Preparation,Properties And Phase Transition Kinetic Of Modified Capric Acid-Expanded Vermiculite Composite Thermal Storage Materials

The phase change heat storage technology is a method of storing and releasing heat energy by using a heat storage medium-phase change material,which can effectively solve the problem that the heat energy does not match in time and space,and achieve the purpose of energy saving and emission reduction.Among them,organic phase change heat storage materials have broad application prospects in the fields of building energy conservation,solar energy utilization and industrial waste heat recovery due to their advantages of large heat storage density and good thermal stability.Commonly,organic phase change materials are prone to leakage in the presence of liquid phase.In view of these shortcomings,Capric Acid（CA）was used as phase change material,and Expanded Vermiculite（EV）,Acid Vermiculite（AV）and Silanization Vermiculite（SV）obtained by acidification and silanization were used as CA support matrix,and CA-EV,CA-AV and CA-SV phase change composites were prepared by direct physical adsorption.Through the modification of vermiculite,the packaging,structure and thermophysical properties of phase change composites were studied.The problem of liquid phase leakage of phase change materials was overcome;the comprehensive performances of phase change composites were improved.The non-isothermal phase transition kinetic thermal analysis was used to derive the phase transition kinetic parameters and mechanism of phase change composites.The thermal stability of phase change composites was evaluated.The mechanism changes of the modification process were discussed.The structures of the three vermiculites were analyzed by transmission electron microscopy（TEM）,X-ray diffractometry（XRD）,Fourier transform infrared spectroscopy（FT-IR）,multi-channel specific surface area and pore size analyzer.EV,AV and SV are three-dimensional porous structure;after acidification,the cations of the vermiculite layer are washed out,resulting in a large number of voids and defects in the silicate hexahedral structure,and the crystal structure is destroyed.The specific surface area is 82 m²/g of EV increased to 544 m²/g of AV;after silanized treatment,silane group was added to the surface of SV,the specific surface area decreases slightly to 210 m²/g,still significantly higher than EV;the adsorption curves of AV and SV represents micro and medium pore structure is back to the hysteresis loop,while the EV does not.The thermal properties of the three types of shaped phase change composites were analyzed by differential scanning calorimetry（DSC）.The saturated adsorption capacity of EV,AV and SV to CA is 55%,65%and 67%respectively;the phase transition enthalpy of CA-SV composite is 104.41 J/g,which is 21.8%higher than that of CA-EV.The modification of vermiculite improves its adsorption capacity.After 1000 cycles of thermal cycling,the thermal enthalpy of the CA-EV shaped phase change composites decreased by 4.43%,the CA-AV composites decreased by3.69%,and the CA-SV composites decreased by 3.21%.The phase change composite prepared by using modified vermiculite as a support material has excellent thermal cycle stability.The kinetic activation energies of CA-EV,CA-AV and CA-SV were calculated by the Kissinger method at the same CA content（55 wt.%）:430.6 k J/mol,482.8k J/mol and 532.2k J/mol,respectively.The modification of vermiculite has an effect on the stability of phase change composites.With the increase of CA content,the activation energy of each phase change composite exhibits a decreasing trend.The composite materials are derived by Boltzmann fitting to reflect the relation between Activation energy and CA content.The phase transition kinetic parameters of CA-EV,CA-AV and CA-SV composites at 55 wt.%CA content were derived by double extrapolation.The original state activation energies E_α→0 of CA-SV,CA-AV and CA-EV were 1264k J/mol,1103 k J/mol and 913 k J/mol,respectively,and the activation energy E_β→0 of the thermal equilibrium state was 1270 k J/mol,1196 k J/mol and 981 k J/mol;the most probable mechanism function of CA-SV is G（α）=[1-（1-α）^（1/3）]²;CA-AV is G（α）=α^2/3;CA-EV is G（α）=[（1+α）^（1/3）-1]².After 1000 thermal cycles,the E_α→0 and E_β→0 of CA-SV increased significantly,reaching 1943 k J/mol and 1992 k J/mol,respectively.The activation energies of CA-AV and CA-EV did not change much.The approximate mechanism function satisfies the Mample first-order one-line rule:G（α）=-ln（1-α）,which is consistent with the mechanism function before and after the pure CA cycle.The mechanism is a random nucleation and subsequent growth process with a reaction order of n=1.The multi-scanning rate curves of CA-EV,CA-AV and CA-SV phase change composites with 55 wt.%CA content at different heating rates（1,5,10,15 and20°C/min）were compared.The temperature rise curve moves toward the high temperature as the heating rate increases.The curve of the phase transition rate and temperature follows the"S"shape,that is,the phase transition rate is slower in the early and final stages,and faster in the middle stage.The increase of interface and micropores,surface modification and reduction of interlayer charge improve the bonding ability of CA and vermiculite,but it does not change its long-term phase transition mechanism.