Ln_1.5Sr_0.5NiO₄（Ln=La、Nd） Polyvinylidene Fluoride Dielectric Composites

Polymer-based dielectric composites can combine the advantages of traditional dielectric ceramic and dielectric polymer, indicating the great potential as novel dielectric materials in microelectronics. Insulate polymer is used as the matrix in polymer-based dielectric composites, and the filler can be dielectric ceramic or conductive material, while the combination of high permittivity and low dielectric loss is a key problem. In this thesis, semi conductive Ln1.5Sr0.5MO4 (Ln=La、Nd) ceramic particles with intrinsic giant permittivity were used as fillers, and the dielectric characteristics and mechanisms of Ln1.sSr0.5NiO4/polyvinylidene fluoride (PVDF) composites were investigated, in which the combination of high permittivity and low dielectric loss was obtained.Nd1.5Sr0.5NiO4 (NSNO)/PVDF dielectric composites were prepared by hot pressing and the dielectric properties were characterized at 213-403 K and 1～107 Hz. The composites showed typical percolative behaviors due to the semiconductivity of Ndi.5Sro.5Ni04 filler, for which the AC conductivity and permittivity increased rapidly with the filler volume fraction near to the percolation threshold (0.276). Different dielectric relaxations were observed when the filler volume fraction was below and above the percolation threshold, and they were attributed to the β relaxation of PVDF matrix and thermal-activated small polaronic hopping of Nd1.5Sr0.5NiO4 filler, respectively. Different from other polymer-based dielectric composites, the present composites exhibited both temperature-stable giant dielectric constant and relatively low dielectric loss above the percolation threshold, which benefited from the intrinsic giant permittivity of Nd1.5Sr0.5MO4 filler. When the filler volume fraction was 0.35, the optimal dielectric properties with permittivity of 9,300-12,000 and dielectric loss of 0.33-0.41 at 1 kHz were obtained in the temperature range of 250-350 K, indicating the great potential of the present composites as embedded capacitors.In order to investigate the microwave dielectric properties of polymer-based dielectric composites, the modified metal cavity method was developed to evaluate the microwave dielectric properties of high loss materials. In this method, small-size sample under test and large-size low-loss reference ceramic were used to reduce the overall loss of the resonant system, so that the measurement for high-loss sample could be conducted. Compared to other methods, this method has the advantages of high accuracy and easy preparation of samples. This method was used to measure the microwave dielectric properties of CaCu3Ti4O12, Ba(Fe1/2Nb1/2)O3 and Sr(Fe1/2Nb1/2)O3 giant permittivity ceramics, and the effects of sintering temperature and annealing atmosphere were focused. Their dielectric constant showed slight frequency dependence, and was insensitive to the preparing conditions, while strong frequency dependence and sensitivity to preparing conditions were observed for dielectric loss. Microwave dielectric loss increased significantly with sintering temperature for CaCu3Ti4O12, while the annealing atmosphere had more notable effect on the dielectric loss of Ba(Fe1/2Nb1/2)O3 and Sr(Fe1/2Nb1/2)O3, indicating the different extrinsic polarization mechanisms at microwave frequencies for the three ceramics.In the further work, La1.5Sr0.5NiO4 (LSNO)/PVDF composites were prepared by hot pressing, and the dielectric properties were characterized up to microwave frequencies at room temperature. The dielectric properties of LSNO/PVDF composites were similar to NSNO/PVDF composites, and the permittivity increased rapidly with the filler volume fraction near to the percolation threshold (0.305) at 1 kHz. However, the permittivity near to the percolation threshold increased more slowly with the filler volume fraction for higher frequency, and it fitted the effective medium theory better rather than percolation theory, indicating that the contribution of interfacial polarization to permittivity was weakened with the increase of frequency. The dielectric properties were dominated by intrinsic mechanisms of LSNO at higher frequencies, while by both interfacial polarization and intrinsic mechanisms at lower frequencies.