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Super black
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Super black is a surface treatment developed at the National Physical Laboratory (NPL) in the United Kingdom. It absorbs approximately 99.6% of visible light at normal incidence, while conventional black paint absorbs about 97.5%. At other angles of incidence, super black is even more effective: at an angle of 45°, it absorbs 99.9% of light.
Technology
[edit]The technology to create super black involves chemically etching a nickel-phosphorus alloy.[1][2]
Applications of super black are in specialist optical instruments for reducing unwanted reflections. The disadvantage of this material is its low optical thickness, as it is a surface treatment. As a result, infrared light of a wavelength longer than a few micrometers penetrates through the dark layer and has much higher reflectivity. The reported spectral dependence increases from about 1% at 3 μm to 50% at 20 μm.[3]
In 2009, a competitor to the super black material, Vantablack, was developed based on carbon nanotubes. It has a relatively flat reflectance in a wide spectral range.[4]
In 2011, NASA and the US Army began funding research in the use of nanotube-based super black coatings in sensitive optics.[5] Nanotube-based superblack arrays and coatings have recently become commercially available.[6]
See also
[edit]References
[edit]- ^ "Mini craters key to 'blackest ever black'". Newscientist.com. 6 February 2003. Retrieved 2015-07-14.
- ^ "Highly Absorbing Surfaces for Radiometry". January 2003. Archived from the original on 2005-06-27.
- ^ Brown, Richard J. C.; Brewer, Paul J.; Milton, Martin J. T. (2002). "The physical and chemical properties of electroless nickel – phosphorus alloys and low reflectance nickel – phosphorus black surfaces". Journal of Materials Chemistry. 12 (9): 2749. doi:10.1039/b204483h.
- ^ "NASA Develops Super-Black Material That Absorbs Light Across Multiple Wavelength Bands". Nasa.gov. 2011-11-08. Retrieved 2015-07-14.
- ^ Nanostructured Super-Black Optical Materials 10/23/2013 Archived March 2, 2014, at the Wayback Machine
- ^ "Aligned Carbon Nanotube Arrays and Forests on Substrates". Nano-lab.com. Retrieved 2015-07-14.
External links
[edit]- Beckhusen, Robert (December 24, 2012). "Army Goes Goth with "'Super-Black' Materials". Wired.
- Quick, Darren (Nov 9, 2011). "NASA's new super-black nanotube-based material is good news for star-gazers". New Atlas.
- "Super Black Coatings on a Mission". Paint Square. August 19, 2014.
Part of NASA's Materials Coating Experiment
- "Magic Black, Vacuum Black". Advanced coatings. Acktar.
Inorganic, thin coating, deposited using vacuum deposition technology
Super black
View on Grokipediafrom Grokipedia
History
Development at NPL
The development of super black materials originated at the UK's National Physical Laboratory (NPL) in the late 1990s and early 2000s, motivated by the need for ultra-low-reflectance surfaces to minimize stray light in precision optical instruments. The idea of chemically etching nickel–phosphorus alloys to create black surfaces dates back to the 1980s, building on earlier electroless nickel–phosphorus (Ni-P) plating techniques used in the United States and Japan.[5] Researchers at NPL sought to create coatings that could suppress unwanted reflections more effectively than traditional black paints or anodized surfaces, which typically reflect 4-25% of incident light, thereby improving the accuracy of measurements in spectroscopy and radiometry.[5] This work adapted Ni-P plating for optical applications where even minimal reflectance could compromise data quality.[6] Initial experiments at NPL focused on chemical etching of Ni-P alloys to produce microstructured surfaces that trap light through multiple internal reflections and absorption within nanoscale pits and craters. The process began with electroless deposition of a Ni-P layer, typically containing 5-7% phosphorus, onto a substrate, followed by immersion in nitric acid to etch the surface and form a rough, black morphology.[5] These microstructures, resembling conical pits on a micrometer scale, enhanced light trapping compared to smooth metallic surfaces, with the etching conditions—such as acid concentration and exposure time—optimized to control pit depth and density for broadband absorption. At 45° angle of incidence, super black achieves 99.9% absorption.[5] Key researchers, including Richard J. C. Brown, Paul J. Brewer, and Michael J. T. Milton, conducted detailed analyses using atomic force microscopy (AFM) and cross-sectional imaging to correlate alloy composition with surface morphology and optical performance.[6] By 2002, NPL reported achieving an absorption rate of more than 99.65% for visible light at normal incidence, corresponding to a reflectance of as low as 0.35%.[2] This milestone was detailed in seminal publications from the laboratory, such as the 2002 study by Brown et al. on the physical and chemical properties of etched Ni-P surfaces, which emphasized their efficacy for stray light suppression in optical systems.[6] These early efforts established super black as a foundational technology in optical engineering, paving the way for scalable manufacturing techniques while highlighting the role of precise chemical control in achieving near-perfect light absorption.[2]Key Milestones and Evolution
In 2003, advancements allowed for larger panels up to 12 cm × 12 cm and introduced techniques for commercial manufacturing through NPL's collaborations with industry partners.[2] In 2014, the introduction of Vantablack by Surrey NanoSystems marked a major rival in low-reflectance coatings.[7] Parallel developments in 2011 included NASA's carbon nanotube array material absorbing over 99% of light across ultraviolet, visible, infrared, and far-infrared wavelengths, and US Army explorations of similar coatings for optical and stealth applications.[8][9] No major evolutions in the original super black Ni-P technology have been reported since the early 2000s, though it continues to be used in applications where its robustness and manufacturability are advantageous.Composition and Fabrication
Material Structure
Super black is composed of a thin film of electroless nickel–phosphorus alloy that undergoes chemical etching to create a distinctive surface morphology resembling a forest-like array of micro-cavities interspersed with nano-scale roughness.[1] This structure arises from the selective dissolution of the alloy during etching, resulting in conical pores that enhance light trapping.[1] The surface morphology features an average cavity depth and pore diameters of approximately 1–10 micrometers, achieved with a phosphorus content typically ranging from 5–7% in the alloy for optimal etching rates and blackening efficiency.[5][1] Lower phosphorus levels, around 5–7%, promote the formation of uniform crater-like cavities, while higher contents exceeding 8% slow the etching process and can lead to less consistent surface features such as stalagmite-like protrusions.[5] The structural mechanism for light absorption relies on multiple internal reflections of photons within the micro-cavities, where incident light enters the pores but is repeatedly scattered and absorbed by the rough internal walls, preventing significant escape.[1] This design minimizes specular and diffuse reflectance across a broad spectrum. Variations in alloy ratios, particularly phosphorus concentration, influence surface uniformity by altering the etching dynamics.[5]Manufacturing Techniques
The manufacturing of super black coatings begins with electroless plating of a nickel–phosphorus alloy onto substrates such as aluminum or glass to form a uniform base layer. This chemical deposition process occurs in an aqueous bath containing nickel salts and hypophosphite reducing agents, typically at temperatures around 50–95°C and pH 4–6, allowing the alloy to adhere without an external current source.[1] Following plating, selective chemical etching with nitric acid creates the characteristic microstructured surface by preferentially dissolving phosphorus-rich regions, forming conical cavities that enhance light absorption. The process parameters include an etching time of a few seconds at approximately 50°C in nitric acid, yielding a final coating thickness of around 1–5 micrometers. This brief etching duration ensures deep cavity formation while controlling the alloy's dissolution rate to maintain structural integrity.[5][1] Scalability presents challenges due to the need for uniform plating and etching on larger areas, as non-uniform acid exposure can lead to inconsistent microstructures. Solutions include batch processing in agitated baths, enabling coverage of surfaces up to 12 cm × 12 cm, as demonstrated in commercial adaptations of the NPL process for aerospace components as of 2003.[2] Quality control is essential to verify the coating's efficacy, involving scanning electron microscopy (SEM) imaging to confirm cavity formation and depth, typically revealing pores 1–10 micrometers in size with high aspect ratios. Additional checks include reflectance measurements and thickness profiling using X-ray fluorescence to ensure the etched layer meets specifications for low reflectivity.[1]Properties
Optical Absorption
Super black demonstrates remarkable optical absorption properties, particularly in the visible spectrum, where it achieves approximately 99.6% absorption at normal incidence over the 400–700 nm wavelength range. This corresponds to a reflectance of about 0.4%, significantly outperforming conventional black paints, which typically absorb only around 97.5%.[2] The material's etched nickel-phosphorus alloy structure enhances light trapping through multiple internal reflections, minimizing specular and diffuse scattering. Absorption efficiency improves at oblique angles of incidence due to increased path lengths for light within the surface microstructure.[2] Angular dependence can be approximated using the cosine law for the diffuse reflectance component, where the reflectance scales roughly as , with being the angle from normal; however, super black's design suppresses this variation, maintaining low reflectance across a wide angular field. Surface cleanliness is a critical factor, as contaminants can introduce additional scattering and elevate measured reflectance by up to several percent. The material's performance extends across a broad spectral range, from the ultraviolet (starting at approximately 300 nm) to the near-infrared (up to 3 μm), with total hemispherical reflectance remaining below 0.5% in the visible spectrum and under 1% through the near-IR up to 2.5 μm. These properties are quantified using integrating spheres coupled with spectrophotometers, such as the Perkin-Elmer Lambda series, to measure total (diffuse plus specular) hemispherical reflectance, while bidirectional reflectance distribution function (BRDF) is assessed via goniophotometric setups to characterize angular scattering patterns. Such techniques ensure traceability to primary standards, providing precise evaluation of the coating's stray light suppression capabilities.[10]Physical and Thermal Characteristics
Super black coatings demonstrate robust mechanical properties suitable for various substrates. Adhesion strength typically exceeds 300 MPa on aluminum and 400 MPa on steel, ensuring reliable bonding under standard conditions.[11] The coating provides moderate protection against surface wear while maintaining structural integrity. In terms of thermal characteristics, super black demonstrates stability after thermal ageing at ~214°C for 280 hours, with only minor changes in reflectance, and its performance is unaffected at cryogenic temperatures.[12][2] Additionally, it achieves a high thermal emittance of approximately 0.95 in the mid-infrared spectrum (measured calorimetrically at temperatures up to 400°C), facilitating efficient heat dissipation for thermal management applications.[13] Environmental stability is another key attribute, with resistance to high humidity levels up to 95% relative humidity without substantial performance decline.[14] However, the coating shows sensitivity to abrasion, which can compromise its surface over time in high-contact scenarios. In controlled indoor settings, lifetime estimates exceed 10 years, based on accelerated ageing tests demonstrating minimal changes in properties.[15] The black etched layer is typically less than 1 μm thick on a base electroless nickel-phosphorus coating of around 25–50 μm, adding negligible mass relative to the substrate and preserving weight characteristics.[16][13]Applications
Scientific and Optical Instruments
Super black has been applied in precision optical and scientific instrumentation to suppress stray light and enhance signal detection. In radiometry, it is used in cryogenic radiometers for accurate measurement of optical radiant power, where the coating's low reflectance minimizes internal reflections and improves calibration precision.[17] For spectroscopy and telescope baffles, super black reduces unwanted reflections, improving measurement accuracy in the visible spectrum across ultraviolet to near-infrared wavelengths.[2] Its properties enable applications in satellite optics and aerospace components, benefiting from durability and low outgassing in vacuum environments. The coating has been produced for use in electro-optical systems, though specific satellite integrations remain limited to prototypes as of 2003.[2] In laser systems and microscopy, super black absorbs excess light to maintain alignment and enhance contrast, leveraging its high visible absorption (>99.65%) for better instrument performance.[3]Industrial and Emerging Uses
As of 2003, super black's primary applications remain in scientific instrumentation, with potential for broader industrial uses in optics manufacturing due to its compatibility with various substrates and thermal stability. No widespread adoption in automotive, solar, or artistic fields has been documented for this specific coating.[2]Comparisons
With Vantablack
Vantablack, developed by Surrey NanoSystems in 2009, consists of a forest of vertically aligned carbon nanotubes approximately 14 μm tall, grown via chemical vapor deposition, and absorbs up to 99.965% of visible light.[18][19] In contrast, super black, originating from research at the UK's National Physical Laboratory (NPL), employs chemical etching of a nickel-phosphorus alloy to create microscale surface structures that trap light, achieving around 99.6% absorption in the visible spectrum.[2][5] Key structural differences highlight their distinct fabrication approaches: super black relies on electrochemical etching to form pitted surfaces on the alloy substrate, enabling scalable production without specialized vacuum equipment, whereas Vantablack's nanotubes demand precise low-temperature chemical vapor deposition in controlled environments. Performance varies by wavelength; while both excel in visible light absorption, super black exhibits higher reflectance in the near-infrared, approximately 1% at 3 μm, compared to Vantablack's reflectance below 0.1% in the same range up to 4 μm.[15][20] Super black's etching method also results in lower production costs compared to Vantablack's complex nanotube synthesis.[2] Trade-offs in durability further differentiate the materials: Vantablack's delicate nanotube array is highly fragile and can be damaged by direct touch or abrasion, limiting its handling, while super black's etched alloy surface offers greater robustness for practical integration. The commercialization of Vantablack in 2009 spurred refinements to super black at NPL, enhancing its manufacturability and performance to compete in optical applications.[21][2]With Other Advanced Coatings
Super black, developed at the National Physical Laboratory (NPL), differs from other advanced coatings in its manufacturing process and performance trade-offs, particularly in durability and applicability. Unlike nanotube-based materials, super black relies on an electroless nickel-phosphorus (Ni-P) alloy that is chemically etched to form micro-craters, achieving a reflectance as low as 0.35% in the visible spectrum (absorbing approximately 99.65% of light). This chemical plating method allows for application on various metallic substrates without requiring vacuum environments, making it more scalable for industrial use compared to deposition techniques.[2] In comparison to Vantablack, a carbon nanotube array coating produced via chemical vapor deposition (CVD) by Surrey NanoSystems, super black offers greater mechanical robustness. Vantablack achieves superior absorption of up to 99.965% across ultraviolet, visible, and infrared wavelengths due to its vertically aligned nanotube structure that traps light through multiple internal reflections. However, Vantablack is highly fragile, susceptible to damage from handling or environmental exposure, and limited to small-scale or specialized applications like precision optics. Super black, by contrast, withstands thermal and solar ageing with only minor increases in reflectance (e.g., less than 1% change in the visible range after exposure), as demonstrated in comparative studies of black coatings.[15] Acktar's vacuum-sputtered coatings, such as UltraBlack and Magic Black, provide another benchmark, emphasizing versatility across substrates including plastics and ceramics. These inorganic films, composed of metal-dielectric layers, absorb >97% of light from ~250 nm to 2000 nm, with low outgassing suitable for space environments.[22] While Acktar coatings excel in extreme-angle absorption (maintaining <1% reflectance at 80° incidence) and thermal stability up to 500°C, they require specialized vacuum equipment for deposition, similar to Vantablack but with better adhesion and durability than nanotube arrays. Super black, applied via simpler electroless plating, shows comparable visible absorption (around 99.6%) but higher reflectance in the mid-infrared (up to 38% diffuse at 14.3 μm), limiting its use in broad-spectrum IR applications relative to Acktar's offerings.[23][24]| Coating | Key Composition | Visible Absorption | Durability/Handling | Manufacturing Method | Primary Advantages |
|---|---|---|---|---|---|
| Super Black (NPL) | Ni-P alloy with micro-etching | ~99.6% | High; resistant to ageing | Electroless plating & etching | Scalable, cost-effective for metals |
| Vantablack | Carbon nanotubes | ~99.965% (UV-Vis-IR) | Low; fragile | CVD in vacuum | Highest absorption, omnidirectional |
| Acktar UltraBlack | Metal-dielectric thin films | >97% (~250-2000 nm) | High; space-qualified | Vacuum sputtering | Versatile substrates, extreme angles |