S. V. Baryshnikov

Dielectric and NMR Studies of Nanoporous Matrices Loaded with Sodium Nitrite

NMR and dielectric studies have been performed on NaNO2 loaded in mesoporous matrices of MCM-41 and SBA-15 with pore sizes of 20, 37, and 52 Å. The spin-lattice relaxation rate and 23Na NMR line shape, as well as the complex impedance, were measured within a broad temperature interval including the ferroelectric phase transition in bulk NaNO2. Two different phases of sodium nitrite, the crystalline and melt phases, are shown to coexist under conditions of a restricted geometry. The crystalline phase undergoes a ferroelectric phase transition. The melt fraction increases with temperature. The existence of two phases accounts for all experimental data on NaNO2 under conditions of a restricted geometry.

NMR studies of structure and ferroelectricity for Rochelle salt nanoparticles embedded in mesoporous sieves

NMR studies were carried out for Rochelle salt embedded in molecular sieves. 23 Na magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectra revealed a complex structure of the confined crystalline material. The major part of particles within nanopores had a structure similar to that of bulk Rochelle salt. The 23 Na spin‐lattice relaxation times at various temperatures associated with this modification were also similar to those for bulk Rochelle salt and showed broad minima that corresponded to the ferroelectric and re-entrant phase transitions under nanoconfinement at temperatures just below the relevant transitions in bulk. This result suggests that the bulk-like modification within pores is ferroelectric in between. Fast spin relaxation in the rest of the confined material reflected high molecular mobility.

Effect of confined geometry on linear and nonlinear dielectric properties of triglycine sulfate near the phase transition

The temperature dependence of the linear permittivity and the third harmonic generation amplitude of nanocomposites representing nanoporous silica matrices (opal matrix and SBA-15) with triglycine sulfate embedded in pores has been studied in the region of the ferroelectric phase transition. A broadening of the phase transition and an increase its temperature in comparison with bulk triglycine sulfate have been revealed. The latter becomes more significant as the pore size decreases. It has been shown that the nonlinear dielectric properties of nanocomposites near the phase transition differ significantly from the properties of bulk triglycine sulfate.

Dielectric and calorimetric investigations of KNO3 in pores of nanoporous silica matrices MCM-41

The temperature dependences of the linear dielectric permittivity, the third harmonic amplitude, and the heat capacity of nanoporous silica matrices MCM-41 with cellular channels (3.7 and 2.6 nm in diameter) filled with KNO3 have been investigated in comparison with those obtained for bulk potassium nitrate. Measurements have been performed during heating and cooling in the range from room temperature to 463 K. Anomalies corresponding to structural phase transitions have been observed. A significant broadening of the temperature region of the existence of the ferroelectric phase III of potassium nitrate upon cooling has been revealed. This broadening increases with a decrease in the size of pores. It has been shown that, in the nanocomposites with potassium nitrate, the ferroelectric phase can also be formed during heating. The efficiency of observation of the third harmonic generation for studying nanocomposites with the ferroelectric phase has been demonstrated.

Phase transitions in K1−xNaxNO3 embedded into molecular sieves

Dielectric studies of molecular sieves MCM-41 filled with K(1-x)Na(x)NO(3) salts (x = 0, 0.05 and 0.10) were carried out and compared with results obtained for their bulk counterparts. The regular increase upon warming of the temperature of the reconstructive phase transition from phase II to phase I with decreasing pore sizes was observed for all x. A thermodynamic model for shifts of reconstructive phase transitions was suggested. Ferroelectric phase III was shown to occur upon cooling in bulk binary salts and for all x in confined geometry under the thermal conditions when phase III does not arise in bulk KNO(3). Ferroelectricity persisted below room temperature in the bulk binary salt with x = 0.10.

Dielectric studies of nanoporous alumina films filled with the Rochelle salt

Dielectric studies of nanoparticles of the Rochelle salt embedded in pores of porous alumina have been performed in the temperature range from 80 K to the decomposition temperature of the bulk Rochelle salt. It has been revealed that the permittivity exhibits an anomaly corresponding to the lower structural transition to the paraelectric phase, whereas the upper ferroelectric transition is shifted above the decomposition temperature in agreement with the recently published data. The temperature of the lower transition for nanoparticles in pores is found to decrease by 10 K. Possible physical factors that can be responsible for the broadening of the region of existence of the ferroelectric phase have been discussed.

Phase transitions in KNO3 embedded in MCM-41 films with regular nanopores

This paper reports on a comparative study of phase transitions in nanocomposites made up of KNO3 embedded in 10-μm-thick MCM-41 films with unidirectional pores 4.0 nm in size on an aluminum substrate and of nanocomposites prepared in the form of potassium-nitrate-filled pressed MCM-41 powders with 3.7-nm pores. The temperature dependences of linear permittivity and the amplitude of third harmonic generation have been measured under heating and cooling. The structural transition from phase II to phase I shifts under heating relative to that occurring in bulk KNO3 toward lower temperatures for potassium nitrate in the film and toward higher temperatures for the pressed MCM-41-based nanocomposite. A significant difference has been observed also within the region of existence of ferroelectric phase III. The data obtained suggest that the shifts of phase transition temperatures observed in the conditions of nanoconfinement are influenced markedly not only by pore size and geometry but also by other factors.

Ferroelectricity in Rochelle Salt Nanoparticles Confined to Porous Alumina

Studies of complex permittivity and third harmonic generation were carried out for a Rochelle salt loaded porous alumina film within a temperature range from 80 to 325 K in comparison with studies of Rochelle salt single crystals. Only one peak of real part of permittivity corresponding to the lower (re-entrant) phase transition in confined Rochelle salt was observed which temperature was reduced by 10 K compared to the relevant temperature in bulk. The upper phase transition was shifted beyond the decomposition temperature in agreement with previous findings. The amplitude of the third harmonic which correlates to the emergence of ferroelectricity was found to be enhanced above 245 K till 325 K. The results obtained evidence the strong extension of the ferroelectric phase in confined Rochelle salt due to the decrease and the increase of the lower and upper phase transition temperatures, respectively. The physical nature of such extension is discussed.

Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH4)2SO4 ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH4)2SO4 nanocomposite as compared to bulk (NH4)2SO4 was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.

Dielectric properties of mixed NaNO2-KNO3 ferroelectrics in nanoporous silicate matrices

Results of dielectric studies of MCM-41 molecular sieves with mesoporous channels (37.0 and 24.3 Å in size) loaded with the ferroelectric (NaNO2)1 − x(KNO3)x (x = 0, 0.05, 0.10) are presented. The experimental data on the permittivity and conductivity of the composite are used to calculate the temperature dependences of the permittivity and conductivity of these solid solutions in pores. The permittivity and conductivity are found to increase with increasing KNO3 concentration both in bulk samples and under restricted geometry conditions. An increase in the permittivity and conductivity due to the restricted geometry as compared to bulk mixtures was found to occur for all values of x. The mechanisms underlying this behavior are discussed.