Lithium niobate piezoelectric transducer

Abstract

The invention relates to a method for manufacturing a piezoelectric transducer (1) comprising a front electrode (10a), a rear electrode (10b) and a piezoelectric ceramic (50) made of lithium niobate. The method comprises: a step (a) of depositing on each opposite face of the ceramic (50) a primary metallic bonding layer (40a, 40b); a step (b) of depositing on each electrode (10a, 10b) a compliance layer (20a, 20b) made of an electrically conductive material with Tmelting > 700°C and δrupture > 20%; a step (c) of placing a foil (30a, 30b) based on an Ag-Cu alloy on each primary bonding layer (40a, 40b); a step (d) of forming an assembly (60) by stacking in this order the elements (10a, 20a, 30a, 40a, 50, 40b, 30b, 20b, 10b); a step (e) of brazing the assembly (60) in compression in a furnace at more than 450°C in a non-oxidizing atmosphere. Figure 1

Description

Title of the invention: Lithium niobate piezoelectric transducer

[0001] The present invention lies in the field of high-temperature ultrasonic translators (or transducers) (TUSHT) which are used in particular for the non-destructive testing of structures by ultrasound. Such translators make it possible to monitor the resistance to aging and the appearance of possible defects in parts, and are also used for telemetry or obstacle detection operations in the nuclear industry characterized by high-temperature and / or high-pressure environments. By high temperature is meant a temperature above 200°C, for example above 600°C.

[0002] [Fig.4] illustrates a measuring device that integrates such a transducer. This measuring device comprises a housing 6 that contains the transducer 100. The transducer 100 comprises a piezoelectric material interposed between a front electrode 10a and a rear electrode 10b. Each electrode (10a, 10b) is connected to an electrical circuit. The front electrode 10a is integrated into an opening provided in the housing 6, such that the front electrode 10a is integrated into the housing 6. Under the effect of the application of an alternating voltage between the electrodes (10a, 10b), the piezoelectric material generates an acoustic wave W. The emitted acoustic wave W propagates towards an external medium. The transducer 100 thus operates in a transmission mode but it can also operate in a reception mode, during which an acoustic wave propagates from the external medium towards the transducer.The received acoustic wave causes the piezoelectric converter to vibrate, resulting in the appearance of an alternating voltage across the electrodes (10a, 10b). The transducer 100 thus operates in a receiving mode. For high-temperature applications, the piezoelectric material may be gallium phosphate, aluminum nitride, langasite, bismuth titanate, or lithium niobate, as described in patent FR2977377. The measuring device also comprises, in the housing 6, an electrically insulating plate 2 that covers the outer face of the rear electrode 10b (i.e., the outer face of the transducer 100), a blocking piece 3 that covers the insulating plate 2, and a tube 4 that is an electrical insulator and whose lower end is in contact with the blocking piece 3. The housing 6 also comprises a coaxial cable 5 that is housed in the tube 4.The coaxial cable 5 exits the tube 4 at its lower end and passes through the blocking piece 3 and the insulating plate 2, and is in contact with the external face of the rear electrode 10b. At its other end, the coaxial cable 5 is connected to a processing unit (not shown) which . analyzes the voltage of the signal coming from the transducer 100 and transmitted by the coaxial cable 5.

[0003] In order to achieve the desired transducer performance under the temperature and pressure conditions of a nuclear reactor, the choice of the various transducer components and their assembly process is essential. Lithium niobate is a material of choice for piezoelectric ceramics because it is one of the few materials to retain its integrity and properties above 1000°C. The front and rear electrodes are made of stainless steel. Lithium niobate is a single crystal (manufactured for example by the Czochralski method), with a Young's modulus of 170 Gpa and a Vickers hardness Hv of approximately 600). Its coefficient of expansion at 25°C is 15.7.106 parallel to the variable "c" of the mesh.

[0004] The piezoelectric transducer 100 described in patent FR2977377 is illustrated in [Fig.5], which details its structure. The piezoelectric transducer 100 is manufactured by joining a piezoelectric ceramic 150 with the electrodes (10a, 10b). This joining is carried out by vacuum diffusion brazing. In order for the brazing to be effective, it is necessary to insert, between each electrode and the piezoelectric ceramic, a multilayer junction. This junction notably comprises adhesion layers and protective layers. In patent FR2977377, there is thus a front junction 111a between the front electrode 10a and the piezoelectric ceramic 150, and a rear junction 111b between the rear electrode 10b and the piezoelectric ceramic 150. The front junction 11a and the rear junction 111b are identical and are each symmetrical with respect to their median plane and made up of eight layers in the following order C1-C2-C3-C4-C4-C3-C2-C1.The Cl layer is a bonding layer, the C2 and C3 layers are solder layers, and the C4 layer is a protective layer. Half of a junction (111a, 111b) showing the Cl, C2, C3, C4 layers is shown in the insert in [Fig.5]. The bonding layer Cl is made of chromium (Cr) or a nickel-chromium alloy (Ni-Cr), the C2 solder layer is made of gold (Au), the C3 solder layer is made of indium (In), and the C4 protective layer is made of gold.

[0005] Thus, a method is known for manufacturing a piezoelectric transducer comprising a front electrode, a rear electrode and a lithium niobate piezoelectric ceramic having a front face and a rear face opposite the front face.

[0006] High scrap rates have been observed during the manufacture of transducers such as described above. These failures are due to the brazing process with an Au-In filler alloy which is characterized by a low brazing rate (ratio between the brazed surface and the surface of the piezoelectric ceramic) and / or by partial or total deterioration of the piezoelectric ceramic.

[0007] For these reasons, attempts have been made to produce transducers using layers of C2 and C3 solder made of silver (Ag) and copper (Cu) alloy instead of Au-In. However, damage to the transducers after cooling was observed, and in particular the appearance of cracks in the lithium niobate. This damage is due to residual stresses generated during cooling at the interfaces between the junctions and the ceramic and between the junctions and the electrodes. Description of the invention

[0008] The present invention aims to remedy these drawbacks.

[0009] The invention aims to propose a method for manufacturing a piezoelectric transducer with a lithium niobate piezoelectric ceramic which makes it possible to obtain transducers which are operational over a long lifetime and in a wide temperature range, in particular at high temperature (greater than 200°C, or even up to 1000°C).

[0010] This aim is achieved by the fact that the method comprises the following steps: (a) A metallic front primer layer is deposited on the front face and a metallic rear primer layer on the rear face without intrinsic or surface transformation of the ceramic; (b) A front compliance layer is deposited on one face of the front electrode, and a back compliance layer is deposited on one face of the back electrode, the material of the compliance layers being an electrical conductor and having a melting point greater than 700°C and an elongation at break greater than 20%. (c) A front foil is placed on the front primer coat and a back foil is placed on the back primer coat, each foil being based on a silver and copper alloy. (d) The front electrode is positioned such that the front compliance layer is in contact with the front foil and the rear electrode is positioned such that the rear compliance layer is in contact with the rear foil to form an assembly along a main axis X which is made up of the piezoelectric ceramic, the primary bonding layers, the foils, the compliance layers and the electrodes. (e) The assembly is brazed in a furnace at a brazing temperature above 450°C in a non-oxidizing atmosphere.

[0011] Thanks to these provisions, the piezoelectric transducer is more robust, more durable, and has better sensitivity, in particular at higher temperatures.

[0012] For example, the deposition in step (a) is carried out either by metallization or by cathodic sputtering.

[0013] For example, the material of the primary bonding layers is chosen from Cr, Ni-Cr, Ti, Ag, Au, Pt.

[0014] For example, this material is a Ni-Cr alloy.

[0015] For example, the material of the compliance layers is copper or an alloy with copper base.

[0016] For example, the strips have a thickness of less than 1 mm.

[0017] For example, in step (c), the alloy is binary at the composition of the eutectic with 72% silver and 28% copper.

[0018] For example, in step (e) the assembly is compressed with a pressure which is less than 10 MPa.

[0019] For example, in step (e) the brazing is carried out under partial gas pressure or under vacuum.

[0020] The invention also relates to a piezoelectric transducer comprising a front electrode, a rear electrode and a lithium niobate piezoelectric ceramic having a front face and a rear face opposite the front face, such that the piezoelectric transducer comprises the following elements stacked along a main axis X between the electrodes: a front compliance layer deposited on one face of the front electrode, a front foil, a metallic front primer layer deposited on the front face without intrinsic or surface transformation of the ceramic, the piezoelectric ceramic, a metallic rear primer layer deposited on the rear face without intrinsic or surface transformation of the ceramic, a rear foil and a rear compliance layer deposited on one face of the rear electrode,the material of the compliance layers being an electrical conductor and having a melting point greater than 700°C and an elongation at break greater than 20% and each of the strips being based on an alloy of silver and copper, the piezoelectric transducer being produced by strong brazing in a furnace at a brazing temperature greater than 450°C in a non-oxidizing atmosphere.

[0021] The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of embodiments shown as non-limiting examples. The description refers to the appended drawings in which:

[0022] [Fig-1] [Fig.l] illustrates a piezoelectric transducer according to the invention.

[0023] [Fig.2] [Fig.2] is an example of a brazing cycle in the process according to the invention.

[0024] [Fig.3] [Fig.3] shows the intensity of a signal received by a piezo transducer electric according to the invention and the intensity of a signal received by a piezoelectric transducer of the prior art.

[0025] [Fig.4] [Fig.4], already described, illustrates a device which integrates a transducer.

[0026] [Fig.5] [Fig.5] illustrates a transducer according to the prior art. Detailed description of the invention

[0027] The invention relates to a method for manufacturing a piezoelectric transducer 1 comprising a front electrode 10a, a rear electrode 10b and a piezoelectric ceramic 50 made of lithium niobate having a front face 51a and a rear face 51b opposite the front face 51a. Such a piezoelectric transducer 1 is suitable for use in a measuring device as described above, as a replacement for the piezoelectric transducer 100.

[0028] The different stages of the process are described below:

[0029] In step (a), a front metallic primer bonding layer 40a is deposited on the front face 51a and a rear metallic primer bonding layer 40b on the rear face 51b. The primer bonding layer (40a, 40b) is necessary to achieve the bonding of the strips (30a, 30b) (see below) on the piezoelectric ceramic 50 during the final brazing operation. The deposition of the primer bonding layers (40a, 40b) must be carried out without intrinsic or surface transformation (such as oxidation, carburization, nitriding) of the deposited elements or of the lithium niobate, this transformation being undesirable because it modifies the properties of the lithium niobate. For example, this deposition is carried out by metallization or by cathodic sputtering. For example, the thickness of each primer layer (40a, 40b) is between 10 nm and 100 nm (109 μm).

[0030] In step (b), a front compliance layer 20a is deposited on a face 11a of the front electrode 10a, and a rear compliance layer 20a on a face 11b of the rear electrode 10b. The compliance layers (20a, 20b) are made of a material, for example metallic, which is a good conductor of electricity, with a melting temperature greater than 700°C and ideally greater than 1000°C. This material is capable of adhering intimately to the material of the electrode (10a, 10b) and without cracking. The front electrode 10a and the rear electrode 10b are for example made of stainless steel, for example 304L. The material of the compliance layer (20a, 20b) has sufficient ductility to absorb the thermal and / or mechanical stresses generated during brazing. Thus, this material has an elongation at break greater than 20%. Advantageously, this material is copper or a copper-based alloy.For example, this alloy is copper with a purity greater than 99%, for example greater than 99.9%.

[0031] Advantageously, the deposition of the compliance layers (20a, 20b) is carried out without intrinsic or surface transformation of the material of these layers (20a, 20b). For example, the thickness of each compliance layer (20a, 20b) is between 20 μm and 300 μm.

[0032] The compliance layers (20a, 20b) have a dual function: to ensure, during the final brazing operation, the role of diffusion material with the metal of the strips (30a, 30b) (see below) during the thermal cycle and also to ensure the role of compliance material to absorb thermal and mechanical stresses during the brazing operation.

[0033] In step (c), a front strip 30a is placed on the front primer layer 40a and a rear strip 30b on the rear primer layer 40b. The strips (30a, 30b) are based on a silver-copper alloy.

[0034] The strips (30a, 30b) comprise an alloy of silver and copper. For example, this alloy is a binary Ag-Cu alloy, for example a eutectic alloy with 72% Ag and 28% Cu. Alternatively, this alloy is ternary Ag-Cu-Ml where Ml is another metal. Alternatively, this alloy is quaternary Ag-Cu-Ml-M2 where M2 is yet another metal. This alloy is the brazing alloy.

[0035] The strips (30a, 30b) have a thickness of less than 1 mm. Advantageously, the thickness is between 10 μm and 100 μm, for example equal to 50 μm.

[0036] In step (d), the front electrode 10a is positioned such that the front compliance layer 20a is in contact with the front strip 30a and the rear electrode 10b is positioned such that the rear compliance layer 20b is in contact with the rear strip 30b to form a symmetrical assembly 60 which is made up of the piezoelectric ceramic 50, the primary bonding layers (40a, 40b), the strips (30a, 30b), the compliance layers (20a, 20b) and the electrodes (10a, 10b).

[0037] The main axis X is perpendicular to the first 51a and second 51b faces and passes perpendicularly through the piezoelectric ceramic 50, the primary bonding layers (40a, 40b), the strips (30a, 30b) and the compliance layers (20a, 20b).

[0038] In step (e), in the case where the main axis X is vertical, each element (layers, strips, ceramic) located between the electrodes (10a, 10b) is subjected to compression along this main axis X which is due to the weight of the element(s) located above this element, if applicable. Optionally, the assembly is compressed along the main axis X with a pressure of less than 10 MPa in order to avoid flow of the solder during the manufacture of the sensor.

[0039] In step (e), the assembly 60 is brazed in a furnace at a brazing temperature Tb greater than 450°C in a non-oxidizing atmosphere and with compression of the assembly 60 along the main axis X. The assembly 60 is heated at a heating rate which is, for example, relatively low (a few degrees per minute) with one or more homogenization stages up to a brazing temperature Tb which is greater than the liquidus temperature L of the brazing alloy. This brazing temperature Tb is then maintained for a minimum duration in order to allow wetting and diffusion between the different materials. The atmosphere of the furnace during brazing is non-oxidizing. For example, the furnace is filled with a gas mixture, for example a neutral gas such as argon. Advantageously, the interior of the furnace is under partial pressure, or even under vacuum. At a given temperature, the total pressure of a gas mixture is equal to the sum of the partial pressures exerted by each of the gases making up the mixture. [Fig.l] illustrates a piezoelectric transducer (1) obtained with this process, in section along the main axis X.

[0040] [Fig.2] shows an example of a thermal cycle during the brazing of step (e), in the case of an alloy of the strips (30a, 30b) which is a binary eutectic (72% Ag, 28% Cu). The abscissa axis is the time t, the ordinate axis is the temperature T. The heating phase includes a homogenization stage at a temperature P substantially equal to 200°C. The liquidus temperature L is equal to 780°C. The brazing temperature Tb, which is approximately equal to 800°C, is maintained for at least 10 minutes, for example 10 to 15 minutes.

[0041] The non-destructive impedancemetry and echometry tests carried out by the inventors show that the acoustic properties of the frequency spectrum for the piezoelectric transducer according to the invention are preserved. The comparison of the echograms of transducers brazed one with an Au-In brazing and the other with the Ag-Cu brazing according to the invention shows a better sensitivity and axial resolution of the piezoelectric transducer according to the invention, as illustrated in [Fig. 3]. The intensity of the signal received by a piezoelectric transducer 1 according to the invention (curve 1) is greater than the intensity of the signal received by a prior art Au-In piezoelectric transducer (curve 2 in bold).

[0042] Advantageously, the various elements of the piezoelectric transducer 1 are machined. For example, this machining is mechanical using machine tools (lathe, milling machine, grinder).

[0043] Advantageously, the faces of the various constituent parts of the assembly 60 to be brazed are polished (for example with a roughness Ra < 3.2 pm) before they are brought into contact. Thus, better adhesion of the parts is obtained. Advantageously, the flatness of these faces is less than 0.1 mm.

[0044] The invention also relates to a piezoelectric transducer 1 comprising a front electrode 10a, a rear electrode 10b and a piezoelectric ceramic 50 made of lithium niobate having a front face 51a and a rear face 51b opposite said front face 51a. The piezoelectric transducer 1 comprises the following elements stacked along a main axis X between the electrodes 10a, 10b: a front compliance layer 20a deposited on a face 11a of the front electrode 10a, a front foil 30a, a front metallic primer bonding layer 40a deposited on the front face 51a without intrinsic or surface transformation of the ceramic 50, the piezoelectric ceramic 50, a rear metallic primer bonding layer 40b metal deposited on the rear face 51b without intrinsic or surface transformation of the ceramic 50, a rear foil 30b and a rear compliance layer 20b deposited on a face 11b of the rear electrode 10b, the material of the compliance layers 20a, 20b being an electrical conductor and having a melting temperature greater than 700°C and an elongation at break greater than 20%, and each of the foils 30a, 30b being made of an alloy comprising silver and copper, the piezoelectric transducer 1 being produced by strong brazing in a furnace at a brazing temperature Tb greater than 450°C in a non-oxidizing atmosphere and, optionally, with compression along the main axis X.

Claims

Claims

1. Method for manufacturing a piezoelectric transducer (1) comprising a front electrode (10a), a rear electrode (10b) and a piezoelectric ceramic (50) made of lithium niobate having a front face (51a) and a rear face (51b) opposite said front face (51a), said method being characterized in that it comprises the following steps: (a) A front metallic primer layer (40a) is deposited on said front face (51a) and a rear metallic primer layer (40b) is deposited on said rear face (51b) without intrinsic or surface transformation of said ceramic (50).(b) A front compliance layer (20a) is deposited on a face (11a) of said front electrode (10a), and a rear compliance layer (20b) is deposited on a face (11b) of said rear electrode (10b), the material of said compliance layers (20a, 20b) being an electrical conductor and having a melting temperature greater than 700°C and an elongation at break greater than 20%. (c) A front foil (30a) is placed on said front primer layer (40a) and a rear foil (30b) is placed on said rear primer layer (40b), each of said foils (30a, 30b) being based on a silver and copper alloy.(d) Said front electrode (10a) is positioned such that said front compliance layer (20a) is in contact with said front strip (30a) and said rear electrode (10b) is positioned such that said rear compliance layer (20b) is in contact with said rear strip (30b) to form an assembly (60) along a main axis (X) which is made up of said piezoelectric ceramic (50), said primary bonding layers (40a, 40b), said strips (30a, 30b), said compliance layers (20a, 20b) and said electrodes (10a, 10b). (e) Strong brazing of said assembly (60) is carried out in a furnace at a brazing temperature (Tb) greater than 450°C in a non-oxidizing atmosphere.

2. A method according to claim 1 such that the deposition in step (a) is carried out either by metallization or by cathodic sputtering.

3. Method according to claim 1 or 2 such that the material of said primary bonding layers (40a, 40b) is chosen from Cr, Ni-Cr, Ti, Ag, Au, Pt.

4. Method according to claim 3 such that the material of said primary bonding layers (40a, 40b) is a Ni-Cr alloy.

5. A method according to any one of claims 1 to 4 such that the material of said compliance layers (20a, 20b) is copper or a copper-based alloy.

6. Method according to any one of claims 1 to 5 such that said strips (30a, 30b) have a thickness of less than 1 mm.

7. A method according to any one of claims 1 to 6 such that in step (c) the alloy is binary at the composition of the eutectic with 72% silver and 28% copper.

8. A method according to any one of claims 1 to 7 such that in step (e) the assembly is compressed with a pressure which is less than 10 MPa.

9. A method according to any one of claims 1 to 8 such that in step (e) the brazing is carried out under partial gas pressure or under vacuum.

10. Piezoelectric transducer (1) comprising a front electrode (10a), a rear electrode (10b) and a lithium niobate piezoelectric ceramic (50) having a front face (51a) and a rear face (51b) opposite said front face (51a), said piezoelectric transducer (1) being characterized in that it comprises the following elements stacked along a main axis (X) between said electrodes (10a, 10b): a front compliance layer (20a) deposited on a face (11a) of said front electrode (10a), a front foil (30a), a front metallic primer bonding layer (40a) deposited on said front face (51a) without intrinsic or surface transformation of said ceramic (50), said piezoelectric ceramic (50), a rear metallic primer bonding layer (40b) deposited on said rear face (51b) without transformation intrinsic or surface of said ceramic (50),a rear foil (30b) and a rear compliance layer (20b) deposited on one face (11b) of said rear electrode (10b), the material of said compliance layers (20a, 20b) being an electrical conductor and having a melting temperature greater than 700°C and an elongation at break greater than 20% and each of said foils (30a, 30b) being based on a silver and copper alloy, said piezoelectric transducer (1) being produced by strong brazing in a furnace at a brazing temperature (Tb) greater than 450°C in a non-oxidizing atmosphere.,

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