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temperature. The latter, self-referencing method, provides higher accuracy and greater robustness against fluctuations in excitation power, optical losses, and environmental conditions [ 11 ], and therefore constitutes the primary focus of the present study. In the second detection approach, the luminescence decay times of the 1 D 2? 3 H 4 transition were analyzed. This transition is characterized by a significantly longer lifetime and a stronger temperature dependence than the 3 P 0? 3 H 4 transition, which makes lifetime-based analysis a sensitive complementary tool for determining the maximum temperature reached during thermal exposure.
2 Materials and methods
The samples of yttrium-aluminum garnet doped with Pr 3 + ions( general formula Y 2. 97 Pr 0. 03 Al 5 O 12) were synthesized using a solid-state reaction assisted by high-energy milling, using high-purity commercial powders of Al 2 O 3( Krahn Ceramics, > 99.99 %), Y 2 O 3( NYC YT3WP, 99.9 %), and Pr 6 O 11( Thermo Scientific Chemicals, 99.996 %). Stoichiometric amounts of the starting materials were mixed and milled with anhydrous ethanol( Chempur, 99.8 %) and tetraethyl orthosilicate( Sigma-Aldrich), added at a concentration of 4.66 lL per gram of total mass. Planetary milling was carried out in a Fritsch Pulverisette 7 Premium Line systemusing5mmSi 3 N 4 balls. The milling process consisted of ten cycles of 10 min at 300 rpm, each followed by a 10 min cooling break. An 80 mL silicon nitride jar and 250 milling balls were used. The slurry was dried on a hot plate at 80 ° C for 24 h to remove ethanol. The powder was then subjected to two stages of thermal treatment in air muffle furnaces. In the first stage, the powder was heated to 600 ° Cat5 ° C / min( 30 min dwell), then to 900 ° Cat7 ° C / min( 30 min dwell). In the second stage, it was heated to 600 ° Cat10 ° C / min( 15 min dwell), then to the target temperature of 1100 – 1600 ° C at 10 ° C / min( 120 min dwell), followed by controlled cooling to 400 ° C at 10 ° C / min.
After annealing, the obtained powders were manually ground in an alumina mortar to obtain a homogeneous powder fraction suitable for subsequent characterization. The XRD patterns were collected in the 2h range of 5 – 100 ° with a step size of 0.01 ° and a scanning rate of 2.0156 ° per minute. Cu Ka 1 radiation( k = 1.5406 Å) with a nickel filter was used. The quantitative phase composition and lattice parameters were evaluated by Rietveld refinement.
Emission spectra and excited-state lifetimes were measured at room temperature using an Edinburgh Instruments FS5 spectrometer with a 150 W xenon lamp, Czerny – Turner monochromators, an optical system optimized for powder measurements, and a temperature-stabilized single-photon-counting PMT( Hamamatsu R928). Spectra were corrected for instrumental response using FLUORACLE. Lifetime measurements were performed in time-resolved mode, and decay curves were fitted using dedicated exponential-fit routines in FLUORACLE to obtain Pr 3 + emission lifetimes.
Figure
1. XRD pattern of YAG: 1 % Pr 3 + annealed at 1200 ° C, 1400 ° C, 1600 ° C, for 2 h, with overlaid reference lines of the best-matching phase standards from crystallographic databases.
3 Results and discussion
3.1 Structural characterization
YAG: 1 % Pr 3 + samples were synthesized and thermally treated at temperatures ranging from 1100 to 1600 ° Cfor2h. Subsequently, XRD measurements were performed to analyse the crystalline structure formed at each annealing temperature. The XRD pattern of the sample annealed at 1600 ° C, 1400 ° C, and 1200 ° C is presented in Figure 1.
Phase identification analysis indicates that the sample annealed at 1200 ° C for 2 h is multiphase, with a predominance of the intermediate YAP crystalline phase( YAP = 71.4 wt.%, YAM = 6.8 wt.%, YAG = 1.79 wt.%, Al 2 O 3 = 19.7 wt.%, Y 2 O 3 = 0.33 wt.%). With increasing annealing temperature, the content of the target YAG crystalline phase increases. At an annealing temperature of 1400 ° C, the YAG phase becomes dominant( YAG = 92.1 wt.%, YAP = 5.01 wt.%, Al 2 O 3 = 2.9 wt.%). Upon further