Thermodynamic Properties of Nano-Silver and Alloy Particles

Abstract

In this chapter, the analytical embedded atom method and calculating Gibbs free energy method are introduced briefly. Combining these methods with molecular dynamic and Monte Carlo techniques, thermodynamics of nano-silver and alloy particles have been studied systematically. For silver nanoparticles, calculations for melting temperature, molar heat of fusion, molar entropy of fusion, and temperature dependences of entropy and specific heat capacity indicate that these thermodynamic properties can be divided into two parts: bulk quantity and surface quantity, and surface atoms are dominant for the size effect on the thermodynamic properties of nanoparticles. Isothermal grain growth behaviors of nanocrystalline Ag shows that the small grain size and high temperature accelerate the grain growth. The grain growth processes of nanocrystalline Ag are well characterized by a power-law growth curve, followed by a linear relaxation stage. Beside grain boundary migration and grain rotation mechanisms, the dislocations serve as the intermediate role in the grain growth process. The isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation. By extrapolating the mean grain size of nanocrystal to an infinitesimal value, we have obtained amorphous model from Voronoi construction. From nanocrystal to amorphous state, the curve of melting temperature exhibits three characteristic regions. As mean grain size above about 3.8 nm for Ag, the melting temperatures decrease linearly with the reciprocal of grain size. With further decreasing grain size, the melting temperatures almost keep a constant. This is because the dominant factor on melting temperature of nanocrystal shifts from grain phase to grain boundary one. As a result of fundamental difference in structure, the amorphous has a much lower solid-to-liquid transformation temperature than that of nanocrystal.

Mechanism and behavior of silver nanoparticles in aqueous medium as adsorbent

“…As temperature goes above the melting point, the free energy of the crystal phase becomes greater than that of the liquid phase, and phase transition will take place. The same holds true for nanoparticles [75].…”

“…As reported by numerous studies, isothermal grain growth behaviors of nanocrystalline silver shows that the small grain size and high temperature accelerate the grain growth [75].…”

Exergy of Nano-Particulate Materials

“…This is illustrated by Figure 2 with Gibbs energy of NP material of different sizes liquid (droplet) or solid compared to "bulk" size material (D >> 100 nm) at melting point equilibrium. (Data for 5 nm Ag nanoparticles that confirm this behaviour is given in [6]. The subject matter of exergy of nano particles can be considered novel.…”

“…With surface atoms becoming increasingly dominant for smaller sizes, thermodynamic properties can be divided into a two parts: a bulk quantity and a surface quantity, where the latter can be related to the surface free energy (SFE) [6][7][8]. For liquid-gas interfaces SFE commonly referred to as surface tension, in the current paper merely gives the energy difference between atoms located at the surface of materials compared to those in the bulk of it.…”

“…Exergy analysis is considered to be the proper tool for such process evaluations [4,5] with the benefit that electricity consumption can be clearly defined as exergy input while at the same time chemical conversions need not be considered for the arc/spark-driven process route. On the other hand, the small size of NP material, covering size range of 1 ~ 100 nm (nm = 10 -9 m) introduces variable thermodynamic properties when compared to "bulk" size material: melting temperatures of nano-particles are lower, and distinct superheating behaviour has been noticed (e.g., [6]). …”