| Conductive polymer composites (CPCs) are new kinds of high performance and multifunctional materials which are prepared by mixing polymer and conductive fillers. The CPCs attracted substantial interests due to their vast applications, for instance, over current protectors, self-control heaters, sensors, stretchable conductors, and so on. This thesis focuses on the temperature dependence of electrical conductivity and the evolution of microstructure and mechanical properties under self-heating (Joule heat) and traditional external heating in terms of tunneling effect, AC electrical response, and the simultaneous measurements.The conductivity of composites with a carbon fiber (CF) content slightly higher than the percolation threshold was measured at increasing temperatures up to0℃. The tunneling effect was theoretically calculated using a rigorous non-parabolic potential barrier whereas the electrical hopping could not explain the temperature dependence of conductivity for the composites. The tunneling barrier width D and the surface area A were determined. For a ultrahigh molecular weight polyethylene (UHMWPE) matrix, good agreement between theoretical and experimental results was obtained using D=1.00nm and A=1.35to1.68nm at a30vol%CF content and using D=1.30nm and A=1.25to1.28nm2at a25vol%CF content. That is, almost perfect agreement between experimental and theoretical results was obtained by adjusting the parameters except over the temperature ranges in which the β and γ relaxation peaks appeared. Dynamic loss modulus and positron annihilation measurements were also conducted. However, a theoretical analysis that was derived using a parabolic potential barrier produced inconsistent results. That is, the tunneling barrier width D was less than the c-axis length of a PE crystal unit, and the surface area A was considerably less than the a-b plane area of a PE crystal unit.The filler content dependence of positive temperature coefficient (PTC) effect was studied for composites of carbon fibers (CF) filled with ultrahigh molecule weight polyethylene/low density polyethylene blend matrix (UHMWPE/LDPE) prepared by kneading method. The PTC effect of electrical resistivity was investigated by direct current (DC) and alternating current (AC) measurements over the frequency range of100to1065Hz from30to150℃. The onsets of PTC effect were found to be strongly depended upon the CF content even the melting behaviors were almost same for all composites. To interpret this phenomenon, a master curve of temperature-frequency-resistivity superposition was constructed for composites with different CF contents based on the AC resistivity. The CF content dependence of correlation length was related to the onset of PTC effect. The transitions of conductor-insulator were studied quantitatively by complex planes of AC impedance, and the calculated capacitances and resistances showed a similar PTC effect under DC. Based on the analysis of AC capacitance, the average distances between CFs were calculated using a plane capacitance model which varied with CF concentration and temperature, and the tendency was consistent to the PTC effect.Aiming to study the microstructure of CPCs under applied field and Joule heat, the simultaneous measurements of X-ray intensity, electrical current and surface temperature (Ts) as a function of time were realized by modifying the clamps in X-ray instruments for the UHMWPE/Nickel coated CF (NiCF) composites which were prepared by gelation/crystallization method. It is interesting to find that without melting flow, linear UHMWPE provided X-ray intensity curve from only amorphous halo at129℃(Ts, arisen by Joule heat) lower than the conventionally known melting point145℃. The calculated radial distribution function revealed the amorphous structure with disordered chain arrangement. The appearance of such amorphous phase was arisen by the phenomenon that the transferring electrons between overlapped adjacent NiCFs by tunneling effect struck together with X-ray photons and some of the transferring electron flown out from the gap to UHMWPE matrix collided against carbon atoms of UHMWPE. The impact by the collision caused disordering chain arrangement in crystal grains.According to the results of last section, the simultaneous measurements of dynamic tensile modulus, electrical current and Ts were also realized by modifying the clamps in viscoelastic spectrometer for UHMWPE/NiCF composites. The dynamic tensile modulus of UHMWPE/NiCF composites with10and4vol%NiCF contents under electrical field were measured in the frequency range of100-0.01Hz. The external bulk strain to place the sample in tension ensuring axial sinusoidal oscillation which had a peak deformation is0.067%to assure linearity for measuring correct dynamic tensile modulus and the conductivity of composites did not change during the measurement. The drastic descent of the storage modulus for a10vol%was verified in lower frequency range with elevating Ts by self-heating (Joule heat). The composite was cut, when Ts was beyond108℃. Incidentally, the measurements by external heating could be done up to130℃for10vol%, but the composite was elongated beyond the above temperatures. Such different properties were analyzed in terms of crystal dispersions, electrical treeing, and thermal fluctuation-induced tunneling effect.The effect of electrical beam (EB) irradiation on the self-heating properties and microstructures was studied for nanocomposites of high density polyethylene (HDPE) and vapor grown carbon nanotubes prepared by kneading method. The composites were studied by DSC, TGA, TMA, and DMA before and after EB radiation. The results showed that the gel content and the thermal decomposition temperature increased and the thermal expansion decreased with increasing the dose. As a result, the surface temperature (Ts) was heightened and the heat efficiency were improved after irradiation. And the mechanical properties of the composite were significantly improved after irradiation, especially at the temperatures which were higher than the HDPE melting point, indicating the improvement of the thermal stability by irradiation crosslinking. It made the irradiated HDPE/CNT composites good candidates for application in the electrical heating. |
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