Acoustic studies of melting and crystallization of sodium nitrite nanocrystals in the pores of mesoporous silicate matrices

Abstract

A conventional phase-pulse acoustic method was used to study melting and crystallization of sodium nitrite embedded in the pores of mesoporous silicate matrices. The pore diameter was 20, 37, and 52 A. The measurements were performed at a frequency of 3–8 MHz in the temperature interval 290–560 K. The temperature dependence of ultrasonic velocity was found to exhibit anomalies corresponding to phase transitions of sodium nitrite. The transitions were smeared in temperature and shifted to lower temperatures from the melting point T b of bulk sodium nitrite; the shift in crystallization temperature was greater than that of the melting temperature. The irreversible character of melting was revealed. The size dependence of the melting temperature of sodium nitrite was obtained. Phenomena observed in the experiments were discussed with the use of different size effect models.