In this paper the thermal conductivity of epoxy-based composite materials is analysed. Twoand three-phase Lewis-Nielsen models are proposed for fitting the experimental values of the thermal conductivity of epoxy-based polymer composites. Various inorganic nano-and microparticles were used, namely aluminium oxide, aluminium nitride, magnesium oxide and silicon dioxide with average particle size between 20 nm and 20 µm. It is shown that the filler-matrix interface plays a dominant role in the thermal conduction process of the nanocomposites. The two-phase model was proposed as an initial step for describing systems containing 2 constituents, i.e. an epoxy matrix and an inorganic filler. The three-phase model was introduced to specifically address the properties of the interfacial zone between the host polymer and the surface modified nanoparticles.
By changing the ratio of resin to hardener, a series of epoxy resin samples has been produced with differing network structures and different retained chemical functionalities. The resulting materials were characterized by thermal analysis, dielectric spectroscopy, DC conductivity, and DC and AC breakdown strength measurements, to explore the effect of network structure and chemical composition on molecular dynamics and electrical properties. Differential scanning calorimetry showed that the glass transition temperature is primarily determined by the crosslinking density and indicates that, under the range of conditions employed here, side reactions, such as etherification or homopolarization, are negligible. Conversely, changes in DC conductivity with resin stoichiometry appear to occur as a result of changes in the chemical content of the system, rather than variations in network structure or dynamics. Specifically, we suggest that the DC conductivity is markedly affected by the residual amine group concentration in the system. While DC conductivity and DC breakdown appear broadly to be correlated, AC breakdown results indicated that this parameter does not vary with changing stoichiometry, which suggests that the AC and DC breakdown strengths are controlled by different mechanisms.
IntroductionCross-linked polyethylene (XLPE) has replaced oil-paper insulated systems as the primary solution for medium and high voltage ac cables decades ago, since they enable marginally higher operating temperatures and can be produced with high throughput and well-controlled extrusion technology [1]. The base for this insulation is polyethylene (PE), which can be crosslinked either with peroxide cure (involving thermal decomposition), or by grafting silane onto the polymer chains, and the use of moisture based cure [2]. Crosslinking is deemed necessary, since commercially available, branched low density polyethylene (LDPE) has more significant melting at temperatures around 100 ºC and the material loses all of its mechanical stability. In contrast, linear high density PE (HDPE) has a higher melting point and can achieve higher operating temperatures. But while HPDE found some success in medium voltage cables, it has not managed to establish a foothold in the high voltage cable sector [3]. After decades of research, conventional XLPE is at the limit of its capabilities, as outlined below, and further development is bound to have diminishing returns. Further, the costs of large extrusion and catenary crosslinking manufacturing facilities and the costs and time of degassing larger cross-section HV and EHV cables present significant sustainability issues for cable manufacturers.
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