Novel solid-state electrolytes, MgZr4P6O24, MgHf4P6O24, MgCe4P6O24 and their solid-state solutions were synthesised using a modified sol-gel chemical procedure. Xerogel of this chemical synthesis was subjected to thermal analysis for possible calcination temperature and high-temperature X-ray diffraction to determine transformation point with temperature. Based on the TGA-DSC analysis, the dried xerogel powders were calcined at 800 and 900oC, yielding a pure single phase homogeneous nanopowders with an average crystallite size. Pellets of 13mm diameter (Ø) and 3.8mm thickness were prepared and sintered at various temperatures in the range, 1000 ≤ T/oC ≤ 1550. However, 1300oC was adopted as the sintering temperature haven achieved optimum density and stable sample composition at that temperature. XRF, FTIR, Laser-Raman scattering, and HR-TEM are some other characterisation techniques deployed for the structural analysis of these ceramic materials.
MgHf4P6O24 and MgCe4P6O24 solid-state electrolytes were synthesised for the novelty and characterised for their electrical and thermodynamic properties. The conductivity and thermodynamic data of the novel solid-state electrolytes, compared with those of MgZr4P6O24 electrolyte, shows some improvement in the trend of ionic conductivity.
Sintered pellets of the end-members electrolytes and their corresponding solid-state solutions were characterised for their electrical properties and thermodynamics studies using the electrochemical method. The ionic conductivity of the end-members solid-state electrolytes identified MgZr4P6O24 as most conductive, with bulk conductivity, σbulk = 7.23 x 10-3 Scm-1 at 998K followed by MgCe4P6O24 with σbulk = 2.14 x 10-3 Scm-1 at 1017K and then, MgHf4P6O24 with σbulk = 4.52 x 10-4 Scm-1 at 1020K in this order. In terms of ionic mobility, MgCe4P6O24 solid-state electrolyte with activation energy, Ea = 0.52 ± 0.04eV has shown the highest mobility, followed by MgHf4P6O24 with Ea = 0.74 ± 0.02eV and MgZr4P6O24 with Ea = 0.84 ± 0.04eV, respectively. Therefore, the end-members solid-state electrolytes depict different ionic conductivity and activation energies but have shown chemical stability at 1300oC. The solid-state solutions further shows a unique trend which depicts that ionic conductivity increases with increasing concentration. This research interest however shows that conductivity decreases at two different concentrations, with chemical mole ratio (x) each, that is, it decreases at x = 0.5 and x = 0.8 for Hf-doped solid-state solution, while for Ce-doped solid-state solution, it decreases at x = 0.3 and x = 0.6, respectively. It has shown that the rate of conductivity varies for the two doped samples at different temperatures.
Based on available ionic conductivity data, stability of the end-members electrolytes in molten Al, OFHC-molten Cu, Cu-0.5 wt.% Mg and Al-Mg alloy, a biphasic powder mixture of the MgCr2O4+Cr2O3 reference electrode, Mg-sensors were fabricated and tested successfully in molten non-ferrous alloy. A linear dependence of sensor emf on logarithm of Mg concentration (logXMg) showing that sensor emf increases as Mg concentration increases in the molten Al-Mg alloy at 700±5oC, which complies with the Nernst equation. The Mg-sensor fabricated with MgZr4P6O24 solid-state electrolyte shows an increase in the sensor emf of about 0.2V, MgHf4P6O24 based Mg-sensor shows sensor emf of 0.12V as Mg concentration increases and, for MgCe4P6O24, the sensor emf increased to 0.48V.
In terms of sensor evaluation and performance, sensor testing characteristics such as response and recovery time were determined; Mg-sensor fabricated with MgHf4P6O24 solid-state electrolyte, shows fast response of the sensor by adding 0.005-1.5 wt.% Mg indicating the Mg-sensor reached a stable emf of 1.92V in about 0.5h, the emf reading equilibrates at this emf for a further 0.5h, the sensor emf then increased by 30mV after the measurand (Mg-rod wrapped in Al-foil) was increased by 0.05 wt.% Mg. After further equilibration for 0.5h, a stable emf of 1.95V was achieved. Subsequently, with additional 0.5 wt.% Mg, the sensor emf increased to about 2.1V which is an increase of about 150mV. The solid-state Mg-sensor then returned to zero after the measurand was discontinued, which indicates that the Mg-sensor has a robust recovery time. For MgCe4P6O24 Mg-sensor with a measurand ranging 0.005-1.0 wt.% Mg, the Mg-sensor reached a stable emf of 1.5V after the initial 0.005 wt.% Mg was added. The sensor emf further increased to 1.75V and then 2.25V after Mg concentration was increased to 1.0 wt.% at an interval of 250mV after each stage of Mg addition. However, for the MgZr4P6O24 Mg-sensor with a measurand of 0.005-1.5 wt.% Mg concentration range, a sensor emf of 2.03V was achieved after 0.005 wt.% Mg sample wrapped in Al-foil was inserted. This insertion was done after a long Al-baseline calibration of molten Al was possible for 2.02h. The sensor emf of 2.07V, an increase of 40mV was achieved after the measurand was increased by 0.05 wt.% Mg at an equilibration time of 2.25h. Further equilibration was allowed before 0.5 wt.% Mg was inserted, this then resulted in a sensor emf increase to 2.09V, an increase of 20mV. The addition of 1.0 wt.% Mg and 1.5 wt.% Mg at separate equilibration intervals resulted in an increase of the sensor emf to 2.15V and 2.21V, a sustained increment of 60mV, respectively. As expected, the Mg-sensor then returned to zero after the measurand was removed showing a good Mg-sensor recovery time.
Thermodynamic activity of Mg in molten Al from the Gibbs energy of formation, ∆Gfo of MgCr2O4 and MgO was determined by electrochemical method at a temperature of 700±5oC. In this system, the thermodynamic activity of Mg in molten Al as a function of concentration (XMg) was computed, showing negative deviation from Raoult’s law. A linear dependence of the theoretical and experimental emfs on the logarithm of Mg concentration (logXMg) in molten Al at 700±5oC was determined and used to compute the average transport number of Mg2+-cation in the solid-state electrolytes. However, the average transport number of Mg2+-cation in the stabilised solid-state electrolytes characterised in this research is believed to be approximately 0.85±0.03.
