Vernon M. Swope

The Physicochemical and Electrochemical Properties of 100 and 500 nm Diameter Diamond Powders Coated with Boron-Doped Nanocrystalline Diamond

There is a need for advanced, corrosion-resistant electrocatalyst support materials for use in fuel cells. To this end, electrically conducting diamond powder was prepared by depositing a layer of boron-doped nanocrystalline diamond on 100 and 500 nm diam diamond powders. The doped layer was deposited by microwave plasma-assisted chemical vapor deposition using an Ar-rich CH 4 /H 2 /Ar/B 2 H 6 source gas mixture. After coating, the 100 nm doped diamond powder had a specific surface area of 27 m 2 /g and an electrical conductivity of 0.41 S/cm. The 500 nm doped diamond powder had a specific surface area of 8 m 2 /g and an electrical conductivity of 0.59 S/cm after coating. The specific surface area of both powders decreased by ca. 50% after diamond coating due mainly to particle-particle fusion. The electrical measurements provided conclusive evidence for a doped diamond overlayer as the uncoated powders possessed no electrical conductivity. Furthermore, the fact that the electrical properties were unaltered by acid washing confirmed that the conductivity arises from the doped diamond overlayer and not any adventitious sp 2 carbon impurity on the particle surface, which is removed by such chemical treatment. Scanning electron microscopy images and Raman spectroscopy yielded further evidence in support of a nanocrystalline diamond overlayer. Both powders exhibited electrochemical responses for Fe(CN) 3 6 -/4- , Ir(Cl)) 6 -2/-3 , and Fe +2/+3 that were comparable to typical responses seen for high-quality, boron-doped nanocrystalline diamond thin-film electrodes. The electrochemical behavior of the powders was assessed using a pipette electrode that housed the packed powder with no binder. The 100 nm doped diamond powder electrodes were more plagued by ohmic resistance effects than were the 500 nm powder electrodes because of reduced particle contact. Importantly, it was found that the doped diamond powder electrodes are dimensionally stable and corrosion-resistant during anodic polarization at 1.4 V vs Ag/AgCl (1 h) in 0.5 M H 2 SO 4 at 80°C. In contrast, glassy carbon powder polarized under identical conditions underwent significant microstructural degradation and corrosion.

Oxidation Resistance of Bare and Pt-Coated Electrically Conducting Diamond Powder as Assessed by Thermogravimetric Analysis

A corrosion-resistant electrocatalyst support was prepared by overcoating high surface-area diamond powder (3-6 nm diameter, 250 m 2 /g) with a thin layer of boron-doped ultrananocrystalline diamond (B-UNCD) by microwave plasma-assisted chemical vapor deposition. This core-shell approach produces electrically conducting (0.4-0.5 S/cm) and high surface-area (150―170 m 2 /g) diamond powder (B-UNCD-D). Accelerated degradation testing was performed by thermogravimetric analysis (TGA) to assess the oxidation resistance (i.e., corrosion resistance) of powder in the absence and presence of nanoscale Pt. The oxidation onset temperature for B-UNCD-D powder decreased with the Pt loading from 0 to 30 wt % (Pt/C). However, compared with the bare powder, the rate of carbon consumption was significantly greater for Pt-(XC-72) as compared to the platinized diamond powder. For example, the temperature of the maximum carbon consumption rate, T d , occurred at 426°C for Pt-(XC-72) (20% Pt/C), which was 295°C lower than the T d for bare XC-72. In contrast, T d for Pt-(B-UNCD-D, 20% Pt/C) was 558°C; a temperature that was only 62°C lower than that for bare diamond. Isothermal oxidation at 300°C for 5 h produced negligible weight loss for Pt-UNCD-D (20% Pt/C) while a 75% weight loss was observed for Pt-(XC-72) (20% Pt/C). The results clearly demonstrate that platinized diamond is more resistant to gas phase oxidation than is platinized Vulcan at elevated temperatures.

Postnatal development of the serotonin signaling system in the mucosa of the guinea pig ileum

Background  Serotonin is an important neurohumoral molecule in the gut but its signaling system is not fully developed in the neonatal gastrointestinal (GI) tract. This study aimed to evaluate the postnatal maturation of serotonin signaling in the small intestine.