2.1 Materials
Lithium acetate (C2H3LiO2), tetrabutyl titanate (TBT) [Ti(OC4H9)4], and acetic acid (C2H4O2) were obtained from Aladdin Co. Ltd. (Shanghai China). Anhydrous ethanol was purchased from Sinopharm Chemical Co. Ltd. (Shanghai, China). These reagents were of analytical reagent grade. LDPE (Kunlun 18D) was purchased from National Petroleum Corporation (Daqing, Heilongjiang, China). EVA copolymer (EVA 7470) was purchased from Formosa Plastics Co. Ltd. (Taiwan China), and conductive CB (VXC-72) was obtained from Cabot Co. Ltd. (Boston, MA, US).
2.2 Synthesis of Li4Ti5O12 nanopowder
To prepare an ionic conductor with uniform morphology, narrow particle size distribution, and high homogeneity, a sol-gel technique was used to synthesize a Li4Ti5O12 precursor, which was subsequently sintered to obtain Li4Ti5O12 compound with a spinel structure. An appropriate amount of anhydrous lithium acetate and TBT were weighed at a Li+: Ti4+ molar ratio of 0.82:1. Anhydrous lithium acetate was dissolved in a mixture composed of anhydrous ethanol, acetic acid, and deionized water (V: V: V = 120: 10: 5) at a molar ratio of 1:50 (Liquid A). TBT was dissolved in anhydrous ethanol at a molar ratio of 1:60 (Liquid B). Liquid B was placed into an oil bath at 55°C and subjected to vigorous stirring. Liquid A was gradually dropped into liquid B to produce a colorless and transparent solution. To obtain a stable colloidal solution, the reaction solution was aged for 4 h at 55°C under vigorous stirring. The resulting gel was heated to 80°C to extract excess anhydrous ethanol, water, and an organic precursor. Next, the organic precursor was dried at 150°C, transferred to a muffle furnace, and carbonized for 4 h at 500°C. After that, the furnace temperature was increased to 800°C, and the sample was sintered for 6 h. As a result, lithium titanate with a spinel structure was obtained [25, 26, 29].
The synthesized Li4Ti5O12 was ground in a planetary ball mill (MiQi YXQM-4L) for a certain period (10 hours, 20 hours and 30 hours). According to the ball milling procedure described elsewhere [30], the sample was divided into two equal parts and placed into a pair of nylon cans, followed by the addition of zirconia balls with different sizes at a weight ratio of 1:20 and anhydrous ethanol in the amount corresponding to 75% of the reactor capacity. The rotating speed was set to 300 rpm, and the ball milling time was increased at 10-h intervals until Li4Ti5O12 was milled into nanopowder with uniform particle sizes. Samples were taken out every 10 h for structural and morphological analysis by X-ray diffraction (XRD, D-MAX 2500/PC) and field emission scanning electron microscopy (FE-SEM, FEI/Nova Nano SEM 450), respectively, to determine the optimal ball milling time.
2.3 Preparation and characterization of Li4Ti5O12 semiconductive composites
Li4Ti5O12 semiconductive composites were manufactured in two steps by a melt blending method. Initially, 45wt.% LDPE, 30wt.% EVA, and 25wt.% CB particles were fused together at 150°C with a two-roll miller (HaPro). Next, 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, and 5 wt.% of the Li4Ti5O12 nanopowder and 2wt.% of the crosslinking agent were blended with the pre-mixed materials for 12 min at 120°C using a mixed torque rheometer (RM-200C, HaPro) to obtain LTO semiconductive materials. Simultaneously, the premixed product was blended without Li4Ti5O12 during the first step under the same conditions. The produced nanocomposites with different Li4Ti5O12 contents were shaped into sheets with various sizes for different electric performance analyses using a hot press apparatus (SKZ401) as follows. First, a nanocomposite was placed in a flat vulcanizer at a temperature of 130°C no pressure for 4 min and then under a pressure of 10 MPa for 6 min. In the next step, a cross-linking reaction was performed at a pressure of 10 MPa and temperature of 180°C for 15 min. Finally, the specimen was cooled for 10 min under a pressure of 10 MPa.
To obtain the distribution of Li4Ti5O12 nanoparticles in the composite matrix, the microstructures of the LTO semiconductive composites with Li4Ti5O12 contents of 1 wt.%, 4 wt.%, and 5 wt.% were observed by FE-SEM and HRTEM (FEI·Tecnai·G2F30) after being sliced in liquid nitrogen, and their elemental energy dispersive spectroscopy (EDS) analyses were performed.
2.4 Resistivity measurements
Because the thermal expansion coefficient of the polymer is three times larger than that of CB particles, the polymer matrix expands with increasing temperature. As a result, the CB particles become diluted, inhibiting their charge transfer and leading to the blockage of conductive channels, which increases the resistivity of the nano-compound [31, 32]. Hence, resistivity represents an important characteristic of the conductive channels inside semiconductive composites.
The specimen for resistivity testing was a sheet with a length of 110 mm, width of 50 mm, and thickness of 0.5 mm. Its resistivity was measured at temperatures of 30, 40, 50, 60, 70, 80, 90, 100, and 110°C using a semiconductive rubber and plastic material resistivity tester (DB-4). The specified temperature range corresponds to the operational conditions of the HVDC cable.
2.5 TSDC testing
TSDC can accurately reflect the space charge amount in an insulating material; thus, TSDC measurements represent a widely used technique for studying the parameters of space charges in the insulation layer [33]. To examine the influence of Li4Ti5O12 concentration on the suppression of carrier transport under the action of a powerful electric field, the TSDC generated in the insulation was determined. For this purpose, the insulating layer was polarized by high voltage under the shield of a LTO semiconductive nanocomposites. The utilized polarization device included a high negative voltage DC source, a couple of copper cylindrical electrodes with diameters of 25 mm. The LTO semiconductive layer was placed between the cathode and the insulating sheet used as the shelter, and a plastic shielding box used for screening (Fig. 1). The thickness of the LTO semiconductive layer was 0.5 mm. The insulating layer is an LDPE film with a thickness of 0.3 mm and uniform structure. The surfaces of the insulating and semiconductive layers were cleaned with anhydrous ethanol and dried for 4 h at 60°C before testing. The polarizing electric field strength was varied between 10, 20, and 30 kV/mm, and the polarization time was 30 min.
After the polarization process was complete, the LDPE depolarization current was measured using a Novocontrol Concept-80 wide band dielectric spectrometer. The starting temperature was 20°C, the heating rate was 3°C/min, and the final temperature was 90°C.
2.6 Space charge distribution measurements
The space charges in the insulation layer can be classified into two types: homo-charges originated from the electrode (metal core) emission, and hetero-charges caused by the material impurities ionized under high stress [34]. When the electric field strength is sufficiently high (for example, 10 kV/mm or more), the implanted charges from the core become predominant, which is the major reason for the space charge accumulation in the insulation layer.
To investigate the inhibitory effect of the produced LTO semiconductive composites on the space charge implantation to the insulating material, PEA measurements were performed. During the test, an LTO semiconductive sheet was placed between the cathode and the insulating layer not only for acoustic impedance matching but also for suppressing the simultaneous charge injection into the insulation layer [35]. In this process, a high voltage was applied through the upper electrode to simulate charge injection, and a high-voltage electric pulse was applied at the same time to cause a small displacement of the charge in the insulating sheet. This slight movement was transferred to the piezoelectric sensor as a sound wave that was subsequently detected and transmitted to a computer for analysis. Thus, the distribution of space charges in the insulating layer was represented by the obtained charge density. A schematic of the utilized PEA measurement setup is shown in Fig. 2.
In this test, the influence of Li4Ti5O12 content in the semiconductive composites on the space charge distribution was explored, and an LDPE layer with a thickness of 0.3 mm was used as the insulation medium. The test electric field strength was varied between 10, 20, and 30 kV/mm, and the applied voltage was maintained for 30 min.