Experimental study of oxygen catalytic recombination on a smooth surface in a shock tube

Accepted Manuscript Experimental Study of Oxygen Catalytic Recombination on a Smooth Surface in a Shock Tube Ikhyun Kim, Gisu Park PII: DOI: Reference:

S1359-4311(19)30470-3 https://doi.org/10.1016/j.applthermaleng.2019.04.054 ATE 13644

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

10 January 2019 8 April 2019 15 April 2019

Please cite this article as: I. Kim, G. Park, Experimental Study of Oxygen Catalytic Recombination on a Smooth Surface in a Shock Tube, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng. 2019.04.054

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Experimental Study of Oxygen Catalytic Recombination on a Smooth Surface in a Shock Tube Ikhyun Kima , Gisu Parka,∗ a Department

of Aerospace Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea

Abstract The effect of oxygen catalytic recombination on various metal-coated surfaces has been experimentally investigated. The approach uses experimental results from shock-tube tests with an existing theory based on binary gas mixture to determine the oxygen catalytic efficiency. Surface heat-transfer rates at shock tube endwall were measured using a thin-film gauge. During the testing, the surface of the gauges was polished to a high degree of smoothness and maintained at near room temperature. Six different surfaces, coated with aluminum, iron, titanium, aluminum oxide, iron oxide, and titanium dioxide. Prior to testing, the surface quality was examined using tools for microscopic and macroscopic analyses, thereby characterizing the initial condition. The present efficiency data are compared with the existing values from different facilities. Keywords: Catalytic Efficiency, Shock Tube, Thin-Film Gauge

∗ Corresponding

author Email address: [email protected] (Gisu Park)

Preprint submitted to Applied Thermal Engineering

April 16, 2019

cp

specific heat at constant pressure, J/(kg·K)

cp

frozen specific heat at constant pressure, J/(kg·K)

H

total enthalpy (mass specific), J/kg

h

enthalpy (mass specific), J/kg

hR

heat of recombination (mass specific), J/kg

Ic

constant electric current, A

k

thermal conductivity, W/(m·K)

kw

catalytic velocity, m/s

Pr

frozen Prandtl number, dimensionless,

p

pressure, Pa

q

heat-transfer rate, W/m2

R

resistance, Ω

Rq

root-mean-square roughness, nm

Sc

Schmidt number, dimensionless,

T

temperature, K

t

time, s

u

velocity, m/s

V

voltage, V

x

distance along the wall measured from the stagnation

µ cp k

µ ρD

point, m α

mass fraction, dimensionless

β

velocity gradient, 1/s,

βR

coefficient of resistivity, 1/K

γ

surface catalytic efficiency, dimensionless

η

boundary layer coordinate normal to the wall

µ

viscosity, kg/m·s

ρ

density, kg/m3

ϕ

correction factor for catalytic effect of binary gas mixtures

due dx

2

Subscript C

conduction

D

diffusion

e

boundary-layer edge

s

stagnation point

shock

primary shock wave

st

steady

w

wall

2

freestream (behind the primary shock wave)

5

reservoir (behind the reflected shock wave)

1. Introduction The desire to maintain thermal loads acting on re-entry vehicles within acceptable limits drives numerous activities pertaining to research and development of thermal-protection materials [1–3]. Metallic thermal-protection materials with low mass densities and life-cycle costs as well as greater strength to withstand thermal loads have been widely employed in the design and manufacture of re-entry as well as reusable launch vehicles [4, 5]. It is well known that exothermic catalytic recombination effects govern the magnitude of surface heat transfer via diffusion, which may be greater compared to conductive heat transfer. It has previously been demonstrated that recombination phenomena account for up to 30% of the total heat transfer during re-entry into Earth’s atmosphere [6]. Thermal-protection materials employed in the base heat shield of re-entry vehicles are subjected to significantly different surface heat-transfer phenomena depending upon their catalytic efficiency. Therefore, to facilitate accurate prediction of thermal loads during re-entry, it is important to accurately determine the catalytic efficiency of materials employed in vehicle construction. In the past, the efficiency has been determined by various researchers using 3

different test facilities over a wide range of potential materials. Several techniques have been employed to determine the catalytic efficiency and they primarily fall into two basic categories—direct methods of atom-loss measurement and indirect methods of heat-flux measurement. Direct atom-loss measurements are mostly accomplished by performing flow-tube reactor experiments. By dissociating molecular gases using low-pressure electrical discharge, reactants are diffused and transformed out of the gas phase via surface reactions on the probe, which is movable probe and usually coated with the material of interest. Changes in atom concentration with distance are a measure of the rate of atom loss on the surface owing to recombination. Indirect methods of estimating catalytic efficiency involve the use of thermocouples or heat-input wires to measure the heat flux. Catalytic efficiencies are deduced from the measured heat flux by using the well-known Goulard’s formula [7]. Existing experimental data concerning catalytic efficiencies of metals provide a useful understanding of recombination effects. Table 1 [8–14] summarizes reported surface oxygen catalytic efficiencies at near-room wall temperature of different metals while Table 2 [8, 15–18] presents corresponding efficiency values for metal oxides. Data from the present study are also included for comparison. In the tables, the subscripts Pt, Al, Fe, Ti, Al2 O3 , Fe2 O3 , TiO2 denote, platinum, aluminum, iron, titanium, aluminum oxide, iron oxide, titanium oxide, respectively. The catalytic recombination efficiency (γw ) is defined as the ratio of the number of atoms recombining at the surface per unit area and time to the total number of atoms striking the surface per unit area and time. Data presented in the tables were mostly obtained using a sidearm or flowtube in accordance with the principle underlying atom-loss measurements. To determine catalytic efficiency during tube flow, gas-pressure levels must be maintained low (of the order of a few pascals) to minimize the occurrence of collisions between atomic species; this, however, causes deterioration in accuracy of the side-arm technique at high efficiency values [19]. When using such techniques as glow discharge tube, and plasma tubular reactor, gas molecules are commonly dissociated by means of electrical discharge. However, erosion of electrodes dur4

ing electrical discharge tends to contaminate specimen surfaces, thereby unintentionally altering the recombination activity. It has been previously demonstrated that dissociated gases produced via electrical discharge contain large amounts of metastable particles, and this may lead to overestimation of the catalytic efficiency [11, 12]. Aforementioned extant studies reveal that the catalytic recombination efficiency is difficult to predict accurately, since microscopic features of the surface condition and the difficulty in characterization of flow tend to significantly affect measurements performed. Table 1: Literature survey of oxygen catalytic recombination on metal surfaces

Catalytic efficiency, γ [-] Reference Year

Technique

γ Pt

γ Al

γ Fe

γ Ti

[8]

1958

Sidearm reactor

-

-

0.036

-

[9]

1961

Flow reactor

0.01

-

-

-

[10]

1965

Glow discharge tube

0.004

0.01

-

-

[11]

1969

Flow reactor

-

0.007

0.017

-

[12]

1971

Flow reactor

0.014

-

0.01

-

[13]

1986

Shock tube

0.01 - 0.025

-

-

-

[14]

2006

Sidearm reactor

0.0053

-

-

-

Present

2018

Shock tube

0.007

0.0034 0.0036 0.0039

Table 2: Literature survey of oxygen catalytic recombination on metal-oxide surfaces

Catalytic efficiency, γ [-] Reference Year

Technique

γ Al2 O3

γ Fe2 O3

γ TiO2

[15]

1959

Sidearm reactor

0.0021

0.0052

-

[16]

1964

Sidearm reactor

-

0.0085

-

[17]

2001

Plasma tubular reactor

0.014

-

0.013

[18]

2005

Plasma wind tunnel

0.02 - 0.09

-

-

Present

2018

Shock tube

0.0015

0.0011

0.0022

Recently, Park [20] assessed the oxygen catalytic efficiency of copper using

5

a shock tube capable of accurately and uniformly simulating the desired flow field. The determined oxygen catalytic efficiency in this case was observed to be lower by approximately one order of magnitude when compared against the well-known Goulard’s [7] value. Use of a thermally dissociated gas within shock tube facilitates elimination of the effect of metastable particles, such as singlet oxygen atoms formed with electrical-discharge equipment, and changes in chemical composition of specimen surfaces [13]. Further, surface structure of the model specimen tends to remain intact throughout the test owing to relatively short test times [20]. These advantages make the shock tube ideal for the proposed investigation. The work performed by Park indicates that reported catalytic efficiencies need to be determined, wherein uncertainties related to surface condition and test-flow environment are reduced to minimum. Therefore, there is a strong need to re-determine the catalytic efficiency of other metals and metal-oxides as well, thereby hopefully to resolve or to understand existing discrepancies. The purpose of this research is to determine the oxygen catalytic efficiency at near-room wall temperature of metals and metal-oxides through use of a shock tube under controlled test conditions and specimen surfaces such that sources of errors in the measurements are eliminated or at least minimized. Specimen surfaces were highly polished, up to the micrometer scale, and were considered smooth surface herein. The coating materials used included Al, Fe, Ti, Al2 O3 , Fe2 O3 , and TiO2 . Surface conditions of specimens prior to testing were quantitatively characterized using microscopic- and macroscopic-analyses tools. By comparing measured values of heat flux for each specimen with those prescribed by heat-transfer-rate theory [7], values of catalytic efficiencies were deduced. Results obtained from this study have been compared against existing values derived using different techniques, and possible causes for discrepancies between results have been discussed.

6

2. Experimental Details 2.1. Test Apparatus Experiments in this study were performed using a conventional shock tube. Fig. 1 shows a schematic setup for shock tube experiment. The shock tube comprises a driver tube, transition section, driven tube, and test section. Their respective lengths and internal diameters, respectively, measured 86 cm and 68 mm (driver tube), 6 cm and 68 mm (transition section), and 330 cm and 47.5 mm (driven tube). All sections of the shock tube were axially symmetric. A 0.35 mm thick polyethylene diaphragm initially separated the driver tube from the transition piece and the driven tube. High-purity helium (99.99%) was used as the driver gas, and the driver tube was equipped with an arrangement to feed high-pressure gas from a 12-MPa-pressurized bottle using an appropriate high-pressure regulator. The diaphragm was observed to rupture passively until the tube was filled with gas at an absolute pressure of 1.65 ± 0.06 MPa. The driven tube was evacuated using a vacuum pump prior to it being filled with the desired test gas. With shock tube, it is well-known (standard) that the regions 1 and 4 denote the initial filling condition of gases in driven and driver tubes, respectively. Upon diaphragm rupture, the high-pressure driver gas expands into the low-pressure driven tube, thereby forming a region of constant thermodynamic properties behind a primary shock wave. This region, which is bounded by the contact surface, is referred to as region 2. As the shock wave propagates, the hot compressed gas is bounded by a contact surface, which forms an interface between gases in regions 2 and 3. When the primary shock wave reaches the endwall of the driven tube, it reflects and propagates against the direction of oncoming flow. This leads to generation of a constant-property region, thereby causing further compression and heating of the driven gas. The region downstream of the reflected shock wave is known as the reservoir region denoted as region 5. Flow within region 5 remains nearly at rest, although the reflected shock itself moves. Since flow in region 5 has encountered two shock waves, values of

7

pressure and enthalpy within this region are considerable large. In the present study, the said flow in region 5 interacted with the surface of thin-film sensors.

Figure 1: Setup for shock tube experiment.

As depicted in Fig. 1, surface heat-transfer rates at the endwall of the shock tube were measured using thin-film gauges flush-mounted on the test model. Thin-film sensors have been extensively used and validated using short-duration impulse facilities that are known to provide a short test time, typically a few milliseconds or less [20–23]. Details concerning the sensor are provided in Sec. 3. The developed endwall model was made of acrylic with inner and outer diameters measuring 4.7 and 9 cm, respectively. A 2-mm diameter hole, through which thin-film gauges were inserted, was drilled through the internal surface of the endwall model. Gauges were flush-mounted and leveled with the surface of the model using an araldite. Vacuum epoxy was additionally employed to

8

ensure existence of the vacuum state within the driven tube prior to test-gas filling. Four or five gauges were attached to the developed endwall model to obtain several heat-transfer data during a single experiment. The gauges were placed close to the centerline of the shock tube endwall to minimize the influence of the non-planar surface of the incident shock wave, which tends to alter the one-dimensional behavior of the flow behind the reflected shock wave [24]. The nearest gauge is 1.5 cm from the side wall insures that the experiment can simulate reflections from an infinite shock tube endwall that are not affected by the side-wall boundary layer. The differences in the level of the averaged heat transfer rates between the gauges were negligible. Pressure measurements were also performed, which form a useful diagnostic tool for confirming test-flow conditions. The pressure at the endwall was measured in advance using a flush-mounted 16852 series PCB piezoelectric pressure transducer independent of heat-transfer-rate measurements. The said pressure transducer was located at the center of the disk-cylinder model, and possessed a sensitivity of 1.431 mV/kPa. The endwall pressure-measurement model was manufactured using stainless steel SUS 630. On wall of the driven tube was equipped with flush-mounted PCB transducers—16011 and 16008 series, respectively—for measuring wall static pressures and speed of the primary shock wave. The two transducers possessed sensitivities of 1.493 mV/kPa and 1.471 mV/kPa, respectively. Time response of each transducer measured less than 2 µs which is well-suited for performing short-duration transient measurements [25]. 2.2. Test Condition Experiments were performed using a single flow condition. The test gas in the driven tube was a mixture of 21% oxygen and 79% argon by volume. Increased weight of the test gas caused by mixing the oxygen with an argon was necessary to increase the enthalpy of the flow so that a sufficient mass fraction of dissociated oxygen could be obtained. Table 3 summarizes the steady flow condition used. Detailed discussions regarding the examination of steady flow in a state 9

of thermochemical equilibrium in region 5 are included in our previous work [26]. Properties of the shock tube were calculated using a nonequilibrium flow solver, known as SHOCK1D [27]. In the SHOCK1D, the quasi-one-dimensional Navier-Stokes equations are numerically solved including the effect of thermochemical nonequilibrium. The well-known Park’s two-temperature model was considered. The reaction rate coefficients were adopted from the work of Park [28]. A detailed description of the code is given in Kim [27]. Table 3: Steady flow condition [26]

Experiment

Calculation

±%

±%

Property Driver gas

He

Test gas

21%O2 - 79%Ar by volume

Behind the primary shock (condition 2) p2 [kPa]

52.7

3.6

51.3

0.9

ushock [km/s]

1.96

0.5

2.01

0.4

Behind the ref lected shock (condition 5) p5 [kPa]

276

1.1

266

1.1

T5 [K]

-

-

3914

0.6

H5 [MJ/kg]

-

-

4.23

0.7

Mass fraction of

-

-

0.601

1.2

O2 dissociation [-]

The measured shock speed (ushock ) as well as pressures downstream of the primary (p2 ) and reflected (p5 ) shock waves were included for comparison. Uncertainties (±) listed in table 3 were obtained via consideration of a shot-to-shot variation in flow properties based on the 95% confidence interval. The measured shock speed (ushock ) and pressures downstream of the primary (p2 ) and reflected (p5 ) shock wave demonstrated close agreement with their corresponding calculated values within 3%, 3%, and 4%, respectively. The mass fraction of dissociated oxygen (αO2 ) downstream the reflected shock wave was determined to be 0.6. It is believed that this is a moderate amount of dissociation when compared to the 67% dissociation used by Park for a O2 - Ar gas mixture [20].

10

3. Surface Heat-Transfer Measurement Surface heat-transfer rates from the test gas to endwall of the shock tube were measured by using thin-film gauges, which operate on the simple principle that resistance of a metal increases with rise in temperature. By passing constant current through a small metallic film by means of an external power supply, the change in resistance owing to voltage change could be measured in accordance with Ohm’s law. Thin-film gauges used in this study were based on the design of Park [20]. The thin-film gauges were constructed by painting and firing a small strip of metallo-organic platinum (type 5051-C) (manufactured by ESL Electro Science) onto the surface of an insulating substrate—quartz. Quartz is an ideal material owing to its negligibly small thermal conductivity and porosity [20]. The diameter of the quartz sample used in this study measured 2 mm, it’s length being 7 mm. Platinum was hand painted on top of quartz using a single hair of a sizezero paint brush, a single stroke of which was sufficient to produce a uniformly distributed strip. Subsequently, the gauge was dried at room temperature (25 ◦

C) for 30 min and fired inside a furnace to 750 ◦ C and maintained at that

temperature for nearly 15 min. Afterwards, the furnace was gradually cooled to room temperature, thereby preventing internal stresses from being induced due to thermal shock [22]. The platinum strip measured roughly 0.3-mm wide and possessed a thickness ranging from 150 to 250 nm [20]. A platinum film of this thickness is nearly negligible, and its temperature could virtually be considered identical to that of the surface of the quartz substrate. Towards both ends of the platinum film, metallo-organic gold, of type 8081-C (manufactured by ESL Electro Science) was hand painted and continued down the sides of the gauge. Gold was painted down, parallel to the sides of the gauge, whilst ensuring that the gold painting is continuous around the edges of the gauge. Like platinum, the gold-painted gauge was also fired in the furnace and maintained at 750 ◦ C for 15 min. Thickness of the gold layer measured approximately (less than) 1000 nm

11

[20]. Subsequently, the rods were kept in the furnace at 150 ◦ C for 12 hours to enhance gauge performance as well as eliminate any form of resistance instability [22]. A heat shrink tube was used to form a joint between gold tabs and a pair of enameled coated copper wires. Subsequently, the wires were twisted together to enhance the strength of each individual lead. Endfest 300 Plus epoxy adhesive (manufactured by UHU GmbH & Co. KG) was applied to the lower part of the gauge to strengthen the connection between the wires and gold tabs. Surface heat-transfer rates were calculated using the direct semi-infinite technique based on the assumption of one-dimensional unsteady heat transfer [22, 29]. It was assumed that the semi-infinite substrate defined the heat capacity of the entire sensor while the thin metallic film possessed no capacity. To maintain consistency with the one-dimensional theory, the quartz rod used was sufficiently long to approximate a semi-infinite solid. According to Kinnear and Lu [29], the substrate must measure at least 3 mm in length to satisfy the semiinfinite assumption. Therefore, the quartz sample used in this study, measuring 7-mm long, was found to be more than sufficient. Also, the extremely small test duration within the shock tube made the assumption of infinite insulation feasible, albeit for a small depth of the substrate. In accordance with the onedimensional unsteady heat-conduction theory, a general numerical expression for calculating the heat-transfer rate could be obtained as follows [22].

q (tn ) =

n 2(ρcp k)1/2 X 1/2

(π)

i=1

T (ti ) − T (ti−1 ) (tn − ti )

1/2

1/2

+ (tn − ti−1 )

(1)

In the above equation, (ρcp k)1/2 denotes the thermal product, which depends on the material of the substrate, and its value was considered to be equal to 1510 Ws2 /m2 K [20], which could only be determined from the substrate material. The constant-current regulator supplied a current Ic of 10 mA through the sensor. The voltage history was recorded by a digital oscilloscope (GDS-2064) (manufactured by INSTEK). The voltage signal was picked up by a high-speed operational amplifier and amplified to approximately 10 times its original value. The variable T (ti ) denotes transformed temperature from the measured time 12

history of V obtained from each sensor using the following relation.

T (ti ) =

V (ti ) Ic · dR/dT

(2)

The value of dR/dT was deduced via static calibration using a convection furnace, which measured changes in resistance values by gradually increasing sensor temperature from room temperature to 50 ◦ C with increments of 5 ◦ C [20, 21]. At each step, the temperature was maintained constant for approximately 10 min to ensure thermal equilibrium. The resistance was then recorded by each gauge. Using the least-square technique for data analysis along with initial estimates of resistance measured by all gauges, dR/dT was determined [21]. The systematic uncertainty of the heat transfer measurements depends on the accuracy of measuring the current and the initial resistance of thin film [30]. A high precision digital multimeter (FLUKE 17B) (manufactured by FLUKE) was used for the current and resistance measurements, both to an accuracy of ±1.5% as manufacturer reported.

4. Test Model with Surface Characterization 4.1. Surface Coating Fig. 2 shows heat-transfer specimens used in this study with and without surface coatings. During heat-transfer testing, as previously mentioned, surfaces of thin-film gauges were coated with different metals and their oxides. In the figure, photographs of the sources of corresponding coating materials are included at the bottom left of each subfigure. All sources were purchased from Thin Films and Fine Materials (THIFINE). The materials used included silicon dioxide (SiO2 ), aluminum (Al), iron (Fe), titanium (Ti), aluminum oxide (Al2 O3 ), iron oxide (Fe2 O3 ), and titanium dioxide (TiO2 ). An uncoated platinum (Pt) surface was used to validate the thin-film assumption by comparing heat-transfer data corresponding to the SiO2 surface, which is assumed to be noncatalytic [20, 21].

13

Figure 2: Endwall model with various surface coating materials.

Deposition of SiO2 was performed using the electron-beam (E-Beam) evaporation technique in conjunction with the process of physical vapor deposition up to an estimated thickness of 1000 ± 50 nm. The SiO2 source was regarded as being 99.99% pure, and the deposition rate was set to roughly 0.3 nm/s under high vacuum. The metal (Al, Fe, Ti) and metal-oxide (Al2 O3 , Fe2 O3 , TiO2 ) coated specimens comprised twin surface-coated layers. Each specimen was first coated with a 1000 ± 50-nm SiO2 layer. Subsequently, using the E-Beam technique, each specimen was deposited with a 180 ± 70-nm thick layer of metal or metal-oxide. The SiO2 layer functions as an insulator offering protecting against occurrence of a short circuit between the metal–metal or metal–metal-oxide layers [20]. Metals were deposited on the SiO2 layer from respective sources having a purity of 99.998%; corresponding purity of metal-oxide sources was of the order of 99.9%. As depicted in Fig. 2, the SiO2 -coated surface demonstrated a green transparent tinge when compared to the Pt (uncoated) surface. Metal-oxide-coated surfaces are, in general, transparent; however, appearance of any color on the surface was caused by interference between the light reflecting off the quartz substrate and that reflecting off the top of coated surfaces. The Al-coated surface appears light gray, whereas the Al2 O3 -coated one demonstrated a transparent white. The first specimen in the second row of the figure depicts a Fe-coated layer, which appears relatively lighter compared to the Al layer. When com14

pared against the uncoated layer, the Fe2 O3 layer appears transparent violet. The Ti-coated layer appears dark gray. The TiO2 -coated layer appears transparent yellow when compared against uncoated Pt. 4.2. Microstructure Characterization Morphologies of coated surfaces were analyzed using a Magellan 400 scanning electron microscope (SEM). Elemental compositions of specimen surfaces after deposition were analyzed using an APOLLO XPP energy dispersive X-ray spectrometer attached to the SEM. X-ray photoelectron spectroscopy (XPS) measurements were also performed to characterize chemical bonding formation within coated surfaces of test samples. XPS analyses were performed using a K-Alpha 1063 spectrometer (Thermo Fisher Scientific). Photoelectron emission was excited through use of a monochromated Al K-α source. The detection angle was set as 90◦ . Pass energies of 200 eV for survey scans and 50 eV for high-resolution scans were used. Fig. 3 shows SEM images of surfaces of Al- and Al2 O3 -coated heat-transfer gauges prior to experiments. Before experiments, all test samples were individually packed in plastic bags and sealed from the surrounding environment [20]. Precautions were taken to ensure that the surface coatings remained uncontaminated by foreign matter. SEM image of the Al-coated specimen depicts many grain-like structures of small and big sizes. Corresponding images of the Al2 O3 -coated surface, on the other hand, shows sheet-like structures composed of nanoparticles. Surface composition of the Al-coated heat-transfer gauge was determined to include 4.16 at.% (2.22 wt%) C, 36.67 at.% (26.13 wt%) O, 55.00 at.% (66.08 wt%) Al, 4.13 at.% (5.16 wt%) Si, and 0.05 at.% (0.41 wt%) Pt. The Al2 O3 -coated surface mainly comprised 55.30 at.% (38.50 wt%) O, and 31.90 at.% (37.50 wt%) Al. The high concentration of carbon C was due to inadvertent contamination, which cannot be prevented in practice, of test samples exposed to ambient air, albeit ever so briefly.

15

Figure 3: SEM images and X-ray spectra of Al- and Al2 O3 -coated surfaces.

Fig. 4 shows variations in the XPS Al(2p) spectra observed for the Al- and Al2 O3 -coated specimens with and without Ar+ sputtering. All binding energies were aligned based on the C1s binding energy line whilst considering that the C1s peak is generally reported at 284.8 eV. Specimen surfaces were bombarded by Argon ions to remove surface impurities as well as analyze the depth of the oxide layer formed on the thin-film-coated surface. For Al-coated surface, there exist two peaks at 71.9 eV and 74.8 eV, which are representative of metallic Al and Al2 O3 , respectively, and demonstrate good agreement with corresponding values previously reported [31]. Post argon sputtering, the oxide peak at 74.8 eV decreases, whereas the pure aluminum peak at 71.9 eV increases. For Al2 O3 -coated surfaces, there exists only one peak, that of Al2 O3 . The aluminum-oxide peak at 74.8 eV increases post argon sputtering. This indicates that both Al- and Al2 O3 -coated surfaces have been slightly oxidized. This could be caused by the samples being exposed to ambient air just before the experiments. Also, surface oxidation could probably have occurred during sample transfer to the vacuum chamber for XPS measurements. Accord-

16

ing to Jeurgens et al. [32], aluminum is a highly reactive metal, and therefore, the as-received metal always has some amount of surface oxides present.

Figure 4: XPS spectra of Al(2p) core level peak for Al- and Al2 O3 -coated surfaces.

Fig. 5 shows SEM images of the surface of Fe- and Fe2 O3 -coated heat-transfer gauges prior to experiments. Corresponding X-ray spectra of the surfaces are also included to depict the elemental composition of the Fe- and Fe2 O3 -coated specimens. As observed in the X-ray spectra, the Fe-coated surface depicts segregated islands comprising small-sized particles that seem to be separated from each other. As regards the Fe2 O3 -coated surface, it is seen that relatively smooth thin films are formed with some visible cracks. As depicted in Fig. 5, surface composition of the Fe-coated surface includes 40.3 at. % (68.5 wt. %) Fe. The Fe2 O3 -coated surface, on the other hand, comprises only 13.11 at. % (25.74 wt. %) Fe.

17

Figure 5: SEM images and X-ray spectra of Fe- and Fe2 O3 -coated surfaces.

XPS Fe(2p) spectra of the Fe- and Fe2 O3 -coated surfaces are shown in Fig. 6. XPS data corresponding to the Fe-coated surface after Ar+ sputtering demonstrate that the Fe(2p) spectrum comprises a large and a small peak around 706.3 and 710 eV. The former value was attributed to metallic Fe in accordance with [33] while the latter peak at ∼710 eV, which demonstrates higher binding energy compared to the metal was ascribed to the formation of Fe2 O3 owing to surface oxidation of Fe nanoparticles [34]. The iron-oxide energy peak at ∼719 eV also confirms that the surface is present as Fe2 O3 [35]. This result confirms the presence of both metallic Fe and Fe oxide on the Fe-coated surface. However, XPS data concerning the Fe2 O3 -coated surface demonstrate peak positions at ∼710 eV and ∼723 eV, and these are located near values exclusively reported for Fe oxides [36].

18

Figure 6: XPS spectra of Fe(2p) core level peak for Fe- and Fe2 O3 -coated surfaces.

Fig. 7 shows SEM images of surface of Ti- and TiO2 -coated heat-transfer gauges prior to experiments. In the Ti-coated surface, a self-organized regular structure of the micron size was observed. The TiO2 -coated surface, in contrast, contained spherical micrograins uniformly distributed over the surface. Surface composition of the Ti-coated surface comprised 38.16 at. % (64.76 wt. %) Ti, whereas that of the TiO2 -coated surface comprised 21.40 at. % (34.97 wt. %) Ti.

19

Figure 7: SEM images and X-ray spectra of Ti- and TiO2 -coated surfaces.

Fig. 8 shows the XPS spectra of Ti(2p) obtained for the Ti- and TiO2 coated surfaces. The Ti-coated surface comprised two peaks at binding energies of 453.9 and 459.8 eV, which correspond to reported values of clean metallic Ti [37]. As regards the TiO2 -coated surface, main features at ∼459 and ∼465 eV correspond to characteristics of TiO2 [38].

20

Figure 8: XPS spectra of Ti(2p) core level peaks for Ti- and TiO2 -coated surfaces.

4.3. Surface Roughness Characterization Prior to hand-painting the platinum film and coating test materials onto the model surface, it was important to make sure that the rods surface is smooth and highly polished [20]. The polishing process is essential because geometric sharp irregularities and discontinuities at the surface can possibly produce faulty sensors [20]. An auto-rotating flat disk was used for treatment of the quartz substrate. The disk was set to rotate at 350 rpm, and specimen surfaces were carefully polished using a slilicon carbide (SiC) open-coat sandpaper (R & B Co. Ltd). The surfaces were polished using sandpapers with increasing grit size—400, 800, 2000, and 4000—thereby corresponding to an average particle diameter of 35.0 µm, 21.8 µm, 10.3 µm, and 2.5 µm, respectively. At each step, the polishing process lasted approximately 30 minutes. During polishing, 1-µmparticle-size filtered water was used to wash away the swarf. Using a crocus 21

cloth mounted on the disk, the process was completed. A 1-µm sized diamond suspension was used as the lubricant. Prior to conducting shock-tube experiments, topographies of the surface were examined by performing an atomic force microscopy (AFM) analysis—an ideal tool for quantitatively characterizing nanometric dimensional surface roughness [39]. Since the AFM process does not require any special sample preparation, it qualifies as one of the most commonly used techniques for analyzing surface topologies. Fig. 9 shows surface topography results for the quartz substrate after being subjected to polishing process. Surface images of the uncoated and coated sensors just before initiation of the shock tube testing has been included. The said AFM analysis was performed using the Innova Scanning Probe Microscope (BRUKER), and images were captured using an n-doped silicon tip type OTESPA-R3 (BRUKER) having a nominal spring constant of 2 N/m, radius measuring 7 nm, and resonant frequency equal to 70 kHz. During each measurement, test samples were scanned across the edge of the scratch over a 5 × 5 µm area, and the tapping mode was employed with an image resolution of 512 × 512 pixels. Scanned data were processed and analyzed using NanoScope analysis (v1.4) to obtain three dimensional (3D) surface images and root-mean-square values of the surface roughness (Rq ). To ensure repeatability and high accuracy of AFM scanning, ten different zones from the sample surface were analyzed; corresponding images depicted in Fig. 9 are representative of each test sample. Values of Rq are indicated at the bottom right of each figure. The uncertainty parameter (±) was calculated based on the 95% confidence interval. Referring to Fig. 9, the column on the left of the figure presents typical 3D AFM images corresponding to each test sample. To make it easier to visually compare different samples, vertical ordinates were uniformly scaled for each sample. Colors in the images indicate different heights with light and dark colors corresponding to higher and lower topographies, respectively. Data corresponding to the surface of the quartz substrate indicated a more-or-less regular pattern. The uncoated Pt wall exhibited a grain-type surface topology, and 22

Figure 9: AFM images and surface profiles for coating materials.

23

similar grain types were observed for the Al- and Ti-coated surfaces. It was observed that the surface of control was nearly smooth at the 10-nm scale. Values of Rq calculated for the different sample surfaces were not largely different from each other within the same order of magnitude and assumed values approximately equal to 10 nm. The right column in Fig. 9 presents typical line profiles captured along the perpendicular to the surface depicted in the 2D AFM images located directly above the said line profiles. Line profiles(denoted by the blue line), perpendicular to specimen surfaces, are depicted in the top-right column corresponding to each subfigure. Quantitative sizes of the valley depth and peak width, which correspond, respectively, to the height and horizontal distance, could be determined using line profiles. According to Melo et al. [40], surface roughness of thin films depend upon thickness of the deposited film. Since the thickness of each coated film is nearly identical, the surface roughness of smooth surfaces used in this study is of the order of approximately 10 nm.

5. Results and Discussion 5.1. Surface Heat-Transfer Rate Unfiltered heat-transfer signals obtained from all coated and uncoated specimens are shown in Fig. 10. Time elapsed was measured from the instant of primary-shock-wave arrival at the shock tube endwall during each measurement. Several shots of heat-transfer signal traces are included to illustrate shot-to-shot repeatability. Average values of the stagnation heat-transfer rate under steady state are quoted at the bottom left of each subfigure. The uncertainty parameter (±) denotes shot-to-shot variations in the steady heat-transfer rate based on the 95% confidence interval. Fluctuations in the each mean signal level under steady flow condition were observed to be less than about ±3%. As depicted in the figure, the steep initial rise in heat-transfer signals was caused by shock-wave arrival at the endwall. A rapid increase in heat transfer at the beginning of measurements for both coated and uncoated specimens indicates that thin-film gauges demonstrate a definite response irrespective of

24

Figure 10: Measured surface heat-transfer rates.

25

the presence of a coating layer on specimen surfaces. However, under steady flow conditions, a significant difference in the average heat-transfer rate was observed between SiO2 -coated and other testing walls. The Pt data shows approximately 28% higher than the SiO2 -coated case. The observed increase in heat flux is a result of the heat released, during catalytic recombination of incoming oxygen atoms, which in turn, causes an increase in the diffusive heat flux. In contrast, the wall coated with SiO2 , which is considered as noncatalytic, measures only the convective heat transfer. The above difference is understandable because the temperature downstream of the reflected shock induced under present conditions is high enough to cause a noticeable chemical reaction, wherein the mass fraction of dissociated oxygen approximately equals 0.6. Results depicted in Fig. 10 also demonstrate a significant difference between cases corresponding to the SiO2 -coated wall and Al-, Fe-, and Ti-coated surfaces. Compared to the SiO2 coated wall, Al-, Fe-, and Ti–coated surfaces demonstrate an increase in heattransfer rate of the order of 17%, 18%, and 19%, respectively. As regard metaloxide-coated surfaces, Al2 O3 -, Fe2 O3 -, and TiO2 -coated surfaces, respectively, demonstrate 8%, 7%, and 12%, increase in heat-transfer rate when compared against the SiO2 case. 5.2. Comparison with Theory Measured heat-transfer rates in conjunction with Goulard’s theory [7] were used to assess the value of catalytic efficiency. Goulard’s method of analysis for estimating total qtotal , conductive qC , and diffusive qD heat transfers, based on the binary gas mixture, could be mathematically represented as

qD

qtotal = qC + qD p −2/3 qC = 0.47 2βµe ρe P rw he p = 0.47Sc−2/3 2βµe ρe hR αe ϕ

(3) (4) (5)

where ϕ denotes a correction factor for the catalytic wall, ϕ=

1 √ 1 + 0.47Sc−2/3 2βµe ρe /ρw kw 26

(6)

e In Eqs. (4) to (6), β = du dx is stagnation-point velocity gradient, and kw = q γw 8kTw 4 πm is catalytic velocity. The γw is the surface catalytic efficiency defined

in the Introduction section. In the calculation, Sc is assumed to be 0.485 to follow the binary gas assumption [7]. Values at the edge of the boundary-layer were obtained using the SHOCK1D code [27] which were regarded as the condition 5 values in shock tube flows. Values of viscosity and frozen Prandtl number at the wall were calculated using the CEA program [41] assuming wall temperature of 293 K for the gas mixture considered. Assuming the Chapman-Rubesin constant of unity (ρw µw /ρe µe = 1) after Goulard, the density at the wall is determined (ρw =1.74 kg/m3 ). When calculating qC and qD using Eqs. (3), (4), and (5), value of the stagnation velocity gradient β is required. In this work, similar to the work performed by Park [20] and Cheung et al. [21], the value of β was obtained using Eq. (3) by substituting measured surface heat-transfer rates for the SiO2 -coated wall under the assumption of it being noncatalytic. Dependence of surface heat transfer on catalytic efficiency is depicted in Fig. 11. Values of qC were calculated using Eq. (3) while those of qD were calculated using Eq. (5) pertaining to binary gas mixtures. Experimental data concerning smooth surfaces for uncoated Pt, coated with metal (Al, Fe, Ti), and their oxides (Al2 O3 , Fe2 O3 , TiO2 ) demonstrate how catalytic efficiency could be obtained based on heat-transfer distribution. For all cases, qD of the coated/uncoated specimen being tested was defined as the difference between the measured heat transfer corresponding to that material and that corresponding to SiO2 . Looking at the theoretical distribution, the heat-transfer ratio is zero near the region of γw ∼ = 10−5 . This region could, therefore, be considered as a noncatalytic wall. In the region of 10−4 < γw < 10−2 , heat-transfer distribution begin to gradually increase, and rise rapidly up to about γw ∼ = 10−1 . Beyond this region, trends for theory tend to attain nearly constant values in the fully catalytic region represented by γw = 1, where the maximum diffusive heat transfer occurs.

27

Figure 11: Dependence of surface heat-transfer rate on catalytic recombination efficiency.

28

Also, in Fig. 11, heat-transfer ratios for all cases of coated surfaces are represented by horizontal lines. Vertical error bars were determined by considering the minimum and maximum values of the measured heat-transfer ratio qD /qC . Experimental values of catalytic efficiencies were obtained by locating the point of intersection between the horizontal line representing measured heat-transfer ratio and the binary-gas-based theoretical distribution curve. Using this approach, the efficiency for all the testing materials was obtained. For the Pt (uncoated) wall, value of γw was observed to be 0.007. As for the smooth Al-coated surface, γw measured 0.0034. The measured efficiency of the Al2 O3 -coated wall was 0.0015. As regard the remaining coated materials, efficiency values for the Fe-, Fe2 O3 -, Ti-, and TiO2 -coated walls measured 0.0036, 0.0011, 0.0039, and 0.0022, respectively. The overall result was found to be consistent with data available in literature, wherein catalytic efficiencies of metal-coated surfaces are generally higher compared to those of metal-oxide-coated surfaces [8–18]. 5.3. Catalytic Recombination Efficiency Variation trends in oxygen catalytic efficiency γw corresponding to changes in the wall temperature Tw of Pt (uncoated) and metals (Al, Fe, Ti) are shown in Figs. 12–15. Corresponding data values pertaining to metal oxides (Al2 O3 , Fe2 O3 , TiO2 ) have been included for comparison. Data values obtained in this study were compared against those reported in extant literature. Values reported in literature mostly correspond to experiments performed at near-room wall temperature. The authors attempted to collect information regarding the partial pressure of oxygen to understand the effect of pressure on the catalytic efficiency at a fixed wall temperature. Because only the wall temperature information is usually available, it was not possible to draw a trend on pressure and so the interpretation of the data may be limited to a certain degree of aspects. With regard to the present study in the figures, Tw denotes the average temperature encountered during steady flow, as measured by the thin-film gauge. Vertical error bars denote shot-to-shot experimental variations in temperature measured during the steady state while horizontal bars indicate the calculated

29

efficiency by considering the minimum and maximum error-bar values of the measured qD /qC ratio.

Figure 12: Oxygen recombination efficiency for platinum.

Filled symbols in Fig. 12 denote experimental results for the Pt (uncoated) wall while empty symbols denote corresponding values reported in literature. As indicated in the figure, the value of γw for the Pt (uncoated) wall obtained in the present study agrees fair to those observed by Hartunian [10], Melin [12], Berkut [13], and Marschall [14]. Discrepancies in the measured value of recombination efficiency, as obtained by various researchers, could be attributed in part to the use of different experimental techniques. A more probable explanation is of differences between gaseous environments in which the experiments are performed, thereby leading to radically different surface reactions. Gas temperature within the side-arm or flow/diffusion tube [10, 12, 14] is essentially equal to the room temperature, whereas temperature within the shock tube is much higher. Such differences in temperature tend to trigger certain surface-reaction processes, such as molecular dissociation or surface diffusion [14]. Also, the level of scatter in efficiency values could be explained by presence of the oxide layer on the platinum surface. It is known that oxide layers measuring 30–50 A◦ in

30

thickness are rapidly formed on platinum surfaces even for short-term exposures to ambient air [42]. However, practically in most of the experimental studies, the oxide layer on platinum surfaces has not been quantitatively characterized owing to technical difficulties in the interpretation of their measurement. Fig. 13 depicts comparison of recombination efficiency values, observed for Al- and Al2 O3 -coated surfaces, between the proposed and extant studies. Open symbols denote experimental data for Al-coated walls while filled symbols denote experimental data for Al2 O3 -coated walls. Data obtained in the present study

Figure 13: Oxygen recombination efficiency for aluminum.

for the Al-coated wall demonstrate a smaller value of the catalytic efficiency when compared against those obtained via measurements performed by Hartunian et al. [10] and Myerson [11]. The same argument applies to efficiency values obtained for the Al2 O3 -coated wall when compared against studies performed by Greaves and Linnett [15]. Experiments by Hartunian et al. [10] were performed within atom flow tubes in which gas dissociation are glow and microwave discharges. A similar argument applies to studies performed by Myerson [11] and Greaves and Linnett [15] that involved use of an arm reactor. Values of partial atomic pressures on the surface are not available in the literature.

31

However, it is known that the erosion of electrodes may contaminate material surfaces, thereby altering recombination activities in an uncontrollable manner. In contrast to extant studies, results in the present study were obtained under a defined environment, wherein material surfaces were maintained relatively clean owing to relatively short test durations. With regards to Al2 O3 -coated-surface data, it can be seen that results reported by Pidan et al. [18] are higher compared to those obtained in the present study. However, since values reported by Pidan et al. were obtained using arc-jet flows, wherein efficiency values are determined in a very different manner, making a direct one-to-one comparison between results obtained by the two studies using different facilities may be difficult. In Fig. 14, recombination efficiencies of Fe- and Fe2 O3 -coated surfaces are presented along with corresponding values reported in extant studies. Open and filled symbols, respectively denote experimental data for Fe- and Fe2 O3 -coated walls.

Figure 14: Oxygen recombination efficiency for iron.

Since the chemical energy accommodation βc , that is the fraction energy released during catalytic recombination that gets actually transferred to the

32

test sample, is different from unity, the obtained value of efficiency could assume largely different values. However, because near-room-temperature conditions were considered in both cases, so the energy level of the oxygen atoms in both studies remained at the ground state. It is, therefore, reasonable to assume βc as unity [21]. The observed discrepancy in efficiencies could be attributed to several factors such as composition and structure of the surface, surface roughness, atomic partial pressure, gas composition, and/or experimental technique used. Fig. 15 presents measured values of catalytic efficiency for Ti- and TiO2 coated surfaces from the work of Guyon [17] along with results obtained in the present study. Open and filled symbols, respectively, denote experimental results for the Ti- and TiO2 -coated surfaces. Values of catalytic recombination efficiency for Ti- and TiO2 -coated surfaces are not often found in literature and so the data provided in the figure are highly limited.

Figure 15: Oxygen recombination efficiency for titanium.

As regards TiO2 -coated-wall data, values obtained in the present study are observed to be lower by a factor of approximately 6 when compared against those observed by Guyon [17] and obtained under a low-pressure plasma environment

33

with operating pressures of the order of 100 Pa. In contrast, the stagnation pressure in the proposed shock tube test measured approximately 290 kPa. The catalytic efficiency can be decreased by increasing partial atomic pressures. Also, efficiencies can be altered at high reactant pressures by changing the extent of species’ surface coverage [14]. Another possibility involves the use of different surface structures of the tested material. Data for TiO2 -coated-wall obtained in the present study was based on use of a highly polished smooth wall, whereas samples used in the study performed by Guyon [17] existed in the form of pellets.

6. Conclusions The stagnation heat-transfer rates of metal- and metal-oxide-coated surfaces have been measured using a thin-film gauge during tests performed in a shock-tube. The stagnation heat-transfer theory was used to deduce catalytic efficiencies using measured data. Results obtained were compared against values reported in former studies involving use of different techniques. Throughout the comparison, due to a large scatter in the existing data including the present study, making a direct comparison was quite limited to draw any definite trend. Sharing information regarding the surface and flow conditions between the investigators seem strongly needed in the future to quantitatively and also qualitatively better understand the trend or the level of scatterness of the efficiencies. For this journey, the accumulation of the present catalytic efficiencies of aforementioned materials in a shock tube are believed to provide a meaningful experimental database under controlled test conditions and specimen surfaces, in particular hopefully serving as the reference smooth surface dataset to the relevant community.

Acknowledgment This work was supported by Defense Acquisition Program Administration and Agency for Defense Development (No. UD160043BD).

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Highlights: 

Oxygen recombination efficiency on various metal- and metal-oxide-coated surfaces has been experimentally determined



Surface heat transfer rate at the shock tube end-wall was measured using thin-film gauges



The gauge surface was polished to a high degree of smoothness and maintained at near room temperature



The reference smooth surface dataset is obtained



The comparison is made with the existing data from different facilities