The growing demand for energy-efficient windows, optoelectronic components, and solar cells has driven the search for high-performance and sustainable optical materials. A unique synthetic material—transparent wood—has emerged as a promising candidate from that search. For one thing, wood is an excellent thermal insulator compared with other sustainable materials. For another, rendering it optically transparent is straightforward. Two steps are usually required for the job: removing the light-absorbing polymer lignin from the cell walls by immersing the wood in a chemical solution and then infiltrating a refractive-index-matching polymer into the delignified wood matrix to minimize light absorption and scattering. Unfortunately, the immersive chemicals impair the wood’s mechanical strength and are difficult to recycle.
A group of University of Maryland materials engineers led by Liangbing Hu has now developed a fabrication technique that avoids excessive chemical consumption and retains some of the natural lignin to enhance the wood’s structural integrity. Instead of immersing a light wood such as balsa, oak, or poplar in solution, Hu’s team lightly brushes its surface with a 15 ml coat of hydrogen peroxide and exposes it to UV light for as little as one hour to remove the light-absorbing chromophores of lignin. The team then infiltrates the wood’s network of pores with an epoxy polymer that renders the material highly transparent, with an absorptivity of less than 0.4% and a transmittance up to 90% over the visible wavelength range of 400–800 nm. The image here illustrates a 400 mm × 110 mm × 1 mm piece of processed balsa wood placed atop grass and a cut flower.
Compared with wood whose lignin has been removed entirely by the conventional method, the lignin-modified wood exhibits 50 times the tensile strength (20.6 MPa versus 0.4 MPa). That’s because the preserved lignin provides a robust scaffold to accommodate the subsequent epoxy polymer into the wood’s pervasive network of pores. In effect, the Hu group has made the fabrication process fast, scalable, inexpensive, and environmentally friendly. And it has done so in a way that preserves wood’s naturally tough mechanical properties and is amenable to selective patterning of the surface. One need brush with H2O2 only those parts on which one wants to imprint a particular bleaching pattern. (Q. Xia et al., Sci. Adv. 7, eabd7342, 2021.)
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February 02, 2021 at 03:22AM
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A fast, scalable approach to making wood transparent - Physics Today
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