Filler Design And Structural Performance Study Of Polyolefin-Based High Dielectric Constant Microwave Dielectric Composite Substrates

Microwave circuits necessitate the utilization of microwave dielectric composite substrates featuring varying dielectric constants（D_k）to accommodate diverse application requirements.The use of substrates with elevated D_k values serves as the most straightforward strategy for the miniaturization of microwave circuits.Presently,investigations into microwave dielectric composite substrates predominantly target the medium to low spectrum of D_k,showcasing a notable deficiency in research dedicated to substrates endowed with ultra-high D_k（>15）.This document advocates for the adoption of polyolefin（polybutadiene,ethylene-propylene-dicyclopentadiene,and styrene-butadiene-styrene copolymer:PB/EPDM/SBS）as the foundational resin matrix of the composite substrate.Through the strategic enhancement of the substrate’s attributes via filler optimization,a suite of polyolefin-based,high-D_k microwave dielectric composite substrates has been innovatively engineered.These substrates not only exhibit exceptional performance but also hold substantial prospects for commercialization,effectively bridging the existing research and market voids in the field of high-D_kmicrowave dielectric composite substrates.The specific research findings are as follows:（1）Preparation and performance study of f-ZNT/polyolefin.This study utilized solid-state sintering and two-stage crushing process to prepare Zn_0.15Nb_0.3Ti_0.55O₂（ZNT,D_k=94.35,D_f=0.0006,10 GHz）ceramic filler powder,which was subsequently surface-functionalized（f-ZNT）with silane coupling agent vinyltrimethoxysilane（VTMS）and then combined with polyolefin（polyolefin:PB/EPDM/SBS）to produce the composite substrate.When the f-ZNT content was 89 wt.%,the f-ZNT/polyolefin composite substrate reached the highest D_k of 16.79（10 GHz）,a D_f of 0.0024（10 GHz）and a temperature coefficient of D_k of-120×10^-6/°C（10 GHz）.The investigation demonstrated that incorporating high-dielectric f-ZNT markedly increased the composite substrate’s dielectric constant.However,an excessive amount of ceramic filler（>89 wt.%）prevented the resin matrix from completely encapsulating all the ceramic interfaces,resulting in substantial filler agglomeration and porosity,thus impairing the substrate’s performance parameters.The inclusion of f-ZNT hindered the mobility of resin molecular chains during the substrate’s curing and thermal degradation phases.This condition resulted in an abundance of uncured small molecules inside the composite substrate,lowering the system’s cross-linking density,while concurrently enhancing the overall thermal stability of the composite substrate.（2）Preparation and performance study of the f-ST/polyolefin.This study utilized the silane coupling agent VTMS for surface functionalization of Sr Ti O₃（ST）powder,which was then combined with polyolefin to prepare the composite substrate.The incorporation of f-ST significantly enhanced the D_k of composite substrate,exhibiting the highest microwave D_k and relatively low D_f（D_k=21.10,D_f=0.0041,10 GHz）when the f-ST content was 87.5 wt.%.It was found that surface functionalization of ST could notably improve the composite substrate’s density,thereby enhancing its overall performance.As the f-ST content increased,the D_k,thermal conductivity（λ）,and flexural strength（R）of the f-ST/polyolefin composite substrate showed a trend of first increasing then decreasing,with higher ceramic filler content（≥88 wt.%）leading to filler agglomeration and the generation of pores,thus deteriorating the substrate’s performance parameters.f-ST significantly raised the initial decomposition temperature and thermal stability of the composite substrate.Given its higher D_k and better commercial prospects,this thesis focuses on further performance improvements of the f-ST/polyolefin system as the main research subject.（3）Investigation into the influence of modified boron nitride nanosheets（f-BNNS）on the performance of f-ST/polyolefin Microwave dielectric composite substrate.This research employed Polydopamine（PDA）for the non-covalent modification of BNNS,subsequently incorporating it into the f-ST/polyolefin composite substrate（with f-ST filler content fixed at 78 wt.%）.The introduction of high-λfillers markedly boosted the composite substrate’sλ.With an f-BNNS content of 8 wt.%,the f-BNNS/f-ST/polyolefin composite substrate material achieved unparalleled microwave D_k（17.70,10 GHz）,substantially low D_f（0.0030,10 GHz）and peakλ（1.60 W/（m·K））,rendering it an exemplary substrate with high D_k and superiorλfor microwave applications.The findings indicated that PDA-modified BNNS not only significantly improved the substrate’s density but also introduced organic interface layers through non-covalent bonds within the thermal conduction pathway,marginally diminishing the thermal conductive efficiency of the composite material.Furthermore,relative to unmodified BNNS,f-BNNS exerted a more pronounced inhibitory effect on the resin molecular chains’mobility during the substrate’s curing and thermal degradation phases.（4）Study on the effect of modified glass fiber（f-GF）on the performance of f-BNNS/f-ST/polyolefin Microwave dielectric composite substrate.GF were surface-modified with the silane coupling agent 3-aminopropyltriethoxysilane（KH550）,and f-GF was subsequently introduced,replacing f-BNNS（8 wt.%）to improve the composite substrate’s mechanical properties and coefficient of thermal expansion.The introduction of f-GF significantly improved the composite substrate’s flexural strength（R）,tensile strength（σ）and both in-plane（CTE_∥）and perpendicular（CTE_⊥）thermal expansion coefficients.With an f-GF content of 4 wt.%,the f-GF/f-BNNS/f-ST/polyolefin composite substrate demonstrated optimal comprehensive performance:D_k=17.81（10GHz）,D_f=0.0037（10 GHz）,λ=1.30 W/（m·K）,CTE_∥=14.68μm/（m·°C）,CTE_⊥=21.82μm/（m·°C）,R=65.5 MPa,σ=17.4 MPa.This configuration established the composite as a high-Dk and high-λMicrowave dielectric composite substrate with considerable prospects for commercial applications.It was found that the surface functionalization of GF markedly enhanced its dispersibility and compatibility within the resin matrix,thereby improving the substrate’s D_k and reducing its D_f.As f-GF replaced f-BNNS,there was a gradual increase in the substrate’s D_k and a decrease inλ,significantly augmenting thermal stability.