Microstructure and microwave dielectric properties of (1 − y)Li3NbO4+yLi2TiO3(Li2SnO3) ceramics

Phase formation, microstructure and microwave dielectric properties of (1 − y)Li3NbO4 + yLi2TiO3(Li2SnO3) ceramics have been studied in this paper. The structure and microstructure of the compounds were investigated using X-ray powder diffractometer (XRD), scanning electron microscope (SEM), Raman spectrometer. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–12 GHz. Li3NbO4 formed ordered solid solutions with the addition of small amount of Li2TiO3 (y ≤ 0.2), whereas no solid solution formed with the addition of small amount of Li2SnO3. Small amount of Li2TiO3 doping suppressed the appearance of impurity phases caused by lithium evaporation for Li3NbO4. The Li2TiO3 doped compositions with 0.02 ≤ y ≤ 0.08 demonstrated homogeneous and dense microstructure after sintering at 1150 °C/2 h, in contrast the 0.2 ≤ y ≤ 0.6 specimens exhibited porous and subgrains microstructure after sintering at 1250 °C/2 h. Short range ordering was observed in the 0.2 ≤ y ≤ 0.6 compositions. Mechanical mixture phases of Li3NbO4 and Li2SnO3 based solid solution (Li2SnO3 (ss)) existed in the Li2SnO3 added specimens. The dielectric permittivity increased with increasing Li2TiO3 addition, but decreased with the increase of Li2SnO3 content. All specimens exhibited negative τf value for the Li2TiO3 added specimens, although its absolute τf value decreased with the increase of Li2TiO3 addition. Whereas, the τf value changed from negative into positive with the increase of Li2SnO3 addition. Optimized combined microwave dielectric properties (ɛr = 19.8, Q × f = 91,200 GHz, τf = −24 ppm/°C and ɛr = 16, Q × f = 75,300 GHz, τf = 3 ppm/°C) could be obtained for the Li2TiO3 added (y = 0.6) and Li2SnO3 added specimens(y = 0.7), respectively. The microwave dielectric properties of the Li2SnO3 end member are ɛr = 13.5, Q × f = 61,600 GHz, τf = 29 ppm/°C.