The Preparation And Heat Transport Behavior Of Nanocrystalline Metals

With the development of science and technology,more new materials,to date,are constantly emerging.Nanocrystalline(NC)materials have a higher yield strength and a better wear resistance in comparison with conventional coarse-grained polycrystalline materials,However,because of various scattering mechanisms which give rise to thermal resistance,NC materials show lower thermal conductivity than their corresponding coarsegrained counterpart.It is generally accepted that the thermal conductivity of NC materials decreases with the decreasing grain size,and a great effort has been made to clarify this phenomenon.Therfore,it is essential to master the heat transfer characteristics of NC materials.The present dissertation elucidates the process of the material micromation,the recent research on micro-scale thermal transport,and discusses the heat transport mechanism of NC materials.In this paper,both experimental and theoretical studies were carried out on NC matels.NC Cuand Cu-Ag are prepared by the high pressure sintering method using a DS6 × 14 MN cubic press.For a better understanding of the effects of grain boundary and size on the thermal conductivity of nc material,a modified model,with special emphasis on the contributions of electron conduction,is presented by incorporating the concept of the Kapitza resistance into an effective medium approach.The theoretical calculations are in good agreement with our experimental results.This work not only possesses very important scientific value,but also provides a solid theoretical foundation for revealing the nanostructures metal crystal heat transport mechanism.In this paper,the main research content and results are summarized as follows.NC material according to coarse-grain materials shows lower thermal conductivity than its coarse grain counterpart,which restricts its engineering applications.Bimodal nanocrystalline Cu-Ag and NC Cu were prepared by the high pressure sintering method in argon atmosphere.The microscopic structures of the samples were studied by X-ray diffraction(XRD)and scanning electron microscopy(SEM),respectively.The thermal conductivity of the samples with average grain sizes ranging from 50 to 270 nm was measured.The test results showed that the thermal conductivity of NC Cu-Ag and NC Cu were reduced significantly from 243.84 W/m·K to 163.45 W/m·K and 259.93 W/m·K to 180.36 W/m·K at 300 K,respectively.In addition,the thermal conductivity increases with the increasing of grain size.For a better understanding of the effects of grain boundary and size on the thermal conductivity of NC material,a modified model,with special emphasis on the contributions of electron conduction,is presented by incorporating the concept of the Kapitza resistance into an effective medium approach.The theoretical calculations are in good agreement with our experimental results.