Crystal Structures And Microwave Dielectric Properties Of Hexagonal Perovskite Ceramics

As h-BaTiO₃hexagonal perovskite compounds were a class of new dielectricmaterials with important potential application values. The rule of phase formation,stability mechanism at room temperature, the dielectric response at microwavefrequency, and the relationships among composition-structure-microwave dielectricproperties had been investigated in this thesis.Firstly, the ceramics with two ions co-substituted Ti4+in BaTiO₃were prepared,and the rule of phase transition affected by co-substitution ions was discussed. Withincreasing x, the crystal structures varied from cubic phase （Pm3m） to hexagonal phase（P63/mmc） and finally to the double cubic perovskite phase （Fm3m） inBa[Ti_1-x(Co_0.5W_0.5)x]O₃and Ba[Ti_1-x(Zn_0.5W_0.5)x]O₃ceramic systems, while the crystalstructures varied from cubic phase （Pm3m） to hexagonal phase （P63/mmc） inBa[Ti_1-x(Co_0.5W_0.5)x]O₃ceramic systems. After substituting by adequate amount, someh-BaTiO₃-based full-filed hexagonal perovskite microwave dielectric ceramics withsingle phase were successfully stabilized at room temperature. But the ranges of thechemical compositions of the single hexagonal phase were different in different systems.The experimental results were carefully analyzed. It further revealed that the stability ofthe hexagonal perovskite was related to the ion radii or the preference of the O-B-Obonding.Secondly, the crystal structures of the typical hexagonal perovskites were refined.The parameters of the crystal structures were identified. With these data fromrefinement, the relationship between the connection type of the oxygen octahedra andmicrowave dielectric properties was investigated. The hexagonal perovskite ceramichad a higher quality factor （Q×f）, a lower dielectric constant （εr）, and a nearer zerotemperature coefficient of frequency （τf） than the cubic one. This was related to thepartial face-sharing oxygen octahedra in the hexagonal crystal lattice. The face-sharingoxygen octahedra resulted that the bond length was shorten, the B-O bond strength wasstrengthen, and the rotation or tilting freedoms of the oxygen octahedra were lower.This led to the hexagonal perovskite had a lower εr, a higher Q×f, and a nearer zero τf.According to the refinement results, it proposed that there were deformations in the face-sharing oxygen octahedra of the hexagonal crystal lattice, which resulted in thepresence of balanced spontaneous dipoles. It fully explained the rule of the microwavedielectric properties of the hexagonal perovskite, and supplied a theoretical basis ofadjusting the microwave dielectric properties.Finally, τfof （1xvol）Ba4LiNb3O12-xvolBaWO4and Ba[Ti0.4Ga0.3Nb0.3（1x）Sb0.3x]O₃ceramics were adjusted to near0ppm/oC, by preparing composite or solid solutionceramics. It obtained two classes of valuable ceramics. Their microwave dielectricproperties were εr=16.9, Q×f=75500GHz, τf=+8.7ppm/oC （xvol=0.69）, and εr=29.0,Q×f=19400GHz, τf=1.1ppm/oC （x=0.50）, respectively. This was enriched thefamily of low and high εrmicrowave dielectric ceramics.