There are many sputtered metal film heaters in the market, but they are opaque and can only be used on the back of the anti-counterfeiting label. Also, the heating rate is slow and the bending resistance is usually poor.
A research team led by Prof. JI Shulin from Hefei Institutes of Physical Science (HFIPS) of Chinese Academy of Sciences (CAS) gave solutions. They developed a novel silver nanowire transparent heating film. It can be used to fabricate a paper-based silver nanowire film heater. Its sheet resistance is much smaller than that of ITO. Under the low voltage of a micro battery, rapid temperature change would be realized. Combined with thermochromic inks of different discoloration temperatures and discoloration directions, the film would achieve intelligent anti-counterfeiting discoloration effect.
Prof Ji, who has been working on this project for years and is now expecting this mature technology to be commercially marketed, gave more explicit introduction of the new system.
The heating system is based on percolative heat exchange of silver nanowires; the film is optically transparent and does not interfere with high-definition display on paper, plastic and other substrates.
Physicists print multiple and overlapping high-resolution thermochromic patterns on the substrate. The corresponding spatial position of each pattern and the discoloration directions of thermochromic inks as well as the discoloration temperatures are different.
The heat exchange coefficient between the heater and the thermochromic object is adjusted by the manipulation of the silver nanowire film percolation structure, and the transformation type and speed of thermochromic patterns can be controlled through the accurate spatio-temporal response of temperature under a series of set voltages.
As the combination of discoloration temperature and direction is unknown, simply applying friction heat or fire discoloration by imitators cannot show the required discoloration switch. In this way, the real intelligent anti-counterfeiting or complex display effect can be realized.
At the same time, thanks to the optical high transmittance of the invisible heater, the heater and the discoloration pattern are located on the same side of the substrate, avoiding the slow heat exchange across the substrate and heat capacity loss. Therefore, low power consumption can be achieved under the built-in micro battery drive.
This heating technology is used to drive the complex changes of several groups of thermochromic high-resolution patterns to achieve high-end visual requirements.
This intelligent discoloration technology solved the limitations of electrochromic, photochromic, hydrochromic and pressure-sensitive discoloration technology. It’s expected to be applied in the fields of anti-counterfeiting, secrecy, intelligent display, etc.
Vivid exhibition of high resolution pattern changes during multiple thermochromic processes: (a-i) photos of the display sample under the ever-increasing voltage of 0-8 V with gradually enhanced heater temperature. (Image by JI Shulin)