Studying On The Phase Transition Process Of N-Alkanes With Middle And Low Carbon Number By Thz Time Domain Spectroscopy

Normal paraffin is the main component of paraffin in petroleum.Under the condition of low temperature,the crystallization of n-paraffin will make petroleum difficult to be transported and exploited.In this study,n-alkenes with medium and low carbon number were selected as the research object.Terahertz time-domain spectroscopy was used to study the phase transition process of middle and low carbon n-alkanes.The main research contents are as follows:1.The n-icosane(C₂₀H₄₄)is scanned by transmissive terahertz time-domain spectroscopy system from high temperature value to low temperature value.Terahertz time-domain spectroscopy of n-icosane at different temperatures is obtained.Optical parameter calculation results show the terahertz time-domain diagram,absorption coefficient diagram and refractive index images both reflect the phase transition of n-icosane.In the time domain graph shows the change in intensity,turning point of the intensity corresponds to the critical point where the rotating phase appears.The state of the sample can be seen on the relationship between the absorption coefficient and the temperature.The state of the sample can be seen on the relationship between absorption and temperature.The absorption coefficient increases from the liquid phase to the rotating phase,and the absorption coefficient decreases gradually from the rotation phase to the solid phase.The change of refractive index is closely related to the state of the sample.According to the difference in the slope of the curve of refractive index and temperature,the temperature corresponding to each state of the sample can be accurately determined.And the refractive index is affected by the density of the sample.2.Terahertz time-domain spectroscopy is used to detect the phase transition of n-dodecane(C₂₂H₄₆).The results were consistent with that of n-icosane.It is proved that the terahertz time-domain spectroscopy can show the entire phase transition of n-alkanes.In particular,the change of refractive index and temperature can show the entire phase change process of n-alkanes,and the temperature range of phase transition can also be clearly obtained.The change of refractive index is very sensitive to the structural change of alkanes.From the liquid phase to the formation of the rotating phase,the interlayer spacing does not change significantly.This process is the continuous formation of grains.the transformation process from the rotating phase to the solid phase.The interlayer spacing decreases with temperature,indicating that the process of grain growth mainly occurs in the sample.The change of layer spacing and the change of refractive index are basically the same,so the change of refractive index is not only affected by the density of the sample,but also is affected by the crystal structure.The phase transition of n-tetracosane(C₂₄H₅₀)is then characterized by terahertz time-domain spectroscopy.This study verifies that terahertz time-domain spectroscopy can be used to characterize the phase transition of n-alkanes,which provides new ideas and methods for the study of n-alkane phase transitions.