Abstract
In this work, the (Na0.5Bi0.5)(Mo1−xWx)O4 (x = 0.0, 0.5 and 1.0) ceramics were prepared via solid state reaction method. All the samples can be well densified at sintering temperature about ~720 °C. Dense and homogeneous microstructure with grain size lying between 2~8 μm can be observed from scanning electron microscopy (SEM). Microwave dielectric permittivity of the (Na0.5Bi0.5)(Mo0.5W0.5)O4 ceramic was found to be temperature-independent in a wide range between 25~120 °C with a temperature coefficient of frequency (TCF) ~−6 ppm/°C, a permittivity ~28.9, and Qf values 12,000~14,000 GHz. Crystal structure was refined using Rietveld method and lattice parameters are a = b = 5.281 (5) Å and c = 11.550 (6) Å with a space group I 41/a (88). The (Na0.5Bi0.5)(Mo1−xWx)O4 ceramics might be good candidate for low temperature co-fired ceramics (LTCC) technology.
Highlights
Different from the (Li0.5Bi0.5)(Mo,W)O4 and (Ag0.5Bi0.5)(Mo,W)O4 system, with a ionic radius of 1.18 Å (Na+) between that of Li+ and Ag+, (Na0.5Bi0.5)(Mo,W)O4 system can crystallize in standard scheelite structure
Crystal structure refinement based on Rietveld method gave the cell parameters a = b = 5.281 (5) Å, and c = 11.550 (6) Å with a space group I 41/a (88)
With increase of W content, microwave permittivity decreased from 34.4 to 25.7 and Qf value increased from 12,300 GHz to 17,500 GHz while TCF shifting from +43 to −18 ppm/°C, which indicates that TCF value is strongly dependent on the [BO4] tetrahedron in scheelite structure
Summary
To better understand the scheelite solid solubility here, the phase compositions, sintering temperatures and microwave dielectric properties of the (A0.5Bi0.5)(Mo,W)O4 (A = Li, Na, K and Ag) ceramics are presented in Table 2 in order of ionic radius of A+. In a wide temperature range 20~120 °C, the permittivity of (Na0.5Bi0.5)(Mo0.5W0.5)O4 ceramic changed slightly around 28.9, while Qf values above 12,000 GHz. It is interesting to note that no matter in forms of solid solution or composite, TCF values can be tailored to be near zero in the (A0.5Bi0.5)(Mo,W)O4 (A = Li, Na, K and Ag) systems, which makes them possible in fabrication of temperature stable devices. Bi3+ neighboring structure was found to account for the shift of macroscopical permittivity due to the large polarization ~6.12 Å3, which is larger than that of Mo6+ and W6+ ions (3.28 and 3.2 Å3, respectively)[26, 27, 42]
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