Eiichirou Narimatsu

Preparation and electrical properties of Li–Si–Al–O–N ceramics

Abstract Ceramic samples were synthesized by hot pressing mixtures of Li3N, Si3N4, AlN, Al2O3, and Li2CO3 with nominal compositions of LiSi2−xAlxOxN3−x (x = 0–0.75) at 20 MPa and 1773–2073 K in a N2 atmosphere of 0.10 MPa. The samples prepared with nominal compositions, x = 0.25 and 0.50, showed electronic conductivities of 2.2 and 4.2 S m−1 at room temperature with activation energies of 3.8 and 3.9 kJ mol−1, respectively. Electronic conductive parts were detected in the sample of x = 0.50 by conductive atomic force microscopy (AFM). In this sample, a glassy thin layer, having a Si/Al atomic ratio of 3.8, was observed between the grains of LiSi2−xAlxOxN3−x solid solution by high-resolution transmission electron microscopy (HRTEM). It was expected that the glassy phase of grain boundaries is an electronic conductive pathway besides the conductive parts observed by AFM.

Effect of Ca-doping on the ionic conductivity of LiSi2N3

Effect of Ca-doping on the ionic conductivity of LiSi2N3 was studied. The compositions of Li1-2xCaxSi2N3 (x=0-0.2) were synthesized by the reaction of Li3N, Si3N4, and Ca3N2 at temperature of 1873K-2073K. Ca was incorporated into the LiSi2N3 host lattice and formed the solid solution, Li1-2xCaxSi2N3. Activation energy for ionic conduction was decreased and ionic conductivity at room temperature was enhanced by Ca doping. At 298K, the ionic conductivity of densified Li1-2xCaxSi2N3 (x=0.075) ceramics achieved 1.6×10−5Sm−1, almost 4 orders of magnitude higher than that of densified Li1-2xCaxSi2N3(x=0) ceramics (3.1×10−9Sm−1). The LiSi2N3 framework change by Ca doping decreased the interaction between the ions and increased the defects of the structure. Thus, mobile Li+ ion could migrate easier. Moreover, incorporation of aliovalent substitutional Ca2+ ions in the LiSi2N3 lattice might be expected to create a Li+ vacancy (VLi) for charge compensation (Li1-2xCaxVLiSi2N3), thereby increasing the number of mobile Li+ ions.