Abstract
A system consisting of at least two components was considered. In this system, nanocrystalline material is formed at high temperature, at which diffusion does not limit the mass transport. The structure results from establishing an equilibrium between surface and volume of the crystallites and their surroundings in isothermal‐adiabatic conditions. The surface of each crystallite is covered with another substance. On the basis of the performed energy‐balance calculations it was concluded that the reduction in the surface area is associated with a decrease in the surface coverage degree and thus with the necessity to provide energy to the system in order to remove chemisorbed atoms. An increase in the temperature of a nanocrystalline substance to a temperature higher than the preparation temperature results in the formation of a new state of equilibrium. At temperatures below the maximum temperature only the equilibrium between the gas phase and the surface exists.
Highlights
There is a common view that substances with a strongly extended surface are not in a state of chemical equilibrium
To determine if materials with an extended surface exist in a state of chemical equilibrium, let us consider a two component system in which the surface of each individual crystallite is covered with substance B to a given degree, θ > 0
Materials consisting of nanometer-sized crystallites are far away from thermodynamic equilibrium
Summary
There is a common view that substances with a strongly extended surface are not in a state of chemical equilibrium. To determine if materials with an extended surface exist in a state of chemical equilibrium, let us consider a two component system in which the surface of each individual crystallite (substance A) is covered with substance B (chemisorption) to a given degree, θ > 0. The total value of the surface energy, γ, which corresponds to the energy minimum (at equilibrium state), results from the tendency of the system to reduce its specific surface area and to increase the degree of surface coverage: γ = γA (1 − θ) + γA(B)θ.
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