Acoustic studies of melting and crystallization of indium-gallium alloy in porous glass

Temperature dependences of the velocity of the longitudinal ultrasonic waves in a nanocomposite on the basis of porous glass filled with gallium-indium alloy have been measured. Acoustic anomalies due to crystallization and melting of the alloy in nanopores have been revealed for the complete and partial cooling-heating cycles. A two-step temperature hysteresis loop between the curves of the velocity change upon cooling and heating has been found, the existence of which is related to the formation in pores of two types of mixtures, with α- and β-Ga. Stabilization of β-Ga in nanopores has been observed. It was shown that the conditions of confined geometry lead to a shift to low temperature of the melting regions for both mixtures in comparison with the regions of the coexistence of the liquid and solid phases in bulk alloy.     

Dielectric Properties of Ferroelectric Nanocomposites Based on KD2PO4

Russian Physics Journal - The temperature dependences of the dielectric permittivity ε' and dielectric loss tangent tgδ have been studied for KD2PO4 embedded into the pores of the...     

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.     

Separation of the impurity and lattice components of the spin-lattice relaxation of Al27 nuclei in corundum crystals under magnetic saturation conditions

The impurity and lattice contributions to the spin-lattice relaxation of Al²⁷ nuclei in nominally pure and lightly chromium-doped corundum (Al₂O₃) crystals are separated experimentally under the conditions of additional stationary resonance magnetic saturation of a pulsed NMR signal. The relaxation time T _i ^lat due to the lattice mechanism of the spin-phonon interaction is determined. Fiz. Tverd. Tela (St. Petersburg) 39, 1041–1043 (June 1997)     

Peculiarities of gallium crystallization in confined geometry

The freezing and melting phase transitions for gallium embedded into a porous glass with a pore size of about 8 nm were studied using acoustic, NMR, and x-ray techniques. It was shown that the broadened solidification and melting transitions upon deep cooling up to complete freezing at 165 K were due to the formation of β-Ga within pores. The offset of confined β-Ga melting was lowered by about 21 K compared to the bulk β-Ga melting point. Both melting and freezing in pores were irreversible. The fulfillment of some special thermal conditions led to gallium crystallization into other modifications. The role of heterogeneous crystallization in freezing of confined gallium is discussed.     

Diffusion slowdown in the nanostructured liquid Ga‐Sn alloy

The diffusion of gallium in liquid Ga‐Sn alloy embedded into different porous silica matrices was studied by NMR. Spin relaxation was measured for two gallium isotopes, ⁷¹Ga and ⁶⁹Ga, at two magnetic fields. Pronounced rise of quadrupole contribution to relaxation was observed for the nanostructured alloy which increased with decreasing the pore size. The correlation time of atomic mobility was evaluated and found to be much larger than in the relevant bulk melt which evidenced a pronounced diffusion slowdown in the Ga‐Sn alloy under nanoconfinement. It is shown that the diffusion was slower by a factor of 30 for the alloy within 7 nm pores. The spectral densities of electric field gradients at zero frequency were found to double for the finest pores. The Knight shift was found to decrease but slightly for the nanostructured alloy.     

Acoustic studies of phase transitions in crystals and nanocomposites

A brief review of acoustic studies of phase transitions in crystals (ferroelectric, ferroelastic, and superionic ones) and nanostructured composite materials made on the basis of porous matrices is presented. The studies were carried out at the Laboratory of Quantum Acoustics and Ultrasonic Spectroscopy of the St. Petersburg State University.     

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.     

Ising model for a ferroelectric phase transition in a system of interacting small particles

An approach based on the Ising model has been proposed for describing a ferroelectric phase transition in a system of interacting identical small particles. It has been found that the shift of the phase transition temperature with respect to the transition point in a bulk sample is affected by both the size effects due to the smallness of the particles and their interaction with each other. The behavior of the dependence of the phase transition temperature on the distance between particles is determined by the nature of the interparticle interaction. An analysis has demonstrated that the interaction between small particles should be taken into account in the interpretation of the ferroelectric properties of nanocomposite materials.