Effect of Melt Superheating Treatment on the Latent Heat Release of Sn
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DIFFERENTIAL scanning calorimetry (DSC) is an analytical method to measure the relationship between the temperature and the power difference of the input sample and the reference sample. It has been extensively applied in monitoring the changes of the system energy such as the material latent heat and the transformation enthalpy.[1,2] However, for the measurement of the transformation enthalpy, the calculation area of the reaction peak is an inevitable problem.[3] This area must be determined with reference to the baseline. If the baseline is same before and after the transformation, it seems to be a simple matter to interpolate it. However, generally speaking, the baseline is appreciably shifted at the end of the transition so that a problem arises from the construction of the baseline equation. The migration of the baseline before and after the transition will bring difficulties to evaluate the areas of the reaction peaks. Since different baseline methods lead to different latent heats with the same DSC curve, some of which deviate from the actual value obviously, a reliable method for determining the baseline needs to be developed. Many studies have shown that melt treatment has influences on the final structures and properties.[4–13] Li, Kahtani, and Zhang found that the superheating temperature and the holding time of melting can affect both the grain size and the number of the Al-Si alloy.[10–12] Fu et al. found that the fine microstructure of large-volume eutectic oxide ceramics can be produced by controlling the melt treatment conditions (superheating temperature).[5] In
addition, the grain refinement has also been found in Mg-Al,[6] Al-4.7 pct Cu,[7] Cu-9Al-4Fe,[9] and Al-Si alloys[10,11] by the superheating treatment. In the theoretical research, on the basis of the micro-inhomogeneity theory, Mi et al. established a calculation model to quantitatively describe the structural information of the metal melts.[8] Chang et al. found that the role of prolonging superheating treatment time is equivalent to increasing the treatment temperature of melt.[13] Wang et al. found that the crystals melt heterogeneously from surfaces or grain boundaries once they are heated to the melting point;[14] the crystals cannot melt completely and some preferential nucleus sites exist if the superheating time is not enough. From those studies, one of these factors for microstructure refinement is the preferential nucleation sites, and the nucleus clusters were melted after the melt superheating treatment, which can be reflected by the slight change of transition enthalpy. Nevertheless, the researches regarding the latent heat changing with the melt superheating treatment time (MTT) are rarely reported. Without employing a reliable baseline method, the effects of the melt treatment are hard to be investigated quantitatively and comprehensively. In this paper, an improved baseline method is introduced to study the enthalpy change in the different treatment processes. Through this method, the change of crystallization latent heat of pur
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