Conductivity Characteristics And Piezo Current Test Of Polymers Materials Under High Pressure

With the proposal of the"dual carbon"goal,new energy generation has received increasing attention.Currently,offshore wind power and photovoltaic power have tremendous potential,and at the same time,more stringent requirements have been placed on submarine high-voltage direct current（HVDC）transmission cables.As the main material for cable insulation,polymer can form charge carrier traps inside due to impurities or additives,leading to the accumulation of space charges and accelerating the insulation aging of cable,which shortens the service life of the cable.To better study the electrical properties of insulation materials,it is particularly important to measure the trap parameters of polymer materials.Currently,the thermally stimulated current（TSC）method is the mainstream test method for measuring the trap characteristics of polymer materials,which calculates the trap energy levels by controlling temperature changes to obtain the thermal stimulated current spectrum.Referring to the TSC method,this paper proposes a new method for measuring the trap characteristics of polymer materials,called the pressure stimulated current（PSC）method,which uses pressure as the control variable.A hydraulic control system was designed to apply a pressure of 10-600MPa to the polymer,and the reliability and accuracy of the hydraulic system were verified through press experiments.In this paper,a sample chamber was designed for testing the electrical properties of materials under high voltage and pressure,and the experimental principles and steps were explained.Resistance tests were performed on nylon 6 and linear low-density polyethylene（LLDPE）,and the results showed that the measured values were more consistent with theoretical values after the sample chamber was dried,and the test system demonstrated accuracy in conducting electrical performance tests.Analysis of the resistance test results at different pressures showed that the resistance increased with increasing pressure.The electrical conductivity behavior of polymers can be divided into ionic conductivity and electronic conductivity.As the pressure increases,the ionic conductivity weakens while the electronic conductivity strengthens.The resistance test experiment demonstrated that under low field strength,nylon 6 and LLDPE mainly exhibit ionic conductivity.The mechanical performance testing was conducted to study the changes in the glass transition pressure point.The stress-strain experiment requires a higher pressure,so a sample chamber with a small diameter for mechanical performance testing was designed,with a maximum pressure of up to 2.4 GPa.Analysis of the stress-strain curves of LLDPE and co-polypropylene（PPR）showed that the glass transition pressure point increased with increasing temperature.This is because at higher temperatures,the molecular chain segments become more active and require higher pressure to restrict their movement.Comparing the glass transition pressures of the two materials at the same temperature,it was found that the glass transition pressure of LLDPE was higher,which is because the chain flexibility of LLDPE is superior to that of PPR.The polymer high-voltage test device was used to test the excitation current of nylon 6,and the reliability of the experimental device was verified by comparing different polarization methods.We quantitatively calculated the molecular activation volume and activation energy of Nylon 6 using the constructed physical model,and the calculated results were consistent with the theoretical values,validating the accuracy of the pressure stimulated current testing experiment.We also tested LLDPE and polyimide（PI）,and calculated the molecular activation volume of LLDPE to be5.9×10^-28m³ and the activation enthalpy to be 0.078e V using the pressure stimulated current spectrum.The pressure stimulated current spectrum of PI indicated the existence of two different trap levels,with the shallow trap level at 0.038e V and the deep trap level at 0.094e V.The calculation results of the trap levels for different materials provided data support for subsequent analysis.