| With the opening of fifth-generation（5G）commercial application,the era of“Internet of Everything”is coming.To meet the tendency of high working frequency,miniaturization and high reliability,microwave circuits and devices put forward more strict demands on the development of high frequency microwave dielectric substrate.Commonly,dielectric substrates with extremely low dielectric loss（tanδ）,adjustable dielectric constant（εr）and near-zero temperature coefficient of dielectric constant（τε）are required in high working frequency.Polytetrafluoroethylene（PTFE）based and ceramics filled composites have attracted more attention in recent years because they can integrate both the excellent dielectric properties of ceramic materials and the easy process of polymers.However,these current researches mainly focused on the analyses of macro-performances,while the surface modification mechanism on inorganic fillers has not been adequately characterized,and the determinants ofτεhave also not been thoroughly studied.This dissertation based on the PTFE composites filled with inorganic fillers,assisted with fluoride modification process on the surface of fillers using C14H19F13O3Si silane coupling agent（F8261）.We are capable of revealing the mechanism on surface modification process,obtaining a series of PTFE based composite substrates which could satisfy diverse requirements during high working frequency.Herein,the main achievements are enclosed as follows:（1）Conventional solid state route was used to prepare Ca Mg0.98Cr0.04/3Si2O6（CMCS,εr=8.1,tanδ=0.0001）ceramic powders.A mass of R-groups（-（CH2）2-（CF2）5-CF3）were grafted on the surface of CMCS particles when modified using 1.5 wt.%F8261.And then,the surface energy of CMCS particles was decreased to 3 mj/m2 representing a high hydrophobic nature,which could promotes a dense microstructure with PTFE.Finally,F8261 modified CMCS/PTFE composite substrates possess relatively low dielectric constant（εr=3.21）,extremely low dielectric loss（tanδ=0.0008）and water absorption（0.13%）at 40 wt.%CMCS filling.（2）Based on one synthetic method,Ca0.61Nd0.26Ti0.99Cr0.01O3（CNT,εr=99,tanδ=0.0002）ceramic powders were prepared.H2O2 was selected to pretreat the surface of CNT particles,which increased the amount and activity of hydroxyl groups（-OH）on the surface of CNT,promoted the surface modification of F8261.The maximum filling amount of CMT particles in PTFE matrix was increased from 40 wt.%to 60 wt.%,resulting in a more excellent dielectric properties（εr=7.93,tanδ=0.0025）as well as a lower water absorption of 0.16%.（3）For alkali metal elements exhibit high activity,which could react with the H2O absorbed on the surface to produce a large amount of-OH,we derived a series of lithium-rich ceramic oxides such as Li2Sn Mg0.5O3.5（LSMO）,Li2Mg3Ti O6（LMT）and Li2Mg2.88Ca0.12Ti O6（LMCT）.These lithium-rich ceramic powders were modified using F8261 and then filled into PTFE matrix.In this dissertation,when the content of F8261was fixed at 2.0 wt.%,the characteristic absorption bands of methylene（-CH2-）in FTIR spectrum was the strongest,and the contact angle of modified LSMO powder reached the maximum value of 164°,indicating the best modification effect.Particularly,the maximum loading content of Li-Based ceramic powders in PTFE was increased up to 70wt.%,together with extremely low water absorption（<0.16%）as well as low dielectric loss（tanδ<0.0026）.Further,the second phase of Ca Ti O3 was loaded into LMT powders to decrease theτεfrom 67 ppm/℃to-16 ppm/℃,resulting in an opposite changing trend in theτεof LMT/PTFE and LMCT/PTFE composites.Typically,theτεvalue of LMCT/PTFE composite was-30 ppm/℃when the filing content of LMCT was 70 wt.%,which is more closer to zero when compared with that of LMT/PTFE composite（τε=120ppm/℃）.Besides,the experimentalεr of Li-Based ceramic powder/PTFE composites were compared with several theoretical models which were commonly used in predicting the dielectric constant of Ceramic-Polymer composite system.It was found that the fitting factor（k）should be 0.1 when the Modified Lichtenecker model was selected in Li-Based ceramic powder/PTFE composites.And we also confirmed that the Effective-Medium Theory（EMT）was an universal model in Li-Based Ceramics/PTFE composites when the morphology parameter（m）was near 0.25.（4）Li2Mg3SnO6（LMS）ceramic particles with low dielectric constant（εr=12.1）and appropriateτεof 38 ppm/℃were selected to load in PTFE matrix.The surface energy of LMS particles was decreased from 560 mj/m2 to 5 mj/m2 after modified with 2.0 wt.%F8261,which is lower than that of deionized water（72 mj/m2）.The modified LMS particles displayed a strong hydrophobicity as the contact angle increased from 0°to 159°.At last,PTFE substrate filled with 46 wt.%LMS particles has low water absorption（0.02%）and good dielectric properties:tanδ<0.001,τε=9.42 ppm/℃.（5）The complex phase ceramic particles of 0.8Ba Ti4O9-0.2Ba Zn2Ti4O11（BZT,εr=34.2,τε=-16 ppm/℃）were synthesized by traditional solid route and then filled into PTFE matrix.To decrease the coefficient of thermal expansion（CTE）and adjust theτεvalue of BZT/PTFE composite substrate,glass fiber was modified and loaded.Particularly,theτεvalue of BZT/GF/PTFE substrates was close to zero（τε=1.57 ppm/℃）when the content of GF was 4 wt.%and BZT was 46 wt.%.In addition,the degree of crystalline（Xc）of BZT/GF/PTFE substrates went down from 55.4 to 49.8 with the BZT loading amount increased from 26 wt.%to 66 wt.%.（6）Orthorhombic perovskite Na1/2Sm1/2Ti O3（NST,εr=100,τε=-400 ppm/℃）ceramic particles with high dielectric constant were synthesized.In this dissertation,NST particles and 4 wt.%GF were filled into PTFE matrix.As the filling content of NST particles raised from 26 wt.%to 66 wt.%,the CTE of NST/GF/PTFE composite substrates along X and Y axes experienced a continuous decrease from 90 ppm/℃ to 19ppm/℃.Typically,the CTE of NST/GF/PTFE along X and Y axes was 68 ppm/℃ when the content of NST was 36 wt.%.At the meantime,NST/GF/PTF substrate exhibits extremely low water absorption（0.0037%）and prominent dielectric properties:εr=3.9,tanδ=0.001 andτε=17 ppm/℃. |
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