## 3 Results and Discussion
### Preliminary Material Characterization: Effects of Processing Parameters
#### 3.1.1 Particle Size Distribution, Microporosity, and Thermal Profiles
The pre-sintered powders registered a BET surface area of 1.7 m\({}^{2}\) g\({}^{-1}\), rising to 2.7 m\({}^{2}\) g\({}^{-1}\) after thermal treatment, within the order of magnitude for BET area reported elsewhere [41]. The adsorption/desorption isotherms displayed a hysteresis loop, suggesting the dominance of a mainly mesoporous structure; their near overlap revealed a relatively poor porosity behavior corresponding to IV-type isotherms. These occur when gas condenses in the miniscule capillary pores of solid media at pressures below their saturation points (capillary condensation), indicating the dominance of finite-length pores; hysteresis occurs due to the difference in interface geometry between adsorption and desorption [59]. The effects of thermal processing were gauged by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) conducted over the 20 \({}^{\circ}\)C to 800 \({}^{\circ}\)C range in an inert atmosphere at the same heating rate to emulate the non-oxidative conditions of calcination (Figure 3). It is noteworthy that thermal analysis in the literature has been conducted largely in synthetic (oxidative) atmospheres, thus severely limiting the derivable information [40; 46]. Samples of untreated NASICON powder collected after ball-milling  
Figure 2: (**a**) BET adsorption/desorption isotherms, with inlet showing BET surface area calculation; and (**b**) BJH microporosity for untreated powders. (**c**) BET adsorption/desorption isotherms, with inlet showing BET surface area calculation; and (**d**) BJH microporosity for sintered powders. Surface area modestly increased after thermal treatment; porosity was conserved at the mesoporous regime. and fully thermally treated powder (pulverized from NASICON pellet after sintering) were compared. Thermal analysis revealed four main regions of thermal events, with an initiatory apparent mass increase (0.48%) preceding the 50 \({}^{\circ}\)C mark--this is a typical artifact due to the Archimedean buoyancy effect caused by the density of the carrier gas decreasing on heating [60]. The initial thermal event (I) occurs in the zone between 51 \({}^{\circ}\)C and 142 \({}^{\circ}\)C, with a dominant endothermic peak at 101 \({}^{\circ}\)C (rate of 0.21 uW mg\({}^{-1}\), enthalpy change \(\Delta H\) of 74.66 uJ mg\({}^{-1}\)) resulting in a mass loss of 4.31%; this can be attributed to the loss of physisorbed water. Events of a lesser magnitude follow with thermal peaks at 168 \({}^{\circ}\)C (II: monoclinic to rhombohedral 2 nd order phase transition, i.e., an A-type anomaly [40,58,61]),  
Figure 3: (**a**) TGA and (**b**) corresponding DSC with derived differential curves (DTG, DDSC) of untreated NASICON vs. thermally treated powders (shaded curves). Major thermal events and corresponding identifiers marked accordingly. 282 \({}^{\circ}\)C (III: NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) decomposition [62]), and 431 \({}^{\circ}\)C (IV: chemisorbed water; susceptibility to hydration: NaAl\({}_{5}\)O\({}_{8}\) (\(\beta^{\prime\prime}\)-alumina) > NaAl\({}_{11}\)O\({}_{17}\) (\(\beta\)-alumina) > Na\({}_{3}\)Zr\({}_{2}\)Si\({}_{2}\)PO\({}_{12}\) (NASICON) > Na\({}_{3}\)Sc\({}_{2}\)(PO\({}_{4}\))\({}_{3}\) (PHOSCAN) [63; 64; 65]) resulting in gradual losses until the residual mass was stabilized at 89.99% by the 800 \({}^{\circ}\)C mark. In contrast, TGA/DSC curves of the thermally treated material did not display event peaks of a significant magnitude, with minor mass losses being limited to less than 5%.