Chemical Bonding Characteristics And Microwave Dielectric Properties Of Series Garnet-type Ceramics

As mobile communication technology enters the 5G era and communication devices move toward high frequency and high performance,it is important to develop new ceramics with low dielectric constant（ε_r）,high quality factor（Q×f）and nearly zero resonant frequency temperature coefficient（τ_f）.In this paper,a series of garnet-type microwave dielectric ceramics with lowε_r were prepared,and the effects of sintering characteristics,crystal structure,microstructure,and cation bonding valence on microwave dielectric properties of different ceramics were investigated by using X-ray diffraction,scanning electron microscopy,Raman spectroscopy,bond valence theory,chemical bonding theory,and other methods.The main content is as follows:（1）A series of cubic garnet structure Eu₂Ca MGa₄O₁₂（M=Ti,Sn,Zr）ceramics were prepared by partially replacing the Eu³⁺at the A site and the Ga³⁺at the B site of Eu₃Ga₅O₁₂ with Ca²⁺and M⁴⁺（M=Ti,Zr,Sn）,respectively.The optimum sintering temperature of the series ceramics is 1475℃,and the relative density is greater than95%.This series of ceramics has lowε_r（13.66,10.74,and 12.34）,the high the Q×f values（59,163,83,496,and 72,788 GHz）and the large negativeτ_f values（-48.15,-60.60 and-57.18 ppm/℃）.Theε_r of Eu₂Ca Ti Ga₄O₁₂ and Eu₂Ca Zr Ga₄O₁₂ is larger than the theoretical estimate of the Clausius-Mosotti equationε_r（C-M）（12.61 and 11.16）,and theτ_f values of both are larger than that of Eu₂Ca Sn Ga₄O₁₂.The differences betweenε_r andτ_f values are mainly due to the“rattling”（hopping）effect of the overall cation.The Q×f values are negatively correlated with the half-height width（FWHM）of Raman mode A_1g.（2）Ca₃M₂Ga₂Ge O₁₂（M=Ti,Sn,Zr）ceramics were prepared by choosing Ca²⁺and M⁴⁺（M=Ti,Zr,Sn）to completely replace Eu³⁺at the A-site and Ga³⁺at the B-site,respectively,and replacing one Ga³⁺at the C-site with Ge⁴⁺to balance the electrovalence and decrease the sintering temperature.The optimal sintering temperatures of Ca₃M₂Ga₂Ge O₁₂（M=Ti,Sn,Zr）ceramics are 1325℃,1475℃and1475℃,respectively,and the corresponding optimal microwave dielectric properties are as follows:ε_r=15.04,9.53 and 11.89,Q×f=59,515,49,355 and 45,587 GHz,τ_f=-47.3,-52.3 and-43.0 ppm/℃.As the radius of M⁴⁺ion increases,the“compression”effect of Ca²⁺in the A-site weakens,the“rattling”effect of M⁴⁺in the B-site weakens,the“compression”effect of Ga³⁺in the C-site weakens,and the“rattling”effect of Ge⁴⁺enhances,so theε_r of Ca₃Ti₂Ga₂Ge O₁₂ and Ca₃Zr₂Ga₂Ge O₁₂ is larger thanε_r（C-M）（11.73and 9.48）,while the values ofτ_f for both are closer to zero than Ca₃Sn₂Ga₂Ge O₁₂.The P-V-L complex chemical bonding theory analysis shows that the chemical bonds at the A and B sites play a dominant role inε_r.Compared with Ca₃Zr₂Ga₂Ge O₁₂ and Ca₃Sn₂Ga₂Ge O₁₂,the lattice energy and packing fraction of Ca₃Ti₂Ga₂Ge O₁₂ are larger,and the FWHM value of Raman mode A_1g is smaller,so its Q×f value is higher.（3）V⁵⁺and M⁴⁺（M=Si,Ti,Zr,Sn）were selected to enter the C-site,and single-phase Ca₃Mg₂MV₂O₁₂（M=Si,Ti）ceramics were prepared,while others were unable to synthesize of a single phase.The optimum sintering temperatures of Ca₃Mg₂Si V₂O₁₂and Ca₃Mg₂Ti V₂O₁₂ are 1240℃and 1260℃,and the microwave dielectric properties are:ε_r=9.70 and 11.70,Q×f=35,680 GHz and 48,530 GHz,τ_f=-60.1 ppm/℃and-43.7 ppm/℃.The overall structure of Ca₃Mg₂Ti V₂O₁₂ is in“rattling”state,which makes itsε_r larger thanε_r（C-M）andτ_f shifts in the direction of zero.And the overall structure of Ca₃Mg₂Si V₂O₁₂ is in“compression”,which is related to its average bond length being compressed.