| Microwave dielectric ceramics with high permittivity, low dielectric loss and near zero temperature coefficient of resonance frequency are the key materials for modern communication technology. High permittivity is important for the miniaturization of high frequency capacitors and microwave resonators.In this thesis, ferroelectric relaxors with complex perovskite structure were modified to obtain high dielectric constant microwave dielectric materials. The investigations were focused on the modification of dielectric loss and temperature coefficient by co-substitution and introducing secondary phases to obtain the ceramics with dielectric constant great than 100, low dielectric loss and near zero temperature coefficient, and the mechanism of modification and dielectric loss for ferroelectric relaxors were also examined.The microstructures and dielectric properties of ferroelectric relaxor Pb(Mg1/3Nb2/3)O3 ceramics modified by CaTiO3 were investigated. CaTiO3 phase with the minimum of 20 mol% was effective to stabilize perovskite structure in Pb(Mg1/3Nb2/3)O3 ceramics, and with increasing the amount of CaTiO3 phase, the structure of Pb(Mgi/aNb2/3)03 ceramics varied from cubic to orthorhombic. Alternatively, introducing CaTiO3 phase significantly reduced the dielectric loss and temperature coefficient of Pb(Mg1/3Nb2/3)O3 ceramics. The microwave properties of Pb(Mg1/3Nb2/3)O3 ceramics also varied with sintering time. The good microwave properties were obtained in Pb0.5Ca0.5[(Mg1/3Nb2/3)0.5Ti0.5]O3 ceramic: =198, Qf = 2810 GHz ,Tf=360 ppm/'C.The temperature coefficient of dielectric constant of Pb0.5Ca0.5[(Mg1/3Nb2/3)0.5Ti0.5]O3 ceramics was adjusted by substituting A-site ions. La3+, Nd3+, Bi3+ions as donors influenced the perovskite stability and lead to the formation of some pyrochlore phase. Ca2+ substitution for Pb2+ could stabilize perovskite structure but changed from cubic to orthorhombic. La3+, Nd3+, Ca2+ substitution for Pb2+ sifnificantly reduced the temperature coefficient of dielectric constant of Pb0.5Ca0.5[(Mg1/3Nb2/3)0.5Ti0.5]O3ceramics where high dielectric constant and low dielectric loss were obtained. Bi3+ substitution for Pb2+ increased the dielectric constant and the dielectric loss sharply while the temperature coefficient of dielectric constant was almost the same. La3+ and Ca2+ substitution for Pb2+ reduced the unit cell volume to form compact structure and then improved Qf value inPbo.jCao 5[(Mgi/3Nb2/3)o sTio s]O3 ceramics. Good microwave properties were obtained in (Pbo.5Cao.5)o.85Laoi[(Mg1/3Nb2/3)o.5Tio.5]03 ( e= 125, Qf = 3150 GHz, xf = +253ppm/'C) and PbcuCaoTKMgi/sNb^o.sTio.sJCb ( e = 103, Qf = 6560GHz, tf = +121ppm/癈).Sn4+ substitution for Nb5+ effectively reduced the dielectric loss of Pbo.5Cao.5[(Mgi/3Nb2/3)o.5Tio.5]O3 ceramics, Sn4+ substitution for Ti4+ significantly reduced the temperature coefficient, and the good properties were obtained for Pbo.5Cao.5[(Mgi/3Nb2/3)o.5Tio5]O3 ceramics: e = 106, Qf = 3208 GHz, Tf = + 148 ppm/ 癈.The temperature coefficient of dielectric constant of Pbo.5Cao.5[(Mgi/3Nb2/3)o.5Tio.5]O3 ceramics could be tailored effectively by introducing Lii/2Ndj/2TiO3 with perovskite structure and positive temperature coefficient of dielectric constant. The near zero temperature coefficient and good dielectric properties were reached in Lii/2Ndi/2TiO3: 0.5(Lii/2Ndi/2Ti03)/ 0.5 Pbo.5Cao.5[(Mgi/3Nb2/3)o.5Ti0.5]03 ceramics: e = 142, tan5 = 0.009, t?= -1.9 ppm/癈.Synthesis and dielectric properties of Pb(Zni/3Nb2^)03 ceramics modified by Ba ion substitution for Pb were also investigated. The theoretical minimum of Ba substitution for Pb to stabilize perovskite structure was 10.0 mol%. The minimum of Ba substitution to stabilize perovskite structure was 20mol% in this practical study. Ba substitution for Pb was effectively to lower the dielectric loss and temperature coefficient of Pb(Zni/3Nb2/3)O3 ceramics: e = 133.5, tan8 = 0.0009, TE = -811 ppm/'C.The modification and dielectric loss mechanism of ferroelectric relaxor with A(B'i/3B"2/3)O3...
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