Process and apparatus for measuring enthalpy changes by...

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Reexamination Certificate

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C219S506000, C219S508000, C219S494000, C374S011000, C373S136000

Reexamination Certificate

active

06239415

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process and apparatus capable of measuring the enthalpy change or eventually its change rate as a function of the temperature of a sample by means of a DTA apparatus operating with quasi-isothermal heating technique.
PRIOR ART
Physical and chemical transformations taking place on heating of materials always involve variation of the enthalpy, the consequence being that the temperature of sample changes differently from that of its environment. For almost a century, the study of these processes has been carried out by means of differential thermal analysers, i.e. by DTA instruments during heating or cooling. However, by using these equipments, in the majority of cases, only a deteriorated picture of processes taking place can be obtained either at heating or cooling of the samples. The reason for this is that the course of curves is significantly influenced by the experimental conditions, such as the rate of heating or cooling, the rate of heat transfer between the sample and its environment, the magnitude of the transformation heat, etc. Thus, according to experience, phase diagrams of unknown structures for multi-component systems are very difficult to be constructed by a traditional DTA apparatus.
This problem can very well be studied in the opposite way, when we compare the results of traditional DTA studies with a well-known phase diagram. In this case, namely, we have a basis for comparison, we know, how and at which temperature the processes studied would have taken place under ideal conditions.
Such a phase diagram is shown in
FIG. 1
a
.
FIGS. 1
b
and
1
c
show the results of DTA studies on two samples with different compositions. Phase diagram
1
a
represents the equilibrium for a eutectic mixture of components A and B in a coordinate system the ordinate of which is temperature, and the abscissa is composition.
Symbols in
FIGS. 1
b
and
1
c
have the flowing meanings:
DTA
t
=traditional DTA curve as a function of time
T
s
=temperature change in the sample as a function of time
T
r
=temperature change in the inert material as a function of time
&Dgr;T change=in the temperature difference T
s
−T
r
proportional to enthalpy variation
The ordinates for these curves are temperature and temperature difference.
If we study the system at cooling by starting from point a, it can be established from
FIG. 1
a
that at reaching the eutectic temperature e, the eutectic mixture begins to precipitate from the melt and the system solidifies fully without any change in the temperature.
At studying the system by the known DTA apparatus, as is shown in
FIG. 1
b
, however, it turns out that this phase transformation has not taken place in an isothennal way, but in a wide temperature range, during quite a long time-period. It is clear from
FIG. 1
b
, namely, that the temperature T
s
, turns back to the temperature T
r
of the reference material, only with a delay. It is obvious that in the opposite case, when the goal is to construct a phase diagram, this phenomenon is very misleading.
Starting from point b of
FIG. 1
a
we can see the following: at reaching the temperature measured in point c, modification B
&bgr;
starts to precipitate, which process continues up to point d, in a non-isothermal way. At this temperature, modification B
&bgr;
already precipitated transforms in isothermal way into modification B
&agr;
, and later only this modification precipitates from the melt, also in a nonisothermal way. At last, at eutectic temperature e, the system solidifies isothermally.
The results of the study performed by the DTA apparatus shown in
FIG. 1
c
are almost impracticable, because, in this case, of the more elementary processes taking place with a strong delay and overlapping. Isothermal and non-isothermal processes cannot be differentiated and identified on the DTA
t
and T
s
curves. Consequently, in the opposite case, no phase diagram could be constructed from these curves.
We have chosen these two examples, since they can be generalized for the whole area of thermal analysis. Phase transformations like that shown in
FIG. 1
b
are characteristic for individual, isothermally occurring thermal transformations, such as certain decomposition reactions, modification and state changes, etc., while that shown in
FIG. 1
c
is characteristic for complex, non-isothermal types of transformations, like e.g. the big family of solid state reactions.
Thus we have seen that in the traditional way, the course and characteristic temperatures of both transformation types can be determined only inaccurately, or not at all.
To the contrary, the apparatus operating according to the principle patented by J. Paulik, F. Paulik and M. Arnold under the number U.S. Pat. No. 3,344,654, the study of the first type of transformations can be performed by a large accuracy. This is illustrated in
FIGS. 3
b
and
4
b
. However, the second type of transformations cannot be investigated satisfactorily either by the use of this known procedure.
This invention relates to a process and apparatus elaborated by a further development of the method applied in the previously mentioned patent, the use of which makes the determination of the enthalpy changes in the second type of complex and non-isothermal transformations also possible.
For the elimination of similar mistakes originating from the conditions of the experiment in the thermal gravimetric measuring technique, J. Paulik, F. Paulik and L Erdey elaborated the first static thermal analytical method, the so-called quasi-isothermal thermogravimetric procedure, characterised thereby that the heating of the furnace is controlled by the rate of the weight change. This method is described in the U.S. Pat. No. 3,344,654. This is a well-established method applied successfully already for decades. Later on, Rouquerol also elaborated a similar procedure, in which heating was controlled by the rate of gas evolution occurring during decomposition.
However, the above methods cannot eliminate the problems caused by the experimental conditions in DTA and DSC, i.e. differential scanning calorimetric measurements. The previously mentioned patent, U.S. Pat. No. 4,812,051 has been elaborated by J. Paulik, F. Paulik and M. Arnold for performing such measurements, i.e. the methods of quasi-isothermal differential thermal analysis, Q-DTA, and quasi-isothermal differential scanning calorimetry, Q-DSC.
In
FIG. 2
, a scheme of the most important elements of the quasi-static temperature controlling system described in patent U.S. Pat. No. 4,812,051 is shown. We present this, in order to make the recognition of the invention more unambiguous. It is seen that reference material
1
and sample
2
are placed into furnace
3
in separate containers, to which furnace
3
programmed temperature controller
4
is connected. The temperatures of both, reference material
1
and sample
2
, are sensed by temperature sensors
5
and
6
, which are thermocouples. By temperature measuring device
7
is the temperature of sample
2
measured. This temperature measuring device
7
is, in the present example, a galvanometer, the output signal of which is recorded by writer
15
2
on paper
16
. A so-called symmetrising resistance
8
is connected parallel to temperature sensor
5
. At the non-common point of symmetrising resistance
8
and temperature sensor
7
, differentiating unit
10
and temperature difference measuring device
9
are connected in series. The latter measures the difference in the thermal potentials of oppositely coupled thermocouples
5
and
6
, i.e. the difference between the temperatures of reference material
1
and sample
2
. The output of temperature sensor
7
measuring the temperature of sample
2
is connected via galvanometer
11
to writer
15
3
. The recorder is equipped with three
15
1
;
15
2
;
15
3
writers, and the paper
16
. Similarly, temperature difference measuring device
9
is also connected to recording paper
16
through another writer,
15
2
. The output of diffe

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