Foundational Issues And Modification Of MRAlO₄ (M=Sr, Ca; R=La, Nd, Sm, Y) Microwave Dielectric Ceramics

Synthesis,structures,and microwave dielectric properties of MRAlO₄-based(M=Ca, Sr;R=La,Nd,Sm,Y) ceramics with I4/mmm space group were systermatically investigated together with their property-structure relationship,and based on these findings,the modification of microwave dielectric properties was carried out for CaSmAlO₄ ceramics through structure tailoring and microstructure control.The following primary findings and/or achiements were obtained:An evaluation system involving the open cavity method and the closed cavity method was built up to characterize the microwave dielectric properties with high accurately.An innovative algorithm for numerical calculation of complex permittivity was developed,and the computational efficiency was improved greatly.With the algorithm,the application scope of the close cavity method was significantly extended.Crystal structure refinements were carried out for MRAlO₄ ceramics by Rietveld method.Along the c axis,AlO₆ octahedra and(M,R)O₉ dodecahedra were heavily stretched and compressed,respectively,while within the ab plane,the situation was reversed.The interlayer size mismatch and the interlayer electric polarization were the primary reasons for these distortions,and they could be evaluated with the tolerance factor and the deformation degree of octahedron(or the distance from(M,R) cation to O(2a) atomic plane),respectively.As the tolerance factor decreased,the distortion of AlO₆ octahedra kept nearly unchanged,while that of(M,R)O₉ increased.Based on the analysis by the bond valence method,the instability of SrYAlO₄ and the metastability of CaLaAlO₄ were attributed to large differences of ionic radius and of apparent bond valence between M and R cations,respectively.To understand the dielectric nature of MRAlO₄ ceramics in terms of phonon modes,Fourier transform infrared reflectivity spectra in the range of 50-4000 cm^-1 were measured and evaluated by means of K-K ananlysis and classical oscillator fit.The data were extrapolated below the measured frequency range to estimate the intrinsic microwave losses.The bending and stretching vibration modes of(M,R)-AlO₆ gave primary contributions to the microwave complex permittivity,therefore the distortion of (M,R)O₉ dodecahedra greatly affected the dispersion parameters of these two modes and consequently dominated polarizabilities and intrinsic dielectric losses of MRAlO₄. This could explain the great deviation of measured polarizabilities from those predicted by the oxide additivity law and the variation of calculated Qf values with the tolerance factor.To minimize the intrinsic dielectric loss,it was the keypoint to reduce the distortion of(M,R)O₉ dodecahedra.Moreover,the fact that the calculated Qf values were much higher than the measured ones suggested a great opportunity to improve Qf through optimizing the microstructures.By changing molar ratios among CaO,Sm₂O₃,and Al₂O₃,the effects of secondary phases on mcirostructures and microwave dielectric properties were investigated for CaSmAlO₄-based ceramics.In(2+2x)CaO·(1-x)Sm₂O₃·Al₂O₃(x=-0.10,-0.05,0,0.05, 0.10,0.20)ceramics,Ca₃Al₂O₆ and CaO secondary phases were observed at x＞0,while SmAlO₃ and Sm₂O₃ secondary phases were observed at x≤0.In (2+2x)CaO·Sm₂O₃·Al₂O₃(x=-0.02,-0.01,0,0.02,0.04) ceramics,CaSmAl₃O₇,CaO, and Sm₂O₃ secondary phases were observed at x＞0,while SmAlO₃ secondary phase was observed at x≤0.The secondary phases Ca₃Al₂O₆ and SmAlO₃ decreased the Qf values greatly,whereas CaSmAl₃O₇,CaO and Sm₂O₃ had much weaker minus effects on Qf values.Good microwave dielectric properties were achieved in (2+2x)CaO·Sm₂O₃·Al₂O₃(x=0.02,0.03,0.04) ceramics:ε_r≈19,Qf =120,000～125,300 GHz,Ï„_f=-10～-9ppm/℃.Modification of CaSmAlO₄ was performed by cosubstitution of Ca on A site and Ti on B site,and Ca_1+xSm_1-xAl_1-xTi_xO₄(0≤x≤0.4) ceramics were prepared and characterized.For Ca_1+xSm_1-xAl_1-xTi_xO₄ phases,as x increased,lattice parameter a increased linearly whereas lattice parameter c decreased linearly;the distortion of (Al,Ti)O₆ octahedra reduced slightly,whereas(M,R)O₉ dodecahedra were further compressed along the c axis,and this was responsible for the decrease in lattice parameter c.Ca_1+xSm_1-xAl_1-xTi_xO₄ ceramics showed very good microwave dielectric properties,especially when 0.06≤x≤0.2(ε_r≈20,Qf=96,500～118,700GHz,andÏ„_f=-8～2 ppm/℃).SmAlO₃ secondary phase was detected at x=0,and trace amount of CaTiO₃ secondary phase was detected at x≥0.15;abnormal grain growth was observed in samples with x=0,0.02 and sintering temperature of 1500℃. Suppression of these microstructures would help Ca_1+xSm_1+xAl_1-xTi_xO₄ ceramics maintain high Qf values.With certain amount of excessive CaO in composition, microwave dielectric properties of Ca_1+xSm_1-xAl_1-xTi_xO₄-based ceramics were further improved and excellent microwave dielectric properties(ε_r=20.2,Qf=100,000GH,Ï„_f=0.5 ppm/℃) were achieved at x=0.15.