Structural Regulation And Microwave Dielectric Properties Of Garnet-type Sm₃Ga₅O₁₂-based Ceramics

Microwave dielectric ceramics are the key materials for making microwave components such as dielectric resonators,filters,and dielectric substrates,and are widely used in mobile communications,satellite communications,radar antennas,global navigation and positioning,and the Internet.With the advent of the 5G era,microwave components continue to develop towards miniaturization,chip-based,integrated,lightweight,high frequency,and high stability.Microwave dielectric ceramics with low dielectric constant（ε_r）,high quality factor（Q×f）,or low dielectric loss,as well as the near-zero temperature coefficient of resonant frequency（τ_f）are becoming an important choice.Some garnet-based A₃B₂C₃O₁₂ceramics have been widely studied for their lowε_rand high Q×f values,but the large negativeτ_fvalues severely limit their practical applications.In this thesis,Sm₃Ga₅O₁₂-based garnet is selected as the research object,and the properties of the garnet are adjusted by substituting A,B,and C ions individually or complexly,taking advantage of the controllable structural gift of the garnet,the influences and mechanism of crystal structure on microwave dielectric properties（ε_r,τ_fand Q×f）was studied from the aspects of bond valence,lattice vibration,atomic packing fraction,lattice energy and bond energy.The details are as follows:（1）Firstly,the effects of the partial substitution of Sm³⁺ions at the A-site by Bi³⁺on the sintering characteristics,phase composition,crystal structure,and microwave dielectric properties of Sm_3-xBi_xGa₅O₁₂（0≤x≤0.4）ceramics were investigated.A significant reduction in the optimum sintering temperature from 1440°C to 1140°C was achieved by the replacement of Sm³⁺by Bi³⁺.The microwave dielectric properties of Sm₃Ga₅O₁₂（x=0）ceramics are significantly different from the previously reported results comparing Q×f andτ_fvalues.When 0≤x≤0.3,the ceramic is a single-phase cubic garnet structure with a space group of Ia-3d（230）.The increase inε_rfrom 12.68 to 13.35 is mainly related to the increasing ion polarisation rate and the increasing"rattling"effect of the A-site cation.The increase in Q×f is mainly related to the increase in total lattice energy and the decrease in the full width at half maximum（FWHM）of the A_1gvibrational mode of the Raman spectrum.The shift ofτ_fin the positive direction is mainly associated with an increase in the total bond energy.ε_rvaries linearly withτ_fas the amount of substitution increases.The changes inε_randτ_fare mainly the result of a synergistic effect of two mechanisms,the dilution increase in the mean ion polarisation rate and the cation"rattling"effect.（2）Secondly,the effect of B-site substitution on the crystal structure and dielectric properties was investigated.Sm₃Mg BGa₃O₁₂（B=Zr,Sn）ceramics were designed and prepared by replacing the smaller B-site Ga³⁺ions in Sm₃Ga₅O₁₂with Mg²⁺/Zr⁴⁺and Mg²⁺/Sn⁴⁺complexes of larger ionic radius.The best sintering temperatures for Sm₃Mg BGa₃O₁₂（B=Zr,Sn）are 1500°C and 1525°C,respectively,both in cubic garnet-type structures with space group Ia-3d（230）and microwave dielectric properties ofε_r=13.46 and11.45,Q×f=63,620 GHz and 92,916 GHz,τ_f=-39.5 ppm/°C and-51.1 ppm/°C,respectively.Bond valence and P-V-L bonding theory suggest that Sm₃Mg Sn Ga₃O₁₂as a whole is“compressed”,while Sm₃Mg Zr Ga₃O₁₂as a whole is rallting,resulting in Sm₃Mg Zr Ga₃O₁₂ceramics with higherε_rand more positiveτ_fvalues.Sm₃Mg Sn Ga₃O₁₂has a higher lattice energy than Sm₃Mg Zr Ga₃O₁₂,in addition to its large packing fraction and smaller FWHM value for the A_1gvibrational mode of the Raman spectrum,which has a higher Q×f value.（3）The effects of synergistic substitution at A and B sites on the crystal structure and dielectric properties were explored.Sm_3-xCa_xTi_xGa_5-xO₁₂（0≤x≤2.1）ceramics were prepared using Ca²⁺/Ti⁴⁺synergistic substitution of Sm³⁺and Ga³⁺at the A and B positions in Sm₃Ga₅O₁₂,and characterised for sintering properties,phase composition,crystal structure and microwave dielectric properties.The densification temperature of the sample decreases from 1440°C to 1260°C as the amount of substitution increases.X-ray powder diffraction data with Rietveld structural refinement confirm the preferential access of Ca²⁺to the A-site and Ti⁴⁺to the B-site.Sm_3-xCa_xTi_xGa_5-xO₁₂for a single cubic garnet phase when 0≤x≤1.7.The microwave dielectric properties areε_r=12.68～17.12,τ_f=-27.9 ppm/°C～-17.5 ppm/°C,Q×f=103,617 GHz～68,895 GHz.Band-valence calculations indicate that the anomalous increase in the dielectric constantε_ris related to the introduction of the"rattling"effect of the B-site Ti⁴⁺ion and the gradual increase in substitution,thatτ_fis mainly influenced by two mechanisms,the increase in the ion polarisation rate and the"rattling"effect at the B-site,and that the decrease in Q×f is related to the increase in the half-height width of the A_1gmode.When 1.8≤x≤2.1,the dielectric constant increases sharply（from 17.52 to 26.51）,τ_fincreases rapidly from-6.7 ppm/°C to+32.3 ppm/°C and Q×f decreases rapidly to 22,923GHz,these are mainly due to the high dielectric constant and the lossy second phase Ca Ti O3.At x=1.8 and 1.9,τ_fvalues are near zero and the microwave dielectric properties areε_r=17.52 and 18.53,Q×f=59,150 GHz and 48,487 GHz andτ_f=-6.7 ppm/°C and+2.9 ppm/°C respectively.（4）The effects of synergistic substitution at the B and C positions on the crystal structure and dielectric properties were explored.Sm₃BGa₃Si O₁₂（B=Mg,Zn）ceramics were prepared by using Zn²⁺or Mg²⁺with larger ionic radii and Si⁴⁺with smaller ionic radii to replace the Ga³⁺ions at the B and C positions in Sm₃Ga₅O₁₂,respectively.X-ray diffraction analysis showed that Sm₃Zn Ga₃Si O₁₂failed to obtain a single-phase garnet structure in the sintering temperature range of 1260°C to 1400°C.Sm₃Mg Ga₃Si O₁₂was sintered at 1500°C/6h to obtain a single phase with microwave dielectric properties ofε_r=11.97,Q×f=89,965 GHz andτ_f=-50.1 ppm/°C.The bond valence calculations show that the overall structure of Sm₃Mg Ga₃Si O₁₂ceramics exhibits a compression effect,resulting in lower values of the corresponding dielectric constantsε_randτ_fthan Sm₃Ga₅O₁₂ceramics.P-V-L complex chemical bonding theory calculations reveal that the total lattice energy of Sm₃Ga₅O₁₂is higher than that of Sm₃Mg Ga₃Si O₁₂ceramics and therefore Sm₃Ga₅O₁₂has a higher Q×f value.（5）Finally,the garnet ceramics Ca₃Zn BGe₃O₁₂（B=Zr,Sn）were designed and prepared with simultaneous substitution of A,B and C in Sm₃Ga₅O₁₂.XRD analysis showed that Ca₃Zn BGe₃O₁₂（B=Zr,Sn）were cubic garnet structures with optimum sintering temperatures of 1270°C and 1230°C,respectively,and microwave dielectric properties ofε_r=10.52 and9.34;Q×f=79,700 GHz and 96,040 GHz;τ_f=-36.1 ppm/°C and-61.5 ppm/°C,respectively.The bond valence and lattice energy calculations reveal that Ca₃Zr Zn Ge₃O₁₂ceramics are in an overall"rattling"state,while Ca₃Sn Zn Ge₃O₁₂is in a slightly compressed state,with the"rattling"effect of Ca₃Zr Zn Ge₃O₁₂being the main factor behind the higherε_randτ_fthan Ca₃Sn Zn Ge₃O₁₂.The lattice energy and packing fraction of Ca₃Sn Zn Ge₃O₁₂is higher than that of Ca₃Zr Zn Ge₃O₁₂,resulting in a larger Q×f value.Extrapolating the fitted far-infrared reflectance spectra of Ca₃Zn BGe₃O₁₂（B=Zr,Sn）to the microwave band,the intrinsic Q×f values of Ca₃Zr Zn Ge₃O₁₂and Ca₃Sn Zn Ge₃O₁₂are very close to the actual measured Q×f values.