Processing and properties of luminescent Cr3+ doped transparent alumina ceramics

Transparent Cr₂O₃-doped alumina ceramics were prepared by slip casting, followed by pre-sintering in ambient atmosphere and hot isostatic pressing. The effect of dopant concentration on material properties, including microstructure and optical properties was evaluated. Real in-line transmittance in the range of 20–44 % was measured for the ceramics with the mean grain size <520 nm: the transmittance decreased with increasing grain size and Cr content. The excitation spectra consisted of two broad bands with maxima at 404 nm and 558 nm, corresponding to ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g transitions of Cr³⁺ ions in octahedral sites of α-Al₂O₃. The intensive deep red narrow emissions under violet/green light excitation, R-lines (²E_g → ⁴A_2g transition), were observed at 692.5 nm and 693.8 nm, that are very close to ruby single crystal. The highest emission was achieved at the Cr³⁺ concentration of 0.4 at.%. The luminescence decay curves exhibited single-exponential behaviour with decay times of ～3.6 ms.