Issue link: https://iconnect007.uberflip.com/i/1104607
APRIL 2019 I PCB007 MAGAZINE 21 Next, we study what substrate we're going to print. We study the surface chemistry of the substrate, check the conduct handling, and see how we can improve the wettability of the ink onto the surface and adhesion. All of these properties are important for ther- mal sintering because the thermoelectric ma- terial and substrate that you're using on both have a different coefficient of thermal expan- sion. Photonic sintering has a very short time and high energy pulse. If you have a very dif- ferent coefficient of thermal expansion, the ma- terials will break apart and the filling doesn't stick very well to the substrates. We can print onto flexible Kapton and tubu- lar substrates. The idea is that if you have heat or a cold pipe, with the 3D conformal printing, one option is to print on a flexible substrate and wrap it around the tube. The other op - tion is to print directly onto the tube. If you print directly onto the tube, you can harvest the heat coming out of the tube easier and more efficiently than by wrapping something around it. That was the idea for conformal printing, and when we started doing that, we found that there was a lot of porosity inside the fill- ing once we print because aerosol jet print- ing is more like spray printing. It can be ran- dom stacking of the material and evaporating solvent while printing on it so that it creates some bubbles and holes in the filling. We ap- plied some isostatic pressing so that it would densify the filling, reduce the interface contact resistance, and allow more contact area. It also helps to have better photonic sintering. Johnson: Is the idea to custom formulate nano- crystals, formulate them into an ink that works in an aerosol spray, and make sure that you've characterized the material you are using to match the ink for adhesion? Because it's an aerosol spray, do you have to do a little bit of compressing? Varghese: You densify the filling so its electri- cal conductivity will be improved much higher than just spraying on it. It will densify and im- prove the conductivity by photonic sintering. Johnson: So, you've characterized and treated an ink for a particular substrate. Varghese: Correct. Once you optimize for Kap- ton substrate, the ink can be modified easily for other substrates, such as glass, transparent polymers, or even photo paper. It is important to modify the substrate surface to improve the adhesion between the material and substrate by using oxygen plasma and chemical washing of the surface. Johnson: That's exactly what I was trying to ask. And then, of course, you also have all of the work with sintering. Varghese: Yes. We did a photonic sintering study. First, we made a model of the material to help us narrow down our sintering condi- tions. By studying the model with different en- ergy and pulse duration, we can understand how much heat is generated in the material. Then, we compare the results with our experi- mental methods to see that it reduces the re- sistance and increase electrical conductivity at those conditions. Comparing the modeling and experimental study it is possible to fine-tune the sintering conditions to get the best material properties using photonic sintering. _____________________________ Sameeksha and Emma explained their work next. While I can't say for certain that Emma was the only undergrad in attendance, she was the only non-doctorate student I met while there. Nolan Johnson: Emma, tell us a little bit about the project. Emma Pedersen: We're trying to address is max- imizing power output by changing the topology of the photovoltaic (PV) ar- rays. On each panel, we have a smart monitoring device (SMD) connected to a switching matrix. The smart monitoring Emma Pedersen