Performance Control Of High Dielectric Constant Microwave Dielectric Ceramics With Tungsten Bronze Structure

The rapid development of the fifth generation(5G)mobile communication system has put forward higher performance requirements for microwave devices such as dielectric resonators and dielectric filters.As the core material for manufacturing microwave devices,microwave dielectric ceramics have always been the focus of research.Compared with other high dielectric constant(?_r)microwave dielectric ceramics,Ba_6-3xM_8+2xTi₁₈O₅₄(M=lanthanide)ceramics with tungsten bronze structure have better comprehensive dielectric properties,but relatively lower quality factor(Q×f)and the larger temperature coefficient of resonance frequency(TCF)limits its application in 5G mobile communications.In this thesis,the effects of lanthanides on sintering characteristics,crystal structure,microstructure,chemical bond characterizations,vibration spectra and microwave dielectric properties of Ba₄M_28/3Ti₁₈O₅₄(M=La,Pr,Nd,Sm)ceramics were investigated,and the relationship between vibration mode,chemical bond parameters and microwave dielectric properties were also studied.Based on it,with the goal of improving the comprehensive microwave dielectric properties of the Ba₄M_28/3Ti₁₈O₅₄ ceramic series,A-site substituted Ba₄Sm_28/3Ti₁₈O₅₄,A/B-site cosubstituted Ba₄Pr_28/3Ti₁₈O₅₄ and B-site substituted Ba₄Nd_28/3Ti₁₈O₅₄were investigated.The main research contents as follows:Single phase Ba₄M_28/3Ti₁₈O₅₄(M=La,Pr,Nd,Sm)ceramics with orthorhombic tungsten-bronze structure were prepared by the traditional solid-state processing.The crystal structure refinement results showed that as the ionic radius of the lanthanide element decreased,the unit cell parameters and unit cell volume gradually decreased,and the average bond length of the Ba-O,M-O,and Ti-O bonds gradually became shorter.Based on the theory of complex crystal chemical bonds,the chemical bond parameters(bond energy,lattice energy and bond ionicity)of Ba-O,M-O and Ti-O bonds were calculated,and the relationship between microwave dielectric properties and the chemical bond parameters were established.The decrease in?_r of Ba₄M_28/3Ti₁₈O₅₄ ceramics was related to the decrease in ionic polarizability,ionicity of chemical bonds and unit cell volume.The increase in Q×f value was closely related to the increase in packing fraction,total lattice energy,and the decrease in full width at half maximum(FWHM)of the Raman peaks of A_gand B_1g.The shift of the TCF value in the negative direction was associated with the decrease of tolerance factor and the increase of total bond energy.The infrared reflectivity spectra indicated that the major dielectric contribution of Ba₄M_28/3Ti₁₈O₅₄(M=La,Pr,Nd,Sm)ceramics in the microwave range may be caused by the absorption of phonon oscillations at the infrared region.As the ionic radius of the lanthanides decreased,the vibration intensity of the infrared active vibration mode at low frequencies became weaker,and the contribution to the dielectric polarization gradually decreased.In order to solve the problem of Ti reduction and large TCF value in Ba₄Sm_28/3Ti₁₈O₅₄ceramics.The effect of a small amount of Pr substitution for Sm at the A-site on the crystal structure,microstructure,vibration spectra,and microwave dielectric properties of Ba₄Sm_28/3Ti₁₈O₅₄ ceramics were studied,and the role of oxidant Pr₆O₁₁ in inhibiting the reduction of Ti⁴⁺ions was analyzed.Pr substitution could effectively improve the"dark core"phenomenon of Ba₄Sm_28/3Ti₁₈O₅₄ ceramics,suppressed the reduction of Ti⁴⁺ions and the generation of oxygen vacancies,thereby reducing dielectric loss.Pr⁴⁺ions with strong oxidizability was the key to inhibit the reduction of Ti⁴⁺ions and the generation of oxygen vacancies.With increasing Pr content,the?_r increased,the Q×f increased significantly,and the TCF shifted in the positive direction.When y=0.15,Ba₄(Sm_1-yPr_y)_28/3Ti₁₈O₅₄(0?y?0.25)sintered at 1375? for 4h showed excellent microwave dielectric properties:?_r=80.5,Q×f=9700 GHz,TCF=-0.9 ppm/?.To improve the comprehensive microwave dielectric properties of Ba₄M_28/3Ti₁₈O₅₄series ceramics,the effects of A/B-site cosubstitution on the crystal structure,microstructure,vibration spectra and microwave dielectric properties of Ba₄Pr_28/3Ti₁₈O₅₄ceramics were investigated.Sm/Al cosubstitution led to a good combination of high?_r(?_r?70),high Q×f(Q×f?12,000 GHz),and near-zero TCF(-10<TCF<+10 ppm/?)in a wide range(0.4?x?0.7).The continuous adjustment of dielectric constant and temperature coefficient of resonance frequency was realized.For Ba₄(Pr_0.5Sm_0.5)_28/3Ti_18-yAl_4y/3O₅₄(x=0.5;0?y?1.5)series,XPS analysis confirmed that Al³⁺ion substitution could effectively inhibit the reduction of Ti⁴⁺and improve the quality factor,and the influence of B-site cation order on microwave dielectric properties was analyzed by Raman spectroscopy and infrared reflectance spectroscopy.When y=1.25,Ba₄(Pr_0.5Sm_0.5)_28/3Ti_18-yAl_4y/3O₅₄ceramics sintered at 1375? for 4h exhibited excellent microwave dielectric properties:?_r=72.5,Q×f=13,900GHz,TCF=+1.3 ppm/?.In order to obtain a relatively higher?_r,Ba₄(Pr_0.4Sm_0.6)_28/3Ti_18-yGa_4y/3O₅₄(x=0.6;0?y?1)solid solution ceramics were successfully prepared by Sm³⁺/Ga³⁺cosubstitution.Compared with pure Ba₄Pr_28/3Ti₁₈O₅₄ ceramics,the Q×f was increased by about 90%after the Sm³⁺/Ga³⁺cosubstitution;the TCF value was adjusted from+150 ppm/? to near zero;the?_r was maintained at a relatively high value.When y=0.75,Ba₄(Pr_0.4Sm_0.5)_28/3Ti_18-yGa_4y/3O₅₄(0?y?1)ceramics sintered at 1375? for 4h showed excellent comprehensive microwave dielectric properties:?_r=78.5,Q×f=12,400GHz,TCF=+2.1 ppm/?.To further improve the comprehensive microwave dielectric properties of Ba₄M_28/3Ti₁₈O₅₄series ceramics,the effect of different types of ion substitution in B-site(a small amount of donor Nb⁵⁺,acceptor Ga³⁺and donor/acceptor(Ga_1/2Nb_1/2)⁴⁺substitution)on the dielectric properties of Ba₄Nd_28/3Ti₁₈O₅₄ ceramics were investigated.Acceptor Ga³⁺substitution could effectively suppress the reduction of Ti⁴⁺ions and promote grain growth,greatly improving the quality factor.Donor Nb⁵⁺substitution aggravated the reduction of Ti⁴⁺ions and suppressed grain growth,which seriously deteriorated the quality factor.The donor-acceptor(Ga_1/2Nb_1/2)⁴⁺cosubstitution could also inhibit the reduction of Ti⁴⁺ions and improve the quality factor to a certain extent,but the grain size had not changed significantly.In addition,all types of ion substitution could effectively improve the TCF of Ba₄Nd_28/3Ti₁₈O₅₄ ceramics.On the basis,the effects of Ga³⁺ions with different substitution amounts on the crystal structure,microstructure,and dielectric properties of Ba₄Nd_28/3Ti₁₈O₅₄ ceramics were studied.When y=1.5 and y=2,Ba₄Nd_28/3Ti_18-yGa_4y/3O₅₄(0?y?2)ceramics sintered at 1400? for 4h exhibited excellent comprehensive microwave dielectric properties:when y=1.5,?_r=72.8,Q×f=14,600 GHz,TCF=+4.1 ppm/?;when y=2,?_r=70.3,Q×f=15,500 GHz,TCF=+3.9 ppm/?.Overall,a series of microwave dielectric ceramics with?_r ranging from 70 to 80,high Q×f value(up to 15,500GHz)and near-zero TCF were obtained by sophisticated adjustment of the ionic polarizability and tolerance factor of Ba₄M_28/3Ti₁₈O₅₄ ceramics and the suppression of Ti reduction,which can be applied to the manufacture of high performance microwave devices.