Assessment of MgHf4P6O24 Solid-State Electrolyte in High-Temperature Electrochemical Mg-Sensors

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The chemical synthesis of MgHf4P6O24 solid-state electrolyte was achieved using a modified sol-gel method by directly substituting the B-site in MgZr4P6O24 solid-state electrolyte [1], resulting in a considerably low activation energy of Ea = 0.74 ± 0.04 eV and a promising ionic conductivity of 4.52 x 10-4 Scm-1 at 747 oC. Using TGA-DSC measurement data, the synthesised xerogel powders were calcined at relatively low temperature of 800-900 oC, while the calcined nanopowders were pressed into pellets of 13 mm diameter (Ø) and 3.8 mm thickness with a uniaxial steel die at 5 kN compressive pressure, and later sintered at 1000 ≤ T oC ≤ 1550. However, 1300 oC was adapted as the sintering temperature in this study because the optimum samples density and composition stability were achieved at 1300 oC. X-ray diffraction reveals a good crystallinity of the calcined nanopowders phase having an average crystallite size of 20±2 nm and 42±2 nm at 800 oC and 900 oC, respectively, indicating that crystallite size increases as a function of calcination temperature, which is consistent with the TGA-DSC profiles and confirmed using HRTEM.The sintered MgHf4P6O24 solid-state electrolyte was subsequently characterised for their electrical and thermodynamic properties, identifying reliable ionic and transport data of the solid-state electrolyte. Furthermore, the average transport number of Mg2+-cations in the MgHf4P6O24 solid-state electrolyte measured as a function of Mg concentration in liquid Al is 0.84 ± 0.03. The ionic conductivity and emf data of the novel solid-state electrolyte in this study was compared with those of MgZr4P6O24 electrolyte [2-4], showing similar trend which defines the similarity in both chemical and structural properties of the B-site elements on the periodic table.Relying on the structural, chemical stability and ionic conductivity data characterised in this study, the electrochemical Mg-sensor was fabricated. The high-temperature electrochemical Mg-sensor was fabricated using the highly conducting Mg2+-cation solid-state electrolyte characterised in this study and a biphasic powder mixture of MgCr2O4 + Cr2O3 + O2 reference electrode in air, and testing in liquid Al alloys at 700 ± 5 oC after successful variation of Mg concentration using electrochemical method was achieved and the test rig is presented in Fig.1 [1-3]. The testing outcome shows a linear dependence of sensor voltage on the logarithm of Mg concentration. MgHf4P6O24 solid-state electrolyte has been identified in this study to have useful applications during refining, virgin metals alloying and scrap metal recycling for the benefit of our environment and depleting climate. Figure 1. Schematic of high-temperature electrochemical Mg-sensor test rig [1-3].