Differential scanning calorimetry thermodynamic parameters

The liquid fraction sensitivity is an important parameter for the determination of the semi-solid forming capability. It is defined as the rate of change of the liquid fraction in the alloy with temperature and is related to the relative slopes, in the phase diagram, of the liquidus and solidus curves. It may be determined by differential scanning calorimetry or predicted by thermodynamic modelling. Examples related to various Al alloys have been reported by Maciel Camacho et al. (2003), Dong (2003). See also several papers in Chiarmetta and Rosso (2000). 

Fig. 4. Profile of a differential scanning calorimetry experiment done on a synthetic lysozyme. The heat capacity (kilocalories per degree per mole) of the unfolding process was monitored as a function of temperature on a Micro-Cal MC2 instrument. The transition midpoint of protein unfolding corresponds to the temperature at the peak of the curve, and the thermodynamic parameters A H and A Cp are evaluated by the procedure of Privalov.33...

Similarly, if a quantity such as the volume exhibits an abrupt change in slope, which occurs at the T, then there is a discontinuity in quantities associated with first derivatives of this parameter, or second derivatives of the free energy (with respect to appropriate thermodynamic variables), such as the specific heat (Figure 10-19). Accordingly, the Tg may be related to a second-order phase transition, but this remains in dispute. The experimentally observed transition is clearly governed by kinetics and the standard method of measuring this transition is by differential scanning calorimetry (DSC), which measures the specific heat. 

Differential scanning calorimetry measures the thermodynamic parameters associated with thermally induced phase transitions. Here, the sample of interest and an inert reference are heated or cooled independently at a programmed rate, and in tandem, such that their temperatures change in unison and the differential temperature is maintained at zero. If the sample undergoes a thermally induced transition, heat must be applied to or withdrawn from the sample in order to maintain the same temperature in both sample and reference compartments. The instrument measures the heat flow into the sample relative to the reference and this dijferential heat flow (or excess specific heat) is recorded as a function of temperature, resulting in a trace, as shown in Fig. 1... 

In previous chapters, the principles and applications of differential scanning calorimetry (DSC) have been outlined, and it should be clear that the technique is both versatile and extremely sensitive. Using DSC, it is possible to analyze a wide range of systems quickly and cheaply so that thermodynamic parameters may be obtained. These qualities have led to the widespread use of DSC for not only pure research but also for routine thermal analysis. DSC does, however, have some drawbacks. To achieve good thermal contact with a sample, most DSC instruments are equipped with a pair of sample holders into which prepared sample and reference materials are placed. These materials are usually encapsulated in crimped aluminum ampoules, a typical sample mass being 5 to 10 mg. Such a small mass of sample contributes... 

Once the probabilities are known, other physical quantities, which are function of the occupation probabilities, can be calculated from (A) — J2yPy y- or order parameters for order-disorder phase transitions. Different examples will appear in the following. For instance, the orientational contribution to the absolute polarization of the ferroelectric compound pyridinium tetrafluoroborate was estimated from 2H NMR temperature-dependent measurements on the perdeuterated pyridinium cations.116 The pyridinium cation evolves around a pseudo C6 axis, and the occupation probabilities of the different potential wells were deduced from the study of 2H NMR powder spectra at different temperatures. The same orientational probabilities can be used to estimate the thermodynamical properties, which depend on the orientational order of the cation. Using a generalized van t Hoff relationship, the orientational enthalpy changes were calculated and compared with differential scanning calorimetry (DSC) measurements.116... 

A study of the relaxational transitions and related heat capacity anomalies for galactose and fructose has been described which employs calorimetric methods. The kinetics of solution oxidation of L-ascorbic acid have been studied using an isothermal microcalorimeter. Differential scanning calorimetry (DSC) has been used to measure solid state co-crystallization of sugar alcohols (xylitol, o-sorbitol and D-mannitol), and the thermal behaviour of anticoagulant heparins. Thermal measurements indicate a role for the structural transition from hydrated P-CD to dehydrated P-CD. Calorimetry was used to establish thermodynamic parameters for (1 1) complexation equilibrium of citric acid and P-CD in water. Several thermal techniques were used to study the decomposition of p-CD inclusion complexes of ferrocene and derivatives. DSC and derivative thermogravimetric measurements have been reported for crystalline cytidine and deoxycytidine. Heats of formation have been determined for a-D-glucose esters and compared with semiempirical quantum mechanical calculations. ... 

The valence-bond isomers of hexakis(trifluoromethyl)benzene (Vol. 1, p. 263) are among the most stable such isomers known. Kinetic and thermodynamic parameters for their thermal rearrangement (Scheme 25) have been determined by differential scanning calorimetry and by conventional techniques... 

The simplest parameter to control is the temperature and in the more sophisticated applications one does not only control this but one is also able to obtain thermodynamic data at the same time by using Differential Scanning Calorimetry (DSQ. ... 

Temperature is the most important quantity in differential scanning calorimetry. With DSC, in essence temperature is the only measured quantity. Everything else is calculated from the changes of temperature, from the difference between the sample and reference temperatures. We can define temperature as a primary thermodynamic parameter of a system, which is a measure of the average kinetic energy of the atoms or molecules of the system. In everyday language, we use the words hot, warm, and cold to characterize the temperature of materials and bodies. 

In series of publications [25,27,29,35-40] several methods were used for eharaeterization of the microphase structure of the semi-IPNs studied. Small-angle X-ray seattering (SAXS), differential scanning calorimetry (DSC) [27, 35-37], dynamic mechanical thermal analysis (DMTA) [27, 30-32], dielectric relaxation spectroseopy (DRS), and thermally stimulated depolarization currents (TSDC) [25, 39, 40] measurements have shown that pure PCN is characterized by a typical homogeneous structure, but for segmented LPU the microphase separation on the level of hard and soft domains due to their thermodynamic immiscibUity was denoted. As for semi-IPNs, the destruction of the microphase separated morphology of LPU was observed and the microphase separation between PCN and LPU phases, expected from the difference of solubility parameters, was not found. 

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