Energy Storage And Electron Irradiation Damagebehavior Of TDPA-Ba TiO₃/PVDF Dielectric Nanocomposites

Poly(vinylidenefluoride)(PVDF)-based polymer has been widely employed in the electrical industry as insulators or capacitors. In recent years, the need for effcient transient energy storage has motivated research for better materials capable of accumulating a large charge per unit volume when compared to current state of the art technology. Although many materials possess either high permittivity or high dielectric strength, the combination of these performances is needed for the development of new materials with large recoverable energy storage. Furthermore, the polymer film capacitor exhibits great potential application in space weapon system. The safety design and practical performance of device benefit from the the study of electron irradiation on the dielectric material in the capacitor. In this thesis, the effect of annealing effect and uniaxial stretching on the crystal structure and dielectric property of PVDF is researched to explore the relationship between the electroactive phase and dielectric constant. Nano-sized barium titanate with high dielectric constant is introduced into PVDF matrix to increase the dielectric property of nanocomposite. In addition, the functionalizatio n of nanoparticle surface has beed investigated to improve the dispersion of nano-fillers and the compatibility between the inorganic particles and the organic host, and finally obtain the nanocomposite with high breakdown strength and energy density. The effect of nanoparticle on crystallization of PVDF is studied by analysis the crystallization behavior and kinetics of nanocomposite film with DSC method. Also the irradiation of nanocomposite is studied to provide the general solution for the applications in the space.The influence of annealing temperature and heat-retained time on the crystal structure and dielectric property of PVDF is investigated. The effect of stretching conditions, such as stretching ratios, temperatures and rates of extension, on th e relative fraction of β-phase and electron structure as well as dielectric properties of PVDF films was investigated. The fraction of β-phase in PVDF film increases greatly and reaches the peak of 93% after uniaxial stretching. Meanwhile, stretched spherulites and micro-strips paralleling with the stretching direction were observed in the matrix by atomic force microscopy(AFM). The results of fine structure of fluorine and carbon elements in PVDF, identified by X-ray absorption near edge structure(XANES), indicate that the coordination between F and H atoms of the adjacent chains is produced and thus enhances the dielectric response of the stretched film. As a result, the dielectric constant of the stretched film increases by 50% and achieves up to 12.1, as well as the dielectric loss being as low as 0.02. The linear relationship between the electroactive phase and dieletric constant is confirmed. Tuning the dielctric constant is realistic by controlling the concentration of β phase in the PVDF film.Surface-functionalization of Ba Ti O3 nanoparticles and subsequent utilization of simple solution casting to yield the good compatible PVDF nanocomposites were investigated. 1-Tetradecylphosphonic acid(TDPA) was synthesized and then used to functionalize the surface of Ba Ti O3 nanoparticles. The prepared TDPA-Ba Ti O3 nanoparticles were characterized by Fourier transform infrared spectroscopy(FTIR), X-ray photoelectron spectroscopy(XPS) and X-ray diffraction(XRD). These results indicated that the surface of nanoparticles was grafted successfully with such phosphonic acid. Also, the existing ligands of surface were examined, and the coverage onto surface of Ba Ti O3 nanoparticles evaluated by thermogravimetric analysis(TGA) was 25.1%. The dispersion of modified nanoparticles in ethanol and dichloromethane was significantly improved. Besides of good flexibility, PVDF nanocomposite film with volume fraction of 40 vol.% modified nanofillers exhibited high dielectric constant and low dielectric loss.The transformation of α to β-phase in PVDF induced by the addition of TDPA-Ba Ti O3 nanoparticles,and subsequently the isothermal crystallization kinetics of pristine PVDF and its nanocomposites have been investigated. The result of infrared spectra showed that the relative crystalline fraction of β-phase was enhanced greatly after the introduction of TDPA-Ba Ti O3 nanoparticles, and reached the peak of 93% when the concentration of nanofillers was 20 vol.%. The interaction between TDPA-Ba Ti O3 nanoparticles and PVDF macromolecular chains induced the change of conformation from trans-gauche to all-trans crystal structure in PVDF segment. The isothermal crystallization of TDPA-Ba Ti O3/PVDF nanocomposites was carried out by the differential scanning calorimetry(DSC). The influence of TDPA-Ba Ti O3 nanoparticles concentration on crystallization rate, activate energy, melting enthalpy and peak temperature were studied. The nanocomposite film loaded 20 vol.% TDPA-Ba Ti O3 nanoparticles exhibited the highest crystallization rate and activate energy, which decreased after loading more nano-fillers in the host because of high volume fraction of nanoparticles leading to steric hindrance and further weakening the mobility of PVDF chains during the crystallization. Three different kinetics models, Jeziorny, Ozawa and Mo models were applied to deal with the non-isothermal crystallization of PVDF and its nanocomposites. Jeziorny method was satisfactory only at the initial and the middle of the crystallization, and Mo method was fit in the entire nonisothermal crystallization.The effect of electron irradiation on the crystal structure and dielectric properties of PVDF films was investigated. XRD and DSC were employed to examine the degree of participation in PVDF and the relative fraction of electroactive phase as well as the thermal property of PVDF films. The results showed that both the exothermal peak temperature and the crystallinity of irradiated film decreased with increasing doses. Huge damage on the morphology of spherulite was observed in irradiated films by AFM. The results of fine structures in PVDF films characterized by XANES implied that the dominance of oxidation reaction occurred during the irradiation, and further induced the regular packed structure destroyed. The results indicate that β-phase exhibits good anti-irradiation ability during the electron irradiation on PVDF film. Because of large portion of electroactive phase maintaining and plus intermediate phases produced in the irradiated PVDF sample, the irradiated film exhibits relative high dielectric constant(around 7.8 under 2200 k Gy) with low loss. The irradiation affects the modified nanocomposite more than original one because the interaction between the surface modifier and PVDF macromolecular chains induces wide molecular weight distribution. Irradited nanocomposite film exhibits lower melting peak temperature, melting enthalpy and crystallinity, and finally decreases the dielectric constant of film.