| With the rapid development of the wireless communications, the requirements for the properties of the microwave device are more than before, which stimulate to improve the properties of the microwave dielectric ceramics. Due to the shortcoming of the traditional solid-state method, the soft chemistry method has attracted considerable attention. As a soft method, hydrothermal synthesis method has been applied to manufacture the microwave dielectric ceramics in this paper. The properties of the ceramics made by hydrothermal synthesis are better than that by the traditional solid-state method. In this paper, the microwave dielectric ceramics based on BaO-TiO2 were synthesized by the hydrothermal method, which has not been reported before by other researchers.In this paper there are the innovation of not only the synthesis method but also many detailed researches. The substitution for A, B sites and dopant for BaO—Ln2O3—TiO2 ceramics with high dielectric constant were studied. The relationships between the properties, composition and structure were also investigated. The research outlines and conclusions are as follows.The pure phase Ba2Ti9O20 ceramics were synthesized by the hydrothermal method. In this paper, the influences of several different Ti precursors on the product were studied. It was concluded that only when the precursor is TiCl4 and the sintering temperature is 360℃ and the duration is 8 hours, the pure Ba2Ti9O20 phase with high crystallinity can be obtained. The ceramics made by this powder without calcined have good properties: εr=39,Q·f=28000GHz.The relationship between the microwave dielectric properties of the Ba6-3xLn8+2xTi18O54(Ln=Nd,Sm)ceramics made by solid-state method and compositions with different x were studied. For the Ba6-3xLn8+2xTi18O54(Ln=Nd,Sm)ceramics, the qualities are the maximum when x=0.67. The properties of the Ba6-3xNd8+2xTi18O54 ceramic at x=0.67 are: εr=82,Q·f=6250GHz,τf =36ppm/℃, and for the Ba6-3xSm8+2xTi18O54 ceramic, they are: εr=76,Q·f=7200 GHz, τf =-12 ppm/℃.The ternary BaO—Ln2O3—TiO2(Ln=La,Nd,Sm) ceramics are synthesized by the hydrothermal method and its conditions were studied. When the conditions were 360℃ 12h, the pure phase can be got. The synthesis dynamics for BaO—Ln2O3—TiO2 system under the hydrothermal condition was analyzed. The super saturation is the key factor to form host crystal. It was certified that the crystal under the temperature of 350℃ was smaller than that under 300℃. The BaO—Ln2O3—TiO2(Ln=La,Nd,Sm) ceramics made by the hydrothermal powders had lower sintering temperature than that made by the conditional solid-state method, and they had a little higher constant and better quality. When x=0.67, the quality of the Ba6-3xNd8+2xTi18O54 ceramic made by hydrothermal method is Q· f=7350GHz; the quality of the Ba6-3xSm8+2xTi18O54 ceramic is Q·f=8120GHz.The BaO—Ln2O3—TiO2(Ln=La,Nd,Sm) ceramics with substitution for A site were synthesized by hydrothermal method, its optimum sintering temperature was about 1250℃, which is lower than those reported by many references. When La was substituted for Sm of Ba4(Sm1-yLay)Ti18O54 ceramic, the lattice parameters increased with y, and the changes of the dielectric constant and the temperature coefficient of resonant frequency are the same as the lattice parameters, but the quality decreased with the increase of y. For the Ba4(Sm1-yLay)Ti18O54 ceramic, when y=0.1, εr=79, Q·f=7600GHz, τf =0ppm/℃. For Ba4(Sm1-yNdy)9+1/3Ti18O54 ceramic, when y=0.4, εr=80, Q·f=7300GHz, τf =0ppm/℃.It is a complex process that Sr substituted for A site in the (Ba1-ySry)6-3xNd8+2xTi18O54 ceramic. The change of x will result in the change of y and the ions the Sr substituted for. When x=0.5, in the range of 0≤y≤0.11, the lattice parameters decreased with y, and in the range of 0.11≤y≤0.2, it increased with y. When y=0.11,the lattice parameters are the smallest, and the properties are εr=82.8,Q·f =5830GHz, τf =20ppm/℃. When x=0.6, in the range of 0≤y≤0.048, the lattice parameters de...
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