Research On Structure And Property Optimization Of MgO-TiO₂ Based Microwave Dielectric Ceramics

In the past 30 years, microwave dielectric ceramics have been widely used in wireless communication systems as dielectric filters, resonators, and patch-type antennas. To meet the requirements for use in such wide applications, the materials must satisfy three major criteria. The high dielectric constant（ er） is very important for realizing the miniaturization of microwave component because the physical length of the dielectric resonator is in proportion to1 /er. The high quality factor（ Q ×f） enables the outstanding microwave selectivity, which can reduce the risk of cross talk within a given frequency. Moreover, the low temperature coefficient of resonant frequency（τ_f） ensures the high stability of the equipment in different temperature environments.The structure and microwave dielectric property of Mg₂TiO₄-based and Mg TiO₃-based ceramic system prepared by the conventional solid-state route were investigated. The main results and conclusions are presented as below.1. The crystal structure of Mg₂TiO₄ ceramic was determined by Rietveld full spectrum fitting method, and the crystal lattice vibration mode and Raman spectrum were analyzed. Mg₂(Ti_1-xSn_x)O₄（x=0-1） ceramics were synthesized by the conventional solid-state reaction route. The effect of Sn substitution on the crystal structure, surface morphology and microwave dielectric properties of Mg₂(Ti_1-xSn_x)O₄ have been investigated. Formation of the solid solution was confirmed by the X-ray diffraction and the measured lattice parameters, which varied linearly from Mg₂TiO₄（a=b=c=8.4402?） to Mg₂ SnO₄（a=b=c=8.6372?）. The minor second phase was Mg O, of which the content increased with x increasing. The reaction mechanism of the presence of Mg O has been found. The crystal structure of these materials was determined by Rietveld refinement. We found that the er, Q ×f and τ_f of these compounds are correlated with the ionic polarizability, packing fraction and bond value, respectively. A fine microwave dielectric properties（ er=12.2, Q ×f =153,270, τ_f =-51.7℃） was achieved for Mg₂(Ti_0.8Sn_0.2)O₄ ceramics sintered at 1510℃ for 4h.2.(Mg_0.97Zn_0.03)₂(Ti_0.95Sn_0.05)O₄ ceramics by adding Ca TiO₃ have been prepared via the solid-state reaction method. The microstructures of samples were systematically studied in order to establish the effects of sintering temperature and additives on microwave dielectric properties of(Mg_0.97Zn_0.03)₂(Ti_0.95Sn_0.05)O₄ ceramics by x-ray diffraction and scanning electron microscopy. A fine combination of microwave dielectric properties（ er=14.6, Q ×f =183,468 GHz, τ_f =-43.7 ppm/℃） was achieved for(Mg_0.97Zn_0.03)₂(Ti_0.95Sn_0.05)O₄ ceramics sintered at 1390℃for 4h. Ca TiO₃, as a τ_f compensator, was added to form a temperature-stable ceramic system. 0.93(Mg_0.97Zn_0.03)₂(Ti_0.95Sn_0.05)O₄-0.07 Ca TiO₃ ceramic sintered at 1390℃ had optimal dielectric properties（ er=18.3, Q ×f =94,715 GHz, τ_f =-4.1 ppm/℃） which satisfied microwave applications in resonators, filters and antenna substrates.3. The crystal structure of Mg TiO₃ ceramic was determined by Rietveld full spectrum fitting method. The tolerance factor, the bond energy, the cation of the cation, the percentage of the covalent interaction and the distortion parameters of the eight surface of oxygen were calculated by using the structural parameters of Mg TiO₃. The lattice vibrational modes and Raman spectra of the crystal lattice were analyzed.4. The effects of Co₂O₃ addition on the sintering behavior, phase compositions, microstructure and microwave dielectric properties of 0.95 Mg TiO₃-0.05 Ca TiO₃ ceramics have been investigated. These results suggested that Co₂O₃ addition not only contributes in lowering the sintering temperature but also inhibits the formation of second phase Mg Ti₂O₅ in 0.95 Mg TiO₃-0.05 Ca TiO₃ ceramics. Moreover, the decomposition of Co₂O₃ would inhibit the Ti⁴⁺ from being reduced to Ti³⁺, which had the positive effect on the Q ×f value. A fine combination of microwave dielectric properties（ er=20.5, Q ×f =76,485 GHz, τ_f =2.4 ppm/℃） was achieved for 0.95 Mg TiO₃-0.05 Ca TiO₃-0.01Co₂O₃ ceramics sintered at 1300 ℃ for 4h, which satisfied microwave applications in resonators, filters and antenna substrates.5. The（1-x）Mg TiO₃-x Mg2.05 Si O_4.05-0.06 Ca TiO₃ ceramic system composite dielectric ceramics with different Mg_2.05 Si O_4.05 addition amounts were prepared by solid-state reaction method. The results indicated that exceeding Mg in Mg_2.05 Si O_4.05 not only hold back the formation of Mg Si O₃ but also inhibits the formation of second phase Mg Ti₂O₅. A new composite ceramics Mg TiO_3-x Mg_2.05 Si O_4.05-y Ca TiO₃ sintered at 1380℃ for 4h showed excellent comprehensive microwave dielectric properties, dielectric constant（ er） and temperature coefficient of resonant frequency（τ_f） can be fully adjustable by change the value of x and y.6. The effects of Li₂ZnTi₃O₈ addition on the sintering behavior, phase compositions, microstructure and microwave dielectric properties of 0.94 Mg TiO_3-0.06 Ca TiO₃ ceramics have been investigated. These results suggested that Li₂ZnTi₃O₈ addition not only contributes in decreasing the sintering temperature, but also inhibits improving grain growth. 0.94 Mg TiO_3-0.06 Ca TiO_3-0.015Li₂ZnTi₃O₈ ceramic sintered at 1220℃ had optimal dielectric properties（ er=21.0, Q ×f =70,832 GHz, τ_f =3.5 ppm/℃） which satisfied microwave applications in resonators, filters and antenna substrates.7. Mg O-Ca O-TiO₂ based ceramics was prepared by solid-state reaction technique and the effect of Mg: Ti ratio（0.873¹.057） on the phase, microstructure and microwave dielectric properties were investigated. Only two phases Mg TiO₃ and Ca TiO₃ appeared in the sample with Mg:Ti=0.955, and Mg Ti₂O₅ and Mg₂TiO₄ appeared in the sample with Mg: Ti ratio lower and higher than 0.955 respectively. The microstructure showed the dark Mg TiO₃ grains and the bright grains with three elements Mg, Ca and Ti in the triangle grain boundary. As increasing the ratio of Mg: Ti, the dielectric constant（ er） displayed down trend from 21.2 to 20.6, and the Q ×f value first increased form 76,300 GHz to 80,000 GHz, thereaτ_fer, it declined to 75,000 GHz. Meanwhile, the temperature coefficient of resonant frequency（τ_f） value first varied from-3.8 ppm/o C to 1.2 ppm/o C and then maintained unchanged. At last, our Mg O-Ca O-TiO₂ based ceramics with Mg:Ti=0.955 sintered at 1310°C for 4h showed good microwave dielectric properties: er=21.1, Q ×f =79915 GHz and τ_f =1.2 ppm/°C.8. In this paper, [(Mg_1-x-y Cax)La_y](Ti_1-y Al_y)O₃（x+y=0.75 and x=0.79⁰.89, y=0.01） ceramics was prepared by the traditional solid-state reaction, moreover, the phase composition, surface morphology and microwave dielectric properties of this ceramic series were investigated in detail. When x+y=0.75, the main phase was Ca TiO₃, and the impurity phases changed from Mg Al₂O₄ to Mg Ti₂O₅ and Mg TiO₃ as increasing the value of x. In addition, La³⁺ ions would enter the lattice point of these phases. The change of the apparent density was influenced much by the phase composition. When increasing Ca content, the dielectric constant of [(Mg_1-x-y Cax)La_y](Ti_1-y Al_y)O₃ kept increasing and the temperature coefficient of dielectric constant（et） went down continuously. Meanwhile, the Q ×f value showed the maximum value at x=0.70, which was mainly determined by the pores and grain uniformity. The well sintered [(Mg_0.25Ca_0.70)La_0.05](Ti_0.95Al_0.05)O₃ ceramics showed microwave dielectric properties: er=84.7, Q ×f =133700 GHz,et ≈-841 ppm/o C. When a little of La Al O₃ was added（y=0.01）, the more Ca content（x=0.79⁰.89） the sample had, the more columnar grains it showed, at the same time, the dielectric constant displayed a small increase, and the change of the temperature coefficient of dielectric constant（et） and Q×f value was small.