This paper focuses on the space charge characteristics in TiO2/cross-linked polyethylene (XLPE) nanocomposites; the unmodified and modified by dimethyloctylsilane (MDOS) TiO2 nanoparticles were added to XLPE matrix with different mass concentrations (1 wt%, 3 wt%, and 5 wt%). The scanning electron microscope (SEM) showed that the MDOS coupling agent could improve the compatibility between TiO2 nanoparticles and XLPE matrix to some extent and reduce the agglomeration of TiO2 nanoparticles compared with unmodified TiO2 nanoparticles; the volume resistivity testing indicated that the volume resistivity of TiO2/XLPE nanocomposites was higher than Pure-XLPE and increased with the increase of filling concentrations. According to the pulsed electroacoustic (PEA) measurements, it was concluded that the space charge accumulation was suppressed by filling TiO2 nanoparticles and the distribution of electric field in samples was improved greatly. In addition, it was found that the injection of homocharge was more obvious in MDOS-TiO2/XLPE than that in UN-TiO2/XLPE and the homocharge injection decreased with the increase of filling concentration.
Cross-linked polyethylene (XLPE) has low dielectric constant, low conductivity, and stable chemical properties, which has been widely used as cable insulation material [
In recent years, the tremendous development of nanotechnology has provided a new direction for researching on properties of polymer. Domestic and foreign scholars have carried out extensive studies on the space charge characteristics of polymer nanocomposites, mainly focusing on the following aspects:
At present, the interface between nanoparticles and polymer matrix has a significant impact on the dielectric properties of the nanocomposite, which has been confirmed by domestic and foreign scholars. Several different models have been proposed to explain the specific structure and the function mechanism of interface regions. Lewis proposed an electric double layer model considering that the interface formed when the low density polyethylene (LDPE) and the nanoparticle are in contact, with LDPE being negatively charged and the nanoparticles being positively charged [
In this paper, the TiO2 nanoparticles unmodified and modified by dimethyloctylsilane (DMOS) coupling agent were added to XLPE matrix with different mass concentrations (1 wt%, 3 wt%, and 5 wt%) in order to research the properties of TiO2/XLPE nanocomposite and explain the experimental phenomenon reasonably. Scanning electron microscopy (SEM) was firstly adopted to observe the TiO2 nanoparticle dispersion in XLPE matrix. Furthermore, the volume resistivity was carried out to analyze the movement of carriers. In addition, the pulsed electroacoustic (PEA) method was used to determine the space charge characteristics of TiO2/XLPE nanocomposite.
Common surface modification agents are shown as follows: coupling agents, surfactants, unsaturated organic acids and oligomer, organosilicon, inorganic surface treatment agents, and so on. A function of surface modification is to increase the dispersion of nanoparticle in the matrix, taking full advantage of the small dimension effect of nanoparticles, and another is to increase the compatibility between polymer matrix and nanoparticles, enhancing the interfacial chemical bond [
Chemical structure of MDOS.
LDPE was the master batch of XLPE, with density of 0.910~0.925 mg/cm3, melt index of 2.1~2.2 g/10 min, and melting point of 112°C. Rutile TiO2 nanoparticles were selected as filler, with diameter of 20~25 nm, and dicumyl peroxide (DCP) was used as a cross-linking agent. Sample preparation method is as follows: TiO2/LDPE mixture was prepared by melting blend in an open type mixer at 115°C; the TiO2/LDPE mixture and the DCP cross-linking agent were mixed in a closed type mixer in order to avoid pre-cross-linking at 115°C; then, the film samples with the thickness of about 170
XLPE thin film samples.
The PC68 digital high resistance meter was used to obtain the volume resistivity. Compression moulded sheet having diameter of 100 mm was inserted into the holder and charged for more than one minute at 5000 V. The volume resistivity measurement was carried out at room temperature (
The space charge measurement was carried out with a pulsed electroacoustic (PEA) system; the measuring principle of the device is shown in Figure
Schematic diagram of the measuring device by PEA method.
Scanning electron microscope (SEM) was used to observe TiO2 nanoparticle dispersion and cross-sectional microstructure in XLPE. The SEM images of UN-TiO2/XLPE and MDOS-TiO2/XLPE (1 wt% and 5 wt%) are shown in Figure
SEM of TiO2/XLPE. (a) UN-TiO2/XLPE (1 wt%), (b) MDOS-TiO2/XLPE (1 wt%), (c) UN-TiO2/XLPE (5 wt%), and (d) MDOS-TiO2/XLPE (5 wt%).
The volume resistivity of Pure-XLPE and TiO2/XLPE nanocomposite is shown in Figure
Volume resistivity of Pure-XLPE and TiO2/XLPE.
Figure
Space charge distribution of Pure-XLPE.
Figures
Space charge distribution of UN-TiO2/XLPE with different concentrations at different times. (a) 1 wt%, (b) 3 wt%, and (c) 5 wt%.
Figures
Space charge distribution of MDOS-TiO2/XLPE with different concentrations at different times. (a) 1 wt%, (b) 3 wt%, and (c) 5 wt%.
The electric field distribution of Pure-XLPE and TiO2/XLPE under DC stress of −30 kV/mm at 1800 s is shown in Figure
Electric field distribution of Pure-XLPE and TiO2/XLPE under DC stress of −30 kV/mm at 1800 s.
In order to further investigate the space charge characteristics of modified material, the mean volume density of space charge was used to quantitatively describe the space charge accumulation into TiO2/XLPE, which can be calculated based on the charge density distribution, as shown in the following formula:
Figure
Mean volume density of space charge in samples.
After the Pure-XLPE sample was polarized under −30 kV/mm DC voltage within 30 min, apparent heterocharge accumulated at XLPE-electrode interface and two charge peaks appeared in the middle of the sample, which was consistent with the results reported in the related literature [
Schematic diagram of space charge accumulation in Pure-XLPE.
Second, the impurities in XLPE medium (cross-linking agents, coupling agents, cross-linked products, and so on) were dissociated into positive and negative ions; the positive ions were transferred to the cathode and the negative ions were transferred to the anode under high electric field. According to the fast PEA testing, it was indicated that the “solitary waves” reached the opposite electrodes and established the equilibrium state, the process of which took several
The heterocharge and total space charge accumulation were both suppressed in XLPE medium, when XLPE was filled with UN-TiO2 and MDOS-TiO2 nanoparticles. For the inhibition mechanism of space charge in nanocomposite media, Tanaka et al. considered that the diffuse layer of XLPE-TiO2 interface accumulated negative charges due to producing negative charges in polyethylene by friction. The diffuse layer attracts holes and rejects electrons, improving the potential barrier of electrons injection and reducing the potential barrier of holes injection [
In this paper, the inhibition of space charge and the disappearance of heterocharge were analyzed through trap characteristics. Figure
Carrier transport and interface electric field model based on trap characteristics. (a) Carrier transport in Pure-XLPE. (b) Carrier transport in TiO2/XLPE.
On the other hand, a large number of charges would be fixed in the vicinity of electrode interface because of the many traps in TiO2/XLPE nanocomposite, leading to the accumulation of homocharge and the increase of interface reverse electric field (as shown in Figure
According to the accumulated charge volume which is net charge volume, some stable negative charges accumulated in the middle of the TiO2/XLPE nanocomposite; this is possibly because the electrons injected from the cathode were more than holes injected from the anode. The carriers were injected into polymer by hot electron emission under DC electric field, and the energy band diagrams for polymer-electrode system are shown in Figure
Energy band diagrams for polymer-electrode system. (a) No contact between sample and electrode, (b) contact between sample and electrode and no electric field, and (c) contact between sample and electrode and applied electric field.
In particular, the homocharge injection was more obvious in MDOS-TiO2/XLPE nanocomposite, and homocharge injection volume decreased with the increase of filling concentration. According to the potential well model proposed by Takada et al., deep traps would form at the interface between nanoparticles and polymer under applied electric field [
The work presented in this paper is concerned with the microstructure and space charge behaviors of Pure-XLPE and TiO2/XLPE nanocomposite. Several significant findings were concluded. The TiO2 nanoparticle agglomeration phenomenon in UN-TiO2/XLPE nanocomposite is serious with the particle radius of 400~600 nm, but that of MDOS-TiO2/XLPE is weak with the corresponding particle radius of 25~200 nm. It is concluded that MDOS coupling agent can improve TiO2 nanoparticle dispersion and reduce the agglomeration of TiO2 nanoparticles in XLPE matrix. The relationship of volume resistivity among the three kinds of samples in this paper is apparent, Pure- The results of the PEA measurements show that TiO2 nanoparticles have a significant influence on space charge accumulation behavior; the electric field distribution in TiO2/XLPE is improved greatly under DC electric field. According to the analysis in this paper, on the one hand, a large number of interface regions generate lots of deep traps, which can shorten the effective distance of “solitary waves” migration and greatly reduce carrier mobility. On the other hand, low carrier mobility enhances the neutralization process and weakens the impurity ionization, leading to the disappearance of heterocharge in TiO2/XLPE. The interface reverse electric field not only suppresses the charge injected from the electrodes, but also strengthens the electric field in the middle of the matrix, so that the accumulated space charge can be transferred out of the material timely. The homocharge injection is more obvious in MDOS-TiO2/XLPE, and homocharge injection volume decreases with the increase of filling concentration. According to the analysis in this paper, the MDOS-TiO2/XLPE nanocomposite has deeper traps than the UN-TiO2/XLPE nanocomposite, causing more homocharges to be injected in the MDOS-TiO2/XLPE. The reverse electric field established by homocharge is the main reason for the homocharge decrease with the increase of nanoparticles, increasing filling concentration.
The authors declare that they have no financial or personal relationship with any people or any organization that may inappropriately influence their work and that there is no professional or commercial interest of any kind in all of the commercial entities mentioned in their paper.
The reported research was performed with funding from the National Key Basic Research Program of China (973 Program), Contract Grant no. 2015CB251003.