The Physical–Chemical Model of Nanoscaled Metal Component Formation …
267
15 nm and the thickness of salt layer varies within h Salt = z 2 − z 1 = (14−30) nm
depending on solution concentration.
To start the passing of the reaction of thermolysis, it is necessary to convert metal
salt into nonequilibrium state through temperature increasing. Let us analyze the
duct of heat delivering to the salt layer in thermoexfoliated graphite.
It is necessary to point out three important conditions determining the character
of heat delivering to the salt layer. The salt and graphite layers are in direct contact
with an ambient medium through their outer ends, which outcrop on the surface of
graphite particle.
The thickness of graphite and salt layers are close to each other.
Hear conductivity coefficient within graphite layer is essentially higher than that
of salt crystalline hydrate.
This permits to state that on heating of “graphite-salt” layered system, the heat
predominantly spreads along the graphite layers and enters the salt layer through
the graphite-salt boundary. This means the arising of temperature gradient along
graphite layer is directed toward the outer end of plane; the temperature gradient in
the bulk of salt layer ∂ T /∂z is directed to the boundary of graphite and salt layers.
The magnitudes of these gradients are determined by the heating regime, i.e., by
the maximal heating temperature and by the heating rate. In any case, the maximal
temperature in the salt layer is believed to reach the boundary of graphite and salt
layers.
The condition of salt thermal stability is the following in equation
G Salt (T ) ≤ G Oxide (T ),
(2)
where G Salt (T ) and G Oxide (T ) are the Gibbs thermodynamic potentials for metal
salt and metal oxide, respectively. The typical behavior of G Salt (T ) and G Oxide (T )
functions is presented in Fig. 2.
The temperature T 0 that corresponds to the equality of G Salt (T ) and G Oxide (T )
determines the limit of salt thermal stability: salt is thermodynamically stable at
T < T 0 , and at T > T 0 it decomposes with the formation of oxide.
Reasoning from the presented qualitative analysis of temperature distribution
over the salt layer thickness, one can conclude that during heating, first of all, the
salt molecules directly adjoining to the boundary of salt and graphite layer turn to
thermodynamically nonequilibrium state since the temperature in this zone is the
highest. The nuclei of the new phase, i.e., metal oxide nuclei, will form just there.
Then each nucleus grows due to thermolysis at “salt-oxide” interphase boundary.
The dominant formation of the oxide phase nuclei at the boundary of graphite and
salt layers is the typical feature of heterogeneous character of oxide phase formation
in the system under consideration.
Selection of the model for the heterogeneous formation of the metal oxide phase
permits to turn to the description of the kinetics of phase formation and, finally, to
determine the influence of these process parameters (graphite surface characteristics,
type of salt, parameters of the reaction, i.e., heating regime, etc.) on the composition
and structure of the oxide phase.
267
15 nm and the thickness of salt layer varies within h Salt = z 2 − z 1 = (14−30) nm
depending on solution concentration.
To start the passing of the reaction of thermolysis, it is necessary to convert metal
salt into nonequilibrium state through temperature increasing. Let us analyze the
duct of heat delivering to the salt layer in thermoexfoliated graphite.
It is necessary to point out three important conditions determining the character
of heat delivering to the salt layer. The salt and graphite layers are in direct contact
with an ambient medium through their outer ends, which outcrop on the surface of
graphite particle.
The thickness of graphite and salt layers are close to each other.
Hear conductivity coefficient within graphite layer is essentially higher than that
of salt crystalline hydrate.
This permits to state that on heating of “graphite-salt” layered system, the heat
predominantly spreads along the graphite layers and enters the salt layer through
the graphite-salt boundary. This means the arising of temperature gradient along
graphite layer is directed toward the outer end of plane; the temperature gradient in
the bulk of salt layer ∂ T /∂z is directed to the boundary of graphite and salt layers.
The magnitudes of these gradients are determined by the heating regime, i.e., by
the maximal heating temperature and by the heating rate. In any case, the maximal
temperature in the salt layer is believed to reach the boundary of graphite and salt
layers.
The condition of salt thermal stability is the following in equation
G Salt (T ) ≤ G Oxide (T ),
(2)
where G Salt (T ) and G Oxide (T ) are the Gibbs thermodynamic potentials for metal
salt and metal oxide, respectively. The typical behavior of G Salt (T ) and G Oxide (T )
functions is presented in Fig. 2.
The temperature T 0 that corresponds to the equality of G Salt (T ) and G Oxide (T )
determines the limit of salt thermal stability: salt is thermodynamically stable at
T < T 0 , and at T > T 0 it decomposes with the formation of oxide.
Reasoning from the presented qualitative analysis of temperature distribution
over the salt layer thickness, one can conclude that during heating, first of all, the
salt molecules directly adjoining to the boundary of salt and graphite layer turn to
thermodynamically nonequilibrium state since the temperature in this zone is the
highest. The nuclei of the new phase, i.e., metal oxide nuclei, will form just there.
Then each nucleus grows due to thermolysis at “salt-oxide” interphase boundary.
The dominant formation of the oxide phase nuclei at the boundary of graphite and
salt layers is the typical feature of heterogeneous character of oxide phase formation
in the system under consideration.
Selection of the model for the heterogeneous formation of the metal oxide phase
permits to turn to the description of the kinetics of phase formation and, finally, to
determine the influence of these process parameters (graphite surface characteristics,
type of salt, parameters of the reaction, i.e., heating regime, etc.) on the composition
and structure of the oxide phase.