Novel MgHf4P6O24 as a Solid Electrolyte in Mg-Sensors
Authors: Adamu Mohammed , Kale Girish M. 
Published: December 2020

In this study, a modified sol-gel chemical route was developed for synthesising MgHf4P6O24 multicomponent nanostructured ceramic oxide electrolytes. The chemical synthesis approach was determined by substituting the Zr4+ B-site in MgZr4P6O24 solid electrolyte [1-3] with Hf4+ reactive oxide, resulting in a considerably low activation energy (Ea = 0.74 ± 0.04 eV) and a promising ionic conductivity (4.52 x 10-4 Scm-1 at 747oC), for the novel MgHf4P6O24 solid electrolyte.The solid electrolyte was first calcined at a relatively low temperature range of 800-900oC, while the resultant calcined nanopowders were pressed into pellets of 13mm diameter (Ø) and 3.8mm thickness using a uniaxial steel press at 5kN compressive pressure. The pressed pellets were later sintered at 1000 ≤ T/oC ≤ 1550 temperature range. Meanwhile, calcination and sintering temperatures of the electroceramic electrolytes were maintained at 900 ≤ T/oC ≤ 1300oC after TGA-DSC thermal analysis and densification measurements; An optimum density and stable thermodynamic composition of the solid oxide pellets were also achieved at 1300oC. X-ray diffraction and rietveld analysis reveals a good crystallinity of the solid electrolyte with an average crystallite size of 20±2 nm and 42±2 nm at 800oC and 900oC, respectively, this indicates that crystallite size increases as a function of calcination temperature, which is consistent with the TGA-DSC thermal analysis profiles and confirmed using HRTEM. Interestingly, the refined crystallographic data and conductivity properties of the novel MgHf4P6O24 solid electrolyte characterised in this study was reported for the first time. The MgHf4P6O24 solid electrolyte characterised for their electrical and thermodynamic properties portray reliable ionic and transport properties of the solid electrolyte; The average transport number for Mg2+-cations in MgHf4P6O24 solid electrolyte measured as a function of Mg concentration in molten Al is 0.84±0.03. Ionic conductivity and thermodynamic stability of the solid electrolyte in this study was compared to those of MgZr4P6O24 electrolyte [1, 3], which shows novel improvement in the ionic conductivity and thermodynamic properties of both solid oxide electrolytes.
Relying on the structural, thermodynamic stability and ionic conductivity data in this study, Mg-sensor probes were designed and fabricated, then tested in molten Al at 700±5oC, using the electrochemical method illustrated in Figure 1[3]. In application, a high-temperature Mg-sensor probe was fabricated using the Mg2+-cation conductors characterised in this study by incorporating a biphasic powder mixture of MgCr2O4+Cr2O3 ceramic reference electrode in air, showing promising trend after successfully sensing Mg dissolved in molten Al alloys at 700±5oC. A linear dependence of sensor voltage on the logarithm of Mg concentration was obtained. The thermodynamic activity of Mg in molten Al shows a rather negative deviation from Raoult's law [4]. MgHf4P6O24 solid electrolyte has useful potential applications in high-temperature Mg-sensors during refining, virgin metals alloying and scrap metal recycling for improved environmental sustainability and climate change.

Figure 1. A schematic of high-temperature Mg-sensor test rig [3].

References

[1] M. Adamu, and G.M. Kale, J. Phys. Chem. C,
120, 17909 (2016).
[2] G.M. Kale, L. Wang, and Y. Hong, Int. J. Appl. Ceram Technol., 1, 180 (2004).
[3] M. Adamu, K.T. Jacob, and G.M. Kale, J. Electrochem. Soc.,
167, 027532 (2020).
[4] G.R.K. Aghdam, and M. Soltanieh, Can. Metall. Quart.
49, 39 (2010).

Figure 1