Extended Abstract submitted to the 49th AIAA Aerospace Sciences Meeting
INVESTIGATION OF CN PRODUCTION FROM CARBON
MATERIALS IN NITROGEN PLASMAS
A. Lutz*, W. Owens†, J. Meyers‡ and D.G. Fletcher§
University of Vermont
Burlington, VT
and
J. Marschall**
SRI International
Menlo Park, CA
Ablating heat shields are typically used for surface heat fluxes greater than 500 W/cm2 as non-ablating
heat shields do not provide the protective capability necessary at these conditions. The recession rates used
to calculate ablative heat shield thickness have come into question as many believe the rates are much too
conservative at the cost of vehicle payload. Few measurements have been performed of one key reaction, the
nitridation of solid carbon. An inductively coupled plasma torch has been designed and fabricated at the
University of Vermont, which simulates the re-entry conditions on a scaled graphite sample. Spectroscopic
measurements, such as laser-induced fluorescence, will be used to assess the atomic nitrogen arrival and
consumption rates at the surface to assess nitridation. Experiments will be conducted in a nitrogen plasma
with surface temperatures between 1400K and 2500K and variable total pressure.
1. Introduction
Understanding the carbon nitridation process is important for earth re-entry applications. Primarily,
the cyanogen (CN) molecule is known to be a powerful radiator at high temperatures, which may influence
the heat flux experienced by the shield. Also, solid-phase CN is known to exist within the boundary layer
where it can react with other gases, producing more exothermic reactions. All of these processes may play
a significant role in ablation. However, a reliable gas-surface CN reaction efficiency has not been measured.
Previous experiments have attempted to measure the carbon nitridation reaction efficiency. One of the
more surprising results was obained by Park and Bogdanoff [1]. In their experiment, tungsten wire coated
in lampblack was subjected to a stream of highly dissociated nitrogen, produced within a shock tube. The
N-atom concentrations were calculated from equilibrium equations and the CN production was inferred
from emission intensity at 386 nm. The authors determined a reaction efficiency of γN = 0.3 from this
experiment. This value is suspicious for two reasons; the value is higher than expected and it does not
have a temperature dependence, which is physically unlikely [2]. For comparison, it is known that carbon
oxidation has a strong dependency on temperature. γO is known to vary between 0.01 and 0.3 for room
*Graduate Student, Mechanical Engineering Department, 211 Perkins Hall, 33 Colchester Ave; [email protected].
†Graduate Student, Mechanical Engineering Department, 211 Perkins Hall, 33 Colchester Ave;
[email protected].
‡Post-doctoral Researcher, Mechanical Engineering Department, 109 Votey Hall, 33 Colchester Ave,
Member AIAA.
§Professor, Mechanical Engineering Department, 201 Votey Hall, 33 Colchester Ave; [email protected]. Fellow AIAA.
**Senior Research Scientist,
Molecular Physics Laboratory,
333 Ravenswood Avenue;
[email protected]. Senior Member AIAA.
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INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
temperature and 3000 K respectively. Also, carbon oxidation should be a more efficient process but the
Park and Bogdanoff results do not reflect this.
The oxidation efficiency with respect to nitridation is easily proven. A sample placed within an air
plasma will show signs of oxidation on the order of minutes. A similar sample will not show signs of
deformation until much later at identical conditions in a pure nitrogen plasma. This is illustrated in Figure
1 where several samples were subjected to different plasma streams at varying exposure times. The sample
exposed to nitrogen was held within the stream on the order of 10 minutes. It appears unchanged from the
virgin sample whereas samples exposed to air plasmas for equivalent or shorter durations show significant
signs of decomposition. A more accurate and precise reaction efficiency will be measured at the University
of Vermont (UVM) Inductively Coupled Plasma Torch (ICPT) facility, which was designed to replicate
atmosphere re-entry conditions.
Figure 1. Graphite samples used in the UVM ICPT test facility. These
samples show the different stages of decomposition from 1) nitrogen plasma
(10 min) 2-3) air-argon plasma in successively longer exposure times up to
10 minutes [3].
2. Boundary Layer Code
Several numerical software packages have been developed at the von Karman Institute (VKI) in Belgium, which compute transport, chemical and thermodynamic properties of gas mixtures at various conditions. The Multicomponent Transport, Thermodynamic and Chemistry Properties for Ionized Gases
(MUTATION) library is a package which calculates gas mixture properties at prescribed temperatures
and pressures [4]. This library is a useful tool that clearly illustrates the behavior of gases in conditions
where experimental methods would be too difficult or time intensive to conduct. MUTATION will play a
significant role in obtaining numerical results within the boundary layer.
INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
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Figure 2 shows the mass fraction of a 7-species N2 /Ar mixture as a function of temperature. The
program ran at a temperature range of 1500 to 7000K and 115 Torr which are the projected conditions
for one set of tests in the ICPT facility. The molar composition of the mixture was set at a 1:1 Ar to N2
ratio. Figure 2 clearly shows that the argon percentage does not change over this temperature range as it
does not dissociate in these conditions. However, N2 does begin to dissociate, beginning at 4000K. Trace
amounts of electrons and ions exist in this mixture, but do not make an appearance on the plot. These
data, along with enthalpy, mass density and viscosity serve as initial conditions for the NEBOULA input.
Figure 2. Mass fraction of the 7-species Ar/N2 mixture created in MUTATION as a function of temperature. The primary species are argon and
molecular and atomic nitrogen. All other species do not appear in large
quantities for these conditions.
Also developed at VKI, the Non-Equilibrium Boundary Layer (NEBOULA) software package is a computational tool that calculates transport, thermodynamic and chemical properties of gas mixtures within
a boundary layer positioned along the stagnation point streamline [5]. Output values include temperature,
density, velocity, enthalpy and mass and molar fractions. The sample body diameter is controlled as a
dependent variable of the velocity gradient around the body.
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INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
Figure 3. Molar composition and mixture temperature of the 7-species
Ar/N2 gas created in NEBOULA. Prominent species include argon and
molecular and atomic nitrogen.
Figure 3 shows a sample output from the NEBOULA code. Molar fractions and the bulk gas temperature
are plotted along the stagnation point streamline. The boundary layer and wall temperatures were set to
7000K and 1500K respectively for this particular case, which is confirmed in the figure. Again, the prominent
species for these conditions are argon, molecular and atomic nitrogen. Trace amounts of ions and electrons
are present as well.
3. Facility Description
The ICPT facility at UVM is designed to test samples in high enthalpy gas flows for simulation of atmospheric re-entry. It operates in the subsonic flow region and is calibrated to match post-shock conditions.
The facility design allows for precise gas mixtures to replicate the heat loads experienced by vehicles for
different planetary atmospheres. The primary components of the ICPT are the 30kW power source, the
gas injection assembly and the test chamber. The test chamber includes the sample and slug calorimeter.
System specifications are shown in Table 1.
INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
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Table 1. UVM ICPT System Specifications [3]
Parameter
Inductive heater power
Enthalpy range
Mach range
Stagnation heat transfer rate
Stagnation pressure
Rating
30 kW (max) @ 4 MHz
10 - 40 MJ kg−1 (for air)
0.3 to 1
10 - 290 W cm−2
0.05 - 1.0 atm
Figure 4 shows the ICPT facility, the gas injector block assembly and a view inside the chamber during
operation. The test chamber, shown in Figure 4a was fabricated from stainless steel to prevent contamination from oxidation byproducts. It is equipped with portholes in four locations to provide optical and
measurement access. Feed-throughs are included in other locations as well, which are used for the sample
holder, slug calorimeter and active cooling lines in the hood and baseplate of the chamber.
[3]
(a)
ICPT
Test Facility
(b) Injector Block
(c) Plasma Test
Figure 4. a. The University of Vermont ICPT test chamber and 30kW
power source. b. The gas injector block assembly is located below the test
chamber and is responsible for injecting steady, laminar gas flow through
the quartz tube [3]. c. Experimental setup with graphite sample face held
perpendicular to major flow axis. Sample shown tested in nitrogen plasma.
The gas injection assembly, shown in Figure 4b, consists of the main injector block, quartz tube and
inductance coil. The main injector block is constructed from brass components which allow for efficient
cooling in hot regions created by the plasma. It is responsible for injecting a laminar gas mixture evenly
along the quartz tube axis, which limits the possibility of permanently damaging the tube. The quartz
tube is 30 mm in diameter and is concentrically centered within the inductance coil loops. A Lepel Model
T-30-3-MC5-TLI RF power supply, shown in Figure 4a, is used to provide a radio frequency AC current to
the inductance coil. The power supply uses de-ionized water as a working fluid to provide a cooling loop
within the inductance coil. The output plasma gases originating within the quartz tube are ejected towards
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INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
the test sample. Figure 4c shows a sample placed in the test configuration. The sample is mounted such
that the leading face is perpendicular to the axis of the flow during operation.
Laser diagnostic instrumentation is installed at the UVM ICPT facility to measure gas-phase flow properties. The full beam path is shown in Figure 5. The instrumentation consists of a Nd:YAG laser, which
pumps a dye laser. The dye laser provides a fundamental output frequency to a tripling system. The UV
output beam is divided in two directions; 90% of the beam power is directed through the test chamber and
10% through the flow reactor. Currently, the NO cell shown in the figure has not been implemented. Before
passing through the test chamber, a Galilean telescope is used to focus the beam diameter below 1 mm. A
microwave discharge flow reactor is used for calibrating the results obtained in the test chamber. Several
previously performed experiments indicate the method for characterizing plasma species concentrations[2].
Figure 5. UVM ICPT facility and LIF data acquisition configuration.
The Nd:YAG laser currently installed will be used to perform a 2-photon laser induced fluorescence
(LIF) technique for species detection. The fundamental laser properties are shown in Table 2.
Table 2. Nd:YAG Laser Specifications
Parameter
YAG pulse energy
Dye fundamental output
Tripled output
Temporal pulse width
Rating
740 mJ @ 532 nm
120 mJ @ 633 nm
3-4 mJ @ 211 nm (N-atom LIF)
7 ns
4. Experiment
For experiments that will be conducted at the UVM ICPT facility, reaction efficiency for carbon nitridation will be measured. The reaction efficiency, γN , is calculated from the following equation [2].
q
8kB T
∆MC
nN
γN = ΓΓCN
=
(
)/(
)
Am
∆t
4
πmN
N
C
INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
7
The reaction rate efficiency is a function of ΓCN and ΓN , which are the rates of CN creation and N-atom
approach to the surface respectively. This is further deconstructed to a function of the sample carbon
mass loss ∆MC , the sample area A, the atomic mass of carbon and nitrogen respectively mC and mN ,
the exposure time ∆t, the N-atom number density nN , the Boltzmann constant kB and temperature T.
Of these parameters, the only values that need to be measured experimentally are the carbon mass loss,
sample area, exposure time, temperature and N-atom number density just above the sample surface.
The sample area is measured directly from the design parameters and geometry. The carbon mass loss is
calculated from pre- and post-experimental mass measurements. Care will be taken as to not contaminate
the surface in a way that would inhibit or promote reaction efficiency. The exposure time will also be
controlled tightly. LIF techniques will be used to measure the temperature and N-atom number densities.
Using a 2-photon excitation procedure, nitrogen atoms will elevate to an higher electronic state through
absorption. The Nd:YAG laser will be frequency tripled to a 211 nm wavelength. This characteristic
wavelength will induce a 2p3 4 S03/2 → 3p 4 D0 transition in the nitrogen atom ground state, where it will
subsequently relax with a 3p 4 D0 → 3s 4 P0J transition at 869 nm as shown in Figure 6 [6]. The relaxation
transition will be captured using a filtered photomultiplier tube (PMT). The PMT will be controlled by a
SR250 Gated Integrator and Boxcar Averager Module so as to collect fluorescence from each laser pulse.
Figure 6. Two-photon nitrogen excitation strategies.
A plot of the intensity of the emission near the surface of the sample will be created by scanning the
laser beam output across a narrow wavelength band near the sample surface. A separate signal will be
obtained from the flow reactor. In the flow reactor, conditions are maintained constant during the ICP
tests and the N-atom number density is known from chemical titration [2]. By carefully measuring other
parameters, the flow reactor N-atom number density is used to calibrate that of the ICP flow. This will
be performed in future tests. Figure 7 shows a plot of the 2-photon LIF signal for atomic nitrogen in the
UVM ICPT facility free-stream, as well as a simultaneous signal recorded in the flow reactor from a recent
test. N-atom number density is then calculated with previously developed techniques [2].
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INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
0.6
ICP free stream (115 torr, 7940 K)
0.5
Signal (AU)
0.4
ICP fit
Flow Reactor (0.5 torr, 293 K)
Reactor fit
0.3
0.2
0.1
0
632.72
-0.1
632.73
632.74
632.75
632.76
632.77
Dye Fundamental Wavelength (nm)
Figure 7. Emission spectroscopy measurements within the UVM ICP
chamber and flow reactor.
Figure 8 shows surface temperature measurements in a nitrogen/argon plasma stream. Measurements
were obtained with a two-color infrared pyrometer aimed at the sample surface through a viewport located
beneath the test chamber.
Figure 8. Surface temperature of graphite sample in nitrogen/argon
plasma mixture. Measurements taken with Marathon high performance
integrated ratio infrared pyrometer [3].
INVESTIGATION OF CN PRODUCTION FROM CARBON MATERIALS IN NITROGEN PLASMAS
9
The surface temperature range tested at the UVM ICPT facility will be 1400K to 2500K. Measurements
will also be conducted at SRI International using a microwave discharge setup in the 300K to 1800K
temperature range, albeit at very low pressure (1 Torr). The full paper will include a comparison of SRI
and UVM results over a common temperature range.
References
[1] C. Park and D. Bogdanoff (2006) Shock-Tube Measurement of Nitridation Coefficient of Solid Carbon
J. Thermophsyics and Heat Transfer, vol 20, no. 3, pp. 487-492
[2] D.A. Pejakovic, J. Marschall, L. Duan and M.P. Martin (2008) Nitric Oxide Production from Surface
Recombination of Oxygen and Nitrogen Atoms Journal of Thermophysics and Heat Transfer, vol. 22
no. 2 10.2514/1.33073
[3] W.P Owens, J. Uhl, M. Dougherty, A. Lutz and D.G. Fletcher (2010) Development of a 30 kW Inductively Coupled Plasma Torch Facility for Aerospace Material Testing University of Vermont
[4] T. Magin (2004) A Model for Inductive Plasma Wind Tunnels von Karman Institute for Fluid Dynamics
[5] P.F. Barbante (2001) Accurate and Efficient Modeling of High Temperature Non-equilibrium Air Flows
von Karman Institute for Fluid Dynamics
[6] D.G. Fletcher and M. Playez (2006) Characterization of Supersonic and Subsonic Plasma Flows von
Karman Institute for Fluid Dynamics AIAA 2006-3294