Acoustic transducer and method of using the acoustic transducer

Abstract

The present invention is an acoustic transducer, comprising a piezoelectric converter (10) formed from a piezoelectric material and interposed between a front electrode (11) and a back electrode (12), and a front opening (15) formed within a housing, with the front electrode disposed between the piezoelectric material and the front opening (15). The acoustic transducer is configured to transmit a sound wave (EW) to the front opening or detect a sound wave (RW) propagating from the front opening, and includes a back component (13) that is applied to or forms the back electrode, the back component forming a backing element of the acoustic transducer, the back component being a porous metal material having a melting point exceeding 200°C.

Description

Technical Field

[0001] The present invention relates to an acoustic transducer intended for use in operations such as non-destructive testing, obstacle detection, and distance measurement in high-temperature and high-pressure environments such as nuclear power plants.

Background Art

[0002] Ultrasonic non-destructive testing is suitable for monitoring structures to track resistance to aging and the occurrence of defects, or for operations such as distance measurement and obstacle detection. In particular, it is carried out in high-temperature and / or high-pressure environments. For example, it is used in the fields of aircraft engines, the oil industry, or nuclear reactors.

[0003] FIG. 1A schematically shows a conventional ultrasonic transducer 1 intended for use in a high-temperature environment AA . High temperature generally means a temperature higher than 200°C or higher than 600°C. The active element of the transducer is disposed within a housing 2. The active element includes a piezoelectric converter 10 formed from a piezoelectric material interposed between a front electrode 11 and a back electrode 12. Each electrode is connected to an electrical circuit 20. The front electrode is disposed facing an inlet opening 15 formed in the housing 2. Such a configuration is described in US Patent Document 2014215784.

[0004] When an alternating voltage is applied between the electrodes, the piezoelectric converter generates a sound wave EW. The transmitted sound wave EW propagates through the opening 15 to an external medium 3 outside the housing 2. The transducer can include a plate 14 that enables acoustic impedance matching between the transducer and the external medium 3. In this way, the transducer operates in a transmission mode.

[0005] The transducer can also operate in a receiving mode in which an acoustic wave RW propagates from an external medium 3 to the transducer. The received acoustic wave RW vibrates the piezoelectric converter. As a result, an alternating voltage is generated between the terminals of the electrodes 11, 12. Therefore, the transducer operates in the receiving mode.

[0006] For high-temperature applications, the piezoelectric material may be lithium niobate as described in U.S. Patent No. 9,425,384.

[0007] Figure 1B shows a timing diagram of the amplitude of the vibration of a prior art electrical transducer after receiving an acoustic wave. The y-axis corresponds to the amplitude of the vibration wave of the piezoelectric converter measured by an electrical circuit, and the x-axis corresponds to time. Transmission is triggered by applying an alternating voltage for a few to several tens of microseconds. However, the vibration of the piezoelectric converter continues for a much longer time on the order of several hundred μs (microseconds). This results in degradation of the transducer's performance.

[0008] To address this problem, it is known to connect the back electrode to an element that forms a damping backing, commonly referred to as a backing element. The main function of the backing element is to damp the vibration of the assembly formed by the piezoelectric converter and the electrode with the back electrode. In the prior art, the backing element of the transducer can be formed from a polymer doped with high-density particles, such as tungsten or lead particles. The expression "composite material particles" is also used. The random distribution of the particles induces multiple reflections, and as a result of destructive interference, the acoustic wave is attenuated. 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 applications.

[0009] The publication "Porous Ceramics as a 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 includes a backing element made of porous ceramic connected to an electrode. The use of ceramic makes it possible to achieve compatibility with high-temperature mounting. However, to manufacture such a transducer, it is necessary to bond the ceramic forming the backing element to the electrode. There remains uncertainty about the durability of such bonding. The resistance of porous ceramics to high radiation is also uncertain.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] International Publication No. 2016 / 124941 describes an apparatus for transmitting sound waves including a backing element formed from a metal foam.

[0011] The inventors have developed an ultrasonic transducer intended for use under high radiation conditions and at high temperatures, or under high temperature gradients, for a long operating time, i.e., for a time longer than several years or decades.

MEANS FOR SOLVING THE PROBLEMS

[0012] A first main subject of the present invention is an acoustic transducer comprising a piezoelectric converter formed from a piezoelectric material and interposed between a front electrode and a back electrode, a housing including the piezoelectric converter, the front electrode, and the back electrode, a front opening formed within the housing and having the front electrode disposed therebetween and between the piezoelectric material and the front opening, and configured to transmit an acoustic wave (EW) to the front opening or detect a propagating wave (RW) from the front opening. Applied to the back electrode or including a back component forming the back electrode, the back component forming a backing element of the acoustic transducer, the back component being a porous metal material having a melting point exceeding 200°C.

[0013] The device is implemented alone or in any technically feasible combination. The device can have any of the following features. - The back component forms the back electrode. - The melting point of the metal material exceeds 600°C. - 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. - The Curie temperature of the piezoelectric material is higher than 1000°C. - The volume ratio of the pores of the porous metal material is between 20% and 60%, or between 25% and 50%, or between 25% and 40%. - The pores are filled with air. - The average size of the pores is less than 100μm corresponding to the average diameter of each pore. - The piezoelectric material is selected from lithium niobate and barium titanate. - The metal material contains at least one element selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, Al. - The metal material is a stainless steel alloy.

[0014] Another subject of the present invention is A method of using an acoustic transducer according to the first subject for transmitting or receiving sound waves, The transmitted or received sound waves propagate through an opening, and the acoustic transducer is disposed in a medium with a temperature exceeding 200°C.

[0015] The present invention will be better understood by reading the disclosure of examples of embodiments described hereinafter in connection with the figures listed below.

Brief Description of the Drawings

[0016]

Figure 1A

Figure 1B

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 4

Figure 5A

Figure 5B

Figure 6A

Figure 6B

Figure 6C

DETAILED DESCRIPTION OF THE INVENTION

[0017] Figure 2A shows a first embodiment of the transducer 1 according to the present invention. As described in relation to the prior art, the transducer comprises a converter 10 formed by a piezoelectric material interposed between a front electrode 11 and a back electrode 12. The assembly consisting of the piezoelectric converter 10, the front electrode and the back electrode is arranged in a housing 2 having an opening 15. The front electrode 11 is arranged between the opening 15 and the piezoelectric converter 10. The device preferably comprises an acoustic impedance matching plate 14 interposed between the front electrode 11 and the opening 15. The impedance matching plate is formed, for example, from aluminum.

[0018] The transducer is connected to an electrical circuit 20 and enables an alternating voltage to be applied or measured between the front electrode and the back electrode. As described in relation to the prior art, when a short alternating voltage is applied, a sound wave EW passes through the opening 15 and propagates in the surrounding medium 3. The surrounding medium may in particular be a liquid or a solid. It may be water, a liquid material or a solid material. Under reception of the sound wave RW, an alternating electrical signal is detected by the electrical circuit 20, and the amplitude of that signal corresponds to the amplitude of the vibration of the piezoelectric converter under reception of the sound wave RW.

[0019] The piezoelectric converter 10 can take the form of a disk with a thickness of 1 mm and a diameter of 5 mm to 50 mm. The material used is suitable for use at high temperatures, for example between 200 °C and 700 °C, or even higher temperatures, for example 1000 °C or more. The piezoelectric material may be lithium niobate (LiNbO3). The resonance frequency of the piezoelectric converter may be from several hundreds of kHz to several MHz, for example 4 MHz or 5 MHz. The thickness of each electrode may be about 1 mm. Each electrode can take the form of a disk, the diameter of which corresponds to the diameter of the piezoelectric converter 10.

[0020] The electric circuit 20 is connected to a central unit 30 configured to control the electric circuit when the transducer is operating in the transmission mode and / or to analyze the voltage measured between the electrodes when the transducer is operating in the reception mode.

[0021] The transducer 1 includes a rear part 13 attached to the rear electrode 12. The rear part is intended to form a "backing element" for the piezoelectric converter 10. As described in the prior art, it is a problem to attenuate the echo of the sound wave transmitted to the rear of the piezoelectric converter. The thickness of the backing element is preferably greater than 5 mm, and further preferably 10 mm or more. It may be 10 mm to 100 mm, for example 40 mm.

[0022] One 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 higher than 200°C, preferably higher than 600°C or 700°C, and preferably higher than 1000°C. For example, it may be steel, such as stainless steel, or aluminum, or a metal selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, Al. It may also be a metal alloy such as bronze or brass. The backing element can be formed of the same material as the rear electrode, which facilitates joining them.

[0023] One advantage of the metal material, especially stainless steel, is its excellent resistance to corrosion and ionizing radiation (especially neutrons or gamma rays), which is suitable for use in nuclear power plants.

[0024] When the backing element is formed from the same material as the back electrode, a conductive metal, the step of joining the backing to the electrode is avoided. Conversely, when the backing element is made of ceramic, adhesives or brazing must be used to join them. Such joints may lack durability, especially when the transducer is exposed to high thermal gradients. Specifically, the coefficients of thermal expansion of ceramic and metal electrodes generally differ. Repeated exposure to particularly high thermal gradients can lead to degradation of the joint over time.

[0025] The material forming the piezoelectric converter 10 has a Curie temperature at which its piezoelectric behavior is thought to disappear. The melting point of the metal material forming the backing element is preferably higher than the Curie temperature of the piezoelectric converter. Lithium niobate has a Curie temperature greater than 1100 °C.

[0026] The use of a conductive metal material is advantageous. FIG. 2B schematically shows an embodiment in which the backing element and the back electrode form the same part. Thus, the back electrode is formed from a conductive porous metal material. Such an embodiment is particularly advantageous as it minimizes the number of parts forming the transducer and simplifies manufacture, especially by avoiding the need to join the backing element to the electrode.

[0027] The backing element 13 is configured to maximize the transmission of vibration waves generated by the piezoelectric transducer and minimize the reflection of said waves. FIGS. 3A and 3B show a configuration as shown in FIG. 2B, in which the piezoelectric converter 10 is arranged in direct contact with the backing element 13, which latter functions as a back electrode. At the interface between the piezoelectric converter 10 and the backing element 13, it is possible to define a transmission coefficient T and a reflection coefficient R. The transmission coefficient T corresponds to the following ratio. The amplitude of a wave called the incident wave that propagates from the piezoelectric converter 10 and is incident on the backing element 13, this amplitude being denoted Ai in FIGS. 3A and 3B. Also, it is the amplitude At of the transmitted wave and corresponds to the portion of the incident wave that propagates through the backing element.

[0028] The reflection coefficient R corresponds to the following ratio. The amplitude of the incident wave Ai. The amplitude of the reflected wave Ar, which corresponds to the portion of the incident wave reflected by the backing element 13 and propagated to the piezoelectric converter 10.

[0029] When Z1 and Z2 specify the acoustic impedances of the piezoelectric converter 10 and the backing element 13, respectively, the coefficients R and T are as follows.

Equation

Equation

Equation

[0030] Figure 3A schematically shows a configuration in which the reflection coefficient is high in absolute value and the transmission coefficient is low. This is a mismatched configuration. An echo is generated by the reflection of the incident wave at the interface between the piezoelectric converter 10 and the backing element 13, and the duration of the sound wave transmitted or received by the transducer becomes long. As a result, temporal degradation of the measurement occurs, and the time for wave transmission or reception is determined with reduced accuracy. When the transducer is used for distance measurement purposes, the spatial resolution of the measurement decreases.

[0031] Figure 3B schematically 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 an echo at the interface between the piezoelectric converter 10 and the backing element 13 is weak. As a result, the transmitted (or detected) sound wave becomes short, and the temporal resolution of the measurement is improved. The object of the present invention is to approach this configuration. Equation (3) shows that such a configuration can be obtained when the acoustic impedances of two adjacent media are close to each other, that is, when Z1 ≈ Z2.

[0032] The acoustic impedance of the piezoelectric converter 10 is generally several tens of MRayl, typically 25 to 40 MRayl (megarayl). In contrast, the acoustic impedance of air is 430 MRayl, and the acoustic impedance of water is 1.5 MRayl. The configuration shown in Fig. 3A corresponds to the interface between the piezoelectric converter and air. The configuration shown in Fig. 3B corresponds to the desired interface between the piezoelectric converter and the porous metal backing element.

[0033] In addition to having a transmission coefficient close to 1, the porous metal backing element needs to attenuate the transmitted sound wave.

[0034] The acoustic impedance and attenuation of the backing element 13 are controlled by the size and volume ratio of the pores filled with air. Various experimental attempts have been made to define the range of pore sizes and volume ratios that can obtain a transmission coefficient close to 1 and sufficient attenuation. Note that the effects of pore size and volume ratio on impedance depend on the material used. The values obtained for one material cannot be applied to another material.

[0035] An important parameter is the acoustic propagation velocity c, and the acoustic impedance Z can be calculated using the following equation.

Equation

[0036] Fig. 4 schematically shows the echo measurement test bench 100. Various porous metal samples 103 were placed between the acoustic transmitter 101 and the acoustic receiver 102. The entire lot was immersed in water 104.

[0037] Figure 5A shows the pulses received by the acoustic receiver 102 without a sample between the transmitter and the receiver. It corresponds to the detection time of the sound wave. The y-axis corresponds to the amplitude and the x-axis corresponds to the time. The propagation of the sound wave through the water 104 is represented by the dotted arrow.

[0038] Figure 5B shows the pulses received by the acoustic receiver 102 after the sample 103 with thickness e is placed 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 sound wave through the water 104 is represented by the solid arrow. Considering the interface formed by the sample 103, two pulses are detected. The first pulse corresponds to the wave that propagated through the sample without reflection at time t1. Regarding the transmission time of the sound wave by the transmitter 101, since the sound wave propagation speed in the sample 103 is faster than that in the water 104, the time t1 is earlier than the time t.

[0039] Based on these measurements, the sound wave propagation speed (or simply speed) c can be defined as follows. The sound wave propagation speed (or simply speed) c is calculated based on Δt = t1 - t. Δt is based on the following formula.

Equation

Equation

[0040] In Equation (5), Cw represents the acoustic propagation speed in water.

[0041] Samples of 316L stainless steel with various thicknesses e (5 mm or 10 mm), various pore volume ratios (between 25% and 53%), and various pore sizes (between an average diameter of 2 μm and 60 μm) were tested. The following table summarizes the characteristics of the tested samples. Each sample 103 is in the shape of a 50 mm × 50 mm plate.

[0042] The porosity of each sample was determined by measuring the mass per unit volume. The average pore size was determined by an optical microscope. The main properties of the tested samples are shown in Table 1.

Table 1

[0043] Figure 6A shows the propagation speed of sound waves obtained using Equation (6) as a function of porosity. In Figure 6A, the y-axis corresponds to the speed (m·s−1), and the x-axis corresponds to the volume fraction of pores (%).

[0044] Figure 6B shows the acoustic impedance calculated using Equation (4) based on the information on the density ρ of the test material and the speed obtained using Equation (6). The y-axis corresponds to the speed (unit: m / s), and the x-axis corresponds to the volume fraction of pores (%).

[0045] The linear attenuation coefficient α (unit: dB / mm) is also determined from the maximum amplitude of the wave detected by the receiver 102. The attenuation was measured by comparing the amplitudes of the detected pulses at times t1 and t2, respectively, defined with reference to Figure 6B.

Equation

[0046] From the calculated attenuations Atta and Attb for each sample, the linear attenuation coefficient per millimeter, denoted by α, was determined considering the following. The thickness of sample e when considering Atta. Twice the thickness e of the sample when considering Attb.

[0047] Figure 6C shows the attenuation coefficient α obtained using Equation (7). The y-axis corresponds to the attenuation (unit: dB / mm), and the x-axis corresponds to the volume fraction of pores (%).

[0048] In FIGS. 6A and 6C, each point corresponds to one measurement value. Also, a function obtained by interpolating each measurement point is plotted with a dashed line. In FIGS. 6A and 6B, the interpolation is linear. In FIG. 6C, the function obtained from the interpolation is a polynomial.

[0049] FIG. 6B defines a range of porosity, expressed as %, where the impedance is sufficiently close to that of the piezoelectric material, i.e., in the range of 10 - 40 MRayls. According to FIG. 6B, this corresponds to a porosity of less than 50%.

[0050] FIG. 6C defines a range of porosity, expressed as %, where the attenuation is sufficient. Sufficient attenuation means an attenuation coefficient α of 1 dB / mm or more. According to FIG. 6C, this corresponds to a porosity of more than 25%.

[0051] The range of the optimal pore volume fraction is as follows. Referring to FIG. 6B, noting that the impedance decreases with the pore volume fraction, the impedance of the porous metal material forming the backing element is sufficiently high (close to the impedance of the piezoelectric material). Referring to FIG. 6C, considering that the linear attenuation coefficient α increases with the pore volume fraction, a sufficiently high attenuation rate can be obtained.

[0052] From the above, it is understood that the attenuation effect is due to the presence of pores, and the impedance matching effect is due to the metal. Therefore, the porosity characteristics of the metal are the result of a compromise regarding the pore volume fraction.

[0053] In the case of 316L stainless steel, considering pore sizes (i.e., average diameters) between 2 μm and 60 μm, the optimal range of the pore volume fraction is between 25% and 50%.

[0054] The optimal range of the pore volume fraction may vary for other materials. Generally, the characteristics of the porous metal material forming the backing element 13 of the transducer are that the average pore diameter is less than 500 μm, and less than 200 μm or 100 μm. And / or the pore volume fraction is between 20% and 60%, preferably between 25% and 50%, more preferably between 25% and 40%.

[0055] In the above-described embodiment, the piezoelectric material forming the converter is made of lithium niobate. Other piezoelectric materials suitable for high-temperature environments, such as barium titanate (BaTiO3), bismuth titanate (BiTiO3) and its derivatives (e.g., addition of sodium), aluminum nitride (AlN), langatate (oxide of lanthanum, gallium, tantalum) can be used.

[0056] The transducer according to the present invention can be used for any high-temperature application for the purpose of non-destructive testing, diagnosis, range detection, obstacle detection, or flow rate measurement.

Claims

1. It is an acoustic transducer, A piezoelectric converter (10) is formed from a piezoelectric material and interposed between the front electrode (11) and the back electrode (12), A housing (2) including the piezoelectric converter, the front electrode and the back electrode, The acoustic impedance matching plate (14) is formed within the housing and has the front electrode positioned between the piezoelectric material and the acoustic impedance matching plate, Equipped with, It is configured to transmit sound waves (EW) to the front opening or to detect sound waves (RW) propagating from the acoustic impedance matching plate, The back component (13) is included, the back component forms the backing element of the acoustic transducer, and the back component is a porous metal material having a melting point of over 200°C. The aforementioned back component forms the back electrode, The thickness of the aforementioned rear component exceeds 5 mm. Acoustic transducer.

2. The thickness of the aforementioned rear component exceeds 10 mm. The acoustic transducer according to claim 1.

3. The melting point of the porous metal material exceeds 600°C. The acoustic transducer according to claim 1 or 2.

4. The piezoelectric material has a Curie temperature, The melting point of the porous metal material is higher than the Curie temperature of the piezoelectric material. The acoustic transducer according to claim 1.

5. The Curie temperature of the piezoelectric material is higher than 1000°C. The acoustic transducer according to claim 4.

6. The volume fraction of pores in the porous metal material is between 20% and 60%, or between 25% and 50%, or between 25% and 40%. The acoustic transducer according to claim 1 or 2.

7. The average size of the aforementioned pores is less than 100 μm, which corresponds to the average diameter of each pore. The acoustic transducer according to claim 1 or 2.

8. The piezoelectric material is selected from lithium niobate and barium titanate. The acoustic transducer according to claim 1 or 2.

9. The porous metal material comprises at least one element selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, and Al. The acoustic transducer according to claim 1 or 2.

10. The porous metal material is a stainless steel alloy. The acoustic transducer according to claim 1 or 2.

11. A method of using an acoustic transducer according to claim 1 or 2 for transmitting or receiving sound waves (EW, RW), The transmitted or received sound waves propagate through the opening (15), and the acoustic transducer is placed in a medium (3) with a temperature exceeding 200°C. How to use an acoustic transducer.

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