Researchers have uncovered a relationship between film temperature and densification in photonic sintering of silver nanoparticle films, namely the use of intense pulsed light, or IPL, to rapidly fuse functional conductive nanoparticles, that may lead to faster production of advanced, flexible electronics.
The findings, by engineers with the College of Engineering at Oregon State University (OSU), indicate that there is a temperature turning point in IPL despite no change in pulsing energy, and that the turning point appears because densification during IPL reduces the nanoparticles' ability to absorb further energy from the light.
Densification in IPL increases the density of a nanoparticle thin-film or pattern, with greater density leading to functional improvements such as greater electrical conductivity.
The previously unknown interaction between optical absorption and densification creates a new understanding of why densification levels off after the temperature turning point in IPL, and further enables large-area, high-speed IPL to realize its full potential as a scalable and efficient manufacturing process.
"For some applications we want to have maximum density possible," said Rajiv Malhotra, assistant professor of mechanical engineering at OSU.
"For some we don't. Thus, it becomes important to control the densification of the material. Since densification in IPL depends significantly on the temperature, it is important to understand and control temperature evolution during the process. This research can lead to much better process control and equipment design in IPL."
With potential applications in printed electronics, solar cells, gas sensing and photocatalysis, IPL sintering allows for densification in a matter of seconds over larger areas compared to conventional sintering processes such as oven-based and laser-based.
Earlier research showed that nanoparticle densification begins above a critical optical fluence per pulse but that it does not change significantly beyond a certain number of pulses.
The OSU study, published in Nanotechnology, explains why, for a constant fluence, there is a critical number of pulses beyond which the densification levels off.
"The leveling off in density occurs even though there's been no change in the optical energy and even though densification is not complete," Malhotra said. "It occurs because of the temperature history of the nanoparticle film, i.e. the temperature turning point."
A smaller number of high-fluence pulses quickly produces high density. For greater density control, a larger number of low-fluence pulses is required. "We were sintering in around 20 seconds with a maximum temperature of around 250 degrees Celsius in this work," Malhotra was quoted as saying in a news release.
"More recent work we have done can sinter within less than two seconds and at much lower temperatures, down to around 120 degrees Celsius. Lower temperature is critical to flexible electronics manufacturing."
"To lower costs, we want to print these flexible electronics on substrates like paper and plastic, which would burn or melt at higher temperatures. By using IPL, we should be able to create production processes that are both faster and cheaper, without a loss in product quality."