Optimization Of Thermal Conductivity Of Polyethylene Composites

Among the heat dissipating devices in electronic components,chips,aerospace equipment and industrial heat exchangers,polyethylene composite materials have many excelent properties,such as good electrical insulation,mechanical properties,fatigue resistance,light weight and low cost.Although the polyethylene and its composite material has been widely used in industrial,its thermal conductivity is still lower than traditional thermal materials,which limits the application in some aspects.In order to further improve the thermal conductivity of polyethylene matrix composites,this thesis used Finite element software coupled with genetic algorithm to optimize the design of polyethylene matrix composites at a lower filling ratio.Besides,the experimental method was adopted to improve the thermal conductivity of the polymer composites of polyethylene matrix filled with different size of flake graphite.In this thesis,the single-objective genetic algorithm was used to optimize the design of polyethylene/alumina?HDPE/A1₂O₃?composites.And the volume filling ratio of alumina particles is V_f=15%.In the optimization process,the coordinates of alumina particles were used as the optimization variables,and the thermal conductivity of the composites was optimized.The alumina particles with the same particle size and two different particle sizes were selected separately,and the distribution of the optimized alumina particles under different conditions was finally obtained.The optimization results show that the thermal conductivity of composites filled with single particle size alumina particles increased from 0.3601 W/?m·K?to 0.7992 W/?m·K?,and the thermal conductivity of composites filled with two different particle size alumina particles increased from 0.3686W/?m·K?to 1.1057 W/?m·K?,which was increased to 2.22 times and 3 times before optimization.In the case of volume filling ratio V_f=15%,the optimized design and comparison of polyethylene composites filled with different amounts of alumina particles of the same particle size were carried out.The optimization results showed that the thermal conductivity of HDPE/A1₂O₃ composites filled with three different alumina particles after optimization was greatly improved compared with that before optimization.The thermal conductivity of HDPE/Al₂O₃ composites with 15 fillers was increased from 0.3601 W/?m·K?to 0.7992W/?m·K?,and the thermal conductivity of HDPE/A1₂O₃ composite with 30 fillers increased from 0.3491 W/?m·K?to 0.9081 W/?m·K?.The thermal conductivity of the 40 fillers HDPE/A1₂O₃ composite increased from 0.3352 W/?m·K?to 1.035 W/?m·K?,which was increased to 2.22,2.61 and 3.08 times before optimization,respectively.In this thesis,the principle of entransy dissipative extremum and the principle of the minimal loss of the exergy were introduced as the performance evaluation index,which was compared with the optimization work using the thermal conductivity as the evaluation index.In the case of volume filling ratio V_f=15%,the same HDPE/Al₂O₃ composite model was optimized using three different evaluation indicators.The optimization results show that the three performance evaluation indexes can obtain the distribution of alumina particles which are beneficial to heat conduction,and form a particle"passage"which can penetrate the composite material and can transfer heat efficiently.In the optimization work of this thesis,due to the disjoint and non-overlapping limitations of each filler particle in the optimization process,a penalty function method was introduced in the optimization process to solve this problem.The penalty function method is a method for transforming constrained optimization problems into unconstrained optimization problems,mainly for dealing with highly nonlinear constraints.Finally,a high thermal conductivity polyethylene/flake graphite?HDPE/FG?composite was prepared by powder blending fusion method,and its thermal conductivity was optimized by changing the ratio of fillers of different particle sizes and the particle size of the matrix material.The experimental results show that when the weight filling ratio of the matrix material HDPE is 45%250?m+10%20?m+5%2?m?HDPE?,and the weight filling ratio is 20%500?m+10%200?m+5%20?m+5%2?m?FG?,the HDPE/FG sample prepared has the best thermal conductivity and the thermal conductivity can reach as high as 2.853 W/?m·K?.