Nanostructured Spacers: Creating Micron-Sized Gaps for Conductive Plates in Thermionic and Thermophotovoltaic Energy Converters
Thermionic (TEC’s) and thermophotovoltaic (TPV’s) devices convert heat energy into electricity and have promising potential to power everyday needs in the near future. TEC’s require micron-sized gaps to reduce the electric field that oppose the flow of electrons between conductive plates. Meanwhile, micron-sized gaps enhance the radiative heat transfer in TPV’s. Efficiency of these devices depend on maximizing the electron flow between conductive plates. Gaps must be created using a spacer that is thermally and electrically insulating, mechanically robust, and thin enough to create the micron-sized gaps.
Different spacer designs are tested using COMSOL simulations and are optimized to find a spacer with the desired mechanical properties. Spacers are then formally designed using MATLAB, where a unit cell is created by calculating the coordinates of each point on the mask at a micro-scale. The unit cell is then patterned over a 5 [cm] disk using LayoutEditor. The masks are used to create a corresponding pattern over a silicon wafer using a spin-coated photoresist. The photoresist is then exposed and developed until the desired mask pattern is left. The silicon wafer is etched by the desired thickness, and the process is repeated for three cycles, creating the silicon mold for the spacer. Alumina (Al2O3) is used to coat the surface of the patterned silicon wafer using atomic layer deposition. The wafer is spin-coated with photoresist, exposed, and developed one last time to create the final pattern. The excess alumina is removed using a XeF2 isotropic etch.
The result is a thin alumina honeycomb-structured spacer with a bumpy surface that maximizes the efficiency of the TEC’s and TPV’s. The bumps minimize contact area, while the alumina provides thermally insulating properties needed to minimize conduction.
Nanofabrication is limited by current technology and the scale of items at which it can produce. The development of technology that is able to create even smaller structures can increase the efficiency of TEC’s and TPV’s. Additionally, new spacer designs and structures that may arise using the constraint of a thinner spacer may also increase the efficiency of the electron flow in the TEC’s and TPV’s.