The partial space qualification of a vertically aligned carbon nanotube coating on aluminium substrates for EO applications Evangelos Theocharous,1,* Christopher J. Chunnilall,1 Ryan Mole,1 David Gibbs,1 Nigel Fox,1 Naigui Shang,2 Guy Howlett,2 Ben Jensen,2 Rosie Taylor,3 Juan R. Reveles,3 Oliver B. Harris,3 and Naseer Ahmed3 2
1 Optical Measurement Group, NPL, Hampton Road, Teddington, TW11 0LW, UK Surrey NanoSystems Ltd, Building 24, Euro Business Park, Newhaven, BN9 0DQ, UK 3 EnerSys ABSL, Culham Science Centre, Abingdon, OX14 3ED, UK *[email protected]
Abstract: The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube Array (VANTA) on aluminium substrates is reported for the first time. The coating on aluminium was realised using a process that employs top down thermal radiation to assist growth, enabling deposition at temperatures below the substrate’s melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths in the 2.5 µm to 15 µm range. VANTA-coated aluminium substrates were subjected to space qualification testing (mass loss, outgassing, shock, vibration and temperature cycling) before their optical properties were re-assessed. Within measurement uncertainty, no changes to hemispherical reflectance were detected, confirming that NanoTube Black coatings on aluminium are good candidates for Earth Observation (EO) applications. © 2014 Optical Society of America OCIS codes: (120.0280) Remote instrumentation; (260.3060) Infrared.
sensing
and
sensors;
(120.3930)
Metrological
References and links 1.
W. R. Blevin and J. Geist, “Influence of black coatings on pyroelectric detectors,” Appl. Opt. 13(5), 1171–1178 (1974). 2. M. J. Persky, “Review of black surfaces for space-borne infrared systems,” Rev. Sci. Instrum. 70(5), 2193–2217 (1999). 3. S. M. Pompea, D. W. Bergener, and D. F. Shepard, “Optically black coating with improved infrared absorption and process of formation,” United states Patent Number 4,589,972 (1986). 4. K. A. Karki, “Process for forming an optical black surface and surface formed thereby,” Patent application number US 05/946,786 (1979). 5. S. Kodama, M. Horiuchi, T. Kunii, and K. Kuroda, “Ultra-black nickel-phosphorous alloy optical absorber,” IEEE Trans. Instrum. Meas. 39(1), 230–232 (1990). 6. R. J. C. Brown, P. J. Brewer, and M. J. T. Milton, “The physical and chemical properties of electroless nickelphosphorous alloys and low reflectance nickel-phosphorous black surfaces,” J. Mater. Chem. 12(9), 2749–2754 (2002). 7. F. J. García-Vidal, J. M. Pitarke, and J. B. Pendry, “Effective medium theory of the optical properties of aligned carbon nanotubes,” Phys. Rev. Lett. 78(22), 4289–4292 (1997). 8. Z. P. Yang, L. Ci, J. A. Bur, S. Y. Lin, and P. M. Ajayan, “Experimental observation of an extremely dark material made by a low-density nanotube array,” Nano Lett. 8(2), 446–451 (2008). 9. K. Mizuno, J. Ishii, H. Kishida, Y. Hayamizu, S. Yasuda, D. N. Futaba, M. Yumura, and K. Hata, “A black body absorber from vertically aligned single-walled carbon nanotubes,” Proc. Natl. Acad. Sci. U.S.A. 106(15), 6044– 6047 (2009). 10. M. A. Quijada, J. G. Hagopian, S. Getty, R. E. Kinzer, and E. J. Wollack, “Hemispherical reflectance and emittance properties of Carbon nanotube coatings at infrared wavelengths,” Proc. of SPIE 8150 (2011). 11. Z. P. Yang, M. L. Hsieh, J. A. Bur, L. Ci, L. M. Hanssen, B. Wilthan, P. M. Ajayan, and S. Y. Lin, “Experimental observation of extremely weak optical scattering from an interlocking carbon nanotube array,” Appl. Opt. 50(13), 1850–1855 (2011).
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7290
12. E. Theocharous, R. Deshpande, A. C. Dillon, and J. Lehman, “Evaluation of a pyroelectric detector with a carbon multiwalled nanotube black coating in the infrared,” Appl. Opt. 45(6), 1093–1097 (2006). 13. S. P. Theocharous, E. Theocharous, and J. H. Lehman, “The evaluation of the performance of two pyroelectric detectors with vertically aligned multi-walled carbon nanotube coatings,” Infr. Phys. & Tech. 55(4), 299–305 (2012). 14. C. J. Chunnilall, J. H. Lehman, E. Theocharous, and A. Sanders, “Infrared hemispherical reflectance of carbon nanotube mats and arrays in the 5–50 µm wavelength region,” Carbon 50(14), 5348–5350 (2012). 15. J. H. Lehman, B. Lee, and E. N. Grossman, “Far infrared thermal detectors for laser radiometry using a carbon nanotube array,” Appl. Opt. 50(21), 4099–4104 (2011). 16. M. A. Quijada, M. Wilson, E. Waluschka, and C. R. McClain, “Optical component performance for the Ocean Radiometer for Carbon Assessment (ORCA),” Proc. SPIE 8153, Earth Observing Systems XVI, (2011), doi:10.1117/12.895938. 17. P. J. Gero, J. A. Dykema, and J. G. Anderson, “A Blackbody design for SI-traceable radiometry for Earth Observation,” J. Atmos. Ocean. Technol. 25(11), 2046–2054 (2008). 18. I. M. Mason, P. H. Sheather, J. A. Bowles, and G. Davies, “Blackbody calibration sources of high accuracy for a spaceborne infrared instrument: the Along Track Scanning Radiometer,” Appl. Opt. 35(4), 629–639 (1996). 19. C. Lijie, R. Vajtai, and P. M. Ajayan, “Vertically Aligned Large-Diameter Double-Walled Carbon Nanotube Arrays Having Ultralow Density,” J. Phys. Chem. C 111(26), 9077–9080 (2007). 20. N. G. Shang, Y. Y. Tan, V. Stolojan, P. Papakonstantinou, and S. R. P. Silva, “High-rate low-temperature growth of vertically aligned carbon nanotubes,” Nanotechnology 21(50), 505604 (2010). 21. G. Y. Chen, B. Jensen, V. Stolojan, and S. R. P. Silva, “Growth of carbon nanotubes at temperatures compatible with integrated circuit technologies,” Carbon 49(1), 280–285 (2011). 22. C. Chunnilall and E. Theocharous, “Infrared hemispherical reflectance measurements in the 2.5 μm to 50 μm wavelength region using an FT spectrometer,” Metrologia 49, S73–S80 (2012). 23. J. M. Palmer, “The measurement of transmission absorption emission and reflection,” in Handbook of Optics, 2nd edition, M. Bass, editor (McGraw-Hill, 1994), Part II, Chapter 25. 24. F. J. J. Clarke and D. J. Parry, “Helmholtz reciprocity: Its validity and application to reflectometry,” Light. Res. Technol 17, 1–11 (1985). 25. S. Berber, Y. K. Kwon, and D. Tomanek, “Unusually high thermal conductivity of carbon nanotubes,” Phys. Rev. Lett. 84(20), 4613–4616 (2000). 26. A. Okamoto, I. Gunjishima, T. Inoue, M. Akoshima, H. Miyagawa, T. Nakano, T. Tanemura, and G. Oomi, “Thermal and electrical conduction properties of vertically aligned carbon nanotubes produced by water-assisted chemical vapor deposition,” Carbon 49(1), 294–298 (2011).
1. Introduction Materials with very low reflectance are highly attractive in a number of areas of science, such as blackbody cavity coatings, absorptive coatings for thermal detectors [1] and coatings for baffles in optical instruments to reduce stray light [2]. It is well known that the reflectance of a surface can be reduced by adding absorbing species such as dyes, as used in Martin Black coatings [3], or carbon black particles, as used in paints such as Nextel black [4]. However, Fresnel reflection at the air/surface boundary resulting from different refractive indices will always limit the fraction of incident radiation which is absorbed. The formation of cavities or projections from a surface has been found to increase its absorbance, as incident radiation is reflected within these features, increasing its probability of being absorbed. The combination of a rough surface with an absorbing species on the surface has produced coatings with directional hemispherical reflectance values below 0.2% in the visible part of the spectrum [5, 6]. It was theoretically predicted that Vertically Aligned carbon NanoTube Array (VANTA) coatings should have an extremely low index of refraction [7], implying Fresnel reflection from such materials will be low. The combination of low Fresnel reflection with the stronglyabsorbing behaviour of a highly-percolated structure suggests the reflectance of VANTA coatings will be extremely low. This paper reports the procedure developed for depositing a NanoTube Black VANTA coating on aluminium substrates intended for Earth Observation (EO) applications, together with its measured performance.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7291
2. Blackbodies in earth observation The very low reflectance of VANTA coatings has been experimentally demonstrated in the visible [8] and infrared wavelengths [9–14]. VANTA coatings have already been used as black coatings on thermal detectors [12, 13, 15]. Carbon nanotubes have also been used to successfully coat both sides of the 600 µm wide slit of the Ocean Radiometer for Carbon Assessment (ORCA) in order to reduce stray light [16]. Importantly, the slit itself was fabricated from silicon, a brittle, glass-like material poorly suited to space applications. However, it was chosen to provide a suitable surface for Carbon NanoTube (CNT) adhesion, and its ability to withstand the high temperatures conventionally required for CNT growth. Blackbodies flown in Earth Observation (EO) satellite missions currently provide the means to calibrate instruments measuring thermal infrared radiance with the lowest uncertainties [17]. As payload is a critical factor in any satellite-borne EO mission, minimising the weight of the blackbody calibration source cavity whilst retaining the required environmental integrity and optical performance necessarily leads to design trade-offs. Highly absorbing VANTA coatings offer the possibility of designing smaller, and consequently lighter, cavities than those that can be realised with lower-emissivity materials. However, the VANTA coating must be suited to application on lightweight engineering alloys to retain these benefits. Aluminium is used almost exclusively in EO blackbody cavity applications as it combines low weight with high thermal conductivity [18]. Unfortunately, conventional growth of VANTA coatings requires the substrate to be maintained at around 750 °C [19], which is beyond aluminium’s 660 °C melting point. Consequently, less absorbing coatings (such as Martin Black) have historically been preferred as the best compromise between performance and weight. However, methods of synthesising VANTA coatings at substrate temperatures as low as 350 °C have been reported [20, 21], which would be suitable for use with low melting point aluminium alloys. 3. Method A total of six NanoTube Black samples were prepared by Surrey NanoSystems (SNS) with a range of total VANTA thicknesses from 22 to 44 microns. The directional-hemispherical reflectance of these samples, plus an Enhanced Martin Black (EMB) reference sample (provided by ABSL), was measured using the NPL infrared hemispherical reflectance facility. Five of the VANTA coated samples were then sent to ABSL where they were subjected to space qualification testing, whilst the sixth and EMB samples were retained by NPL as a control. On completion of space qualification testing, the NanoTube Black samples were returned to NPL for re-measurement of the hemispherical reflectance. The control and EMB samples were also re-measured at this time. 3.1 Fabrication of CNT coating on aluminium substrate Six test coupons (40 mm x 40 mm x 3 mm) were manufactured from 6061-T6 aluminium alloy using typical machining processes for this type of material. This particular alloy is commonly used, finding application in optical baffles [18] and more generally in aerospace industries. The coupons were laser part-marked for traceability, and then chemically cleaned prior to being coated with an electro-deposited finish that improves VANTA film adhesion to the substrate. A multi-layer barrier/catalyst, designed to absorb IR energy from the radiation source whilst ensuring robust adhesion of the CNTs to the substrate, was deposited on the prepared coupons. Following deposition of the catalyst, the samples were subjected to an activation step under a reducing atmosphere at 450 °C. After catalyst activation, the samples were transferred to a CVD reactor configured for plasma-assisted, photo-thermal chemical vapour deposition (PTCVD). The PTCVD process provides rapid top-down heating of the catalysed
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7292
surface of the sample, whilst maintaining the bulk of the coupon at a much lower temperature via cooling of its support platen. This technique allows the higher catalyst temperatures required for low defect, aligned growth at controllable CNT mass density [21], whilst preventing melting or gross mechanical changes of the underlying alloy. CNT growth was initiated at reduced pressure using acetylene as the carbon source in a mixed carrier gas at a temperature of 425 °C. The growth time was varied to achieve different CNT lengths on the coupons. The structure and morphology of the deposited CNT films on the aluminium coupons was characterised by Raman scattering and scanning electron microscopy. Figure 1 shows a wide area view of a VANTA coating grown on an aluminium substrate. The VANTA coating growth mirrors the underlying substrate microstructure exactly. 3.2 The NPL hemispherical reflectance measurement facility The directional-hemispherical reflectance of the NanoTube Black coatings grown on the test coupons was measured (before and after they were subjected to the space qualification tests) using the NPL infrared directional-hemispherical reflectance measurement facility [22]. This facility illuminates the test sample uniformly over almost a 2π solid angle by imaging the output of a cylindrical Oppermann source using a hemispherical mirror. The radiation reflected by the test sample at an angle of 7° from the normal to the plane of the test sample is allowed to escape though a small hole on the hemisphere. Although this measurement represents the hemispherical-directional reflectance of the test sample [23], using the Helmholtz Reciprocity Principle [24], it is equivalent to the directional-hemispherical reflectance measurement of the test sample [23]. The NPL infrared directional-hemispherical reflectance measurement facility utilises a Fourier Transform (FT) interferometer to analyse the reflected radiation in order to provide spectral measurements anywhere in the 2.5 µm to 50 µm wavelength range while the temperature of the sample substrate is maintained at a predetermined level (usually 20 °C). A reference blackbody is used to compensate for the heating of the surface of the test sample caused by the beam used for illumination [22]. Although this effect is large for some black coatings, it was small in the case of VANTA coatings because carbon nanotubes have a very high thermal conductivity along their lengths [25]. A correction is normally applied to account for the inter-reflections between the test sample and the Oppermann source but the low reflectance of the NanoTube Black samples rendered this unnecessary.
Fig. 1. Wide area view of a VANTA coating grown on an aluminium substrate. The VANTA growth mirrors the underlying substrate microstructure exactly.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7293
4. Coupon Space qualification tests 4.1 Mass loss and outgassing tests Mass loss tests were conducted on the VANTA coated substrates using the European Cooperation for Space Standardisation test protocol: ECSS‐Q‐ST‐70‐02C. Materials are considered to be suitable for space applications provided the Total Mass Loss (TML) is less than 1%, the Collected Volatile Condensable Mass (CVCM) is less than 1% and the Recovered Mass Loss (RML) is less than 0.10%. For critical optical instruments, these requirements might be a factor of 10 lower if there is a large amount of coating material in question. Table 1 lists the results of the mass loss and outgassing tests. Table 1. Mass loss and outgassing test results (highest values taken) TML = 0.003%
RML = 0.01%
CVCM = 0.00%
As part of the mass loss and outgassing tests, the samples were also analysed under vacuum by a residual gas analyser (RGA), scanning from 1 to 148 AMU. No molecular species were detected at significant levels other than water, which would be driven off by the preliminary bake-out in the event of NanoTube Black being used in the manufacture of a blackbody cavity. 4.2 Vibration test Figure 2 shows the vibration test set-up with Particle Fall Out (PFO) monitoring, where the NanoTube Black coupon is mounted face down within a sealed aluminium box. The lower part is much like a ‘dish’ and has a smooth surface. The assembly was sealed in Class 100 conditions prior to transport to the vibration facility and only opened again once returned to the Class 100 environment. The resonant jig was designed to give one single resonance at ~400 Hz in the 20 Hz to 2000 Hz frequency range. Particle fall out was measured via solvent transfer of any particulates within the ‘dish’ to a PFO plate. PFO levels were then measured using the PFO photometer in the cleanroom at ABSL.
Fig. 2. Assembly CAD model of the aluminium sample box for vibration testing. The blackened sample (1) is mounted face down and the seal is made between lid (2) and base (4) using a PTFE gasket (3).
The test was carried out using a resonating fixture to amplify the acceleration provided by the shaker head. A mass dummy was used in the tailoring of the jig response and then the blackened samples were subjected to the vibration test; PFO monitoring was performed following vibration runs. The applied mechanical loads are considered worst case; the input
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7294
being that seen in random vibration (out of plane) testing of instruments developed by ABSL. These loads were taken from finite element analysis models of flight hardware and correlated test results. Figure 3 provides the target vibration profile for the NanoTube Black-coated coupons. The amplifying jig was tailored to give an output that best meets this profile, but since an exact replication was technically difficult, the main aim was to produce a profile with the correct resonant frequency and which envelopes (i.e. is equal or greater than) the response previously seen during testing of flight-hardware. The target resonant frequency of the vibration jig was 400 Hz ± 20%, with an output root mean square acceleration gRMS of 84 g ± 5% and a peak acceleration spectral density (ASD) of 70 g2 Hz−1. The performance of the vibration jig was found suitable for the application, displaying a resonance at the required frequency. Tailoring of the input signal was done on-the-fly and notching and capping of the input ASD signal was required due to the input signal amplification at resonance.
Fig. 3. Out-of-plane axis random vibration response (solid blue line) for a mass dummy during jig tailoring exercise. This is the profile which the blackened coupons were subsequently subjected to. Solid red line shows the response from analysis of flight hardware when subjected to random vibration in out-of-plane axis; this profile was the target for coupon vibration.
4.3 Shock test Shock loading is a transient dynamic condition all flight-hardware is exposed to during launch and subsequent orbit injection. The purpose of this test is to simulate the mechanical loads resulting from rocket staging and separation devices, which typically spans frequencies from 100 Hz to 2000 Hz and accelerations from 20 g to a plateau of 2000 g beyond 1000 Hz. Two coupons were tested concurrently by attaching them to a solid steel block which was in turn bolted onto a ringing plate. A trial and error approach together with mass dummies were used to verify the correct output shock response spectrum (SRS) was obtained prior to testing the actual test coupons. The trial and error exercise consisted of dropping a mass from a height onto the ringing plate. The drop height and the level of damping at the impact point were varied according to the response recorded by a tri-axial accelerometer mounted on the steel block. Once the sought parameters were established, the actual test coupons were exposed to the SRS- provided in Table 2.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7295
Table 2. Shock response spectrum requirement (tolerance + 6dB −6dB) Frequency (Hz)
Acceleration (g)
100
20
1000
2000
10000
2000
It was expected that the three orthogonal axes had to be tested independently. However, after inspection of the dummy mass results it was established that a single shock event resulted in meeting the required SRS in all axes. A sample SRS response graph is provided in Fig. 4.
Fig. 4. SRS requirement (shown as a solid black line), tolerance envelope (red and brown lines) and tri-axial accelerometer response during the shock event (blue, green and orange lines).
4.4 Thermal cycling test During the thermal cycling the test coupons were mounted facing downwards on the thermal test jig inside a thermal vacuum chamber. The coupons were then thermally cycled between the minimum and maximum temperature limits, as given in Table 3. The temperatures of the coupons were monitored via thermistors mounted on the thermal jig to ensure that the coupons achieved the required temperatures limits. A calibrated residual gas analyser (RGA) and thermal-crystal quartz meter (TQCM) were used to monitor outgassing and cleanliness levels and a PFO witness plate provided PFO monitoring during testing. The thermal cycling limits were designed to encompass levels that might be seen on space-borne optical instruments where survival, without performance degradation, often has to be demonstrated for non-operational temperature limits at qualification levels. The coupons were exposed to 6 full cycles.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7296
Table 3. TV cycling limits Maximum temperature limit + 100°C Tolerance −0/+3°C
Minimum temperature limit −100°C Tolerance −3/+0°C
Number of cycles
Dwell time
6
2 hours
Temperature rate of change
1 Optical Measurement Group, NPL, Hampton Road, Teddington, TW11 0LW, UK Surrey NanoSystems Ltd, Building 24, Euro Business Park, Newhaven, BN9 0DQ, UK 3 EnerSys ABSL, Culham Science Centre, Abingdon, OX14 3ED, UK *[email protected]
Abstract: The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube Array (VANTA) on aluminium substrates is reported for the first time. The coating on aluminium was realised using a process that employs top down thermal radiation to assist growth, enabling deposition at temperatures below the substrate’s melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths in the 2.5 µm to 15 µm range. VANTA-coated aluminium substrates were subjected to space qualification testing (mass loss, outgassing, shock, vibration and temperature cycling) before their optical properties were re-assessed. Within measurement uncertainty, no changes to hemispherical reflectance were detected, confirming that NanoTube Black coatings on aluminium are good candidates for Earth Observation (EO) applications. © 2014 Optical Society of America OCIS codes: (120.0280) Remote instrumentation; (260.3060) Infrared.
sensing
and
sensors;
(120.3930)
Metrological
References and links 1.
W. R. Blevin and J. Geist, “Influence of black coatings on pyroelectric detectors,” Appl. Opt. 13(5), 1171–1178 (1974). 2. M. J. Persky, “Review of black surfaces for space-borne infrared systems,” Rev. Sci. Instrum. 70(5), 2193–2217 (1999). 3. S. M. Pompea, D. W. Bergener, and D. F. Shepard, “Optically black coating with improved infrared absorption and process of formation,” United states Patent Number 4,589,972 (1986). 4. K. A. Karki, “Process for forming an optical black surface and surface formed thereby,” Patent application number US 05/946,786 (1979). 5. S. Kodama, M. Horiuchi, T. Kunii, and K. Kuroda, “Ultra-black nickel-phosphorous alloy optical absorber,” IEEE Trans. Instrum. Meas. 39(1), 230–232 (1990). 6. R. J. C. Brown, P. J. Brewer, and M. J. T. Milton, “The physical and chemical properties of electroless nickelphosphorous alloys and low reflectance nickel-phosphorous black surfaces,” J. Mater. Chem. 12(9), 2749–2754 (2002). 7. F. J. García-Vidal, J. M. Pitarke, and J. B. Pendry, “Effective medium theory of the optical properties of aligned carbon nanotubes,” Phys. Rev. Lett. 78(22), 4289–4292 (1997). 8. Z. P. Yang, L. Ci, J. A. Bur, S. Y. Lin, and P. M. Ajayan, “Experimental observation of an extremely dark material made by a low-density nanotube array,” Nano Lett. 8(2), 446–451 (2008). 9. K. Mizuno, J. Ishii, H. Kishida, Y. Hayamizu, S. Yasuda, D. N. Futaba, M. Yumura, and K. Hata, “A black body absorber from vertically aligned single-walled carbon nanotubes,” Proc. Natl. Acad. Sci. U.S.A. 106(15), 6044– 6047 (2009). 10. M. A. Quijada, J. G. Hagopian, S. Getty, R. E. Kinzer, and E. J. Wollack, “Hemispherical reflectance and emittance properties of Carbon nanotube coatings at infrared wavelengths,” Proc. of SPIE 8150 (2011). 11. Z. P. Yang, M. L. Hsieh, J. A. Bur, L. Ci, L. M. Hanssen, B. Wilthan, P. M. Ajayan, and S. Y. Lin, “Experimental observation of extremely weak optical scattering from an interlocking carbon nanotube array,” Appl. Opt. 50(13), 1850–1855 (2011).
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7290
12. E. Theocharous, R. Deshpande, A. C. Dillon, and J. Lehman, “Evaluation of a pyroelectric detector with a carbon multiwalled nanotube black coating in the infrared,” Appl. Opt. 45(6), 1093–1097 (2006). 13. S. P. Theocharous, E. Theocharous, and J. H. Lehman, “The evaluation of the performance of two pyroelectric detectors with vertically aligned multi-walled carbon nanotube coatings,” Infr. Phys. & Tech. 55(4), 299–305 (2012). 14. C. J. Chunnilall, J. H. Lehman, E. Theocharous, and A. Sanders, “Infrared hemispherical reflectance of carbon nanotube mats and arrays in the 5–50 µm wavelength region,” Carbon 50(14), 5348–5350 (2012). 15. J. H. Lehman, B. Lee, and E. N. Grossman, “Far infrared thermal detectors for laser radiometry using a carbon nanotube array,” Appl. Opt. 50(21), 4099–4104 (2011). 16. M. A. Quijada, M. Wilson, E. Waluschka, and C. R. McClain, “Optical component performance for the Ocean Radiometer for Carbon Assessment (ORCA),” Proc. SPIE 8153, Earth Observing Systems XVI, (2011), doi:10.1117/12.895938. 17. P. J. Gero, J. A. Dykema, and J. G. Anderson, “A Blackbody design for SI-traceable radiometry for Earth Observation,” J. Atmos. Ocean. Technol. 25(11), 2046–2054 (2008). 18. I. M. Mason, P. H. Sheather, J. A. Bowles, and G. Davies, “Blackbody calibration sources of high accuracy for a spaceborne infrared instrument: the Along Track Scanning Radiometer,” Appl. Opt. 35(4), 629–639 (1996). 19. C. Lijie, R. Vajtai, and P. M. Ajayan, “Vertically Aligned Large-Diameter Double-Walled Carbon Nanotube Arrays Having Ultralow Density,” J. Phys. Chem. C 111(26), 9077–9080 (2007). 20. N. G. Shang, Y. Y. Tan, V. Stolojan, P. Papakonstantinou, and S. R. P. Silva, “High-rate low-temperature growth of vertically aligned carbon nanotubes,” Nanotechnology 21(50), 505604 (2010). 21. G. Y. Chen, B. Jensen, V. Stolojan, and S. R. P. Silva, “Growth of carbon nanotubes at temperatures compatible with integrated circuit technologies,” Carbon 49(1), 280–285 (2011). 22. C. Chunnilall and E. Theocharous, “Infrared hemispherical reflectance measurements in the 2.5 μm to 50 μm wavelength region using an FT spectrometer,” Metrologia 49, S73–S80 (2012). 23. J. M. Palmer, “The measurement of transmission absorption emission and reflection,” in Handbook of Optics, 2nd edition, M. Bass, editor (McGraw-Hill, 1994), Part II, Chapter 25. 24. F. J. J. Clarke and D. J. Parry, “Helmholtz reciprocity: Its validity and application to reflectometry,” Light. Res. Technol 17, 1–11 (1985). 25. S. Berber, Y. K. Kwon, and D. Tomanek, “Unusually high thermal conductivity of carbon nanotubes,” Phys. Rev. Lett. 84(20), 4613–4616 (2000). 26. A. Okamoto, I. Gunjishima, T. Inoue, M. Akoshima, H. Miyagawa, T. Nakano, T. Tanemura, and G. Oomi, “Thermal and electrical conduction properties of vertically aligned carbon nanotubes produced by water-assisted chemical vapor deposition,” Carbon 49(1), 294–298 (2011).
1. Introduction Materials with very low reflectance are highly attractive in a number of areas of science, such as blackbody cavity coatings, absorptive coatings for thermal detectors [1] and coatings for baffles in optical instruments to reduce stray light [2]. It is well known that the reflectance of a surface can be reduced by adding absorbing species such as dyes, as used in Martin Black coatings [3], or carbon black particles, as used in paints such as Nextel black [4]. However, Fresnel reflection at the air/surface boundary resulting from different refractive indices will always limit the fraction of incident radiation which is absorbed. The formation of cavities or projections from a surface has been found to increase its absorbance, as incident radiation is reflected within these features, increasing its probability of being absorbed. The combination of a rough surface with an absorbing species on the surface has produced coatings with directional hemispherical reflectance values below 0.2% in the visible part of the spectrum [5, 6]. It was theoretically predicted that Vertically Aligned carbon NanoTube Array (VANTA) coatings should have an extremely low index of refraction [7], implying Fresnel reflection from such materials will be low. The combination of low Fresnel reflection with the stronglyabsorbing behaviour of a highly-percolated structure suggests the reflectance of VANTA coatings will be extremely low. This paper reports the procedure developed for depositing a NanoTube Black VANTA coating on aluminium substrates intended for Earth Observation (EO) applications, together with its measured performance.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7291
2. Blackbodies in earth observation The very low reflectance of VANTA coatings has been experimentally demonstrated in the visible [8] and infrared wavelengths [9–14]. VANTA coatings have already been used as black coatings on thermal detectors [12, 13, 15]. Carbon nanotubes have also been used to successfully coat both sides of the 600 µm wide slit of the Ocean Radiometer for Carbon Assessment (ORCA) in order to reduce stray light [16]. Importantly, the slit itself was fabricated from silicon, a brittle, glass-like material poorly suited to space applications. However, it was chosen to provide a suitable surface for Carbon NanoTube (CNT) adhesion, and its ability to withstand the high temperatures conventionally required for CNT growth. Blackbodies flown in Earth Observation (EO) satellite missions currently provide the means to calibrate instruments measuring thermal infrared radiance with the lowest uncertainties [17]. As payload is a critical factor in any satellite-borne EO mission, minimising the weight of the blackbody calibration source cavity whilst retaining the required environmental integrity and optical performance necessarily leads to design trade-offs. Highly absorbing VANTA coatings offer the possibility of designing smaller, and consequently lighter, cavities than those that can be realised with lower-emissivity materials. However, the VANTA coating must be suited to application on lightweight engineering alloys to retain these benefits. Aluminium is used almost exclusively in EO blackbody cavity applications as it combines low weight with high thermal conductivity [18]. Unfortunately, conventional growth of VANTA coatings requires the substrate to be maintained at around 750 °C [19], which is beyond aluminium’s 660 °C melting point. Consequently, less absorbing coatings (such as Martin Black) have historically been preferred as the best compromise between performance and weight. However, methods of synthesising VANTA coatings at substrate temperatures as low as 350 °C have been reported [20, 21], which would be suitable for use with low melting point aluminium alloys. 3. Method A total of six NanoTube Black samples were prepared by Surrey NanoSystems (SNS) with a range of total VANTA thicknesses from 22 to 44 microns. The directional-hemispherical reflectance of these samples, plus an Enhanced Martin Black (EMB) reference sample (provided by ABSL), was measured using the NPL infrared hemispherical reflectance facility. Five of the VANTA coated samples were then sent to ABSL where they were subjected to space qualification testing, whilst the sixth and EMB samples were retained by NPL as a control. On completion of space qualification testing, the NanoTube Black samples were returned to NPL for re-measurement of the hemispherical reflectance. The control and EMB samples were also re-measured at this time. 3.1 Fabrication of CNT coating on aluminium substrate Six test coupons (40 mm x 40 mm x 3 mm) were manufactured from 6061-T6 aluminium alloy using typical machining processes for this type of material. This particular alloy is commonly used, finding application in optical baffles [18] and more generally in aerospace industries. The coupons were laser part-marked for traceability, and then chemically cleaned prior to being coated with an electro-deposited finish that improves VANTA film adhesion to the substrate. A multi-layer barrier/catalyst, designed to absorb IR energy from the radiation source whilst ensuring robust adhesion of the CNTs to the substrate, was deposited on the prepared coupons. Following deposition of the catalyst, the samples were subjected to an activation step under a reducing atmosphere at 450 °C. After catalyst activation, the samples were transferred to a CVD reactor configured for plasma-assisted, photo-thermal chemical vapour deposition (PTCVD). The PTCVD process provides rapid top-down heating of the catalysed
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7292
surface of the sample, whilst maintaining the bulk of the coupon at a much lower temperature via cooling of its support platen. This technique allows the higher catalyst temperatures required for low defect, aligned growth at controllable CNT mass density [21], whilst preventing melting or gross mechanical changes of the underlying alloy. CNT growth was initiated at reduced pressure using acetylene as the carbon source in a mixed carrier gas at a temperature of 425 °C. The growth time was varied to achieve different CNT lengths on the coupons. The structure and morphology of the deposited CNT films on the aluminium coupons was characterised by Raman scattering and scanning electron microscopy. Figure 1 shows a wide area view of a VANTA coating grown on an aluminium substrate. The VANTA coating growth mirrors the underlying substrate microstructure exactly. 3.2 The NPL hemispherical reflectance measurement facility The directional-hemispherical reflectance of the NanoTube Black coatings grown on the test coupons was measured (before and after they were subjected to the space qualification tests) using the NPL infrared directional-hemispherical reflectance measurement facility [22]. This facility illuminates the test sample uniformly over almost a 2π solid angle by imaging the output of a cylindrical Oppermann source using a hemispherical mirror. The radiation reflected by the test sample at an angle of 7° from the normal to the plane of the test sample is allowed to escape though a small hole on the hemisphere. Although this measurement represents the hemispherical-directional reflectance of the test sample [23], using the Helmholtz Reciprocity Principle [24], it is equivalent to the directional-hemispherical reflectance measurement of the test sample [23]. The NPL infrared directional-hemispherical reflectance measurement facility utilises a Fourier Transform (FT) interferometer to analyse the reflected radiation in order to provide spectral measurements anywhere in the 2.5 µm to 50 µm wavelength range while the temperature of the sample substrate is maintained at a predetermined level (usually 20 °C). A reference blackbody is used to compensate for the heating of the surface of the test sample caused by the beam used for illumination [22]. Although this effect is large for some black coatings, it was small in the case of VANTA coatings because carbon nanotubes have a very high thermal conductivity along their lengths [25]. A correction is normally applied to account for the inter-reflections between the test sample and the Oppermann source but the low reflectance of the NanoTube Black samples rendered this unnecessary.
Fig. 1. Wide area view of a VANTA coating grown on an aluminium substrate. The VANTA growth mirrors the underlying substrate microstructure exactly.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7293
4. Coupon Space qualification tests 4.1 Mass loss and outgassing tests Mass loss tests were conducted on the VANTA coated substrates using the European Cooperation for Space Standardisation test protocol: ECSS‐Q‐ST‐70‐02C. Materials are considered to be suitable for space applications provided the Total Mass Loss (TML) is less than 1%, the Collected Volatile Condensable Mass (CVCM) is less than 1% and the Recovered Mass Loss (RML) is less than 0.10%. For critical optical instruments, these requirements might be a factor of 10 lower if there is a large amount of coating material in question. Table 1 lists the results of the mass loss and outgassing tests. Table 1. Mass loss and outgassing test results (highest values taken) TML = 0.003%
RML = 0.01%
CVCM = 0.00%
As part of the mass loss and outgassing tests, the samples were also analysed under vacuum by a residual gas analyser (RGA), scanning from 1 to 148 AMU. No molecular species were detected at significant levels other than water, which would be driven off by the preliminary bake-out in the event of NanoTube Black being used in the manufacture of a blackbody cavity. 4.2 Vibration test Figure 2 shows the vibration test set-up with Particle Fall Out (PFO) monitoring, where the NanoTube Black coupon is mounted face down within a sealed aluminium box. The lower part is much like a ‘dish’ and has a smooth surface. The assembly was sealed in Class 100 conditions prior to transport to the vibration facility and only opened again once returned to the Class 100 environment. The resonant jig was designed to give one single resonance at ~400 Hz in the 20 Hz to 2000 Hz frequency range. Particle fall out was measured via solvent transfer of any particulates within the ‘dish’ to a PFO plate. PFO levels were then measured using the PFO photometer in the cleanroom at ABSL.
Fig. 2. Assembly CAD model of the aluminium sample box for vibration testing. The blackened sample (1) is mounted face down and the seal is made between lid (2) and base (4) using a PTFE gasket (3).
The test was carried out using a resonating fixture to amplify the acceleration provided by the shaker head. A mass dummy was used in the tailoring of the jig response and then the blackened samples were subjected to the vibration test; PFO monitoring was performed following vibration runs. The applied mechanical loads are considered worst case; the input
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7294
being that seen in random vibration (out of plane) testing of instruments developed by ABSL. These loads were taken from finite element analysis models of flight hardware and correlated test results. Figure 3 provides the target vibration profile for the NanoTube Black-coated coupons. The amplifying jig was tailored to give an output that best meets this profile, but since an exact replication was technically difficult, the main aim was to produce a profile with the correct resonant frequency and which envelopes (i.e. is equal or greater than) the response previously seen during testing of flight-hardware. The target resonant frequency of the vibration jig was 400 Hz ± 20%, with an output root mean square acceleration gRMS of 84 g ± 5% and a peak acceleration spectral density (ASD) of 70 g2 Hz−1. The performance of the vibration jig was found suitable for the application, displaying a resonance at the required frequency. Tailoring of the input signal was done on-the-fly and notching and capping of the input ASD signal was required due to the input signal amplification at resonance.
Fig. 3. Out-of-plane axis random vibration response (solid blue line) for a mass dummy during jig tailoring exercise. This is the profile which the blackened coupons were subsequently subjected to. Solid red line shows the response from analysis of flight hardware when subjected to random vibration in out-of-plane axis; this profile was the target for coupon vibration.
4.3 Shock test Shock loading is a transient dynamic condition all flight-hardware is exposed to during launch and subsequent orbit injection. The purpose of this test is to simulate the mechanical loads resulting from rocket staging and separation devices, which typically spans frequencies from 100 Hz to 2000 Hz and accelerations from 20 g to a plateau of 2000 g beyond 1000 Hz. Two coupons were tested concurrently by attaching them to a solid steel block which was in turn bolted onto a ringing plate. A trial and error approach together with mass dummies were used to verify the correct output shock response spectrum (SRS) was obtained prior to testing the actual test coupons. The trial and error exercise consisted of dropping a mass from a height onto the ringing plate. The drop height and the level of damping at the impact point were varied according to the response recorded by a tri-axial accelerometer mounted on the steel block. Once the sought parameters were established, the actual test coupons were exposed to the SRS- provided in Table 2.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7295
Table 2. Shock response spectrum requirement (tolerance + 6dB −6dB) Frequency (Hz)
Acceleration (g)
100
20
1000
2000
10000
2000
It was expected that the three orthogonal axes had to be tested independently. However, after inspection of the dummy mass results it was established that a single shock event resulted in meeting the required SRS in all axes. A sample SRS response graph is provided in Fig. 4.
Fig. 4. SRS requirement (shown as a solid black line), tolerance envelope (red and brown lines) and tri-axial accelerometer response during the shock event (blue, green and orange lines).
4.4 Thermal cycling test During the thermal cycling the test coupons were mounted facing downwards on the thermal test jig inside a thermal vacuum chamber. The coupons were then thermally cycled between the minimum and maximum temperature limits, as given in Table 3. The temperatures of the coupons were monitored via thermistors mounted on the thermal jig to ensure that the coupons achieved the required temperatures limits. A calibrated residual gas analyser (RGA) and thermal-crystal quartz meter (TQCM) were used to monitor outgassing and cleanliness levels and a PFO witness plate provided PFO monitoring during testing. The thermal cycling limits were designed to encompass levels that might be seen on space-borne optical instruments where survival, without performance degradation, often has to be demonstrated for non-operational temperature limits at qualification levels. The coupons were exposed to 6 full cycles.
#201883 - $15.00 USD (C) 2014 OSA
Received 27 Nov 2013; revised 4 Mar 2014; accepted 5 Mar 2014; published 21 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007290 | OPTICS EXPRESS 7296
Table 3. TV cycling limits Maximum temperature limit + 100°C Tolerance −0/+3°C
Minimum temperature limit −100°C Tolerance −3/+0°C
Number of cycles
Dwell time
6
2 hours
Temperature rate of change
