With the miniaturization and lightweight of electronic equipment, graphite film materials with high thermal conductivity were widely concerned recently. In this paper, the preparation of polyimide (PI) based graphite film was reviewed, and the influence factors of their performance, which included molecular structure, molecular orientation, and the inducement of other materials, were introduced in detail. The research and patent situation of graphite film composite materials were summarized, and the future research and development direction were suggested and prospected.

In order to develop a high thermal conductive epoxy potting adhesive for dry-type transformer, we prepared a potting adhesive by adding self-made modified silica to epoxy anhydride system. The thermal conductivity and electrical insulation performance were tested, and the suitable pouring temperature and best curing process of the potting adhesive were studied. The results show that when the filling content is 75%, the potting adhesive has high thermal conductivity and excellent electrical insulation performance, the thermal conductivity is 1.494 W/(m·K), the dielectric loss factor is only 0.41%. When the pouring temperature is 70℃, the potting adhesive has good pouring process, the viscosity is low than 2 800 MPa·s within 2 h. The best pouring process for potting adhesive is 80℃ vacuum/0.5 h + 80℃/4 h + 90℃/3 h + 110℃/2 h + 140℃/5 h. Under the condition, the deposition of powder in potting adhesive is small, and the cured product has the best comprehensive performance.

Spherical alumina was introduced in drawing and filtering process of graphene nanosheets to build a binary porous structure of “pea-pod-like” alumina-graphene, and an alumina-graphene binary structure reinforced epoxy resin composite was prepared. Its thermal conductivity was tested, and the mechanism of “pea-pod-like” alumina-graphene binary structure enhancing the thermal conductivity of epoxy resin was analyzed. The results show that the horizontal arranged graphene generates partial orientation transformation under the action of spherical alumina, showing a “pea-pod-like” structure, in which graphene provides an efficient heat transfer channel for epoxy composites in plane and out of plane directions and greatly enhances the thermal conductivity of epoxy composites. The thermal conductivity of “pea-pod-like” binary alumina-graphene reinforced epoxy composite is up to 13.3 W/(m·K) and 33.4 W/(m·K) in plane and out of plane directions, respectively with 12.1% of graphene and 42.4% of alumina loading. The “pea-pod-like” alumina-graphene binary structure has a significant effect in improving the thermal conductivity of epoxy resin, which has potential application prospects in the field of electrical packaging.

Polymer materials such as epoxy resin have hidden dangers of thermal failure and insulation failure during long-term service since its low thermal conductivity. In this study, a high thermal conductive composite insulating material was prepared by filling micron boron nitride and nano alumina with high thermal conductivity and high insulation properties to epoxy resin, and the effect of filling amount and ratio of fillers on the thermal conductivity and insulation properties of composite materials were studied. The results show that when the total filling content is 30% and the mass ratio of micron boron nitride to nano alumina is 3∶1, the thermal conductivity, breakdown time, and imaginary part of complex permittivity (ε″) of the composite materials is 1.182 0 W/(m·K), 31.9 s, and 0.034, respectively, which is improved by 697%, 21.7%, and 406% compared with epoxy resin, respectively. The composite material has good resistance performance under high frequency and high electric field.

In order to improve the thermal conductivity and environmental resistance of motor insulation system, we analyzed the conventional properties, heat resistance, thermal conductivity, and environmental resistance of a high thermal conductive and high temperature resistant epoxy encapsulating resin. A prototype was made, and the application performance of the epoxy encapsulating resin in low voltage motor was tested. The results show that the epoxy encapsulating resin has excellent mechanical and electrical properties, excellent low temperature and thermal shock resistance, and good compatibility with enameled wire, and the thermal conductivity and temperature index reach 1.18 W/(m·K) and 187.5℃, respectively. The application of high thermal conductive insulating resin could effectively improve the thermal conductivity of the motor insulation system, under the same conditions, the temperature rise of motor decreases by 20.7℃ compared to the motor with ordinary high temperature resistant insulating varnish. At the same time, compared with the vacuum pressure impregnation process, the insulation system of the motor made by the vacuum encapsulation process has better integrity, electrical properties, and humidity resistance, and the disadvantage of low paint hanging at the groove is avoided.

The thermal conductivity of insulating varnish could be improved by adding inorganic oxide thermal conductive material into the epoxy modified unsaturated polyester resin. The effect of particle size and surface modification of fillers on the sedimentation of insulating varnish and effect of different filler content on the thermal conductivity and viscosity of insulating varnish were studied. A high thermal conductive and solvent-free insulating varnish was prepared, and its performance was characterized. The results show that the surface modification of filler could reduce its sedimentation rate. When the filler content is 40%, the viscosity of insulating varnish is 72 s, the thermal conductivity reaches 0.46 W/(m·K), and the insulating varnish exhibit no sedimentation at 40℃ after 6 days, which indicates that it has practical application value.

First of all, hexagonal boron nitride (h-BN) micro-powder and sericite micro-powder (Mica) were used as raw materials, boron nitride nanosheets (BNNSs) and mica nanosheets (MNS) were exfoliated by freeze-thaw cycle combined with ultrasonic technology. Then the BNNSs and MNS were used as insulating thermal conductive fillers, mica/boron nitride nanohybrid polyimide films (MNS/BNNS nanohybrid PI films) were prepared by in-situ polymerization and two-step water-based polyimide (PI) process. The influence of different MNS/BNNS filling amounts on the performance of PI composite films was studied. The morphology and structure of BN, BNNS, Mica, and MNS were characterized by XRD, TEM, and AFM, and the thermal conductivity, dielectric constant and electric strength of the MNS/BNNS nanohybrid PI films were measured. The results show that when the ratio between MNS and BNNSs is 1∶2, the MNS/BNNS nanohybrid PI films have better comprehensive properties, and the thermal conductivity is significantly improved compared with pure PI. The thermal conductivity is 0.743 W/(m·K), the electric strength is 246 MV/m, and the dielectric constant is 5.28.

The high thermal conductive insulation structure applied in H-class AC high voltage motor was studied. The coil samples with the insulation structure were conducted turn-to-turn impulse, withstand voltage to ground, dielectric loss factor, breakdown voltage, voltage durability (electrical ageing), thermal evaluation and classification (thermal ageing) experiments, and the thermal conductivity of several dry mica tapes were tested and analysed. The results show that the application of dry mica tape with high thermal conductivity could improve the thermal conductivity of insulation structure for motor, and the temperature rise reduces effectively. The insulation structure also show good electrical performance, and the temperature index reaches 181℃, which can meet the requirements of the insulation structure for H-class motor.

The increasingly developed microelectronic devices have put forward higher requirements on thermal conductive and electrical insulating thermal interface materials, and the silicone rubber composites with high thermal conductivity and electrical insulating can be prepared through construction of micro-nano hybrid heat conduction network. First, a nano-alumina coated graphene oxide was prepared by a simple and green self-assembly strategy, and it was reduced into TRGO@Al₂O₃ nano-hybrid filler by high temperature treatment. Then it was mixed with micron alumina and filled into liquid silicone rubber. The influence of different nano alumina coated amounts on the thermal conductivity and volume resistance of the system was studied by adjusting the ratio of nano alumina to graphene oxide. The synergistic thermal conductive effect of the micro-nano hybrid system was studied by controlling the ratio of nano-hybrid filler to micron alumina. The results show that when the 3% of TRGO@Al₂O₃ and 54% of micron alumina are compounded, the thermal conductivity of the composite reaches about 2.5 W/(m·K), and the volume resistance is greater than or equal to 10⁹ Ω·cm.

The thermal conductive and insulating composite material with polymer matrix and high thermal conductive filler is an ideal solution to settle insulation protection of live working equipment and heat dissipation problem of electrical and electronic equipment. In this study, micron alumina (Al₂O₃), surface modified by silane coupling agent KH550, mixed with high thermal conductive carbon nanotubes (CNT) as thermal conductive filler, silicone rubber (SR) with wide temperature range resistant and corrosion resistant was selected as polymer matrix, an SR composite material was prepared, and its performance was tested. The results show that when the total content of Al₂O₃/CNT mixed filler is 10%, the proportion of carbon nanotubes is 0.2%, the thermal conductivity of the SR composite is as high as 0.268 W/(m·K), which is improved by 103.1% compared with SR, the resistivity is 10.5×10¹² Ω·cm, the relative dielectric constant is almost unchanged, and the Shore hardness A and Young’s modulus increase slightly .

In order to study the effect of micron particle fillers on the thermal conductivity of filled type high thermal conductive composites, we constructed a finite element model of composites with randomly distributed particle fillers in this study. The effects of filling ratio, particle size, thermal conductivity, particle shape of filler on the thermal conductivity of the composites were calculated and discussed, respectively. The results show that with the increase of filling ratio and length-to-diameter ratio of filler particle, the thermal conductivity of the composites increases significantly. The particle filler size has little effect on the thermal conductivity without considering the interface thermal resistance and particle agglomeration. The thermal conductivity of filler has little effect on the thermal conductivity of the composites. Without considering the interface thermal resistance, whether the heat conduction channel can be formed effectively is the key to determine the thermal conductivity of the filled type composites.

In order to expand the new energy vehicle market and improve the power transmission efficiency of new energy vehicles, it is necessary to improve the current-carrying capacity of its internal cables. But at the same time, with the increase of load current, the heat dissipation problem of vehicle cable has become prominent. In this study, a finite element simulation model of vehicle cable considering electric field and thermal field was established by COMSOL software, the ampacity was solved by the Nelder-Mead method, and then the effectiveness of Nelder-Mead method was verified by the analytical methods. The improvement effect of the increase of insulation material thermal conductivity on the current-carrying capacity and heat dissipation of the vehicle cable was simulated analyzed. The results show that when a silicone rubber/boron nitride nanosheets (20%) composites with 0.833 W/(m·K) of thermal conductivity was used in vehicle cable, the ampacity of vehicle cable obtained by simulation increases by 5.12%, which can promote heat dissipation more significantly at overload, but it would increase the critical radius of insulation, resulting in a slight increase of conductor temperature when the insulation thickness decreases within the critical radius of insulation.

A polyacrylonitrile (PAN)/styrene-isoprene-styrene (SIS) composite fiber membrane was prepared by electrospinning method. The effects of different PAN/SIS ratios on its porosity, liquid absorption, thermal stability, mechanical properties were studied. The results show that when the ratio of PAN to SIS is 8∶2, the SIS/PAN composite membrane fiber prepared has the most cross-linked structures, uniform size, and the best mechanical properties. The tensile strength is 20.29 MPa, the porosity and absorption rate reach 47.8% and 310.7%, respectively, and the ionic conductivity is 2.03×10^-4 mS/cm. Under the condition of 0.2 C multiplier, the initial discharge specific capacity of Li-ion battery assembled by the composite fiber membrane is 146.4 mAh/g, the discharge specific capacity fluctuates little after 50 cycles, and the capacity retention rate is as high as 98.02%, showing good cycle stability.

Polyethylene cables would generate crack defects inevitably during long-term operation, which will cause partial discharge faults and threaten the normal operation of power grid. Polyethylene composite materials doped with microcapsules could realize the self-healing of cracks. In order to study the effect of microcapsules on the insulating properties of polyethylene materials before repair, we prepared pure polyethylene samples and polyethylene composite samples doped with different concentrations (0, 0.5%, 1%, 5%, 10%) of microcapsules, and their basic performance, volume resistivity, and AC electric strength were measured. The results show that the volume resistivity of composite samples doped with a small amount of microcapsules (≤1%) is significantly higher than that of pure polyethylene samples. But when the concentration is larger (>1%), the volume resistivity decreases. Compared with the pure polyethylene sample, the AC electric strength of the composite materials decreases, but when the concentration is no greater than 1%, the decrease amplitude is smaller, which could meet the normal operation requirements of cable. This is mainly related to the crystallinity of material, the interface effect between microcapsule and polyethylene, and the characteristics of microcapsule itself. In general, when the doping concentration of microcapsules is no greater than 1%, the insulating properties of the polyethylene composite material can meet the requirements of normal operation for cable.

A silicon carbide/organic montmorillonite/epoxy resin micro-nano non-linear corona resistant composite material was prepared. The influence of silicon carbide and organic montmorillonite content on the dielectric properties of the corona resistant material was studied. Bars were prepared using the corona resistant material, and their corona resistance and surface temperature were tested. The results show that the addition of a certain amount of nano organic montmorillonite could effectively improve the non-linear characteristic of the corona resistant composite, reduce the surface temperature of anti-corona area, and improve the corona resistance of bars.

In order to deeply grasp the breakdown characteristics of AC 500 kV cross-linked polyethylene (XLPE) submarine cable insulation materials under electrical-thermal stress and establish an electrical-thermal combined lifetime model, we conducted electrical-thermal breakdown experiments on the XLPE material at 25, 40, 55, 70℃ under step stress firstly. The AC electric strength and voltage duration time were analyzed by Weibull distribution to obtain the equivalent AC electric strength and voltage duration time at different temperatures. Then the FALLOU, SIMONI, CRINE models were established by multiple linear regression method, and their error was analyzed. Finally, an E-T lifetime model for the AC 500 kV XLPE material was constructed. The results show that at the same temperature, the equivalent electric strength decreases with the increase of voltage duration time of each voltage stage. At the same voltage duration time of each stage, with the increase of temperature, the equivalent AC electric strength and voltage duration time both firstly increase and then decrease. The analysis on the electrical-thermal lifetime models indicate that the fitting error of FALLOU, SIMONI, CRINE models is large, and their fitting goodness do not meet the accuracy requirements. An improved electric-thermal combined ageing lifetime model is obtained by using stepwise regression to calculate the significance and correlation between electric-thermal variables and lifetime, and the error analysis show that it has better fitting accuracy.

To investigate the key factors of design for thermo-expandable sheets, we made thermo-expandable sheets from two types of glass mat with different fiber diameter and PA thermoplastic film. Different thermo-expandable sheets were prepared by changing the fiber diameter, resin content and density, and their thermal expansion rate and effect were studied. The results show that the thermal expansion effect of thermo-expandable sheets is codetermined by the elastic potential energy of fiber and the viscous resistance of melting resin. Under the same composition and density condition, the thermal expansion rate of thermo-expandable sheets increases with the increase of fiber diameter. There is a critical resin content for thermo-expandable sheets. When the resin content is below the critical value, the thermal expansion rate increases with the increases of resin content. When the resin content is above the critical value, the thermal expansion rate decreases with the increase of resin content. The critical resin content decreases with the increase of fiber diameter. When the single fiber diameter and resin content is unchanged, the higher the density of thermo-expandable sheets, the greater the thermal expansion rate.