Novel Sol-Gel Synthesis of MgZr4P6O24 Composite Solid Electrolyte and Newer Insight into the Mg2+-Ion Conducting Properties Using Impedance Spectroscopy

Magnesium zirconium phosphate, MgZr4P6O24 (MZP), is a magnesium ion conducting ceramic material with potential for application as solid electrolyte in high temperature electrochemical sensor in non-ferrous scrap metal refining and virgin metal alloying operations. In this work, MZP was synthesized using a simple but novel and economical sol-gel route at a significantly reduced temperature. An insight into the calcination process and possible phase transformation at higher temperature was obtained using simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC). Phase identification of the synthesized material was studied after calcining the powder at 900 degrees C for 3 h using X-ray diffraction (XRD); a single monoclinic phase was observed at that temperature. However, a trace amount of possible minor second phase, zirconium oxide phosphate [Zr-2(PO4)(2)O], was formed after heating at temperatures T >= 1000 degrees C. Impedance spectroscopy measurements on platinized sintered-MZP pellets were carried out in the frequency range 100 mHz to 32 MHz and in a temperature range of 30-800 degrees C to determine the electrical properties of MZP. Ionic conductivity of MZP was found to be equal to 7.23 x 10(-3) Omega(-1) cm(-1) at relatively lower temperature, 725 degrees C. Furthermore, the Nyquist and modulus plots measured at 764 and 390 degrees C show single semicircles suggesting contribution from only grain interiors (GIs). The ion-hopping rate was calculated by fitting the conductance spectra to the power law variation, sigma(ac) (omega) = sigma(dc) + A omega(n). The ac and dc conductivity of MZP shows Arrhenius-type of behavior with activation energies in the range 0.84 <= E-a(eV) <= 0.87. SEM of the fractured MZP pellet sintered at 1300 degrees C for 24 h revealed a highly dense microstructure with clearly visible grain boundaries and low porosity which is in good agreement with the relative density of similar to 99% determined using the Archimedes principle and the theoretical density calculated from the crystal structure. EDS confirms the presence of Mg, Zr, P, O in appropriate atomic ratio to yield MgZr4P6O24. Finally, TEM on MZP particles with crystallite size of similar to 50 nm also confirmed MZP as stable at 900 degrees C with no observable second phase, Zr-2(PO4)(2)O.