Fiber optic temperature sensor whose free end forms an optical probe comprising a matrix in which thermoluminescent particles are incorporated, and a layer covering the matrix, reflecting and / or diffusing at the emission wavelengths of the particles.

Abstract

Fiber optic temperature sensor, the free end of which forms an optical probe comprising a matrix in which thermoluminescent particles are incorporated, and a layer covering the matrix, reflecting and / or diffusing at the emission wavelengths of the particles. The invention relates to a temperature sensor, comprising an optical fiber, a free end of which forms an optical probe comprising at least one thermoluminescent material capable of emitting a peak of light at at least two wavelengths, and at least one layer covering the matrix, the layer being made of at least one material reflecting and / or diffusing at least at the two emission wavelengths of the thermoluminescent material. Figure for abstract: Fig. 8

Description

Title of the invention: Optical fiber temperature sensor whose free end forms an optical probe comprising a matrix in which thermoluminescent particles are incorporated, and a layer covering the matrix, reflecting and / or diffusing at the emission wavelengths of the particles. Technical field

[0001] The present invention relates to the field of instrumentation, in particular temperature sensors.

[0002] It relates more particularly to fiber optic temperature sensors.

[0003] The invention aims to propose a solution to at least overcome or reduce the environmental constraints on the actual measurement of this type of sensor.

[0004] The invention is described with reference to a use for measuring temperatures within electrochemical accumulators or batteries, in particular of the metal-ion type, in order to better understand the phenomena which cause overheating and / or thermal runaway of the accumulators.

[0005] Although described with reference to a Lithium-ion accumulator, the invention applies to temperature measurements of any metal-ion electrochemical accumulator, i.e. also sodium-ion, Magnesium-ion, Aluminum-ion accumulators, etc. or more generally to any electrochemical accumulator.

[0006] Generally speaking, a sensor according to the invention can be implemented in any industrial, medical or biological application requiring, at one time or another, the determination of a temperature, in particular in a range from -180°C to 400°C.

[0007] By "thermoluminescence" is meant here and within the framework of the invention, the capacity of a material to emit almost instantaneously, at a given temperature and under the effect of light radiation called absorption or excitation radiation, light radiation of the same wavelength or of a different wavelength called emission radiation.

[0008] The light radiation emitted by the thermoluminescent material is characterized by an emission spectrum comprising one or more peaks whose intensity and / or luminescence lifetime varies depending on the temperature to which the material is subjected. Prior art

[0009] Fiber optic temperature sensors have been developed to collect precise thermal data on structures or in devices, in particular with a view to detecting anomalies which could affect their safety and reliability.

[0010] These fiber optic sensors have many advantages, including the ability to be miniaturized and therefore able to be installed in an environment constrained in terms of available space. They are also non-conductive and allow the properties of light to be used to probe different physical or chemical parameters at the heart of an element, in particular an accumulator or battery: [1], [2], [3].

[0011] Fiber optic sensors based on thermoluminescent particles implement a principle of thermoluminescence by radiometry of emission peaks.

[0012] According to this principle, a luminescent material has an emission spectrum with several peaks at distinct wavelengths, at least one of which has an intensity which varies with temperature while others will remain constant.

[0013] [Fig.l] illustrates the luminescence spectrum of a fiber optic sensor based on thermoluminescent particles whose two emission peaks evolve as a function of the temperature to which they are subjected.

[0014] If we choose among the peaks having a thermal coupling a constant peak and a peak dependent on the temperature we can then go back to the temperature measurement by the luminescence measurement using the relation according to equation 1 as follows:

[0015] [Equation 1]

[0016] in which FIR is the intensity ratio (or integrals) of the two emission peaks, T is the temperature, kB is the Boltzmann constant, AE is the energy gap between the two energy levels corresponding to the two luminescence peaks and which are thermally coupled, B a constant.

[0017] This logarithmic response of the sensor according to [Fig.l] is illustrated in [Fig.2].

[0018] A classic embodiment of a thermo-particle-based fiber optic sensor luminescent, consists of producing a probe based on said particles deposited at one end of an optical fiber and / or on sites along the length of an optical fiber by a sol-gel process.

[0019] In operation, absorption or excitation light radiation is sent through the optical fiber to reach the probe. The emission radiation The resulting signal is recovered and returned by the same fiber to a detector (photodiode, photomultiplier, spectrophotometer, etc.) which allows the measurement of the fluorescence signal and therefore the measurement of the temperature by signal processing.

[0020] An example of this type of fiber optic sensor is described in patent application EP4155700A1.

[0021] The inventors carried out temperature measurement tests during cycling of a Li-ion accumulator with liquid electrolyte impregnated in a conventional separator, by means of such a sensor by positioning it between the positive NMC (Nickel Manganese Cobalt) electrode and the separator.

[0022] [Fig.3] shows a current profile applied to a 1Ah Li-ion accumulator corresponding to a cycling of the latter.

[0023] [Fig.4] illustrates the variation in the intensity of the two peaks of interest, i.e. the two emission peaks of the thermoluminescent material of the sensor, as a function of time, for a 1Ah Li-ion accumulator during the cycling according to [Fig.3] to which it is subjected.

[0024] The temperature of the Li-ion accumulator being stable during this cycling, it is therefore noted that the response of the sensor is disturbed by the operation of the accumulator itself while the temperature of the accumulator, under these cycling conditions, does not present any measurable heating with this sensor.

[0025] Other tests were carried out on Li-ion accumulators comprising the same electrodes as the previous accumulator but with a separator coated with a layer of alumina (A12O3), by positioning the sensor in the same position, i.e. between the positive electrode and the separator.

[0026] With such an alumina layer separator, the inventors did not observe any variation in the intensity of the emission peaks, with the accumulator temperature always remaining stable during the cycling identical to the previous one.

[0027] [Fig.5] illustrates from top to bottom, the current profile identical to that of [Fig.3] as well as the voltage of the accumulator during cycling, the stability of the temperature during this cycling as well as the intensity of the two luminescence peaks of the material, called H and S.

[0028] It is clear from this [Fig.5] that there is no longer any visible impact of the variation in current on the signal from the optical sensor: in fact, there is no variation in the intensity of the H and S luminescence peaks during cycling.

[0029] The inventors were able to identify the origin of the problem observed in [Fig.4]: it corresponds to an optical interaction between the sensor and the negative graphite electrode of the accumulator. Indeed, when the latter undergoes lithiation or delithiation during cycling, it changes color and therefore the optical absorption of the environment changes the optical response of the sensor.

[0030] This means that in this type of configuration of the measurement sensor within a battery, in particular as described in patent application EP4155700A1, the sensor is sensitive to a change in optical environment on its response.

[0031] [Fig.6] schematically shows an existing fiber optic sensor 1. It comprises an optical fiber 2 consisting of a core 20 adapted to propagate light and a sheath 21 surrounding the core. A free end 22 of the fiber carries an optical probe 3 consisting of a matrix 30 comprising thermoluminescent particles also called luminophores 31.

[0032] Figures 7A to 7C show the impact or not of the optical environment on the operation of this sensor 1.

[0033] In the case of [Fig.7A], the optical response of the sensor 1 is not impacted by the environment, because the luminescence emitted by the probe 3 is not modified by the environment and returns integrally to the fiber, as symbolized by the single arrow which returns to the core 20 of the latter.

[0034] Figures 7B and 7C correspond to two cases where the negative graphite electrode of the accumulator 4 is respectively lithiated or delithiated: it can then be seen that this environment of the accumulator 4 has an impact on the response of the sensor 1, since part of the luminescence emitted by the probe is absorbed by the nearby environment, as symbolized by the additional emission arrow on the left, before being collected by the fiber 2 for the measurement.

[0035] To corroborate their analysis, the inventors carried out additional tests with accumulators comprising the same electrodes but for some with a usual separator, that is to say without any layer deposit on its surface, and for others with a separator coated with a layer of alumina on the surface.

[0036] When soaked with electrolyte, a conventional separator, without deposit, is optically transparent, while the separator coated with a layer of alumina on the surface appears opaque.

[0037] Thus, it is the opacity of the alumina separator on the surface which optically isolates the sensor from the negative electrode.

[0038] This therefore limits the use of an existing sensor to temperature measurements within accumulators where it is certain that the separator is transparent, i.e. which is not coated with a surface layer which changes the optical behavior of the separator, no longer sees the variations in absorption of the negative electrode during cycling even if the sensor is placed on the side of the positive electrode, or within the separator.

[0039] More broadly, an existing sensor cannot be used reliably when the measurement environment has optical variations that are in the same emission wavelength ranges of the peaks of interest of the thermo-lu- minescent of the optical probe of the sensor.

[0040] There is therefore a need to make the optical response of fiber optic sensors independent of the nearby measurement environment, in order to obtain reliable measurements.

[0041] The aim of the invention is to meet at least part of this need. Statement of the invention

[0042] To do this, the invention relates to a temperature sensor, comprising an optical fiber, one free end of which forms an optical probe comprising at least one thermoluminescent material capable of emitting a peak of light at at least two wavelengths, and at least one layer covering the matrix, the layer being made of at least one material reflecting and / or diffusing at least at the two emission wavelengths of the thermoluminescent material.

[0043] By "layer of optical diffusing material", it is specified that what is meant here and within the framework of the invention is a layer whose material will deflect light at the two emission wavelengths of the thermoluminescent material in various directions and predominantly not in the direction of the optical fiber.

[0044] According to an advantageous embodiment variant, the optical probe comprises a matrix in which thermoluminescent particles are incorporated.

[0045] Advantageously, the reflective material of the layer is chosen from gold, aluminum, platinum, aluminum, copper, molybdenum, indium, tantalum or a mixture thereof.

[0046] The reflective material of the layer may also be chosen from silica, alumina, zirconia or a mixture thereof. The layer is then preferably a porous layer deposited by a sol-gel process.

[0047] Preferably, the thickness of the layer of reflective material is between 90 and 500 nm, more preferably between 90 and 150 nm.

[0048] According to an alternative embodiment, the layer of reflective and / or diffusing material covers the matrix directly or indirectly via at least one interlayer of refractive index, in particular a layer of poly(methyl methacrylate) (PMMA).

[0049] The invention also relates to the use of a temperature sensor as described above, for measuring temperature within an electrochemical accumulator, in particular a metal-ion accumulator.

[0050] Thus, the invention essentially consists of an optical fiber temperature sensor carrying at the end an optical probe with thermoluminescent material with at least two emission peaks which is coated with at least one reflective and / or diffusing layer forming an optical barrier, to the peaks considered, to the nearby environment. whose temperature we are trying to measure.

[0051] The deposition of the reflective and / or diffusing layer is therefore adapted to the measurement wavelength peaks of the sensor.

[0052] The material of the reflective and / or diffusing layer must be compatible with the measurement environment. In the case of temperature measurement of Li-ion accumulators, the material is chemically and electrochemically compatible with the accumulator during its cycling.

[0053] This frees us from the impact of the environment, the optical characteristics of which, in particular light absorption, can change during their operation.

[0054] Ultimately, a sensor of the invention allows a temperature measurement by radiometry on two peaks by thermoluminescence which is reliable and precise, because its optical probe is optically isolated from the environment and therefore from its potential optical influence.

[0055] Other advantages and characteristics will become more apparent upon reading the detailed description, given for illustrative and non-limiting purposes, with reference to the following figures. Brief description of the drawings

[0056] [Fig-1] [Fig.l] illustrates in form the luminescence spectrum of a fiber sensor optical base of thermoluminescent particles whose two emission peaks evolve according to the temperature to which they are subjected.

[0057] [Fig.2] [Fig.2] is the logarithmic response line of the sensor according to [Fig.l].

[0058] [Fig.3] [Fig.3] is the curve of an example of cycling to which a battery is subjected Li-ion emulator whose light is measured with a fiber optic sensor and thermoluminescent material.

[0059] [Fig.4] [Fig.4] illustrates the variation in the intensity of the two emission peaks of the thermoluminescent material of the sensor, as a function of time, for a 1Ah Li-ion accumulator during the cycling according to [Fig.3] to which it is subjected.

[0060] [Fig.5] [Fig.5] illustrates respectively the current profile identical to that of [Fig.3] as well as the voltage of the accumulator during cycling, the stability of the temperature during this cycling as well as the intensity of the two luminescence peaks of the material, called H and S.

[0061] [Fig.6] [Fig.6] is a longitudinal sectional view of a fiber optic measuring sensor according to the state of the art.

[0062] [Fig.7A], [Fig.7B], [Fig.7C] Figures 7A, 7B, 7C schematically show the optical probe of the sensor according to [Fig.6], when the measuring environment constituted by a Li-ion accumulator has respectively no impact on the measurement, an impact due to lithiation, and delithiation of the negative graphite electrode of the accumulator.

[0063] [Fig.8] [Fig.8] is a longitudinal sectional view of a fiber measuring sensor optics according to the invention.

[0064] [Fig.9] [Fig.9] is a reprographic reproduction of a Li-ion accumulator in which a measuring sensor according to the state of the art is arranged, such as that of [Fig.6], and a measuring sensor according to the invention, such as that of [Fig.8].

[0065] [Fig. 10] [Fig. 10] illustrates from top to bottom the voltage and current curves of the accumulator according to [Fig.9], during the cycling it undergoes, the external temperature curve of the accumulator measured with a thermocouple, and the variation of the maximum luminescence intensity of the peak S of the sensor according to the invention over time. Detailed description

[0066] Figures 1 to 7C have already been described in the preamble. They will therefore not be detailed later.

[0067] [Fig.8] shows a fiber optic measuring sensor 1 according to the invention.

[0068] It comprises an optical fiber 2 consisting of a core 20 adapted to propagate light and a sheath 21 surrounding the heart.

[0069] A free end 22 of the fiber carries an optical probe 3 consisting of a matrix 30 comprising thermoluminescent particles also called luminophores 31.

[0070] The materials of the matrix 30 and the luminophore particles 31 as well as the production techniques, in particular by sol-gel deposition, can advantageously be those described in patent application EP4155700A1.

[0071] A layer of opaque material 32 is deposited on the surface of the matrix 30. This layer 32 is reflective in the emission wavelengths of the luminescent material of the particles 31.

[0072] Thus, this layer 32 optically isolates the probe 3 from its measurement environment.

[0073] The inventors have in particular carried out the deposition of this layer 32 in gold by a vacuum evaporation technique, on the tip of fiber 2, directly on the matrix 30 of probe 3. The thickness of the deposit is of the order of a hundred nm.

[0074] This sensor 1 with a gold layer 32 was then inserted into a Li-ion accumulator identical to that tested, under the same conditions as [Fig.3], and in the same place, i.e. between the positive electrode (NMC) and the separator without external coating.

[0075] For comparison purposes, a state-of-the-art optical fiber sensor 1 was also tested, such as that in [Fig.6], comprising the same optical fiber 2 and an optical probe 3 with the same matrix 30 and phosphors 31 as those of the sensor 1 according to the invention.

[0076] [Fig.6] shows the arrangement of the two sensors 1 respectively according to the invention and according to the state of the art, as inserted in a Li-ion accumulator with graphite negative electrode, separator without external coating and NMC positive electrode.

[0077] This accumulator was cycled according to a protocol at different charge and discharge rates and the luminescence signal from the sensor was monitored over time as well as the surface temperature of the accumulator using a thermocouple.

[0078] The conditions of this cycle as well as the results obtained with a sensor 1 according to the invention whose matrix 30 of probe 3 is covered with a layer of gold 32 are presented in [Fig. 10].

[0079] It can be seen that at slow cycle speed, the temperature of the accumulator changes very little. The variations that can be seen are less than one degree Celsius and probably correspond on the curve to an external event, such as an opening of the test enclosure.

[0080] There is no correlation between these variations and the cycling of the accumulator.

[0081] It can be seen that the luminescence signal of the sensor 1 according to the invention, which is translated by the maximum of the emission peak S is almost constant (bottom curve of [Fig. 10]). The very small variation that we see is barely more important than the noise and comes from the variation in the intensity of the source.

[0082] It is noted more broadly that there is no variation in the intensity of the S and H peaks as a function of the charging and discharging conditions of the cycling, as was visible with a sensor according to the state of the art.

[0083] This therefore proves that the sensor 1 according to the invention with an outer layer 32 of gold on its optical probe 3 is insensitive to the optical conditions of the measurement environment.

[0084] Other variants and improvements may be envisaged without departing from the scope of the invention.

[0085] The sensor 1 according to the illustrated example is with the constituent materials and is produced according to the techniques, in particular by sol-gel according to patent application EP4155700A1. The invention can be applied to any other type of thermoluminescence optical probe using another type of particles or molecules as luminophores and another type of material for the matrix.

[0086] More generally, the invention can also be applied to other types of fiber optic temperature sensors with a probe manufactured at the end of the fiber and which operates by either reflectance or luminescence and whose signal must be isolated from optical interactions with the external environment. List of cited references:

[0087] [1]: Wang, R., Zhang, H., Liu, Q„ Liu, F., Han, X., Liu, X., Li, K., Xiao, G., Albert, J., Lu, X. & Guo, T. « Operando monitoring ofion activities in aqueous batteries with plasmonic fiber-optic sensors. » Nature Communications 13, 547 (2022).

[0088] [2] : Lu, X., Tarascon, J.-M. & Huang, J. « Perspective on commercializing smart sensing for batteries ». eTransportation 14, 100207 (2022).

[0089] [3] : Hedman, J., Mogensen, R., Younesi, R. & Bjôrefors, F. « Fiber Optic Sensors for Détection of Sodium Plating in Sodium-Ion Batteries ». ACS Applied Energy Materials (2022) doi:10.1021 / acsaem.2c00595

Claims

Claims

1. Temperature sensor (1), comprising an optical fiber (2) of which a free end (22) forms an optical probe (3) comprising at least one thermoluminescent material (31) capable of emitting a peak of light at at least two wavelengths, and at least one layer (32) covering the matrix, the layer being made of at least one material reflecting and / or diffusing at least at the two emission wavelengths of the thermoluminescent material.

2. A temperature sensor according to claim 1, the optical probe comprising a matrix (30) in which thermoluminescent particles are incorporated.

3. A temperature sensor according to claim 1 or 2, the reflective material of the layer being selected from gold, aluminum, platinum, aluminum, copper, molybdenum, indium, tantalum or a mixture thereof.

4. Temperature sensor according to one of the preceding claims, the reflective material of the layer being chosen from silica, alumina, zirconia or a mixture thereof.

5. A temperature sensor according to claim 4, the layer being a porous layer deposited by a sol-gel process.

6. Temperature sensor according to one of the preceding claims, the thickness of the layer of reflective material being between 90 and 500 nm, preferably between 90 and 150 nm.

7. Temperature sensor according to one of claims 2 to 6, the layer of reflective and / or diffusing material covering the matrix directly or indirectly via at least one interlayer of refractive index, in particular a layer of poly(methyl methacrylate) (PMMA).

8. Use of a temperature sensor according to one of the preceding claims, for measuring temperature within an electrochemical accumulator, in particular a metal-ion accumulator.

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