| In the experimental study of the dynamic properties of materials,transparent windows are often attached after the samples as anvils,and maintain a high temperature and pressure state for a period of time,which is in order to observe and measure the interfacial particle velocity and spectral radiance.In this way,the information on wave structure evolution caused by changes in physical properties of the measured sample can be obtain.In this type of experiment,the window material will also undergo elastic-plastic transition and shock-induced structural phase transition or even melting due to compression of the shock wave.The optical properties of window material including refractive index and optical absorption rate will change.Therefore,the study of the dynamic properties of the window material is a very basic work in the field of shock wave physics.It has an extremely important practical significance,because it is directly related to proper interpretation for the experimental signal.Gadolinium gallium garnet(GGG)single crystal(initial density:7.1 g/cm3)is selected as a potential high-impedance window material,which has attracted the interest of many re-searchers and carried out more systematic research.As so far,the elastic-plastic transition and its elastic precursor wave amplitude changes with shock stress,Hugoniot equation of state,shock-induced phase transition,optical refractive index of single-wave region,shock-induced luminescence,shock-induced temperature rise,shock-induced melting,and high-pressure elec-trical conductivity have been reported by relevant experimental work and theoretical work.The initiation pressure of shock-induced phase transition in GGG is about 76 GPa,and melting oc-curs around 200 GPa.While,in static high pressure experiment,GGG undergoes a solid-solid phase transition from cubic garnet to perovskite cubic at around 88 GPa.However,the opti-cal transmittance(shock transparency)of the GGG single crystal under high pressure and its influence on the kinetics of phase transition have not been reported in the literature.In the first part of this work,shock compression experiments have been performed on[111]GGG single crystals with the planar impact method at light-gas gun.The history of the interface and free surface velocity of the sample are obtained with a Doppler Pin System(DPS),operating at 1550 nm,and the pressure range is 6-201 GPa.We have investigated its refractive index of double-wave and three-wave structure region changes with the shock pressure.The results show that the refractive index of GGG changes little before the shock-induced phase transition;and the change of refractive index of high pressure phase with pressure increases in a higher rate;there is no step change at the phase transition kinetic process pressure.In the second part of this work,we used two-stage light-gas gun dynamic high-pressure loading device and a multi-channel radiation pyrometer system,in combination with the in-situ light source produced by the high-temperature target,simultaneous measurement of the trans-mission and emission histories of the GGG sample.The measurement wavelength in the range of 400-800 nm,and the experimental pressure in the range of 47-211 GPa.The quantitative relationship of GGG extinction(absorption)coefficient ? as a function of wavelength and pres-sure was obtained.Overall,the results show that a coefficients of different wavelengths increase with increasing shock pressure,show a transparent-opaque transition and peak at 75 GPa,and then at a pressure range of 75-100 GPa,? coefficient decreases with increasing pressure and an opaque-transparent transition occurs;then a rapid increase again occurs at higher pressures.The initiation pressure of the decreasing a coefficient is consistent with the phase transition pressure,suggesting that the recovery of GGG shock transparency is most likely due to the microstruc-ture evolution associated with the plastic deformation and the solid-solid phase transition.The compression of inelastic shock waves causes the distribution of various defects including shear bands inside the crystal to change its optical transparency.And when a shock-induced phase transition occurs,the nucleation of the daughter phase is often coupled with or occurs at defec-tive sites.And its nucleation and growth reduce the deformation-induced defects and give rise to transparency recovery or retaining during the phase transition.However,the refractive index of GGG does not change significantly near the phase transition pressure,indicating that the re-fractive indices of the daughter phase is similar to the parent phase,and thus the appearance of the new phase does not lead to changes in the transparency of GGG.Therefore,the decrease in defect concentration caused by the formation of a new phase is the reason for the transparency recovery.The innovations in this work:1.Establish and develop simultaneous measurement tech-niques for transmission and emission histories;2.Measured the refractive index n for GGG single crystals including elastic-plastic transition dual-wave structure and phase-change three-wave structure.The quantitative relationship between refractive index and shock pressure show that n is continuous at the phase transition kinetic process pressure;3.The quantitative rela-tionship between the extinction(absorption)coefficient of GGG single crystal and the change of wavelength and shock pressure was measured.It was found that the transparency recovery of GGG appeared near the phase transition pressure,and a reasonable explanation is given. |
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