Introduction

Polymer dielectric is thermally insulating and has a thermal conductivity under 0.5 W/(m·K). The heat generated by overload operation or partial discharge could lead to the temperature rise of insulating materials, which would cause the loss of dielectric performance gradually. With the development of new materials and technology, the voltage level and capacity of the transformer were improved constantly and then the overheat problems became more serious. Moreover, a considerable portion of fault in transformer is believed to be due to thermal accumulation. The development of materials with high thermal conductivity has provided a new approach to reduce operating temperature and then prolong the service life.

Intrinsic thermal conductive polymer dielectrics

Generally, there are two ways to improve the thermal conductivity of the polymer. Usually, crystal has relatively high thermal conductivity due to its highly ordered structure. So change in the molecule structure of the polymer to form crystal-like structure could improve the thermal conductivity. The crystal-like structure could reduce phonon scattering, so the ability of conducting heat would be improved for the polymer.

Hitachi and Hitachi Chemical have recently developed a novel method for increasing the thermal conductivity of epoxy resin by controlling its higher-order structure. Fig. 1 is a schematic of the higher-order structure of the developed epoxy resin. It shows three main features: (i) microscopic anisotropy with crystal-like structures of oriented mesogens in di-epoxy monomers, (ii) macroscopic isotropy induced by randomly oriented domains of crystal-like structures and (iii) indistinct boundaries between the crystal-like regions, each connected with an amorphous region by covalent bonds. The highly ordered structure would be expected to suppress phonon scattering so that the resin should have high thermal conductivity. Another property resulting from the highly ordered structure is high flexibility, which is important for resins used in manufacturing. The amorphous regions improve mould and process ability but tend to reduce thermal conductivity.

Thermal conductivities of epoxy are in the range 0.25–.96 W/ (m·K), which is ∼1.5–5.0 times greater than the conventional ones and have the highest thermal conductivities of all macroscopically isotropic organic insulating substances [0.46–0.51 W/(m·K)]. Fig. 2 shows atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of a conventional resin and of the developed resin. The TEM image shows clearly that the latter has a lattice structure, whereas large domains with sizes of several micrometres are seen in the AFM image.

Studies have shown that the orientation stretching of the polymer can effectively reduce the phonon scattering, and thus significantly improve the thermal conductivity of the polymer. Nysten studied the thermal conductivity of semi-crystalline oriented polymers and proposed the thermal-transfer mechanisms along and across the chains by measuring the thermal conductivity parallel and perpendicular to the polymer chains as a function of temperature.

When polyethylene (PE) is stretched to more than 25 times, thermal conductivity of the direction parallel to the molecular chain is 13.4 W/(m·K) at room temperature. The thermal conductivities of unidirectional gel-spun PE fibre-reinforced composites have been measured parallel (K∥ ) and perpendicular (K⊥) to the fibre axis from 15 to 300 K. The thermal conductivity of gel-spun PE fibre at 300 K is 38 and 0.33 W/(m·K) along and perpendicular to the fibre axis. The axial thermal conductivity is exceptionally high for polymers and this high value arises from the presence of a large fraction of long (>50 nm) extended chain crystals in the fibre.

Some researchers studied the thermal conduction anisotropy in polymers by reviewing currently available theories and experimental methods for studying oriented polymers. The anisotropic thermal conductivity and diffusivity of oriented polymers originate from the difference between the thermal energy transport mechanisms parallel and perpendicular to their molecules. Kato prepared the liquid crystal acryl compounds which the molecular directions were aligned by the rubbing method. The compounds were polymerised by ultraviolet irradiation and the free-standing aligned films of 200 μm thickness were made. The relation between the thermal conductivity and the aligned molecular direction of the films was investigated. The homogeneous film showed the largest magnitude of the thermal conductivity at the direction along the molecular long axis [0.69 W/(m·K)], which was 3.6 times greater than that of poly(methyl methacrylate). It is indicated that the additional thermal transmission effect, which the increase of the thermal conductivity may be induced, would exist in the twisted films.