Recent progress in understanding how nanopaper effectively conducts heat shows its potential as a transparent, cooling material for flexible electronics, according to a review published in the journal Science and Technology of Advanced Materials.
Electronics need a cooling system to diffuse excessive heat and prevent thermal failure, but such systems are normally bulky and unsuitable for thin technologies. Researchers are learning how nanocellulose— the biopolymers making up plant cell walls—could be used to solve this problem. Nanocellulose has various potential applications ranging from electronics to buildings, thanks to its intrinsic high physical strength and heat management properties. Its thin-sheet form, called nanopaper, is an ideal candidate as a cooling material for paper-thin electronic components such as conductive films and foldable solar cells.
Until recently, a standardized method for measuring heat conductivity in 2D materials was lacking, making it difficult to know how effectively heat travels in nanopaper. Heat travels in 2D materials in two directions – along their length, which is called ‘in-plane’, and through their thickness, called ‘through-plane’. Both metrics need to be measured separately.
Kojiro Uetani at Rikkyo University and Kimihito Hatori of plastics company Bethel reviewed studies reporting on the best ways to measure heat conductivity in nanopapers, and comparing nanopaper performance with other materials, such as polymers, composites and papers.
The studies find that heat dissipates faster in nanocellulose than in general crystalline polymers, which have a defective structure that obstructs heat transfer. Mixing nanocellulose with composites increases its mechanical properties and transparency while enhancing its thermal conductivity. White print papers are not good at staying cool because air within the pulp structure inhibits heat transfer.
A team led by Uetani also found that heat conductivity in nanopaper is affected by the size of fibre grains and their orientations. This made them suspect that nanopaper can be designed to carry heat in desired directions. Their experiment showed that nanopaper strips conducted heat better along the length of the fibres, more so than across or perpendicular to them.
Other research found that nanopapers disperse heat well because the material has less thermal resistance between its fibres than other materials, such as carbon nanotubes. One study reported that nanopapers have pores that are generally too small for heat to travel through, which helps direct the heat to move along the fibres themselves.
To measure heat conductivity in nanopapers, Uetani’s team used a technique that applies temperature waves to the surface of a 2D material and then checks the temperature response on the backside of the sample. This method is quick, applicable to small samples or highly thermal conducting materials, and allows separate measurements along and perpendicular to the sample thickness. Other techniques that have been investigated take a long time, or do not support measurements in both directions, making them less appealing.
Further research may reveal other heat management mechanisms hidden in plant-derived material, the reviewers conclude.
For further information contact:
Assistant Professor Kojiro Uetani | E-mail: [email protected]
Institute of Scientific and Industrial Research (ISIR)
Mikiko Tanifuji | E-mail: [email protected]
Science and Technology of Advanced Materials
National Institute for Materials Science