Phase Transition in Potassium Nitrate Ferroelectric in a Nanoporous Matrix
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Transition in Potassium Nitrate Ferroelectric in a Nanoporous Matrix Yu. F. Markova*, V. M. Egorova, E. M. Roginskiia, and E. V. Stukovab a
Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia b Amur State University, Blagoveshchensk, 675027 Russia *e-mail: [email protected] Received April 28, 2020; revised May 28, 2020; accepted June 10, 2020
Abstract—Differential scanning calorimetry has been used to carry out a precision study of KNO3 introduced into a silicate nanoporous matrix. Heat capacity peaks associated with ferroelectric phase transitions were found and examined. The characteristics of these transitions were examined in terms of the theory of diffuse phase transitions. A hypothesis was made that there exists a minimum size of pores in the nanocomposite at which KNO3 is in the fully ferroelectric state at room temperature. Keywords: phase transitions, ferroelectric, potassium nitrate, heat capacity. DOI: 10.1134/S1063785020090254
The ferroelectric properties of both bulk KNO3 and finely dispersed KNO3 powders with particle sizes of ~40–200 μm have been extensively studied [1, 2]. However, the analysis of ferroelectric composite materials based on potassium nitrate is in its initial stage. A particular place among composites of this kind is occupied by composites based on nanosize matrices having ferroelectric particles several nanometers in size in their pores. This is so because the physical properties of these structures are mostly affected by effects associated with the size, configuration, and ordering of pores [3]. Heterostructures of this kind, based on ordered nanosize matrices filled with a ferroelectric, can be used to fabricate memory elements of long-term information carriers, in microscopic power supply units, pyroelectric detectors, high-capacitance capacitors, and transducers for determining the humidity [4–6]. In this Letter, we present the results of a study by the method of differential scanning calorimetry (DSC) of nanocomposites based on KNO3 into MCM-41 matrices (MCM stands for “mobile composition of matter”) in comparison with bulk potassium nitrate. The goal of the study was to obtain calorimetric data on the behavior of the corresponding heat capacity peaks, change in the existence range of the ferroelectric phase in nanocomposite materials, and change in its origination conditions. The thermal properties of samples were examined with a Perkin-Elmer DSC-2 calorimeter in the atmosphere of nitrogen under heating and cooling. The temperature scale was calibrated against melting points of ice (273.1 K) and indium (429.7 K), and the
heat flux scale was calibrated against the heat capacity of sapphire. Measurements were made in the temperature range of 300–450 K. It is known that three polymorphic phases exist in KNO3 crystals [7, 8]. Mutual transitions between these phases are accompanied by endothermic and exothermic effects under heating and cooling, respectively. All these phase transitions are manifested for KNO3 samples under study i
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