Impacts of impurities and thermal history on the electrical conduction and charge trapping characteristics in crosslinked polyethylene thin films
Open Access
- Author:
- Walker, Roger Craig
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 17, 2020
- Committee Members:
- Michael T Lanagan, Dissertation Advisor/Co-Advisor
Michael T Lanagan, Committee Chair/Co-Chair
Ramakrishnan Rajagopalan, Outside Member
Ralph H Colby, Committee Member
Tze-Chiang Chung, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- polyethylene
XLPE
LDPE
dielectric spectroscopy
thermally stimulated depolarization current
BDS
TSDC
Meyer-Neldel rule
compensation law
current-voltage measurement
conduction current measurement
IVM
CCM
high-voltage power cable
HVDC
HVAC - Abstract:
- The work presented in this dissertation is primarily aimed at improving the understanding of the electrical properties of the insulating polymer crosslinked polyethylene (XLPE) in order to support its use as power cable insulation. XLPE has been used for decades as the material of choice in this application due to its low cost, the ease of maintaining it, its improved environmental friendliness over what it was replacing, and – most importantly – its low DC conductivity and AC losses. However, it has several unaddressed issues associated with its use in the long term due to extrinsic impurities (such as water and acetophenone) and intrinsic variability (due to its semicrystalline nature). A better understanding of how these factors influence the electrical properties of XLPE is needed both to enhance the fundamental knowledge regarding this prominent polymer and to improve its utility as power cable insulation. As such, an analysis of the electrical properties of XLPE was carried out by using four separate types of analyses. Broadband dielectric spectroscopy (BDS) was used to analyze the AC response. Conduction current measurement (CCM) and current-voltage measurement (IVM) were used to analyze the DC response. Thermally stimulated depolarization current (TSDC) technique was used to analyze the charge trapping characteristics of polyethylene. XLPE thin film samples were generated by melt pressing pellets of low-density polyethylene (LDPE) that were infused with the crosslinking agent dicumyl peroxide (DCP). Some LDPE pellets were not infused with DCP and used to make LDPE thin films for comparative purposes. After fabrication, a selection of films was degassed in order to remove impurities. Some of those films were then soaked in specific impurities to intentionally re-introduce that one in particular. Aluminum electrodes were then applied to the samples in the same high-vacuum evaporation chamber to a thickness of 50 nm at a deposition rate of 2.5 A/s. Electrode diameters were fixed at either 1 cm or 3 cm, and the thickness was measured after electrode deposition. Chapter 3 goes into the details of the AC analysis of polyethylene. Samples were characterized electrically by their dielectric loss, a measure of inefficiency during the AC cycle that can also be related to the AC conductivity. It was found that the presence of impurities such as DCP byproducts had a strong impact on the AC response in two different ways. One is that the presence of impurities above a certain threshold led to significant increases in the dielectric loss at room temperature. The other is that excess impurities modified the structure as was seen by alterations in the temperature coefficient of capacitance (TCC). The impurities are polar in nature and created an internal pressure that enhanced thermal expansion when compared to degassed XLPE and to as-received LDPE. Chapter 4 goes into the details of the phenomenon of electrical compensation in polyethylene. Compensation is an observed trend where the activation energy and pre-exponential factor for conductivity are correlated. It had been previously observed in the DC conduction of LDPE and it was found that it can also be observed in the DC conduction of XLPE via CCM. Additionally, it was also found in AC conduction as observed in the BDS results. Electrical compensation was not observed in as-received XLPE samples that were water soaked instead of degassed for AC conduction, and those samples generally had low activation energy of 0.2 eV or less. All other samples exhibited variation in the range of 0.2 eV to 1.4 eV in AC. All samples showed compensation in DC in the range of 0.2 eV to 1.0 eV for activation energies. The observed compensation was determined to arise from sample to sample variation in polar impurities such as water and was sensitive to the thermal history of each sample. Chapter 5 goes into the details of charge trapping in polyethylene as examined using the TSDC technique. TSDC analysis results in a spectrum of current measured as the temperature rises, indicating what temperatures were needed to release trapped charges as well as the amount of stored charge and the energy associated with that release. As-received XLPE samples tended to exhibit one peak in the TSDC spectra associated with impurities. Their removal via degassing meant that three peaks could be observed – that same impurity peak, but also peaks associated with molecular motions near the glass transition temperature and with charge injection in the melting range of polyethylene. Harsher degassing was shown to reduce the impacts of these impurities and of charge injection. Acetophenone was found to be the key DCP byproduct in determining the overall trapping characteristics of XLPE. Chapter 6 goes into the details of a brief examination of the response of XLPE to applied DC bias using IVM. Both the time response of the current and the current-voltage spectra were examined. It was found that that overall response of XLPE could be altered depending on one of two things: the thermal history via changing the degassing temperature and the presence of impurities via the addition of acetophenone. Both the standard 65°C degassed and the test 90°C degassed samples exhibited true conduction current with the 90°C samples having reduced conductivity and higher activation energy in comparison. The addition of ACP into the standard degassed samples reduced the activation energy and the conductivity but the current response was now due to polarization rather than true conduction. Additionally, standard samples were found to exhibit space charge limited current while the others had ohmic or sub-ohmic response. Chapter 7 contains a summary and directions for future work. In general, it is suggested that future investigations should combine experiment and simulation to best determine how thermal history and impurities determine the electrical properties of XLPE samples. What is needed to know is how precisely these two factors alter the resulting structure of the XLPE and contribute to changes in these various responses to applied electrical fields. It was also suggested to look into the impacts of other impurities not related to DCP such as antioxidants and nanoparticles.