UGA researchers invent new material for warm-white LEDs
Light emitting diodes, more commonly called LEDs, are known for
their energy efficiency and durability, but the bluish, cold light of current
white LEDs has precluded their widespread use for indoor lighting. Now, University
of Georgia scientists have fabricated what is thought to be the world's first
LED that emits a warm white light using a single light emitting material, or
phosphor, with a single emitting center for illumination. The material is
described in detail in the current edition of the Nature Publishing Group
journal Light: Science and Applications.
"Right
now, white LEDs are mainly used in flashlights and in automotive lamps, but
they give off a bluish, cool light that people tend to dislike, especially in
indoor lighting," said senior author Zhengwei Pan, an associate professor
in the department of physics in the UGA Franklin College of Arts and Sciences
and in the College of Engineering. "Our material achieves a warm color
temperature while at the same time giving highly accurate color rendition,
which is something no single-phosphor-converted LED has ever been shown to
do."
Two
main variables are used to assess the quality of artificial light, Pan
explained. Correlated color temperature measures the coolness or warmth of a
light, and temperatures of less than 4,000 kelvins are ideal for indoor
lighting. Correlated color temperatures above 5,000 kelvins, on the other hand,
give off the bluish color that white LEDs are known for. The other important
measure, color rendition, is the ability of a light source to replicate natural
light. A value of more than 80 is ideal for indoor lighting, with lower values
resulting in colors that don't seem true to life.
The
material that Pan and his colleagues fabricated meets both thresholds, with a
correlated color temperature of less than 4,000 kelvins and a color rendering
index of 85.
Warm
white light can commonly be achieved with a blue LED chip coated with light
emitting materials, or phosphors, of different emitting colors to create what
are called phosphor-based white LEDs, Pan said. Combining the source materials
in an exact ratio can be difficult and costly, however, and the resulting color
often varies because each of the source materials responds differently to
temperature variations.
"The
use of a single phosphor solves the problem of color stability because the
color quality doesn't change with increasing temperatures," said lead
author Xufan Li, a doctoral student in the College of Engineering.
To
create the new phosphor, Pan and his team combine minute quantities of europium
oxide with aluminum oxide, barium oxide and graphite powders. They then heat
the powdered materials at 1,450 degrees Celsius (2,642 degrees Fahrenheit) in a
tube furnace. The vacuum of the furnace pulls the vaporized materials onto a
substrate, where they are deposited as a yellow luminescent compound. When the
yellow luminescent compound is encapsulated in a bulb and illuminated by a blue
LED chip, the result is a warm white light.
Although
his team's results are promising, Pan emphasized that there are still hurdles
to be overcome before the material is used to light homes, businesses and
schools. The efficiency of the new material is much lower than that of today's
bluish white LEDs. Scaling the production to an industrial scale will be
challenging as well, since even slight variations in temperature and pressure
in the phosphor synthesis process result in materials with different
luminescent colors.
The new
yellow phosphor also has a new lattice structure that has not been reported
before. The researchers currently are working to discern how the ions in the
compound are arranged in hopes that a better understanding of the compound at
an atomic level will allow them to improve its efficiency.
"We
still have more work to do," Pan said, "but the color temperature and
rendition that we have achieved gives us a very good starting point."
Source: University
of Georgia
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