The preparation and characterization of dense, highly conductive Na₅GdSi₄O₁₂ nasicon (NGS)

Na5GdSi4O12 has been prepared via conventional ball-milling technique and through spray-freezing/freeze-drying. The ball-milled materials were calcined at 700°C or 925°C and sintered at 1050°C/3.5h to 89% dense multiphase ceramics. Spray-frozen/freeze-dried powders were calcined at 530°C and sintered at 1050°C/25 min to 99 ± 1% theoretical density. The latter material was single-phase NGS nasicon with a 300°C resistivity of 3.8 Ω·cm, an activation energy for Na+-conduction of 4.4 kcal/mol, a flexural strength of 105 MPa, a duplex grain structure with average sizes 0.4 μm and 3 μm and a unique linear thermal expansion coefficient (25–540°C; α = 12.6 × 10−6/°C±4%).