The result is an efficient solar heating metamaterial that can heat up rapidly to 83 degrees C (181 degrees F) in an open environment with minimal heat loss. Proposed applications for the film include thermal energy harvesting and storage, thermoelectricity generation, and seawater desalination.<\/p>\n
Suppressing\u00a0thermal emission loss\u2014also known as blackbody radiation\u2014while simultaneously absorbing solar light is critical for an efficient solar thermal absorber but is extremely challenging to achieve, says Baohua Jia<\/a>, founding director of CTAM. \u201cThat\u2019s because, depending on the absorbed heat and properties of the absorber, the emission temperature differs, which leads to significant differences in its wavelength,\u201d she explains. \u201cBut we\u2019ve developed a three-dimensional structured graphene metamaterial (SGM) that is highly absorbent and selectively filters out blackbody radiation.\u201d<\/p>\n The 3D SGM is composed of a 30-nanometer-thick film of alternating graphene and dielectric layers deposited on a trench-like nanostructure that does double duty as a copper substrate to enhance absorption. More importantly, the substrate is patterned in a matrix arrangement to enable flexible tunability of wavelength-selective absorption.<\/p>\n The graphene film is designed to absorb light between 0.28- to 2.5-micrometer wavelengths. And the copper substrate is structured so that it can act as a selective bandpass filter that suppresses the normal emission of internally generated blackbody energy. This retained heat then serves to further raise the metamaterial\u2019s temperature. Hence, the SGM can rapidly heat up to 83 degrees C. Should a different temperature be required for a particular application, a new trench nanostructure can be fabricated and tuned to match that specific blackbody wavelength.<\/p>\n \u201cIn our previous work<\/a>, we demonstrated a 90 nm graphene heat-absorbing material,\u201d says Baohua. Though it could heat up to 160 degrees C, \u201cthe structure was more complicated, [comprising] four layers: a substrate, a silver layer, a layer of silicon oxide, and a graphene layer. Our new two-layer structure is simpler and doesn\u2019t require vacuum deposition. And the method of fabrication is scalable and low cost.\u201d<\/p>\n The new material also uses\u00a0less graphene by significantly reducing the film thickness to one third, and its thinness aids in transferring the absorbed heat more efficiently to other media such as water. Additionally, the film is hydrophobic, which fosters self-cleaning, while the graphene layer effectively protects the copper layer from corrosion, helping to extend the metamaterial\u2019s lifetime.<\/p>\n \u201cBecause the metal substrate\u2019s structural parameters are the main factors governing overall absorption performance of the SGM, rather than its intrinsic features, different metals can be used according to application needs or cost,\u201d says Keng-Te Lin<\/a>, lead author of a paper<\/a> on the metamaterial recently published in Nature Communications<\/em>, and who is also a research fellow at Swinburne University. Aluminum foil can also be used to replace copper without compromising the performance, he notes.<\/p>\n To test the metamaterial\u2019s design and stability, the researchers fabricated a prototype using standard laser nanofabrication, self-assembly graphene oxide coating, and photo-induced reduction.<\/p>\n \u201cWe used the prototype film to produce clean water and achieved an impressive solar-to-vapor efficiency of 96.2 percent,\u201d says Keng-Te. \u201cThis is very competitive for clean water generation using a renewable energy source.\u201d<\/p>\n He adds that the metamaterial can also be used for energy harvesting and conversion applications, steam generation, wastewater cleaning, seawater desalination, and thermoelectricity generation.<\/p>\n One challenge still remaining is finding a manufacturing method for making the substrate scalable.<\/p>\n![\"\"](\"https:\/\/www.fie.undef.edu.ar\/ceptm\/wp-content\/uploads\/2020\/04\/MzU5Njc1Nw.jpeg\")