Tag Archives: More of the ‘blackest black’

Super-black wood from the University of British Columbia (UBC)

The researchers have developed prototype watches and jewelry using the new super-black wood. Photo credit: UBC Forestry/Ally Penders

Generally stories about very black materials will mention carbon nanotubes but not this time. A July 30, 2024 University of British Columbia (UBC) news release (also on EurekAlert and received via email) announces the discovery of a technique for making super-black wood,

Thanks to an accidental discovery, researchers at the University of British Columbia have created a new super-black material that absorbs almost all light, opening potential applications in fine jewelry, solar cells and precision optical devices. 

Professor Philip Evans and PhD student Kenny Cheng were experimenting with high-energy plasma to make wood more water-repellent. However, when they applied the technique to the cut ends of wood cells, the surfaces turned extremely black. 

Measurements by Texas A&M University’s department of physics and astronomy confirmed that the material reflected less than one per cent of visible light, absorbing almost all the light that struck it. 

Instead of discarding this accidental finding, the team decided to shift their focus to designing super-black materials, contributing a new approach to the search for the darkest materials on Earth.

“Ultra-black or super-black material can absorb more than 99 per cent of the light that strikes it – significantly more so than normal black paint, which absorbs about 97.5 per cent of light,” explained Dr. Evans, a professor in the faculty of forestry and BC Leadership Chair in Advanced Forest Products Manufacturing Technology.

Super-black materials are increasingly sought after in astronomy, where ultra-black coatings on devices help reduce stray light and improve image clarity. Super-black coatings can enhance the efficiency of solar cells. They are also used in making art pieces and luxury consumer items like watches.

The researchers have developed prototype commercial products using their super-black wood, initially focusing on watches and jewelry, with plans to explore other commercial applications in the future.

Wonder wood

The team named and trademarked their discovery Nxylon (niks-uh-lon), after Nyx, the Greek goddess of the night, and xylon, the Greek word for wood. 

Most surprisingly, Nxylon remains black even when coated with an alloy, such as the gold coating applied to the wood to make it electrically conductive enough to be viewed and studied using an electron microscope. This is because Nxylon’s structure inherently prevents light from escaping rather than depending on black pigments.

The UBC team have demonstrated that Nxylon can replace expensive and rare black woods like ebony and rosewood for watch faces, and it can be used in jewelry to replace the black gemstone onyx.

“Nxylon’s composition combines the benefits of natural materials with unique structural features, making it lightweight, stiff and easy to cut into intricate shapes,” said Dr. Evans.

Made from basswood, a tree widely found in North America and valued for hand carving, boxes, shutters and musical instruments, Nxylon can also use other types of wood such as European lime wood.

Breathing new life into forestry

Dr. Evans and his colleagues plan to launch a startup, Nxylon Corporation of Canada, to scale up applications of Nxylon in collaboration with jewellers, artists and tech product designers. They also plan to develop a commercial-scale plasma reactor to produce larger super-black wood samples suitable for non-reflective ceiling and wall tiles. 

“Nxylon can be made from sustainable and renewable materials widely found in North America and Europe, leading to new applications for wood. The wood industry in B.C. is often seen as a sunset industry focused on commodity products—our research demonstrates its great untapped potential,” said Dr. Evans.

Other researchers who contributed to this work include Vickie Ma, Dengcheng Feng and Sara Xu (all from UBC’s faculty of forestry); Luke Schmidt (Texas A&M); and Mick Turner (The Australian National University).

Here’s a link to and a citation for the paper (and hat’s off to the writers for an accessible introduction),

Super-Black Material Created by Plasma Etching Wood by Kenneth J. Cheng, Dengcheng Feng, Luke M. Schmidt, Michael Turner, Philip D. Evans. Advanced Sustainable Systems DOI: https://doi.org/10.1002/adsu.202400184 First published: 16 June 2024

This paper is open access.

I can’t resist; this is such a good introduction, keeping in mind it’s written for an academic journal, from Super-Black Material Created by Plasma Etching Wood.

Super-black materials have very low reflectivity due to structural absorption of light.[1] They are attracting considerable scientific and industrial attention because of their important applications in many fields: astronomy,[2, 3] photovoltaics,[4, 5] and optical science,[6] among others. In these applications, super-black materials minimize unwanted reflection of light enabling devices to operate more accurately or efficiently.[6] In other fields, for example art and design, the attraction of super-black materials lies in their ability to create bizarre visual effects because of huge contrast between black and adjacent colored objects or surfaces.[7] This artistic application of super-black materials is analogous to the juxtaposition of super-black and brightly colored courtship display patches in birds and peacock spiders.[8, 9] In birds, super-black patches have been defined as those having less than 2% directional reflectance at normal incidence.[8] Reflectance values of super-black patches in 32 bird species ranged from 0.045 to 1.97% with an average of 0.94% (300–700 nm).[8] Other studies have associated super-blackness with reflectance values of 1%[10] or 0.5%.[3] Far lower reflectance values have been achieved with materials containing aligned carbon nanotubes (CNT), for example a low-density CNT array (0.045%),[11] the coating Vantablack (0.035%)[7] and a CNT-metal foil (0.005%).[12] The current holder of the “record” for a low reflectivity material (<0.0002%) is an ion-track micro-textured polymer with anti-backscatter matrix.[13]

The low reflectivity of materials such as Vantablack is due to the high absorption of light by graphene and the ability of vertical arrays of CNT to lower surface reflection.[6, 7] In the case of a low-density CNT array, its low reflectivity was ascribed to its random surface profile and presence of a loose network of entangled nanotubes, in addition to vertically oriented nanotubes.[11] Other structures can also be used to reduce reflectivity of synthetic materials including nanopores, and microcavities.[6] Even more diverse structures are found in natural super-black materials, including complex barbule microstructures in birds,[1] cuticular micro-lens arrays in peacock spiders,[9] and polydisperse honeycomb configurations in the wings of butterflies.[14] The structural features of butterfly wings have been used as biomimetic models to create super-black polymer films.[4, 10] This biomimetic route to creating super-black materials has the advantages that “the films are thinner than known alternatives and can be fabricated at lower temperatures via plasma-enhanced chemical vapor deposition, instead of being grown from CNT.”[4, 14]

Biomimicry of nature’s structural material par excellence, wood, is being used to create lightweight stiff and tough composites,[15, 16] but wood is not a model for the creation of super-black materials because even the darkest woods such as ebony (Diospyros spp.) or African blackwood (Dalbergia melanoxylon Guill. & Perr.) lack structural features that reduce reflectivity. Nevertheless, there is interest in using wood in applications where blackness is advantageous such as solar steam generation and desalination of water,[17-20] because wood is widely available, inexpensive, sustainable and can be fabricated into panels and objects. In these applications, wood is carbonized and retains its porous microstructure creating a black material with reflectivity of 3%.[18] The creation of additional porosity by micro-drilling the wood prior to carbonization further reduced reflectivity to 2%.[18] We serendipitously created a super-black wood during undirected investigations into the use of plasma etching to “machine” novel microstructures at basswood (Tilia americana L.) surfaces. We called this material Nxylon, a neologism created from Nyx (Greek goddess of the night) and xylon (Greek for wood materials). One of us published the reflectivity data for Nxylon in 2020.[21] Here we report on the structural features responsible for the super-blackness of Nxylon, describe how it is made and discuss its possible practical uses. During the preparation of this manuscript, we became aware of a novel approach to creating super-black wood involving high temperature carbonization of delignified balsa wood (Ochroma pyramidale (Cav. ex Lam.) Urb.).[22] This material is produced using “mature processing technologies” and can be used to create solid wood products with complex geometries. The surface plasma process we describe is liquid free, generates little waste and is more suited for the creation of super-black veneer which can be used on a small scale to manufacture luxury consumer products. Therein lies the novelty and significance of our work.

The most comprehensive piece I’ve published on the topic of the ‘really, really black’ is in a December 4, 2019 posting, “More of the ‘blackest black’.” At that point, some new work on creating the blackest black (up to 99.99% and 99.995% light absorption, respectively) had come from the US National Institute of Standards and Technology (NIST) and the Massachusetts Institute of Technology (MIT). I also included the latest about an artistic feud over Vantablack (mentioned in the paper’s introduction) and its 99.8% light absorption and provided a link back to my earliest stories on Vantablack.

The coolest paint

It’s the ‘est’ of it all. The coolest, the whitest, the blackest … Scientists and artists are both pursuing the ‘est’. (More about the pursuit later in this posting.)

In this case, scientists have developed the coolest, whitest paint yet. From an April 16, 2021 news item on Nanowerk,

In an effort to curb global warming, Purdue University engineers have created the whitest paint yet. Coating buildings with this paint may one day cool them off enough to reduce the need for air conditioning, the researchers say.

In October [2020], the team created an ultra-white paint that pushed limits on how white paint can be. Now they’ve outdone that. The newer paint not only is whiter but also can keep surfaces cooler than the formulation that the researchers had previously demonstrated.

“If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts. That’s more powerful than the central air conditioners used by most houses,” said Xiulin Ruan, a Purdue professor of mechanical engineering.

Caption: Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab’s sample of the whitest paint on record. Credit: Purdue University/Jared Pike

This is nicely done. Researcher Xiulin Ruan is standing close to a structure that could be said to resemble the sun while in shirtsleeves and sunglasses and holding up a sample of his whitest paint in April (not usually a warm month in Indiana).

An April 15, 2021 Purdue University news release (also on EurkeAlert), which originated the news item, provides more detail about the work and hints about its commercial applications both civilian and military,

The researchers believe that this white may be the closest equivalent of the blackest black, “Vantablack,” [emphasis mine; see comments later in this post] which absorbs up to 99.9% of visible light. The new whitest paint formulation reflects up to 98.1% of sunlight – compared with the 95.5% of sunlight reflected by the researchers’ previous ultra-white paint – and sends infrared heat away from a surface at the same time.

Typical commercial white paint gets warmer rather than cooler. Paints on the market that are designed to reject heat reflect only 80%-90% of sunlight and can’t make surfaces cooler than their surroundings.

The team’s research paper showing how the paint works publishes Thursday (April 15 [2021]) as the cover of the journal ACS Applied Materials & Interfaces.

What makes the whitest paint so white

Two features give the paint its extreme whiteness. One is the paint’s very high concentration of a chemical compound called barium sulfate [emphasis mine] which is also used to make photo paper and cosmetics white.

“We looked at various commercial products, basically anything that’s white,” said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology who worked on this project as a Purdue Ph.D. student in Ruan’s lab. “We found that using barium sulfate, you can theoretically make things really, really reflective, which means that they’re really, really white.”

The second feature is that the barium sulfate particles are all different sizes in the paint. How much each particle scatters light depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun.

“A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering, which contributes to the highest reflectance,” said Joseph Peoples, a Purdue Ph.D. student in mechanical engineering.

There is a little bit of room to make the paint whiter, but not much without compromising the paint.”Although a higher particle concentration is better for making something white, you can’t increase the concentration too much. The higher the concentration, the easier it is for the paint to break or peel off,” Li said.

How the whitest paint is also the coolest

The paint’s whiteness also means that the paint is the coolest on record. Using high-accuracy temperature reading equipment called thermocouples, the researchers demonstrated outdoors that the paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient surroundings at night. It can also cool surfaces 8 degrees Fahrenheit below their surroundings under strong sunlight during noon hours.

The paint’s solar reflectance is so effective, it even worked in the middle of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the paint still managed to lower the sample temperature by 18 degrees Fahrenheit.

This white paint is the result of six years of research building on attempts going back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners.

Ruan’s lab had considered over 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material. Their previous whitest paint was a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells.

The researchers showed in their study that like commercial paint, their barium sulfate-based paint can potentially handle outdoor conditions. The technique that the researchers used to create the paint also is compatible with the commercial paint fabrication process.

Patent applications for this paint formulation have been filed through the Purdue Research Foundation Office of Technology Commercialization. This research was supported by the Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research [emphasis mine] through the Defense University Research Instrumentation Program (Grant No.427 FA9550-17-1-0368). The research was performed at Purdue’s FLEX Lab and Ray W. Herrick Laboratories and the Birck Nanotechnology Center of Purdue’s Discovery Park.

Here’s a link to and a citation for the paper,

Ultrawhite BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling by Xiangyu Li, Joseph Peoples, Peiyan Yao, and Xiulin Ruan. ACS Appl. Mater. Interfaces 2021, XXXX, XXX, XXX-XXX DOI: https://doi.org/10.1021/acsami.1c02368 Publication Date:April 15, 2021 © 2021 American Chemical Society

This paper is behind a paywall.

Vantablack and the ongoing ‘est’ of blackest

Vantablack’s 99.9% light absorption no longer qualifies it for the ‘blackest black’. A newer standard for the ‘blackest black’ was set by the US National Institute of Standards and Technology at 99.99% light absorption with its N.I.S.T. ultra-black in 2019, although that too seems to have been bested.

I have three postings covering the Vantablack and blackest black story,

The third posting (December 2019) provides a brief summary of the story along with what was the latest from the US National Institute of Standards and Technology. There’s also a little bit about the ‘The Redemption of Vanity’ an art piece demonstrating the blackest black material from the Massachusetts Institute of Technology, which they state has 99.995% (at least) absorption of light.

From a science perspective, the blackest black would be useful for space exploration.

I am surprised there doesn’t seem to have been an artistic rush to work with the whitest white. That impression may be due to the fact that the feuds get more attention than quiet work.

Dark side to the whitest white?

Andrew Parnell, research fellow in physics and astronomy at the University of Sheffield (UK), mentions a downside to obtaining the material needed to produce this cooling white paint in a June 10, 2021 essay on The Conversation (h/t Fast Company), Note: Links have been removed,

… this whiter-than-white paint has a darker side. The energy required to dig up raw barite ore to produce and process the barium sulphite that makes up nearly 60% of the paint means it has a huge carbon footprint. And using the paint widely would mean a dramatic increase in the mining of barium.

Parnell ends his essay with this (Note: Links have been removed),

Barium sulphite-based paint is just one way to improve the reflectivity of buildings. I’ve spent the last few years researching the colour white in the natural world, from white surfaces to white animals. Animal hairs, feathers and butterfly wings provide different examples of how nature regulates temperature within a structure. Mimicking these natural techniques could help to keep our cities cooler with less cost to the environment.

The wings of one intensely white beetle species called Lepidiota stigma appear a strikingly bright white thanks to nanostructures in their scales, which are very good at scattering incoming light. This natural light-scattering property can be used to design even better paints: for example, by using recycled plastic to create white paint containing similar nanostructures with a far lower carbon footprint. When it comes to taking inspiration from nature, the sky’s the limit.