The phase change hysteresis characteristics ofNa2HPO4·12H2O: A molecular dynamics study

• The hysteresis degree of DSP is decreased after encapsulation. • The reason for the hysteresis is the difference in the molecular structure during the heating and cooling process. • The system energy of encapsulated DSP is mainly decided by “randomly distributed” molecules. • Molecular dynamics simulation of pure DSP and encapsulated DSP. There is a phenomenon of inconsistent melting and solidification behavior of PCM, which is called phase change hysteresis, generally expressed by the difference between melting temperature (T m ) and solidification temperature (T s ). The hysteresis degree (T m -T s ) is proportional to the energy loss of the system, if the temperature is beyond the operating temperature range of the system, the latent heat of the PCM cannot be exploited. The phase change hysteresis and supercooling were regarded as the same concept in previous studies no matter simulations or experimental research, which will lead to large errors. It is believed that there is a large hysteresis degree of inorganic PCM besides higher supercooling degree, poor chemical stability, and serious phase separation phenomenon, like Na 2 HPO 4 ·12H 2 O(DSP), which is prone to dehydrate from Na 2 HPO 4 ·12H 2 O to Na 2 HPO 4 ·7H 2 O during the heating and cooling process.DSP is adsorbed by SiO 2 via hydrogen bond, van der Waals force, and capillary effect, which can solve the above shortcomings. There are certain limitations in studying the hysteresis properties of PCM via experimental methods. As so far, the hysteresis properties of inorganic hydrated salts are rarely discussed in related papers, the reason for the hysteresis phenomenon is not explained, and the nucleation mechanism is not clear. Therefore, a composite phase change material (CPCM) model with DSP as the core and mesoporous silica MCM-41 as the shell was established in this work. The phase change hysteresis (PCH) characteristics are studied by analyzing the radial distribution function (RDF), self-diffusion coefficient(D), and density. The molecular motion of the interface layer was studied by the molecular dynamics(MD) method, and the arrangement of microscopic particles before and after adsorption was compared. The results showed that the hysteresis degree of DSP decreases to 1.5 °C after encapsulation by SiO 2 , which is because the effect of “Near-range ordered” molecules tend to disappear. After experimental verification, there is a 20% error between the simulation and experimental results. In addition, the reason for the hysteresis is the difference in the molecular structure that determines the energy of the system during the heating and cooling process. The purpose of this paper is to explain the reasons for the hysteresis from the view of microscopic and to provide a basis for the design of LHTES systems.