Dielectric properties and phase transitions of KNO3 embedded in porous aluminum oxide

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 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. 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. 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. 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.