Calorimetric and photoacoustic investigation of KNO3 phase transitions

Abstract

Results of a thermodynamic study of the phase transitions of powdered KNO3 at two impurity concentrations (99.999% pure and 99.9% pure) are reported. Transition onset and peak temperatures have been measured between 350 and 420 K using both heat flow and photoacoustic calorimetry. The transition temperatures and characteristics observed with both techniques show excellent agreement. Specific-heat values calculated from scanning and stepped temperature heat-flow data, utilizing both positive and negative temperature increments, compare well with previously reported values. Enthalpies for the II-I, I-III, and III-II phase transitions were measured as 5.065, 2.603, and 2.084 kJ/mol, respectively, for 99.999% pure KNO3 powder using heat-flow calorimetry. The transition enthalpies and temperatures measured for 99.9% pure powder were slightly lower for all three transitions. Entropy changes for the II-I, I-III, and III-II transitions were 12.53, 6.61, and 5.30 J/mol K, respectively. Enthalpy, entropy, and Gibbs free-energy curves are presented with emphasis on the temperature range over which the ferroelectric phase transition occurs. The experimental data and calculated thermodynamic functions indicate that the ferroelectric phase III in bulk KNO3 is a metastable state at atmospheric pressure, which was not observed to exist below 350 K contrary to previous reports.

Preparation and Characterization of the Ferroelectric Potassium Nitrate: Poly(vinyl alcohol) Composite Films

“…It has been reported that the preparation of the composite films of nano 2 , selenide with PVa using solvent cast technique was helpful in controlling the growth of nano 2 , and selenide nanoparticles [10], [22]. Many studies using techniques such as far infrared [25], neutron diffraction [26], inelastic neutron scattering [27], raman spectroscopy [28], [29], X-ray [30]- [32], differential scanning calorimetric [33]- [35], dielectric [36], electrical and thermal conductivity [36], [37], and ferroelectric [38]- [41], have been carried out in the past by various researchers to examine the nature of the phase transitions in Kno 3 . The technological interest in Kno 3 thin films originates due to the reason that these films have low operating voltage and fast switching response.…”

Ferroelectric Properties of Polyvinyl Alcohol: Potassium Nitrate Composite Films

“…However, in the DSC scan of the composite film, no peak appears below 114.4 • C up to room temperature, which indicates that phase III exists up to room temperature. The ferroelectric composite film can possess ferroelectric phase III at room temperature whereas KNO 3 does not possess phase III at room temperature [11]. Therefore, the composite can have the advantage to be ferroelectric at room temperature in thin film form.…”

Ferroelectric Phase Stability Studies in Spray Deposited KNO3:PVA Composite Films

“…Many studies such as far infrared, 4,7 neutron diffraction, 8 inelastic neutron scattering, 9 Raman spectroscopy, 1,10,11 X‐ray, 11–13 differential scanning calorimetric, 14–16 dielectric, 5,17 electrical 17 and thermal conductivity, 18 and ferroelectric, 19–22 have been carried out in past by many researchers to examine the nature of the phase transitions in KNO 3 .…”