Printing Composites with Salt Hydrate Phase Change Materials for Thermal Energy Storage

Salt hydrate phase change materials are important in advancing thermal energy storage technologies for the development of renewable energies. At present, their widespread use is limited by undesired undercooling and phase separation, as well as their tendency to corrode container materials. Herein, we report a direct ink writing (DIW) additive manufacturing technique to print noncorrosive salt hydrate composites with thoroughly integrated nucleating agents and thermally conductive additives. First, salt hydrate particles are prepared from nonaqueous Pickering emulsions and then employed as rheological modifiers to formulate thixotropic inks with polymer dispersions in toluene serving as the matrix. These inks are successfully printed at room temperature and cured by solvent evaporation under ambient conditions. The resulting printed and cured composites, containing up to 70 wt % of the salt hydrate, exhibit reliable thermal cyclability for 10 cycles and suppressed undercooling compared to the bulk salt hydrate. Remarkably, the composites consistently maintain their structural integrity and thermal performance throughout the entirety of both the melting and solidification processes. We demonstrate the versatility of this approach by utilizing two salt hydrates, magnesium nitrate hexahydrate (MNH, Tm = 89 °C) and zinc nitrate hexahydrate (ZNH, Tm = 36 °C), to achieve desired thermal characteristics across a wide range of temperatures. Further, we establish that the incorporation of carbon black in these inks enhances the thermal conductivity by at least 33%. This approach consolidates the strengths of additive manufacturing and salt hydrate phase change materials to harness customizable thermal properties, well suited for targeted thermal energy management applications.

Phase Change Composite Microcapsules with Low-Dimensional Thermally Conductive Nanofillers: Preparation, Performance, and Applications

Phase change materials (PCMs) have been extensively utilized in latent thermal energy storage (TES) and thermal management systems to bridge the gap between thermal energy supply and demand in time and space, which have received unprecedented attention in the past few years. To effectively address the undesirable inherent defects of pristine PCMs such as leakage, low thermal conductivity, supercooling, and corrosion, enormous efforts have been dedicated to developing various advanced microencapsulated PCMs (MEPCMs). In particular, the low-dimensional thermally conductive nanofillers with tailorable properties promise numerous opportunities for the preparation of high-performance MEPCMs. In this review, recent advances in this field are systematically summarized to deliver the readers a comprehensive understanding of the significant influence of low-dimensional nanofillers on the properties of various MEPCMs and thus provide meaningful enlightenment for the rational design and multifunction of advanced MEPCMs. The composition and preparation strategies of MEPCMs as well as their thermal management applications are also discussed. Finally, the future perspectives and challenges of low-dimensional thermally conductive nanofillers for constructing high performance MEPCMs are outlined.

Synthesis and Properties of Inositol Nanocapsules

Sugar alcohols are phase-change materials with various advantages but may suffer from leakage during applications. In this study, inositol nanocapsules were synthesized at various conditions, including the amount of precursors and the time for adding the precursors. The effects of synthesis conditions on the properties of the nanocapsules were studied. The morphology, chemical composition, microstructure, phase-change characteristics and size distribution of the nanocapsules were investigated by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscope (TEM), differential scanning calorimeter (DSC) and a zeta potential analyzer. The results confirm that inositol was well-encapsulated by an SiO₂ shell. The shell thickness increased, while the supercooling degree of the nanocapsules decreased with increasing time for adding the precursors. In order to obtain nanocapsules with good morphology and phase-change characteristics, the time for adding the precursors should increase with the amount of precursors. The nanocapsules with the best properties exhibited high melting enthalpy, encapsulation ratio and energy storage efficiency of 216.0 kJ/kg, 83.1% and 82.1%, respectively. The size of the nanocapsules was remarkably affected by the triethoxysilane (TES) amount.

Highly Efficient Thermal Energy Storage Using a Hybrid Hypercrosslinked Polymer*

In this work, an organic/inorganic hybrid polymer containing siloxyl functional groups was synthesized and applied to encapsulate phase change materials (PCMs). Owing to the mild conditions of the hypercrosslinking reaction, which only requires the addition of a catalytic amount of aqueous alkaline solution, both organic and inorganic PCMs are tolerated. It is noteworthy that the initial homogeneous state of the reaction mixture allowed the ultimate encapsulation rate of the PCMs and the uniform blending of the third nano-additives with the aim of thermal conductivity enhancement. Further study reveals that the presence of this hybrid hydrophobic polymer in a phase change composite endows the latter with a unique self-cleaning property. This novel PCM encapsulation protocol is suitable for nanoparticles including carbon-based nanomaterials, metal oxide nanoparticles, and inorganic oxide nanoparticles. A thermal conductivity enhancement of 600 % was achieved along with 93.7 % light-to-thermal conversion efficiency with a latent heat of 180 J g^-1 without leakage.

Manifestations of Weak O-H···F Hydrogen Bonding in M(H2O) n(B12F12) Salt Hydrates: Unusually Sharp Fourier Transform Infrared ν(OH) Bands and Latent Porosity (M = Mg-Ba, Co, Ni, Zn)

Eight M(H₂O) _n(Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z^2- = B₁₂F₁₂^2-): M = Ca, Sr, n = 7; M = Mg, Co, Ni, Zn, n = 6; M = Ba, n = 4, 5. Weak O-H···F hydrogen bonding between the M(H₂O) _n²⁺ cations and Z^2- resulted in room-temperature Fourier transform infrared (FTIR) spectra having sharp ν(OH) bands, with full widths at half max of 10-30 cm^-1, which are much more narrow than ν(OH) bands in room temperature FTIR spectra of most salt hydrates. Clearly resolved ν_asym(OH/OD) and ν_sym(OH/OD) bands with Δν(OH) as small as 17 cm^-1 and Δν(OD) as small as 11 cm^-1 were observed (Δν(OX) = ν_asym(OX) - ν_sym(OX)). The isomorphic hexahydrates ( R3̅) have two fac-(H₂O)₃ sets of H₂O ligands and nearly octahedral coordination spheres. They exhibited four resolvable ν(OH) bands, one ν_asym(OH)/ν_sym(OH) pair for H₂O ligands with longer O(H)···F distances and one ν_asym(OH)/ν_sym(OH) pair for H₂O ligands with shorter O(H)···F distances. The ν(OH) bands for the three H₂O molecules with shorter, slightly stronger O(H)···F hydrogen bonds were broader, more intense, and red-shifted by ca. 25 cm^-1 relative to the bands for the three other H₂O molecules, the first time that such small differences in relatively weak O(H)···F hydrogen bonds in the same crystalline hexahydrate have resulted in observable IR spectroscopic differences at room temperature. For the first time room temperature ν(OH) values for salt hexahydrates showed the monotonic progression Mg²⁺ > Co²⁺ > Ni²⁺ > Zn²⁺, essentially the same progression as the p K_a values for these metal ions in aqueous solution. A further manifestation of the weak O-H···F hydrogen bonding in these hydrates is the latent porosity exhibited by Ba(H₂O)_5,8(Z), Sr(H₂O) _n,m(Z), and Ca(H₂O)_4,6(Z). Finally, the H₂O/D₂O exchange reaction Co(D₂O)₆(Z) → Co(H₂O)₆(Z) was ca. 50% complete in 1 h at 50 °C in N₂/17 Torr H₂O( g).