Study On The Structure-Property Relationship Of Microwave Ceramics With Olivine And Scheelite Structures

With the rapid development of communication technology,in order to meet the increasing demand for the performance of electronic components,the performance of microwave dielectric ceramics is getting higher and higher.Microwave dielectric ceramics are gradually moving towards higher operating frequency ranges（millimeter wave or sub-millimeter wave）.In this paper,BaCu（B₂O₅）-doped CaMgGeO₄ ceramics,LiMnPO₄ ceramics,and AMoO₄（A=Ca,Ba）ceramics were synthesized by conventional solid-phase reaction methods.The effects of doping and sintering processes on the sintering behavior,structure and properties of ceramics were studied.The phonon characteristics and intrinsic properties of scheelite structural ceramic were explored.In this paper,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,Raman scattering（Raman）,and Fourier far-infrared（FTIR）spectroscopy are used to deeply analyze the ceramics structures,dielectric properties,and the relationship between them.（1）CaMgGeO₄（CMG）ceramic with olivine structure was fabricated by traditional solid-phase reaction method,which was dense at 1300℃/6h and has excellent dielectric properties（εr=6.83,Q×f=125,432 GHz,f=14.9 GHz）.The effects of BaCu（B₂O₅）（BCB）on the sintering characteristics,crystal structures and microwave dielectric properties of CMG systems were studied.BCB can reduce the sintering temperature（Ts）of CMG ceramic effectively,and when the amount of BCB is 16 wt.%,Ts can be lowered to 875℃.CMG ceramic containing 8 wt.%BCB shows excellent dielectric properties（sr=7.01,Q ×f=73,962 GHz,f=14.8 GHz）at 940℃/6h.It is found that the Raman mode 11 moves to a lower wavenumber with the increase of BCB content,which is opposite to the change of the dielectric constant,however,the FWHM values of mode 11 correlate with Q ×f values negatively.In addition,the change of the packing fraction is similar to that of the Q ×f values with increasing BCB.Decrease in the packing fraction means the increase in the vibrational space and possibly make the non-resonance increasing,which in turn reduces the Q ×f value.In addition,the ceramic has good chemical compatibility with Ag electrode,indicating a broad application prospect in LTCC for CMG.（2）LiMnPO₄（LMP）microwave dielectric ceramics suitable for low temperature firing applications were manufactured at different temperatures via a standard solid-state reaction method,which belongs to olivine structure.LMP ceramics sintered at 750℃ adapt excellent dielectric properties:εr=7.82,Q × f=29,189 GHz（f=12.7 GHz）.The lattice vibrational characteristics were analyzed utilizing Raman and infrared reflection spectroscopy.Based on infrared reflection spectroscopy,the four-parameter semi-quantum model was performed to simulate their microwave dielectric properties,which were proved to be similar to the measured values.The Raman shift of LMP ’s mode 9(attributed to the[PO₄]^3-tetrahedron)possesses the opposite trend with the change of P-O bond length.With increasing temperature,the packing fraction is positively correlated with the Q×f value,and simultaneously,the bond length and the permittivity are also positively correlated.That is,the structure-property relationship of LMP ceramics were established as a function of sintering temperature.（3）The phase-pure CaMoO₄ and BaMoO₄ ceramics were densified under the sintering conditions of 1150℃/4h and 1100℃/3h,respectively（both relative densities>95%）.AMoO₄（A=Ca,Ba）ceramics have a tetragonal scheelite structure with the space group I41/a.The Raman mode in the low wavenumber range is related to the vibration of the[MoO₄]tetrahedron relative to the cation at the A site.The internal modes v3（Eg）and v4（Bg）correspond to the internal vibrations of the[MoO₄]tetrahedral element.The IR spectrum were fitted by the 4-P model,and the main contribution to the dielectric response of AMoO₄（A=Ca,Ba）is the Eu mode,that is,the translation vibration of Ca/Ba-O bonds.In addition,the vibration parameters independently obtained by the K-K transform agree well with the fitting results based on the 4-P model,confirming the reliability of the fitting results.