Baozhong Han

QM/MD simulations on the role of SiO2 in polymeric insulation materials

Quantum chemical molecular dynamics (QM/MD) simulations based on a self-consistent charge density functional tight-binding (SCC-DFTB) method on SiO2 filler in polyethylene (PE) showed that: in the absence of SiO2, the PE was quickly charged by high-energy electrons, which resulted in C–C or C–H bonds breaking; on the contrary, in the presence of SiO2 nanoclusters, electron trapping and accumulating were dominated by SiO2 nanoclusters rather than polyethylene, which made polyethylene be preferentially protected and increased the initial time of electrical treeing. In our calculations, we also observed double electric layers around the SiO2 nanocluster, in agreement with recent suggestions from experimental observations. Furthermore, compared with some other investigated nanoclusters, SiO2 was regarded as the most promising candidate attributed to the highest electron affinity. We further observed that once the high-energy electrons were supersaturated in the nanoclusters, the polyethylene chains would be unavoidably charged and C–H bond breaking would occur, which resulted from the interaction between H and O or Si in the nanoclusters. Following that, decomposing and cross-linking of the polyethylene chains were involved in the initial growth of electrical treeing. The current observation can potentially be used in power cable insulation.

Theoretical study on the radical reaction mechanism in the cross-linking process of polyethylene

Theoretical investigation on the radical-initiated reactions of the polyethylene cross-linking process with the voltage stabilizer grafting is accomplished by density functional theory (DFT) calculations. At the B3LYP/6-311+G(d,p) level, the molecular properties of 22 reactions and reaction potential energy information of 18 radical-initiated reactions are obtained. The HOMO–LUMO energy gaps, ionization potentials, and electron affinities of the cross-linking agent, voltage stabilizer, antioxidant, and by-product in the polyethylene insulation composite are calculated. The results show that the by-product, cross-linking agent, and voltage stabilizer can effectively increase the electrical breakdown strength. In addition, aromatic ketone voltage stabilizer and hindered phenol antioxidants of the studied molecule can be grafted to polyethylene chain easily during the polyethylene cross-linking process, and they have excellent compatibility with polymers matrix. The investigation is expected to provide reliable information for optimizing process and for the development of the insulation material of high-voltage cable exceed 500 kV in real applications.

Origin of large field-induced strain of azobenzene/polyurethane blend dielectric elastomers

Herein a diaphragm type dielectric elastomer actuator by blending azobenzene dyes into a polyurethane matrix (Azo/PU) is described. The effects of azobenzene content on the dielectric, mechanical, and electromechanical properties of the Azo/PU blends are discussed. The resultant blends with azobenzene content ranging from 1 to 8 wt% were characterized. The dielectric permittivity of PU increased from 8.0 to 11.1 (increasing of 38.5%) at 1 wt% azo content and reached a maximum of 35.6 (increasing of 343%) at 4 wt% azo content. By blending with a small content of azo the dielectric breakdown strength reached 57 kV mm−1, which is 86% higher than that of the neat PU. The 30 μm thick diaphragm of Azo/PU blend with 1 wt% azo content exhibited the highest actuated displacement at the center around 700 μm. Meanwhile the neat PU gave a displacement of 25 μm. That is a 26-fold increase in actuation is obtained by blending only 1 wt% azo dyes into the PU matrix. Infrared spectroscopy was used to characterize the structure of the Azo/PU blends. The particular chemical structure the Azo/PU blend containing 1 wt% of azo dyes is proposed to be responsible for the improvement in electromechanical actuation.

Theoretical study on the reaction of triallyl isocyanurate in the UV radiation cross-linking of polyethylene

Herein, a theoretical investigation on the reaction of triallyl isocyanurate (TAIC) in the UV radiation cross-linking process of polyethylene is conducted at the B3LYP/6-311+G(d,p) level for the production of high voltage cable insulation materials, where the reaction potential energies of 10 reaction channels are identified. The HOMO–LUMO energy gaps, ionization potentials, and electron affinities of the raw materials, product, and by-product in polyethylene insulation composite products are obtained. Furthermore, the optimized process for the production of UV radiation cross-linking polyethylene insulation materials for high voltage cables is described. The results indicate that the UV radiation cross-linking reaction of polyethylene is initiated by benzophenone, and the multi-functional cross-linker TAIC is required for the cross-linking process to occur. This investigation is expected to provide reliable information for the optimization of the polyethylene UV radiation cross-linking process and the development of insulation materials for high-voltage cables that can withstand more than 500 kV in real applications.

Theoretical study on the tailored side-chain architecture of benzil-like voltage stabilizers for enhanced dielectric strength of cross-linked polyethylene

A theoretical investigation on the mechanism of the tailored side-chain architecture of benzil-like voltage stabilizers for enhanced dielectric electrical breakdown strength of cross-linked polyethylene at the atomic and molecular levels is accomplished. The HOMO–LUMO energy gaps, the ionization potentials, and the electron affinities at the ground states of a series of benzil-like molecules are obtained at the B3LYP/6-311+G(d,p) level. The 8 isomerization reactions at the S0 state, including the harmonic vibration frequencies of the equilibrium geometries and the minimum energy paths (MEP) found using the intrinsic reaction coordinate (IRC) theory, are obtained at the same level. The substituent group effect, functional group effect, and electronic effect of the different heteroatoms (O, N, S) in substituent groups have been evaluated. The results show that benzil-like molecules, which have much smaller HOMO–LUMO energy gaps, much lower ionization potentials, and much larger electron affinities than those of aliphatic chains, can increase the electrical breakdown strength effectively as voltage stabilizers in cross-linked polyethylene. This result is consistent with Jarvid’s suggestions (Journal of Polymer Science, Part B: Polymer Physics, 2014, 52(16): 1047–1054).

Theoretical study on the reaction mechanism in the UV radiation cross-linking process of polyethylene

A theoretical investigation on the benzophenone-initiated UV radiation cross-linking reactions of polyethylene is accomplished at B3LYP/6-311+G(d,p) level for high-voltage cable insulation materials. The reaction potential energies of 9 reaction channels are identified in the T1 state. The HOMO–LUMO energy gaps, ionization potentials, and electron affinities of the polyethylene, photoinitiator, voltage stabilizer, antioxidant, and by-products in the polyethylene insulation composite products are obtained. The results show that the by-products, photoinitiator, voltage stabilizer, and antioxidant can effectively increase the electrical breakdown strength. In addition, aromatic ketone voltage stabilizer and hindered phenol antioxidants of the studied molecule can be grafted to the polyethylene chain easily during the polyethylene UV radiation cross-linking process, and they have excellent compatibility with the polymer matrix. The investigation is expected to provide reliable information for optimizing the polyethylene UV radiation cross-linking process and for the development of insulation material for high-voltage cable for use at voltages exceeding 500 kV in real applications.

Space charge and conductivity characteristics of CB/XLPE nanocomposites

In order to improve the DC dielectric properties of cross-linked polyethylene (XLPE) insulation materials, carbon black (CB)/ XLPE nanocomposites were prepared with the melt blending method. The pulsed electro-acoustic (PEA) method was used to measure the space charge distribution of XLPE and CB/XLPE nanocomposites. The relationship between the DC electrical conduction and the applied electric field strength of each material was measured under several constant temperatures. The research results show that the amount of space charges significantly reduces when a small quantity of CB was added into XLPE. The ability of composites to inhibit space charges is strong when the CB content is 1 phr. It was found that the electrically conductive properties of XLPE present nonlinear characteristics under lower electrical field intensity. The electrical conductivity significantly increases with temperature. The electrical conductivity of the nanocomposites with CB is almost a constant value under the electrical field below 20 kV/mm, and the effect of temperature on the DC electrical conductivity obviously decreases, when temperature goes up. The addition o CB into the XLPE can effectively inhibit the internal space charge accumulation and improve the DC conduction characteristics of the composites.

A density functional theory study of the role of functionalized graphene particles as effective additives in power cable insulation

The role of a series of functionalized graphene additives in power cable insulation in suppressing the growth of electrical treeing and preventing the degradation of the polymer matrix has been investigated by density functional theory calculations. Bader charge analysis indicates that pristine, doped or defect graphene could effectively capture hot electrons to block their attack on cross-linked polyethylene (XLPE) because of the π–π conjugated unsaturated structures. Further exploration of the electronic properties in the interfacial region between the additives and XLPE shows that N-doped single-vacancy graphene, graphene oxide and B-, N-, Si- or P-doped graphene oxide have relatively strong physical interaction with XLPE to restrict its mobility and rather weak chemical activity to prevent the cleavage of the C–H or C–C bond, suggesting that they are all potential candidates as effective additives. The understanding of the features of functionalized graphene additives in trapping electrons and interfacial interaction will assist in the screening of promising additives as voltage stabilizers in power cables.

Research on DC dielectric properties of polyaniline nanofibers/LDPE composites

In this article, polyaniline (PANI) nanofibers were prepared and added to low-density polyethylene (LDPE) to produce PANI nanofibers/LDPE composites. LDPE and the composites were tested for direct current (DC) conductivity, breakdown strength, and space charge characteristics. The results suggested that DC breakdown strength of PANI nanofibers/LDPE composites significantly declined once PANI was added, and the decline was more evident with the increase of PANI nanofibers. Meanwhile, the addition of PANI nanofibers contributed to a decrease in the conductivity of LDPE. As the content of PANI nanofibers increased, the conductivity of the composites declined first and then raised. DC conductivity properties of LDPE could be improved by adding an appropriate amount of PANI nanofibers. Compared with LDPE, the space charge distribution was changed in LDPE due to the addition of PANI nanofibers. With the increase of content of PANI nanofibers, the amount of space charges close to the electrodes decreased gradually.

Electric field control in DC cable test termination by nano silicone rubber composite

The electric field distributions in high voltage direct current cable termination are investigated with silicone rubber nanocomposite being the electric stress control insulator. The nanocomposite is composed of silicone rubber, nanoscale carbon black and graphitic carbon. The experimental results show that the physical parameters of the nanocomposite, such as thermal activation energy and nonlinearity-relevant coefficient, can be manipulated by varying the proportion of the nanoscale fillers. The numerical simulation shows that safe electric field distribution calls for certain parametric region of the thermal activation energy and nonlinearity-relevant coefficient. Outside the safe parametric region, local maximum of electric field strength around the stress cone appears in the termination insulator, enhancing the breakdown of the cable termination. In the presence of the temperature gradient, thermal activation energy and nonlinearity-relevant coefficient work as complementary factors to produce a reas...