Analysis of a Heat-Flux Differential Scanning Calorimetry Instrument
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MPERATURE lags are inherent to differential scanning calorimetry (DSC) measurement systems, because (a) temperatures are recorded from thermocouples that are placed away from the sample and reference materials (Figure 1), and (b) there is a nonhomogeneous temperature distribution within the DSC instrument. By performing a computational analysis of the measurement process, the temperature lags can be estimated and their effect can be taken into account in determining the thermophysical properties. Gray[2] proposed one of the first models to describe the heat flow in differential thermal analysis cells that has been adopted for the study of DSC instruments. Dong and Hunt[1] developed an analytical model for the DSC heat flux instrument by considering that the instrument can be represented by a certain number of regions of uniform temperatures. However, their model includes some heat-transfer features that do not physically exist in the instrument, such as conduction paths between the sample plates and furnace. Kempen et al.[3] modeled the Netzsch DSC 404C heat flux instrument (Selb, Germany) using an oversimplified heat-transfer ADRIAN S. SABAU and WALLACE D. PORTER, Research Staff Members, are with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 378316083, USA. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled ‘‘Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,’’ which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee. Article published online June 9, 2007. 1546—VOLUME 38A, JULY 2007
model; e.g., only the plates and pans are considered in the model and a conduction path between the sample plates and furnace is considered. The analysis of DSC instruments is useful not only in improving desmearing of the data but also in identifying the area of improvements, such as in Danley[4] and Dong et al.[5] Danley[4] introduced a new DSC sensor design and a new method for reducing the baseline. Their method comprises two differential temperature measurements instead of one and an additional temperature measurement. Dong et al.[5] showed that a single-pan calorimeter was an alternative to the conventional two-pan calorimeter for the study of phase changes in alloys. Dong et al.[5] showed that a direct calculation of fraction solid from enthalpy data was only possible when there was no change in the average composition in the solid and liquid phases during the phase change. Dong et al.[5] showed that the fraction solid data must be obtained from enthalpy data by taking into account microsegregation, which always occurs during solidification of alloys. The Netzsch DSC 404C heat flux instrument was considered in this study (Figure 1). This instrument was different from that considered by Dong and Hunt,[1] because it contained a large component that surrounds the two pans. Following Dong a
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