WO1996018886A1

WO1996018886A1 - Multiharmonic calorimetric spectroscopy

Info

Publication number: WO1996018886A1
Application number: PCT/IT1994/000210
Authority: WO
Inventors: Giuseppe Salvetti; Elpidio Tombari; Davide Bertolini
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 1994-12-13
Filing date: 1994-12-13
Publication date: 1996-06-20
Also published as: AU1279495A

Abstract

A method of calorimetric spectroscopy is described including the use of excitation signals being the sum of a constant level plus a multiplicity of signals at frequency wi, which are selected at will, with the sole limitation of being harmonics of a fundamental frequency wo; and the analysis of the response signal in terms of the discrete Fourier transform at the excitation frequencies wi and at their second harmonics 2wi.

Description

DESCRIPTION

"MULTIHARMONIC CALORIMETRIC SPECTROSCOPY"

Technical Field

Until a few years ago, the thermal capacity Cp and the thermal conductivity k were considered to be physical observable quantities of systems in equilibrium; consequently, the measurement techniques developed and currently available are of static type, in which it is sought to maintain the specimen under conditions close to those of equilibrium. Recently, it has been demonstrated that Cp and k are dependent upon the pulsation frequency w of the power transmitted to the specimen during the experiment. This has revealed the dynamic nature of Cp and k, which can be associated with the relaxation processes internal to the specimen.

Background Art

Two calorimetric techniques are currently known, operating in the field of frequency. Both operate at a single frequency; one utilizes a thermoresistive wire directly immersed in the specimen, which acts at the same time as heater and as sensor of the calorimeter; the other is a modification of conventional differential scanning calorimetry (DSC) , in that it superposes upon the scanning signal a modulation at a certain frequency. The first technique can be utilized only under particular conditions (liquids which can be recovered from the measurement cell) as a result of which it exhibits significant limitations of application capability. The second is proposed principally as a method for the simultaneous measurement of Cp and of the process enthalpy during a transformation of the specimen.

Disclosure of Invention

The present invention permits the performance of the calorimetric investigation of any specimen (including thermosetting polymers) in the field of frequency, giving:

Cp (w) or k (w) at a set of different frequencies w. by a single measurement;

Cp and k simultaneously, in that area of the spectrum in which their dependence upon the frequency is negligible.

In both cases, a further possibility is the simultaneous measurement of the process enthalpy, where a chemical transformation or a transformation of physical state takes place within the specimen.

The calorimetric method which has been devised is innovative in that:

A) it makes use of; excitation signals which are the sum of a constant level plus a multiplicity of signals and frequencies selected at will, with the sole limitation of being harmonics of a fundamental frequency; analysis of the response signal in terms of the discrete Fourier transform at the excitation frequencies w. and at their respective second harmonics 2w. ,

B) it makes use of: representation of the calorimetric cells in terms of equivalent electrical circuits, linear response and capability of application of the principle of superposition,

C) it permits the repeatability of the measurement at time intervals equal to the period corresponding to the fundamental frequency selected.

In practice, having available a specimen cell and a reference cell, the method concerned provides the simultaneous transmission of signals, superposition of a plurality of frequencies, both on the specimen cell and on the reference cell; there is then provided the analysis, i.e. the deconvolution of the signal, by means of the discrete Fourier transform at the excitation frequency w. and at the harmonics 2w.. Advantageously, there is provided the use of cells of plane or cylindrical geometry.

The optimal frequency range is defined by the following relation l/τ_Q < w < l(τ_i in which T and τ . are respectively the characteristic times for the transmission of the heat from the calorimetric cell to the thermal bath in which it is immersed and from the heater to the specimen inside said cell. The measurement technique is applicable to various configurations of calorimetric cells (cylindrical symmetry, plane, etc.). By way of example, one of its applications is a cell such as that according to EP-A_0 532 480 or the corresponding US-A-5.295.745 (the content of which is incorporated in the present description) , which is particularly suitable, it being possible therein to vary broadly the values so and si which determine the operative frequency range thereof.

The instrumentation is substantially the same as that according to Italian Patent No. 1,216,296 (Application No. 9528 of 21/11/86) , and/or Italian Patent Applications No. 9471 A 90 of 23/8/90 and EP-A-0 532 480. The content of said monopoly rights is incorporated in the present description. The apparatus comprises a calorimetric head the temperature of which is controlled, containing two twin cells in differential configuration. On both the cells there are present a heating element which is utilized to transmit the input signal and a thermoresistor element which is utilized as sensor, the resistance value of which, being proportional to the temperature variations,^* is measured by means of a Wheatstone bridge circuit. The cell containing inside it the specimen and on the walls the heater and the sensor is immersed in a suitable inert gas which determines the rate of dissipation of the heat from the cell to the thermal bath, which is represented by the measurement head maintained at constant temperature. It is understood that the drawing shows only an exemplification given only by way of practical demonstration of the invention, it being possible for this invention to vary in forms and arrangements without nevertheless departing from the scope of the concept which forms said invention.

Claims

1. Method of calorimetric investigation (multiharmonic calorimetric spectroscopy) , characterized in that it includes the use of excitation signals being the sum of a constant level plus a multiplicity of signals at frequencies w., selected at will, with the sole limitation of being harmonics of a fundamental frequency, and characterized by the analysis of the response signal in terms of the discrete Fourier transform at least at the excitation frequencies w..

2. Method according to Claim 1 , characterized in that the response signal is also analysed in terms of the discrete Fourier transform at the second harmonics 2w. of the excitation frequencies, to subtract the effects of the enthalpy release on the thermal capacity Cp (w) .

3. Method according to Claim 1 or 2, characterized in that it adopts: the representation of the calorimetric cells in terms of equivalent electrical circuits; and the linear response and the application capability of the principle of superposition.

4. Method according to Claim 1, 2 or 3, characterized in that it includes the repeatability of the measurement at time intervals equal to the period corresponding to the fundamental frequency w selected.

5. Method according to one or more of the preceding claims, characterized: by the simultaneous transmission of signals, superposition of a plurality of frequencies, on the specimen cell and on the reference cell; and by the analysis or deconvolution of the signal by means of the discrete Fourier transform at the excitation frequency w. and at the harmonics 2w..

6. Method according to one of Claims 1 to 5, characterized by the use of cells of plane or cylindrical geometry.

7. Method for the calorimetric investigation of a specimen - including thermosetting polymers - in the field /18886 PC--7IT94/00210

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of frequency, characterized in that the thermal capacity Cp(w) or the thermal conductivity k(w) is provided at a set of different frequencies w. by a single measurement.

8. Method for the calorimetric investigation of a specimen in the field of frequency, characterized in that Cp and k are provided simultaneously, in the area of the spectrum in which their dependence upon the frequency is negligible.

9. Method according to Claim 7 or 8, characterized in that it includes a simultaneous measurement of the process enthalpy, where a chemical transformation or a transformation of physical state takes place within the specimen.