| As an energy storage device,dielectric capacitors play a vital role in electronics,electrical equipment,and systems,and the most important part is dielectric materials.Polymer dielectric materials have been widely used because of their high voltage withstand capability,low loss,easy processing,good flexibility,and other excellent properties.At present,the most widely used polymer dielectric material-biaxially oriented polypropylene(BOPP),its breakdown strength exceeds 700 MV/m,the low loss is less than 0.00018,and it also has good processing properties and low manufacturing costs.However,the lower dielectric constant(2.2)of BOPP also leads to its lower energy density(2 J/cm~3).At the same time,the maximum operating temperature of BOPP is only 105°C,which largely limits its use at higher temperatures.Under the application,such as aerospace,oil and gas exploration,supersonic aircraft,wind power generation system,etc.This thesis takes polyaramids with excellent heat resistance and good dielectric properties as the research object.Through molecular structure design,a series of amorphous and crystalline polyaramids are synthesized,and the introduction of different naphthalene ring positions and fluorine-containing groups are studied,the change of the aggregated structure affects the thermal and mechanical properties of the polymer and the dielectric energy storage performance at room temperature and high temperature.By comparing the properties of polymers with different chemical structures,an attempt was made to summarize the relationship between the intrinsic properties and structure of polymer dielectrics.The main research contents are as follows:First,by optimizing the polymerization reaction conditions,two polyaramids(1,4-PEMNA and 2,6-PEMNA)with different naphthalene ring substitution positions were synthesized.The molecular weights of the prepared polyaramids were 2.8×10~5 Da and2.4×10~5 Da,respectively.The transition temperatures are 238°C and 224°C,respectively,showing high heat resistance.At the same time,the polymer also has good thermal stability,and the 5%weight loss temperature exceeds 420°C.The dielectric properties and energy storage properties of polyaramid with different naphthalene ring substitution positions are studied in detail.Due to the smaller free volume,the dielectric constant of 2,6-PEMNA is higher than that of 1,4-PEMNA(4.3),which is 4.7.The dielectric loss of both is below 0.02.By comparing the breakdown strength,charge and discharge efficiency,and energy storage density of the two polyaramids at 150?,it is found that the performance of 1,4-PEMNA is better than that of 2,6-PEMNA,and its breakdown strength is 534 MV/m.The discharge energy density reached 2.2 J/cm~3while maintaining 90%charge and discharge efficiency.It can be seen from the above research results that for polyaramid systems,a high dielectric constant will bring high dielectric loss,and high dielectric loss is unfavorable for high-temperature breakdown strength,high-temperature energy storage,and other properties.Therefore,we selected large-volume fluorine-containing groups to reduce the overall polarization response,prepared fluorine-containing polyaramid(1,4-PEFMNA),and studied in detail the thermal and mechanical effects of the introduction of fluorine-containing groups on the polymer,dielectric,energy storage,and other performance.The molecular weight of the prepared polymer is 2.9×10~5 Da,which is close to 1,4-PEMNA,which eliminates the influence of molecular weight on thermal,mechanical,and dielectric properties.The introduction of fluorine-containing groups increases the glass transition temperature to 251°C.5%The thermal decomposition temperature reaches 450?,but it has little effect on the mechanical properties.The introduction of fluorine-containing groups also improves the breakdown strength at high temperatures and reduces the dielectric loss while maintaining a high dielectric constant,improving solubility and transparency.The dielectric constant of 1,4-PEFMNA at 1000 Hz is 4.1,and the dielectric loss is 0.008.At 150°C,the breakdown strength reached 579 MV/m,showing a higher energy storage density(2.44 J/cm~3)and high charge and discharge efficiency(>90%).To further reduce the dielectric loss and improve the breakdown strength at high temperatures,a crystalline polyaramid thin film dielectric material(1,4-PEENA)was prepared by the method of high-temperature solvent-induced crystallization.The prepared polymer has an intrinsic viscosity of 1.9,a glass transition temperature of248°C,and a tensile strength of 100 MPa.The dielectric constant of 1,4-PEENA is 4.0.It is particularly noteworthy that the presence of the crystalline region in the polymer can inhibit the relaxation of the amorphous region segment and reduce the participation of the amorphous region to reduce relaxation.At 1000 Hz,the dielectric loss of the polymer film is as low as 0.007.Also,due to the protection of the crystal region,the modulus attenuation of the polymer at 250°C is small,and at 300°C,it still has a certain modulus,which makes the polymer film's breakdown strength reach 615 MV/m at150°C,and show enhanced discharge energy density(4.9 J/cm~3)and high charge and discharge efficiency(>90%).Moreover,at 200?,it still shows high discharge energy density(2.2 J/cm~3)and high charge and discharge efficiency(90%).In summary,this thesis has studied in detail the effects of the naphthalene ring structure,fluorine-containing groups,and aggregate structure on the thermal,mechanical,and normal temperature,high-temperature dielectric,and energy storage properties of polyaramid.The crystalline polyaramid film was prepared by molecular structure control and high-temperature solvent induction method.The film exhibits excellent discharge energy density and high charge and discharge efficiency at 150°C,and still maintains good dielectric properties and energy storage at 200°C.These research work provides the theoretical basis and design guidance for the preparation of polyaramid film dielectric materials with excellent dielectric properties. |
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