Differential pulse voltammetry of toxic metal ions at the boron-doped CVD diamond electrode

Boron-doped polycrystalline diamond films were grown over a molybdenum substrate by a microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35 ± 1 Torr. Boron doping of diamond was achieved in situ by using a solid boron source while growing diamond in the CVD processxu. We have observed a negligible background current (l) for diamond by differential pulse voltammetry in 0.5 M NaCl, 0.5 M H₂SO₄, and 0.5 M HNO₃ solutions over a wide potential range. Therefore, diamond will certainly have a use as an electrode material in electroanalytical applications to detect trace toxic/nontoxic metal ions such as cadmium, lead, copper, and silver. Differential pulse voltammetry was used to detect and evaluate the presence of lead ions in 0.5 M NaCl and cadmium ions in 0.5 M H₂SO₄ supporting electrolyte solution using highly conducting boron-doped diamond coated molybdenum electrode material. Furthermore, reverse differential pulse voltammetry was used to evaluate the presence of copper and silver ions in 0.5 M H₂SO₄ and 0.5 M HNO₃ solution, respectively. Diamond electrode has been used in this study to detect metallic ions in the solution over a wide potential range that covers + 0.8 V to –0.4 V vs., SHE.     

Cyclic voltammetric, a.c. and d.c. polarization behavior of boron-doped CVD diamond

Boron-doped polycrystalline diamond films have been deposited over a molybdenum substrate by the microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35.7 Torr. Boron doping of diamond has been achieved in situ by using a solid boron source while growing diamond in the CVD process. The a.c. impedance of boron-doped diamond films in 0.5 M NaCl solution has been determined and compared with the results obtained with a molybdenum substrate. Capacitance, solution resistance, and polarization resistance (corrosion rate) have been determined using the experimental data plotted in Nyquist and Bode formats. D.C. polarization techniques such as linear and Tafel polarization have been used to evaluate the doped diamond coated molybdenum and molybdenum for corrosion resistance characteristics in terms of charge-transfer coefficients and corrosion rate. Cyclic voltammetry has been used to evaluate the molybdenum, platinum, and doped and undoped diamond coated molybdenum materials in 0.5 M NaCl solution. We have observed two time constants with a doped diamond electrode / solution interface. Solution resistance was found to be constant irrespective of the electrode in the same electrolyte solution.     

Polishing of polycrystalline diamond by hot nickel surface

A microwave plasma technique has been employed to deposit polycrystalline diamond film over a molybdenum substrate button using a gas mixture of hydrogen and methane at a substrate temperature of 851°C. A CVD diamond coated molybdenum substrate button was mounted with a load against hot nickel plate and rotated for 3.45 h in a hydrogen ambient. Hot tungsten filament was used as a heat source to maintain the temperature of the nickel block and CVD diamond coated molybdenum button at 848°C. This experiment has reproducibly shown the successful polishing of polycrystalline CVD diamond by hot nickel. A Tencor profilometer and scanning electron microscope have been used to evaluate the surface smoothness and morphology before and after polishing the polycrystalline diamond thin films.     

Fabrication of diamond microstructures for microelectromechanical systems (MEMS) by a surface micromachining process

Selective polycrystalline diamond thin film has been grown on a silicon dioxide/silicon substrate using high pressure microwave plasma-assisted chemical vapor deposition from a gas mixture of methane and hydrogen at a substrate temperature of 950°C. A simple process flow has been developed to fabricate diamond microstructures such as diamond beams and cantilever beams using surface micromachining and photolithography for the first time. Scanning electron and optical microscopy has been used to characterize the surface micromachined diamond microstructures.     

Electronic nose for space program applications

The ability to monitor air contaminants in the shuttle and the International Space Station is important to ensure the health and safety of astronauts, and equipment integrity. Three specific space applications have been identified that would benefit from a chemical monitor: (a) organic contaminants in space cabin air; (b) hypergolic propellant contaminants in the shuttle airlock; (c) pre-combustion signature vapors from electrical fires. NASA at Kennedy Space Center (KSC) is assessing several commercial and developing electronic noses (E-noses) for these applications. A short series of tests identified those E-noses that exhibited sufficient sensitivity to the vapors of interest. Only two E-noses exhibited sufficient sensitivity for hypergolic fuels at the required levels, while several commercial E-noses showed sufficient sensitivity of common organic vapors. These E-noses were subjected to further tests to assess their ability to identify vapors. Development and testing of E-nose models using vendor supplied software packages correctly identified vapors with an accuracy of 70-90%. In-house software improvements increased the identification rates between 90 and 100%. Further software enhancements are under development. Details on the experimental setup, test protocols, and results on E-nose performance are presented in this paper along with special emphasis on specific software enhancements.     

Selective deposition of diamond films

Polycrystalline diamond films have been selectively deposited on a silicon surface. A novel process was developed which exposes a patterned, scratch damaged silicon area, surrounded by SiO₂, to a high pressure microwave plasma of hydrogen containing methane. The hydrogen plasma dissociates the methane injected into the reaction chamber, resulting in diamond deposition, which occurs only on the exposed silicon. Under the process conditions described in this work, some in situ plasma etching of the oxide is observed, resulting in little or no growth of diamond in unwanted areas, and further enhancing the selectivity. A variety of patterns and structures have been fabricated. Raman spectroscopy confirmed the films were diamond.     

fabrication of diamond membranes for MEMS using reactive ion etching of silicon

Polycrystalline diamond thin film has been grown on a silicon substrate using high pressure microwave plasma-assisted chemical vapor deposition from a gas mixture of methane and hydrogen at a substrate temperature of 950°C. A simple process flow has been developed to fabricate optically transparent polycrystalline synthetic diamond membranes/windows employing reactive ion etching (RIE) of a single crystal silicon substrate using an electron beam evaporated aluminum thin film mask pattern formed by photolithography. Scanning electron microscopy has been used to study the morphology of as-grown diamond thin films.