Study On The Correlation Between The Microstructures And Thermal Conductivity Of Particle Reinforced Aluminum Matrix Composites

With the integration and miniaturization of microelectronic and optoelectronic devices,chip level thermal power density increases rapidly,therefore,how to make the electronic devices dissipate heat timely and effectively has became an urgent problem for new packaging materials.The particle reinforced aluminum matrix composites as a new generational electronic packaging applications have advantages of performance design,but in order to ensure closer heat matching with the semiconductor devices,higher content particles need be added to the composites,due to the poor compatibility between the particles and the matrix,composites need be prepared under higher temperature,therefore very complex microstructures will be formed which brings great difficulty to the regulation design of macroscopic properties and the predictability for the composites.Therefore,reasonable micro structure design of the composites and the relationship study between the micro structure and the effective properties are a very important topic.This paper takes SiCp/Al composites as the research object and experiment,finite element simulation and theoretical analysis as the research method,investigates systematically the regulation of the typical micro structures(component,interface and pore)that affects the heat conduction of the composites and the macro and the micro correlation between thermal conductivity and micro structure of the composites and also the heat conduction model.The main research content and conclusions are as follows:(1)The regulation and the characterization of the complex components(the intrinsic thermal conductivity of matrix,the intrinsic thermal conductivity of particle and the formed relative structure of matrix and particle)and the law that thermal conductivity varies with the it are investigated systematically.The results suggest that: a)by the ceramic mold pressureless infiltration,particle content has small regulating range,namely,single particle diameter is 48% to 52% and double particle diameter is 50% to 63%,and by preformed body pressureless infiltration,it can reach the range of 35% to 62% volume fraction through adjusting the pore forming agent content.b)Three kinds of micro-structure that the reinforcement body diffuses,interpenetrates and interconnects in the matrix are well characterized by methods of the dispersion degree and the topology.c)When a higher content particles are added to the matrix,the conductivity decreases significantly and the resistivity becomes no rules with the changes of nodes,but the thermal conductivity values changes little and the changes that thermal conductivity of the composites varies with the intrinsic thermal conductivity of matrix or particle are not linear;when the particle content is high,the effects of the relative structure formed in the matrix to the thermal conductivity of the composites are not obvious,but when the particle forms interpenetrating networks with the matrix,it can significantly improve the flexural strength and decrease the coefficient of thermal expansion.(2)The regulation and the characterization of the complex interface phase(interface phase combination,type,thickness and distribution ratio)and the law that thermal conductivity varies with the complex interface phase are investigated systematically.The results suggest that: a)adding Mg,Si to the matrix or raising infiltration temperature can improve the wetting combination of the Al/Si C system;nickel coating on the surface of the particles,pre-oxidation treatment or adding Si to the matrix can effectively inhibit the formation of the harmful interfacial product A14C3.b)The shape,the distribution and the size of the interface phase can be characterized well by particle-chemical extraction and field emission scanning method,during the fabrication process of pressureless infiltration,the interface reaction phase is almost not continuous apart from the coating layer.c)When the interface phase and matrix particles combine not well,the degradation effect on the thermal conductivity is almost equal to the air layer with different thickness,when the air layer is about 20 nm,the interface phase layer almost turns into the insulation layer;when the interface phase improves the combination and the distribution is continuous,it acts as the interface layer;when the thermal conductivity of the coating interface layer is too low,even if the improvement of particles thermal conductivity is great,it is also difficult to reflect the high thermal conductivity of the particles;when the interface distribution is not continuous,regardless of the intrinsic thermal conductivity of the interface phase is high or low,it has little effect on the overall thermal conductivity of the composites.its contribution to the thermal conductivity is mainly for reacting with chemicals in the interface and enhancing the interface bonding,by means of the elastic-plastic mechanics,the wetting angle measurement and the first principle calculation,the stress and the work of adhesion on the interface are characterized quantitatively,and further more,the effect of interface phase combination to the thermal conductivity are explained when the interface phase is not continuous,meanwhile,some good methods for improving interface phase combination are put forward.(3)The interface thermal conductivity characterization and the effect of interface phase to interface thermal conductivity are investigated.For two different combination interface,the calculated method for the interface thermal conductivity of are presented;the interface thermal conductivity value under different kinds,different combination,different distribution or without interface phase is acquired.The research results show that: a)the interface thermal conductivity value is very sensitive to the interface phase combination conditions,the span of the orders of magnitude for regulation is from from 10~5 to 10~8,when the effective interface length is fixed to 2?m,the equivalent thermal conductivity of interface can vary in the range of 0.002 to 200W/m.K,and the span of the orders of magnitude is 5,which is almost approximated to the overall span of the thermal conductivity whose span is 6,to a certain extent,it reflects that the regulation space of the interface thermal conductivity is large.b)When the interface phase improves the interface combination and the distribution is continuous,thermal conductivity of interface phase itself has a greater influence on interface thermal conductivity,when the thermal conductivity of interface phase itself is higher,its magnitude of interface thermal conductivity is 10~8,which is almost identical when the interface is well bonded and not contained inter-phase,when the thermal conductivity of interface phase itself is lower,the interface thermal conductivity declines,when the interface phase distribution is discontinuous,the orders of magnitude of interface thermal conductivity is from 10~6 to 10~7.c)When the interface phase is not conducive to the interface combination,the interface thermal conductivity is low and its magnitude range is from 10~3 to 10~5.(4)The regulation and the characterization of the complex pore(porosity,pore size and pore shape)and the law that thermal conductivity varies with the complex pore are investigated systematically.The results suggest that: a)by the ceramic mold,preform pressureless infiltration methods,changing the infiltration temperature and time can adjust the porosity(from 0.7% to 9.4%)and the average pore size(from 21.4 to 49.7?m),and the porosity and the pore size distribution is more uniform under preform pressureless infiltration methods,and in this way,the pore forming agents types can be changed and can achieve a large size range pore(from 46.5 to 96.2?m)and pore shape.b)The pore parameters can be well characterized by the combination of ultrasonic microscopic examination and microscopic image analysis.c)The thermal conductivity of composites decreased significantly with the increase of porosity,and the pore size and pore shape has little effect on the thermal conductivity;the simulation process shows the heat flow phenomenon is obvious due to the existence of pore and there is "hot choking effect" on the junction of the pore and material,and local heat flux density peak appears,when there are more pores and the distribution is mixed,or the pore shape is irregular(including sharp corners),there will be heat flux in opposite direction of heat transfer,which has great impacts on the thermal conductivity of the materials;when the porosity is larger than 2%,the simulation value compared with the calculated value by the Maxwell and MEMA model is more close to the experimental value.(5)The thermal conductivity correction model for composites is studied in more detail.The research focuses on the actual microstructure of the composites,combines with the micro mechanical model and synthesizes micro structure(interaction between the particles with high volume fraction,porosity,interface phase and etc.)parameters,and modifies the thermal conductivity DEM model.The research results show that: the simulation value is in good agreement with the measured values.The research results above can provide some theoretical guidance and technical support for the thermal properties control,the thermal interface materials design and the prediction of heat conduction properties of composites,and it also can enrich the related theory of the interface heat conduction of composites.