Ultrasonic transducer for high-temperature applications

Abstract

Acoustic transduction device, comprising a piezoelectric converter (10), interposed between a front electrode (11) and a rear electrode (12), formed of a piezoelectric material; a front opening (15), arranged such that the front electrode is disposed between the piezoelectric material and the front opening (15); the device being configured to emit an acoustic wave (EW) towards the front opening or to detect an acoustic wave (RW) propagating from the front opening; the device comprising a rear component (13), applied against the rear electrode or forming the rear electrode, the rear component forming an acoustic damping back of the device, the device being characterized in that the rear component comprises a porous metal having a melting temperature greater than 200°C.

Description

Title of the invention: Ultrasonic transducer for high temperature application Technical field

[0001] The invention relates to an acoustic transducer intended for non-destructive testing operations, obstacle detection, telemetry, etc. in environments which may be placed under high temperature, under high pressure, for example in a nuclear installation. PREVIOUS ART

[0002] Ultrasonic non-destructive testing is well suited to the monitoring of structures, in order to monitor resistance to aging and the appearance of possible defects, or for telemetry or obstacle detection operations. Some applications are carried out in high temperature and / or high pressure environments. This is the case, for example, in the field of aircraft engines, or the oil industry, or in nuclear reactors.

[0003] [Fig.1A] shows schematically an ultrasonic transducer Iaa of the prior art, intended to be used in a high temperature environment. By high temperature, we generally mean a temperature above 200°C, or above 600°C. The active elements of the transducer are arranged in a housing 2. The active elements comprise a piezoelectric converter 10, formed of a piezoelectric material, interposed between a front electrode 11 and a rear electrode 12. Each electrode is connected to an electrical circuit 20. The front electrode is arranged facing an inlet opening 15 provided in the housing 2.

[0004] Under the effect of the application of an alternating voltage between the electrodes, the piezoelectric converter generates an acoustic wave EW. The emitted acoustic wave EW propagates towards a medium 3 outside the enclosure 2, through the opening 15. The transducer may comprise a blade 14 allowing an adaptation of acoustic impedance between the transducer and the external medium 3. The transducer thus operates according to an emission mode.

[0005] The transducer can also operate in a reception mode, during which an acoustic wave RW propagates from the external medium 3 towards the transducer. The received acoustic wave RW causes a vibration of the piezoelectric converter. This results in the appearance of an alternating voltage at the terminals of the electrodes 11 and 12. The transducer thus operates in a reception mode.

[0006] For high temperature applications, the piezoelectric material may be lithium niobate, as described in patent US9425384.

[0007] [Fig. 1B] represents a timing diagram of the vibration amplitude of an electrical transducer of the prior art, following reception of an acoustic wave. The ordinate axis corresponds to the amplitude of the vibration wave of the piezoelectric converter, as measured by the electrical circuit, while the abscissa axis corresponds to time. The emission is triggered by an application of an alternating voltage for a few microseconds to a few tens of microseconds. However, the vibration of the piezoelectric converter extends over a much longer duration, of the order of a few hundred ps (microseconds). This results in a degradation of the performance of the transducer.

[0008] In order to address this problem, it is known to connect the rear electrode with an element, forming a "damper back", usually referred to as a "backing element". The main function of the damping back is to attenuate, at the rear electrode, the vibration of the assembly formed by the piezoelectric converter and the electrodes. In the prior art, the damping back of a transducer can be formed from a polymer doped with high density particles, for example tungsten or lead particles. This is also referred to as a particulate composite. The random distribution of the particles induces multiple reflections and the acoustic wave is attenuated by destructive interference. This makes it possible to reduce the duration of the acoustic pulse of the transducer. However, such a composition is not suitable for high temperature application.

[0009] The publication “Porous ceramics as backing element for high temperature transducers”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 62, n°12, pp 360-372, 2015, describes a transducer, intended for use at high temperatures. The transducer comprises a damping back, connected to an electrode, made of a porous ceramic. The use of a ceramic makes it compatible with implementation at a high temperature. However, the manufacture of such a transducer requires an assembly of the ceramic, forming the damping back, on the electrode. Uncertainty remains as to the durability of such an assembly over time. The resistance of porous ceramics to exposure to high irradiation is also uncertain.

[0010] The inventors have developed an ultrasonic transducer, intended to be used under strong irradiation, and under high temperature, or exposed to strong temperature gradients, and this for a significant operating period, greater than a few years, or even a few decades. Statement of the invention

[0011] A first object of the invention is an acoustic transduction device, comprising - a piezoelectric converter, formed of a piezoelectric material, interposed between a front electrode and a rear electrode; - a housing, containing the piezoelectric converter, the front electrode and the rear electrode; - a front opening, provided in the housing, arranged such that the front electrode is arranged between the piezoelectric material and the front opening;

[0012] the device being configured to emit an acoustic wave toward the front opening or to detect an acoustic wave propagating from the front opening;

[0013] the device comprising a rear component, applied against the rear electrode or forming the rear electrode, the rear component forming a shock-absorbing back of the device, the device being characterized in that the rear component is a porous metallic material whose melting temperature is greater than 200°C.

[0014] The device may comprise one of the following characteristics, taken in isolation or according to technically feasible combinations. - the rear component forms the rear electrode. - the melting temperature of the metallic material is higher than 600°C. - the piezoelectric material has a Curie temperature and the melting temperature of the metallic material is higher than the Curie temperature of the piezoelectric material; - the Curie temperature of the piezoelectric material is greater than 1000°C. - the volume fraction of the pores of the porous metallic material is between 20% and 60% or between 25% and 50% or between 25% and 40%. - the average pore size is less than 100 pm, the average size corresponding to an average diameter of each pore. - the piezoelectric material is chosen from lithium niobate or barium titanate. - the metallic material comprises at least one element chosen from: Ni, Fe, Pd, Ag, Au, Cu, Pd, Al. - the metallic material is a stainless steel type alloy.

[0015] Another object of the invention is a use of a device according to the first object of the invention for emitting or receiving an acoustic wave, the emitted or received acoustic wave propagating through the opening, the device being arranged in a medium whose temperature is greater than 200°C.

[0016] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0017] [Fig.lA] [Fig.lA] shows a diagram of a transducer according to the prior art.

[0018] [Fig.lB] The [Fig.lB] a vibration of a transducer.

[0019] [Fig.2A] [Fig.2A] represents a first embodiment of a transducer according to the invention.

[0020] [Fig.2B] [Fig.2B] represents a second embodiment of a transducer according to the invention.

[0021] [Fig.3A] [Fig.3A] shows schematically a poor impedance match between a piezoelectric converter and a component forming a damping back, assembled to the converter.

[0022] [Fig.3B] [Fig.3B] shows a diagram of a good impedance match between a piezoelectric converter and a component forming a damping back, assembled to the converter.

[0023] [Fig.4] [Fig.4] shows a diagram of a test bench, aimed at determining a speed of propagation of an acoustic wave through different materials.

[0024] [Fig.5A]

[0025] [Fig.5B] Figures 5A and 5B show detection echograms obtained with the test bench shown schematically in [Fig.4], respectively in the presence and absence of the sample studied.

[0026] [Fig.6A] [Fig.6A] represents the propagation speed (or celerity) of an acoustic wave (ordinate axis - unit ms') as a function of the volume fraction of the pores (abscissa axis - %).

[0027] [Fig.6B] [Fig.6B] represents the acoustic impedance of a porous stainless steel material (y-axis - Rayls unit x 107) as a function of the pore volume fraction (x-axis - %).

[0028] [Fig.6C] [Fig.6C] represents a linear attenuation coefficient (ordinate axis - unit dB / mm) as a function of the volume fraction of the pores (abscissa axis - %). PRESENTATION OF SPECIAL EMBODIMENTS

[0029] [Fig.2A] illustrates a first embodiment of a transducer 1 according to the invention. As described in connection with the prior art, the transducer comprises a converter 10 formed by a piezoelectric material, interposed between a front electrode 11 and a rear electrode 12. The assembly, formed by the piezoelectric converter 10, the front electrode and the rear electrode, is arranged in a housing 2 comprising an opening 15. The front electrode 11 extends between the opening 15 and the piezoelectric converter 10. The device preferably comprises an acoustic impedance matching blade 14, interposed between the front electrode 11 and opening 15. The impedance matching blade is for example made of aluminum.

[0030] The transducer is connected to an electrical circuit 20, making it possible to apply or measure an alternating voltage between the front electrode and the rear electrode. As described in connection with the prior art, under the effect of an application of a brief alternating voltage, an acoustic wave EW is emitted through the opening 15, and propagates through an ambient medium 3. The ambient medium may in particular be liquid or solid. It may be water, a liquid material or a solid material. Under the effect of reception of an acoustic wave RW, an alternating electrical signal is detected by the electrical circuit 20, the amplitude of which corresponds to the vibration amplitude of the piezoelectric converter under the effect of reception of the acoustic wave RW.

[0031] The piezoelectric converter 10 may take the form of a disk with a thickness of 1 mm, and a diameter of between 5 mm and 50 mm. The material used is compatible with use at high temperatures, for example between 200°C and 700°C, or even more, for example above 1000°C. The piezoelectric material may be Lithium Niobate (LiNbO3). The resonant frequency of the piezoelectric converter may be from a few hundred kHz up to several MHz, for example 4 MHz or 5 MHz. The thickness of each electrode may be of the order of 1 mm. Each electrode may take the form of a disk, the diameter of which corresponds to that of the piezoelectric converter 10.

[0032] The electrical circuit 20 is connected to a central unit 30, configured to control the electrical circuit when the transducer operates in transmission mode and / or analyze the voltage measured between the electrodes when the transducer operates in reception mode.

[0033] The transducer 1 comprises a rear component 13, applied against the rear electrode 12. The rear component is intended to form a “damper back” with respect to the piezoelectric converter 10. As mentioned in the prior art, this involves attenuating the echoes of the acoustic wave transmitted towards the rear of the piezoelectric converter. The thickness of the damping back is preferably greater than 5 mm, or even 10 mm. It may be between 10 mm and 100 mm, for example 40 mm.

[0034] An important aspect of the invention is that the shock-absorbing back 13 is formed by a porous metallic material (pure metal or metallic alloy), the melting temperature of which is greater than 200°C, and preferably greater than 600°C or 700°C, and preferably 1000°C. It may for example be a steel, for example stainless steel, or aluminum, or a metal chosen from: Ni, Fe, Pd, Ag, Au, Cu, Pd, Al. It may be a metallic alloy such as bronze or brass. The shock-absorbing back may be formed from the same material as the rear electrode, which facilitates their assembly.

[0035] An advantage of the metallic material, in particular stainless steel, is its good resistance to corrosion and ionizing radiation, in particular neutrons or gamma radiation, which makes it compatible with use in a nuclear installation.

[0036] When the damping back is formed from the same material as the rear electrode, in this case a conductive metal, a phase of assembling the back on the electrode is avoided. Conversely, when the damping back is made of a ceramic, the assembly involves the use of an adhesive or brazing. Such an assembly may not be resistant over time, particularly when the transducer is subjected to strong thermal gradients. Indeed, the respective thermal expansion coefficients of a ceramic and a metal electrode are generally different from each other. This can lead to degradation of the assembly over time, particularly during repeated exposure to strong thermal gradients.

[0037] The material forming the piezoelectric converter 10 has a Curie temperature, beyond which it is considered that the piezoelectric behavior disappears. It is preferable that the melting temperature of the metallic material forming the damping back is higher than the Curie temperature of the piezoelectric converter. Lithium niobate has a Curie temperature higher than 1100°C.

[0038] The use of an electrically conductive metallic material is advantageous. [Fig.2B] shows schematically an embodiment in which the damping back and the rear electrode form a single part. Thus, the rear electrode is formed from the porous, electrically conductive metallic material. Such an embodiment is particularly advantageous because it minimizes the number of parts forming the transducer, and simplifies manufacturing, in particular by avoiding the assembly of the damping back on the electrode.

[0039] The damping back 13 is configured to maximize transmission of a vibration wave produced by the piezoelectric transducer, and minimize reflection of said wave. In Figures 3A and 3B, a configuration as shown in [Fig.2B] is shown, in which the piezoelectric converter 10 is directly arranged in contact with the damping back 13, the latter acting as a rear electrode. At the interface between the piezoelectric converter 10 and the damping back 13, a transmission coefficient T and a reflection coefficient R can be defined. The transmission coefficient T corresponds to a ratio between - the amplitude of a wave, called the incident wave, propagating from the piezoelectric converter 10 and incident on the damping back 13, this amplitude being noted A; in figures 3A and 3B; - the amplitude of a transmitted wave At, which corresponds to the part of the incident wave propagating through the damping back.

[0040] The reflection coefficient R corresponds to a ratio between: - the amplitude of the incident wave A; ; - the amplitude of a reflected wave Ar, which corresponds to the part of the incident wave reflected by the damping back 13 and propagating towards the piezoelectric converter 10.

[0041] If Zi and Z2 respectively designate the acoustic impedances of the piezoelectric converter 10 and the shock-absorbing back 13, the coefficients R and T are such that:

[0042] R + T= l (1) 100431 «= > Ù) 100441 T= (3)

[0045] [Fig.3A] shows schematically a configuration according to which the reflection coefficient is, in absolute value, high and the transmission coefficient is low. This is an unsuitable configuration: the reflection of the incident wave at the interface between the piezoelectric converter 10 and the damping back 13 causes the formation of echoes increasing the duration of the acoustic wave emitted or received by the transducer. This results in a temporal degradation of the measurement, in the sense that the instants of emission or reception of the wave are determined with degraded precision. When the transducer is used for telemetry purposes, this results in a degradation of the spatial resolution of the measurement.

[0046] Figure 3B shows a configuration in which the reflection coefficient is close to 0, and the transmission coefficient is close to 1, which corresponds to an ideal case. The formation of echoes at the interface between the piezoelectric converter 10 and the damping back 13 is low. This results in a shorter emitted (or detected) acoustic wave, which promotes the temporal resolution of the measurement. The invention aims to approach this configuration. Expression (3) shows that such a configuration is obtained if the acoustic impedances of the two contiguous media are close to each other, in other words » Z2.

[0047] The acoustic impedance of the piezoelectric converter 10 is generally a few tens of MRayls, typically between 25 and 40 MRayls (Mega Rayls), to be compared with the acoustic impedance of air, which is 430 Rayls or that of water, which is 1.5 MRayls. The configuration shown in [Fig.3A] corresponds to a piezoelectric converter / air interface. The configuration shown in [Fig.3B] corresponds to a piezoelectric converter / porous metal damper back interface that it is desired to obtain.

[0048] In addition to a transmission coefficient close to 1, the porous metal shock absorber back must attenuate the transmitted acoustic wave.

[0049] The impedance and acoustic attenuation of the damping back 13 are controlled by the size and volume fraction of the pores. Various experimental tests have been carried out, in order to define ranges of pore sizes and volume fractions allowing to obtain a transmission coefficient close to 1 and sufficient attenuation. It should be noted that the effect, on the impedance, of the pore size, and their volume fraction, depends on the material used. The values ​​obtained on one material cannot be transposed to another material.

[0050] A key parameter is the acoustic propagation speed c, from which the acoustic impedance Z can be calculated according to the expression:

[0051] Zf)C (4)

[0052] where is the density of the metallic material, expressed in kg.m3 etc is expressed in ms 1

[0053] [Fig.4] shows a diagram of an echometry test bench 100. Metal samples 103 of different porosities are placed between an acoustic transmitter 101 and an acoustic receiver 102. The assembly is immersed in water 104.

[0054] Figure 5A represents a pulse received by the acoustic receiver 102 in the absence of a sample interposed between the transmitter and the receiver. 1 corresponds to an instant of detection of the acoustic wave. The ordinate axis corresponds to the amplitude and the abscissa axis corresponds to the time. The propagation of the acoustic wave through the water 104 is shown by a dotted arrow.

[0055] Figure 5B shows the pulses received by the acoustic receiver 102 after a sample 103, of thickness e, has been interposed between the transmitter and the receiver. The y-axis corresponds to the amplitude and the x-axis corresponds to the time. The propagation of the acoustic wave through the water 104 is shown by a solid arrow. Taking into account the interfaces formed by the sample 103, two pulses are detected: a first pulse, at time 6, corresponds to the wave having propagated through the sample, without reflection. Compared to the time of emission of the acoustic wave by the transmitter 101, time 6 is earlier than time z, because the propagation speed of the acoustic wave is greater in the sample 103 than in the water 104.

[0056] From these measurements, the acoustic propagation speed (or celerity) c can be defined: - on the basis of A t = trt, using the expression: —(5) - based on A tn = t2-th using the expression: ~ ~

[0057] In (5), denotes the acoustic propagation speed in water.

[0058] Samples of 316L type stainless steel of varying thicknesses e (5 mm or 10 mm) were tested, with different porosity volume fractions (between 25% and 53%) and different average pore sizes (average diameter between 2 μm and 60 μm). The table below shows the characteristics of the samples tested. Each sample 103 was in the form of a plate measuring 50 mm by 50 mm.

[0059] The porosity volume fraction of each sample was determined by measuring the density. The average pore sizes were determined by optical microscopy. Table 1 shows the main characteristics of the tested samples. Reference Average thickness (mm) Porosity (%) Average pore diameter (pm) 02-10 10.00 25.20 1.7 05-10 10.30 36.91 7.6 10-10 10.10 34.97 10.9 15-10 9.90 44.12 13.5 25-10 9.90 45.14 26.5 40-10 10.30 52.75 39.0 60-05 5.10 52.95 59.5

[0060] Table 1.

[0061] [Fig.6A] shows the acoustic wave propagation speeds determined from expression (6), as a function of the volume fraction of porosity. In [Fig.6A], the ordinate axis corresponds to the speed (unit ms ') and the abscissa axis corresponds to the porosity (%).

[0062] Figure 6B shows the acoustic impedance, calculated according to (4), from the knowledge of the density P of the materials tested and the speeds obtained by implementing expression (6). The ordinate axis corresponds to the speed (unit m / s) and the abscissa axis corresponds to the volume fraction of the pores (%).

[0063] A linear attenuation coefficient a, (in dB / mm) was also determined from the maximum amplitudes of the waves detected by the receiver 102. The attenuation was measured by comparing the amplitudes of the pulses respectively detected at the instants and ^2 defined in relation to [Fig.6B].

[0064] ^=ioi0g(^) (?)

[0065] From the attenuations Atta and Attb calculated for each sample, a linear attenuation coefficient, per millimeter, noted a, was determined, taking into account: - the thickness of the sample e when considering Atta; - twice the thickness e of the sample when considering Attb.

[0066] Figure 6C represents the attenuation coefficients a obtained from expression (7). The y-axis corresponds to the attenuation (unit dB / mm) and the x-axis corresponds to the pore volume fraction (%).

[0067] In Figures 6A and 6C, each point corresponds to a measured value. A function resulting from the interpolation of each measurement point has also been plotted in dotted lines. In Figures 6A and 6B, the interpolation is linear. In [Fig.6C], the function resulting from the interpolation is polynomial.

[0068] [Fig.6B] makes it possible to define a range of porosity volume fraction, expressed in %, for which the impedance is sufficiently close to the impedance of the piezoelectric material, i.e. in the range 10 - 40 MRayls. According to [Fig.6B], this corresponds to a porosity volume fraction of less than 50%.

[0069] [Fig.6C] makes it possible to define a range of porosity volume fraction, expressed in %, for which the attenuation is sufficient. By sufficient attenuation, we mean an attenuation coefficient a greater than or equal to 1 dB / mm. According to [Fig.6C], this corresponds to a porosity volume fraction greater than 25%.

[0070] The optimal porosity range is that for which: - the impedance of the porous metallic material forming the shock-absorbing back is sufficiently high (close to the impedance of the piezoelectric material), knowing that the impedance decreases with the volume fraction of porosity: see [Fig.6B]; - the attenuation is sufficiently high, knowing that the linear attenuation coefficient a increases with the volume fraction of porosity: cf. [Fig.6C].

[0071] It is understood from the above that the attenuation effect is due to the presence of the pores, while the impedance adaptation effect is due to the metal. The porosity characteristics of the metal therefore result from a compromise on the volume fraction of the pores.

[0072] With regard to type 316L stainless steel, taking into account a pore size (i.e. average diameter) of between 2 pm and 60 pm, the optimal range of porosity volume fraction is between 25% and 50%.

[0073] The optimal range of porosity volume fraction may be different for another material. Generally speaking, the characteristics of the porous metallic material forming the damping back 13 of the transducer are: - an average pore diameter of less than 500 pm, and less than 200 pm or 100 pm; - and / or a volume fraction of porosity between 20% and 60%, and preferably between 25% and 50%, and more preferably between 25% and 40%.

[0074] In the embodiment described above, the piezoelectric material forming the converter is made of lithium niobate. Other piezoelectric materials Suitable for high temperature environments are usable, for example barium titanate (BaTiO3), bismuth titanate (BiTiO3) and its derivatives (e.g. addition of sodium), aluminum nitride (AIN), langatates (lanthanum, gallium and tantalum oxide).

[0075] The transducer according to the invention may be used in any high temperature application, for non-destructive testing or diagnostic or telemetry or obstacle detection or flow measurement purposes.

Claims

Claims

1. Acoustic transduction device, comprising - a piezoelectric converter (10), formed of a piezoelectric material, interposed between a front electrode (11) and a rear electrode (12); - a housing (2), containing the piezoelectric converter, the front electrode and the rear electrode; - a front opening (15), provided in the housing, arranged such that the front electrode is arranged between the piezoelectric material and the front opening (15); the device being configured to emit an acoustic wave (EW) towards the front opening or to detect an acoustic wave (RW) propagating from the front opening;the device comprising a rear component (13), the rear component forming a shock-absorbing back of the device, the device being characterized in that the rear component is a porous metallic material whose melting temperature is greater than 200°C, the device being such that: - the rear component forms the rear electrode; - the thickness of the rear component is greater than 5 mm.;

2. A device according to claim 1, wherein the melting temperature of the metallic material is greater than 600°C.

3. Device according to any one of the preceding claims, in which: - the piezoelectric material has a Curie temperature; - the melting temperature of the metallic material is higher than the Curie temperature of the piezoelectric material.

4. A device according to claim 3, wherein the Curie temperature of the piezoelectric material is greater than 1000°C.

5. A device according to any preceding claim, wherein the volume fraction of the pores of the porous metallic material is between 20% and 60% or between 25% and 50% or between 25% and 40%.

6. A device according to any preceding claim, wherein the average pore size is less than 100 pm, the average size corresponding to an average diameter of each pore.

7. A device according to any preceding claim, wherein the piezoelectric material is selected from lithium niobate or barium titanate.

8. Device according to any one of the preceding claims, wherein metallic material comprises at least one element selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, Al.

9. A device according to any preceding claim, wherein the metallic material is a stainless steel type alloy.

10. Use of a device according to any one of the preceding claims for emitting or receiving an acoustic wave (EW, RW), the emitted or received acoustic wave propagating through the opening (15), the device being arranged in a medium (3) whose temperature is greater than 200°C.

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