| Alumina(Al2O3)powder has been widely used in industry because of its excellent mechanical properties,high temperature resistance and chemical stability.The spherical Al2O3 powder with regular morphology and good dispersion has better performance because of its large packing density and good fluidity.Based on the characteristics of spherical Al2O3 powders prepared by thermal plasma,this paper focuses on its application in two fields.First,because of the high bond energy of Al-O,it needs higher sintering activation energy and higher sintering temperature to prepare Al2O3 ceramic.The pressureless sintering is widely used in practical production due to its advantages of simplicity and industrialization,but it is difficult to prepare ceramics with high density and high mechanical properties.Secondly,Al2O3 powder is used as filler to prepare polymer composites with high thermal conductivity.Due to the addition of fillers,a large number of filler-matrix interfaces are formed in the composite.The appearance of the interface seriously reduces the efficiency of heat transfer,that is,the thermal resistance is generated at the interface,which affects the overall heat transfer performance of the composite.In view of the two problems in the practical application of Al2O3 powders,we investigate the characteristics of spherical Al2O3 powders prepared by thermal plasma,and carry out research in the two application fields of high-performance Al2O3 ceramics and high thermal conductive polymer composites.The main research contents are as follows:(1)Firstly,we study the characteristics of the spherical Al2O3 powders prepared by thermal plasma,including the morphology,particle size,dispersion,crystal structure,element composition and the properties of the prepared slurry.It is found that Al2O3 powder not only has narrow particle size distribution,but also has good particle dispersion.In addition,the surface of the particles is smooth,the internal structure is compact and the crystallinity is good.The size of Al2O3 powder can be controlled by changing the feeding rate of aluminum powder.The slurry prepared by dispersing Al2O3 powder into water possesses good dispersing stability,and the particles are suspended in the slurry for a long time without settling.Further studies show that Al2O3 particles show high zeta potential in neutral aqueous solution.With the increase of solid content,the viscosity of Al2O3 slurry increases and the slurry is shear thinning.(2)The spherical Al2O3 prepared by thermal plasma is used to prepare Al2O3 ceramics with high performance.Compared with the untreated powder,the ball milled powder shows higher sintering activity.The relative density of the ceramic can reach 98.7%after sintering at 1600℃ for 2 h,and the ceramic owns high elastic modulus(400.2 GPa)and hardness(22.9 GPa).The results show that the sintering activation energy of ball milled powder(498.8 kJ·mol-1)is much lower than that of untreated powder(685.9 kJ·mol-1)at the initial stage of sintering.Further characterizations of the particles show that there are more strains in the ball milled particles,amorphous phase on the surface and more dislocations in the particles,which greatly improve the sintering activity of the powder and reduce the sintering temperature.(3)Based on the advantages of the spherical Al2O3 prepared by plasma,such as good dispersion and fine particle size,together with the sintering mass transfer of powder,the branched Al2O3(b-Al2O3)is prepared and filled into the phenolic resin(PR)to prepare polymer composite with higher heat transfer performance.The heat transfer mechanism of composite is studied,it is found that the b-Al2O3 particles form continuous network structure in matrix through the overlapping of the branch structure,which reduces the thermmal resistance of the interface during the heat transfer,and then improves the thermal conductivity of the PR(1.481 Wm-1K-1).In addition,due to the continuous network structure formed by b-Al2O3 in PR matrix and the better interface bonding force between filler and matrix,the thermal expansion performance of PR composite is greatly improved.(4)Based on the third part of the study,network Al2O3 spheres(N-Al2O3)with three dimensional continuous structures are prepared by spray drying and high temperature sintering process.The structural characterizations show that N-Al2O3 has more continuous structures compared with b-Al2O3,and possesses uniform and connected pores.After fully embedded into PR,PR matrix infiltrates into the pores of N-Al2O3,thus the prepared N-Al2O3/PR composite has interpenetrating structure of N-Al2O3 and PR.Due to the more continuous structures of N-Al2O3,the interfacial thermal resistance of PR composites is reduced to a greater extent,and the thermal conductivity of PR material is greatly increased to 4.01 Wm-1K-1,which is more than 18 times of that of pure PR.Furthermore,the mechanical properties,thermal stability and dielectric properties of the PR composite are also improved. |
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