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The effects of magnesium and niobium substitution for titanium on the microwave dielectric properties of Ba 3.75 Nd 9.5 Ti 18Àz (Mg 1/3 Nb 2/3 ) z O 54 (0 £ z £ 3) ceramics were studied. The temperature coefficient of resonant frequency (s f ) decreased from about +60 ppm/°C to +17 ppm/°C when z £ 1. Excellent quality factor (Qf = 7300 GHz) as well as high dielectric constant (e r = 80.96) were obtained. For z ‡ 1.5, Nd 2 (Ti,Mg,Nb) 2 O 7 secondary phase appeared which would obviously influence the microwave dielectric properties. As z varied from 0 to 3, matrix grain size degraded which would obviously deteriorate the microwave dielectric properties by conducting more pores, especially the Qf value. The s f value was found to be related to b-site bond valence (V B-O ) and unit cell volume (V m ). Average ionic polarizability (a D ) and relative density evidently influenced the dielectric constant.

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According to the solid-state reaction, microwave dielectric ceramics of Ba 6À3x Nd 8+2x [Ti 1Àz (Ni 1/3 Nb 2/3 ) z ] 18 O 54 (x = 0.75, 0 ≤ z ≤ 0.2) were fabricated. Microwave dielectric properties, including relative dielectric constant (e r ), quality factor (Q f ), and temperature coefficient of resonant frequency (s f ), were investigated as a function of substitution content. As z increasing from 0 to 0.2, dielectric constant was decreased from 87.3 to 80.4, Qf value declined from 9700 to 1200 GHz, and s f dropped from +60.0 to À27.0 ppm/°C. Appropriate substitution conducted excellent microwave dielectric properties with z = 0.05 sintered at 1375°C for 2 h in air: e r = 82.25, Q f = 6100 GHz, and s f = +18 ppm/°C. *

The addition of BaO–ZnO–B2O3 (BZB) glass was proven to effectively reduce sintering temperature of BaTi5O11 ceramic from 1100 to 900 °C in the presence of the liquid phase while maintaining excellent microwave dielectric properties. Effects of BZB glass on the phase composition, wetting behavior, microstructure, activation energy, and microwave dielectric properties of BaTi5O11 ceramics were studied in detail to understand the basic mechanism of the low-temperature sintering kinetics. The results revealed that the sintering process of BaTi5O11 was significantly promoted at a low temperature, owing to the existence of the liquid phase caused by BZB glass, which effectively lowers the activation energy, called liquid-phase sintering.

According the solid state reaction routine, Ca 0.61 Nd 0.26 Ti 1¹x Al 4x/3 O 3 (x = 0, 0.05, 0.1, 0.15, 0.2) ceramics were synthesized and their microwave dielectric properties were determined. The X-ray diffraction results showed a single perovskite phase (JCPDS Card No. 01-082-0229) for 0¯x¯0.1. Dielectric constant (¾ r ) decreased from 102.8 to 72.24, the Q©f value almost doubled, from 8560 to 15437 GHz, and the temperature coefficient of resonant frequency (¸f ) dropped from +247 to +155 ppm/°C. Then the Xray diffraction patterns showed the appearance of secondary phase that degraded microwave dielectric properties of the system rapidly. At last, a ceramic possessing microwave dielectric properties of ¾ r = 72.24, Q©f = 15437 GHz and¸f = +155 ppm/°C could be obtained when sintered at 1400°C for 2 h in air, x = 0.1.

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