Robert R. Harl

Graphene: Corrosion-Inhibiting Coating

We report the use of atomically thin layers of graphene as a protective coating that inhibits corrosion of underlying metals. Here, we employ electrochemical methods to study the corrosion inhibition of copper and nickel by either growing graphene on these metals, or by mechanically transferring multilayer graphene onto them. Cyclic voltammetry measurements reveal that the graphene coating effectively suppresses metal oxidation and oxygen reduction. Electrochemical impedance spectroscopy measurements suggest that while graphene itself is not damaged, the metal under it is corroded at cracks in the graphene film. Finally, we use Tafel analysis to quantify the corrosion rates of samples with and without graphene coatings. These results indicate that copper films coated with graphene grown via chemical vapor deposition are corroded 7 times slower in an aerated Na(2)SO(4) solution as compared to the corrosion rate of bare copper. Tafel analysis reveals that nickel with a multilayer graphene film grown on it corrodes 20 times slower while nickel surfaces coated with four layers of mechanically transferred graphene corrode 4 times slower than bare nickel. These findings establish graphene as the thinnest known corrosion-protecting coating.

Amplification of Surface-Initiated Ring-Opening Metathesis Polymerization of 5-(Perfluoro-n-alkyl)norbornenes by Macroinitiation

This article reports the enhanced rate of the surface-initiated polymerization (SIP) of 5-(perfluoro-n-alkyl)norbornenes (NBFn) by combining two SIP techniques, namely surface-initiated atom-transfer polymerization (SI-ATRP) to grow a macroinitiator and surface-initiated ring-opening metathesis polymerization (SI-ROMP) to produce the final coating. This polymerization approach promotes the rapid growth of dense partially fluorinated coatings that are highly hydrophobic and oleophobic and yield thicknesses from 4-12 μm. Specifically, the growth rate and the limiting thickness of pNBFn with different side chain lengths (n = 4, 6, 8, and 10) at various monomer concentrations and temperatures are evaluated through two approaches: growing the polymer from an initiator-terminated monolayer (control) or from a modified poly(2-hydroxyethyl methacrylate) (PHEMA) macroinitiator. X-ray photoelectron spectroscopy (XPS) analysis shows that 38% of the hydroxyl termini in the macroinitiator react with a norbornenyl diacid chloride (NBDAC) molecule, and 7% of such anchored norbornenyl groups react with a catalyst molecule. The kinetic data have been modeled to determine the propagation velocity and the termination rate constant. The PHEMA macroinitiator provides thicker films and faster growth as compared to the monolayer, achieving a 12 μm thick coating of pNBF8 in 15 min. Increasing the monomer side chain length, n, from 4 to 10 improves the growth rate and the limiting polymer thickness. Performing the polymerization process at higher temperature increases the growth rate and the limiting thickness as evidenced by an increase in the film growth rate constant. Arrhenius plots show that the reactions involved in the macroinitiation process exhibit lower activation energies than those formed from a monolayer. Electrochemical impedance spectroscopy reveals that the films exhibit resistance against ion transport in excess of 1 × 10(10) Ω·cm(2).

Radiation Effects on the Photoluminescence of Rare-Earth Doped Pyrochlore Powders

This work examined the sensitivity of the luminescence spectra of europium-doped lanthanum zirconate to different types and amounts of radiation exposure. For the samples and radiation sources used in this work (X-rays and protons), changes in the photoluminescence of europium-doped lanthanum zirconate were minimal. However, when the phosphor was paired with an alternate luminescent material, which had a differing radiation response, relative changes in the photoluminescence of the two materials were correlated to the radiation exposure.

Low emissivity high-temperature tantalum thin film coatings for silicon devices

The authors study the use of thin (∼230 nm) tantalum (Ta) layers on silicon (Si) as a low emissivity (high reflectivity) coating for high-temperature Si devices. Such coatings are critical to reduce parasitic radiation loss, which is one of the dominant loss mechanisms at high temperatures (above 700 °C). The key factors to achieve such a coating are low emissivity in the near infrared and superior thermal stability at high operating temperatures. The authors investigated the emissivity of Ta coatings deposited on Si with respect to deposition parameters, and annealing conditions, and temperature. The authors found that after annealing at temperatures ≥900 °C the emissivity in the near infrared (1−3 μm) was reduced by a factor of 2 as compared to bare Si. In addition, the authors measured thermal emission at temperatures from 700 to 1000 °C, which is stable up to a heater temperature equal to the annealing temperature. Furthermore, Auger electron spectroscopy profiles of the coatings before and after anne...

Accelerated oxidation of silicon due to x-ray irradiation

Enhanced rates of oxide growth have been observed on silicon upon exposure to 10-keV X-ray irradiation. Oxide thicknesses were determined using spectroscopic ellipsometry on irradiated and control samples, and confirmed via X-ray photoelectron spectroscopy. The oxidation rate varied with the radiation total dose and dose rate. The increased oxidation rate is attributed to the generation of ozone, which decomposes into molecular oxygen and highly reactive atomic oxygen at the surface of the Si wafer. The generation of ozone by 10-keV X-rays was found to increase linearly with increasing dose rate. UV irradiation led to similarly enhanced oxidation rates. The potential application of this phenomenon to dosimetry is explored.

Comparison of VO2 thin films deposited by pulsed laser, electron-beam and sputter deposition

The optical performance and morphology of VO2 thins films deposited by electron beam evaporation, rf magnetron sputtering and pulsed laser deposition are compared. Laser-deposited films are strongly affected by substrate dewetting and epitaxial mismatch.