Energy/Data Storage Of Dielectric Polymers

Dielectric polymers are desired for capacitor,transistor and electronic packaging,which play an important role in energy,medicine,military,and so on.Here,VDF-based and urea-based dielectric polymers are employed to investigate the polarization mechanism and electrical applications,which are affected by the dipoles,crystal structure and space condition.There are three kinds of chain conformations in VDF-based polymers,including TTTT(all trans),TGTG' and TTTGTTTG'.The chains are arranged to form ?,?,?,?and ? phases,which have different polarization performances.Along with the dipole switching,their crystal phases transform from one to another under mechanical force,electric field,heat process,light and so on.The polarization mechanism alters at the same time,which is closely related with their performance in the flexible electronic devices.The large dipole moments and benzene rings of urea-based polymers facilitate their high thermal stability,high breakdown strength,high energy density and low loss.But their further capacitor application is limited by the small dipole density,fragility and indissolubility in common polar solvents.To intervene or tailor the crystal structures of the dielectric polymers,different physical and chemical methods are developed,including uniaxial stretching,green synthesis,polymer composite and different heat treatments.Their crystal structures and applications for energy storage,memory device and negative capacitance are studied based on the polarization mechanism.In the thesis,we focus on five parts as follows:(1)The typical semi-crystalline relaxor ferroelectric polymer P(VDF-TrFE-CFE)is the most complex and characteristic VDF-based dielectric polymer.To improve the film quality and manipulate their crystal structures and resulting capacitor properties,two film samples with different initial phases are uniaxially stretched with a homemade drawing device.And WAXS in situ is employed to monitor the crystal structure evolutions of P(VDF-TrFE-CFE),and the stress-strain curves are obtained simultaneously.Subsequently,the phase transition and energy storage evolutions of the two samples under cold drawing are investigated.The chain orientation and dipole coupling work synergistically in the effective dielectric constant and breakdown strength,which are related with the energy storage performance.To quantitatively descript the relations between energy density and dielectric constant of the nonlinear dielectric polymer,the effective dielectric constant is presented by drawing an analogy among the typical linear dielectric polymers,VDF-based copolymers and terpolymers.The nano-capacitor,composed of confined and sandwiched order phase between the amorphous layers with certain volume space,is proposed for high energy density capacitor applications,which provides a guideline for device optimization.The amorphous phase(Keff)acts as the electrodes,and the order phase plays as the dielectric layer to bear the high voltage(Eb).In addition,the tensile rate has less effect on the structure than the tensile ratio and temperature.(2)Developing dielectric polymers with higher dielectric constant without sacrificing loss and thermal stability is of great importance for next generation of high energy density capacitors.Under the guidance of Frohlich equation,the amorphous aromatic linear dielectric polymer poly(arylene ether urea)is designed and synthesized in a green route with no solvent,no catalyst and low cost,which exhibits high dielectric constant,high thermal stability and high energy storage,but low loss.The conduction losses of PEEU perform well both at room temperature and elevated temperature,which are better than the conductivity of BOPP at room temperature.As a result,PEEU is an attractive candidate for capacitor with high thermal stability,high energy density and low loss.In addition,we study the molecular structure,free volume and dielectric constant of urea-based aromatic dielectric polymers for advanced designs of capacitor applications.(3)Aromatic linear dielectric polymer PTU is introduced into VDF-based dielectric polymer P(VDF-CTFE)by grind.The composite,P(VDF-CTFE)-PTU,combines the merits of both polymers,i.e.high dipole density and easy processability of P(VDF-CTFE),and large dipole moment and high charge-discharge efficiency of PTU.The breakdown strength,charge-discharge efficiency,energy density,discharge speed and the loss are all improved for the composite P(VDF-CTFE)-2%PTU.Its discharge energy density is as large as 17.8 J/cm3 at 450MV/m.2%immiscible aromatic PTU introduced into the P(VDF-CTFE)results in the reduced size of the crystallite,and increased number of the crystal nucleus and interface between the amorphous and crystalline phases.In addition,PTU brings structural defects into P(VDF-CTFE),leading to less ordered crystals and decreased crystallinity.Nonpolar phases,including amorphous regions and paraelectric crystals,accelerate the discharge process and play an important role in the depolarization of PVDF and its copolymers in the microsecond range.The polar groups and benzene rings of aromatic PTU reduce the high-field conduction current.Thus,the breakdown is enhanced with low conduction.(4)Because of the anti-parallel bistable state under low coercive field,relaxor ferroelectric polymer P(VDF-TrFE-CFE)is employed to develop non-volatile memory storage.Three sets of nano-scale films,Quench,Spin-coating and Ann 110,with different heat processes are obtained.As a result of differences in ferroelectric phase content,surface morphology and spatial distribution of the domains,these three samples demonstrate different performance of memory storage.For the Spin-coating sample,the ferroelectric phase content is prior to the paraelectric phase.Its average diameter of the grains is the minimum.The smooth and homogeneous surface of Spin-coating film with few defects makes it possible for memory switching within 10ms.More importantly,the information memorized in the Spin-coating film can be read at both vertical and horizontal directions,which is unexpected and different from other memory devices.This 2D read access helps us to understand the spatial distribution and the polarization mechanism of the domains.The additional read access at the horizontal direction increases the number of read access and extends its life.This 2D read access by tuning the surface morphology and domain arrangement under 3D architectural environment may offer a new direction to develop memory device with graceful read-failure.(5)By putting a large resistance between the power source and the capacitor,the free charges provided by the power source are slowed down,which may be less than the ferroelectric charges from spontaneous polarization.At this point,the negative capacitance can be directly characterized when the power switches.Besides the ferroelectric polymer P(VDF-TrFE),the negative capacitance of the relaxor ferroelectric polymer P(VDF-TrFE-CFE)is directly measured for the first time.Because of the multi-states of relaxor,the domains switch as a function of the applied electric field.With the negative capacitance value adjusted by the multi-states,the terpolymer P(VDF-TrFE-CFE)acts as voltage-controlled transformer.Integrated with the transistor,the negative capacitance of the relaxor terpolymer could effectively improve its subthreshold slope and reduce the energy dissipation.