The study of homonuclear diatomic molecules under high pressure and high temperature is a fundamental problem of condensed matter physics. In this research work, a cryogenic target (CT) was built to liquefy the gas and execute shock compression. The characteristics of the CT and the diagnostic system are explained in detail. We performed a shock compression of liquid nitrogen by using a two-stage light-gas gun at pressures up to 93 GPa (0.93 Mbar). Impactor velocities were measured with the magnetic velocimetry system, with a precision of 0.2%. The optical waveforms were recorded with the Doppler pin system, then further fast Fourier transform obtained velocity profiles in the sample. The measured velocity profiles were used to identify optical reflectance and obtain first-shock velocities, independent of the sample thickness above dissociative pressure (>30 GPa). The measured shock velocities had an uncertainty of less than 1%. First particle velocities were calculated by impedance matching, and the second velocities were directly calculated from the velocity profiles in an LiF anvil. The experimental shock Hugoniot results were observed to be consistent with those of the previous work. However, the principal Hugoniot softened above 27 GPa, and the uncertainties in the first and second-shock volumes were less than 0.7% and 3%, respectively.
Optical transmittance measurement plays an important role in studying the phase transition mechanism of transparent liquids. A new real-time transmittance monitoring system was constructed to investigate the phase transition in transparent liquids under dynamic compression. The results show that for water, there are a transition occurring on sub-µs time scales during the second shock compress, which presents good agreement with Dolan's result. This finding demonstrates that the water response is indeed strongly time-dependent. A similar experiment has also been done for the shocked benzene, but no obviously phase transition information has been obtained.
The behavior of 98 wt% hydrogen peroxide in delay period under shock loading was researched by optical transmissivity in-situ detection system. The time evolution of the transmissivity of 98 wt% hydrogen peroxide in the delay period is firstly presented. The experimental phenomenon shows that when the shock pressure in hydrogen peroxide reaches the ignition level, a large number of clusters will be produced inside the hydrogen peroxide. The formation of new clusters provides the reserve of energy for ignition. Those clusters formed by hydrogen peroxide in the delay period may act as hot spots, which leads the homogeneous explosives to reach the heterogeneous state before ignition. This conclusion is reasonable for revealing the ignition mechanism of homogeneous explosive. The particle behavior in the delay period gives us a new method to detect the ignition pressure. The experiments also confirm that the ignition pressure of 98 wt% hydrogen peroxide is just between 9.3 GPa and 12.1 GPa.
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