Thermal expansion, heat capacity and phase transformations in nanocrystalline and coarse-crystalline silver sulfide at 290–970 K

Abstract

The temperature dependences of the thermal expansion coefficient and heat capacity of nanocrystalline and coarse-crystalline silver sulfide Ag₂S have been studied using dilatometry and differential scanning calorimetry. When the temperature increases from 290 to 970 K, the phase transformations “acanthite α-Ag₂S ⇔ argentite β-Ag₂S” and “argentite β-Ag₂S ⇔ γ-Ag₂S phase” occur sequentially in the silver sulfide. The thermal expansion coefficient α and heat capacity C _p of α-Ag₂S, β-Ag₂S and γ-Ag₂S phases in nanocrystalline state (with particle size ≤60–70 nm) in the temperature regions of existence of these phases are larger than α and C _p of the same phases in coarse-crystalline state (with particle size ≥400 nm). It is shown that the heat capacity and the thermal expansion coefficient of nanocrystalline silver sulfide Ag₂S contain an additional positive contribution due to the restriction of the phonon spectrum on the side of low and high frequencies caused by a small particle size. It is established that the acanthite α-Ag₂S to argentite β-Ag₂S and argentite β-Ag₂S to γ-Ag₂S phase transformations are the first-order phase transitions, and the temperatures and enthalpies of these transformations have been determined.