Incorporating paraffin@SiO2 nanocapsules with abundant surface hydroxyl groups into polydimethylsiloxane to develop composites with enhanced interfacial heat conductance for chip heat dissipation.

Incorporating phase change capsules into polymeric matrices is an effective approach for developing flexible composites with both heat storage capacity and good thermal reliability, while the interfacial heat conductance between the capsules and the matrix has seldom been considered. Herein, paraffin@SiO2 nanocapsules synthesized by an interfacial polycondensation process using a basic catalyst were incorporated into a polydimethylsiloxane matrix for the first time to prepare phase change composites at different loadings. Furthermore, the composites containing the nanocapsules were systematically compared with the composites containing the paraffin@SiO2 microcapsules synthesized using an acidic catalyst. It is shown that, at every identical mass fraction, the composites containing the nanocapsules not only possessed larger latent heat than those containing the microcapsules, but also exhibited higher thermal conductivity and lower hardness. The enhancement in thermal conductivity as well as the decline in hardness for the composite containing the nanocapsules are revealed to originate from a larger amount of hydroxyl groups at the surfaces of the nanocapsules than the microcapsules, which could form more hydrogen bonds with the polymer matrix. This bonding favored the interfacial heat conductance between the nanocapsules and the matrix together with decreasing the crosslinking density of the matrix. Subsequently, composites with enhanced thermal conductivity were developed by combining the nanocapsules with a BN filler. By evaluating the performance for chip heat dissipation, it was found that, when the chip was heated at a power of 10 W, the incorporation of the paraffin@SiO2 nanocapsules at a loading of 36 wt% into the polymer matrix made a remarkable decrease in the chip equilibrium temperature by 31.7 °C, and a further decline by 8.9 °C occurred when combined with 16 wt% BN. This work sheds light on facilitating the interfacial heat conductance between phase change capsules and the polymer matrix by hydrogen bonding.

Compounding MgCl2·6H2O with NH4Al(SO4)2·12H2O or KAl(SO4)2·12H2O to Obtain Binary Hydrated Salts as High-Performance Phase Change Materials.

Developing phase change materials (PCMs) with suitable phase change temperatures and high latent heat is of great significance for accelerating the development of latent heat storage technology to be applied in solar water heating (SWH) systems. The phase change performances of two mixtures, NH4Al(SO4)2·12H2O-MgCl2·6H2O (mixture-A) and KAl(SO4)2·12H2O-MgCl2·6H2O (mixture-B), were investigated in this paper. Based on the DSC results, the optimum contents of MgCl2·6H2O in mixture-A and mixture-B were determined to be 30 wt%. It is found that the melting points of mixture-A (30 wt% MgCl2·6H2O) and mixture-B (30 wt% MgCl2·6H2O) are 64.15 °C and 60.15 °C, respectively, which are suitable for SWH systems. Moreover, two mixtures have high latent heat of up to 192.1 kJ/kg and 198.1 kJ/kg as well as exhibit little supercooling. After 200 cycles heating-cooling experiments, the deviations in melting point and melting enthalpy of mixture-A are only 1.51% and 1.20%, respectively. Furthermore, the XRD patterns before and after the cycling experiments show that mixture-A possesses good structure stability. These excellent thermal characteristics make mixture-A show great potential for SWH systems.

Characterization of MgCl2·6H2O-Based Eutectic/Expanded Perlite Composite Phase Change Material with Low Thermal Conductivity.

The melting points of the phase change materials (PCMs) incorporated into the walls of buildings should be within the human thermal comfort temperature range. In this paper, 15 wt.% of MgCl2·6H2O was mixed with CaCl2·6H2O to obtain the eutectic with a melting point of 23.9 °C. SrCl2·6H2O suppresses the supecooling of the eutectic. The combination with expanded perlite (EP) via the impregnation method overcomes the phase separation and liquid leakage of the CaCl2∙6H2O-MgCl2∙6H2O mixture. The composite PCM is form-stable with the maximum loading mass fraction up to 50 wt.% and latent heat of 73.55 J/g. EP also significantly reduces the thermal conductivity of the CaCl2∙6H2O-MgCl2∙6H2O from 0.732 to 0.144 W/(m·K). The heating-cooling cycling test reveals that the composite PCM is thermally stable. The cheap eutectic salt hydrate, with little supercooling, no phase separation and liquid leakage, low thermal conductivity and good thermal reliability, show great potential as envelope materials to save energy consumption in buildings.