Background
In this project, we tested graphene foils in a space plasma instrument to increase their technology readiness level (TRL). Foils, until now made of amorphous carbon, are used for coincidence and time-of-flight (TOF) measurements. The interaction of particles with the foil results in multiple processes that either enable or negatively affect the measurement. For example, 1) the particle-induced secondary electron emission enables coincidence and time-of-flight measurements, and 2) the particle charge state modification enables detection and measurement of energetic neutral atoms (ENAs). However, 3) the angular scattering perturbs the particle trajectory and may result in particle loss in the detector, and 4) the energy straggling reduces the particle energy. Processes 3) and 4) scale with foil thickness, while 1) and 2) are marginally affected by it, at most. Thus, using the thinnest practical foil is always a goal. Current state-of-the-art carbon foils are upwards of about 9 nm thick.
Recently, the emergence of graphene (single atomic layer of carbon, 0.345 nm thick) has opened the possibility of reducing the foil thickness and, thus, tremendously advancing foil performance by reducing angular scattering and energy straggling. In recent studies (partially funded by SwRI’s IR&D program and a NASA grant), the team showed that graphene foils outperform carbon foils in angular scattering by a factor of 2 to 3. Graphene foils could therefore replace carbon foils in future space plasma instruments.
Approach
We divided the project into three objectives:
Perform environmental tests on graphene and carbon foils by themselves
Compare the performance of graphene foils with carbon foils in an instrument with time-of-flight
Perform environmental tests on graphene and carbon foils in an instrument
Accomplishments
We developed, in collaboration with Texas State University, a reliable method to transfer graphene foils onto fine nickel grids. The graphene foils consist of four stacked single layer graphenes reinforced using a combination of low and high molecular weight PMMA during the stacking and transfer processes. The PMMA is dissolved once the graphene has been transferred onto the grid.
We characterized the graphene foils by performing three measurements: 1) optical measurement of the coverage of graphene, 2) angular scattering of H+ as a function of energy, and 3) energy straggling of H+ at 50 keV. The coverage of graphene foils was better than 95 percent for the majority of samples. The angular scattering was mostly lower than for the thinnest carbon foils and as low as a factor of about 1.7 less than for carbon. The energy straggling was comparable to that of carbon foils.
We tested the survivability of side-by-side graphene and carbon foils under very high vibration levels and large temperature swings. In both cases, graphene and carbon showed extreme robustness to these harsh conditions. Very little to no damage was observed even though we over-tested the foil samples.
We partially tested graphene and carbon inside the HPCA instrument. The preliminary results obtained from this short amount of testing already revealed that a time-of-flight based instrument could operate at lower voltage which will reduce risk and cost, as well as decrease the resources needed for similar or better overall performance.