An experiment was carried out to study the heat transfer performance of a horizontal micro-grooved heat pipe using CuO nanofluid as the working fluid. CuO nanofluid was a uniform suspension of CuO nanoparticles and deionized water. The average diameter of CuO nanoparticles was 50 nm. Mass concentration of CuO nanoparticles varied from 0.5 wt% to 2.0 wt%. The experiment was performed at three steady operating pressures of 7.45 kPa, 12.38 kPa and 19.97 kPa, respectively. Effects of the mass concentration of CuO nanoparticles and the operating pressure on both the heat transfer coefficients of the evaporator and the condenser sections, the critical heat flux (CHF) and the total heat resistance of the heat pipe were discussed. Experimental results show that CuO nanofluid can improve the thermal performance of the heat pipe and there is an optimal mass concentration which is estimated to be 1.0 wt% to achieve the maximum heat transfer enhancement. Operating pressure has apparent influences on both the heat transfer coefficients and the CHF of nanofluids. The minimum pressure corresponds to the maximum heat transfer enhancement. Under an operating pressure of 7.45 kPa, the heat transfer coefficients of the evaporator can be averagely enhanced by 46% and the CHF can be maximally enhanced by 30% when substituting CuO nanofluids for water.
INTRODUCTIONLead-free solder alloys are intended to completely substitute for traditional Pb-Sn compositions in conventional processes in the future. Among them, tinsilver eutectic solder is an attractive candidate alloy for meeting the requirements for demanding hightemperature service environments, such as automotive applications. 1 However, Sn-Ag eutectic has a higher melting temperature and poorer wettability compared to Sn-Pb eutectic. 2 To improve the properties and decrease the melting temperature, some additional elements, such as Bi and Cu, are added to the Sn-Ag eutectic.As surface-mounted devices have been used under thermomechanical-cycling conditions, mechanical deformation and cracking could develop in solder joints. Therefore, knowledge about their mechanical properties, in particular, fatigue deformation and fracture is required to ensure the reliability of the joints in electronic devices. 1,3-7 Although Bi additions can decrease the melting temperature, degradation of mechanical properties and fatigue resistance by over-addition of Bi above 3-5% has been reported. 5,6 Kariya and Otsuka 6 investigated the effect of Bi on the isothermal fatigue behavior of the Sn-Ag solder. They indicated that the fatigue life drastically decreased with increasing content of Bi, and this was attributed to the decrease of ductility. McCabe and Fine 7 investigated the creep properties of precipitation-strengthened Sn-based alloys and found that the additions of very small particles improved the creep resistance at moderate to high stress levels but could possibly adversely affect the diffusional-creep resistance of the materials at low stresses. However, very little is known about the fatigue crack-growth behavior of the Sn-Ag-Cu-Bi alloys.Because fatigue is a process of crack initiation and propagation, studies on fatigue-crack growth are essential for fully understanding the fatigue behavior of the solders. The results of the studies on Fatigue crack-growth behavior and mechanical properties of Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders have been investigated at room temperature (20°C). The tensile strength and hardness of the solders increased with increasing Bi content. However, the yield strengths of Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders were nearly similar, but the 3Bi solder exhibited the lowest ductility. Fatigue crack-growth behavior of the solders was dominantly cycle dependent in the range of stress ratios from 0.1-0.7 at a frequency of 10 Hz, except for the Sn-3Ag-0.5Cu solder tested at a stress ratio of 0.7. Mixed intergranular/transgranular crack propagation was observed for the Sn-3Ag-0.5Cu solder tested at the stress ratio of 0.7, indicating the importance of creep in crack growth. The Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders had higher resistance to time-dependent crack growth, resulting from the strengthening effect of the Bi constituent. It appears that the addition of Bi above a certain concentration is harmful to the mechanical properties of Sn3Ag-0.5Cu.
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