Studing Thermodynamics Of Metastable States

FACTA UNIVERSITATIS Series: Physics, Chemistry and Technology Vol. 3, No 2, 2005, pp. 115 - 128 STUDYING THERMODYNAMICS OF METASTABLE STATES UDC 536.7 Yuri Kornyushin Maître Jean Brunschvig Research Unit,Chalet Shalva, Randogne, CH-3975 Abstract. Simple classical thermodynamic approach to the general description of metastable states is presented. It makes possible to calculate the explicit dependence of the Gibbs free energy on temperature, to calculate the heat capacity, the thermodynamic barrier, dividing metastable and more stable states, and the thermal expansion coefficient. Thermodynamic stability under mechanical loading is considered. The influence of the heating (cooling) rate on the measured dynamic heat capacity is investigated. A phase shift of the temperature oscillations of an ac heated sample is shown to be determined by the relaxation time of the relaxation of the metastable nonequilibrium state back to the metastable equilibrium one. This dependence allows one to calculate the relaxation time. A general description of the metastable phase equilibrium is proposed. Metastable states in AB3 alloys are considered. Reasons for the change from the diffusional mechanism of the supercritical nucleus growth to the martensitic one as the heating rate increases are discussed. The Ostwald stage rule is derived. 1. INTRODUCTION Metastable states are common ones in physics, nanophysics, materials science, and chemistry. Many amorphous materials (including some amorphous metals and alloys), superheated and supercooled phases are examples of the metastable states. The problem of metastable states is related to the problem of phase transformations. The common feature is a transition from one phase to the other one. But what is different here is reversibility. Phase transformations are reversible or almost reversible changes. The secondorder transition is completely reversible. The first-order transition is often reversible in a sense that it is possible to transform two phases, one into another, and reversely by means of cycling temperature. Direct and reverse first-order phase transitions almost never occur at the same temperature. Usually a low-temperature phase requires overheating to be transformed into a high-temperature one, and vice versa, the high-temperature phase requires overcooling at reverse transformation. This phenomenon is called hysteresis, and apart from this the first-order phase transitions are usually reversible. On the contrary, the majority of transitions from _________________________________ Received January 17, 2005 116 Y. KORNYUSHIN metastable states to more stable ones are irreversible. For example, a crystallization of amorphous materials and transformation of diamond to graphite are co