Meet the Researcher: Ranjeet Rao Discusses 3-D Printing and Co-extrusion

Ranjeet Rao is a Researcher at PARC specializing in colloidal processing and the printing of complex fluids. His current work focuses on PARC’s advanced deposition techniques, such as Co-extrusion (CoEx), a printing technology for creating structured fluid coatings. 

Ranjeet, please tell us about your background and how you arrived at PARC.

I grew up in the suburbs of Chicago and completed my undergraduate and graduate degrees at the University of Illinois at Urbana-Champaign, where I majored in Materials Science and Engineering. That itself was serendipitous, as I hadn’t even heard of Materials Science until high school, where I had a Student Inquiry project with a Materials Science professor at the University of Illinois – Chicago. I was intrigued at how this field underpins so many other aspects of science and engineering, and it just so happened that my state’s flagship school had a top Materials Science department. 

During my undergraduate years, I specialized in ceramic engineering. For graduate school I joined Jennifer Lewis’ Ceramic Processing Research Group, where we focused on colloidal science, particularly that of polymers and ceramics.  A colloidal suspension is one where small particles of one phase are suspended in another material. For example, fog is a colloidal suspension of water droplets in air, while milk is a colloidal suspension of fat particles in water. By understanding the forces between the particles, you can engineer fluids that have the properties that you want. For instance, if you can make it so that all of the particles repel each other, you can make stable suspensions that don’t aggregate or settle. If you change those forces to be attractive ones, you can form strong gels that can withstand gravity and maintain their structure. One of our major research activities was Direct Ink Writing, where I used this control over rheology (the study of how things flow) to make ceramic inks that were fluid enough to flow through a fine nozzle but immediately coagulated after deposition to maintain their structure. In this way, we could make complex 3-D structures without any need for support material, like is needed in a lot of commercial 3-D printers today. 

As a result of my background in creating inks and 3-D printing, I was hired by PARC after completing my graduate studies to help with one of their relatively new technologies, Co-extrusion.

Can you tell us more about your work at PARC?

I work in the Hardware Systems Lab, and specifically in the Advanced Materials and Deposition Systems group. Very simply, we explore new ways to print or deposit materials that would be otherwise difficult to print. We also develop new ways to assemble structures that are difficult to make through conventional means. This could include 3-D printing of graded material structures, or it could be the production of structured thick films, which we’ve demonstrated using our Co-extrusion technology.

What exactly is Co-extrusion?

Co-extrusion, or CoEx, is a PARC technology where we take two or more fluids and put them through a custom multi-fluidic stack. This stack routes the fluids in such a way that they interdigitate or interweave themselves before exiting the printhead nozzle. By controlling the rate of flow of the two fluids, we are able to create interesting structures that can be used for a wide range of applications.  

What are some applications or uses for PARC’s Co-extrusion technology?

The first application area that we focused on was improving the performance of photovoltaic (solar) cells. For conventional cells, metallic traces need to be put onto the silicon surface in order to conduct the photogenerated current off into the electric grid. When figuring out the best way to put down these traces, there is a design trade-off – wider traces will have lower resistive losses, but narrow lines will shade less of the cell and result in more current. At the time, the most cost-efficient technology was screen-printing of silver lines, which was limited to relatively wide lines. We found that CoEx could improve upon this process. We used two different fluids, (1) a functional ink containing silver particles, and (2) a sacrificial fluid that burns off during later processing of the wafer. By adjusting the pressures of the two fluids, we could create narrow lines that were more than twice as tall as that created with screen printing, resulting in reduced shading but comparable conductivity.  This increases the efficiency and power generated by solar cells.  

For a cost-conscious industry such as solar, however, simply having better performance isn’t good enough. We had to develop our technology to work as a drop-in replacement for screen printing, meaning that a manufacturer would not have to make a whole new factory, but just retrofit their existing one. In other words, our technology had to be at least as fast and reliable as screen printing and fit into a similar design envelope.  

One of the challenges (and opportunities) of working at a place like PARC is that you have to consider the value proposition of the technologies you’re developing to make sure that a path to commercialization is possible. In the end, we licensed the CoEx technology to a solar manufacturer, successfully passing a factory acceptance test in a 2,700 wafer/hr. pilot line.

Another application for Co-extrusion is for improving battery performance.  There is a trade-off in conventional lithium ion batteries between high power and high energy density.  If you make your battery electrodes very thick, you can pack more energy into the same volume. However, the thicker the electrode, the longer it takes for lithium ions to travel though them, thereby limiting power.  You can make the electrodes very thin, so that lithium ions can travel through them very quickly, for fast charging and discharging. But then your battery is full of many current collectors and separators instead of active material, and so you have low energy density.  To address this limitation, we’ve used CoEx to create a structured battery cathode. In this case, we’ve used two fluids with varying drying shrinkage rates, such that the printed and dried film has fine scale, narrow, periodic grooving.  During battery assembly, these grooves fill with electrolyte, creating regions of high ionic transport.  In this way, we can create overall thicker and higher loaded cathodes that perform better than electrodes made with a conventional slot die manufacturing process. As with the production of solar cells, our Co-extrusion technology can serve as a drop-in replacement for the slot dies with minimal disruption to the existing manufacturing line.

We just wrapped up a research project that was funded by the U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) under the Vehicle Technologies Office. The goal of the study is to scale up the technology from making small-scale button cells that would only be appropriate for small portable devices, to larger sized pouch cells that could be used for electric vehicles. These high-efficiency batteries would help reduce the cost and improve the cell energy density in large electric vehicles.

Aside from batteries, other potential applications for CoEx include thermoelectric generators, fuel cells and even superconductors. Because CoEx is high-speed and creates a structure with a lot of internal surface area, it is well-suited for processes like catalysis that depend on creating multi-phase boundaries. Our printhead is designed to be very flexible and modular, enabling a variety of structures with varying geometry and composition.

What excites you most about Co-extrusion?

Because CoEx is a processing technique, it is not limited to one particular application or one particular material. For instance, in the case of batteries, a great deal of research is focused on improving the active materials that go into the electrodes. Whatever improvements are made to these materials, we feel that we can make >10% improvements in performance just by structuring the electrode.    

What do you enjoy most about working at PARC?

I enjoy the diversity of the projects that we get to work on, as it’s rare that someone is only working on one thing at a time. I like the fact that we are individually pushed to come up with new ideas, and encouraged to write proposals and seek funding for promising new technologies. Finally, I love the people here. PARC attracts researchers from so many different areas, with the common thread that they like to work on interesting problems.  PARC is rare in that it has such deep expertise in both printing and processing development as well as machine learning and artificial intelligence, and I’m excited to see where that takes us.  

What are your interests outside of the lab?

I’ve always enjoyed hiking, and there are a lot of spectacular trails in the area.  I’m a huge fan of walking through redwood groves. The Bay Area has a vibrant maker community, and I’ve been working on improving my crafting skills. I also enjoy conquering Escape Room and spoiling my cats. 

To learn more about PARC’s Co-extrusion technology, download our information sheet.

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