# Demonstration of Phase Change Thermal Energy Storage in Zinc Oxide Microencapsulated Sodium Nitrate
## 3 Results  
Microencapsulation of NaNO\({}_{3}\) Is Persistent in the Storage Tank, over Multiple Thermal Cycles  
Our simplified protocol for NaNO\({}_{3}\) microencapsulation produced microcapsules 100 to 300 microns in diameter. The microcapsule shell is made of ZnO particles arranged in a nonuniform layer ranging from a few nanometers up to 3 microns thickness. Based on relevant SEM samples, a gross estimation of the whole population of microparticles suggests a majority of well-coated and thermally stable microparticles, with some prone to collapse from thermal cycles. To test their collective thermal stability, the NaNO\({}_{3}\)-ZnO microcapsules were loaded in the storage tank and exposed to multiple melting-solidification cycles.  
Scanning electron microscopy (Figure 2) was employed to demonstrate that microencapsulation was effective and that it is lasting after charge-discharge cycles in the tank. Just after preparation, the microcapsule material was a powder consisting of relatively large (\(\sim\)300 \(\upmu\)m) grains. Some of the grains presented a nonuniform gray surface, with light and dark spots (Figure 2A). Figure 2D confirmed that light gray areas are ZnO particles encapsulating NaNO\({}_{3}\) grains (dark gray). The correlation was established by EDS (Figure 2C) and grayscale analysis on the small surface spots was pictured in Figure 2E. Thus, elemental concentration (%) of each spot was plotted against the spot intensity on the digital image (grayscale: 0 to 255). On microcapsules that are similar to the microcapsule in the center of Figure 2A, due to the interruptions in the ZnO cover layer, the thickness of the layer can be estimated at the edges of the 2D grain shape. Figure 2B shows the microcapsule-based material after multiple thermal storage cycles in the tank. It compacted into a porous block with microcapsules (smaller than 100 \(\upmu\)m) that are often distinctly arising and are bonded by interstitial NaNO\({}_{3}\). Whereas before storage cycles some microcapsules were nonuniformly covered with ZnO particles, with areas of exposed NaNO\({}_{3}\) core, partially covered microcapsules are absent from material samples after repeated melting-solidification.  
The chemical composition was analyzed on small samples taken from 14 spots at various depths (0 to 45 mm from the front section surface) and radial positions within the storage material. The NaNO\({}_{3}\)-ZnO mass ratio diminished in most of the spots from an initial value 4:1 to 3:41 (\(\pm\)0.14 standard deviation). Table 1 groups in three intervals the NaNO\({}_{3}\):ZnO mass ratio detected in the 14 spots. Most of the microcapsule samples had a mass ratio around 4:1, showing that the chemical composition was preserved locally. For the spots outside the average range, a plausible explanation is that during the liquid phase of the storage cycles, the NaNO\({}_{3}\) content of weakly formed microcapsules leaked in the interstitial space and bonded at solidification with stable microcapsules, in a porous material. The ZnO coating of the collapsing microcapsules redistributed to surroundings. Gravitational stratification occurred, proven by composition analysis of six spots from the upper side versus six spots from the lower side of the tank. Thus, average NaNO\({}_{3}\)-ZnO mass ratios of 3.4 \(\pm\) 0.1 at the upper half of the tank and 4.9 \(\pm\) 1.6 at the bottom were obtained.  
Figure 2: SEM micrograph of NaNO\({}_{3}\)–ZnO microcapsule before (**A**) and after repeated melting–solidification cycles (**B**). EDS analysis (**C**) of 4 spots in a same picture (**E**), confirming (**D**) that light grey spots contain Zn and do not contain Na or N.