| Polymeric PTC composite is a novel functional material with importance both in research and application fields. This kind of composites can be widely applied in many fields such as self-regulating heaters, over-current protectors and sensors due to their advantages as thermoelectrical-switching characteristic, adjustable conductivity, light weight, corrosion resistance, low cost and simple processing technique. However, further research and improvement are necessary for the PTC materials in both theory and practical application respects, which focus on the conductive mechanism, enhance of PTC intensity, elimination of NTC phenomenon and improvement of stability and reproducibility at high temperature, low heat efficiency and so forth.Firstly, carbon fiber (CF) or nickel-coated carbon fiber (Ni-CF) filled ultra-high-molecular weight polyethylene (UHMWPE) composites were prepared by gelation/crystallization method. DSC, SEM, WAXS, DMA and gel fraction measurements were employed to investigate the electrical and mechanical properties of composites with respect to the effect of content of fillers, gamma ray irradiation and chemical crosslink. Results demonstrated that lower percolation threshold value of composites was obtained by NiCF than CF due to its higher conductivity. PTC effect of composites was improved by gamma ray irradiation, while high and stable conductivity and mechanical properties were realized by means of chemical crosslink method.Secondly, CF filled UHMWPE and ethylene-methyl methacrylate (EMMA) blend composites were prepared by gelation/crystallization method in solutions with xylene and decalin as solvents, respectively. Then DSC, SEM, WAXS, TGA and DMA were carried out to investigate systematically the effect of factors such as content of EMMA, CF and gamma irradiation on the properties of electrical, thermal, mechanical properties and morphology as well, and the mechanism of gamma ray irradiation was discussed in detail. Results demonstrated that crosslinking structure was formed in the composites after irradiation. PTC intensity of irradiated composites increased, which of composites with matrix ratio of UHMWPE to EMMA reached the maximum of 1010 at 500kGy. NTC effect was removed, and reproducibility was improved by irradiation.Thirdly, self-heating property of UHMWPE-EMMA-CF composites was discussed in detail. It revealed that surface temperature of composites with matrix ratio of UHMWPE to EMMA of 1/6 was higher than that of composites with matrix ratio of 1/1 at the same concentration of CF and the set applied voltage. The self-heating property of composites was enhanced significantly by gamma ray irradiation. Surface temperature of irradiated composites with 500kGy and matrix ratio of 1/1 was 30℃higher than that of original composites at applied voltage of 20V. The optimal irradiation dose for composites with matrix ratio of 1/1,1/3 and 1/6 was 1000kGy,500kGy and 200kGy, respectively.Finally, thermal volume expansion experiment of UHMWPE-based composites was conducted to explore the mechanism of PTC effect of composites. Results indicated that volume expansion was one of the important factors but not the only one for PTC effect. On the other hand, results of dielectric property of composites showed that resistivity and dielectric parameters of composites were depend strongly on temperature and frequency. Results of dielectric analysis were in good agreement with that of PTC effect in mechanism of electrical conductivity. Furthermore, it reflected the change of microstructure and the effect of gamma ray irradiation on the electrical properties. |
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