The study of the conversion of heat into motion (= thermodynamics) was born out of the Industrial Revolution and the need to understand and improve the operation of steam engines. Indeed, the efficiency of the first " fire engines " did not exceed 2 % ! In the course of the 19th century, the determination of thermodynamic data (enthalpy, entropy, reaction constants, etc.) proved to be crucial in predicting and even controlling the direction of evolution of a reactive system. In addition to steam engines, the simple and governing laws of thermodynamics are applicable to all bio-physical-chemical reactions and transformations. This also applies to industrial transformations carried out at high temperatures, such as metallurgical reactions, another major component of the industrial revolution.
Following an historical review of the discovery of the laws of thermodynamics, we will address this metallurgical aspect by describing the major contribution of solid-state electrochemistry, from the early 20th century onwards, to the study of high-temperature reactions and to the building of the libraries of thermodynamic data and concepts universally used today. This fertile experimental approach required the development of solid electrolytes (or melts) that were stable at high temperatures and good conductors of ions, anions or cations. In today's context, a brief historical review of the evolution of ideas and concepts relating to these solid electrolytes is perhaps not superfluous. Finally, we will describe how the simple measurement of the voltage of an electrochemical cell at equilibrium can also tell us something about the relative stability of crystalline polymorphs, the stability of grain surfaces and their growth.
