Novel Dielectric Foam Ceramics And Their Cyanate Ester Resin Composites With High Dielectric Constant

With the rapid development of microelectronic technology,higher demand has put forward about the miniaturization and signal stability of electronic devices.Due to the ability of storing electric energy and balancing the distribution of electric field,high dielectric constant?high-k?materials show great potential in the fields of microelectronics,energy,electrical engineering,biomedical engineering,aerospace and so on.Ceramic/polymer composite is an important type of high-k materials.However,in order to obtain desirable high-k,the ceramic content generally should be higher than 50 vol%;this often brings deteriorated preparation process and enlargened dielectric loss and leakage current.Therefore,fabricating novel ceramic/polymer composites with high-k,low dielectric loss and excellent stability of dielectric properties at a low ceramic loading has important scientific significance and application value.Our project reported herein focuses on this topic.First,barium titanate?BT?foam?BTF?was designed and prepared using high-k BT as raw material by a polymer foam replication method?PFR?.On this basis,BTF/cyanate ester?CE?resin composites were prepared.The preparation process of BTF and the integrated properties including thermal,dielectric and flame retardancy properties of BTF/CE composites were systematically evaluated and the mechanism behind was also discussed.Results show that BTF is unqiue foam ceramic with single chemical composition,three-dimensional interconnecting foam channels and high-k skeleton,and the BT content of BTF is adjustable through changing number of coatings of BT slurries.With the increase of sintering temperature or the number of coatings,the bulk density of BTF increases and the porosity gradually decreases.When the sintering temperature is controlled at 1200?,the resultant BTF has the best dielectric properties.In the BTF/CE composites,BTF is uniformly distributed in CE resin,has good compatibility with CE resin and retains its unique three-dimensional continuous structure.Compared with CE resin,BTF/CE prepolymer not only has about 100? lower curing peak temperature,but also shows higher thermal stability.When the BTF content is as low as 33.5 vol%,the dielectric constant of BTF6?1200?/CE composite reaches 141.3?100 Hz?,about 8.4 and14.1 times of those of traditional BT/CE and BTP powder?BTFp?/CE composites,respectively,meanwhile the dielectric loss of BTF6?1200?/CE composite is only about0.276.In addition,the presence of BT endows CE resin with good flame retardancy,and BTF?1200?/CE composites are flame retardant composites.Second,hyperbranched polysiloxane modified BTF?BTF@HSi?was prepared through in-situ hydrolysis and polymerization of?-glycidoxypropyltrimethoxy silane?KH560?.On this basis,BTF@HSi/cPES/CE composites were fabricated using phenolphthalein poly?ether sulfone??cPES?/CE as the matrix.Meanwhile,three kinds of composites,BTF/CE,BTF/cPES/CE and BTF@HSi/CE,were prepared for comparative study.The effects of interface and resin composition of dielectric ceramic/polymer composites on thermal and dielectric properties of composites were systematically studied.Results show that epoxy groups on the surface of BTF@HSi can catalyze theself-polymerization of CE and copolymerize with CE,and thus building good interfacial interaction with the matrix.The introduction of cPES into CE matrix effectively blocks the cracks caused by the stress concentration of composites and maintaining a complete 3D continuous network of BTF in composites.With the surface modification of BTF or?and?matrix modification,the composites have higher thermal stability and effectively improved dielectric properties.Specially,with the same amount of loading,BTF@HSi/cPES/CE composites have higher dielectric constant and lower dielectric loss.For BTF@HSi/cPES/CE composite with 34.1 vol%BTF,its dielectric constant at 100 Hz is as high as 162.0 and dielectric loss is only 0.007?@100 Hz?.In addition,the dielectric properties of BTF@HSi/cPES/CE composites exhibit excellent frequency stability.Third,novel BTF@HSi/CNT/cPES/CE composites were prepared through using BTF@HSi as the skeleton,carbon nanotube?CNT?as conductor and cPES/CE as the matrix.The effects of CNT content?f?on thermal,electrical conductivity and dielectric properties of composites were studied.Results show that the presence of CNT reduces the initial decomposition temperature(T_di),while the structure of the ceramic skeleton is favorable for high thermal stability.Compared with?BTF@HSi?p/CNT/cPES/CE composites,BTF@HSi/CNT/cPES/CE composites with the same f not only have lower percolation threshold?f_c?and dielectric loss,but also show higher electrical conductivity and dielectric constant.At 100 Hz,the dielectric constant of BTF@HSi/0.13CNT/cPES/CE compositereaches222.6,about4.6and4.2timesofthoseof?BTF@HSi?p/0.15CNT/cPES/CE?48.9?and BTF@HSi/cPES/CE?53.2?,respectively,while the dielectric loss of BTF@HSi/0.13CNT/cPES/CE composite is only 0.017.Fourth,novel foam ceramic?dBTF?with three-dimensional interconnecting skeleton was prepared through doping nano BT with Nb₂O₅,Na₂CO₃ and K₂CO₃ with one-step PFR.On this basis,dBTF/cPES/CE composites were prepared.The effects of morphology,filler loading and content of dopants on the thermal and dielectric properties of composites were systematically studied and compared with dBTF powder?dBTFp?/cPES/CE composites.Results show that the crystalline form of dBTF transforms from cubic phase into tetragonal one with the increase of the dopant content.When the ceramic loading is 31.5 vol%,the dielectric constant of 6dBTF5/cPES/CE composite reaches 99.1?100 Hz?,about 6.3 and24.8 times of those of 6dBTFp5/cPES/CE composite and cPES/CE,respectively;moreover,the dielectric loss is only about 0.008?100 Hz?.Specially,the dielectric properties of6dBTF/cPES/CE composites exhibit excellent temperature and frequency stabilities from-100 to 110?.Fifth,BN foam?BNF?was prepared using boron nitride?BN?with high thermal conductivity by PFR.On this basis,BNF/CNT/cPES/CE composites were fabricated.Compared with BNFp/CNT/cPES/CE composites,the effects of BN form and CNT content on electrical conductivity,dielectric properties and thermal conductivity of composites were studied.Results show that the porosity of BNF reaches 86.8%,and the interpenetrating three-dimensional skeleton is formed among BN.Sintering does not change the crystalline form?hexagonal structure?of BN;moreover,BN crystallization is enhanced.Compared with BNFp/CNT/cPES/CE composites,BNF/CNT/cPES/CE composites have lower f_c and dielectric loss,higher electrical conductivity and dielectric constant at the same f.At 100 Hz,the dielectric constant of BNF/0.15CNT/cPES/CE composite reaches 31.6 and the dielectric loss is 0.26,while the thermal conductivity reaches 0.80 W·m^-1·K^-1.