Jochen Marschall

Catalytic Atom Recombination on ZrB2/SiC and HfB2/SiC Ultrahigh-Temperature Ceramic Composites

Results are presented of an experimental investigation into the efficiency of zirconium diboride/silicon carbide and hafnium diboride/silicon carbide ultrahigh-temperature ceramic composites for catalyzing the surface recombination of dissociated oxygen and nitrogen at moderate surface temperatures. Experiments were conducted with a diffusion-tube side-arm reactor, together with laser-induced fluorescence species detection diagnostics. Experiments reveal recombination coefficients in the range between silica glasses and oxidized metals, as well as evidence of environment-induced surface modification.

Experimental Investigation of Surface Reactions in Carbon Monoxide and Oxygen Mixtures

During hypersonic entry into the CO 2 atmosphere of Mars, competing exothermic chemical reactions may oceur on a spacecraft heatshield surface. Two possible surface reactions are O+O → O 2 and CO+O → CO 2 . The relative importance of these reactions on quartz is investigated using a diffusion tube side-arm reactor together with two-photon laser-induced fluorescence for both O and CO species detection. The experiments show 1) that the presence of CO in the gas phase does not -significantly affect the oxygen recombination reaction on quartz and 2) that the gas-phase CO concentration is not significantly altered by the presence of atomic oxygen. These results indicate that for our experimental conditions the dominant surface reaction on quartz in oxygen-carbon monoxide mixtures is O+O → O 2 . Current heating computations for Martian entries assume CO oxidation to be fully catalytic. The resulting entry heating values are significantly higher than those computed using the assumption of fully catalytic oxygen recombination. The data presented here indicate that the assumption of fully catalytic CO oxidation may be overly conservative for heatshleld sizing purposes

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.

Temperature Jump Phenomenon During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramics

U LTRAHIGH temperature ceramic (UHTC) materials containing hafnium diboride (HfB2) and zirconium diboride (ZrB2) with a silica former, most commonly silicon carbide (SiC), have been studied extensively over the last decade as materials for leading-edge and control surface components on hypersonic vehicles [1–3]. Such components experience extreme aerothermal heating in chemically aggressive, partially dissociated air environments. Promising aspects of diboride-based UHTC materials include the very high melting points of HfB2 and ZrB2 and their refractory oxides hafnia (HfO2) and zirconia (ZrO2), as well as the high thermal conductivities of HfB2 and ZrB2, which enables efficient heat conduction away from stagnation point regions [4]. Zirconium diboride has some advantages over hafnium diboride as an aerospace material, because it is lighter and less expensive. The oxidation of ZrB2 produces both zirconia and boron oxide (B2O3). Significant oxidation of ZrB2 in atmospheric air begins at about 1050 K. The softening temperature for amorphous B2O3 is in the range of 830–900 K [5]; below about 1500 K, the oxide scale consists of a porousZrO2 network filled with liquidB2O3 that acts as an effective oxygen diffusion barrier [6,7]. However, the vapor pressure of B2O3 increases rapidly with temperature [8], resulting in rapid loss of B2O3 above 1500 K. The residual porous zirconia scale provides little resistance to inward oxygen transport and further oxidation [9,10], making the oxidation resistance of pure ZrB2 insufficient for high-temperature hypersonic vehicle applications. The addition of a silica former to ZrB2 improves its oxidation resistance [11–14]. Compositions containing from 10 to 30% (by volume) SiC have generally been found to be optimal in this regard. The virgin ZrB2-SiC surfaces oxidize through parallel reactions that generate ZrO2,B2O3, and SiO2. LiquidB2O3 mixes with amorphous SiO2 to form a borosilicate glass that seals the ZrO2 scale [15]. With increasing temperature, boron oxide evaporates preferentially from Received 10 August 2011; revision received 13 February 2012; accepted for publication 19 February 2012. Copyright

Gas permeability of lightweight ceramic ablators

Many lightweight thermal protection system (TPS) materials have a large degree of open porosity, which can make them highly permeable to gas flow. Recently, a permeability measurement apparatus was constructed to test rigid, porous TPS materials. This note presents further gas permeability, made with this apparatus, on two lightweight ceramic ablator materials; viz., phenolic impregnated carbon ablator and silicone impregnated reusable ceramic ablator

Validation of a volume-averaged fiber-scale model for the oxidation of a carbon-fiber preform

The oxidation of FiberForm, an industrial carbon-fiber preform, has been studied in an oxidation reactor. The microscopic oxidation behavior of the fibers has been analyzed by scanning electron microscopy. The carbon fibers ablate showing progressive reduction of their diameter. The overall material recession occurs when the fibers are consumed. A reaction/diusion -convection competition is shown to drive the oxidation process and control the depth of oxidation. A fiber-scale model is proposed for the prediction of carbonfiber preform oxidation. A macroscopic model is derived by volume-averaging the microscopic model and a porous-medium formulation is used to model mass transport in the preform. The proposed model has been implemented in a Carbon Oxidation Analysis Code based on OpenFOAM (COACO). Using inverse analysis, it was possible to estimate the intrinsic fiber reactivity and then validate the model. The reactivity obtained is surprisingly high compared to literature data. This is explained by the fact that the carbon fibers contain traces of calcium and potassium, which are known to be catalysts for oxidation. They progressively accumulate at the surface in the form of combustion residues.

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

Direct Detection of NO Produced by High-Temperature Surface-Catalyzed Atom Recombination

The surface-catalytic recombination of oxygen and nitrogen atoms to form nitric oxide was confirmed by the direct detection of product NO molecules, using single-photon laser-induced fluorescence spectroscopy. Experiments were performed from room temperature to 1200 K in a quartz diffusion-tube sidearm reactor enclosed in a high-temperature tube furnace. Atomic nitrogen was generated using a microwave discharge, and atomic oxygen was produced via the rapid gas-phase titration reaction N + NO → O + N 2 . The use of isotopically labeled titration gases 15 N 16 O and 15 N 18 O allowed for the unambiguous identification of nitric oxide produced by the O + N surface reaction. The absolute number densities of surface-produced NO were determined from separate calibration experiments using 14 N 16 O. Observed variations of the NO number density with temperature and varying O/N atomic ratios at the sidearm entrance are generally consistent with the predictions of a simple reaction-diffusion model of the sidearm reactor that includes surface-catalyzed NO production as a species boundary condition.

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.

An analytic model for atom diffusion and heterogeneous recombination in a porous medium

A second-order nonlinear differential equation is developed to model the diffusion and heterogeneous recombination of atomic species in a porous medium. The model incorporates adsorption, thermal desorption and Eley–Rideal recombination of atoms on pore surfaces. Analytic solutions and numerical examples are presented, and their application to laboratory investigations of heterogeneous chemistry is discussed.