Thermodynamics Of Chemical Systems Research Articles

Thermodynamic Origin of Negative Thermal Expansion Based on a Phase Transition-Type Mechanism in the GdF3-TbF3 System.

Multicomponent fluorides of rare earth elements (REEs-R) are phase transition-type negative thermal expansion (NTE-II) materials. NTE-II occurs in RF3-R'F3 systems formed by "mother" single-component dimorphic RF3 (R = Pm, Sm, Eu, and Gd) with a giant NTE-II. There are two structural types of RF3 polymorphic modifications: low-temperature β-YF3 (β-) and high-temperature LaF3 (t-). The change in a structural type is accompanied by a density anomaly: a volume of one formula unit (Vform) Vβ- >Vt-. The empirical signs of volumetric changes ΔV/V of NTE-II materials were considered. For the GdF3-TbF3 model system, an "operating-temperature window ΔT" and a two-phase composition of NTE-II materials follows from the thermodynamics of chemical systems: the phase rule and the principle of continuity. A necessary and sufficient sign of NTE-II is a combination of polymorphism and the density anomaly. Isomorphism in RF3-R'F3 systems modifies RF3 chemically by forming two-component t- and β- type R1-xR'xF3solid solutions (ss). Between the two monovariant curves of ss decay, a two-phase area with ΔTtrans > 0 (the "window ΔT") forms. A two-phase composite (t-ss + β-ss) is an NTE-II material. Its constituent t-ss and β-ss phases have different Vform corresponding to the selected T. According to the lever rule on a conode, Vform is calculated from the t-ss and β-ss compositions, which vary with T along two monovariant curves of ss decay. For the GdF3-TbF3 system, ΔV/V = f(T), ΔV/V = f(ΔT) and the "window ΔT" = f(x) dependencies were calculated.

Thermodynamics of chemical systems far from equilibrium

A critique is presented of some recent work in this and other journals on the relation of thermodynamics to the mass action law of kinetics. For most chemical reactions, the thermodynamic variables change on the same time scale as the progress variable and there is no need for an extended thermodynamics.

Analytical Calorimetry in Biochemical and Clinical Applications

I. INTRODUCTION Calorimetry has played a fundamental historical role in the development of the basic thermodynamics of chemical systems. In this sense, calorimetry is one of the oldest methodologies in the field of chemistry. However, the application of calorimetric measurements to analytical problems has been much slower to develop, the most dramatic increases in use occurring within the last 15 to 20 years. In the fields of biochemistry and analytical biochemistry, only in the past 7 to 10 years has there been significant growth in the applications of calorimetery, largely as a result of new instrumentation capable of the high sensitivity required in dealing with the small quantities of reactants generally involved in biochemical systems. These recent developments have resulted in widespread interest in the capabilities of various calorimetric methods in the study of problems as diverse as enzyme substrate determination, the detailed structural information in lipid-bilayer thermal transitions, and the s...

Thermodynamics of chemical systems in external fields

Subsequent investigations, in particular of the heat transfer, would be useful in establishing the extent to which approximate methods such as the one considered herein agree with more exact solutions or experimental results. It is of note that the calculation of the heat transfer to the wall is less clear, there being numerous specifications, i.e., by being consistent with that obtained from the similarity solutions, by using the similarity solutions directly with p as the parameter,' by using the integral form of the energy equation, or by other methods suggested for the one parameter method e.g., Hays and Probstein and Chan, but there is little heat transfer data to appraise the various methods in flows over a large range of speeds and accelerations such as in supersonic nozzles.