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Abstract  

A numerical program has been written to treat a heat-flux DSC. The model operates in two modes. In the first,experimental data is used as input and the enthalpy is calculated as a function of the sample temperature rather than the sample thermocouple temperature. This allows accurate enthalpies and transition temperatures to be obtained without smearing. In the second mode, enthalpy is used as an input and the responses of the calorimeter are calculated. Using this mode it is possible to investigate the effects of sample size, heating rate and alloy composition. Non-equilibrium effects and difficulty in nucleation can also be included.

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In an attempt to explain how the calibration factor of a heat flux DSC cell depends both on the standard utilized and on the experimental variables, a study has been undertaken of the entire DSC trace.

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Abstract  

The temperature calibration of a TA Instruments 3200-2920 DSC has been performed on cooling using the isotropic → nematic, isotropic → cholesteric and other liquid crystal → liquid crystal transitions of thermally stable, high purity liquid crystals. The thermal stability of these liquid crystals has been verified by measuring the temperature of the mentioned transitions during cyclic heating and cooling experiments. Correspondence has been established between the real and indicated temperature during cooling for all combinations of heating and cooling rates of practical interest: correction values were determined to the indicated temperature in order to obtain the real temperature on cooling. These correction values were calculated as the average from the temperatures of four or five different liquid crystal transitions for each heating-cooling rate combination. The accuracy of the temperature calibration on cooling is ca. 0.2‡C for heating and cooling rates up to 20‡C min−1.

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A general mathematical treatment for heat-flux differential scanning calorimetry is given. It combines equations derived for heat transfer in the calorimeter cell with an approach to the solidification of metal or alloy carried out in this type of instrument. The differences are discussed between temperature evolution, kinetics of latent heat and undercooling evolution within the sample, and temperature evolution, recorded signal and measured undercooling at the monitoring station.

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Abstract  

The determination of heat capacity data with sawtooth-type, temperature-modulated differential scanning calorimetry is analyzed using the Mettler-Toledo 820 ADSC™temperature-modulated differential scanning calorimeter (TMDSC). Heat capacities were calculated via the amplitudes of the first and higher harmonics of the Fourier series of the heat flow and heating rates. At modulation periods lower than about 150 s, the heat capacity deviates increasingly to smaller values and requires a calibration as function of frequency. An earlier derived correction function which was applied to the sample temperature-controlled power compensation calorimeter enables an empirical correction down to modulation periods of about 20 s. The correction function is determined by analysis of the higher harmonics of the Fourier transform from a single measurement of sufficient long modulation period. The correction function reveals that the time constant of the instrument is about 5 s rad−1 when a standard aluminum pan is used. The influence of pan type and sample mass on the time constant is determined, the correction for the asymmetry of the system is described, and the effect of smoothing of the modulated heat flow rate data is discussed.

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Abstract  

In this paper problems associated with a conventional heat-flux DSC are discussed. A single pan calorimeter has been designed and built which eliminates many of the errors that occur in a conventional DSC. It was found that: enthalpy changes and heat capacity were repeatable to better than1%; the apparent latent heat and heat capacity did not depend on specimen size or significantly on rate of heating as often occurs in a two-pan heat-flux DSC; during the melting of pure Al, more than 80% of the latent heat was evolved over a temperature of 0.04 K; in alloys, separate heat capacity peaks for different reaction less than 1 K apart were resolved.

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on a Du Pont 910 heat flux DSC cell which has been interfaced to an in-house control and data processing system. The rate of heating is controlled directly from the DSC signal and the excellent temperature control obtained using the new system is

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Heat transfer in a disc-type DSC apparatus

Part 5. Theoretical equation of melting with a heat flux coupled cells DSC

Journal of Thermal Analysis and Calorimetry
Authors: K. Alouani, M. Siniti, F. Schiets, and P. Claudy

Abstract  

The literature describes models of DSC apparatuses. However, the use of these models require a precise determination of the values of the resistors and the capacitors of the apparatus. Theoretical equations of a first order transition in a coupled cells heat flux DSC at a constant heating rate are given. It is shown that the value of the resistors and capacitors may be obtained. The influence of resistors product — crucible or crucible — detectors is established.

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Abstract  

The application of non-linear heating programs to a heat-flux DSC apparatus has attracted much attention. On the basis of thermodynamics, the change in enthalpy of a sample during a temperature change ΔT is due, on the one hand to the true heat capacity of the sample ΔT C and on the other, to the enthalpy of some transformation occurring in the sample ΔrH Δξ. These contributions can be separated on the basis of the kinetics of the transformation. The coupled cells model of a disc type, heat flux DSC apparatus has been tested, using true heat capacities and a sine modulation of the temperature of the furnace around a constant temperature. In the range from 2 to 60 mHz, the amplitude and phase shift of the calorimetric signal were measured at several frequencies. Theoretical equations, based on the model, and using the thermal Ohm's law explains the results with a reasonable accuracy. A non-linear DSC experiment affords two ways of determination of the heat capacity of a sample making possible a distinction between the enthalpic effect and heat capacity.

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