Synthesis and Electrical Properties of γ-Na x Co2O4 via a Citrate Sol–Gel Method with Polyethylene Glycol 400

In this work, a citrate sol–gel method (Sol–Gel) with polyethylene glycol 400 (Sol-Gel-PEG400) was developed to prepare γ-Na x Co2O4 by using sodium and cobalt nitrates as the raw materials, citric acid as a complexing agent, and PEG400 as a dispersant. At 800°C, single-phase γ-Na x Co2O4 crystals were obtained using Sol-Gel-PEG400. With the addition of 1 vol.% PEG400, smaller, flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. Moreover, polycrystalline sintered bulk γ-Na x Co2O4 with more highly oriented crystals and greater compact density was fabricated using the Sol-Gel-PEG400 synthesized powders compared with the powders synthesized by citrate Sol–Gel. The electrical conductivity (σ) values of Sol-Gel-PEG400 samples were higher than those of Sol–Gel samples between 400 K and 900 K. The σ value of Sol-Gel-PEG400 increased to 3.13 × 104 Sm−1 at 400 K and to 1.84 × 104 Sm−1 at 900 K. Between 400 K and 850 K, the Seebeck coefficient (α) values of Sol-Gel-PEG400 samples were slightly lower than those of Sol–Gel samples. Near 900 K, the α values of these two methods were nearly equal, at 164 μV K−1. Between 400 K and 900 K, the power factor (P) of Sol-Gel-PEG400 was evidently larger than that of Sol–Gel.