The Abnormal Resonant Frequency Against Temperature Of CaSnSiO₅-Based Low-Permittivity Microwave Dielectric Ceramics

In the 5 G era of wireless communication,the high requirements are placed on the low latency and loss of communication systems.Low-permittivity microwave dielectric ceramics are widely used in high-end microwave dielectric devices and substrates due to their low dielectric constant(εr)and high quality factor(Q×f).However,the temperature coefficient of resonance frequency(τf)of low-permittivity microwave dielectric ceramics is generally a large negative value,and the the devices with near-zero τf value guarantee its the temperature stability of the resonance frequency during working.CaSnSiO5 ceramic with low εr,high Q×f,and abnormal positive τf values may be a new τf regulator.However,CaSnSiO5 ceramic exhibits the disadvantages of high sintering temperture(1525℃)and difficulty in synthesizing single phase.Therefore,the CaSnSiO5 ceramic is researched in this paper.The preparation process of CaSnSiO5 ceramic is optimized,its regulating effects are investigated,and the mechanisms of its abnormal positive τf value are revealed,which provides the new research ideas and experimental references for the controlling τf value of low-permittivity microwave dielectric ceramics to near zero.The specific research content is as follows:Different specifications of SnO2 raw materials and sintering process parameters on the sintering performance of CaSnSiO5 ceramics were investigated,and the best preparation process of CaSnSiO5 ceramic was obtained.The CaSnSiO5 ceramic exhibited less sintering shrinkage at a sintering temperature of 1550℃,and the second phase appeared at CaSnSiO5 ceramic.The CaSnSiO5 ceramic sintered at 1450℃ exhibited high relative density using Aladdin SnO2(50-70nm)as the raw material,and no second phase existed in CaSnSiO5 ceramic sintering at 1450℃ for 5 h.Therefore,the single-phase CaSnSiO5 ceramic could be synthesized at a stoichiometric ratio using Aladdin SnO2(50-70nm)as a raw material.The phase compositions and microwave dielectric properties of non-stoichiometric CaSnSiO5 ceramics were studied,and the effect of CaSnSiO5 on controlling nagetive τf value was verified.The CaSiO3 second phase in CaSnSiO5 ceramic could be induced at Sn-deficient CaSnSiO5 ceramics.CaSn0.1SiO3.2(0.1CaSnSiO5-0.9CaSiO3)ceramics exhibited the nearzero τf value(τf=-0.2ppm/℃).The phase compositions of Si-deficient CaSnSiO5 ceramics were complex.The Ca3SnSi2O9 phase could greatly improve the Q×fvalues of CaSn1-xSiO5-2x ceramics,and the τf values of CaSn1-xSiO5-2x ceramics were controlled by the CaSnSiO5 phase.CaSnSi1-xO5-2x(x=0.2)ceramic exhibited the high Q×f and near-zero τf values.The two-phase coexistence of CaSnSiO5 and Ca3SnSi2O9 phases were obtained at Ca1+2xSnSi1+xO5+4x ceramics by controlling the ratio of Ca:Sn:Si,and Ca1.8SnSi1.4O6.6(0.4CaSnSiO50.6Ca3SnSi2O9)ceramic exhibited the excellent microwave dielectric properties(εr=10.2、Q×f=81100 GHz、τf=-4.8 ppm/℃).The CaSnSiO5 was proven to be an effective τf regulator.The enhancement mechanisms of τf values of CaSn1-xTixSiO5 ceramics were studied by changing lattice parameters and shifting εr anomaly peaks with the small Ti4+substitution for Sn4+.With Ti4+substitution for Sn4+,the εr anomaly peaks of CaSnSiO5 ceramic were shifted to 196℃.The τf values of CaSn1-xTixSiO5 ceramics were increased from+62.5 ppm/℃ to+98.3 ppm/℃,and the negative correlations between oxygen octahedral distortions and τf value were obtained.The Q×f values of CaSn1-xTixSiO5(0 ≤x ≤0.4)ceramics gradually decreased from 44600GHz to 3100GHz by Ti4+substitution for Sn4+,which was attributed to the increase of intrinsic loss and extrinsic loss due to the decrease in relative covalency of Sn/Ti-O bond and the increase in the concentration of defect dipoles.The reduction mechanisms of microwave dielectric loss of Ca1-xSrxSnSiO5 ceramics were studied by increasing the relative covalency of Sn-O bond with the large Sr2+substitution for Ca2+.With a small amount of Sr2+substitution for Ca2+,the Sn-O bond of Ca1-xSrxSnSiO5 could be compressed by elongated Ca/Sr-O bond,and the relative covalency of Sn-O bond increased.The Q×f values of Ca1-xSrxSnSiO5 ceramics increased from 44600 GHz to 60100 GHz.The oxygen octahedral distortions of Ca1-xSrxSnSiO5 increased with Sr2+substitution for Ca2+,which made the τf values of Ca1-xSrxSnSiO5 ceramics decrease from+62.5 ppm/℃ to+49.6 ppm/℃.The τf values of CaSnSiO5-based ceramics were related to the redius of substituted ions,and the large oxygen octahedral distortions corresponded to nearer-zero τf value.The synergistic modification mechanism of the microwave dielectric loss and τf values of CaSnSi1-xGexO5 ceramics were studied by increasing oxygen octahedral distortions and the relative covalency of Sn-O and Si/Ge-O bonds with the large Ge4+substitution for Si4+.With Ge4+substitution for Si4+,the elongated Si/Ge-O bond compressed the Sn-O bond,and the relative covalency of Sn-O and Si/Ge-O bonds increased.The Q×fvalues of CaSnSi1-xGexO5 ceramics increased to 51500 GHz.The Q×f values of CaSnSi1-xGexO5(0 ≤x ≤0.6)ceramics were mainly affected by the relative covalency of Sn-O and Si/Ge-O bonds.The oxygen octahedral distortions of CaSnSi1-xGexO5(0≤x<0.6)ceramics with monoclinic structure(A2/a space group)increased to the maximum value of 0.4386%with Ge4+substitution for Si4+,which caused the decrease of τf value from+62.5 ppm/℃ to+18.5 ppm/℃.With the more Ge4+substitution for Si4+,the monoclinic structure(A2/a space group)of CaSnSi1xGexO5 ceramics destabilized and transformed into a triclinic structure(Alspace group)with low symmetry due to the excessive oxygen octahedral distortion,and the εr anomaly peaks shifted from-63℃(x=0.6)to 207℃(x=0.9),which causeed the change of τf values from+18.5 ppm/℃ to a large nagetive value(-61.2 ppm/℃).The τf values of CaSnSiO5-based ceramics were mianly affected by the shifting and broadening of εr anomaly peaks and oxygen octahedral distortions.