Investigation On The Phase Change And Thermal Conduction Properties Of Organic Nanocomposite Phase Change Materials

The rapid development of electronic technology poses huge challenges for the heat dissipation of the devices.The organic phase change materials(PCMs)possess wide application prospects in fields related to thermal management of electronic devices.However,the extreme low thermal conductivity badly limits their large-scale applications.To solve this problem,we advocated experimental method,molecular dynamics simulation and theoretical cross-scale model to explore the new strategies to improve the thermal conductivity of PCMs.The main contents and conclusions in this thesis are summarized as follows:1.The experimental method was adopted for the preparation and characterization of different types of lauric acid(LA)/carbon nanotubes(CNTs)nano-encapsulated phase change materials(NEPCMs)with different sizes of diameters.Besides,the prepared PCMs were used as TIMs in the heat dissipation experiment.The conclusions were drawn as follows: the PCMs can be successfully filled into the inner cavities of CNTs using the solvent impregnation pressurization method;there exist two states in CNTs,which induces two melting points below the value of bulk materials;the phase change properties are not only relevant to the size of CNTs but also connected with the functionalization of CNTs.2.The molecular simulation method was used to investigate the structure and diffusion characteristics of PCMs in nano-constrain spaces.The effects of temperature and filling rate on the structure and diffusion behavior of the constrained molecules and the thermal conductivity of the composites were discussed as well.The results show that the molecular structure of LA filled in the tube changes dramatically.The molecules in the tube arrange in tubular layers.The axial diffusion ability of the molecules in the tube is significantly improved.The thermal conductivity of the composite is higher than that of the free state,which can be increased nearly 200 times than that of bulk LA;the filling rate affects the degree of molecular crowding inside the tube and thus affects the structure,diffusion and thermal conductivity of the composite.3.Molecular dynamics method was used to simulate the filling process of PCMs into CNTs.The structure,diffusion and phase change properties of PCMs in confined spaces were also investigated to explore the size effect on the phase transition and diffusion properties of restricted molecules.The results show that when the diameter of the nanotube is small,the molecules in the tube can enter the central region of the nanotube cavity.When the diameter is large,the molecules in the tube are only arranged near the wall surface of the nanotube.When the diameter of the carbon nanotube is 6nm,the filling rate of PCMs and the latent heat efficiency of nano-capsules reach to the maximum value.The phase change of PCMs inside CNTs starts from the outer region and gradually spreads into the inner region.Under the confinement effects,the melting point and self-diffusion coefficient of the molecules in the tube linearly correlates with the reciprocal of the restricted size.4.The effects of different types of functionalization on the thermal conductivity of boron nitride nanosheets(BNNS)and interface heat transfer between BNNS and PCMs were investigated.The effects of functionalization on the thermal conductivity of composites were predicted by cross-scale prediction model.The results show that the functionalization will cause the phonon scattering of the filler to reduce the thermal conductivity of the filler,but the introduction of functionalization can improve the interfacial heat transfer between the filler and the substrate.Under the combined effect of these two aspects,the functionalization will be helpful for the thermal conductivity enhancement of the composite only when the filler size is smaller than a certain size.