Study On Preparation And Dielectric Properties Of Polyimide-based All-organic Composite Materials

The dielectric capacitor is a kind of high-power energy storage devices with the advantages of fast charging and discharging capabilities,which can be widely used in many fields such as modern electronic equipment,national defense and military,and new energy.The performance of dielectric capacitors was significantly determined by the properties of the dielectric material.Therefore,the synthesis of dielectric materials with high dielectric constant,low dielectric loss,and high breakdown strength has attracted wide attention.At present,most researches use inorganic substances as fillers into a polymer matrix to synthesize inorganic/polymer composites.However,there are few studies on the synthesis of all-organic dielectric materials using organic molecules as fillers with excellent properties.In this paper,polyimide was used as matrix materials to prepare several all-organic dielectric materials with high dielectric constant and low dielectric loss.The main research contents are as follows:1.PI/PVDF composites were prepared by solution blending or in-situ polymerization,and the properties of the composites synthesized by the two methods were systematically studied.Results indicated that the composites prepared by solution blending exhibited superior morphology,thermal stability,moisture resistance,and dielectric properties,compared with the composites prepared by in-situ polymerization.The dielectric constant of the composites synthesized by the two methods increases with the PVDF content,and also with a small dielectric loss.The maximum value of the dielectric constant of the composites prepared by solution blending was 7.745(1 k Hz).Based on solution blending,PI/PVDF dielectric composites were synthesized by using different chemical imidization temperature.Results indicated that the hydrophobic properties and dielectric constant of the composites increases with the reaction temperature.When the reaction temperature was 50°C,the composites show a stable and minimum dielectric loss(<0.04).In addition,the composites have good thermal stability when the reaction temperature did not exceed50°C.2.PVDF was used as a high dielectric filler,and composite films were prepared through thermal imidization.Results indicated that the composite films exhibited outstanding thermal stability,hydrophobic properties,and dielectric properties.In particular,the dielectric loss of the composite film was small.When the PVDF fraction was 30 wt%,the dielectric constant sharply increased to 5.7 at 1 k Hz,and its dielectric loss was only0.009.Moreover,the breakdown strength,energy storage density,and resistivity of the composites slowly decreased with the increase in PVDF amount.At the same time,the charge-discharge efficiency of the composite decreased,but it remained on a high level.Under the electric field of 200 k V/cm,the charge-discharge efficiency of the composite with 50 wt%PVDF was as high as 95.77%.3.Modified PI/PVDF composite films with excellent properties were synthesized by introducing the coupling agent(KH570).Results indicated that KH570 can improve the compatibility between matrix and fillers,which is beneficial to improve the performance of the composite film.The modified composite exhibited superior morphology and thermal stability than the unmodified composite.The mechanical properties of the modified composite increases when the amount of KH570 increases from 0 wt%to 3 wt%.The hydrogen bond formed between hydroxylated KH570 and fillers led to a decrease in the dielectric constant and dielectric loss of the modified composite film,but the dielectric constant was still high(the minimum value was still 4.82,1 k Hz).Noticeably,the dielectric loss of the modified composite was very low.In the tested frequency range,the dielectric loss was less than 0.03.In addition,the breakdown strength and maximum energy storage density of the modified composite obviously increased.When 3 wt%KH570 was added,the breakdown strength and maximum energy storage density increased to 169.65 k V/mm and 0.651 J/cm~3,respectively.The charge-discharge efficiency of the modified composite decreased,but it remained on a high level.When the content of KH570 was 1 wt%,the charge-discharge efficiency was as high as 93.56%under the electric field of 500 k V/cm.4.PI/LNBR composite films were synthesized by introducing LNBR as organic fillers into the PI matrix.Results indicated that PI/LNBR composite films have outstanding thermal stability,mechanical properties,and dielectric properties.Compared to pure PI,the composite film with 5 wt%LNBR shows an improvement of 41%and 31%in tensile strength and tensile modulus,respectively.When LNBR ratio was 15 wt%,the dielectric constant of the composite film increased to 5.58 at 1 k Hz,but the dielectric loss was only0.012.The breakdown strength of the composites slowly decreased with an increasing LNBR amount,but it remained on a high level.Even when the LNBR content was as high as 30 wt%,the breakdown strength was still about 183.16 k V/mm.When the LNBR content was 5 wt%,the composite film shows the largest energy storage density of 1.95J/cm~3.In addition,the composites also have good charge-discharge efficiency and resistivity.Under the electric field of 500 k V/cm,the charge-discharge efficiency of the composite with 5 wt%LNBR can reach 98.44%.When the LNBR content was as high as25 wt%,the charge-discharge efficiency was still as high as 96.37%under the electric field of 500 k V/cm.