| Summary: | 碩士 === 國立成功大學 === 資源工程學系碩博士班 === 93 === In the microwave ceramic material, we focus on two types of dielectric systems and discuss their structures and properties, respectively.
(1)Complex perovskite structure of Ba1-xCax(Mg1/3Nb2/3)O3 system: As far as we're concerned, the microstructure and crystal structure considerably influence the dielectric properties of BCMN system. In view of crystal structure, the dielectric constant is associated with the distorted octahedra in the unit cell, the lower degree of distorted octahedra, the more free space for ions displacement which will result in the increasing dielectric constant. Besides, Ca2+ could be an effective dopant for the ordering degree. The quality factor of ratio 1:2 perovskite ceramics which is very sensitive to the B-site cation ordering could be improved as the degree of order increases. Nevertheless, the second phase and low relative density of sintered bulk would both decrease quality factor. The unit cell parameters decreased due to the contents of Ca2+ increased promote the τf increased.
(2)Ti-O and Ba-Ti-O system: The stable phase of Ti-rich of Ba-Ti-O compounds, such as BaTiO3, Ba6Ti17O40, Ba4Ti13O9, BaTi4O9, and Ba2Ti9O20 are more useful in microwave ceramic material. According to variation of Ti/O ratio, we found out the relationship between connected types of Ti-O octahedra and their dielectric properties.
With the increasing of Ti/O ratio , more octahedra connected to each other. When this happened, the center of cation ion has higher potential energy to deviate from the center, then the distorted octahedra formed. So with the Ti/O ratio increased, The type of shared octahdera tends to transform from perovskite-like structure to rutile-like structure.
Besides, with Ba/Ti ratio decreased, fewer Ba2+ ions with more octahedra connection cause the displacement of Ba ions and form octahedral tilt in the meantime due to more free space around. The displacement of center ion in octahedra and tilted octahedra also causes octahedral distortion and results in polarization which gives better dielectric properties.
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