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Background: The research of nanocomposites based on ferroelectrics has been recently stimulated by the discovery of a number of their unique properties. These properties are of particular interest from both fundamental and applied points of view Objective: This paper presents the results of comparative studies of the linear and nonlinear dielectric properties of potassium nitrate, embedded from the solution and from the melt into aluminum oxide films with a pore diameter of 100 nm. Methods: An E7-25 impedance meter with a frequency range of 25 Hz – 1 MHz was used to investigate the linear dielectric properties. The setup for researching nonlinear dielectric properties has a sinusoidal oscillator with an operating frequency of 2 kHz. Results: The temperature dependences of the permittivity ε' and the third harmonic coefficient γ3ω were measured in the heating and cooling mode. It was found that for a nanocomposite, obtained from the solution, the ferroelectric phase of KNO3 was formed only upon cooling in the temperature range 397 – 360 K. At the same time, when KNO3 was embedded into the Al2O3 film from the melt, the polar phase occurred both upon heating and cooling in the temperature range of 300 – 432 K and 300 – 421 K, respectively. Conclusion: Thus, the conducted studies of the dielectric properties showed a significant difference in the phase transition temperatures for the KNO3/Al2O3 nanocomposites, obtained from the solution and from the melt, compared to the bulk sample. The phase transition shifts during heating had a different sign for the nanocomposites, obtained from the solution and from the melt. The temperature range of the existence of the ferroelectric phase significantly depends on the method of embedding KNO3 into aluminum oxide films. For the nanocomposite, obtained from a solution, the polar phase is formed only upon cooling, whereas when potassium nitrate is embedded from the melt, the polar phase is formed both upon heating and cooling.

Introduction: 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. Experiment: 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. Results & Conclusion: 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.

Temperature dependencies of linear dielectric permittivity and third harmonic coefficient of nanoporous matrices SBA-15 with channel pore size of 7,5 nm filled with ferroelectric NH4HSO4 compared to bulk ammonium hydrogen sulphate were studied. The measurements were performed in a heating and cooling mode with a temperature range from 100 K to room temperature. Low-temperature phase transition shift of 7-8 degrees to low temperatures was found for nanostructured ammonium hydrogen sulphate compared to bulk NH4HSO4. The experimental data were treated within the framework of the models developed for isolated small ferroelectric particles and arrays of coupled particles. The rise of the space charge polarization effects was also observed.

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