High Speed Transient Thermometry Using Phonon-Assisted Anti-Stokes Luminescence

An in-depth understanding of the underlying mechanisms responsible for the formation and evolution of hot spots in energetic materials under impact loadings is essential to improve the performance and safety of energetic materials.However,it is extremely challenging to monitor the evolution processes of impact induced hot spots,characterized as single event transients,in real-time and in situ due to its fast heating rate,short duration and small size.Non-contact thermometry techniques with high temporal-and spatial-resolution and high sampling rate are required.It is very difficult to temporally resolve the fast temperature transitents at low and medium temperature regions(?1000 K)using traditional pyrometry and Raman thermometry because of the low signal levels.This issue can be addressed using luminescence thermometry.However,most of the reported thermographic phosphors(TGPs)have relatively long decay lifetimes(10^-3–10^-5 s),which makes them unsuitable for applications involving extremely fast(heating rate?10⁶ K/s)thermal transients.In this dissertation,cerium doped yttrium aluminum garnet(YAG:Ce)is used as a TGP due to its very short radiative lifetime,large absorption cross section,and high quantum efficiency;and a high speed spectral thermometry method for real-time monitoring of extremely fast thermal transient events is developed,which provides a technical foundation for in-depth study of the hot spot formation and evolution processes in energetic materials.Firstly,the luminescence emission spectra and decay kinetics of YAG:Ce phosphor were studied as a function of temperature(80–800 K)under excitation in the long wavelength tail of the 4f¹(?)4f⁰5d absorption spectrum.Both the anti-Stokes and Stokes emission intensities were found to exhibit an unusual non-monotonic temperature dependence when compared to those commonly excited in the absorption band.Based on the theory of strong vibronic coupling,a phonon-assisted anti-Stokes luminescence model was proposed.It reveals that the single-photon frequency-upconverted anti-Stokes emission of YAG:Ce originates from phonon-assisted light absorption,and the thermally induced anti-Stokes and Stokes luminescence intensity enhancement is due to a rapid increase in the phonon-assisted excitation probability,while the thermal quenching of Ce³⁺anti-Stokes and Stokes luminescence at high temperatures is caused by thermal ionization of the 5d electron into the conduction band.The proposed model provides a theoretical basis for the development of the high speed transient thermometry technique.Secondly,it was found that the logarithm of the anti-Stokes luminescence intensity of the YAG:Ce probe is proportional to the emitted photon energy,and its slope is a monotonically decreasing function of temperature.Based on these characteristics,two novel spectral thermometry methods,i.e.,spectral slope method and anti-Stokes to Stokes fluorescence intensity ratio(ASFIR)method were proposed.The measurement models of the two proposed methods were established using calibration measurements.The average relative thermal sensitivity and single-shot precision of the two methods in the calibration temperature range(300–700 K)were derived,and the values of the corresponding parameters are(0.19%/K,3.6–1.1%)and(0.24%/K,2.3–1.0%),respectively.The performance of the two methods was compared to those of the conventional spectral thermometry methods,and their comprehensive performance indexes are comparable to those of the commonly used TGP Ba Mg₂Al₁₀O₁₇:Eu²⁺.Finally,a high speed spectral luminescent thermometer prototype with 5 k Hz sampling rate and 170 ns temporal resolution was developed using a compact high speed fiber-optic spectrometer,a high repetition rate pulsed green laser,and a fast decaying TGP YAG:Ce.The functionality of the prototype was demonstrated by its application in real-time measurement of local transient temperature changes(heating rate?10⁵ K/s)that were induced by a repetitive long pulsed long-wave infrared(LWIR)laser.The single-shot temperature uncertainty was shown to be better than4.7%.A two-dimensional(2D)axisymmetric laser heating model was developed,which compares well with the transient surface temperature evolutions measured at various heating pulse widths and repetition rates.The energy transport mechanism inside the material during pulsed laser heating was analyzed systematically using the proposed model.It reveals that the rate of temperature rise of the surface region decreases monotonically with heating time,and this effect is caused by an increase of the temperature gradient.The high speed luminescent thermometer developed in this dissertation shows great promise in the real-time measurement of local transient temperature induced by long pulsed lasers,the characterization of thermo-physical parameters of materials,and the diagnosis of transient hot spot events induced by low-and medium-speed impacts.