Novel Surface Treatment Method Of Dielectric Fillers And Application In PVDF Based Flexible Dielectric Composites

Materials with high dielectric constant are of potential applications in various electronic and energy storage devices. Recently, with the growing trends of electronic appliances being miniaturization, portable, and high performance, as well as the transformation of electronic components from surface mount technology to be embedded in printed circuit board, dielectric materials with high performance are required. The polymer based composites are one of the intensively investigated materials for developing the next generation high performance dielectrics with. Although a great progress has been made in developing high dielectric constant and flexible materials, there is still a problem that high dielectric constant always accompanies with high dielectric loss. The main reason for high dielectric loss is lack of the interfacial compatability between filler and matrix. In this thesis, the effects of the surface modification methods on the dielectric properties, especially reducing the dielectric loss, are investigated. Follows are main results:1. Three kinds of core-shell structured organic-inorganic nanocomposites, BaTiO₃@PPFOMA, BaTiO₃@PTFEMA and BaTiO₃@PMMA, were synthesized via surface initiated atom transfer radical polymerization. The effects of polymer types, grafting contents, component ratios, and frequency on dielectric properties of the prepared composites were studied. The results showed that the dielectric constant of BaTiO₃@PPFOMA changed slightly with the increasing of frequency, e.g., the attenuation was only 3.9% when in the range of 10 Hz-10 MHz. The dielectric constant was increased and dielectric loss was decreased with the decreasing of grafted PPFOMA content. For example, for pure PPFOMA polymer, the dielectric constant was 2.6 and dielectric loss was 0.045 at 100 kHz. However, for BaTiO₃@PPFOMA with 30.2% grafted PPFOMA, the dielectric constant was 7.6 and dielectric loss was 0.07 at the same frequency. Meanwhile, the dielectric loss at high frequency （δ=0.002 at 10 MHz） was lower than that at low frequency （δ=0.008 at 10 Hz）. For BaTiO₃@PMMA/PVDF or BaTiO₃@PTFEMA/PVDF polymer based composites, it showed a significant increase of dielectric constant and decrease of dielectric loss with the increasing of BaTiO₃ loading. Moreover, with the same BaTiO₃ content, the thinner the shell was, the bigger the dielectric constant was. For example, when BaTiO₃ loading was 80%, dielectric constant of BaTiO₃@PTFEMAl/PVDF composites was 34 at 100 kHz, in which the shell thickness of PTFEMA was 4.5 nm. However, dielectric constant of BaTiO₃@PTFEMA2/PVDF composites was 26 at 100 kHz, in which the shell thickness of PTFEMA was 5.5nm. Also, it can be observed that different grafted polymer has different effect on dielectric constant of composites. In detail, from 10 Hz～100 kHz, the attenuation of dielectric constant was 16.6% for BaTiO₃@PMMAl/PVDF and 10.7% for BaTiO₃@PMMA1/PVDF composite, in which the grafted content of PMMA1 was 5.5% and PMMA2 was 8.5%, respectively. However, the attenuation of dielectric constant was 5.55% for BaTiO₃@PTFEMAl/PVDF and 4.0% for BaTiO₃@PTFEMA2/PVDF composite, in which the grafted content of PTFEMA1 was 1.5% and PTFEMA2 was 2.0%, respectively.2. The effects of fluorosilane 1H,1H,2H,2H-Perfluorooctyltrimethoxysilane （FAS-13, CAS:85857-16-5） and 1H,1H,2H,2H-Perfluorodecyltriethoxysilane （FAS-17, CAS:101947-16-4） modified BaTiO₃ nanoparticles on dielectric properties of PVDF based polymer composites were studied. The results showed that compared with untreated BaTiO₃ nanoparticles, the addition of modified BaTiO₃ nanoparticles enhanced the interfacial force between filler and matrix, and lowered the attenuation of dielectric constant. At 80% BaTiO₃ content and in the range of 10Hz～100 kHz, the attenuation of dielectric constant was 5.76% for BaTiO₃-F13-15/PVDF and was 8.02% for BaTiO₃-F17-15/PVDF; however, that was 35.71% for untreated BaTiO₃/PVDF. Meanwhile, at the same BaTiO₃ content, the dielectric loss of fluorosilane modified BaTiO₃/PVDF was lower than that of BaTiO₃/PVDF composites, and dielectric loss kept at a low level （0-0.06）. Meanwhile, the dielectric constant increased and dielectric loss kept stable with the increasing of fluorine content on the surface of BaTiO₃ nanoparticles, e.g., the dielectric constant is 27 and dielectric loss is 0.035 for BaTiO₃-F 17-5/PVDF composite at 100 kHz when the BaTiO₃ content is 80%, and dielectric constant is 35 and dielectric loss is 0.032 for BaTiO₃-F17-10/PVDF composite under same condition. The results showed the effects of BaTiO₃ modified with fluorosilane coupling agent on dielectric properties was equivalent to that of grafted with fluoride polymer, and offered us a simple and practical approach for filler modification.3. On the basis of above research, PVDF based composites filled with fluorosilane coupling agent modified BaTiO₃ nanofibers was fabricated. The results showed that the dispersity of modified BaTiO₃ nanofibers was better than that of the untreated ones. Meanwhile, the dielectric constant of composite was increased and dielectric loss was decreased with the increasing of fluorosilane modified BaTiO₃ nanofibers content. For example, the dielectric constant increased two times and dielectric loss kept at 0.06 when the volume fracture of fluorosilane modified BaTiO₃ nanofibers was 20%, which offered the composite good flexibility.4. Two kinds of hybrid fillers, i.e. graphene NP-BaTiO₃ and graphene NP-Ag, were prepared, in which graphene nanoplatelets were respectively loaded by BaTiO₃ nanoparticles and Ag nanoparticles. Both of them were compounded with PVDF to prepare polymer based dielectric composites, respectively.The results showed that graphene NP-BaTiO₃ and graphene NP-Ag both can be dispersed uniformly in PVDF matrix. The BaTiO₃ and Ag nanoparticles can provide binding sites, which enhance the interfacial force between the nanoparticles and PVDF matrix. Graphene NP-BaTiO₃ can increase dielectric constant significantly, even at low filler content, e.g., even when the filler content was 3%, the dielectric constant of composites reached 300 at 100 kHz and composites kept good flexibility. Furthermore, dielectric constant of graphene NP-Ag/PVDF composites was affected significantly by the diameter of Ag nanoparticles. When the diameter of Ag nanoparticles was 50 nm, filler content of 4% have been closed to the threshold value of filling, dielectric constant at 100 kHz was 80, and composites kept good flexibility.