Low Temperature Sintering And Microwave Dielectric Properties Of Niobate M²⁺Nb₂O₆(M²⁺= Zn,Ni) Microwave Dielectric Ceramics

This thesis first sketched the development of the LTCC (low temperature co-fired ceramic). In LTCC, the firing temperature lower than 950℃(≤950℃) is favored since Ag (melting point～961℃) or Cu (melting point～1,064℃) can be used instead of more expensive electrodes such as Ag-Pd binary or Pt-Pd-Au ternary alloys. Since materials technology has not been received enough attention, it hindered the development of the LTCC. The project realized the low temperature fired and co-fired of Ag of materials used for LTCC. Meanwhile, the theory basement, for dielectric properties of LTCC microwave dielectric materials was reasoned and demonstrated. Therefore, we gained the critical factor affecting dielectric properties of LTCC microwave dielectric materials, which can serve as the guidance for our experiments.The binary niobate ceramics, with the formula M2+Nb2O6 where (M= Zn, Mg, Co, Ca, Ni), have the orthorhombic columbite structure. There is a growing interest in the columbites as microwave dielectric ceramics, due to their lower processing temperatures, less complicated processing due to the simple chemistry of the binary compounds, and the lower cost of niobium compared with tantalum, and with incorporation of Cu2+ they are approaching low-temperature cofired ceramics (LTCC) temperatures. ZnNb2O6 and NiNb2O6 were chosen to be investigated.ZnNb2O6 ceramics has promising microwave dielectric properties (Q×f= 83,700 GHz,εr= 25 andτf=-56 ppm/℃). However, the relatively small permittivity of 25 and the large negative temperature coefficient of resonant frequency (τf) of-56 ppm/℃could limit its application in co-fired ceramics devices. Because most dielectric ceramics with high permittivity have a high positiveτf value, it is necessary to search for materials in order to form multiphase composite ceramics by mixing with ZnNb2O6 ceramics. TiO2 (rutile) hasτf=+400 ppm/℃,εr=100 and Q×f=50,000 GHz, and CaTiO3 hasτf=+859 ppm/℃,εr=162 and Q×f=12,960 GHz, therefore it is logical to speculate that the ZnNb2O6-TiO2 composite ceramics have adjustableτf andεr values.Theτf of ZnNb2O6, was adjusted by doping TiO2 at 1,200℃, and BaCu(B2O5) additives were used as sintering aids to effectively lower its sintering temperature. The dielectric properties of ZnNb2O6-1.8TiO2 samples with BaCu(B2O5) additives sintered in 900-1,000℃were investigated. The results indicated that the permittivity and Qxf were dependent on the sintering temperature and the amounts of BaCu(B2O5). As a result, theτf was successfully modified to near 0 ppm/℃with good Q×f andεr, and the sintering temperature of ceramics was effectively reduced to 950℃. Addition of 3.0 wt.% BaCu(B2O5) in ZnNb2O6-1.8 TiO2 ceramics sintered at 950℃showed excellent dielectric properties ofεr=40.9, Q×f=12,200 GHz (f=5.015 GHz) andτf=+0.3 ppm/℃. Addition of 5.0 wt.% BaCu(B2O5) in ZnNb2O6-xCaTiO3(x=8 wt%) ceramics sintered at 950℃showed excellent dielectric properties ofεr=20.2, Q×f=14,100 GHz (f=7.3 GHz) andτf=0ppm/℃. Moreover, the materials were compatible with Ag electrodes, making them a very promising candidate material for LTCC applications.ZnTiNb2O8 ceramics have a permittivity of 34, a Q×f of 42,500 GHz and aτf-of-52 ppm/℃, making them suitable for application in dielectric components. But, the sintering temperature of ZnTiNb2O8 is about 1,250℃, which is too high to be applicable to LTCC. The sintering temperature of ZnTiNb2O8 ceramics was effectively lowered from above 1250℃to 950℃due to the BaCu(B2O5) liquid-phase effect. Addition of 3 wt% BaCu(B2O5) in ZnTiNb2O8ceramics sintered at 950℃showed good dielectric properties ofεr= 32.56, Q×f= 20,100 GHz (f=5.128 GHz) andτf=-64.87 ppm/℃. However, the large negativeτf of-64.87 ppm/℃could limit its practical application in LTCC. Consequently, the near-zeroτf(τf～0) is expected to prevent the disturbance from temperature variation. So attempts have been made to achieve the near-zeroτf of the ZnTiNb2O8 ceramics by adding TiO2 (rutile), and to lower the sintering temperature below 950℃(T≤950℃) by adding BaCu(B2O5) as the sintering aid. The results showed that the microwave dielectric properties were strongly dependent on densification, grain sizes and crystalline phases, which were both influenced by the amount of BaCu(B2O5), the sintering temperature and the amount of TiO2. The sintering temperature of ceramics was effectively reduced from 1,250℃to 950℃and theτf was successfully modified to 0 ppm/℃with reasonably good Qxf andεr. Addition of 2 wt % BCB to ZnTiNb2O8-xTiO2 (x=0.8) ceramics sintered at 950℃showed excellent dielectric properties ofεr=38.89, Q×f=14,500 GHz (f=4.715 GHz) andτf=0 ppm/℃. Moreover, the materials were compatible with Ag electrodes, making them a very promising candidate material for LTCC applications.(Ni1/3Nb2/3)1-xTixO2,where x=0.3,(NiNb2O6-1.3TiO2) exhibit high dielectric permittivity of 68.7, a Q×f of 19,300, aτf of 56.6ppm/℃. However, the sintering temperature of NiNb2O6-1.3TiO2 is about 1,250℃, which is too high to be applicable to LTCC. But the addition of CuO lowered the sintering temperature of NiNb2O6-1.3TiO2 ceramics from 1,200 to 935℃due to the CuO liquid-phase. The NiNb2O6-1.3TiO2 ceramics with the addition of 3.2 wt% CuO sintered at 935℃afforded excellent dielectric properties ofεr= 60.5, Q×f=10,039 GHz (at 3.8 GHz) andτf= 62ppm/℃, which represented very promising candidates for LTCC dielectric materials.