Effect Of Injection By Metal Electrode And Semiconductor Electrode On Space Charge In Polyethylene

In this paper,silicon wafers of N-type and P-type with different doping concentrations were used as semiconductor electrodes,which were respectively mixed with aluminum electrodes to form high voltage terminals.The charges injected into low density polyethylene?LDPE?by high voltage terminals were tested by Pulsed Electro-Acoustic method?PEA?and thermally stimulated current method?TSC?.The quantity of space charge accumulated in LDPE was calculated by integral formula.The carrier mobility and diffusion coefficient in semiconductor were obtained by Einstein relation.The results show that for the silicon wafers of N-type and P-type with different doping concentrations,the charge accumulation in LDPE is the least when the doping concentration is medium.The phenomenon is related to carrier concentration,carrier mobility and diffusion coefficient.The lower carrier concentration cause less charge to be injected into the medium when they are used as electrodes.The higher carrier mobility and diffusion coefficient make the charge dissipate faster.Meanwhile,temperature changes affect carrier mobility,and then affects the charge injection.Titanium and tungsten films were deposited on the copper electrodes by plasma chemical vapor deposition?PCVD?.The surface morphology was characterized.The surface work function and oxidation state of copper were measured.The charge was injected into LDPE by copper electrodes with different coatings and space charge accumulation in LDPE was tested by TSC.It is found that a dense spherical is deposited on the surface of copper by scanning electron microscope?SEM?.The results of X-ray photoelectron spectroscopy?XPS?analysis show that the surface of the copper electrode and the coated?titanium and tungsten?electrodes are in the oxidation state or have been oxidized.After modification,the work function of the electrode material is improves and the charge injected into LDPE is reduced at specific electric field strengths.In addition,the trap level calculated from TSC curves is also smaller.Especially,Ti-coated electrode injects less charges into LDPE under 30kV/mm and W-coated electrode under 70 kV/mm.This is because oxygen vacancies in metal oxide with positive charges have an ability of attraction for the negative charges,it causes electrons hard to transfer.Therefore,the effect of suppression on charge injection is remarkable.Semiconductive shielding materials of different doping concentrations of carbon black?CB?were used as cathode materials for field emission.Copper rod was used as anode.I-U curves were measured and the apparent electric field and threshold electric field were calculated respectively.In particular,when the concentration of CB is 30%,apparent electric field and threshold electric field are the largest,which makes it more difficult to generate field emission electrons.It has better electrical properties as a semiconductive shielding material in high voltage DC cable.Further,the PEA test results are consistent with the previous conclusions.The reason may be that there are many depression and holes on the surface of cathode material after field emission,which changes the field enhancement factor?and leads to the change of the local electric field E_loc.