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
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  and coatings for baffles in optical instruments to reduce stray light . 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 , or carbon black particles, as used in paints such as Nextel black . 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 , implying Fresnel reflection from such materials will be low. The combination of low Fresnel reflection with the strongly-absorbing 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.
2. Blackbodies in earth observation
The very low reflectance of VANTA coatings has been experimentally demonstrated in the visible  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 . 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 . 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 . Unfortunately, conventional growth of VANTA coatings requires the substrate to be maintained at around 750 °C , 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.
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  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 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 , 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 . 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 , using the Helmholtz Reciprocity Principle , it is equivalent to the directional-hemispherical reflectance measurement of the test sample . 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 pre-determined 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 . 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 . 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.
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.
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.
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 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.
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.
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.
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.
Figure 5 shows an SEM of a VANTA coating which was grown on an aluminium substrate before to it was subjected to the space qualification tests. The VANTA coatings exhibited a well aligned structure with an area density of 6x1010 cm−2. Figure 6 shows an SEM of a VANTA coating grown on an aluminium substrate after it was subjected to the space qualification tests. Visually, no detectable changes in the coating structure of its density could be observed.
Figures 7, 9, 11, 13 and 15 show the directional-hemispherical reflectance of the test samples 1, 2, 4, 5 and 6, measured before and after the space qualification tests, respectively. Figure 8, 10, 12, 14 and 16 show the absolute change in the directional-hemispherical reflectance of the tests samples 1, 2, 4, 5 and 6, measured before and after the space qualification tests, respectively (see Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, and Fig. 16 below).
Figure 17 shows the directional-hemispherical reflectance of the NanoTube Black sample 3, measured before and after the space qualification tests, to which it was not subjected. Figure 18 shows its absolute change in the directional-hemispherical reflectance.
Figure 19 shows, for comparison, the directional-hemispherical reflectance of the Martin Enhanced Black sample measured before and after the space qualification tests (to which it was not subjected). Figure 20 shows the absolute change in its directional-hemispherical reflectance.
The uncertainty in the measurement of hemispherical reflectance of the test samples was 0.0045 (k = 2) . The reflectances of the samples before and after the space qualification tests are in good agreement. Only the change in the reflectance of sample 2 over a small wavelength range (18 µm to 23 µm) exceeds the sum of the uncertainties of the two measurements (before and after the space qualification tests). We can conclude that no changes in the hemispherical reflectance of these samples were observed as a result of the space qualification tests. Measurements on the control samples (the samples not subjected to the space qualification tests) confirm that there was no significant change in the reflectance data as a result of the space qualification tests.
Figure 21 shows the hemispherical reflectance of all six samples measured before they were subjected to the space qualification tests. Figure 21 shows that there is a significant variability in the hemispherical reflectance of the six samples. This is as expected as a range of thicknesses (ranging from 22 µm to 44 µm) and densities, were grown to better understand the relationship between physical VANTA characteristics (length and density) and directional-hemispherical reflectance. The change in the hemispherical reflectance of the five samples which were subjected to the space qualification tests remained within the measurement uncertainty of the NPL directional-hemispherical measurement facility.
Table 4 summarises some of the conditions which a black coating has to satisfy before it is adopted as a blackbody cavity coating for EO applications, together with comments indicating how well NanoTube Black coatings satisfy each condition.
The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube (VANTA) coating grown on aluminium substrates, is reported for the first time. The growth of this coating on aluminium was achieved using a modified process that employs top down thermal radiation to assist the CNT growth, enabling deposition at temperatures below the melting point of aluminium. The coatings were shown to exhibit directional-hemispherical reflectance of less than 1% across the 2.5 µm to 15 µm wavelength range, and further to have retained this performance following space qualification testing. VANTA coatings are chemically inert, have a high resistance to vibration and shock, and exhibit excellent environmental, thermal and outgassing stability. VANTA coatings are, therefore, good candidates for Earth Observation applications and in particular as coatings for blackbody cavities and baffles.
The authors wish to thank TSB for the financial support to complete this project. Support from the National Measurement Office of the UK Department of Business, Innovation and Skills is also gratefully acknowledged. This work is subject to British Crown owned copyright (2014).
References and links
2. M. J. Persky, “Review of black surfaces for space-borne infrared systems,” Rev. Sci. Instrum. 70(5), 2193–2217 (1999). [CrossRef]
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). [CrossRef]
6. R. J. C. Brown, P. J. Brewer, and M. J. T. Milton, “The physical and chemical properties of electroless nickel-phosphorous alloys and low reflectance nickel-phosphorous black surfaces,” J. Mater. Chem. 12(9), 2749–2754 (2002). [CrossRef]
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). [CrossRef]
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). [CrossRef] [PubMed]
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). [CrossRef] [PubMed]
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). [CrossRef] [PubMed]
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). [CrossRef] [PubMed]
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). [CrossRef]
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). [CrossRef]
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 SystemsXVI, (2011), doi:. [CrossRef]
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). [CrossRef]
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). [CrossRef] [PubMed]
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). [CrossRef]
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). [CrossRef] [PubMed]
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). [CrossRef]
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). [CrossRef]
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).
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). [CrossRef]