Investigating the formulation of conductive Cu nanoparticles ink and its properties and behaviours

Abstract

Printable functional materials opened up a new area of applications that have immense market potential of flexible electronics. There are a few types of ink and conductive ink are focused in this research. There is a rising interest to develop copper based ink because of its low cost and low resistivity. However, copper nanoparticles are highly prone to agglomeration and oxidation which requires the use of surfactant for stabilization. Sintering is required to remove the organic compound in the ink and it is a challenge to achieve a low temperature sintering. In addition, there are various properties that the ink needs to be fulfil in order to be compatible with specific printing technologies. In this study, conductive copper inks are formulated and coated on polyimide substrate followed by sintering to achieve copper thin film. 1-hexanol, 1-octanol and 1-decanol and the corresponding surfactant was used in the formulation for the ink and their wetting capabilities on substrate are evaluated based on the contact angle measurement. Different substrates such as glass, silicon, polyimide and polyester substrate was tested. Polyimide had the lowest contact angle (<10°) with the inks while glass had the highest. Surface tension and conductivity of the ink was found to increase with the alcohol chain length. TGA results showed that higher temperature is required to remove the longer chain 1-decanol. Hence, 1-octanol was found to be the optimum to achieve low sintering temperature and high conductivity. Subsequent samples uses 1-octanol ink and polyimide substrate. Different sintering temperature, 50,100,150,200 and 250 °C was conducted on the 1-Octanol ink. Sintering removes the organic compound and allows the Oswald ripening of copper nanoparticles followed by necking in the film to enable better conductivity. TGA analysis showed 150°C was significant enough to remove majority of the organic compound. SEM shows higher density of fused particles at 150°C and this increases when sintered at higher temperature. Resistivity drops with higher temperature and 200°C was found to be optimal between the balance of low temperature and resistance value. Study on the conductive ink’s pH (12-8) was done and found little impact on the stability. Resistivity was found to be the lowest at pH 10 ink, thus could be the threshold pH. SEM revealed crack density increases as the ink pH decreases. Viscosity of ink was modified with an additive (EC) and the viscosity increases with the concentration. SEM reveals this additive remain in the film and the resistivity increases with the concentration. It was concluded that the optimum concentration of EC in the ink was 1wt%. This study presented the formulation of ink that were able to tune the mentioned properties. It is hoped that this research will provide a stepping stone for the subsequent studies to improve on the formulation of this ink to cater to different printing technologies. This will then set a new path in being a part of fabricating devices for many industries such as medical, communication, defence, security, manufacturing etc. in the future.