Excited rare-earth ions relax radiatively and non-radiatively. The non-radiative relaxation rate and hence the lifetime of the excited level exhibit temperature dependence. These materials can be used to sense temperature below the melting point of the crystal. We have developed calibration standards using rare-earth ion doped crystals. The sensitivity of the technique and the applicable temperature range depends on the ionic energy levels and the type of crystal. We will discuss some of the results obtained in rare-earth ion doped fluoride and oxide crystals.
Lead magnesium niobate-lead titanate,
Pb(Mg, Nb)O3-PbTiO3 is a piezoelectric, ferroelectric crystal at room temperature with large electromechanical coefficient. The crystals were grown by PbO-B2O3 flux method. Typically the crystals were colorless and transparent, but a small fraction of them were Brown/reddish colored and show interesting photoluminescence (PL) properties. The PL studies were conducted under XeCl (308nm) excitation and under Argon Ion (Ar+) laser excitation. The excitation with 308nm gave broad PL centered at 500nm and intense emission at 710nm. The emission at 710nm in colorless crystals is very weak. The excitation with Ar+ laser coinciding with electronic absorption in brown samples gave rich and sharp PL particularly with 514.5 nm excitation. The PL with
514.5nm-Ar+ laserexcitation, consisted of intense anti stokes emission in addition to intense red and near infrared emission, is a result of photo-transfer optically stimulated luminescence (PT-OSL). This involved electron-hole recombination at photoinduced
magnetic polaron site. The PL emissions and the centers identified are the followings: 718nm emission due to magnetic polaron and 360nm emission due to cooperative emission from two polarons, Fe-R line at 660nm due to Fe3+ coupled to a cation; 380nm and 399nm due to Pb2+ clusters and Nb5+ center; 630 and 860 nm due to 6p→6s transition of Pb3+ and 760nm due to isolated Fe3+ ions. Raman spectrum of brown samples revealed the presence of nano particles/wires of orthorhombic β PbO. The FTIR spectrum gives evidence of significant amount of hydroxyl impurity.
Fluorescence lifetime sensing is useful to measure temperature in harsh environments. We implemented the technique
using samarium doped materials. The sample was exposed to a pulsed laser beam and the resulting lifetime was
measured. A plot of lifetime versus temperature exhibited a linear dependence. A small change in temperature produced
a significant change in lifetime, which is important in measuring temperature accurately. This technique is an alternative
when conventional techniques are not suitable.
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