical process breaks down the wood fibres' original structure into much smaller cellulose nanofibres.
Experiments show that a 55-μm-thick sheet of nanofibre paper transmits between 60% and 80% of visible light (in the wavelength range 400 to 800 nm). As well as being transparent, the material offers some important advantages over other optically transparent materials. For example, glass has good thermal characteristics but is fragile and rigid, whereas plastics, which can be flexible, are typically not thermally stable and are prone to unwanted expansion and deformation. The nanofibre paper has the thermal characteristics of glass but is flexible and as foldable as ordinary paper.
Future research needs to explore issues such as the flammability and hydrophilic nature of nanofibre paper, but it is expected that these can be addressed by additional processing. As the nanofibre paper can be produced using existing paper-making facilities, it should be cost-effective and easy to mass-produce. Perhaps it may not be long before it finds its way into applications such as transparent electronics or flexible substrates, and as a host material for flexible displays including e-paper.
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Pile, D. Transparent nanofibre paper. Nature Photon 3, 314 (2009). https://doi.org/10.1038/nphoton.2009.81