Douglas G. Fletcher

Oxidation of ZrB2-SiC Ultrahigh-Temperature Ceramic Composites in Dissociated Air

The oxidation behavior and surface properties of hot-pressed ZrB 2 -SiC ultrahigh-temperature ceramic composites are investigated under aerothermal heating conditions in the high-temperature, low-pressure partially dissociated airstream of the 1.2 MW Plasmatron facility at the von Karman Institute for Fluid Dynamics. Samples are oxidized at different flow enthalpies for exposure times of up to 20 min at surface temperatures ranging from 1250 to 1575°C. The microstructure and composition of the resulting oxide layers are characterized using electron and optical microscopies, x-ray diffraction, and energy-dispersive x-ray analysis. Comparisons are made with samples oxidized under similar temperature and pressure conditions in a furnace test environment in which atomic oxygen concentrations are negligible. Changes in surface optical properties are documented using spectral reflectance measurements, and effective catalytic efficiencies are estimated using computational fluid dynamics calculations together with measured surface temperatures and heat fluxes.

Laboratory Investigation of the Active Nitridation of Graphite by Atomic Nitrogen

reactive-flow model. The reaction efficiency for graphite nitridation was derived from the interpolated N-atom concentration and the measured graphite mass loss for a given test time. The reaction efficiency, defined as the fractionof N-atom collisions with the surfacethatresult in the title reaction, wasfoundto increase from � 0:2 � 10 � 3

Optical Emission Spectroscopy During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramic Composites

Optical emission spectroscopy is used to investigate the oxidation of a hot-pressed ZrB 2 -SiC ultrahigh-temperature ceramic composite tested in the 1.2 MW Plasmatron facility at the von Karman Institute for Fluid Dynamics. Time-resolved spectra enable the in situ detection and temporal characterization of electronically excited B, BO, and BO 2 species concentrations directly adjacent to the oxidizing sample surface. The evolution of these boron species correlates well with the transient formation of a complex multilayer oxide scale containing a silica-rich glassy outer layer that limits oxide growth.

Diode Laser Absorption Sensor Design and Qualification for CO2 Hypersonic Flows

The design of a diode laser absorption spectroscopy sensor to probe freestream conditions of hypervelocity CO2 flow in the Longshot Free-Piston Facility is presented. The Longshot Facility of the von Karman Institute is an important facility for ground test and numerical validation applications as it can produce high Mach number and highReynold’s number conditions for bothN2 andCO2 flows.Currently, freestreamconditions are determined from measurements of reservoir pressure, pitot pressure, and stagnation-point heat transfer to a hemisphere using a thermodynamic model for the expanding test gas. A nonintrusive absorption sensor can provide a direct measurement of freestream gas dynamic variables, which could be used to evaluate the data reduction process. Detailed descriptions of the design, analysis, and development of the diode laser absorption sensor are presented.

Laser Spectroscopic Investigation of Surface-Catalyzed Reactions for Mars Exploration Vehicles

Discrepancies exist as to whether appreciable amounts of CO2 recombine at the surface of Martian entry vehicles. This is a significant concern as overly conservative design measures need to be used to accommodate this uncertainty owing to the the variation in heat-fluxes that arise for different recombination models used. The University of Vermont (UVM) has recently developed an 30kW Inductively Coupled Plasma (ICP) Torch Facility. This facility is designed for investigation of material testing and gas-surface interaction chemistry. Two-photon absorption laser induced fluorescence (TALIF) and diode laser absorption spectroscopy (DLAS) strategies to characterize O, CO and CO2 concentrations near sample material surfaces are presented here.

Near Surface CO2 Detection in an Inductively Coupled Plasma Facility Using Diode Laser Absorption

Uncertainty exists as to whether appreciable amounts of CO2 are formed by surface recombination for Mars entry vehicles. It is well understood that highly catalytic thermal protection system (TPS) material can lead to nearly two times the heat flux to the vehicle surface as a non-catalytic material. Conservative design practice assumes a supercatalytic surface to accommodate this uncertainty. Risk avoidance leads to oversized TPS geometries that add mass to entry vehicle designs that could be otherwise employed. Determining whether significant near surface production of CO2 occurs at trajectory heating conditions would add significant insight. The University of Vermont (UVM) has developed a 30kW Inductively Coupled Plasma (ICP) Facility to provide a pure high temperature plasma environment for aerospace material testing and gas-surface interaction chemistry investigations. The goal of the present work is to develop and test a diode laser absorption spectroscopy (DLAS) sensor to target near sample surface CO2 concentrations. A DFB diode laser at 2744 nm with a tuning range of around 3 nm is chosen as the light source. Current investigations show no appreciable amount of CO2 near a highly catalytic surface (water cooled copper) within the sensors’ detectivity limits. Work is ongoing to further improve these sensitivity limitations.

Surface Catalyzed Reaction Efficiencies in Nitrogen and Oxygen Plasmas from Laser Induced Fluorescence Measurements

Recent efforts to advance laser diagnostics in plasma facilities have resulted in the capability to measure gradients of temperature and species in the reacting zone above a high-temperature material. These measurements allow a direct qualitative estimate of the surface-catalyzed reaction efficiency when the sub-millimeter spatial distribution of the reactive species is compared with measurements over reference materials. Measurements of nitrogen atom and oxygen atom distributions are presented for reference materials and silicon carbide for both nitrogen and oxygen plasmas to elucidate the relative efficiencies for homogeneous surface catalyzed recombination.

Investigations of Surface-Catalyzed Recombination Reactions in the Mars Atmosphere

In the design of a thermal protection system (TPS) for a planetary entry vehicle, accurate modeling of the trajectory aero-heating poses a significant challenge owing to large uncertainties in chemical processes taking place at the surface. Even for surface-catalyzed reactions, which have been investigated extensively, there is no consensus on how they should be modeled; or, in some cases, on which reactions are likely to occur. Current TPS designs for Mars missions rely on a supercatalytic boundary condition, which assumes that all dissociated species recombine to the free stream composition. While this is recognized to be the the most conservative approach and leads to an increased TPS mass, discrepancies in aero-heating measurements in ground test facilities preclude less conservative design options. The present work is aimed at providing more information about surface catalyzed reactions for Mars exploration missions. Measurements of dissociated species above a catalytic surface are obtained using two-photon absorption laser-induced fluorescence (TALIF) implemented in a new 30 kW inductively coupled plasma torch facility.

Investigation of Non-Equilibrium Nitrogen Plasmas

High-enthalpy nitrogen flow generated within an inductively-coupled plasma torch facility is examined using emission spectroscopy. Evidence of a non-equilibrium distribution of multiple vibrational levels within the N2(B Πg) electronic state created by inverse predissociation of ground-state nitrogen atoms is shown within the N2 1 st-Positive system. The Specair software package is used to estimate the degree with which the flow deviates from equilibrium by determining the overpopulation values for each vibrational level in the upper electronic state. Results yield a maximum overpopulation value of 4.91±1.5 for the B Πg(v’ = 13) level. The overpopulation of the v’=13 level allows for an estimation of the overpopulation of ground state nitrogen atoms to be 2.22±0.25.

Surface Catalyzed Reaction Efficiencies in Air Plasmas Using Laser Induced Fluorescence Measurements

Recent efforts to advance laser diagnostics in plasma facilities have resulted in the capability to measure gradients of temperature and species in the reacting zone above a hightemperature material. These measurements allow a direct quantitative estimate of the surface-catalyzed reaction efficiency when the spatial distribution of the reactive species is compared with measurements over reference materials. Measurements of temperature, relative nitrogen atom density, relative oxygen atom density, and relative NO molecule density in a chemically reacting boundary layer were performed in air plasmas to elucidate the behavior of catalytic efficiencies.