Studies On The Thermal Conductivities And Thermal Rectifications Of Laminated Micro-nano Structures

The application of electronic diodes has greatly motivated the development of Information Age,while similarly,thermal rectifiers,as thermal manipulation devices,might have broad applications in the renewable energy engineering.Thermal rectification is an important physical property,and finding suitable materials(geometry structure or mechanisms)for the development of thermal rectification is a major subject.With the increase of power density in electronic devices,the requirement of thermal managment also increase.Besides,the applications of insulation equipment,energy storage and thermal diodes also rely on the development thermal rectification.In the present work,we observed thermal rectification in several materials with different microstructured geometry by measuring the effective thermal conductivities experimentally.We studied the effect of different geometry structured thermal rectifiers,and discussed the influences of relative factors by experimental measurements,analytical derivation and finite element simulation(e.g.,thermal rectifiers made by biomorphic materials and porous silicon).(1)The effective thermal conductivity(ETC)of laminated material(bulk material)is studied by experimental measurement,analytical derivation and finite element simulation.We measured the axial thermal conductivity of rGO and hBN samples with different densities separately and observed their microstructures.After analytical derivation,we obtained the conclusions:(a)The bigger thickness of samples,the smaller value of ETC.(b)The higher contact thermal conductivity of samples,the bigger value of ETC.(c)In homogeneous bulk materials,the thermal conductivity increases with the increase of density(contact area).The results of finite element simulation verify our experimental and analytical models.Further more,we proposed a method to calculate the contact thermal conductivity by combining of the experimental and analytical models.(2)We reported a significant thermal rectification phenomenon observed by using a thermal rectifier solid-state device comprising microstructured cellular biomorphic materials and by measuring the thermal conductivities in the forward and reverse directions.This thermal rectifier broke several limitations:e.g.,low operation temperature,nanomanufacturing and finding iso-structured materials with suitable thermal conductivities.After repeated experiments,theoretical analyses and FE simulations,we proved that the thermal rectification could be realised in a single biomaterial device which comprised a cellular graded geometric structure at macroscale and exhibited non-separable temperature dependent thermal conductivity.Additionally,we confirmed that the increasing cellular size gradient or temperature-dependent thermal conductivity slope could enhance the thermal rectification effect.(3)We prepared a series of porous silicon samples and measured their axial thermal diffusivities in forward and reverse directions respectively to calculate the thermal rectification effect.We found that only when the corrosion current was setted at 100mA/cm2 and 150mA/cm2,thermal rectification phenomenon produced clearly,and the thermal diffusion coefficient in forward direction always larger than that in reverse direction.The credibility of the experiment is verified by repeated experiments and the finite element simulation.Further more,we provided that the effect of thermal rectification related to the depth of porous silicon,the porousivity,and temperature difference.(4)Taking the La0.7Sr0.3CoO3/LaCoO3 system and biomaterials(bamboo shell)as examples,we studied the influence of time on thermal rectification by finite element simulation.We obtained the conclusions:In the La0.7Sr0.3CoO3/LaCoO3 system,the coefficient of thermal rectification with time was not monotonous,and it reached the biggest value at the first peak point.Diversely,the coefficient of thermal rectification increased gradually with time in biomorphic materials,and finally reached the biggest value at steady state.