IOX-2 The present study has considered the

The present study has considered the generation of the third harmonic. The ratio of the voltage amplitude at the frequency 3ω to the amplitude of the voltage applied to the sample, i.e., the third harmonic coefficient γ3ω[14], was used as the quantity characterizing the generation intensity. The field strength in the samples was about 300V/cm. Prior to the fabrication of the composites, moisture was partially removed from the NCC gel films with filter paper so that IOX-2 the sample\'s thickness was reduced by half. Potassium iodate was embedded from a saturated aqueous solution at a temperature of about 300K. The sample was then dried at 370K for 10h. Electrodes were applied to the obtained samples using indium–gallium paste. The orientation of the NCC nanochannels was selected so that the electrodes were parallel to them. Fig. 1 shows the surface of the dried NCC sample (Fig. 1a) and the surface of the NCC–KIO3 nanocomposite (Fig. 1b). It can be seen from Fig. 1b that there is a large amount of bulk potassium iodate, which did not embed into the pores, on the surface of the NCC. Consequently, the dielectric response of the obtained samples should contain the contributions from both bulk and nanostructured KIO3.
Experimental results and discussion The temperature dependence of the real part of the dielectric constant ε\' for the polycrystalline KIO3 sample, determined at the frequency of 1kHz, is shown in Fig. 2a. Evidently, the ε\' (Т) dependence has four anomalies in the 80–500K temperature range: pronounced dielectric permittivity peaks were observed at temperatures of 113, 263 and 345K, along with a sharp decrease in ε\' (485K), which is typical for improper ferroelectric phase transitions [15]. No anomalies were found near the temperature of 428K. The dashed lines in Fig. 2a show the temperature phase boundaries in potassium iodate. [11] According to the measurement results, the third harmonic coefficient γ3ω depends on temperature and has anomalies at the V→IV, IV→III, III→II phase transitions (Fig. 2b). No anomalies have been observed for the II→I phase transition. Dashed lines also mark the phase boundaries. Let us discuss the potential causes for the behavior of the obtained γ3ω(Т) dependence. According to [16], full spontaneous polarization in monoclinic potassium iodate consists of two qualitatively different, mutually perpendicular components:
These components have been termed the non-reorientable pyroelectric polarization and the reorientable ferroelectric polarization . Both components and are temperature-dependent. The properties of potassium iodate are such that the inequality >> holds true. The phase transition at 485K is accompanied by the vanishing of the ferroelectric component , while the total polarization of the sample is equal to . However, it is known that pyroelectric crystals do not exhibit nonlinear dielectric properties even in strong electric fields, and the component cannot be the cause for the third harmonic coefficient γ3ω taking such a high value in the paraelectric phase [17]. It can be assumed that high non-linearity is associated with the piezoelectric properties of crystals of potassium iodate in the paraelectric phase due to its non-centrosymmetric structure [12]. The temperature dependences of the capacitance C and the third harmonic coefficient γ3ω for the NCC filled with KIO3 are shown in Fig. 3. Four anomalies are present on the С(Т) curve at temperatures of 264 ± 1, 283 ± 1, 346 ± 1 and 370 ± 1K, corresponding to phase transitions (Fig. 3a). No other anomalies could be observed in the temperature range of 80–360K. The presence of bulk potassium iodate not embedded into the pores manifests as a small maximum on the С(Т) curve and a minimum on the temperature dependence γ3ω(Т) (Fig. 3b) at a temperature of 264 ± 1K, which corresponds to the phase transition from phase IV to phase III. The sharp peak at a temperature of 283 ± 1K on the С(Т) curve corresponds to the phase transition from phase IV to phase III for nanostructured KIO3 located in the channels of nanocrystalline cellulose.