Research On The Application Of Polymer-derived SiCN Ceramics For High-dielectric Constant Composite Films

As one of the most common electronic components,capacitors are of great importance in the energy storage areas.Recently,fully solid-state multilayer capacitors have become the focus due to their characteristics of high energy storage density,charge and discharge quickly,safety and no pollution.In order to obtain capacitors with good performances,the dielectric materials with good flexibility,easy to process and have a high dielectric constant（?）is of vital importance.In recent years,a lot of contributions and efforts have been paid to meet these require.One method is to disperse high ceramic particles,such as barium titanate（BaTiO₃）and calcium copper titanate(Ca Cu₃Ti₄O₁₂),etc.into the polymer matrix.Although great progress has been made in recent years,there still some problems to be solved.For example,the?of composites is still very limited,it is hard to meet the growing practical needs.Moreover,in order to achieve a high?,high ceramic contents（?50vol%）are usually required,which results in composites with poor flexibility.Therefore,researching and developing novel filler for high?polymer-matrix composites is especially significant.Polymer-derived SiCN ceramics have a high?,especially their low frequency?is much higher than the traditional BaTiO₃ ceramics.It is a potential dielectric material with wide application in energy storage field.In this work,a commercially available liquid polysilazane（PSN,provided by institute of chemistry,Chinese academy of sciences）was used as precursor,polymer-derived SiCN ceramic were prepared by PDC technique.HighεSiCN/polymer composites were prepared by different processes and with different polymer matrix.The dielectric properties of the SiCN/polymer composites were investigated under a wide frequency range from 10^-1Hz to 10⁶Hz.The main contents and results of this thesis were listed as follow:（1）A novel polymer-derived silicon carbonitride（SiCN）-filled polyvinylidene fluoride（PVDF）composites were prepared by the tape-casting method.For comparison,commercial BaTiO₃-filled PVDF composites were synthesized following the same process.Their dielectric properties were investigated over a broad frequency range(10^-1～10⁶Hz).The results show that the SiCN/PVDF composites have much higher?than the BaTiO₃/PVDF composites,espacially at low frequencies.The?of the 40%SiCN/PVDF composite at 100Hz（54）was 474%higher than that of pure PVDF（9.4）and 67.2%higher than that of BaTiO₃/PVDF（32.3）.At 0.1Hz,the?of the 40vol%SiCN/PVDF composite was as high as2600,which was ca.23.9 times higher than that of BaTiO₃/PVDF（109）.Although the dielectric breakdown strengths of the SiCN/PVDF composites were slightly lower than those of the BaTiO₃/PVDF composites,the calculated maximum energy storage density of the 40vol%SiCN/PVDF composites(17.5 J·cm^-3)was much higher than that of 40vol%BaTiO₃/PVDF(0.773 J·cm^-3)at 0.1Hz.（2）SiCN/PI composites with differentvolume fractions of SiCN ceramics were prepared by in situ polymerization.The resulting SiCN/PI composite showed good thermal stability below 500?C,with the SiCN filler being homogeneously dispersed in the PI matrix.The dielectric properties of the SiCN/PI composites were investigated over a broad frequency range(10^-1Hz～10⁶Hz).The dielectric properties of the SiCN/PI composites at low frequencies revealed a weak percolation phenomenon similar to that shown by conductive/polymer composites.The?at 0.1Hz increased from 9.6 to 157 upon increasing the SiCN content from 15 to 20vol%.At 25vol%SiCN,?reached a maximum of 190,which is 57 times higher than that of pure PI（3.3）.The calculated percolation threshold was ca.22.8vol%.Unlike conductive filler/polymer composites,which readily form a conductive network when approaching the percolation threshold,the conductivity of the 25vol%SiCN/PI composite was as low as 8.5?10^-10 S·cm^-1,revealing excellent insulation characteristics.This ensures the reliability of SiCN/PI composites in practical applications.（3）Regular spherical-shaped and amorphous SiCN ceramic powders were fabricated via a facile hydrotherma method combined with a polymer-derived ceramic（PDC）process.In the first step,polymer microspheres were cross-linked from the solvothermal product of the liquid polysilazane precursor.The effects of reaction conditions,（including solvent proportioning,dosageof the DVB,concentration of the PSN,hydrothermal temperature and hydrothermal time）on the morphology and size of the products were studied.Next,the polymer microspheres converted to ceramic particles by pyrolysis technique.The XRD and SEM analysis on ceramic particles pyrolyzed below 1400°C showed that they were amorphous and maintained a smooth spherical morphology.When pyrolyzed at 1500°C,crystalline SiC/Si₃N₄ ceramic was fabricated and the spherical structure was partly destroyed.（4）Spherical polymer-derived SiCN ceramics were mixed into the polyvinylidene fluoride（PVDF）polymer host,SiCN/PVDF composites were synthesized by hot-press method.The dielectric properties of the SiCN/PVDF composites were investigated by broadband dielectric spectroscopy from 10^-1Hz to 10⁶Hz.The results revealed that the SiCN/PVDF composites exhibited superior low-frequency dielectric properties（higher dielectric constant and lower dielectric loss）to those of many other ceramic-filled PVDF composites.Composites filled with 40vol%SiCN had a high dielectric constant of 360 at 0.1Hz,which is 20 times the value of pure PVDF（18）,a low dielectric loss of 1.5 and excellent insulation.The dielectric constant of the composites at 0.1Hz fit the effective medium theory（EMT）model well.（5）Hydroxylated SiCN particles（H-SiCN）were obtained by treating spherical SiCN with hydrofluoric acid and hydrogen peroxide solution successively.The SEM of the cross-section of the H-SiCN/PVDF composites showed that the interfacial bond between H-SiCN and PVDF matrix was enhanced,which could be attributed to the formation of hydrogen bond between the surface hydroxyl group of SiCN and PVDF.The dielectric property test shows that the dielectric constant of the H-SiCN/PVDF composite is slightly higher than that of the SiCN/PVDF composite,and the dielectric loss is significantly reduced,the breakdown strength of the H-SiCN PVDF composite was also increased.When thevolume fraction of SiCN is 20vol%,the maximum energy storage density of SiCN/PVDF composite material is6.07 J·cm^-3,while the energy storage density of H-SiCN/PVDF composite is 12.23 J·cm^-3.It indicates that good interface bonding can effectively improve the dielectric properties and energy storage density of composites.