Investigation of lithium-containing dielectric oxides for microwave applications

The dissertation describes the preparation, structure (studied by X-ray diffraction and TEM), and dielectric properties of the several new Li-based oxides. M-phases Li1+x−yM1−x−3y Tix+4yO3 in the Li2O-M2O 5-TiO2 systems, M=Nb, Ta, were found to have an intergrowth structure consisting of LiNbO3-type slabs separated by layers of a corundum type with a composition close to [Ti2O3] 2+. High-density ceramics could be obtained by sintering the samples at 1100°C (Nb M-phase) or 1125–1175°C (Ta M-phase). Dielectric constant values range from 84 to 61 (Nb) and from 68 to 52 (Ta). Tunability to zero was observed in both systems for the temperature coefficient of the resonant frequency measured in the microwave region. The systems show quite good quality factors at microwave frequencies up to Q × f = 9,000 at 6GHz (Nb) and 10,500 at 6.7 GHz (Ta). The potential of the Nb M-phase for low-temperature cofired ceramics (LTCC) applications was explored. V2O5 was found very effective in reducing sintering temperature of doped ceramics to ≤900°C without degrading the dielectric properties, such that a sample of the composition LiNb0.588V0.012Ti0.5 O3 has &egr;r = 66, Q × f = 3,800 at 5.6 GHz; τf of 11. Preliminary investigations suggest that V 2O5-doped M-phase is compatible with silver metallization at 900°C. Microstructural studies confirm presence of V2O 5-rich liquid phase. Single-phase, high-density V M-phase ceramics can be produced by a reactive sintering process from a mixture of constituent oxides in 1 hr. New families of ordered perovskites containing Li on the B-site were prepared with 1:1, 1:2 and 1:3 type cation order. Four new 1:2 ordered perovskites were discovered, including LaLi1/3Ti2/3 O3, the first titanate perovskite with 1:2 order, and several 1:2 Li-Nb, Ta compounds. In contrast to its Nb analogue, Ca2/3La 1/3Li1/3Ta2/3O3 was found to form a 1:1 ordered structure. A new 1:3 ordered perovskite, CaLi1/4Ta 3/4O3, with a structure analogous to that of a metastable polymorph of Ca4Nb2O9 was prepared. SrLi 1/4Nb3/4O3, which was previously claimed to be disordered, was found to form two different 1:3 ordered polymorphs. The structure of the low-temperature polymorph is different from any reported system. Several of the new perovskites exhibit dielectric properties comparable with those of commercial microwave ceramics