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New developments in photonic sintering pave the way for flexible electronics.

It is hardly news that printed circuits are everywhere--they're the key component in computer memory, processors, and all kinds of electronics. A new discovery promises to fuel a revolution in the technology of producing these circuits, ushering in a new generation of ubiquitous, cheap electronic devices made of circuits printed onto paper or even plastic. A new understanding of what happens when metal nanoparticles are exposed to extremely high-intensity lights--the usual process of creating printed circuits--is changing the way scientists and engineers think about the technology.

The usual way of making printed circuits is to layer on thin coatings of material, then etch away most of it to leave microscopic lines. This process requires complex machinery and noxious chemicals. It's far simpler to print the lines directly onto the substrate using metal nanoparticle inks. Nano-size silver and copper particles are nearly perfect for printing fine structures with standard rotary presses or inkjet printers. But to turn them into functional electronic circuits, the particles have to be fused so they lose their "weird" nanoscale properties and act like classic metal wire.

The process of fusing metal nanoparticles is called sintering. Broadly, sintering simply means using heat or pressure to form a solid mass of material, without melting the material to the point of liquefying. Sintering occurs naturally deep in the earth; anyone who has fired a pot has seen the effects of sintering in the transformation of the clay that occurs in the kiln.

While sintering of powdered metals like those used in nanoparticle inks occurs below the normal melting point of these metals, it still requires heating the materials, which limits the selection of substrates that can be used. And the sintering takes minutes in the oven, making roll-to-roll printing impractical.

Photonic sintering, which uses laser, infrared, and pulsed high-energy lights rather than heat to fuse the metal, has been around for several years. But the physics behind the process was not well understood. In fact, researchers at Oregon State University have shown that much of what people thought they did know about photonic sintering was wrong. The common belief had been that nanoparticles had to melt to achieve the high densities required for functional electronic circuitry. Given that nanoparticle melting points are size dependent, that meant smaller nanoparticles would result in higher densities.

As the OSU team discovered, that was a mistake. The relationship between temperature change and degree of fusion, they found, had been overestimated. When they examined sintered material with a scanning electronic microscope, they found that the smaller nanoparticles in their sample had not actually melted. "We found that it's slightly unusual physics and not what it was conventionally thought to be," said Rajiv Malhotra, an assistant professor of mechanical engineering in the OSU College of Engineering. "Eventually we were able to do some experiments, develop some models which show that you can give a set of optimum parameters to do it as fast as possible at the lowest temperature possible." In fact, the team showed, photonic sintering could occur at much lower temperatures than had been thought, and with greater control of the material porosity. That discovery, announced in December 2015, had the immediate effect of opening the way for sintering printed nanoparticle lines literally in a flash, at room temperature.

What surprised the scientists when they examined the material with scanning electron microscopes was that the smaller silver nanoparticles had not actually melted. Rather, the sintering had been accomplished through a phenomenon called "interparticle neck growth." As Malhotra explained it: "You start with two particles that are just round shapes, and just touching and you heat it up or you put pressure on it, it turns into a dumbbell shape. What is happening is that the mass is being transferred from the surface of the particles, from the volume inside the particles, and so on, into the neck. The neck keeps growing, and as it grows it fills out pores between the particles." The neck growth, not the melting, caused the densification of the metal lines. Thus as Chih-hung Chang, a chemical engineering professor at OSU, explained, "Our study shows that high densification can be achieved without melting of the nanoparticles, contrary to common assumptions."

The team also found that the size of the particle had a significant effect on the temperature at which sintering happened. Malhotra said the team tried mixing particles of different sizes together, starting with 40-nanometer and 5-nanometer wide silver particles, and found that the sintering process took much less time, using less energy in aggregate and working at lower overall temperatures, with the smaller particles. "We found that the smaller particles do not have to go through the melting process but were able to density and form into conducting lines," Chang said. "The larger particles absorb the heat much faster but they don't sinter as well. So a combination of the two can give you some optimal results."

Malhotra said that referring to this as an "ambient temperature operation," as he does, should not be taken to mean that the nanoparticles themselves don't get heated up. "Of course they do," he said. "Otherwise sintering would not occur. It also doesn't mean that the polymer or the glass or metal--whatever the substrate is--doesn't get heated. It does. It's just that it doesn't get heated up that much."

The study also showed that photonic sintering is an inherently self-damping process, meaning that densification slows the pace of photonic heating even before optimal density has been achieved. With this knowledge, scientists can now develop better models that capture the experimental observations of sintering temperature and densification, improving efficiency in the production of electronic circuits. The reduction in the heat required to accomplish the sintering will mean lower energy consumption.

And it also means that photonic sintering can be used to print circuits onto plastic or paper substrates, or other kinds of materials that would not stand up to the heat of traditional sintering methods. The result is likely to be a new generation of inexpensive, flexible devices, from smart tags and fabric embedded with electronic circuits to wrap-around solar batteries and paper-based sensors. Other potential uses include gas sensors and radio-frequency identification tags. "Lower temperature is a real key," said Malhotra. "To lower costs, we want to print these nanotech products on things like paper and plastic, which would burn or melt at higher temperatures." The low-temperature process--and clearer understanding of the physics involved--will also allow enhanced control over the process, which can produce improvements in higher-end electronics such as biomedical sensors and photovoltaic cells.

Finally, since the pulsed light lasts just a fraction of a second, it can be incorporated into high-speed roller-type presses like the ones used to print newspapers and magazines, allowing for mass production of these low-cost circuits. Beyond that, Chang said, even more complicated systems can be envisioned, ones that will be able to process multiple layers of different materials with lights tuned to penetrate only so far. Those kinds of developments could enable adaptive and adaptable smart materials. Chang said the team is working with a private company to commercialize the work but declined to provide details.

By March, researchers in Munich led by Andreas Albrecht had already built on this work, demonstrating a production process for inkjet printing and photonic sintering of silver and copper oxide nanoparticles. The process takes just a few seconds, making it fully roll-to-roll compatible, for ultra-low-cost manufacturing of circuit boards, wiring, antennas, solar cells, and sensors--and, maybe one day, even flexible, roll-up computer screens.

Manny Frishberg, Contributing Editor

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Title Annotation:PERSPECTIVES
Author:Frishberg, Manny
Publication:Research-Technology Management
Geographic Code:1USA
Date:Sep 1, 2016
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