Experimental method to determine specific heat capacity and transition enthalpy at a first-order phase transition: Fundamentals and application to a Ni-Mn-In Heusler alloy

Highlights: • New method to discriminate specific heat and enthalpy transformation in first-order transitions. • Adaptive amplitude-modulated calorimetry. • Specific heat at constant molar fraction is determined. • Smooth step-like behaviour for the specific heat of the martensitic transition in a NiMnIn alloy. A new method that characterizes thermal properties during a first-order phase transition is described. The technique consists in exciting the sample by a series of constant frequency thermal pulses which one in every N pulses –N is a small number like four—being exceedingly large in amplitude. This pulse induces phase transformation which is inhibited during the following smaller pulses due to thermal hysteresis. That way the specific heat capacity for a given mixture of phases can be determined. The results obtained are independent of experimental parameters like the rate and the amplitude of the pulses, unlike what happens in other calorimetric techniques. The method also provides the enthalpy excess by analysing the energy balance between the dissipated heat and the heat flowing during each pulse of measurement. The protocol is tested to analyse the phase transitions of a Heusler alloy Ni_50.53Mn_33.65In_15.82. The paramagnetic-ferromagnetic transition for the austenite phase is continuous and the specific heat capacity shows a lambda anomaly. The martensitic phase transition shows a first-order character and the specific heat capacity follows a step-like behaviour.