Min Han Kim

Effect of B2O3 and CuO on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics

Sintering temperature of the Ba(Zn1/3Ta2/3)O3 (BZT) ceramic is about 1550°C and it decreased to 950°C when B2O3 was added. The BaB4O7 second phase whose melting temperature is 889°C was found in the B2O3 added BZT ceramics. The BaB4O7 second phase assisted the sintering of the BZT ceramics at 950°C. However, the B2O3 added BZT ceramic was not sintered below 950°C and the microwave dielecric properties were not satisfactory. On the other hand, when the B2O3 and the CuO were added, the BZT ceramic was sintered even at 870°C. The BaCu(B2O5) phase which was found in the CuO and B2O3 added BZT ceramics, existed as the liquid phase during the sintering and helped the densification of the BZT ceramics at temperatures lower than 950°C. Good microwave dielectric properties of Q×f=11,000 GHz, er=26 and τf=0.0 ppm/°C were obtained from the BZT + 5.0 mol% B2O3 + 10.0 mol% CuO ceramic sintered at 870°C for 2 h.

Effect of BaCu(B2O5) Additive on the Sintering Temperature and Microwave Dielectric Properties of BaTi4O9 Ceramics

BaCu(B2O5) (BCB) additive was used to decrease the sintering temperature of the BaTi4O9 ceramics. The amount of Ba4Ti13O30 second phase increased with the addition of BCB, whereas that of the BaTi4O9 phase decreased. The bulk density and dielectric constant (er) considerably increased with the addition of BCB. An increase in the Q-value was also observed for the BaTi4O9 ceramics with a small amount of BCB. Good microwave dielectric properties with values of er=32, Q ×f=10800 GHz and τf=32 ppm/°C were obtained in the BaTi4O9 ceramics with 12.0 mol % of BCB sintered at 875 °C for 2 h.

Dielectric Properties of (1-x)NdGaO3–xCaTiO3 Solid Solution at Microwave Frequencies

Microstructure and microwave dielectric properties of (1-x)NdGaO3–xCaTiO3 ceramics have been investigated. Crystal structure of all the specimens was orthorhombic and the lattice parameters decreased with an increase of x. For specimens with x ≤0.5, a Nd3Ga5O12 phase was observed. The Q value of Nd3Ga5O12 ceramics was about 10,000 at 12 GHz and the dielectric constant (er) was about 12. A liquid phase was found in the specimens with x ≥0.5, containing a high concentration of Ca and Ga ions. The dielectric constant and the temperature coefficient of resonant frequency (τf) of the specimens increased with an increase of x. The Q ×f value of the NdGaO3 specimen was about 85,000 GHz and it decreased inversely with x. Excellent microwave dielectric properties of Q ×f=46,000 GHz, er=45 and τf=-2.0 ppm/°C were obtained for (1-x)NdGaO3–xCaTiO3 ceramics.

Effect of Microstructure on Microwave Dielectric Properties of Al2O3-Added Ba(Zn1/3Ta2/3)O3 Ceramics

The effects of Al2O3 additive on the microstructure and microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 (BZT) ceramics were investigated. The BZT ceramics has a 1:2 ordered hexagonal structure. The degree of the 1:2 ordering did not decrease with the addition of Al2O3. Grain growth occurred and the liquid phase formed for the Al2O3-added BZT ceramics sintered above 1580°C. The liquid phase contains high concentrations of Ba and Al ions, which could be responsible for the grain growth. When a small amount of Al2O3 was added, the relative density slightly decreased but the Q value was markedly enhanced. The increase in the Q value was not related to the relative density or the 1:2 ordering. In contrast, the improvement in the Q value occurred in specimens in which grain growth occurred. Therefore, the increase in the grain size is considered to be responsible for the improvement in the Q value of Al2O3-added BZT.

Microstructure and microwave dielectric properties of SnO2-Added Ba(Zn1/3Ta2/3)O3 ceramics

The Ba(Zn1/3Ta2/3)O3 (BZT) ceramic normally has a 1:2-ordered hexagonal structure, but it changes to the 1:1-ordered cubic structure with the addition of SnO2. When SnO2 was added to the BZT ceramics, grain growth occurred and the liquid phase formed. The liquid phase was responsible for the grain growth. The Q×f value of the BZT ceramic significantly increased with the addition of a small amount of SnO2. Since the degree of 1:2 ordering and the relative density decreased with the addition of SnO2, the enhancement of the Q value cannot be explained by the 1:2 ordering and the relative density. On the other hand, the improvement of the Q value could be related to the increase in the grain size because the average grain size increased with the addition of SnO2. However, when a large amount of SnO2 was added, the Q value was very low. The low Q value was explained by the low relative density and the presence of a large amount of Ba3Ta5O15 phase.

Effect of Ta2O5 on microstructure and microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics

When Ta2O5 was added to Ba(Zn1/3Ta2/3)O3 (BZT) ceramics, the degree of the 1:2 ordering increased and the Ba3Ta5O15 second phase was formed. Grain growth and liquid phase were also found in Ta2O5 added BZT ceramics sintered above 1580°C. The liquid phase, which is responsible for grain growth, is related to the Ba3Ta5O15 second phase. The Q-value of BZT ceramics sintered above 1580°C was considerably increased with the addition of a small amount of Ta2O5. Since the relative density decreased with the addition of Ta2O5, the increase of the Q-value is not related to the relative density. In contrast, the improvement of the Q-value could be explained by the increase in the grain size because the average grain size increased with the addition of Ta2O5. When a large amount of Ta2O5 was added, however, the Q-value became very low even though the grain size was very large. The low Q-value might be attributed to the low relative density and the presence of a large amount of the Ba3Ta5O15 second phase.