Unfortunately, with all that is happening, I seemed to have taken my
eye off of sharing some interesting ‘Hey, what are the rare earths and
where are they used?’ pieces. So here goes…
Some of us will remember that in early 2000, several European countries
abandoned their old currencies and switched to the Euro. As most of
us know, bank notes are prime targets for counterfeiters. But did you
know that the application of luminescence is one of the measures that
guard against this kind of mayhem? Well, I was reminded of the
aplication yesterday afternoon by Professor Andries Meijerink of the
University of Utrecht, at one of the pre-conference seminars at Phosphor
Global 2010 (I’ll be speaking later this morning on the delicacy of the
global REE supply-demand balance)
Professor Meijerink noted that some years ago, researchers at the Department of Chemistry at Utrecht University were curious about what caused the luminescence of the Euro notes and guess what they found? … Yup! Europium.
Europium (Eu), one of the rare earth elements (or lanthanides) and that most of the trivalent REEs are luminescent. This means that ions within these materials can be excited by shining a light of a particular wavelength at them. When the ions relax again, they emit light – of a different wavelength. That is luminescence.
As noted in an article posted on the website smarterscience.com,
“The luminescence of the REEs is unique: relatively long-lasting and
producing sharp bands. It is therefore a widely used tracer and
analytical tool. If you use a very intense excitation source, you can
detect very low concentrations of fluorescent compounds.
Euro notes luminesce in the red, green and blue. The red light is
clearly linked to europium and most likely to a Eu3+ according to the
Utrecht researchers. They found it less easy to identify the source of
the blue and green luminescence. They apparently believe (or at least
they did at the time of the initial investigation) that the likely
source of the green color is SrGa2S4:Eu2+. As for the blue colour, the
researchers suspected that Euro notes printer may integrate
(BaO)x.6Al2O3:Eu3+ in some manner or another.
Apparently the Dutch Central Bank would never comment when asked to
clarify the technology applied to the euro. Understandable – although
I’m not sure I’ll every fully grasp the chemistry and physics that
underlie phosphor technologies … save that rare earths provide the keys
to CFLs, LEDs, etc, etc. and Oh yeah… the Euro.