Melting process of nanometer-sized In particles embedded in an Al matrix synthesized by ball milling
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Melting process of nanometer-sized In particles embedded in an Al matrix synthesized by ball milling H. W. Sheng, J. Xu, L. G. Yu, X. K. Sun, Z. Q. Hu, and K. Lu State Key Laboratory for RSA, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China (Received 22 June 1995; accepted 10 May 1996)
Dispersions of nanometer-sized In particles embedded in an Al matrix (10 wt. % In) have been synthesized by ball milling of a mixture of Al and In powders. The as-milled product was characterized by using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectrometer (EDX), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HREM), respectively. It was found that In and Al are pure components immiscible with each other, with nanometer-sized In particles dispersively embedded in the Al matrix. The melting behavior of In particles was investigated by means of differential scanning calorimeter (DSC). The calorimetric measurements indicate that both the melting point and the melting enthalpy of the In nanoparticles decrease with increasing milling time, or refinement of the In particles. Compared to its bulk melting temperature, a melting point depression of 13.4 K was observed when the mean grain size of In is 15 nm, and the melting point depression of In nanoparticles is proportional to the reciprocal of the mean grain size. The melting enthalpy depression was interpreted according to the two-state concept for the nanoparticles. Melting of the interface was deduced to be an exothermal process due to its large excess energy/volume.
of fine particles as12,16 :
I. INTRODUCTION
Melting behavior is a common phenomenon in materials research, but it is far from being completely understood. During recent decades, the melting behavior of fine particles has been extensively studied. Methods utilized including analytical, computational, and experimental techniques, abound in various materials.1,2 Results show the melting points of fine particles exhibit a strong dependence on their sizes. Finite metal particles, e.g., In, Sn, Bi, Pb, Cu, Ag, and Au, have often been found to melt below their bulk melting temperatures, and the melting temperature, in most cases, is inversely proportional to their particle sizes.3 Meanwhile, superheating of entrained particles was also observed.4,5 Recently, In and Pb embedded in Al matrixes prepared by using rapid quenching6–8 and ion implantation techniques,9 were reported to be greatly superheated above their bulk melting temperatures. Arguments on the melting mechanism for finite particles have never reached an agreement (see e.g., Refs. 10 –15). Couchman and Jesser12 stressed the importance of the heterogeneous nucleation of melting at the crystalline surface. The melting point of thin films or fine particles can be either depressed or increased, depending on the nature of the interface. A thermodynamics equation was thus derived to de
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