| In recent years, with the rapid growth in communication technology, the microwave dielectric materials have been widely used for microwave device applications, such as microwave resonators, dielectric filters, dielectric oscillators, dielectric antenna and so on. Microwave dielectric ceramics have become the key materials in electronic communication system, however, the materials which possessed a high dielectric constant(rε), a low dielectric loss(or high f Q × value) and a near-zero temperature coefficient of resonance frequency(fτ) can be used in microwave circuits. Therefore, the development of materials with excellent microwave dielectric properties is the inevitable trend of the modern information technology.In our paper, the dielectric ceramic samples were prepared by the traditional solid-state route. This paper takes two kinds of Ti based microwave dielectric materials Li2 Zn Ti3O8 and Zn Ti O3 as the research object. The phase composition, microstructure and microwave dielectric properties have been investigated through the non-stoichiometric ratio of the elements, doping and substitution. The results are as follows:Ⅰ. In order to obtain the ceramic with near-zero temperature coefficient of resonance frequency fτ value, the multiphase Li2 Zn Ti3O8-Ti O2 materials were prepared by one-step synthetic process. Li2 Zn Ti3O8 phase and Ti O2 phase can coexist to form a stable system. When x was increased from 0 to 1.00, the relative content of Ti O2 phase gradually increased in the Li2 Zn Ti3+x O8+2x materials. Porosity decreased with the increase of Ti O2 content. Meanwhile, the dielectric constant of the ceramic increased from 26.5 to 35.6. The f Q × value decreased from 61577 GHz to 51250 GHz. And the temperature coefficient of resonance frequency fτ value decreased in an almost straight line from-15 ppm/℃ to +61.4 ppm/℃. In general, when x=0.17, the Li2 Zn Ti3+x O8+2x samples sintered at 1160℃ for 4h had good microwave dielectric properties of rε =28.5, f Q× =58511 GHz,fτ =+2.3 ppm/℃.Ⅱ. In order to improve the f Q× value and thefτ value, the non-stoichiometric ratio of the elements in Li2 Zn Ti3O8 ceramic was studied. For Li2Zn1-x Ti3O8-x(x=0.01~0.30) materials, when x value was increased from 0.01 to 0.30, the single Li2 Zn Ti3O8 phase was obtained with x=0.01~0.06, however, a stable Li2 Zn Ti3O8-Ti O2 two phases system was formed with x=0.09~0.30. The cell volume of Li2 Zn Ti3O8 phase decreased first, and then increased at x=0.09. The result revealed that the decrease of cell volume is helpful to improve the f Q × value of the ceramic. The basic reason for the decrease of density was proved by calculating the theory density. With x increasing from 0.01~0.30, the dielectric constant of ceramics firstly decreased from 27.1 to a minimum value of 26.3(x=0.06), and then increased to 29.8(x=0.30). The f Q × value firstly increased from 72627 GHz to a maximum value of 74268 GHz(x=0.06), and then decreased to 69613 GHz(x=0.30). And the temperature coefficient of resonance frequency fτ value increased from-13.5ppm/℃ to +1.5 ppm/℃. In general, when x=0.30, the Li2Zn1-x Ti3O8-x samples sintered at 1050 ℃ for 4h showed good microwave dielectric properties of rε =29.8,f Q × =69613 GHz,fτ =+1.5 ppm/℃.Ⅲ. In order to reduce the sintering temperature of the Li2 Zn Ti3O8-Ti O2 composite material to meet the requirements of LTCC technology, three different sintering aids were chose to add in Li2 Zn Ti3O8-Ti O2 material. Three kinds of additives doped ceramics can obtain excellent microwave dielectric properties at about 900℃ during sintering.(1) When the amount of Li2O-Al2O3-B2O3(LAB) increased, Ti O2 phase gradually melted into the glass. An appropriate amount of doping was beneficial to sintering. However, when the content of additive exceeded, micro-cracks appeared at grain boundaries of ceramics. In general, the 1wt% LAB added Li2 Zn Ti3O8-Ti O2 ceramics sintered at 900℃ for 4h had good microwave dielectric properties of rε =26.8, f Q× =27615 GHz,fτ =+2.3 ppm/℃.(2) For Ca O-B2O3-Si O2(CBS) doped Li2 Zn Ti3O8-Ti O2 material, when the amount of CBS increased, the grain size increased at first and then decreased. The decrease of grain size was believed to relate to “pinning effect” which was caused by the excessive liquid phase. Meanwhile, it might also connect with the unknown phase resulting from the chemical reaction between CBS and the ceramic. All of these were the main reasons for the sharply decrease of microwave dielectric properties. In general, the 0.5wt% CBS added Li2 Zn Ti3O8-Ti O2 ceramics sintered at 920℃ for 4h showed a compact microstructure and had good microwave dielectric properties of rε =27.6, f Q× =36567 GHz,fτ =+1.1 ppm/℃.(3) With the increase of Li2WO4 additive, the grain size gradually increased because of the liquid phase sintering mechanism. When the content of the Li2WO4 additive exceeded 4wt%, the pores was appeared on the surface of grains which led to the decrease of microwave dielectric properties. In general, the 2wt% Li2WO4 added Li2 Zn Ti3O8-Ti O2 ceramics sintered at 860℃ for 4h showed a dense microstructure and had excellent microwave dielectric properties of rε =27.1, f Q× =51123 GHz,fτ =-3.8 ppm/℃.Ⅳ. To tune the fτ value of the Li2 Zn Ti3O8 ceramic, one-step synthetic process was used to prepare the multiphase Li2 Zn Ti3O8-Li2 Ti O3 ceramics. When x value was increased from 0.05 to 0.4, the rε value of the Li2 Zn Ti3O8-x Li2 Ti O3 ceramic gradually decreased from 27.2 to 26.4, the f Q × value slightly declined from 61827 GHz to 59214 GHz, and the fτ value increased in an almost straight line from-12.4 ppm/℃ to-3.5 ppm/℃. When x=0.4, the Li2 Zn Ti3O8-x Li2 Ti O3 ceramic sintered at 1075℃ for 4h showed a fine microstructure and had good microwave dielectric properties of rε =26.4, f Q× =59214 GHz,fτ =-3.5 ppm/℃.In addition to this, Bi2O3-B2O3 added 0.65Li2 Zn Ti3O8-0.35Li2 Ti O3 ceramics were investigated. When the content of Bi2O3-B2O3 additive was 0.5 wt%, the ceramic sintered at 840℃ for 4h had had good microwave dielectric properties of rε =26.6, f Q× =40683 GHz,fτ =+1.0 ppm/℃.Ⅴ. When Sn4+ ion substituted for Ti4+ ion in(Zn0.3Co0.7)Ti1-x Snx O3 ceramics, the Sn4+ ion can fully enter into the location of Ti4+ ion lattice, which can form a solid solution. With the increase of Sn4+ ion substitution, the(Zn Co)(Ti Sn)O3 phase gradually began to decomposed into(Zn Co)2(Ti Sn)O4 phase and Ti O2 phase. The rε value of the(Zn0.3Co0.7)Ti1-x Snx O3 ceramic increased from 21.9 to 29.2. The f Q × value obviously increased from 56536 GHz to 66700 GHz, and then decreased to 48630 GHz. And the fτ value increased from-46.2 ppm/℃ to +94.5 ppm/℃. The improve of f Q × value might be due to the elimination of pores and the enhancement of the covalent bond in the(Zn Co)Ti O3 crystal which were resulted from Sn4+ ion substitution for Ti4+ ion. When x=0.02, the(Zn0.3Co0.7)Ti1-x Snx O3 ceramic sintered at 1220℃ for 4h had excellent microwave dielectric properties of rε =24,f Q × =66700 GHz,fτ =-5.4 ppm/℃. |
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