Method and device for measuring the electromagnetic properties of a sample heated by microwaves

Abstract

The present invention relates to a device and a method for acquiring at least one electromagnetic property as a function of temperature, of a sample (10) placed in a resonance cavity (21) and heated by a microwave generator (MWG). A control unit (ECU) is configured to control means (50, 30) for adjusting the resonance frequency or the cutoff frequency range (fc) of said cavity as a function of the frequency (fg) of the waves generated and established in the cavity by the microwave generator (MWG). Figure 1

Description

Title of the invention: Method and device for measuring the electromagnetic properties of a sample heated by microwaves

[0001] The present invention relates to a device and a method for acquiring at least one measurement of electromagnetic property as a function of temperature, of a material arranged in a resonance cavity and heated by microwaves.

[0002] It is known from the state of the art to use the resonant cavity perturbation method to determine the electromagnetic properties of materials.

[0003] The resonant cavity method is based on a comparative analysis of certain electromagnetic characteristics between said vacuum cavity and said cavity partially loaded by the material to be characterized.

[0004] More specifically, a comparative analysis is carried out of the total energy stored in the cavity before and after the disturbance.

[0005] However, this resonant cavity method is generally used to characterize materials around room temperature (20°C), or as a function of temperature via a heating process separate from the electromagnetic excitation and measurements. This leads to significant approximation errors, because the transfer and adjustment times of the samples to be characterized, between the heating station and the electromagnetic excitation and measurement station, are not taken into account.

[0006] As a result, possible resulting temperature variations are neglected, leading to measurement errors.

[0007] Furthermore, the dimensional physical state of the materials is considered to be invariable because an oblong measuring sample is positioned in the cavity such that the electric field is parallel to the longest dimension, whereas during the phase change this dimension ratio changes.

[0008] As a result, dimensional variations of the materials, mainly during phase change, are also neglected, also inducing measurement errors.

[0009] This is why the invention proposes to resolve the drawbacks set out above by combining in a single station electromagnetic heating under microwaves and the measurement of electromagnetic properties.

[0010] The invention relates in particular to a device for acquiring at least one measurement of an electromagnetic property as a function of the temperature, of a sample placed in a resonance cavity and heated by microwaves, the device comprising:

[0011] - said resonance cavity itself equipped with frequency adjustment means resonance or cutoff frequency range of said cavity,

[0012] - a microwave generator and means of transmission, recovery and analysis of reflected waves, each connected to a switch itself connected to said resonance cavity,

[0013] - means forming a waveguide arranged between the resonance cavity and the switch,

[0014] - means for measuring the temperature of the sample,

[0015] - a control unit configured to control the adjustment means of the resonance frequency or the cut-off frequency range of said cavity as a function of the frequency of the waves emitted (and more particularly of the waves generated and established in the cavity) by the microwave generator, as well as the microwave generator, the means of transmission, recovery and analysis of the reflected waves, and the switch as a function of the temperature measurements taken by the measuring means.

[0016] Optional, complementary or substitutive features of the invention are set out below.

[0017] According to certain characteristics, the means for adjusting the resonance frequency or the cut-off frequency range of the resonance cavity may advantageously comprise a short-circuit piston.

[0018] According to other characteristics, the means for adjusting the resonance frequency or the cut-off frequency range of the resonance cavity may advantageously comprise a capacitive and inductive iris.

[0019] According to a preferred aspect, the means for transmitting and recovering the reflected waves may comprise a first RF power measuring probe.

[0020] According to another preferred aspect, the means for analyzing the reflected waves may comprise a network vector analyzer.

[0021] Advantageously, the temperature measuring means may consist of an infrared measuring device such as a fiber optic pyrometer.

[0022] Also advantageously, the means forming a waveguide are equipped with an impedance adapter, preferably provided with at least two resistive screws.

[0023] Preferably, the acquisition device may comprise a second RF power measuring probe disposed in the resonance cavity.

[0024] The invention also relates to a method for acquiring a series of measurements of an electromagnetic property of a sample heated by microwaves as a function of the temperature, implementing a device for measuring at least an electromagnetic property in accordance with an embodiment of the invention, characterized in that it comprises a cycle of successive steps defined as follows:

[0025] - El: the control unit regulates the frequency and duration of heating of the generator microwave,

[0026] - E2: the control unit activates the heating of the sample by the generator of microwave according to the set frequency and duration,

[0027] - E3: the control unit activates the temperature measuring means of the sample at the end of the set time, to collect a temperature measurement,

[0028] - E4: the control unit controls the adjustment means to adjust the frequency of resonance or the cut-off frequency range of the cavity to a value, depending on the frequency of the waves generated and established in the cavity by the microwave,

[0029] - E5: the control unit activates the communication of the switch with the means of transmission and recovery and analysis of reflected waves,

[0030] - E6: the control unit activates the acquisition then the analysis of at least one parameter of reflected waves,

[0031] - E7: the control unit converts the at least one parameter to obtain the at least one parameter. least one electromagnetic property.

[0032] Optional, complementary or substitutive features of the invention are set out below.

[0033] According to one aspect of the invention, the control unit iteratively implements a plurality n of cycles to obtain a series of n measurements of the electromagnetic property.

[0034] According to another aspect of the invention, the means for transmitting and recovering the reflected waves of the device for measuring at least one electromagnetic property, comprise an RF power measuring probe, and the at least one parameter of the reflected waves is the reflection coefficient, the at least one electromagnetic property being the permittivity or the complex permeability.

[0035] Advantageously, step E7 of converting the parameter to obtain the electromagnetic property comprises a mathematical filter making it possible to eliminate the impact of the sample support on the disturbance of the resonance cavity.

[0036] According to yet another aspect of the invention, the frequency range emitted by the microwave generator extends between 800 MHz and 80000 MHz.

[0037] Other advantages and particularities of the invention will appear on reading the detailed description of implementations and embodiments which are in no way limiting, and the following appended drawings:

[0038] [Fig. 1] This figure represents a schematic view of a device for measuring the electromagnetic properties of a sample heated by microwaves according to one embodiment of the invention.

[0039] [Fig.2] This figure represents a schematic view of the resonance cavity of said device.

[0040] [Fig.3] This figure represents a schematic view of a detail of the resonance cavity of said device.

[0041] [Fig.4] This figure represents curves of results of measurements of electrical properties electromagnetic as a function of temperature.

[0042] Figures 1 and 2 represent a device for acquiring at least one measurement of electromagnetic property as a function of the temperature, of a sample 10 placed in a resonance cavity 21 and heated by microwaves, according to the invention.

[0043] The device comprises a resonator 20 comprising a resonance cavity 21 and means forming a waveguide 22 arranged between the resonance cavity 21 and a COMUT switch.

[0044] The resonance cavity is equipped with means 50, 30 for adjusting the resonance frequency or the cut-off frequency range of said cavity.

[0045] The device also comprises a microwave generator MWG at frequencies fg, transmission and recovery means 90 and analysis VNA of the reflected waves, each connected to the COMUT switch itself connected to the resonator 20.

[0046] The device also comprises means 60 for measuring the temperature of the sample.

[0047] The device also comprises a control unit ECU configured to control the adjustment means 50, 30 of the resonance frequency or the cut-off frequency range fc of said cavity as a function of the frequency of the waves emitted (and more particularly of the waves generated and established in the cavity) by the microwave generator MWG.

[0048] Thus, the adjustment means 50, 30 make it possible to have a single monomode standing wave in the resonance cavity.

[0049] The ECU control unit is also configured to control the microwave generator MWG, the transmission and recovery means 90 and VNA analysis of the reflected waves, and the COMUT switch.

[0050] The ECU control unit is also configured to control the temperature measuring means 60 and store the temperature readings.

[0051] The VNA means are used to analyze in particular the SU reflection coefficients of the reflected waves.

[0052] Thus, the acquisition device combines both in situ microwave electromagnetic heating and measurement of electromagnetic properties. The measurement and heating are carried out via a single input 90 (also called a “port”) on the com COMUT mutator. A high-precision switch was used at the input 90 of the resonator 20 allowing rapid switching with a minimum speed largely sufficient for monitoring a thermo-electromagnetic phenomenon between the microwave generator MWG, and the transmission and recovery means 90 and VNA analysis of the reflected waves.

[0053] Advantageously, the means for adjusting the resonance frequency or the cut-off frequency range fc of the resonance cavity 21 comprise a short-circuit piston 50. Thus, the control unit ECU makes it possible to control the state of the switch COMUT between the transmission of microwave power coming from the generator and the acquisition 90 and the VNA analysis of the reflected waves while positioning the short-circuit piston at the distance adapted to each state of acquisition and analysis.

[0054] Also advantageously, the means for adjusting the resonance frequency or the cut-off frequency range fc of the resonance cavity also comprise a capacitive and inductive iris 30, which couples the resonance part 21 with the means forming a waveguide 22.

[0055] As shown in [Fig.3], the capacitive and inductive iris 30 makes it possible to modify the coupling inductance and capacitance jB. The iris 30 comprises a set of tabs 32 making it possible to adjust the size of the slot made through the surface 31 of contact with the copper waveguide flange. Ferromagnetic ceramic elements surround the two flanges, that on the waveguide side, and that on the cavity side.

[0056] The iris acts as a parallel impedance-capacitance (LC) circuit. By reducing the local width of the waveguide, perpendicular to the magnetic field (H), the iris modifies the magnetic field (inductive iris) and by modifying the local height of the waveguide, the edges of the iris, which act as a capacitance, are perpendicular to the electric field (E). The electric field is then modulated at the discontinuity. This is called a capacitive iris.

[0057] The adaptation of the resonance frequency or the cut-off frequency range of the resonance cavity between heating on the one hand and acquisition and analysis on the other hand, is carried out thanks to the iris and the short-circuit piston integrated into the resonance cavity. This adaptation of the frequency is essential in order to constantly position the peak of the electric field on the sample 10.

[0058] The variation of the electromagnetic properties, under the effect of heating, in fact induces a slight deviation in the distribution of the electric field, which must be corrected by the short-circuit piston and the iris during acquisition and analysis.

[0059] According to a preferred embodiment, the means 90 for transmitting and recovering the reflected waves comprise a first RF power measuring probe, the end of which is a connection point of an RF cable with the switch COMUT. The first 90 RF power measuring probe provides access to the SI 1 reflection coefficient.

[0060] Advantageously, the acquisition device comprises a second RF power measurement probe 91 arranged in the resonance cavity 21 behind the sample 10, so that the sample is arranged between the second probe 91 and the waveguide 22. The second RF power measurement probe 91 makes it possible to access the transmission coefficient S21 and S12.

[0061] The joint use of two RF power measuring probes thus makes it possible to access the transmission and reflection coefficients SI 1, S21 and S12 at the same time. The transmission coefficient provides an additional check for the acquisition of electromagnetic property measurements.

[0062] Preferably, the VNA analysis means of the reflected waves comprise a vector network analyzer, the network analyzer making it possible to measure the modulus and the phase of the S parameters of a quadrupole.

[0063] Thus, the VNA analysis means make it possible to quantify the transmission coefficient SI 1 in the reflected wave.

[0064] Depending on whether the second RF power measuring probe 91 is connected to the ECU or to the VNA analysis means, the ECU or the VNA analysis means make it possible to quantify the transmission coefficients S12 and S21 in the transmitted signal.

[0065] The temperature measuring means 60 are based on an Infrared (IR) measuring device and in this case comprise a fiber optic pyrometer. Thus, the temperature measurement time is sufficiently fast so as not to generate a lag between the temperature measurement and the acquisition of the electromagnetic properties.

[0066] The means forming a waveguide 22 are advantageously equipped with an impedance adapter 40.

[0067] This impedance adapter is advantageously composed of at least two resistive screws (called stubs in English), adjustable in depth and which have the role of reducing the level of reflected power and adapting the transmission line (for example to 50 Ohm or to 75 Ohm), thanks to a micrometric adjustment.

[0068] The method for acquiring at least one measurement of an electromagnetic property ei of a sample 10 heated by microwaves as a function of the temperature, implementing a device according to the invention will now be described.

[0069] The method implements at least one cycle Ci which comprises the following successive steps, and which makes it possible to acquire a measurement of an electromagnetic property e; at the temperature Ti.

[0070] In step E1, the control unit ECU sets the frequency fg; and the duration d; of heating of the microwave generator MWG.

[0071] This adjustment of the frequency fg; and of the duration d; of heating of the microwave generator MWG, is carried out via a servo-control system based on the MPC method of predictive control. This makes it possible to determine the necessary duration of irradiation by the EM power, while analyzing the system "feedback", in other words the target temperature. This method is not exclusive and can be replaced by another method based on the choice of the permutation frequency between two successive acquisitions.

[0072] In step E2, the control unit ECU activates the heating of the sample 10 by the microwave generator MWG according to the frequency fg; and the duration d,, which were set in the previous step.

[0073] In step E3, the control unit ECU activates the means 60 for measuring the temperature of the sample 10 at the end of the duration d,, to collect a temperature measurement Tj.

[0074] In step E4, the control unit ECU controls the adjustment means 50, 30 to adjust the resonance frequency or the cut-off frequency range fc of the cavity to a value fc;.

[0075] This adjustment can be carried out by sweeping the short-circuit piston in both directions, within a limited interval relative to the initial position (for example [-5.5] mm for the WR340 cavity), until the resonance quality factor exceeds a limit value (45000 for this example). In case of failure, the previous step is repeated while widening the transverse direction of the iris.

[0076] The cavity has a total length equal to 1.5 of the guided frequency of the chosen waveguide standard (3 / 2 Xg), while the short circuit piston has a useful stroke of 1.2 Xg.

[0077] In step E5, the control unit ECU activates the communication of the switch COMUT with the transmission and recovery means 90 and VNA analysis of the reflected waves. Thus, the wave (or pulse) emitted by the VNA analysis means, then reflected, can be recovered by the transmission and recovery means 90 and analyzed by the VNA.

[0078] In step E6, the ECU control unit activates the acquisition then the analysis of at least one parameter Si of the reflected waves.

[0079] In step E7, the control unit ECU converts the at least one parameter Si to obtain the at least one electromagnetic property e;.

[0080] By iteratively implementing a plurality n of cycles Ci, the control unit ECU proceeds to progressively heat the sample 10 and acquires, as a function of the evolution of the temperature T; for each cycle, the measurements of the electromagnetic property e;.

[0081] In the case where the means of transmission and recovery 90 of the reflected waves of the device include an RF power measuring probe, the parameter If reflected waves is the reflection coefficient.

[0082] By means of a calculation code integrated into the ECU control unit program, this parameter is converted into the permittivity or complex permeability of the material tested. A series of permittivity measurements are then obtained, as shown in [Fig.4], using a plurality of cycles.

[0083] The conversion of the parameter Si into permittivity or complex permeability of the material tested can be carried out according to the principle of calculating e' and e” from the reflection coefficients (or quality factor) as follows:

[0084] Sr -^- +

[0085]

[0086] l'r~ 4vs

[0087] HAS /

[0088] Where: £r: complex permittivity / 0: signal frequency at which the gain is maximum [Hz] / : empty cavity frequency [Hz] f: sample-perturbed cavity frequency [Hz] A / ; dominant signal bandwidth [Hz] Vv: empty cavity volume [m3] V4: sample volume [m3] Q: Empty cavity quality factor Q: Filled cavity quality factor

[0089] Advantageously, step E7 of converting the parameter (Si) to obtain the electromagnetic property (e;) comprises a mathematical filter making it possible to eliminate the impact of the support of the sample 10 on the disturbance of the resonance cavity 21. This makes it possible to avoid the calibration step using an empty container.

[0090] For this, a mixing law is introduced in order to link the dielectric characteristics of the sample and that of the sample holder, weighted by their respective volumes, and to define a single equivalent material in the disturbance of the cavity.

[0091] By applying the two approximations of Maxwell Gamett and Bruggeman for an effective medium (here the cylindrical assembly of sample and sample holder), the mixing law is expressed as follows:

[0092] _ j AW(2-A)4) eff~

[0093]

[0094] Where:

[0095] £eff ; effective permittivity of the sample assembly and the sample holder, ee : permittivity of the sample, eP : permittivity of the sample holder, / e ; radius of the sample, rP : radius of the sample holder, he ; height of the sample, hp ; height of the sample holder.

[0096] The frequency range fg emitted by the microwave generator extends between 800 MHz and 80000 MHz. The frequency range studied can vary between 1.7 and 3 GHz for a WR340 standard and between 750 and 1120 MHz for a WR975 standard.

[0097] The device and the method according to the invention make it possible to study and measure with high precision the variation of the electromagnetic properties throughout the heating and the change of state (melting, in particular) on materials with low dielectric value (in particular organic and inorganic polymers, ceramics and any liquid composed in particular of organic molecules). Some examples of dielectric measurements at a temperature of 27°C and the monitoring of the permittivity measurement of polymer (in this case Polycaprolactone PCL), varying as a function of temperature, are presented in Table 1 below as well as in [Fig.4].

[0098] Table 1 below attests to the reliability and precision of the device and method according to the invention when comparing the permittivity value of some known dielectrics and the measurement carried out using the device and method according to the invention. Material Reference value in literature. At room temperature Measurement carried out via this device at 27°C. e' e" e' e" PTFE 2-2.1 0.0001-0.0005 2.004119603 2.29e-4 PEEK 3.1 0.0053 3.1061877451 6.lle-3 Alumina (Ceramic) 9.89 0.00198 9.8156959515 0.00185

[0099] The advantages of the invention can be summarized in four axes, namely:

[0100] - The possibility of synchronized and concatenated measurement between microwave heating (temperature and phase or state variation) and the complex reflection coefficient in the same device without resorting, throughout the test, to manipulation and movement of study samples which would cause errors and approximations;

[0101] - the possibility of carrying out measurements over a wide frequency range and variable, while keeping the possibility of targeting narrower ranges depending on the chosen guidance standard (for example WR340 or WR975, which are standards defining adapted dimensions guaranteeing a single-mode propagation mode, according to the desired frequency ranges), but widely used in industry and research laboratories (civil domain) for microwave heating, which is essential for all wave-material interaction studies, whether numerical or experimental in order to be able to model the heating process and estimate the temperature fields within polymers treated under microwaves;

[0102] - the possibility of using a mathematical model of mixing based on laws of mixing between sample holder and sample, in order to define a single equivalent medium, in order to eliminate the participation of the measuring container in the disturbance of the cavity whereas in most existing devices this step has been carried out by means of a vacuum calibration;

[0103] - the controlled and automated adaptation of the waveguide, makes it possible to ensure the position maximum electromagnetic field on the sample being measured in order to better correlate the dielectric measurement to the measurement point temperature, compensating for the deviation of the electromagnetic field due to the variation in sample permittivity and permeability.

[0104] The device and the method according to the invention find their application in the fields of radiofrequency, piezoelectricity, optics, polymerization, esterification, chemical reactions in particular induced by microwave heating, recycling of plastic products by selection of properties.

[0105] The embodiments described below being in no way limiting, it will be possible in particular to consider variants of the invention comprising only a selection of described characteristics, isolated from the other described characteristics (even if this selection is isolated within a sentence comprising these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one characteristic, preferably functional without structural details, or with only a part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0106] Note that the various features, forms, variants and embodiments of the invention may be associated with each other, in various combinations. to the extent that they are not incompatible or mutually exclusive.

Claims

Claims

1. Device for acquiring at least one measurement of an electromagnetic property as a function of the temperature, of a sample (10) arranged in a resonance cavity (21) and heated by microwaves, the device comprising: - said resonance cavity (21) itself equipped with means (50, 30) for adjusting the resonance frequency or the cut-off frequency range (fc) of said cavity, - a microwave generator (MWG) at frequencies (fg) and means for transmitting and recovering (90) and analyzing (VNA) the reflected waves, each connected to a switch (COMUT) itself connected to said resonance cavity (21), - means forming a waveguide (22) arranged between the resonance cavity (21) and the switch (COMUT), - means (60) for measuring the temperature of the sample, - a control unit (ECU) configured to control the adjustment means (50,30) of the resonance frequency or the cut-off frequency range (fc) of said cavity as a function of the frequency fg of the waves emitted by the microwave generator (MWG), as well as the microwave generator (MWG), the means of transmission and recovery (90) and analysis (VNA) of the reflected waves, and the switch (COMUT) as a function of the temperature measurements taken by the measuring means (60).,

2. Device according to claim 1, characterized in that the means for adjusting the resonance frequency or the cut-off frequency range (fc) of the resonance cavity comprise a short-circuit piston (50).

3. Device according to claim 1 or 2, characterized in that the means for adjusting the resonance frequency or the cut-off frequency range (fc) of the resonance cavity comprise a capacitive and inductive iris (30).

4. Device according to any one of the preceding claims, characterized in that the transmission and recovery means (90) reflected waves comprise a first RF power measuring probe.

5. Device according to any one of the preceding claims, characterized in that the means for analyzing (VNA) the reflected waves comprise a vector network analyzer.

6. Device according to any one of the preceding claims, characterized in that the temperature measuring means (60) consist of an Infrared (IR) measuring device such as a fiber optic pyrometer.

7. Device according to any one of the preceding claims, characterized in that the means forming a waveguide (22) are equipped with an impedance adapter (40), preferably provided with at least two resistive screws.

8. Device according to any one of claim 4, characterized in that it comprises a second RF power measuring probe arranged in the resonance cavity (21).

9. Method for acquiring a series of measurements of an electromagnetic property (e;) of a sample (10) heated by microwaves as a function of the temperature, implementing a device for measuring at least one electromagnetic property according to any one of the preceding claims, characterized in that it comprises a cycle (Ci) of successive steps defined as follows: - El: the control unit (ECU) sets the frequency (fg;) and the duration d; of heating of the microwave generator (MWG), - E2: the control unit (ECU) activates the heating of the sample (10) by the microwave generator (MWG) according to the frequency (fg;) and the duration (d;), - E3: the control unit (ECU) activates the measuring means (60) of the temperature of the sample (10) at the end of the duration d,, to collect a temperature measurement (T)), - E4: the control unit (ECU) controls the adjustment means (50, 30) to adjust the resonance frequency or the cut-off frequency range (fc) of the cavity to a value (fc;), as a function of the frequency (fg;), - E5: the control unit (ECU) activates the communication of the switch (COMUT) with the transmission and recovery means (90) and analysis (VNA) of the reflected waves,; - E6: the control unit (ECU) activates the acquisition then the analysis of at least one parameter (Si) of the reflected waves, - E7: the control unit (ECU) converts the at least one parameter (Si) to obtain the at least one electromagnetic property (Ei).

10. Acquisition method according to claim 9 in which the control unit (ECU) iteratively implements a plurality n of cycles (Ci) to obtain a series of n measurements of the electromagnetic property (e;).

11. Acquisition method according to claim 9 or 10, and in which the means for transmitting and recovering (90) the reflected waves of the device for measuring at least one electromagnetic property, comprise an RF power measuring probe, characterized in that the at least one parameter (Si) of the reflected waves is the reflection coefficient, the at least one electromagnetic property (e;) being the permittivity or the complex permeability.

12. Acquisition method according to any one of claims 9 to 11, characterized in that step E7 of converting the parameter (Si) to obtain the electromagnetic property (e;) comprises a mathematical filter making it possible to eliminate the impact of the sample support (10) on the disturbance of the resonance cavity (21).

13. Acquisition method according to any one of claims 9 to 12, characterized in that the frequency range (fg) emitted by the microwave generator extends between 800 MHz and 80000 MHz.

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