Phase Transitions in Nanocomposites Obtained by Introducing KNO3 into the Pores of Nanosized Al2O3 Films

Comparative studies of phase transitions in nanocomposites on the base of KNO3 and Al2O3 films with pores of 240 and 45 nm were carried out. The temperature dependences of the linear dielectric constant and the amplitude of the third harmonic were measured. The measurements were carried out in the heating and cooling regime in the range from room temperature to 463 K. Anomalies, corresponding to structural phase transitions, were observed. A considerable expansion of the temperature range of the potassium nitrate ferroelectric phase III was observed upon cooling, which increased with decreasing pore size of the Al2O3 films. The structural transition upon heating from phase II to phase I for nanocomposites with a pore size of 45 nm shifted relative to the transition in bulk KNO3 by 2 K toward high temperatures, whereas the temperature change for the sample in the Al2O3 film with pores of 240 nm did not occur.