A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 2 1 2 STATIONARY HEAT ENGINE A research team designed a heat engine with no moving parts in a collaboration between engineers at MIT, Cambridge, and the National Renewable Energy Laboratory in Golden, Colo. They demonstrated that the heat engine converts heat to electricity with over 40% efficiency—a performance better than that of traditional steam turbines. The engine is a thermophotovoltaic (TPV) cell that passively captures high-energy photons from a white-hot heat source and converts them into electricity. The team’s design can generate electricity from a heat source of between 1900° to 2400°C, or up to about 4300°F. The researchers plan to incorporate the TPV cell into a grid-scale thermal battery. The system would absorb excess energy from renewable sources such as the sun and store that energy in heavily insulated banks of hot graphite. When the energy is needed, such as on overcast days, TPV cells would convert the heat into electricity, then dispatch the energy to a power grid. With the new TPV cell, the team has now successfully demonstrated the main parts of the system in separate, small-scale experiments. They are now working to integrate the parts to demonstrate a fully operational system. From there, they hope to scale up the system to replace fossil-fuel-driven power plants and enable a fully decarbonized power grid, supplied entirely by renewable energy. mit.edu, nrel.gov. RICE HUSK LEDs Scientists from Japan’s Hiroshima University created the world’s first silicon quantum dot (SiQD) LED light using recycled rice husks. Searching for a scalable method to fabricate quantum dots, the researchers looked to agricultural waste. Milling rice to separate the grain from the husks produces about 100 million tons of rice husk waste globally each year. The new environmentally friendly, low-cost method transforms this waste into state-of-the-art light-emitting diodes. Nontoxic and abundant in nature, Si has photoluminescent properties, stemming from its microscopic quantum dot structures that serve as semiconductors. Waste rice husks, it turns out, are an excellent source of high-purity silica (SiO2) and value-added Si powder. The team used a combination of milling, heat treatments, and chemical etching to process EMERGING TECHNOLOGY Researchers from the University of Wuppertal and the University of Cologne along with four other German universities and institutes developed a tandem solar cell that reaches 24% efficiency. This sets a new world record as the highest efficiency achieved so far with this particular combination of organic and perovskite-based absorbers. www.uni-wuppertal.de. BRIEF the rice husk silica. First, they milled rice husks and extracted SiO2 powders by burning off organic compounds of the husks. Next, they heated the resulting silica powder in an electric furnace to obtain Si powders via a reduction reaction. Third, the purified Si powder product was further reduced to three nanometers in size by chemical etching. Finally, its surface was chemically functionalized for high chemical sta- bility and high dispersivity in a sol- vent, producing SiQDs that luminesce in the orange-red range with efficiency of over 20%. The scientists suggested that the method they developed could be applied to other plants, such as sugar cane bamboo, wheat, barley, or grasses that contain SiO2. These natural products and their wastes might hold the potential to be transformed into nontoxic optoelectronic devices. The scientists would like to see commercialization of their ecofriendly approach to creating luminescent devices from rice husk waste. www.hiroshima-u.ac.jp/en. Graphical depiction of the world’s first LED light created from rice husks and chemically obtained products. Courtesy of ACS. A TPV cell (size 1 x 1 cm) mounted on a heat sink is designed to measure the cell’s efficiency. Courtesy of Felice Frankel.
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