Ultrasonic transducer for high temperature applications

Abstract

The invention relates to an acoustic transducer arrangement comprising:-a piezoelectric transducer (10) located between a front electrode (11) and a rear electrode (12) and made of a piezoelectric material; -a front aperture (15) arranged such that the front electrode is arranged between the piezoelectric material and the front aperture (15); -said device being configured to emit an acoustic wave (EW) to the front orifice, or to detect an acoustic wave (RW) propagating from the front orifice; -the device comprises a rear part (13) abutting against or forming the rear electrode, the rear part forming an acoustic damping backing of the device, the device being characterized in that the rear part comprises a porous metal having a melting point above 200 DEG C.

Description

Technical Field

[0001] The present invention relates to an acoustic transducer intended for use in non-destructive testing, obstacle detection, ranging and the like in an environment that may be at high temperature and high pressure, such as in a nuclear power plant. Background Art

[0002] Ultrasonic non-destructive testing is very suitable for monitoring structures to track aging resistance and the appearance of any defects, or for distance measurement or obstacle detection operations. Some applications are carried out in high temperature environments or under high pressure. This is the case, for example, in the field of aircraft engines, or in the oil industry or in nuclear reactors.

[0003] Figure 1A The schematic diagram shows a conventional ultrasonic transducer 1. AA , which is intended for use in high temperature environments. High temperatures are generally understood to be temperatures above 200°C or above 600°C. The active element of the transducer is arranged in the housing 2. The active element comprises a piezoelectric converter 10 formed of a piezoelectric material, which is 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 to face an inlet orifice 15 formed in the housing 2. Such a construction is described in US2014215784.

[0004] Under the effect of the AC voltage applied between the electrodes, the piezoelectric converter generates an acoustic wave EW. The emitted acoustic wave EW propagates through the orifice 15 to the external medium 3 outside the housing 2. The transducer may include a plate 14 that matches the acoustic impedance between the transducer and the external medium 3. The transducer thus operates in a transmission mode.

[0005] The transducer can also be operated in a receiving mode, in which an acoustic wave RW propagates from the external medium 3 to the transducer. The received acoustic wave RW causes vibrations of the piezoelectric converter. This causes an alternating voltage to appear between the terminals of the electrodes 11 and 12. The transducer is thus operated in a receiving mode.

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

[0007] Figure 1B A timing diagram showing the vibration amplitude of a prior art electric transducer after receiving an acoustic wave. The y-axis corresponds to the amplitude of the vibration wave of the piezoelectric converter measured by the circuit, while the x-axis corresponds to time. The emission is triggered by applying an alternating voltage of a few microseconds to tens of microseconds. However, the vibration duration of the piezoelectric converter is much longer, on the order of several hundred μs (microseconds). This can lead to a degradation in the performance of the transducer.

[0008] In order to solve this problem, it is known to connect the rear electrode with an element to form a "damping backing", usually called a "backing element". The main function of the damping backing is to damp the vibrations of the assembly formed by the piezoelectric converter and the electrode at the rear electrode. In the prior art, the damping backing of the transducer can be formed from a polymer doped with high-density particles, such as tungsten or lead particles. The expression "particle composite" is also used. The random distribution of the particles causes multiple reflections, and the sound waves are attenuated due to destructive interference. This makes it possible to reduce the duration of the acoustic pulse of the transducer. However, such composites are not suitable for high-temperature applications.

[0009] The publication "Porous ceramics as backing element for high temperature transducers", IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Volume 62, Issue 12, Pages 360-372, 2015, describes a transducer intended for use at high temperatures. The transducer comprises a vibration-damping backing connected to an electrode and made of a porous ceramic. The use of ceramics makes it possible to achieve compatibility with implementation at high temperatures. However, the manufacture of such a transducer requires bonding the ceramic forming the vibration-damping backing to the electrode. There is uncertainty about the durability of such a bond. There is also uncertainty about the tolerance of the porous ceramic to exposure to high radiation.

[0010] WO 2016 / 124941 describes a device for emitting sound waves comprising a backing element formed from a metal foam.

[0011] The inventors have developed an ultrasonic transducer that is intended for use at high radiation and at high temperatures or exposure to high temperature gradients for extended operating times (ie, events exceeding several years or even decades). Summary of the invention

[0012] The first object of the present invention is an acoustic transducer device, comprising:

[0013] - a piezoelectric transducer formed of a piezoelectric material, interposed between the front electrode and the back electrode;

[0014] - a housing which houses the piezoelectric converter, the front electrode and the back electrode;

[0015] - a front aperture formed in the housing and arranged such that the front electrode is arranged between the piezoelectric material and the front aperture;

[0016] The device is configured to transmit sound waves to the front aperture or detect sound waves propagating from the front aperture;

[0017] The device comprises a rear part which bears against or forms a rear electrode, the rear part forming a backing element of the device, the device being characterized in that the rear part is a porous metal material having a melting point above 200°C.

[0018] The device can have one of the following features implemented individually or in a technically feasible combination.

[0019] - The rear component forms the rear electrode.

[0020] - The melting point of metal materials is higher than 600℃.

[0021] - the piezoelectric material has a Curie temperature, and the melting point of the metal material is higher than the Curie temperature of the piezoelectric material;

[0022] - The Curie temperature of piezoelectric materials is above 1000°C.

[0023] - The volume fraction of pores in the porous metal material is 20% to 60%, or 25% to 50%, or 25% to 40%.

[0024] - The pores are filled with air.

[0025] - The average size of the pores is less than 100 μm, which corresponds to the average diameter of each pore.

[0026] - The piezoelectric material is selected from lithium niobate and barium titanate.

[0027] - The metal material includes at least one element selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, and Al.

[0028] - Metal material is stainless steel alloy.

[0029] Another object of the invention is to use the device according to the first object of the invention for emitting or receiving sound waves, the emitted or received sound waves propagating through an orifice, the device being arranged in a medium having a temperature higher than 200°C.

[0030] The invention will be better understood by reading the disclosure of examples of embodiments presented in the remainder of this specification in conjunction with the drawings listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1A A transducer according to the prior art is schematically shown.

[0032] Figure 1B The vibration of the transducer is shown.

[0033] Figure 2A A first embodiment of a transducer according to the invention is shown.

[0034] Figure 2B A second embodiment of a transducer according to the invention is shown.

[0035] Figure 3A The schematic shows a poor impedance match between a piezoelectric transducer and a component joined to the transducer and forming a backing element.

[0036] Figure 3B The schematic shows a good impedance match between the piezoelectric transducer and a component joined to the transducer and forming the backing element.

[0037] Figure 4 Schematic representation of a test rig designed to determine the propagation speed of sound waves through different materials.

[0038] Figure 5A and Figure 5B The results of using Figure 4 Detection echoes obtained by the test bench shown schematically.

[0039] Fig. 6A The propagation speed (or wave velocity) of the acoustic wave (y-axis - in ms) is shown according to the volume fraction of pores (x-axis - %). -1 ).

[0040] Figure 6B The acoustic impedance of porous stainless steel (y-axis - Rayleigh × 10) is shown according to the volume fraction of pores (x-axis - %). 7 ).

[0041] Figure 6C The linear attenuation coefficient (y-axis - in dB / mm) is shown as a function of the volume fraction of pores (x-axis - %). DETAILED DESCRIPTION

[0042] Figure 2A A first embodiment of a transducer 1 according to the invention is shown. As described in connection with the prior art, the transducer comprises a converter 10 made of a piezoelectric material, the converter 10 being 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 aperture 15. The front electrode 11 is located between the aperture 15 and the piezoelectric converter 10. The device preferably comprises an acoustic impedance matching plate 14, the acoustic impedance matching plate 14 being interposed between the front electrode 11 and the aperture 15. The impedance matching plate is formed, for example, of aluminum.

[0043] The transducer is connected to a circuit 20 so that an alternating voltage can be applied or measured between the front electrode and the rear electrode. As described in conjunction with the prior art, under the effect of the applied short-term alternating voltage, an acoustic wave EW is emitted through the orifice 15 and propagates through the surrounding medium 3. The surrounding medium can in particular be a liquid or a solid. It can be water, a liquid material or a solid material. Under the effect of the received acoustic wave RW, an alternating electrical signal is detected by the circuit 20, the amplitude of which corresponds to the vibration amplitude of the piezoelectric converter under the effect of the received acoustic wave RW.

[0044] The piezoelectric converter 10 may be in the form of a disc having a thickness of 1 mm and a diameter of 5 to 50 mm. The material used is compatible with use at high temperatures, for example 200°C to 700°C, or even higher, for example more than 1000°C. The piezoelectric material may be lithium niobate (LiNbO3). The resonant frequency of the piezoelectric converter may be from hundreds of kHz 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 be in the form of a disc having a diameter corresponding to the diameter of the piezoelectric converter 10.

[0045] The circuit 20 is connected to a central unit 30 configured to control the circuit when the transducer operates in a transmit mode and / or to analyse the voltage detected between the electrodes when the transducer operates in a receive mode.

[0046] The transducer 1 comprises a rear part 13 against the rear electrode 12. This rear part is intended to form a "backing element" relative to the piezoelectric converter 10. As mentioned in the prior art, this is to attenuate the echo of the sound waves emitted to the rear of the piezoelectric converter. The thickness of this backing element is preferably greater than 5 mm, or even 10 mm. The thickness can be 10 mm to 100 mm, for example 40 mm.

[0047] An important aspect of the present invention is that the backing element 13 is formed of a porous metal material (pure metal or metal alloy) having a melting point above 200° C., preferably above 600° C. or 700° C., preferably above 1000° C. The porous metal material may be, for example, steel (e.g. stainless steel), aluminum or a metal selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, Al. The porous metal material may be a metal alloy, such as bronze or brass. The backing element may be made of the same material as the rear electrode, which facilitates their bonding.

[0048] The advantages of metallic materials, especially stainless steel, are good resistance to corrosion and resistance to ionizing radiation (especially neutron or gamma radiation), which makes them compatible with use in nuclear power plants.

[0049] When the backing element is formed of the same material as the rear electrode (in this case a conductive metal), the stage of bonding the backing element to the electrode is avoided. In contrast, in the case where the backing element is made of ceramic, adhesives or welding must be used to bond the backing element to the electrode. Such a bond may not be durable, especially in the case where the transducer is exposed to high temperature gradients. In particular, the respective thermal expansion coefficients of the ceramic and the metal electrode are generally different from each other. This leads to degradation of the bond over time, especially in the case of repeated exposure to high temperature gradients.

[0050] The material forming the piezoelectric converter 10 has a Curie temperature above which the piezoelectric behavior of the piezoelectric converter 10 is considered to disappear. Preferably, the melting point of the metal material forming the backing element is higher than the Curie temperature of the piezoelectric converter. The Curie temperature of lithium niobate is higher than 1100°C.

[0051] It is advantageous to use an electrically conductive metallic material. Figure 2B An embodiment is schematically shown in which the backing element and the rear electrode form the same part. The rear electrode is thus formed of an electrically conductive porous metal 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 need to bond the backing element to the electrode.

[0052] The backing element 13 is configured to maximize the transmission of the vibration waves generated by the piezoelectric transducer and to minimize the reflection of said waves. Figure 3A and Figure 3B Shown as Figure 2B The configuration shown, in which the piezoelectric converter 10 is arranged in direct contact with the backing element 13, the backing element 13 serves as the rear electrode. At the interface between the piezoelectric converter 10 and the backing element 13, a transmission coefficient T and a reflection coefficient R can be defined. The transmission coefficient T corresponds to the ratio between the following two terms:

[0053] - the amplitude of the wave propagating from the piezoelectric converter 10 and incident on the backing element 13 (referred to as the incident wave), which is Figure 3A and Figure 3B A i ;

[0054] -Transmitted wave A t The amplitude of the transmitted wave A t Corresponds to the portion of the incident wave that propagates through the backing element.

[0055] The reflection coefficient R corresponds to the ratio between:

[0056] -Incident wave A i the magnitude of

[0057] -Reflected Wave Ar The amplitude of the reflected wave A r This corresponds to the portion of the incident wave that is reflected by the backing element 13 and propagates to the piezoelectric converter 10 .

[0058] If Z1 and Z2 represent the acoustic impedance of the piezoelectric converter 10 and the backing element 13, respectively, the coefficients R and T are:

[0059]

[0060] Figure 3A The schematic diagram shows a configuration in which the absolute value of the reflection coefficient is high and the absolute value of the transmission coefficient is low. This is a mismatched configuration: the reflection of the incident wave at the interface between the piezoelectric converter 10 and the backing element 13 generates an echo, which increases the duration of the acoustic wave emitted or received by the transducer. This leads to a time degradation of the measurement, since the time of emission or reception of the wave is determined with degraded accuracy. When the transducer is used for distance measurement purposes, the spatial resolution of the measurement is degraded.

[0061] Figure 3B The schematic diagram shows a configuration in which the reflection coefficient is close to 0 and the transmission coefficient is close to 1, which corresponds to the ideal situation. The echo formation at the interface between the piezoelectric converter 10 and the backing element 13 is weak. This results in a shorter emitted (or detected) sound wave, thereby improving the time resolution of the measurement. The present invention aims to get closer to this configuration. Expression (3) shows that if the acoustic impedances of two adjacent media are close to each other, or in other words , then such a configuration is obtained.

[0062] The acoustic impedance of the piezoelectric transducer 10 is generally tens of mega-Rayles, typically 25 to 40 mega-Rayles, compared to the acoustic impedance of air (430 Rayles) or water (1.5 mega-Rayles). Figure 3A The configuration shown in corresponds to the interface between the piezoelectric transducer and air. Figure 3B The configuration shown in corresponds to the interface between the piezoelectric transducer and the porous metal backing element.

[0063] In addition to having a transmission coefficient close to 1, the porous metal backing element must also attenuate transmitted sound waves.

[0064] The acoustic impedance and attenuation of the backing element 13 are controlled by the size and volume fraction of the pores filled with air. Various experimental tests have been carried out to define the range of size and volume fraction of the pores that allow to obtain a transmission coefficient close to 1 and sufficient attenuation. It should be noted that the effect of the size and volume fraction of the pores on the impedance depends on the material used. The values ​​obtained with one material cannot be transferred to another material.

[0065] The key parameter is the speed of sound propagation c, from which the acoustic impedance Z can be calculated using the following expression:

[0066]

[0067] Where ρ is the density of the metal material, in kg.m -3 denoted by ms -1 express.

[0068] Figure 4 An echometry test bench 100 is schematically shown. Metal samples 103 of different porosities are placed between an acoustic transmitter 101 and an acoustic receiver 102. The whole is immersed in water 104.

[0069] Figure 5A A pulse received by the acoustic receiver 102 is shown without a sample between the transmitter and the receiver. t corresponds to the detection time of the acoustic wave. The y-axis corresponds to the amplitude and the x-axis corresponds to time. The propagation of the acoustic wave through the water 104 is represented by the dashed arrows.

[0070] Figure 5B Pulses received by the acoustic receiver 102 are shown after a sample 103 of thickness e has been placed between the transmitter and the receiver. The y-axis corresponds to amplitude and the x-axis corresponds to time. The propagation of the acoustic wave through the water 104 is represented by the solid arrows. Considering the interface formed by the sample 103, two pulses are detected: the first pulse at time t1 corresponds to the wave that has propagated through the sample without reflection. Relative to the time of emission of the acoustic wave by the transmitter 101, time t1 is earlier than time t, since the propagation speed of the acoustic wave in the sample 103 is greater than the propagation speed in the water 104.

[0071] Based on these measurements, the sound propagation speed (or wave speed) c can be defined as follows:

[0072] - based on , using the expression:

[0073] - based on , using the expression:

[0074] In (5), c w Represents the speed of sound in water.

[0075] 316L stainless steel samples of different thicknesses e (5 mm or 10 mm), various porosity volume fractions (25% to 53%) and various average pore sizes (average diameters of 2 μm to 60 μm) were tested. The following table shows the characteristics of the tested samples. Each sample 103 was in the form of a plate with dimensions of 50 mm by 50 mm.

[0076] The volume fraction of porosity of each sample was determined by measuring the mass per unit volume. The average pore size was determined by optical microscopy. Table 1 shows the main characteristics of the tested samples.

[0077]

[0078] Table 1

[0079] Fig. 6A The propagation velocity of the acoustic wave determined using expression (6) according to the change in the volume fraction of the porosity is shown. Fig. 6A The y-axis corresponds to the wave speed (in ms -1 ), the x-axis corresponds to the porosity (%).

[0080] Figure 6B The acoustic impedance calculated using (4) based on knowledge of the density ρ of the material being tested and the wave velocity obtained using expression (6) is shown. The y-axis corresponds to the velocity (in m / s) and the x-axis corresponds to the volume fraction of pores (%).

[0081] The linear attenuation coefficient a (in dB / mm) can also be determined based on the maximum amplitude of the wave detected by the receiver 102. The attenuation is calculated by comparing the Figure 6B The amplitude of the pulse detected at defined times t1 and t2 is measured.

[0082]

[0083] According to the attenuation Att calculated for each sample a and Att b , the linear attenuation coefficient per millimeter (denoted as α) is determined considering the following:

[0084] When considering Att a When , the thickness of the sample is e;

[0085] When considering Att b When , the thickness of the sample is twice e;

[0086] Figure 6C The attenuation coefficient α obtained using expression (7) is shown. The y-axis corresponds to the attenuation (unit: dB / mm), and the x-axis corresponds to the volume fraction (%) of pores.

[0087] exist Fig. 6A and Figure 6C In , each point corresponds to a measured value. The function obtained by interpolating each measured point has also been drawn as a dotted line. Fig. 6A and Figure 6B In , the interpolation is linear. Figure 6C In the example, the function obtained by interpolation is a polynomial.

[0088] Figure 6B The range of porosity volume fraction (expressed in %) is defined within which the impedance is close enough to the impedance of the piezoelectric material, i.e., in the range of 10 megarylis to 40 megarylis. Figure 6B , which corresponds to a porosity volume fraction of less than 50%.

[0089] Figure 6C The range of volume fraction of porosity (expressed in %) is defined within which the attenuation is sufficient. Sufficient attenuation means that the attenuation coefficient α is greater than or equal to 1 dB / mm. Figure 6C , which corresponds to a porosity volume fraction greater than 25%.

[0090] The optimum porosity range is such that:

[0091] - the impedance of the porous metal material forming the backing element is sufficiently high (close to the impedance of the piezoelectric material), noting that the impedance decreases with increasing volume fraction of porosity: see Figure 6B ;

[0092] - the attenuation is high enough to note that the linear attenuation coefficient α increases with the volume fraction of porosity: see Figure 6C .

[0093] From the above, it can be seen that the attenuation effect is caused by the presence of pores, while the impedance matching effect is caused by the metal. The porosity characteristics of the metal are therefore the result of a compromise between the volume fraction of the pores.

[0094] For 316L stainless steel, considering that the pore size (ie, average diameter) is 2 μm to 60 μm, the optimal range of the volume fraction of the porosity is 25% to 50%.

[0095] For another material, the optimum range of volume fraction of porosity may be different. In general, the metal material forming the backing element 13 of the transducer is characterized by:

[0096] - average pore diameter less than 500 μm, and less than 200 μm or less than 100 μm;

[0097] - and / or the volume fraction of porosity is 20% to 60%, preferably 25% to 50%, more preferably 25% to 40%.

[0098] In the embodiments described above, the piezoelectric material forming the transducer is made of lithium niobate. Other piezoelectric materials suitable for high temperature environments may be used, such as barium titanate (BaTiO3), bismuth titanate (BiTiO3) and its derivatives (e.g. with sodium additions), aluminum nitride (AlN), lanthanates (lanthanum oxide, gallium oxide, and tantalum oxide).

[0099] The transducer according to the invention will be useful in any high temperature application for non-destructive testing or diagnostics or ranging or obstacle detection or flow measurement purposes.

Claims

1. An acoustic transducer device, comprising: - a piezoelectric converter (10) formed of a piezoelectric material, the piezoelectric converter (10) being interposed between a front electrode (11) and a rear electrode (12); - a housing (2) which accommodates the piezoelectric converter, the front electrode and the rear electrode; - a front aperture (15) formed in the housing and arranged such that the front electrode is arranged between the piezoelectric material and the front aperture (15); The device is configured to emit acoustic waves (EW) toward the front aperture or to detect acoustic waves (RW) propagating from the front aperture; The device comprises a rear part (13) forming a backing element of the device, the device being characterized in that the rear part is a porous metal material having a melting point above 200° C., the device being such that: - said rear component forms said rear electrode; - The thickness of the rear component is greater than 5 mm.

2. A device according to any preceding claim, wherein the thickness of the rear component is greater than 10 mm.

3. The device according to claim 1 or 2, wherein the melting point of the metal material is higher than 600°C.

4. An apparatus according to any preceding claim, wherein: - the piezoelectric material has a Curie temperature; - The melting point of the metal material is higher than the Curie temperature of the piezoelectric material.

5. The device of claim 4, wherein the Curie temperature of the piezoelectric material is greater than 1000°C.

6. A device according to any preceding claim, wherein the volume fraction of pores in the porous metal material is from 20% to 60%, or from 25% to 50%, or from 25% to 40%.

7. A device according to any preceding claim, wherein the average size of the pores is less than 100 μm, said average size corresponding to the average diameter of each pore.

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

9. The device according to any preceding claim, wherein the metallic material comprises at least one element selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, Al.

10. An apparatus according to any preceding claim, wherein the metallic material is a stainless steel alloy.

11. Use of a device according to any preceding claim for emitting or receiving sound waves (EW, RW), the emitted or received sound waves propagating through the orifice (15), the device being arranged in a medium (3) at a temperature above 200°C.

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