Preparation And Properties Of Microwave Board Composited By Rutile TiO₂ With PTFE Or Epoxy Resin

Demands for Communication terminal module have become higher,given the rapid development of wireless communication system especially satellite navigation,wireless positioning system,and mobile base station system.The antenna,as a terminal device of wireless communication,is now getting smaller and thinner.There is a negative correlation between the size of microwave aerial and the permittivity,indicating that a high permittivity leads to a small size.As a result,miniaturization of the aerial could be achieved by a substrate with a high permittivity.Epoxy resin is not only cheap but also good in mechanical property,and its dielectrical property is able to maintain stable in certain temperature and frequency ranges.Likewise,polytetrafluoretyhylene（PTFE）also owns a low dielectric loss tangent（less than 0.0004）and remains stable during the changes of end-use temperature.Additionally,both rutile TiO₂ powders and fibers have a permittivity value of about 100,with a low dielectric loss tangent of around 0.001.For the sake of low-cost production,TiO₂ powders filled epoxy resin composite substrates were synthesized and properties of these high-permittivity microwave composite substrates were explored in this thesis.To reduce the dielectric loss tangent,TiO₂ particles filled PTFE composite substrates as well as TiO₂ fibers and particles together filled ones were both acquired and investigated in detail.Firstly,sol-gel method was used to obtain precursor sol,which was then processed in to TiO₂ and BaTiO3 fiber precursors on a centrifugal spinning machine.Following that,the fiber precursors were calcined to get TiO₂ and BaTiO3 ceramic fibers.After a series of exploations regarding rotation speed,spinning head aperture,and viscosity of the sol,a rotation speed of 6000r/min,PVP proportion of 7%,and a spinning head aperture of 400μm or 600μm were sorted out to get TiO₂ and BaTiO3 ceramic fibers with satisfactory morphologies.Secondly,microwave composite substrate with epoxy resin as the matrix were prepared to reduce the cost.Permittivity was enlarged by adding rutile TiO₂ powders,and PTFE contributed to the reduction of dielectric loss.Additionally,flexural strength was improved with the most suitable layer array of fiberglass meshes and PTFE sheets.The results showed that the TiO₂ additive amount of 48vol%showed a well microwave performance,with a permittivity of 13.7,a dielectric loss angle tangent of 0.0098,a density of 2.5g/cm3,a water absorption rate of 0.15%,a thermal conductivity of 0.65W/（m K）,and a flexural strength of 115MPa.Moreover,four theoretical formulas were applied to calculate the permittivity of composite substrates,and analog values obtained were compared with the experimental ones.The comparison provided theoretical basis for synthesis of microwave composite substrates with different permittivity,where epoxy resin played the role of the matrix.Finally,TiO₂ particles filled PTFE composites with high permittivity were synthesized,followed by careful property study.Conclusions were that addition of 39vol%TiO₂ powders led to the best combination property.At this point,the density was 2.7g/cm3,a water absorption rate of 0.074%,a thermal conductivity of 0.4562W/（m K）,a permittivity of 9.7,dielectric loss angle tangent of 0.00080 and a flexural strength of 26.5MPa.For this reason,the total amount of TiO₂ particles and fibers was fixed at 39vol%during afterward researches.The results indicated TiO₂ fibers proportion from lvol%to 16vol%made little difference to properties of composite plates,namely,the density,water absorption rate,thermal conductivity permittivity dielectric loss tangent,and flexural strength were 2.49g/cm3,0.062%,0.462W/（m·K）,9.6,0.00074,and 29.3MPa,respectively.Four theoretical formulas were also applied to calculate permittivity of composite substrates,and analog values obtained were compared with the experimental ones.The comparison provided theoretical basis for synthesis of microwave composite substrates with different permittivity,where PTFE played the role of the matrix.