Preparation,Properties And Modification Of Sr_2-xLa_xTi_1-xAl_xO₄ Microwave Dielectric Ceramics

At present,ceramic systems used in microwave communication such as mobile communication and satellite communication have problems such as high price,harsh preparation conditions and large dielectric loss,and with the extensive development of modern electronic devices and wireless communication systems,the scale and consumer demand of microwave dielectric ceramics in China are growing rapidly.Therefore,it is necessary to develop a series of microwave dielectric ceramic materials with suitable dielectric constant,low dielectric loss,near-zero temperature coefficient of resonant frequency and suitable sintering temperature.Among them,microwave dielectric ceramics with K₂NiF₄type structure have attracted more and more attention because of their ultra-low dielectric loss.In microwave dielectric ceramics with K₂NiF₄structure,SrLaAlO₄（ε_r=16,Q×f=119300 GHz,τ_f=-33.9 ppm/℃）and Sr₂TiO₄（ε_r=42,Q×f=145200 GHz,τ_f=130 ppm/℃）have excellent microwave dielectric properties,but the temperature stability is extremely poor.Therefore,we selected these two ceramic materials with K₂NiF₄structure,prepared high-Q microwave medium solid solution ceramics with good temperature stability by high-temperature solid phase method,and further studied their optimal components.The following is the main research content of this paper:（1）Sr_2-xLa_xTi_1-xAl_xO₄microwave medium solid solution ceramics with tetragonal layered perovskite structure（K₂NiF₄structure）were prepared by the traditional solid-phase reaction method,and the relationship between the structure and microwave dielectric properties was established by calculating complex chemical bond theory and studying Raman spectroscopy,and the effects of Raman shift and half-peak width of bond ionism,lattice energy,A₁g（1）characteristic peaks and TiO₆octahedral distortion on the dielectric properties of ceramic samples were analyzed.The change in dielectric constant is attributed to the increase in the average bonding ionity of Sr/La-O bonds and the polarity mode of lattice vibration.The decrease in the figure of merit is attributed to the decrease of lattice energy and the decrease of crystal order.The change of temperature coefficient of resonant frequency is closely related to the distortion of TiO₆octahedron.At the sintering temperature of 1575℃,the ceramic sample Sr_1.6La_0.4Ti_0.6Al_0.4O₄with x=0.4 obtain the optimal microwave dielectric properties:ε_r=22.5,Q×f=75464 GHz,τ_f=-7.3 ppm/℃.（2）On the basis of the previous work,the(Sr_1.6-xCa_x)La_0.4Al_0.4Ti_0.6O₄（x=0.1,0.2,0.3,0.4）microwave dielectric ceramics were prepared by ion substitution method,and the performance was successfully further improved,so that the dielectric constant increased while the quality factor was also increased by 13%,and the temperature stability became better,and the sintering temperature also decreased by 25℃.With the increase of components,the dielectric constant of ceramics increases,which is related to the change of the relative density of ceramics,and the quality factor first increases and then decreases,attributed to octahedral distortion and changes in tolerance factors,similar to the change ofτ_fandε_r,from negative monotonic to positive value.Sr_1.6-xCa_xLa_0.4Al_0.4Ti_0.6O₄ceramics（x=0.2）sintered at 1550℃ obtain the best optimal dielectric properties,ε_r=23.3,Q×f=85442 GHz,τ_f=4.5 ppm/℃（3）Based on the first work,Sr_1.6La_0.4Ti_0.6Al_0.4O₄was selected as the base,and the sintering additive calcium fluoride was added,which successfully reduced the sintering temperature without reducing the dielectric properties.The phase and microstructure of ceramic samples were studied by XRD,scanning electron microscopy observation and Raman spectroscopy,and the relationship between Raman shift of A₁g（1）characteristic peak,half-peak width and macroscopic dielectric properties was explored:XRD diffraction proves the presence of quadrilateral K₂NiF₄and cubic fluorite phases in ceramic samples,calcium fluoride dielectric constant and quality factor are much lower than the matrix ceramics,so with the increase of the amount of addition,the dielectric constant,quality factor and resonant frequency temperature coefficient of ceramic samples decreased.By adding calcium fluoride,the optimal sintering temperature of Sr_1.6La_0.4Ti_0.6Al_0.4O₄ceramics was reduced by 225℃,and the microwave dielectric properties were improved.Sr_1.6La_0.4Ti_0.6Al_0.4O₄-1 wt%CaF₂ceramics sintered at 1350℃ showed the best dielectric properties:ε_r=22.2,Q×f=72125 GHz,τ_f=-6.3ppm/℃.