Recycling critical raw materials from waste electronic equipment
Recycling critical raw materials from waste electronic equipment
Recycling critical raw materials from waste electronic equipment
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<strong>Recycling</strong> <strong>critical</strong> <strong>raw</strong> <strong>materials</strong> <strong>from</strong> <strong>waste</strong> <strong>electronic</strong><br />
<strong>equipment</strong><br />
declines slowly) in contrast to incandescent lamps whose end of life is exactly defined when<br />
the filament breaks. In practice other factors also determine the actual lifespan of LED lights.<br />
It is reduced if the LED semiconductor chip is at too high a temperature, for example if there<br />
is inadequate heat dissipation through the lamp housing. The lifespan is also limited if the<br />
current densities in the LED are too high. The ballast required for current stabilization in LED<br />
lights is also a limiting factor. Depending on its design and quality it can sometimes have a<br />
shorter lifespan than the actual LED. As there is an increasing trend to combine the LED and<br />
ballast in a single module, when the ballast fails the LEDs in a light also have to be disposed<br />
of.<br />
5.1 Rare earths, gallium and indium<br />
Amongst the <strong>materials</strong> used in an average LED (see Figure 23) are also a range of <strong>critical</strong><br />
metals. Rare earths in LEDs are mainly used in the luminescent substance which converts<br />
the high-frequency blue or near UV light <strong>from</strong> the actual LED chip into a continuous spectrum<br />
in the visible wavelength range. The composition of the luminescent material is <strong>critical</strong> for the<br />
performance of a WLED. Manufacturers' specifications on the exact composition of the<br />
luminescent substances they use are not usually published and are therefore almost<br />
impossible to obtain. The data which follow in this section are based on empirical values <strong>from</strong><br />
professional experience<br />
Cold white LEDs usually contain only a yellow luminescent substance whilst a red<br />
luminescent substance is added to warm white LEDs. The basis of the yellow luminescent<br />
substance in white LEDs is mainly Ce 3+ -doped yttrium aluminum garnet (YAG) or gadolinium<br />
aluminum garnet (Y,Gd) AG:Ce 3+ . The luminescent substance added in warm white LEDs<br />
obtains its red color spectrum <strong>from</strong> the activator ion Eu 2+ as a doping. However, depending<br />
on the carrier medium, this can also emit in the yellow-orange spectral range (see Figure 22).
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