Selenium liquid, heat capacity

The selection of entropy and heat capacity data for trigonal selenium in the temperature range 298.15 to 494.2 K and for the liquid in the temperature range 494.2 to 1500 K. The thermodynamic properties of the metastable monoclinic phase and the supercooled liquid are assessed for use as auxiliary data. The major source of information is the review by Gaur, Shu, Mehta, and Wunderlich [81GAU/SHU] which has been combined with other information for phase transformations. 

The liquid selenium is easily supercooled and undergoes a glass transition at 303.4 K. The heat capacity expression derived by [8IGAU/SHU] for the liquid is based on the work in [73GR02] and [80SHU/GAU] in the temperature ranges 500 to 1000 K and 330 to 520 K, respectively. The expression is selected here,... 

No experimental values are available for the heat capacity of liquid selenium above 1000 K. For this work a constant value equal to the value at 1000 K was used in evaluations of data above 1000 K ... 

Monoclinic selenium is metastable and its enthalpy of formation and entropy are needed as auxiliary data for some evaluations. They were derived from a thermodynamic cycle involving the enthalpy and entropy of fusion at the melting point 413 K, the selected data for trigonal and liquid selenium, and the heat capacity of monoclinic selenium. The selected enthalpy of fusion is that in [81GAU/SHU] ... 

The enthalpy of formation of Seg(g) has been determined from mass spectro-metric investigations of selenium vapour using Knudsen cells. The measurements can be separated into the approximate temperature ranges 420 to 494 K for equilibria with solid selenium and 494 to 700 K for equilibria with liquid selenium. The various investigations have used different methods and auxiliary data for deriving enthalpies of formation at 298.15 K from the measurements. For the purpose of this review the measurements were recalculated (see Appendix A) using the adopted values for heat capacities and entropies of the gaseous species and condensed phases. The values of the enthalpies of formation of Seg(g) are summarised in Table V-21. 

No heat capacity measurements of liquid selenium dioxide are available. 

The total vapour pressure of liquid selenium at equilibrium was measured by a static method in the temperature range 573 to 1167 K and given as logn, (p/bar) = 5.722 - 5468 T A third law evaluation of the enthalpy of formation of Sc2(g) at 298.15 K was made by this review using the vapour pressure at 1150 K and the selected data for the heat capacities and entropies of Se(l) and Sc2(g). A mole fraction of Se2(g),... 

The DTA curves show no evidence for a Tp mechanism in the case of liquid selenium. The specific heat at constant pressure, Cp, vs. T curve of liquid selenium shows a minimum near 500°C . The increase after this minimum is ascribed to depolymerization, which is endothermic. However, care must be exercized in determining the polymer-ring equilibrium mechanism from the heat capacity measurements. The following contribute to the specific heat ... 

The analysis of heat capacity of a given homopolymer thus starts with the evaluation of the experimental crystalline and amorphous heat capacities over as wide a temperature range as possible. For amorphous polymers, the glassy and liquid heat capacities are directly measurable. For crystallizing polymers, the crystalline and amorphous heat capacities may have to be extrapolated, as illustrated in the polyethylene example in Fig. 5.17. Only in rare cases are almost completely crystalline polymers samples available (as for example, for polyethylene, polytetrafluoroethylene, polymeric selenium, and polyoxymethylene). 

To conclude the treatment of heat capacities a brief mention is made of the special effects found in the heat capacity of liquid selenium, which is also a linear macromolecule. The heat capacity for many liquid, linear macro-... 

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