Investigation On The Electromagnetic Performance Adjustment Of Organic Inorganic Composite Materials And Flexible Electronic Devices

With the rapid development of electronic information technology,especially for Internet of things,artificial intelligence,mobile terminal,wearable electronics and 5 G communication technology,higher requirements are put forward for electronic devices and circuit modules.Therefore,due to growing trend of miniaturization,lightweight and integration,flexible electronic technology with unique advantages can meet the application requirements of next generation mobile electronics.The core components and circuits technology in this technology are based on flexible films,so it’s great significant to study microstructure and electromagnetic properties of the flexible film.However,there are many problems in high-frequency application of the flexible film.For example,low dielectric constant can lead to the large size of devices.Therefore,it’s very vital for expanding application of flexible electronics to prepare flexible film with improved high-frequency performance.In this dissertation,due to different application requirements,PDMS and PVA flexible film materials widely used in flexible electronics were investigated,to modify their high-frequency performances.The dissertation analyzed the critical factors and physical mechanism that affect high-frequency performances of the flexible films,prepared composite flexible films with good electromagnetic properties,and carried out related application research based on these composite film materials.First,the influences of NiCuZn ferrite particles with high permeability on high-frequency magnetic properties of PDMS flexible thin films were systematically studied.In order to synthesize NiCuZn ferrite ceramics with excellent magnetic properties and uniform grain size,the effects of the composite addition of Nb₂O₅ and Bi₂O₃ on the microstructure and magnetic properties of the ferrite were investigated.The results showed that when the additive amount of Nb₂O₅ is 0.25 wt.%,the NiCuZn ferrite with uniform and compact grains could be achieved at low sintering temperature by regulating the additive amount of Bi₂O₃.Finally,when 0.25 wt.%Nb₂O₅ and 0.50 wt.%Bi₂O₃ were selected as composite additives of the NiCuZn ferrite and sintering temperature was 875 ^oC,the average grain size was about 1.2μm,the permeabilityμ_r=478.2@1.0 MHz,the cut-off frequency f_r was about 11 MHz,the saturation magnetization M_s=61.71 emu/g,and the dielectric constantε_r=15.3.Then,the high-frequency properties of NiCuZn/PDMS composite flexible film were studied based on the above NiCuZn ferrite ceramic.The results showed that the rapid curing enabled NiCuZn ferrite particles to be uniformly dispersed in PDMS films because the particles are coated by liquid PDMS during the preparation process.In addition,for the the composite flexible film,PDMS was equivalently dispersed in NiCuZn ferrite grain gap,i.e.,"grain boundary"was enlarged.Finally,when the filling content of magnetic particles m=16 and the frequency was 200 MHz,the dielectric constantε_r=4.76,the dielectric loss tanδε=5.8×10^-3,the permeabilityμ_r=2.96,the magnetic loss tanδμ=0.28,the miniaturization factor n=3.75,and the normalized characteristic impedance Z/Z₀=1.26。Secondly,the high-frequency magnetic performance and related applications of high saturation magnetization LiZnTi ferrite and PDMS composite flexible films were studied.The studies showed that:（1）when moderate low-melting Bi₂O₃ was added to LiZnTi ferrite,Bi³⁺can replace the Fe³⁺in B-site and thereby reduce the reaction activation energy of LiZnTi ferrite,which enabled the ferrite to achieve densification at low sintering temperature.When excessive Bi₂O₃ were existed in the grain boundary in the form of liquid phase,the Bi₂O₃ can hinder the grain displacement,deteriorate the uniformity of grain,and lead to the reduction of magnetic properties.（2）When the addition amount of Bi₂O₃ y=0.003 and sintering temperature was 920 ^oC,the average grain size of LiZnTi ferrite sample was about 2.5μm,permeabilityμ_r was 142.8@1.0MHz,cut-off frequency f_r was about 10 MHz,and dielectric constantε_r was 16.81.（3）For the LiZnTi/PDMS composite flexible film,when the added amount of ferrite particles n=8 and frequency was 1.0 GHz,the dielectric constantε_r=5.4,the dielectric loss tanδε=2.79×10^-2,the permeabilityμ_r=3.04 and magnetic loss tanδμ=0.58.（4）The simulated results of dipole antenna based on the LiZnTi/PDMS（n=8）composite flexible film and PDMS flexible film showed that compared with antenna of PDMS film,the antenna based on composite film had a smaller size（80 mm*120 mm）,and the change of antenna bandwidth was little.However,the results of gain direction diagram showed that the increase of magnetic loss and dielectric loss of PDMS flexible film material leaded to the decrease of antenna gain（the maximum gain decreased from 2.06dB to-2.31 dB）.Then,the effects of TiO₂ nanoparticles with high dielectric constant on the dielectric properties and degradation characteristics of PVA flexible film were systematically studied,and the application of TiO₂/PVA composite flexible film on transient electronics（RF antenna）was explored.The main conclusions were shown as follows:（1）Due to existence of pores in PVA flexible film,TiO₂ nanoparticles with high dielectric constant cannot improve the dielectric constant of PVA film.When a small amount of TiO₂（x=1）was filled,the dielectric constant of the flexible film decreased,and the dielectric loss also decreased.（2）The hydroscopicity（dissolution rate in water）of TiO₂/PVA composite film first decreased and then increased with the increase of TiO₂ nanoparticles amount.This is because the surface groups of TiO₂ and PVA easily form Ti-OH,which leaded to the decline of hydroscopicity.However,excessive TiO₂（x>4）caused the decline of the crystallinity and the destruction of the relatively integrated chain structure of PVA flexible film,so the hydroscopicity started to improve again.（3）Dielectric performance of TiO₂/PVA（x=1）composite flexible film at the frequency of 1.0 GHz:ε_r=3.24,tanδε=0.038.（4）Transient microstrip antennas based on pure PVA flexible film and TiO₂/PVA（x=1）composite flexible film not only possessed good radiation characteristics at the center frequency of 2.5 GHz(S₁₁=-15 dB and G=0 dB),but also can self-degrade in water within 1.0 hours.Finally,the changes of dielectric properties of PVP/PVA flexible film under different humidity conditions was studied,and the application of PVA flexible film in moisture sensitive devices was explored.The results showed that:（1）The flexible PVA film prepared by solution casting method had a good humidity sensitivity within a certain humidity range.The method that adding PVP with good moisture absorption characteristics cannot improve the moisture sensitivity,but reduce light transmittance of the film.（2）The transparent coplanar waveguide（CPW）resonator based on PVA/PET double layer substrate had good moisture sensitivity.The test results of CPW resonators found that when the relative humidity changed from 11.3%to 97.3%,the resonant frequency f_r of the resonator shifted from².6 GHz to².1 GHz,and the amplitude of the S₁₁ parameter changed from¹9 dB to⁸ dB.Thus,the change signal of environmental humidity can be converted into change signal of resonance frequency or amplitude of S₁₁ parameter of the resonator.