Stephen O. Hay

CVD diamond deposition processes investigation: Cars diagnostics/modeling

The driving force behind the strong interest in diamond deposition processes is the outstanding combination of unique natural properties of this material. A wide variety of techniques has been employed to generate diamond coatings including hot filament, thermal plasma, CVD, PACVD (rf, dc, and microwave), low energy carbon ion beam, laser beam, oxyacetylene torch, and numerous hybrid dual-beam configurations. Thus, there are many routes available for producing diamond coatings in the form of small individual crystals, amorphous coatings, polycrystalline films or single crystal films under conditions far removed from the thermodynamically stable region nominally associated with diamond growth. CVD of diamond coatings from hydrocarbon containing gases can have an almost infinite number of compositions and structures; each with differing amounts of sp{sup 3} (diamond) and sp{sup 2} (graphite) bonding. This variation has contributed to confusion both in the working definition of diamond coatings and in understanding the controlling processes of forming these films. In fact, the mechanisms involved in the gas phase processes, the nucleation and growth structures, and especially their correlation are poorly understood.

The viability of photocatalysis for air purification.

Photocatalytic oxidation (PCO) air purification technology is reviewed based on the decades of research conducted by the United Technologies Research Center (UTRC) and their external colleagues. UTRC conducted basic research on the reaction rates of various volatile organic compounds (VOCs). The knowledge gained allowed validation of 1D and 3D prototype reactor models that guided further purifier development. Colleagues worldwide validated purifier prototypes in simulated realistic indoor environments. Prototype products were deployed in office environments both in the United States and France. As a result of these validation studies, it was discovered that both catalyst lifetime and byproduct formation are barriers to implementing this technology. Research is ongoing at the University of Connecticut that is applicable to extending catalyst lifetime, increasing catalyst efficiency and extending activation wavelength from the ultraviolet to the visible wavelengths. It is critical that catalyst lifetime is extended to realize cost effective implementation of PCO air purification.

Diagnostic Techniques for Hot Hydrogen Testing of Materials for SNTP

A key requirement in the USAF Space Nuclear Thermal Propulsion (SNPT) program is the ability to test fuel element coupons and other components in a hot (approx. 3000K), flowing hydrogen environment. The results of such tests may lead to the modification of current requirements of over 100 atm pressure and a hydrogen flow rate of 1 kg/s for the testing of complete fuel assemblies. To date, there is considerable debate regarding the relative ranking of these test parameters. Establishing the proper test parameters is essential to understanding the complex phenomena of material erosion and the local conditions that trigger its onset; this understanding is imperative if fuel elements and structural components are to be safely designed for even the short duration missions typical of SNTP. Specifically, accurate measurement and verification of temperatures are essential since coupon mass loss rates and the local chemical environment are highly dependent on both gas and sample temperature. Laser/optical diagnost...

In-situ CARS detection of H2 in the CVD of Si3N4

Silicon nitride (Si3N4) has been demonstrated to be an effective high temperature anti-oxidant, especially when deposited in its -ciysta1line form. UTRC has developed a pilot scale chemical vapor deposition (CVD) reactor capable of depositing cv-Si3N4 from ammonia (NH3) and silicon tetrafluoride (SiF4) at 1450 C. Coherent anti-Stokes Raman spectroscopy (CARS) has been applied to this reactor which has been fitted with line-of-sight optical access ports. Temperature and concentration measurements have been performed on gas phase species during the deposition of Si3N4. Based on the CARS detection of H2, the importance of high temperature surface (SiN4) catalyzed decomposition of NH3:

Comparison Of Model And Experimental Data Of The CVD Diamond Deposition Process

In order to understand the complicated chemical and physical processes that occur during the deposition of hard face coatings such as diamond, experiments that are remote, nonintrusive and sensitive to critical chemical species have been performed. Coherent anti-Stokes Raman spectroscopy (CARS) has been used to measure temperature and detect species such as methane and acetylene under low pressure, CVD environments. Results of these experiments for both an rf PACVD and heated-filament apparatus are described. In addition, these results and literature studies are interpreted using modeling (kinetic and equilibrium) calculations. Intepretations of the experimental results confirm the importance of high concentrations of hydrogen atoms, suggest that (hydrocarbon) radical species play a negligible role, and support proposals that in the presence of reactive hydrogen atoms virtually any hydrocarbon (or hydrocarbon oxygenate) can lead to diamond growth. The results in other laboratories on diamond deposition in acetylene/oxygen flames strongly support the first of these interpretations. In order to understand the competive process of soot/amorphous carbon formation, the equilibrium analysis of Stein and Fahr has been extended to low pressure, diamond forming conditions. This study indicates that a thermodynamic barrier exists to the growth of polyaromatic hydrocarbons at temperatures above 1300 to 1400K, pressures of 25 torr and hydrogen/acetylene ratios of 200.