| High performance dielectric composite substrate is not only the carrier of microwave components,the core components of module of internal interconnection and passive components with electrical interconnection,and also is the key materials for realizing the function module of small,lightweight,high reliability and low cost.This substrate has a large number of applications in the key components and high frequency circuit like T/R modules,antenna unit,and phase shifter in the field of microwave communication and early warning.The development of emerging technologies,such as millimeter wave radar,satellite navigation,5G communication and assisted driving system,has put forward new performance requirements for dielectric substrates.Fluoropolymer has good electronical characteristics and excellent chemical stability,but poor thermal properties like high thermal expansion coefficient.And the mutual restriction and contradiction between the thermodynamics and electrical performance requirements has become the basic problems that must be solved in the above technical fields and industrial development.In this paper,the method of filling high performance microwave dielectric ceramic was used to realize acquiring the tunable dielectric properties and reducing the thermal expansion.The influences of properties of ceramic and polymer on the structure and dielectric properties of composites were studied.The surface modification technology is used to change the interface structure and combination mode of ceramic polymer,so as to reduce the dielectric loss.The influence of ceramic shape and interface on dielectric properties and the synergistic regulation law of dielectric properties and synergistic correlations between dielectric materials are discussed.The main research contents and innovative conclusions are as follows:（1）The perovskite ceramic（Ca,Li,Sm）TiO3（CLST）with good temperature stability and fluoropolymer polytetrafluoroethylene（PTFE）and polyvinylidene fluoride（PVDF）with excellent dielectric properties are chosen as the filling phase and the matrix materials,respectively,to produce microwave dielectric composite materials,and their dielectric response mechanisms under the external field action are discussed.The results show that the dielectric response mechanisms of the composites vary among wih different polymers.At low frequency（40Hz-1MHz）,the dielectric loss of PVDF matrix composite increases with the increase of frequency,indicating that the loss of space charge polarization is low,while the loss of dipole relaxation polarization is large,which is caused by the fact that PVDF is a polar material.The dielectric loss of PTFE matrix composite decreases with the increase of frequency,indicating that the loss caused by space charge polarization is larger,while the loss caused by dipole relaxation polarization is smaller,which is related to the fact that PTFE is a non-polar material.PVDF with polarity is easy to mix with ceramics to form homogeneous composite material,while PTFE is difficult to combine with ceramics,and it is easy to produce holes and other defects,resulting in a great difference in the contribution of space charge polarization and dipole polarization.In the microwave frequency band,only electron displacement and ion displacement contribute,so the main source of dielectric loss is non-intrinsic conductivity loss caused by organic/inorganic interface defects.The polarity of PVDF is easy to be mixed with ceramics to form homogeneous composites.Therefore,the ceramic filling amount has little influence on the microwave dielectric loss of PVDF matrix composites in the microwave frequency band,while PTFE is difficult to bond with ceramics.As the ceramic content increases,the dielectric loss of composites increases.When the volume fraction of ceramic is 40-50VF%,the microwave dielectric properties of CLST/PVDF show as follows:εr=10.41～14.26,tanδ=2.1～2.2×10-3（～10GHz）,while the CLST/PTFE show as follows:εr=7.92～10.87,tanδ=1.2～2.3×10-3（～10GHz）（2）The effects of different filling amounts of CLST ceramics and Ba(Mg1/3Nb2/3)O3（BMN）ceramic on the dielectric properties of PTFE matrix composites are studied,respectively,which reveals that the relationship between dielectric constant and the types and compositions of ceramic satisfies the Rother-Lichtenecker formula.The dielectric properties of composites could be regulated by adjusting the type and compositions of the ceramic.When the volume fraction of ceramic is too high（more than 40%）,the agglomeration effect by direct contact between ceramic particles leads to weaken the interaction between PTFE and ceramic particles and to produce a three-phase like low-dielectric hole.As a result,the actual test result of dielectric constant is lower than the theoretical calculation,and the dielectric loss is significantly higher than the theoretical calculation value,which is mainly due to the dielectric loss of the composites caused by interface defects.The dispersibility of ceramic and PTFE can be significantly improved by controlling size,sphericity and other parameters of ceramic particles,which can be explained by Modified Rother-Lichtenecker formula and finite element simulation.When the ceramic filler is 50VF%,the dielectric properties of the BMN/PTFE composite are as following:εr=5.76,tanδ=1.4×10-3,while the CLST/PTFE shows:εr=10.87,tanδ=2.3×10-3.（3）The composites were also prepared by heterogeneous cocoagulation method.Uniform dispersion of ceramic particles and polymers can be achieved by adjusting Zeta potential,which can reduce the loss caused by interface defects.Furthermore,composite conditions and effects can be achieved through precise calculation of surface charge.The mechanical blending method is simple and easy to operate,but the uniformity dispersion is difficultly controlled.In this study,coupling agents,active small molecules,surface in-situ grafting by PMMA and fluid surface coating etc.,are used to modify the ceramic surface.After surface modification,most of the modified composites can reduce dielectric loss.And the effects of high molecular silane coupling agent,modified silane coupling agent and azide coupling agent are obvious.This is mainly because different modification methods constraint the free charge and carriers by defects at the bound interface to a certain extent,reducing the conductivity loss caused by leakage current,so the different loss is reduced.The effects of some modifiers are obvious,which may be mainly related to the higher probability of chemical bonding with the composite interface.（4）The PTFE causes phase transition at 19℃,and its molecular structure changes from 13/6 helix to 15/7 helix,which shows obvious volume expansion.Filling of CLST and fiber glass in PTFE can prevent the molecular spiral change during PTFE phase transition,and can significantly reduce the thermal expansion coefficient of the composites.With continuous addition of inorganic filler content,the thermal expansion coefficient decreases further.The results show that the after modification,glass fiber with 1-5μm,a moderate aspect ratio and good flexibility could achieve better compatibility with PTFE.When the glass fiber content was 5%,the composite could achieve better comprehensive regulation of dielectric and thermal properties. |
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