M. K. Lee

Ferroelectricity in an Array of Electrically Coupled Confined Small Particles

The influence of the pore network geometry and interparticle long-distance electric coupling on the ferroelectric phase transition in small particles embedded into mesoporous matrices is considered. It was shown that the temperature of the ferroelectric phase transition in a system of electrically linked particles can be well different from that in isolated small particles. In particular, the model suggests an explanation for the weakening of size-effects on the ferroelectric phase transition in confined 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.

Dielectric Properties of Mesoporous Sieves Filled with NaNO2

Studies of dielectric properties were carried out for MCM-41 molecular sieves whose pores were filled with NaNO 2 particles and whose pore sizes vary from 20 to 37 Å. The experimental data obtained were used to calculate the permittivity of confined particles on the base of a theoretical model for two-component composites with hexagonal structure and cylindrical inclusions. Additional NMR measurements confirmed that sodium nitrite in pores occurs in two coexistent modifications. A modification with high spin relaxation was suggested to respond for elevated conductivity of the samples under study. It was shown that the increase of the real part of permittivity is caused by the Maxwell-Wagner polarization.

Superionic phase transition in AgI embedded in molecular sieves

Superionic AgI particles embedded in MCM-41 and SBA-15 molecular sieves with different pore sizes were studied using NMR and dielectric methods. The measurements confirmed that the structure of the confined particles did not change noticeably compared to that in bulk AgI. A regular increase in the temperature of the superionic phase transition with decreasing pore size was observed and treated on the basis of the Landau theory of thermodynamic size effects. The maximum increase found upon warming and cooling was about 10 and 8 K, respectively. The ionic conductivity in β-AgI under confinement was shown to be due to silver mobility on the particle surface.

Influence of pore size on the Knight shift in liquid tin and mercury in a confined geometry

119Sn and 199Hg NMR studies were carried out for metallic tin and mercury embedded in synthetic opals and porous glasses. The Knight shift for confined liquid tin and mercury was found to decrease monotonically with decreasing pore size, evidence for the reduction of electron susceptibility. Size-induced alterations in the Knight shift were more pronounced for confined mercury than for tin. The influence of pore filling on the NMR line shape and Knight shift was observed for tin within opal. The reasons for the decreasing Knight shift for liquid metals in a confined geometry are discussed. Correlations between the alteration in the Knight shift and atomic number are shown, the changes in fractional values of the Knight shift remaining almost identical.

Influence of size effects on the Knight shift of NMR lines in the gallium-indium alloy

A study is reported of the Knight shift of 71Ga, 69Ga, and 115In NMR lines in a liquid gallium-indium alloy with a composition of 90 at % Ga and 10 at % In, introduced into porous glasses with pore sizes of 5 and 200 nm, relative to the corresponding shifts in the bulk alloy. The measurements have been performed at room temperature. The study has revealed a size-dependent decrease in the Knight shift. A sample with a 5-nm pore size has demonstrated a noticeable difference in the magnitudes of the Knight shift of both gallium isotopes measured in magnetic fields of 9.4 and 17.6 T, which implies a dependence of the electronic susceptibility of the melt on the magnetic field under the nanoconfinement conditions.

Evolution of NaNO2 in porous matrices

NMR and dielectric studies of NaNO2 loaded into an SBA-15 mesoporous matrix are reported. The spin-lattice relaxation rate and the 23Na NMR line shift, as well as the permittivity, were measured within a broad temperature interval including the ferroelectric phase transition in NaNO2. The phase transition temperature of sodium nitrite in as-prepared samples was shown to differ substantially from that characteristic of a bulk crystal. The permittivity grows strongly in the vicinity of the phase transition. Heating a sample causes the properties of NaNO2 embedded in pores to gradually approach those of bulk crystals.

Dielectric and NMR Studies of the superionic conductor AgI embedded in mesoporous silicate matrices

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Magnetic properties of some opal-based nanocomposites

Na NMR studies of the Na-K eutectic alloy embedded into porous glass with 7-nm pores showed that melting of Na