Study On Microwave Dielectric Properties Of CaO-La₂O₃-TiO₂ Ceramics And Their Composites

With the rapid development of communication industry,more requirements of microwave dielectric materials have been raised by various high-tech products.In microwave circuits,the size of the device is inversely proportional to the square root of the dielectric constant of the material.Therefore,microwave dielectric materials with high dielectric constant are significant to the miniaturization of devices.In addition,the devices require high performance of signal-to-noise ratio,and the materials should have a low dielectric loss.The brittleness and high density of traditional microwave dielectric ceramics have seriously limitted their application.Based on this background,microwave dielectric composites start to be widely studied.Microwave dielectric composites are composed of microwave dielectric ceramics and polymer matrix,which possess excellent dielectric properties of ceramics and high reliability of polymer.This kind of material could be made into Copper Clad Laminate after copper cladding.And microwave dielectric composites can be used in many fields such as antenna,radar system,multilayer circuit board and so on.So far,there are a lot of researches on composites with a dielectrc constant below 10.As for the composites possessing a dielectric constant over 10 usually have high dielectric loss.In this paper,basing on the needs of composites with high dielectric constant,the A,B site substitution of Ca Ti O₃ ceramics were studied for obtaining ceramics with high dielectric and low dielectric loss.On this fundation,the ceramics with different morphology and particle size were compounded with polytetrafluoroethylene(PTFE)to study the effect of ceramic filler on the dielectric properties of composites.After that,to obtain a composite with higher dielectric constant,polyarylacetylene(PSAE)with higher dielectric constant was selected as polymer matrix.Finally,when the intrinsic properties of the two phases are close to the limit,the breakthrough of dielectric constant can be achieved by structural design.The main work and results are as follows:(1)Ca Ti O₃ possesses a suitable dielectric constant and low dielectric loss.Moreover,the loss can be reduced by solid solution of lanthanum.In this paper,the dielectric properties of lanthanum substituted calcium titanate(Ca_1-xLa_2x/3Ti O₃/CLT)ceramics were studied.The results show that the dielectric constant decreased with the increase of La³⁺substitution,which is mainly due to the inhibition of Ti-O interaction in[Ti O₆]octahedron by A-site substitution.In addition,the effect of La³⁺on the quality factor of Ca Ti O₃ ceramics can be divided into four stages.When 0<x<0.05,the ordering of the material increased which results in the significantly increase of quality factor.when 0.05<x<0.1,the improvement of the quality factor is mainly attributed to the increase of the density of the samples;when 0.1<x<0.3,the ordering decreased and the anharmonic vibration of lattice is suppressed,and the quality factor increased slowly under the simultaneous action of the two factors;when 0.3<x,the phase transition of ceramics deteriorated the quality factor.When x=0.3,the optimal quality factor can be obtained,?_r?124.7,Q×f?14419GHz.(2)The phenomenon of dark center in Titanium-based ceramics will decrease the quality factor.In this paper,Al₂O₃ was used to inhibit the reduction of Ti⁴⁺and eliminate the dark center phenomenon.The XPS results of the samples before and after doping showed that the reduction of Ti⁴⁺at high temperature could be inhibited by Al₂O₃.The temperature coefficient of resonate frequency of CLT ceramics decreased with the increase of Al₂O₃.Since the doping of Al₂O₃ decreases the tolerance factor and increases the bond valence of B site.When the content of Al₂O₃ reached 1.11mol%,the dielectric constant is 141.0,the quality factor is 14482 GHz and the temperature coefficient of resonant frequency is 576.8 ppm/?.(3)PTFE with ultra-low dielectric loss was selected as the polymer matrix and filled with CLT ceramic powders with different particle sizes and morphologies.The results showed that when the particle size is around 20?m,less interface lower the dielectric loss.Compared with the spherical and the non-spherical particles,the spherical boundary between powders PTFE exist less pores,which result in lower dielectric loss.When the filling volume fraction of spherical particles with particle size of 20?m reached 40 vol%,the optimal dielectric properties could be obtained:?_r?10.25,tan??7.52×10^-4.(4)Compared with the traditional thermoplastic polymer,as a thermosetting resin,PSAE possesses a better temperature resistance.Moreover,PSAE has a high dielectric constant of 2.8,which is of great significance for the preparation of higher dielectric constant composites.Therefore,in this paper,PSAE was used as the second resin matrix to compound with CLT ceramics.And PSAE possesses a low viscosity,which is beneficial to prepare a composite with high ceramic volume fraction.When the ceramic ceramic volume fraction reached 50vol%,the composite with an optimal performance was obtained:?_r?23,tan??3.4×10^-3.(5)In this paper,after optimizing the intrinsic dielectric properties of polymer matrix and ceramics,the dielectric constant of the composite was further improved by structural design.According to the calculation equation of theoretical dielectric constant in series and parallel model,the dielectric constant of composites with parallel model was much higher than that of composites with series model.Actually,the composite with parallel structure could be prepared by ice template method combined with resin vacuum impregnation.The experimental results showed that when the solid content of the ceramic is 53.52vol%,the comprehensive performance is the best,while the dielectric constant reached 44.2.The thermal conductivity along Z axis reached 1.40(W/(m·K)),and that along X/Y axis reached 1.31(w/(m·K)).Compared with the 0-3composite,the dielectric constant and thermal conductivity of this composite are increased by about one time.According to the maximum solid content,CLT was replaced by equivalent volume of Al₂O₃ Fiber.When the volume fraction of Al₂O₃ fiber reached 5vol%,the comprehensive performance is the best,the dielectric constant reached 42.1.The thermal conductivity along Z axis reached 1.72(w/(m·K)),and that along X/Y axis reached 1.64(w/(m·K)).