Chemistry Reference
In-Depth Information
(“dilution”) of the reactant with an inert compound (the “diluent”) with a high ther-
mal conductivity. This approach is characterized by a number of advantages.
Firstly, the possibility of thermal explosion for exothermal reactions is com-
pletely eliminated (a transfer to completely degenerated modes takes place due to
increasing
). This allows one to study processes with higher thermal effect at higher
temperatures and to use standard thermal analysis heating rates.
Secondly, the calculations result in relatively accurate results due to the almost
complete elimination of temperature gradients.
Thirdly, the thermophysical characteristics of the studied material are not re-
quired, since the heating conditions are determined by the properties of the diluent.
In nonisothermal kinetic experiments, the change in the sample composition cannot
be described analytically, even if the expression describing the reaction kinetics is
available. Therefore, the thermophysical characteristics of the sample material can-
not be taken into account quantitatively. This circumstance is an additional argument
in favor of the methods developed by us. Besides, a reference substance for use in
DTA is not required, since the role of this substance is played by the diluent.
Two methods of dilution were developed: “mechanical” and “thermal.” In the first
method, the reagent is blended with powder of the heat-conducting chemically inert
diluent and placed into a vial. In the second one, a thin (
<
1 mm) layer of reagent is
placed between heat-conducting blocks (Fig. 6.1).
The first method is applied for the study of homogeneous solid and liquid mate-
rials, while the second one is used for homogeneous and heterogeneous condensed
materials. In both cases the diluent is used as a reference substance. The size and
shape of the reference cell are similar to those of the working cell. A typical dilution
ratio used is 1/100 (which does not affect the accuracy of TGA data recorded in the
experiments using analytical adjusted beam scales).
The diluent should not exhibit any catalytic behavior in the studied reaction. This
condition is especially important in case of mechanical dilution, since the reagent-
diluent contact area is very large. In the case of thermal dilution, the contact area
is relatively small, and the catalytic effect of the heat-conducting blocks can be ne-
glected. In some cases, to completely eliminate the influence of the block material,
thin layers of inert insulator (for example, mica) can be placed between the sample
and the blocks. Obviously, a high degree of ballasting results in output signal de-
pression. However, this problem can be resolved by using commercially available
photovoltaic amplifiers.
γ
Fig. 6.1a-b
Schematic
illustrating “mechanical” (
a
)
and “thermal” (
b
)dilutionof
a reagent with an inert
compound in thermography;
1
, working cells;
2
, reference
cells;
3
, reagent;
4
, differential thermocouples
b
