Microencapsulation of Zn Phase Change Material via Dry Synthesis Method and Its Shell Formation Mechanism

Decarbonizing industrial process heat and concentrating solar thermal power requires compact thermal energy storage at 300–500 °C. Metals offer high volumetric latent-heat density, but melt leakage, oxidation and sintering necessitate robust microencapsulation, and microencapsulated Zn ( T m ≈ 420 °C) has been little studied. Here we report a solvent-free route to Zn metal-core microencapsulated phase change materials (MEPCMs) by high-speed impact blending (HIB) of 10–50 μm Zn particles with submicron ZnO or AlO(OH), followed by oxidative rigidification at 800 °C in O 2 . SEM-based particle-size segmentation shows a single-mode right shift after HIB, consistent with single-particle coating build-up and an apparent radial buildup of ∼0.95–2.6 μm. ZnO served as a self-oxide control: after oxidation, the sample weight exhibited a net increase of 21.36% and near-complete conversion to ZnO, yielding a sintered, non-PCM product. By contrast, AlO(OH) coatings (20–40 vol %) dehydroxylate and consolidate into composite ZnO/ZnAl 2 O 4 shells (ZnAl 2 O 4 = 4.14–15.2 wt %), retaining Zn and preserving melting and solidification at 418–421 °C with latent heats of 58.36–61.77 J g –1 . The AlO(OH)-shell MEPCMs exhibited comparable latent heats of 58.36–61.77 J g –1 . Considering the estimated ±5% analytical uncertainty, no statistically resolved ranking among the three coating loadings is inferred. Their similar enthalpies are consistent with competing contributions from the retained metallic-Zn fraction and dilution by the shell-derived phases. After 1000 melt–solidification cycles in air, the capsules remain discrete and retain 53.23–58.88 J g –1 . Stepwise oxidation with ex situ XRD implicates γ-Al 2 O 3 formation (≥500 °C) and ZnAl 2 O 4 growth during the 800 °C hold as key steps in establishing cycling durability.