Jesse D. Fowler

Practical Chemical Sensors from Chemically Derived Graphene

We report the development of useful chemical sensors from chemically converted graphene dispersions using spin coating to create single-layer films on interdigitated electrode arrays. Dispersions of graphene in anhydrous hydrazine are formed from graphite oxide. Preliminary results are presented on the detection of NO(2), NH(3), and 2,4-dinitrotoluene using this simple and scalable fabrication method for practical devices. Current versus voltage curves are linear and ohmic in all cases, studied independent of metal electrode or presence of analytes. The sensor response is consistent with a charge transfer mechanism between the analyte and graphene with a limited role of the electrical contacts. A micro hot plate sensor substrate is also used to monitor the temperature dependence of the response to nitrogen dioxide. The results are discussed in light of recent literature on carbon nanotube and graphene sensors.

Graphene flash memory.

Graphenes single atomic layer of sp(2) carbon has recently garnered much attention for its potential use in electronic applications. Here, we report a memory application for graphene, which we call graphene flash memory (GFM). GFM has the potential to exceed the performance of current flash memory technology by utilizing the intrinsic properties of graphene, such as high density of states, high work function, and low dimensionality. To this end, we have grown large-area graphene sheets by chemical vapor deposition and integrated them into a floating gate structure. GFM displays a wide memory window of ∼6 V at significantly low program/erase voltages of ±7 V. GFM also shows a long retention time of more than 10 years at room temperature. Additionally, simulations suggest that GFM suffers very little from cell-to-cell interference, potentially enabling scaling down far beyond current state-of-the-art flash memory devices.

Chemical Vapor Deposition of Graphene on Copper from Methane, Ethane and Propane: Evidence for Bilayer Selectivity

To study the effects of hydrocarbon precursor gases, graphene is grown by chemical vapor deposition from methane, ethane, and propane on copper foils. The larger molecules are found to more readily produce bilayer and multilayer graphene, due to a higher carbon concentration and different decomposition processes. Single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane from 250 to 1000 mTorr. The bilayer graphene is AB-stacked as shown by selected area electron diffraction analysis. Additionally propane is found to only produce a combination of single- to few-layer and turbostratic graphene. The percent coverage is investgated using Raman spectroscopy and optical, scanning electron, and transmission electron microscopies. The data are used to discuss a possible mechanism for the second-layer growth of graphene involving the different cracking pathways of the hydrocarbons.

Temperature dependent Raman spectroscopy of chemically derived graphene

Reduced graphite oxide (GO) has shown promise as a scalable alternative to mechanically exfoliated specimens. Although many measurements show that reduced GO has properties approaching those of pristine graphene, it has been difficult to quantify the extent to which the graphitic network is restored upon reduction. Raman spectroscopy is widely used for the characterization of mechanically exfoliated graphene, but has not been fully explored for reduced GO. In this work, hydrazine suspensions of reduced GO are deposited on micro-hot-plates and examined over a range of temperatures by Raman spectroscopy. The work highlights the benefits of solution processing.

Stroboscopic Imaging Interferometer for MEMS Performance Measurement

The insertion of microelectromechanical systems (MEMS) components into aerospace systems requires advanced testing to characterize performance in a space environment. Here, we report a novel stroboscopic interferometer test system that measures nanometer-scale displacements of moving MEMS devices. By combining video imagery and phase-shift interferometry with an environmental chamber, rapid visualization of the dynamic device motion under the actual operational conditions can be achieved. The utility of this system is further enhanced by integrating the interferometer onto the chamber window, allowing for robust interferometric testing in a noisy environment without requiring a floating optical table. To demonstrate these unique capabilities, we present the time-resolved images of an electrostatically actuated MEMS cantilevered beam showing the first-order to sixth-order plate modes under vacuum.

Effect of concurrent UV irradiation and contamination on silver-coated teflon radiator surface (Conference Presentation)

Silver coated Teflon (SCT) has been used as a radiator material for spacecraft thermal control. In order to reduce the specular reflection, an attempt was made to roughen the heritage smooth SCT surface via sanding, leading to abraded surfaces. The objective of this study is to gain insight into the relative thermal performance degradation of smooth and abraded SCT radiator materials under identical exposure of concurrent UV irradiation and contaminant deposition. Contaminant molecules outgassed from representative spacecraft materials were deposited onto the smooth and abraded SCT samples with quartz crystal microbalances (QCMs) in close proximity to monitor real-time contaminant deposition. Thermal performance degradation is characterized by measuring solar absorptance () change on the SCT samples before and after contaminant film accumulation. Atomic force microscope (AFM) was used to examine the extent of surface roughness before and after contaminant deposition on smooth SCT samples. The preliminary findings indicate that less contamination accumulation was observed on SCT surfaces in comparison to the gold coated crystal surface of QCMs. In addition, the roughness of SCT surface appears to play a role in contributing a more pronounced  change, suggesting the possibility of faster performance degradation of the abraded SCT materials in comparison to that of smooth SCT surfaces.

Outgassing study of spacecraft materials and contaminant transport simulations

Contamination control plays an important role in sustaining spacecraft performance. One spacecraft degradation mechanism involves long-term on-orbit molecular outgassing from spacecraft materials. The outgassed molecules may accumulate on thermal control surfaces and/or optics, causing degradation. In this study, we performed outgassing measurements of multiple spacecraft materials, including adhesives, Nylon Velcro, and other assembly materials through a modified ASTM E595 test method. The modified ASTM E595 test had the source and receiver temperature remained at 125°C and 25°C, respectively, but with prolonged outgassing periods of two weeks. The condensable contaminants were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography/Mass Spectrometry (GC/MS) to determine their spectral transmission and chemical composition. The FTIR spectra showed several spacecraft materials, primarily adhesives and potting materials, exhibiting slight absorption from contaminants consisting of hydroxyl groups and carboxylic acids. To gain insight into molecular contaminant transport, simulations were conducted to characterize contaminant accumulation inside a hypothetical space system cavity. The simulation indicated that contaminant molecules bouncing inside the hypothetical payload cavity can lead to deposition on colder surfaces, even though large openings are available to provide venting pathways for escaping to space. The newly established molecular contaminant transport simulation capability holds the promise of providing quantitative guidance for future spacecraft and its venting design.

Molecular transport modeling for spaceborne instrument contamination prediction

We present a finite element model for the prediction of molecular contamination through narrow pathways in a hypothetical spaceborne instrument using the commercially available COMSOL Multiphysics software. The free molecular flow module of COMSOL uses the angular coefficient method as an alternative to particle based methods. In the angular coefficient method, the microscopic dynamical aspect of the material transport problem is reduced to a macroscopic problem by calculating emission and incident fluxes at each surface rather than the trajectories of individual molecules. The model was validated by comparing the simulated and experimentally measured pressure differential between two chambers separated by a mechanical test structure. The mechanical test structure was designed to exhibit narrow pathways with characteristic size that can be found on spaceborne optomechanical structures. It is shown that materials can slowly migrate through these pathways in a spaceborne instrument to cause noticeable performance degradation within a time scale of a few months. The model for material transport through the test structure was also verified using a stochastic method. To simulate water infiltration through narrow pathways of a hypothetical spaceborne instrument, nominal payload temperature profile was used in addition to setting empirical input parameters such as the desorption energy of water and the outgassing rate of water from multilayer insulator thermal blankets to the appropriate surfaces in the modeling domain. The rate of growth of ice films on low temperature optical components and how optical performance can be degraded over time are discussed in this paper.

Identification of collected volatile condensable material (CVCM) from ASTM E595 of silicone damper fluid

Polydimethylsiloxane damping fluids used for structural deployment mechanisms are not required to be low outgassing. During normal use, these damping fluids are typically encapsulated; however, an unintentional leak may occur which would cause an undesirable contamination at the leak point and form volatile condensable that could reach contamination-sensitive surfaces, degrading the performance of satellites. The collected volatile condensable material (CVCM) at 25 °C from ASTM E595 of a damping fluid, MeSi-300K, was < 0.10%, when the damping fluid was maintained at 125 °C for 24 hours under 10-6 Torr vacuum. MeSi-300K viscosity is 300,000 cSt, which indicates an average molecular weight (MW) of 204,000. This large MW polymer would contain about 2,756 dimethyl siloxane (DMS) units in the chain. These long chains are not expected to be volatile; however, during manufacture, linear chains and cyclic compounds of a smaller number of DMS units produced are volatile. Gas chromatography mass spectrometry (GC-MS) was used to identify the CVCM. Characterization of these materials revealed that the CVCM contained higher MW siloxanes, straight chain and cyclic, in the range of 682 to 1196 (9 to 16 DMS units), whereas CVCM from spacequalified, silicone-based materials have lower MW, 222 to 542 (3 to 7 DMS units). Consequently, contamination from MeSi-300K material would produce greater amounts of higher-MW siloxanes than space-qualified silicones. These higher-MW species would be harder to remove by evaporation and could remain on sensitive surfaces.

Assessment of Particle Deposition inside Payload Fairing from Launch Vehicle Plume Contribution

Concerns were raised for potential payload contamination inside payload faring (PLF) contributed from the soot particles in the launch vehicle ignition plume. Soot particles, once ingested into PLF through vents, can pose potential payload contamination risks due to their light absorbing characteristics. To gain insights into the extent of soot particle contamination inside the PLF, analytical calculations and laboratory experiments were performed using a PLF simulator to determine the rate of soot particle deposition onto surfaces. The analysis assumed a non-venting setting as the worst case scenario, in which particles were trapped inside the PLF simulator and allowed to deposit onto available surfaces. Soot particles were briefly introduced inside a PLF mockup and after the soot generation source ceased, particle deposition rates were examined by measuring the particle concentration decay as a function of time. Based on the experimentally determined particle deposition rates and other parameters including the venting scenarios, the impact of soot particle deposition for the full scale PLF and payload was evaluated. The effects of soot particles contamination were also studied, and pronounced transmission degradation toward the UV region on a fused silica substrate was observed.