MATERIALS SCIENCE
50,000 hours
LED lighting produces a lot less waste heat than Edison’s light bulb, but heat dispersal is still an important consideration. Oxfordshire-based Selectronic produces LED lighting with long lifetime of up to 50,000 hours by building electronics around the chips to ensure a smooth current and good disposal of excess heat.
develop ways of producing white light using only blue LEDs, with the help of a luminescent material – phosphor – known as YAG:Ce (Y3
Al5 O12 :Ce), <10%
LED-based white lights made by combining colours – including blue LEDs – are even more energy efficient than CFLs, using less than 10% of the electricity that an incandescent bulb with the same light output would need.
White light can be produced by using only blue LEDs, with the help of the luminescent material – phosphor - YAG:Ce (Y3
Al5 O12 :Ce),
which converts part of the light to a wavelength that creates the overall impression of white light.
which converts part of the light to a wavelength that, together with the remaining blue light, creates the impression of white light. Many other phosphors producing better approximations to the spectral distribution of white light have also since been developed. The developers have often faced
a trade-off between good coverage of the visible spectrum and energy efficiency. Phosphors that worked well in creating a broad component of red light similar to that of natural sunlight had the tendency to emit part of their radiation in the infrared. If efficiency is calculated based on the visible part of the spectrum alone, that leakage into infrared counts as wasted energy, so some LED products that perform better in terms of the colour spectrum have a poorer energy efficiency. In 2014, however, the group of
Wolfgang Schnick, at the Ludwig Maximilian University (LMU) Munich, made significant progress when they discovered that a nitridoaluminate material they were studying had promising qualities as a phosphor. Specifically, in the europium-doped strontium nitride-lithoaluminate Sr[LiAl3
N4]:Eu2+ (SLA), the europium
emitted in a sufficiently narrow band of red to produce good natural colour without major losses in the infrared.1 The company Lumiled has since
commercialised this phosphor and uses it in several product lines, including the so-called chip-on- board (COB) products trade named Luxeon – a new type of LED packaging where multiple diode chips are arranged as closely as possible on a plane giving the impression of a lighting panel rather than separate lights. They are made for a wide range of applications, including high bay, downlights, spotlights and streetlights. Still, researchers hope to
further improve LEDs by shifting the red emission slightly to shorter wavelengths, in order to have both the natural-looking light and the optimal energy saving. Schnick’s group has tried to replace the strontium in SLA with the heavier homologue barium, but the synthesis has not succeeded yet. However, replacing half the
strontium atoms with barium and the other half with calcium led to a new material with a double emission peak at 638nm and 790nm, producing red and infrared light, respectively.2 This is due to the europium dopant behaving differently depending on whether it replaces a barium or a calcium atom in the crystal lattice.
The researchers found that at very low dopant concentrations, they can suppress the infrared emission entirely, leading to the narrowest emission band thus far observed in a europium-doped material. Further avenues remain to be
tested. For instance, Schnick and colleagues mention that replacing the calcium with magnesium may help to confine the europium dopant to the barium sites and thus to achieve the target of pure red emission without any spill-over into the infrared. Meanwhile, the groups of Joanna
McKittrick and Shyue Ping Ong, at the University of California, San Diego, working with Wong Bin Im, at National University in South Korea, have also identified a promising new phosphor, Sr2
LiAlO4 . They used a systematic,
high-throughput computational approach developed in Ong’s lab to explore as yet uncharted chemical space, namely the possible combinations of the elements strontium, lithium, aluminium and oxygen. Although each of these elements occurs in known phosphors, a combination of the four had not been tested before. The calculations yielded a combination of these four elements predicted to make an efficient phosphor for LEDs.3 McKittrick’s team then came up
with a way of producing the new compound in the lab and confirmed
32 06 | 2019
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