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April 2017 • The PCB Design Magazine 61 Over time, sedimentation had occurred and the product was proving increasingly difficult to reincorporate back into the resin mix. More- over, there was a slight bleed of resin through the gap between the resin and LED unit. With a bit of lateral thinking, the logical solution would be to increase the thixotropic nature of the resin, which would help to slow down the rate of sedimentation to an acceptable level, as well as preventing the resin bleeding so readily through the gap. The polymer used in UR5097 is also highly resistant to the transmission of water even at various pressure differences expe- rienced due to the depth of the water. A couple of weeks of lab work and testing with a number of different options produced a material that had sufficient thixotropy to slow down the sedimentation and avoid leakage into the gap, while still being easy to mix and pour into the unit. In the end, the actual increase in the mix viscosity of modified resin was only slightly higher than that of the original mate- rial. So, there you have it: a job well done, de- spite the setbacks, and one that met with the full approval of our customer. Don't forget to check back next month. PCBDESIGN Alistair Little is global business/ technical director for Electrolube's Resin Division. RESINS MAINTAIN A "POOL" OF LIGHT DOWN UNDER Researchers from AMBER and Trinity College in Dub- lin, in collaboration with TU Delft, have fabricated printed transistors consisting entirely of two-dimensional nanoma- terials for the first time. These 2D materials combine prom- ising electronic properties with the potential for low- cost production. This research could unlock the potential for applications such as food packaging that displays a digital countdown to warn you of spoiling, wine labels that alert you when your white wine is at its optimum temperature, next- generation banknote security and even flexible solar cells. The Trinity College researchers, from the groups of profs. Jonathan Coleman and Georg Duesberg, used standard printing techniques to combine graphene nanosheets as the elec- trodes with two other nanomaterials, tungsten diselenide and boron nitride, as the channel and separator to form an all-printed, all-nanosheet transistor. Two-dimensional transistors have been made before with methods such as chemical vapour deposition. While devices cre- ated in this manner perform well, the costs of these meth- ods are high. Printable elec- tronics, on the other hand, have until now been mostly based on carbon-based mol- ecules. These molecules can cheaply and easily be turned into printable inks, but such materials are somewhat unstable and have well- known performance limitations. Collaborating with Toyota's Dr. Sachin Kinge, Dr. Jannika Lauth from the Laurens Siebbeles group at TU Delft tested the electrical transport characteristics of the transistors, proving they combine the best of both worlds. "By using tera- hertz spectroscopy, we were able to determine the conductivity of the semiconductor materi- als," said Lauth. The material that makes up the team's printed electronics consist of many different nanosheets (or "flakes") of varying sizes. A promising next step is to print 2D-structures that are made up of a single nanosheet, which will drastically improve the performance of the printed electronics. Researchers Print Promising Two-Dimensional Transistors

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