Process for analyzing relaxation spectra and resonances in materials

The present invention relates to the use of thermal stimulated processes for analyzing relaxation spectra and resonances in materials. The process is characterized in that at least two coupled excitation fields are applied to the sample of material analyzed along with a programmed temperature variation, with the objective to deconvolute during the thermally stimulated recovery stage the global deformation resulting from the excitation stage, i.e. obtain one by one the individual and elementary relaxation motions responsible for the global deformation, whether these elementary internal motions be of mechanical, electrical or magnetic origin. The process is characterized in the fact that the relaxation spectra for the motions resulting from the coupling between mechanical and electrical (and/or electromagnetical) excitations are obtained at the same time and are inter-related.

FIELD OF THE INVENTION 
The present invention relates to the use of thermal stimulated processes 
for analyzing relaxation spectra and resonances in materials by 
application of programmed disturbances of the internal state of the 
material, during the excitation stage, and by the study of the thermally 
stimulated recovery of these deformations, during the return to 
equilibrium stimulated by heating. 
BACKGROUND OF THE INVENTION 
Such thermally stimulated processes are already well documented in the 
literature (Chapter 10 in the ACS Book Polymer Characterization, 
"Characterization of Polymers by Thermally Stimulated Current Analysis and 
Relaxation Map Analysis Spectroscopy, by J. P. Ibar, et al., Polymer 
Characterization Advances in Chemistry Series No. 227, Edited by Clara D. 
Craver and Theodore Provder; and for the TSCR: Chapter in the ACS Book 
Polymer Characterization, "Thermally Stimulated Creep for the Study of 
Copolymers and Blends" by Philippe Demont, et al.). The aim of such 
processes, which will be examined in greater detail hereinafter, is to 
understand the behavior of materials by studying the relaxations and 
internal motions which take place in order to optimize their mechanical, 
electrical, magnetic, etc., performances. 
More generally speaking, the recovery process of a system applies to the 
phenomenon of recovering its initial state, after the application of a 
deformation has taken the system out of equilibrium. The recovery process 
is stimulated by a (linear) temperature increase, or can occur 
isothermally over time. Relaxation phenomena in materials during recovery 
are the results of internal motions due to disturbances either of a 
mechanical, electrical, magnetic or electromagnetic nature. Materials 
processed in industry have physical properties which depend on the ability 
to have local motion within the internal structure irrespective of whether 
this motion occurs at the level of the molecules, the atoms, or the 
macromolecules (for polymeric materials), or at the sub-atomic level. 
Deformation at one level or the other depends upon the type of excitation 
field involved to bring the material out of its equilibrium state at a 
given temperature. 
Essentially, three types of methods for studying relaxation phenomena and 
resonances can be distinguished: (1) resonance methods, (2) damping 
analysis methods and (3) heat stimulated methods. In the resonance method, 
the material is subjected to a periodic excitation at a fixed frequency of 
a mechanical, electrical or magnetic nature at a determined temperature 
and fixed pressure. The periodic excitation frequency can be adjusted to 
enable the determination of the resonance frequency for this temperature 
and pressure. The frequency of resonance corresponds to the frequency of 
the internal motion occurring under these conditions. An alternative 
method, which is frequently used, consists in subjecting the material to 
an excitation at a determined frequency and programming a variation in 
temperature. When the temperature reaches a level capable of allowing the 
internal movements sought to be characterized, a resonance peak for the 
selected excitation frequency is observed. It is possible to operate at 
various (fixed) frequencies and thus analyze the dependence between 
frequency and temperature which provides access to the mechanism 
responsible for internal motion under investigation. 
In many instances, the internal motion is kinetically controlled, and the 
variation in the resonance peak frequency (fm) varies with the maximum 
temperature of the peak Tm, and the results are often collected as the ln 
(fm) versus 1/Tm, a so-called Arrhenius diagram (Tm is in degrees Kelvin 
and ln is the natural logarithm). The linearity of the Arrhenius line is 
indicative of an activated phenomenon. The slope of the straight line in 
the Arrhenius diagram is related to the activation enthalpy of the process 
due to internal motions and the intercept is proportional to the 
activation entropy, i.e., to the jump frequency between the activated 
states allowing motion. By determining the values of the entropy and 
enthalpy, one can determine the origin of the movements occurring inside 
the material irrespective of their origin, whether it is viscous, atomic 
or sub-atomic. Mechanical deformation fields allow movements of the 
viscous type to occur in the material (so does ultrasonic excitation) and 
electrical fields (voltages) applied to the material allow the study of 
motions related to the electronic interactions between the atoms inside 
the material. The sub-atomic movements are delocalized by applying 
electromagnetic excitations. 
In the characterization processes which are the subject of this invention, 
the temperature program is always the same regardless of the origin of the 
excitation, and consists of exciting the material at a particular 
temperature, then quenching it, interrupted by partial isothermal 
relaxation if necessary, and finally heating it up linearly to "develop" 
the response to the excitation stage during a thermally stimulated return 
to equilibrium. 
The analysis methods using damping in the material consist in the 
application of a deformation of the material for a given length of time at 
a given temperature, cutting off the source of the excitation and 
analyzing the return to equilibrium (recovery curve) at that temperature 
by recording the freely oscillating damping curve. The equation of the 
recovery curve gives direct access to the damping factor at that 
corresponding temperature. The frequency of the oscillation and the 
damping factor relate to internal friction, and provide the relaxation 
time at the corresponding temperature. The frequency of oscillation and 
the damping factor vary with the temperature at which the material is 
being deformed. This enables the determination of the damping factor at 
different frequencies and different temperatures. As above, the origin of 
the internal motions may be found by studying the corresponding Arrhenius 
diagrams in plots of log of 
EQU ln (fm) vs. 1/Tm 
The so-called "thermal stimulated" methods comprise purely calorimetric 
methods and methods combining the influence of temperature and a 
"stimulant" variable which may be a mechanical, electrical or an 
electromagnetic variable. Differential scanning calorimetry (DSC) consists 
in comparing the calorific energy flux supplied to two crucibles located 
in the same thermostatic atmosphere, a device in which one of the two 
crucibles contains the material to be analyzed. The temperature in the 
chamber may be programmed to increase, decrease, or to stay constant 
(isothermal mode). In a DSC the calorimeter is servo regulated in such a 
way that the temperature of the two crucibles is exactly the same. The 
variable energy flux supplied or subtracted from the crucibles is recorded 
as the temperature of the cell varies, or as a function of time under 
isothermal conditions. Differential Thermal Analysis (DTA) is a slight 
variant of this microcalorimetric DSC process, for which the fine 
difference in the temperature between the two crucibles is recorded as a 
function of the cell temperature. The difference in the temperature 
between the two crucibles changes when there is an alteration in the 
physical structure or in the physical and/or chemical structure resulting 
in the variation in enthalpy within the material. In a DSC analysis, the 
energy flow differential to maintain the two pans at the same temperature 
is recorded, and a peak is observed when there is a modification in the 
thermodynamic state of the material. The peak characteristics relate to 
the state of the material, and transcribe the extent of internal motion 
and local reorganization, for instance due to molecular relaxations. 
Differential scanning calorimetry is a rapid and streamlined method of 
determining phase transitions in materials, for example in order to 
determine fusion and solidification temperatures, and the glass transition 
temperature in the case of the amorphous phase of non-crystalline or 
semi-crystalline materials. It should be noted that in this 
characterization technique, temperature essentially plays two roles, that 
of stimulator by contributing thermal energy capable of initiating 
activated internal motions, and that of sensor, by comparative 
measurements of the temperature or the flow of energy of the two 
crucibles, one containing the material to be characterized. 
One variant of this process consists in obtaining calorific heat capacity 
curves as a function of the temperature at different atmospheric 
pressures. 
Atmospheric pressure plays an important role with respect to the kinetics 
of relaxation phenomena. It is presently known that an increase in 
pressure is accompanied by a restriction of internal movements, which is 
observed in differential microcalorimetry by an increase in the 
temperature at which the internal movements are released during a thermal 
analysis. Apparatus currently marketed enable microcalorimetry curves to 
be obtained at pressurized atmospheres. The pressure remains constant 
during the heating or cooling cycle of these analyses. It is one of the 
characteristics of the present invention to provide means to submit the 
crucibles and their content to a pressure history treatment to enable the 
fine characterization of internal motions inside the material under 
investigation. 
A further important type of analytical instruments for measuring internal 
movements in materials by the thermal-stimulated effect is described in 
the works of several authors, and concerns thermal-stimulated current 
techniques (TSC), and thermal-stimulated creep techniques (TSCR). These 
techniques are relatively original with respect to the previously 
described techniques. In these techniques temperature plays the role of 
developer while the external variables imposed during the excitation stage 
play the role of "marker". 
In a variant of the process, described in further detail hereinafter, 
temperature also plays the role of "filter" for the relaxation times; this 
is the "thermal-windowing" filtering method. The aim of the excitation, in 
the form of a mechanical, electrical or magnetic field, etc., imposed on 
the material at a given temperature, is to induce orientation, or more 
generally to cause an imbalance in the system, by the effect of the field 
on the free activation energy value. The field intensity imposed remains 
fixed for a given time, the time for the new state of equilibrium to 
establish itself, and the temperature is lowered very quickly (tempering) 
to a temperature at which the new thermodynamic state of the material is 
no longer able to modify itself, for kinetic reasons; consequently a 
"frozen-in picture" of the state obtained at high temperature is produced. 
Analysis by the thermal stimulated effect consists in suppressing the 
field at low temperatures and reheating the material, which is now free of 
all stresses, and in so doing freeing up the internal motions which are 
thermally activated to allow their return to equilibrium. The kinetics for 
the return to equilibrium, induced by the temperature, can be analyzed 
quantitatively and is a function of the processing parameters of the 
material and its chemical structure. It is also a function of the 
morphology. 
The thermal stimulated effect reveals all the relaxation modes occurring in 
a global manner. If the local motions inside the material are not simple 
in the sense of a pure relaxation of the Debye type, or when there is a 
large degree of interactive coupling between the relaxation modes 
responsible for the global response of the material, it is then very 
difficult to attribute to the recovery curve any particular local motion 
occurring in the material. Since the entire response of the material to a 
given excitation is global, it is generally essential to deconvolute the 
global response and define the relaxation time distribution, corresponding 
by analogy to different coupled resonators. The coupling between the 
elementary modes of relaxation is subject to a specific kinetics, itself a 
function of structural, chemical and morphological parameters. The 
description of the elementary modes, their thermo-kinetic characteristics 
(activation energy and entropy) and the description of the coupling is 
essential for understanding the macroscopic properties of materials. The 
TSC (thermally stimulated current) and TSCR (thermal stimulated creep 
processes) are thermal stimulated techniques which use the application of 
a field, either electrical (for TSC) or mechanical (for TSCR) at a given 
temperature in order to orient the dipoles in the material (TSC) or the 
chain segments (TSCR), with the aim of disclosing their individual 
existence when heated up in a controlled manner after cooling, and after 
the application of the field has been removed. 
The two techniques, TSC and TSCR, have been described in the literature 
(for the TSC: Chapter 10 in the ACS Book Polymer Characterization, 
"Characterization of Polymers by Thermally Stimulated Current Analysis and 
Relaxation Map Analysis Spectroscopy, by J. P. Ibar, et al., Polymer 
Characterization Advances in Chemistry Series No. 227, Edited by Clara D. 
Craver and Theodore Provder; and for the TSCR: Chapter in the ACS Book 
Polymer Characterization, "Thermally Stimulated Creep for the Study of 
Copolymers and Blends" by Philippe Demont, et al.). 
The principle of the thermal stimulated windowing technique is summarized 
herewith. The technique has been used a great deal by the scientists of 
the Laboratory of Physique des Solides in Toulouse, France. These 
researchers, headed by Professor Lacabanne, concentrated on the 
application of the thermal windowing method with the aim of isolating one 
by one the individual relaxations making up a cooperative complex 
spectrum. The method consists in applying an excitation field (electrical 
or mechanical) to induce orientation in the material at a selected 
temperature of excitation T.sub.p. The temperature is subsequently lowered 
by a few degrees, with the field still applied. At that temperature 
T.sub.d, the excitation field is then removed and the material is free to 
return to its state of equilibrium at this temperature T.sub.d. However, 
it can only do so for a small time t.sub.d and therefore the material 
cannot relax completely at T.sub.d, and the remaining orientation induced 
in the material is then frozen in by quenching to a very low temperature 
T.sub.0. The subsequent reheating at a controlled heating speed, discloses 
the elementary kinetics of the relaxation mode isolated in the window 
temperature range (T.sub.p -T.sub.d). The curve obtained during this 
recovery stage at a constant rate of heating is of a Debye nature, which 
may be analyzed directly and quantitatively according to the Arrhenius 
formulation to determine the activation enthalpy and activation entropy 
parameters for this isolated deconvoluted elementary relaxation. By 
changing the value of T.sub.p around the global temperature peak observed 
in either TSC or TSCR, all the relaxation modes co-operating in an 
interactive manner and contributing to the global response observed 
without thermal windowing can be isolated one by one. This represents the 
description of the prior art according to the processes described as 
thermal stimulated processes. 
However, these known methods for analyzing and characterizing materials by 
the thermal stimulated effect have major negative drawbacks: the method 
using thermal stimulated current cannot be applied to conductors or 
semiconductor materials for which the electrical resistance is smaller 
than 10.sup.7 ohms/meter of thickness; the method using thermal stimulated 
creep is not easy to apply to pasty or liquid materials and does not allow 
temperatures close to the fusion point of the materials to b reached; and 
there is no simple correlation between the distribution spectra for the 
relaxation times obtained by TSC and TSCR analysis. This is a major 
drawback which casts a doubt on the validity of the results obtained by 
these techniques. The relationship between the mechanical and electrical 
spectrum of relaxation appears to be complex. In addition, the thermal 
stimulated method presented in the prior art appears to disturb the 
structural state of the sample owing to the very nature of the experiment 
itself: the TSC or TSCR methods consist in applying an electrical or 
mechanical field at a temperature T.sub.p in the vicinity of the 
temperature at which the internal motions occur. The effect of bringing 
the material to this temperature T.sub.p enables the latter to relax from 
its internal stresses, if there are any present, or to modify its 
morphology, if it is capable of crystallizing, or even modifying its 
degree of curing for curable materials and thermoset resins. It is 
therefore clear that thermal stimulated processes are restricted to the 
study of internal motions undisturbed by morphological changes at the 
analysis temperature T.sub.p. 
It is an object of this invention to add a variable to the existing prior 
art in order to remedy that important drawback by the use of windowing 
processes which do not alter the morphology the way thermal-windowing 
does, as will be explained hereafter. In other words, it is a 
characteristic of the present invention to describe an excitation field 
profile which rheologically freezes the material at a constant given 
temperature instead of changing the temperature in order to create the 
window necessary to induce the filtering process (thermal windowing). 
The main disadvantage of differential microcalorimetry or of differential 
thermal analysis is that the instrument response is a global response 
which integrates the co-operative plurality of internal relaxations. A 
further main disadvantage is the low sensitivity in detecting "secondary" 
internal movements for which the activation enthalpy is low. Finally, this 
technique also presents difficulties, especially a lack of sensitivity, in 
studying certain phenomena such as the orientation of plastic materials or 
the physical aging phenomena. For instance, it is not rare to observe 
great variations in the mechanical properties of plastic materials and not 
to lack such evidence of any difference on the basis of the corresponding 
traces in DSC analyses. Differential microcalorimetry appears not to be 
very sensitive to internal stresses relaxed kinetically during physical 
aging. 
A further major disadvantage of the thermal stimulated processes described 
in the prior art is that the sample must be changed for each temperature 
T.sub.p when the object of the analysis is to study physical aging or 
internal stresses. This results in a long and expensive analysis process. 
In the prior art, a technician using the TSC analysis cell or TSCR 
analysis cell must prepare a variety of samples and introduce them in 
succession one after the other. The thermal windowing experiments are then 
run according to the previous description and a new sample has to be 
entered into the chamber for each T.sub.p since the sample which has been 
analyzed has lost its initial condition, which is what is being studied. 
The above procedure is repeated for each excitation temperature T.sub.p 
with a new sample until the complete relaxation spectrum is obtained. This 
method of analysis for isolating simple modes in materials having internal 
stresses requires a large number of samples and a great deal of labor. 
A further major disadvantage of the prior art is that the TSC and TSCR 
cells are different and the two techniques cannot be used simultaneously 
on the same sample. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to remedy these problems and 
provide means to analyze the mechanical and electrical spectrum for a 
given sample. 
It is also an object of this invention to provide means to characterize 
several samples simultaneously in order to allow the technique to adapt to 
the situation of a change in the internal structure of the material with 
the change of temperature of excitation T.sub.p. 
Analysis methods based on resonance and internal damping are "global" 
methods which do not enable elementary relaxation modes to be isolated one 
by one, and very often the apparent activation energy which is obtained 
from the Arrhenius diagrams (ln fm vs. 1/Tm) is frequently too large to be 
realistic, which reveals the fact that the internal motions observed in a 
global peak are in fact coupled and that the response of the apparatus 
results from a cooperative coupling between a plurality of relaxations 
acting globally. This implies a major drawback of the resonance and 
damping methods in their attempt to give an interpretation to the origin 
of internal motion. It should be mentioned that the thermal analysis 
processes described as prior art in the literature (TSC, TSCR) are capable 
of reconstructing the global response of the material from the thermal 
windowed experiments, and therefore calculating the resonance curves and 
damping characteristics of the material, i.e., providing the same output 
as the more traditional thermal analysis equipment. This is why the 
process of this invention is said to be capable of providing both the 
spectra of relaxation of a given material, and the characteristic 
resonance at any given frequency or temperature, by calculation from the 
spectrum of relaxation. 
An object of the present invention is to overcome the disadvantages of the 
known processes described in the prior art as thermal stimulated processes 
and the invention proposes to create an efficient process which can be 
applied in a relatively general manner to a large number of materials and 
to a large number of transitions in materials enabling the analysis to be 
carried out either to characterize mechanical, electrical, magnetic or 
electromagnetic transitions. To this end, the invention concerns a process 
of the type mentioned in the description of the prior art as thermal 
stimulated processes, but characterized in that at least two coupled 
excitation fields are applied to the sample of material analyzed. In 
accordance with a further characteristic of the invention, the excitation 
fields are selected from the group consisting of electrical excitation 
fields (ac or dc voltage), mechanical excitation fields (hydrostatic 
pressure, force, shear stress, oscillating or static), magnetic excitation 
fields, or electromagnetic excitation fields (ac or dc). According to a 
further characteristic of the invention, the variables which are used as 
output to characterize the resonance and relaxation behavior of the 
material are chosen from among the current, strain rate, the stress rate, 
and the energy flux differential to keep two crucibles at the same 
temperature. 
According to a further characteristic of the invention, if the invention 
applies to a micro calorimetric device such as a DTA or a DSC, the special 
characteristics consist in detecting the changes between at least two 
samples (one being used as a reference) which are assigned constantly the 
same temperature but which are subjected to two thermal pressures. 
As a sub-characteristic of the above-mentioned process, the pressure in the 
crucibles' chamber changes in time according to a predetermined program.

DETAILED DESCRIPTION OF THE INVENTION 
According to one of the characteristics of the invention, the temperature T 
varies according to a predetermined program which may be divided into 
several intervals, for instance five zones--Z1, Z2, Z3, Z4, Z5, as shown 
in FIG. 1A. Thus, during the analysis, the temperature evolves in a manner 
which is programmed, for instance by means of equipment capable of PID 
controls. 
During this variation of the temperature, the material sample is subjected 
to the action of at least two excitation fields P(t) and Q(t) shown 
clearly in FIG. 1B and FIG. 1C. The variation of P(t) and Q(t) is coupled 
and the coupling depends on the transition characteristics that the user 
has selected to analyze. The excitation fields vary with time t in a 
programmed manner. 
In each zone, Z1 to Z5, the evolution of each of the excitation fields P(t) 
and Q(t) is represented by the corresponding curve sections in each 
zone--P1(t), Q1(t); P2(t), Q2(t); P3(t), Q3(t); P4(t), Q4(t); P5(t), 
Q5(t). The different curve sections may be continuous or discontinuous 
depending on the material analyzed and the nature of the analysis. The 
type of the excitation fields applied to the sample may be either 
electrical, magnetic, electromagnetic or mechanical. The force exerted on 
the material might be a magnetic, mechanical or a hydrostatic force. The 
selection of the type of forces and fields depends on the nature of the 
transition which is to be characterized. 
The excitation fields are coupled in order to isolate the elementary Debye 
modes of relaxation which contribute to the global response. In a further 
embodiment of the temperature variation according to the present 
invention, the temperature varies as a function of time according to a 
program similar to those described in the thermal stimulated methods TSC 
and TSCR. Thus the sample of material analyzed according to the process 
described in the invention, is heated to an excitation temperature T.sub.p 
at which the sample remains for a length of time t.sub.p. Subsequently, 
this temperature is lowered by a few degrees in 
order to arrive at the depolarization temperature T.sub.d. The sample 
remains at this temperature for the duration of time represented in FIG. 
1A by zone Z2. At the end of this recovery period at T.sub.d, the sample 
undergoes thermal quenching in order to lower its temperature to the 
freezing temperature T.sub.o, and from this temperature a linear variation 
of temperature T=f(t) is applied. This temperature profile would be a 
characteristic of the invention in FIG. 1A, according to this embodiment 
of the invention. However, the process, according to the present 
invention, differs from the prior art in that it superimposes the effect 
of at least two excitation fields as the temperature variation program 
changes. The objective is to create a filtering of the various relaxation 
modes by means other than pure thermal-windowing effects. Variables which 
are capable of individually modifying the recovery kinetics associated 
with local internal movements in the material can be coupled in the 
process described here to create the desired filtering effect. 
Owing to the coupling between several types of excitation modes and the 
resulting effect it has on the spectrum of relaxation, the present 
invention enables the understanding and the decoupling of the interactions 
between local movement occurring in the material, such as dipolar 
relaxations, and the global movements, such as the mechanical induced or 
viscous relaxations. The new process according to the invention makes it 
possible to characterize such a coupling between the global aspect of the 
deformation and the local aspect. For example, in one embodiment of the 
invention, an electrical voltage field applied to a material at a given 
temperature is coupled up to a mechanical field applied to the material at 
the same time. The mechanical variable may be either a hydrostatic 
pressure or a stress tensor. The variable, as measured during recovery, 
may either be the electric current produced by the material during heating 
or in isothermal condition, or the strain displacement as resulting from 
recovering the effect of deforming the material at T.sub.p, or both of 
these at the same time. Coupling between applied excitation fields enables 
one to sort out the origin and the differences between the dielectric 
relaxation resulting from electrical motions and the mechanical relaxation 
due to viscous motions. The coupling laws between these internal motions 
due to either viscous causes and electronic interactive causes can thus be 
determined according to this characteristic of the invention. The 
excitation profiles P(t), Q(t) describe how to apply the excitation fields 
such as to bring the material out of equilibrium, and back to equilibrium 
as a function of T. The excitation profile of the fields may be identical 
or different so as to demonstrate one or another particular characteristic 
of the recovery kinetics. This explains why there are many types of 
profiles for the two fields variations P(t) and Q(t). Each profile of P(t) 
has to be programmed in and coupled with the program used for Q(t) in 
order to determine an excitation history, which then enables decoupling of 
both dielectric and mechanical relaxation giving rise to a global answer 
in the material. 
In another embodiment of the invention, the pressure in the measuring 
chamber of a TSC or TSCR analysis apparatus varies according to a program 
and is coupled either with the voltage field (in the case of TSC) or with 
a force field (in the case of stress of TSCR). In the case of the 
application of an electromagnetic or magnetic field to excite the 
sub-atomic structure of magnetic materials, the pressure will be coupled 
with the magnetic field itself. In the particular case of coupling 
pressure effect with another field, the pressure plays a role identical to 
temperature, in particular in the creation of a windowing effect (P.sub.d 
-P.sub.p) with the aim of isolating rheologically simple relaxation modes. 
For instance, at T.sub.d, temperature of partial recovery, the pressure 
may be increased to delay the recovery of a given set of relaxation times 
influenced by the effect of both pressure and temperature. Release of the 
pressure, still at T.sub.d, results in the recovery of the relaxation 
modes which have not yet relaxed due to the effect of pressure. In the 
case of an electrical field coupled with pressure, if the motion of the 
dipoles activated by the excitation voltage is influenced by a pressure 
effect, the pressure/voltage field coupling enables one to obtain the full 
relaxation spectrum in a much more rapid manner than for a classical 
normal TSC analysis, as described by the prior art. In other words, the 
"thermal windowing" may be carried out by other means than lowering the 
temperature. In this case here, the window is created by a pressure effect 
which offers the additional advantages of being fairly easily implemented, 
and also the window width can be very small, resulting in an increase 
resolution to resolve the elementary peaks. 
In the above illustration of a preferred embodiment of the invention, 
hydrostatic pressure is shown as a variable capable of stimulating a 
change of state in the material around a phase transition, but the 
application of a mechanical, electrical or electromagnetic vibration may 
serve the same objectives. The application of a vibration to a material 
induces changes in the value of the phase transition temperature, due to 
modification of the internal state of the material. For instance, the 
transition temperature increases as the vibration frequency increases (a 
phenomenon well known to material scientists and rheologists working with 
relaxation phenomena). The phenomenon of increasing the temperature at 
which a transition occurs for a given material is equivalent to a lowering 
of the temperature with respect to the transition temperature. In other 
words, by changing the position of the temperature of the phase transition 
at a given temperature, by vibrational means, one is able to change the 
window width between the temperature of the test and the transition 
temperature under investigation. For activated phenomena, an increase in 
the vibration frequency between two activated levels is equivalent to a 
decrease in temperature. Again, this effect may be used to define the " 
windows" similar to those created with the thermal windowing method used 
by the prior art. Coupling thermal windowing effects (which are created by 
pure changes of temperature) and "frequency or pressure simulated" windows 
(created by the action of a vibrating field or by the effect of changing 
the hydrostatic pressure) enables one to characterize the local motions 
with respect to their origin, whether it be of viscous or electronic 
interactive nature. 
It should be noted that the nature of the vibration applied during the 
excitation state (either at temperature T.sub.p or during the recovery at 
T.sub.d) may be identical or different from the nature of the static 
field, applied in conjunction to it in order to create the coupling, and 
that the detecting variable during the sensing stage (during the 
programmed rise in temperature to reveal a relaxation mode in the recovery 
zone) may be of the same nature as the vibratory variable or the static 
field. For example, it is possible, in another embodiment of this 
invention, to use coupling between a mechanical field (hydrostatic 
pressure or a shear stress applied during a time t.sub.p at temperature 
T.sub.p) with a vibratory field of electrical nature or of electromagnetic 
nature (with a predetermined frequency and amplitude of vibration), that 
vibratory excitation being applied at T.sub.p or at T.sub.d for a 
programmed time. The recovery curve may be studied either with electric 
variables (in such a case a measurement of the depolarization current is 
performed), or with a mechanical variable (variation of the strain and 
strain rate during recovery), or lastly with a purely thermal measurement 
(measurement of the heat capacity changes during recovery). 
In a particularly important embodiment of the invention, heat sensing means 
such as those used in a DSC or a DTA are used to detect motions during the 
recovery stage, after an initial excitation stage which comprises thermal 
and pressure windowing to filter out singular relaxation modes. The cell 
chamber which includes the samples to be analyzed is divided into two 
compartments, one at pressure P.sub.1, the other at pressure P.sub.2. The 
two compartments are strictly at the same temperature irrespective of the 
temperature program T in FIG. 1A, or the pressure in each compartment, 
whether this is during the excitation or the recovery phase. In a 
particular embodiment of the above arrangement, only two crucibles are 
located in each compartment, one of the crucibles in each compartment 
containing a sample of the material to be analyzed. 
It is believed that those skilled in the art will understand how to adapt a 
DSC or DTA apparatus as described in the forgoing paragraph and, 
accordingly, an illustration of such a modified apparatus is believed 
superfluous. 
In a variant of the previous embodiment, the compartments may contain a 
plurality of crucibles, each containing a sample of the material to be 
analyzed in addition to a control reference sample. This configuration is 
particularly suitable for studying physical aging phenomena and curing or 
crosslinking or crystallization phenomena, or for studying the state of 
internal stresses in the material. Note that in this embodiment of the 
invention, a single run will provide the measurement of several samples at 
once and submit it to the same temperature variation. 
The rate of change of the microcalorimetric differentials between the 
several samples and between the crucibles are automatically recorded 
regardless of the compartment they are in and the temperature or pressure 
which is programmed to vary. The temperature and pressure variations 
inside the cell chambers are programmed by a computer to create windowing 
effects which make it possible to separate out the singular relaxation 
modes, provided that the relation occurring by the change of temperature 
or pressure result in a modification in the heat capacity or the enthalpy 
of the material. The microcalories supplied to the crucibles may be 
compared for the crucibles located inside the same compartment or for 
crucibles containing samples of identical origin but located in two 
different compartments and therefore at different pressures. The analysis 
of the enthalpy difference leads to the characterization of the 
distribution of enthalpies attributed to a spectrum of relaxation modes. 
In another embodiment of the invention, the pressure may be programmed in a 
specific way to rapidly study the kinetic characteristics of a pressure 
sensitive phase transition, such as the glass transition temperature of 
glass forming materials. The action of hydrostatic pressure on the 
material may be used to "create" at will a transition effect, since the 
transition itself occurs at a higher temperature if the pressure is 
suddenly increased in the chamber. The sample, which is slowly heated up, 
is subjected to rapid pressurization (simulating quenching across the 
transition under investigation), resulting in states across the transition 
temperature, and subsequently depressurized at a controlled rate in order 
to analyze the kinetic curve of the change of state during the return to 
equilibrium, since the effect of relaxing the pressure will be to cross 
the transition in the other direction, giving an opportunity to record the 
kinetic changes occurring during this partial return to equilibrium. The 
temperature changes during that process can be slow enough to be 
considered constant and therefore the process can correspond to the study 
of a recovery return to equilibrium under isothermal conditions. This 
process of pressurizing and relaxing may be performed a number of times 
during the slow increase in temperature in the chamber, thus providing a 
series of kinetic relaxation curves which can be analyzed with the 
classical tools of rheology and relaxation kinetics. 
In a further embodiment of the invention, for coupling a mechanical field 
and a hydrostatic pressure, a plurality of samples to be compared may be 
introduced at the same time into the cell in order to be subjected 
simultaneously to the same pressurizing programs and the temperature 
variation programs. The responses obtained simultaneously for the various 
samples during recovery enable the differences existing initially in the 
materials to be compared very quickly and in a single operation. This is 
particularly useful for studying the internal stresses set in an object, 
for which these stresses vary from one point of the object to the other 
owing to molding conditions. For example, in the process used to 
manufacture compact discs or optical discs by injection molding, it is 
important to eliminate internal stresses in the direction parallel to the 
reading laser beam. It is thus of prime importance that the material 
properties do not vary over time and from point to point in the radial 
direction. This particular embodiment of the invention can be used for the 
simultaneous study of a plurality of samples in order to determine an 
internal stress intensity curve. 
In a still further embodiment of the invention, the excitation field 
applied during the windowing process is electromagnetic or corpuscular, 
for example luminous or sub-radiating (X ray, gamma rays, UV radiation 
etc.). This excitation mode may be more suitable for the analysis of thin 
layers of conductor or semi-conductor materials, such as in the 
characterization of the electronic behavior of the amorphous component, in 
particular for testing the structure of energy which the global energy is 
composed of. 
One characteristic of the invention is that the same cell base which is 
normally used for Thermally Stimulated Current (TSC) measurement can also 
be used for the apparatus carrying out the means necessary for the present 
invention. This represents a clear advantage of this embodiment since the 
same apparatus (with a few modifications) can be used for simultaneous 
excitation and measurement of thermally stimulated mechanical or 
electrical relaxation occurring in samples, in order to characterize their 
internal motion. 
In FIG. 1A, the temperature profile is programmed via heating and cooling 
the conducting gas which constitutes the sample environment. When P(t) or 
Q(t) of FIG. 1A represents the pressure in the chamber, the conducting gas 
is put under pressure by external means known to a person skilled in the 
art. A preferred range for the pressure is between 1 bar and 700 bars, and 
obviously, the thickness of the wall of the cell is modified accordingly 
in order to accommodate the larger pressures. 
In cases where the signal P(t) or Q(t) is a stress imposed on the sample, a 
load cell is located outside the cell assembly and is controlled by 
computer means 12 via the load cell sensor. The stress applied can be a 
torsion, or a flat force, and the displacement can be measured by optical 
means (transmitted light), or by means of a LVDT, RVDT, Moire fringes, or 
capacitance. 
The application of the stress on the sample holder can be done by means of 
a stepper motor. The range of modulus preferred for the material studied 
here goes from 102 to 1011 dyn/cm.sup.2. A preferred embodiment of the 
invention is to use the stepper motor in direct connection to the sample, 
which provides the stress on the sample. The strain induced by the stress, 
and the change of strain during recovery (strain rate) are best measured 
by means of an encoder disc with a dual laser counting. A preferred 
embodiment for the encoder consists of an optically polished metal with 
laser etched markings with a predetermined resolution (typically 10 .mu.). 
The encoder disk is preferably of minimum thermal mass, must be supported 
without friction, and must be able to withstand the heat generation during 
laser marking as well as the heat that may be transported from the heater 
area up through the central high modulus shaft to the strain detecting 
encoder disc. 
The temperature of use is preferably between -150.degree. C. to 350.degree. 
C., (with liquid nitrogen as the coolant) or -260.degree. C. to 0.degree. 
C. (with liquid helium). 
The preferred current detector, when P(t) or Q(t) is a voltage, is an 
electrometer capable of measuring current as low as 10-17 amperes up to 
10-9 amperes. 
The excitation of the two coupled signals P(t) and Q(t) is done 
simultaneously by programming before the experiment the ramps of the 
signals at given predetermined intervals. Each of the signals (electrical 
power supply ac and/or dc) and stresses (torsion or flection, or 
compression) is electronically sensed, conditioned and controlled by PID 
means to conform to the programmed variation. Such procedure is known to 
persons skilled in the art of PID controls. A computer is used to record 
the outputs from the sensors and send signal to the exciters. The data 
(current of polarization, depolarization, stress on the sample, strain 
(angular, longitudinal or vertical depending on the type of stress), and 
its derivative strain rate, are continuously computed and archived on the 
computer storage medium. The analysis of the data is done as either strain 
rate versus temperature for the mechanical deformation, or current versus 
temperature for the electrical signal. The measurements are done 
simultaneously. The direct outputs provide direct information on the 
resonance characteristics, either mechanical or electrical, of the 
material at the equivalent frequency of excitation. 
The relaxation spectrum can be calculated by using the method of 
thermal-windowing, as explained in the prior art, depending on the 
function T(t) in FIG. 1A. 
When ac signals are used for P(t) and/or Q(t), the detecting devices can be 
such that the ac response of the material is continuously compared with 
the ac excitation, in order to, during the analysis stage, obtain the 
variation of the storage and loss moduli and dielectric constant. The use 
of ac signals during the excitation stage serves, however, another 
purpose, in the present invention, since its use is primarily to induce 
windowing effects which will be revealed in the sensing stage, upon 
recovery when the sample is heated up linearly at the end of the 
experiment. The use of ac signals is to serve as an additional windowing 
technique, in the sense described herein. The use of hydrostatic pressure 
serves the same purpose. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification, as indicating the scope of the invention.