On models to describe the volume in the context of establishing high-pressure Gibbs energy databases

TL;DR

This paper investigates models for describing volume in high-pressure thermodynamics, identifying key issues and proposing new approaches to improve the accuracy of Gibbs energy databases at elevated pressures and temperatures.

Contribution

It systematically evaluates existing volume models based on the Grover law and introduces novel descriptions using thermal pressure and temperature-pressure equivalence.

Findings

Identified inconsistencies in SGTE extrapolation methods.

Evaluated multiple volume models for high-pressure applications.

Proposed new models based on thermal pressure and temperature-pressure equivalence.

Abstract

High-pressure Gibbs energy approaches are promising for establishing multi-component thermodynamic databases. However, so far, they have often been considered unsuccessful, because of their tendency to lead to unphysical extrapolations of the thermodynamic properties at elevated temperatures and pressures. Beyond this symptom, the root causes of the problem are rarely investigated. In this work, it is identified that these shortcomings are caused by (i) an inconsistent treatment of the SGTE method of extrapolation, and (ii) an insufficient knowledge of the models to describe the volume that are the key to extend CALPHAD databases toward high pressures. Because the first step toward solving the problem is to focus on the later issue, several models to describe the volume that are built upon the Grover empirical law are investigated, namely, the Lu-Grover, Joubert-Lu-Grover, Jacobs-Grover, and the Anand-Saxena-Grover approaches. In addition, an original description built upon the concept of thermal pressure, and a new scheme based on the equivalence between temperature and pressure when computing the volume are discussed. For each of these models, limitations and possibilities are identified from a practical standpoint.