Sn3.5Ag （mass fraction, %） nanoparticles were synthesized by an improved chemical reduction method at room temperature. 1,10-phenanthroline and sodium borohydride were selected as the surfactant and reducing agent, respectively. It was found that no obvious oxidation of the synthesized nanoparticles was traced by X-ray diffraction. In addition, the results show that the density of primary particles decreases with decreasing the addition rate of the reducing agent. Moreover, the slight particle agglomeration and slow secondary particle growth can result in small-sized nanoparticles. Meanwhile, the effect of surfactant concentration on the particle size can effectively be controlled when the reducing agent is added into the precursor at an appropriate rate. In summary, the capping effect caused by the surfactant molecules coordinating with the nanoclusters will restrict the growth of the nanoparticles. The larger the mass ratio of the surfactant to the precursor is, the smaller the particle size is.
The preparation and solidification of metallic droplets attract more and more attention for their signiifcance in both engineering and scientiifc ifelds. In this paper, the preparation and characterization of Sn-based al oy droplets using different methods such as atomization and consumable electrode direct current arc （CDCA） technique are reviewed. The morphology and structure of these droplets were determined by optical microscopy, X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The solidiifcation behavior of single droplet was systematical y studied by means of scanning calorimetry （DSC）, and the nucleation kinetics was also calculated. In particular, the development of fast scanning calorimetry （FSC） made it possible to investigate the evolution of undercooling under ultrafast but control able heating and cooling conditions. The combination of CDCA technique and FSC measurements opens up a new door for quantitative studies on droplet solidiifcation, which is accessible to demonstrate some theories by experiments.
The size-dependent solidification undercooling was investigated for single micro-sized particles of pure Sn employing differential scanning calorimeter(DSC).The particles were obtained from a solvent-encapsulation remelting and quenching(SERQ) process.Because of the basically unchanged spherical shape of the measured single particles during a series of continuous heating and cooling processes,it allows studying the independent effect of particle size on undercooling.Applying classical nucleation theory in conjunction with available thermodynamic data yields an increasing undercooling with decreasing particle size.The theoretical description is in good agreement with the experimental data.
YANG BinGAO YuLaZOU ChangDongZHAI QiJieZHURAVLEV.ESCHICK.C
Undercooling of Sn droplets in different atmospheres was studied by fast scanning calorimetry(FSC)at cooling rate of 1,000 K/s.It is found that the undercooling decreased with increasing partial pressure of oxygen.Randomly distributed SnO2islands were observed to form on the droplet surface,which likely has promoted the heterogeneous surface nucleation.As the partial pressure of oxygen changes,the nucleation rate and growth of SnO2led to different oxide islands,which resulted in various potential catalytic sites for the nucleation of the molten Sn droplet.The results showed that the nucleation process of the Sn droplets was sensitive to the solidification environment,and therefore the atmosphere should be taken into account in the study of the nucleation behavior of the single Sn droplets.
The commercial market of Sn–Pb solder is gradually decreasing due to its toxicity,calling for Pb-free substitute materials.Sn–Ag alloy is a potential candidate in terms of good mechanical property.The major problematic issue of using Sn–Ag is their high melting temperature,consequently this study is dedicated to lowering the melting temperature of Sn3.5Ag(wt%)alloy by developing nanomaterials using a chemical reduction approach.The resultant nanocrystalline Sn3.5Ag is characterized by field emission scanning electron microscope.The size dependence of the melting temperature is discussed based on differential scanning calorimetry results.We have reduced the melting temperature to 209.8°C in the nanocrystalline Sn3.5Ag of(32.4±8.0)nm,compared to*221°C of the bulk alloy.The results are consistent with the prediction made by a relevant theoretical model,and it is possible to further lower the melting temperature using the chemical reduction approach developed by this study.
