Microstructure–conductivity relationship of Na₃Zr₂(SiO₄)₂(PO₄) ceramics

Abstract

The ionic conductivity of solid electrolytes is dependent on synthesis and processing conditions, ie, powder properties, shaping parameters, sintering time (ts), and sintering temperature (Ts). In this study, Na3Zr2(SiO4)2(PO4) was sintered at 1200 and 1250°C for 0‐10 hours and its microstructure and electrical performance were investigated by means of scanning electron microscopy and impedance spectroscopy. After sintering under all conditions, the sodium super‐ionic conductor‐type structure was formed along with ZrO2 as a secondary phase. The microstructure investigation revealed a bimodal particle size distribution and grain growth at both Ts. The density of samples increased from 60% at 1200°C for 0 hours to 93% at 1250°C for 10 hours. The ionic conductivity of the samples increased with ts due to densification and grain growth, ranging from 0.13 to 0.71 mS/cm, respectively. The corresponding equivalent circuit fitting for the impedance spectra revealed that grain boundary resistance is the prime factor contributing to the changing conductivity after sintering. The activation energy of the bulk conductivity (Ea,bulk) remained almost constant (0.26 eV) whereas the activation energy of the total conductivity (Ea) exhibited a decreasing trend from 0.37 to 0.30 eV for the samples with ts = 0 and 10 hours, respectively—both sintered at 1250°C. In this study, the control of the grain boundaries improved the electrical conductivity by a factor of 6.

Solid-state synthesis of NASICON (Na3Zr2Si2PO12) using nanoparticle precursors for optimisation of ionic conductivity

“…This abnormal grain growth could be the result of multiple grain growth speeds within the sample caused by anisotropic conditions such as anisotropic grain boundary energy or high grain boundary mobility [34]. A time temperature effect is suggested as a potential contributor to abrupt changes in grain size and boundaries [16,35]. Since this was only evident in the macro-precursor, this suggests that this phenomenon is a function of both time and the precursor powder characteristics.…”

“…Na 3 Zr 2 Si 2 PO 12 is the most common formulation of NASICON and is found to have ionic conductivity in the higher range when compared with other types [14]. This is often known as Hongtype NASICON and was pioneered by the early work by Hong and Goodenough [2,15], and the reported conductivity via the most common conventional solid-state sintered method ranges from 9.2 9 10 -5 to 1.2 9 10 -3 S cm -1 [16] with variance due to different densities and microstructures due to the different processing conditions. The solid-state method is the most common method described in the literature and offers a simpler and less expensive method compared to the alternatives.…”

“…The solid-state method is the most common method described in the literature and offers a simpler and less expensive method compared to the alternatives. It typically consists of several stages including mixing/milling of powdered precursors, calcination and a final sintering stage [11,[16][17][18][19][20][21][22]. The usual laboratory method of sintering is to compress the pre-sintered powder into a pellet and sinter in a box furnace.…”

“…Each synthesis method and its associated processing conditions will yield different pre-sintering powder properties such as particle size distribution, morphology and composition [16]. Lee et al [20] looked at the effect of different pre-sintered particle sizes by comparing ball milling and jet milling methods after calcination.…”

“…NASICON pellets were then prepared from these powders, at a range of sintering durations (10, 24 and 40 h) with microstructure, crystalline structure, density and finally electrical performance analyses performed. [22] 1.13 9 10 -3 Solid state with excess sodium 1100°C for 12 h Kim et al [18] 1.03 9 10 -3 Solid state 1280°C for 10 h Naqash et al [3] 1 9 10 -3 Solution-assisted steady state, 1250°C for 5 h Naqash et al [16] 0.71 9 10 -3 Solid state 1250°C for 10 h Lee et al [23] 1.8 9 10 -3 Spark plasma sintering at 1200°C Lee et al [20] * 0.2 9 10 -3 at 50°C jet milled * 1 9 10 -5 at 50°C ball milled…”