Add time:08/26/2019         Source:sciencedirect.com

Thermodynamic studies of a polycrystalline sodium niobate (NaNbO3) under equilibrium conditions by the method of Knudsen effusion mass spectrometry (KEMS) were conducted in the temperature range 1250–1485 K. The vaporization of pure Na2O(c) was investigated at temperatures of 900–1165 K. Thermodynamic quantities of sublimation enthalpy, activity, and entropy were derived from the partial pressures of gaseous Na, Na2O, and O2 over the sodium niobate and pure sodium oxide reference samples. The sublimation enthalpy of Na from NaNbO3(c) at T=1370 K was ΔsubH1370K0(Na)=465±9kJmol−1. Heat capacity measurements under constant pressure (Cp(T)0) by Dynamic Scanning Calorimetry (DSC) in the temperature range 298–1450 K resulted in six polynomial equations for each branch of the five transitions. Below the first phase transition at 639 K the Cp(T)0 values increased from 97 to 123 J mol−1 K−1. Beyond this transition and up to the third one at 792 K, Cp(T)0 gently inclined from about 120 to a maximum value of 123 J mol−1 K−1. Finally, beyond the fifth transition at 907 K it approximated to an almost constant value of 115 J mol−1 K−1. The enthalpy and the entropy values were calculated from the Cp(T)0 polynomials and from the vaporization studies. The enthalpy of the five transitions in sodium niobate, ΔtrsH2980(NaNbO3(c)) were measured by drop calorimetry. Deduced from empirical and experimental approaches, the heat of reaction for oxides was ΔrH2980(NaNbO3(c))=−94.3±7 kJ mol−1 and the heat of formation was ΔfH2980(NaNbO3(c))=−1250.8±7 kJ mol−1. The thermodynamic activity of Na2O in sodium niobate was temperature dependent, and its values varied from 2.6×10−3 to 8.3×10−4 in the range 1100–1250 K. NaNbO3(c) has a long-term stability in the temperature range including the transition to cubic phase at 907 K.

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