Microwave Dielectric Ceramics With Perovskite And Layered Perovskite Structure

Microwave dielectric ceramics have received the increasing scientific and commercial interests because of their important applications as the key materials in the field of microwave communications.Especially,with the rapid development of 5G mobile communication,the medium-k microwave dielectric ceramics with ultra-high Q value and near-zero temperature coefficient of resonant frequency are strongly required.In the present dissertation,the structure and microwave dielectric characteristics of(Sri?xCax)2TiO4 and Sr2[Ti1-x(Al0.5Nb0.5)x]O4 ceramics with R-P structure and CaTii-x(Al0.5Nb0.5)xO3 ceramics with perovskite structure were systematically investigated,and the following primary conclusions were obtained:(1)(Sr1-xCax)2TiO4 ceramics(0?x?0.15)were synthesized by a standard solid state reaction route.Tetragonal Ruddlesden-Popper(R-P)solid solutions(Sr1-xCax):2TiO4 with space group I4/mmm were obtained in the composition range of x=0-0.15,while Sr3Ti2O7 was observed as minor secondary phase in x>0.10.In the present ceramics,both the dielectric constant ?r and temperature coefficient of resonant frequency ?f decreased at first and turned to increase again with increasing x,while the quality factor Qf decreased slightly at first and turned to decrease sharply after x=0.1.The improved microwave dielectric properties with the best combination were obtained at x=0.075:?r.=39.3,Qf=93,550 GHz,?f=119 ppm/? The obvious improvement was achieved in temperature coefficient of resonant frequency(from 140 ppm/? to 119 ppm/?)with Ca2+-substitution for Sr2+ in Sr2TiO4 ceramics due to the increased B-site bond valance.(2)Sr2[Ti1-x(Al0.5Nb0.5)x]O4(x=0,0.10,0.25,0.30,0.5)ceramics were synthesized by a standard solid state reaction process.Sr2[Ti1-x(Al0.5Nb0.5)x]O4 solid solutions with tetragonal Ruddlesdon-Popper(R-P)structure in the space group/4/mmm were obtained within x<0.50,and only minor amount(1-2wt%)of Sr3Ti2O7 secondary phase was detected for the compositions x>0.25.The temperature coefficient of resonant frequency ?f of Sr2[Ti1-x(Al0.5Nb0.5)>x]O4 ceramics was significantly improved from 132 ppm/? to 14 pprm/? correlated with the increase in degree of covalency with increasing x.The dielectric constant ?r decreased linearly with increasing x,while high Qf value was maintained though it decreased firstly.The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure.Good combination of microwave dielectric properties was achieved for x=0.50:?r=25.1,Qf=77,580 GHz,?f=14 ppm/?.The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta(3)Modified CaTi1-x(Al0.5Nb0.5)xO3(x=0,0.025 0.10,0.15,0.30,0.45)ceramics have been synthesized by a standard solid state reaction method.CaTi1-x(Al0.5Nb0.5)xO3 solid solutions with orthorhombic structure in space group Pbnm were obtained successfully throughout all composition range(x?0.45).The Rietveld refinement and surface morphology confirmed the formation of single phase ceramics.The temperature coefficient of resonant frequency ?f and dielectric constant ?r decreased almost linearly as a function of x whereas,the Qf was increased continuously with increasing the x value.The dielectric constant decreased to a value of 55.1,while the ?f was modified remarkably from 750 ppm/?(at x=0)to near zero value 5 ppm/?(at x=0.45).An obvious increase in Qf value was observed from 11,500 GHz to 49,700 GHz.A combination of outstanding microwave dielectric characteristics is obtained at x=0.45(?f=55.1,Qf=49.700 GHz,?f=5ppm/?).The present modified ceramics could be suitable for the commercial application of wireless communication system.