Research On Enhancing Method Of Thermal Conductivity Of Composites Based On Sand Canal And Stone Canal Transformation Mechanism

With the development of science and technology,polymer-based thermal conductive composites have attracted more and more scholars’ attention,and they have become new heat-conduction and dissipation materials in many fileds,such as the aerospace industry,the automotive industry,electronic equipment,lithium-ion batteries,solar cells and microelectronics packaging.However,the inherent low thermal conductivity of the polymer matrix also greatly limits the application of the composite.In order to improve thermal conductivity of the composite,scholars have proposed a variety of method to build a thermal conductive network by adding thermal conductive fillers.However,The problem is why the thermal network built inside the polymer cannot increase the thermal conductivity of the composite by an order of magnitude like conductive network?Based on this problem,this master’s thesis proposes the conversion mechanism of the sand canal and stone canal to enhance the thermal conductivity,and designs the experiment to verify.Then,the mathematical theory model was derived for the in-depth analysis and research.In the transformation mechanism of the sand canal and stone canal,the heat conduction network is compared to "canal",the surrounding polymer matrix is compared to "sand and stone",and the heat flow is compared to "water flow".When the "water" is transported in the "canal",the "water" will quickly leak into the surrounding "sand and stone",making the "water-rich canal"quickly become water-less canal" or even "water-free canal".At this time,the canal with a large amount of water seepage is called "sand canal",resulting in that the thermal conductivity of the composite material cannot be greatly improved.The reason for the "leakage" is that although the thermal conductivity of the polymer matrix around the network is very low,it has a strong heat absorption ability.When the heat is transmitted along the network,a considerable amount of heat will be absorbed by the surrounding polymer and dissipated on the spot.In order to reduce the energy dissipation of the polymer matrix and prevent the "leakage" of the canal,the "stone canal" was constructed by spacial confining forced network assembly method(SCFNA)and the adding rigid particles.SCFNA method can improve the degree of network compaction,reduce the filler gap,and decrease the heat dissipation between the fillers gaps.Rigid particles can replace part of the polymer matrix,reduce the energy dissipation of the polymer matrix,and scatter part of the heat back to the thermal conductivity network,reducing the network leakage to the surrounding polymer matrix.At this time,when the "flow" conducts in the "canal",not only the amount of transport greatly increased but no leakage occurred,indicating that"sand canal" has been transformed into "stone canal" by the above two methods.To prove the effectiveness of sand canal and stone canal conversion mechanism,PDMS with low viscosity was selected as the polymer matrix in this study.The main research contents include:(1)Al2O3 was selected as the thermal conductivity filler and SCFNA method was used to prepare the thermal conductivity composite,so as to prove that SCFNA method play a role in improving the densification of thermal conductivity network in the transformation mechanism of sand canal and stone canal.Thermal conductivity of composite material versus compression ratio and filler content was studied.The higher the compression ratio,the higher the proportion of thermal conductivity improvement.When the Al2O3 content was 30wt%and the compression ratio was 0.875,thermal conductivity reached 3.425W/mK,increased by 12.6 times,more than one order of magnitude,comparing to the case of no compression at the same filler content.The formation of stone canal network was observed by SEM.When the content of Al2O3 content was 30wt%and the compression ratio increased from 0 to 0.875,the gap between fillers decreased to about 1μm.(2)In order to prove that rigid particles can reduce the polymer dissipation,reflect part of the energy returns to the thermal conductive network and be good to form the heat conduction path of the stone canal.The glass bubbles with low thermal conductivity(0.15W/mK)are specially selected,which is lower than the thermal conductivity of the polymer matrix PDMS(0.27W/mK),and the short carbon fiber(SCF)is selected to as the thermal conductive filler of the composite system to prepare PDMS/SCF/GB composites.Besides,the formation of the stone canal network with GB was observed by SEM.The role of the SCFNA method and rigid particles in the transformation mechanism is proved.The SCFNA method contributes to increase the density of the thermal conductive network.The compression ratio of the PDMS/SCF composite increases from-3 to 0.8,and the thermal conductivity increases by nearly 10 times.A low content of GB,mainly distributed around the heat conduction network,was added into the compound system.When the compression ratio is 0.8 and the GB content is 2wt%,the heat conductive network of the stone canal is constructed,and the thermal conductivity is again increased by 14%,comparing to the case of at the same compression ratio and no GB.(3)Based on the synergistic effect of SCFNA method and rigid particles,a mathematical theoretical model of the stone canal heat conductive network is constructed.The trend of thermal conductivity of composite was predicted,and the effects of thermal conductivity filler content,compression ratio,carbon fiber orientation and other factors on the thermal conductivity of the composite were analyzed.When the content of GB is within a certain range,the thermal conductivity increases with the increase of the content of GB.In the same GB case,the larger the compression ratio,the higher the thermal conductivity.