New fluorescent lighting phosphors slash use of rare-earth elements
Phosphors are essential to fluorescent lighting, and thus office parks the world over, but their use of rare-earth elements makes them less than ideal. To address that issue, new types of phosphors have been developed that use substantially less rare-earth elements than current phosphors found in fluorescent bulbs. This could reduce the reliance on the limited supplies of rare-earth elements until fluorescent lighting can be completely replaced by LED lighting, which isn't expected to occur for over a decade.
The inner lining of fluorescent tubes are coated with phosphors, which absorb the ultraviolet light from electrically charged mercury vapor and re-emit visible light based on a mixt of blue, green and red emitters. And while the adoption of LED lighting technology is growing rapidly, there are still about 2.3 billion fluorescent light sockets in the U.S., which will probably continue for the foreseeable future. Fluorescent lights consume over 1,000 tonnes (1,102 tons) of rare-earth oxides each year. Yttrium (Y) oxide is used most, along with europium (Eu), terbium (Tb), cerium (Ce) and lanthanum (La).
These rare-earth elements are expensive and difficult to come by, while their mining can have serious environmental consequences if not managed properly. Some rare-earth elements are relatively abundant, but lack of concentration in ore deposits make them hard to mine. Currently, more than 95 percent of the world’s rare-earth supply is produced in China. Mine slurry tailings can also be mildly radioactive, while toxic acids are used in the refining process
A team of researchers from General Electric, Lawrence Livermore National Laboratory and Oak Ridge National Laboratory working with the Critical Materials Institute (CMI) at Ames Laboratory has now created alternate phosphors that drastically reduce or eliminate two of these five aforementioned rare-earth elements found in fluorescents. They discovered a green phosphor that eliminates lanthanum completely and reduces the terbium content by 90 percent, while a newly-identified red phosphor is rare-earth element-free by eliminating both europium and yttrium. The blue phosphor in current use is already low in rare-earth elements.
Except for the lack of rare-earth elements, the new phosphors (five samples on right in above image) are very similar to the ones they’re replacing (six samples from bottom left of semi-circle), according to Nerine Cherepy, lead researcher on the project for Lawrence Livermore Labs.
"The new phosphors include manganese-doped nitrides with zero percent rare earth in its composition," said Cherepy, "and terbium-doped phosphates with reduction critical rare earth content to about 20 percent of what it is in the standard phosphor."
Another issue is cost. The market price for rare earths has fluctuated by more than two-times over the last few years. During the most recent peak in rare earth prices, the cost of fluorescent bulbs roughly doubled.
The new phosphors are on their way to meeting strict requirements, including high efficiency, lamp survivability, precise color rendition and low-cost. Meanwhile, the researchers are assessing the next steps in the phosphors' feasibility for commercial lighting by looking at chemical issues such as slurry compatibility while improving the synthetic process.