Direct Measurement Of Solidification Transition Of Benzene At High Pressure And Discussion Of Transformation Kinetics

Liquid-solid transitions are certainly among the most widespread in nature and many of them also of importance. Whereas the methods of direct measurement for the process of solidification under shock wave have not been well developed. Kinetics of solidification of liquid benzene under high pressure has attracted much attention of scientists, and it is also one of long-standing disputes. In present work, the gas-gun technique is combined with a recently developed technique of light transmittance, and the time dependence of light transmittance of liquid benzene is directly observed during successive multiply shock-compressions. The analysis shows that the transmittance decreasing is caused by light scattering effects in a non-uniform optical media, which is formed by local liquid-solid transformation during shock compression, and that the features of light scattering reflects both the time relaxation and spacial accumulation of transition process. The arguments whether the liquid-solid transformations happens under multi-shock compression are clarified, and some important experimental proofs are provided for studies of phase transformation kinetics. The main results are as followed:1. The liquid-solid transition of benzene was firstly observed by a new developed light transmittance method in process of quasi-isentropic loading. The phase transition is found to happen in time scale of nanoseconds, and the process lasted more than one microsecond.2. The durations of multi-shock compressions estimated by the thermodynamic calculation are consistent with those indicated by the light transmission signal. When the benzene was firstly shocked, the light transmittance kept unchanged, but it began to decrease in the process of the second compression. Compared with the theoretical calculation and the phase diagram, the state of the second shock is confirmed to step into the solid phase region of benzene. Therefore, the observed decrease of the light transmittance is interptetted as the occurrence of the nucleation and growth of solification of benzene driven by the shock wave and the super-cooling.3. Based on the phase transformation kinetics model, the characteristics of nucleation and growth of solificaton were discussed. It is found that the process of solidification consists of two steps:the nucleation early induced by shock wave and the successive nucleation driven by supper-cooling, and both the number density and radius of nucleus were time-dependent. By fitting the experimental data, the present work firstly determined the quantative relationship among the number density of shock-induced nucleation, the rate of nucleation driven by supper-cooling, the critical radius of nucleus, and its speed of growth.