PIEZOELECTRIC TRANSDUCER, MANUFACTURING PROCESS PERTAINING THERETO, AND RESONANT ULTRASOUND SPECTROSCOPY DEVICE

Abstract

Disclosed is a piezoelectric transducer of the type including a stack of a contact piece, a piezoelectric pellet and a support of very great stiffness. Specifically, the contact piece forms tips, the respective apex of which constitutes contact points that are spatially isolated from one another. Also disclosed is a process for the manufacture of such a contact piece as well as a resonant ultrasound spectroscopy device including one or more of such piezoelectric transducers.

Claims

1. Piezoelectric transducer (9, 9a, 9b) comprising a piezoelectric pellet (2) and a contact piece (1) stacked on the piezoelectric pellet (2), wherein the contact piece (1) forms tips (10), the respective apex of which constitutes contact points (11) spatially isolated from one another, and wherein is arranged in order to establish a mechanical contact, by at least one of the contact points (11), with the sample (E1) to be analyzed.

2. Piezoelectric transducer according to claim 1, wherein the contact points (11) are distributed over a convex surface (S1).

3. Piezoelectric transducer according to claim 1, wherein the contact points (11) are distributed such that no geometrical plane passes less than 0.5 m from more than three of said contact points.

4. Piezoelectric transducer according to claim 1, wherein the tips (10) have a height (D2) comprised between 1 m and 10 mm.

5. Piezoelectric transducer according to claim 1, wherein the contact points (11) are regularly spaced apart with a pitch (D1) comprised between 1 m and 10 mm.

6. Piezoelectric transducer according to claim 1, wherein the contact piece (1) is made from a material the ratio {square root over (E.sub.c)}/d.sub.c of which is greater than 1 GPa.sup.1/2*cm.sup.3/g, with E.sub.c the Young's modulus and d.sub.c the density of this material.

7. Piezoelectric transducer according to claim 1, wherein the contact piece (1) comprises or consists of polycrystalline diamond and/or polycrystalline cubic boron nitride and/or beryllium and/or aluminium and/or magnesium.

8. Piezoelectric transducer according to claim 1, wherein in a transverse plane, the contact piece (1) forms a radially truncated disk.

9. Piezoelectric transducer according to claim 1, further comprising a support (3, 3b) and wherein the piezoelectric pellet (2) is mounted between the support (3, 3b) and the contact piece (1).

10. Piezoelectric transducer according to claim 9, wherein the support (3, 3b) is made from a material the ratio {square root over (E.sub.s)}/d.sub.s of which is greater than 3 GPa.sup.1/2*cm.sup.3/g, with E.sub.s the Young's modulus and d.sub.s the density of this material.

11. Piezoelectric transducer according to claim 9, wherein the support (3, 3b) comprises or consists of polycrystalline diamond and/or polycrystalline cubic boron nitride and/or beryllium and/or silicon carbide and/or aluminium.

12. Piezoelectric transducer according to claim 9, wherein the support (3) is axisymmetric.

13. Piezoelectric transducer according to claim 12, wherein the cylindrical support (3) has a ratio h/ds comprised between 0.3 and 2, with h the height and ds the diameter of this cylindrical support.

14. Piezoelectric transducer according to claim 12, wherein the support (3, 3b) is a sphere (3b) truncated axially so as to form a mounting surface, and wherein the piezoelectric pellet (2) is mounted on this mounting surface.

15. Piezoelectric transducer according to claim 9, further comprising: a bearing element (7, 7a, 7b), and one or more positioning elements (41, 42, 43), this or these positioning elements (41, 42, 43) linking the support and the bearing element (7, 7a, 7b) and being made from a material the stiffness of which is less than 200 Mpa.

16. Piezoelectric transducer according to claim 15, wherein the positioning element(s) (41, 42, 43) is (are) arranged in order to filter the frequencies of acoustic waves greater than 5 kHz.

17. Piezoelectric transducer according to claim 1, wherein the piezoelectric transducer is arranged in order to transmit or receive acoustic waves having frequencies comprised between 5 kHz and 1 Mhz.

18. Piezoelectric transducer according to claim 1, wherein the contact piece (1) comprises at least nine tips (10).

19. Resonant ultrasound spectroscopy device comprising at least one piezoelectric transducer (9, 9a, 9b) according to claim 1.

20. Device according to claim 19, comprising two of the piezoelectric transducers (9a, 9b), wherein the directions in which the tips of one (9a) of said piezoelectric transducers extend are: parallel, or intersect with the directions in which the tips of the other (9b) one of said two piezoelectric transducers extend, the points of intersection of said directions being situated in the direction in which the tips extend from their base to the contact points.

21. Process for the manufacture of a contact piece (1) for a piezoelectric transducer (9, 9a, 9b) according to claim 1, comprising a step of machining the tips (10) by wire electrical discharge machining.

22. Process according to claim 21, wherein the step of machining the tips (10) comprises: a first series of lines cut by electroerosion of the contact piece (1), parallel to a first axis of erosion, a second series of lines cut by electroerosion of the contact piece (1), parallel to a second axis of erosion, the first axis of erosion not being parallel to the second axis of erosion.

Description

DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0056] Other advantages and characteristics of the invention will become apparent on reading the detailed description of implementations and embodiments that are in no way limitative, and from the attached figures:

[0057] FIG. 1 is a perspective view of a contact piece of a transducer according to the invention;

[0058] FIG. 2 and FIG. 3 are perspective views of a transducer according to a variant of the invention;

[0059] FIG. 4 is a perspective view of a transducer according to another variant of the invention;

[0060] FIG. 5 is a perspective view of a part of a resonant ultrasound spectroscopy device according to the invention, comprising the transducer in FIGS. 2 and 3;

[0061] FIG. 6 is a partial perspective view of a resonant ultrasound spectroscopy device according to the invention, comprising two transducers;

[0062] FIG. 7 is a cross section view of a contact piece of a transducer according to the invention;

[0063] FIG. 8a and FIG. 8b diagrammatically represent a tip of a contact piece of a transducer according to the invention.

[0064] As the embodiments described hereinafter are in no way limitative, variants of the invention can in particular be considered comprising only a selection of the characteristics described, in isolation from the other characteristics described, even if this selection is isolated within a sentence comprising these other characteristics, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0065] Piezoelectric Transducer

[0066] FIGS. 2 to 4 show piezoelectric transducers according to the invention. These transducers comprise a stack, typically with bonding, of an axisymmetric support 3, a piezoelectric pellet 2 and a contact piece 1, the piezoelectric pellet 2 being mounted between the support 3 and the contact piece 1.

[0067] Transducer Support

[0068] The transducer support 3 in FIGS. 2 and 3 is cylindrical and can typically have a ratio h/ds comprised between 0.3 and 2, preferably comprised between 0.4 and 1.5, more preferentially comprised between 0.6 and 1, with h the height (typically comprised between 2 and 20 mm) and ds the diameter (typically comprised between 2 and 30 mm) of this cylindrical support 3.

[0069] The support 3b of the transducer in FIG. 4 is a sphere (the diameter typically comprised between 3 and 30 mm) truncated axially. The truncation forms a mounting surface on which is mounted the piezoelectric pellet 2.

[0070] From the point of view of the composition of the support 3, the latter can: [0071] be made from a material the ratio {square root over (E.sub.s)}/d.sub.s of which is greater than 3 GPa.sup.1/2*cm.sup.3/g, preferably greater than 4 GPa.sup.1/2*cm.sup.3/g, more preferentially greater than 6 GPa.sup.1/2*cm.sup.3/g, with E.sub.s the Young's modulus and d.sub.s the density of this material; and/or [0072] comprise or consist of polycrystalline diamond and/or polycrystalline cubic boron nitride and/or beryllium and/or silicon carbide and/or aluminium.

[0073] Contact Piece of the Transducer

[0074] FIG. 1 shows the contact piece 1 of the transducer in FIGS. 2 to 4.

[0075] It can be seen from FIG. 1 that the contact piece 1 forms tips 10 the respective apex of which constitutes contact points 11 spatially isolated from one another.

[0076] The contact points 11 of the transducer are provided in order to place the transducer in contact, by a limited number of these contact points 11, with a sample to be analyzed, such as illustrated in FIG. 7. FIG. 7 shows a sample E2 in contact with two contact points 11b of the contact piece 1b.

[0077] In other words, the transducer according to the invention is arranged in order to establish a mechanical contact, by at least one of the contact points, with a sample to be analyzed.

[0078] Of course, taking account of the known machining techniques, in fact the contact points 11c form not a tip as such, but a surface 11d, at least below a certain scale of observation of a tip 10c (see FIGS. 8a and 8b). However, this surface must have negligible dimensions with respect to the dimensions of the transducer.

[0079] A distribution of contact points 11b over a convex surface S1 is apparent from FIG. 7.

[0080] The surface S1 is called convex because it appears convex when viewed from outside the transducer. In other words, the surface S1 is concave on the side of the tips 10 and convex on the other side.

[0081] Thus, the contact points 11 of the contact piece 1 in FIGS. 1 to 4 preferably form a discrete convex surface.

[0082] The contact points 11 are distributed such that no geometrical plane passes through more than three of said contact points 11, or no geometrical plane passes less than 0.5 m from more than three of said contact points 11.

[0083] From a dimensional point of view: [0084] the contact points 11 can be regularly spaced apart with a pitch D1 (preferably, but not necessarily, constant) comprised between 1 m and 10 mm, preferably comprised between 20 m and 3 mm, more preferentially comprised between 50 m and 1 mm, and/or [0085] the tips 10 can have a height D2 comprised between 1 m and 10 mm, preferably comprised between 20 m and 3 mm, more preferentially comprised between 50 m and 1 mm.

[0086] Concerning the composition thereof, the contact piece 1 can: [0087] be made from a material the ratio {square root over (E.sub.c)}/d.sub.c of which is greater than 1 GPa.sup.1/2*cm.sup.3/g, preferably greater than 2 GPa.sup.1/2*cm.sup.3/g, more preferentially greater than 2.5 GPa.sup.1/2*cm.sup.3/g, with E.sub.c the Young's modulus and d.sub.c the density of this material; and/or [0088] comprise or consist of polycrystalline diamond and/or polycrystalline cubic boron nitride and/or beryllium and/or aluminium and/or magnesium.

[0089] With reference to FIGS. 1 to 6, in a transverse plane, the contact piece 1 forms a radially truncated disk.

[0090] As shown in FIG. 5, such a truncation frees a space and despite the stacking of the piezoelectric pellet 2 with the contact piece 1, makes it possible to solder the cables 81 and 82 onto two terminals of the piezoelectric pellet 2 situated on a top of this pellet, a first one of these terminals being electrically connected to one face of the piezoelectric pellet 2 and a second one of these terminals being electrically connected to the other face of the piezoelectric pellet 2.

[0091] The contact piece 1 can comprise at least nine tips, preferably at least sixteen tips, more preferentially at least twenty-five tips.

[0092] Piezoelectric Pellet 2

[0093] For the pellet 2, typically a PZT (Lead Zirconate Titanate) pellet is used, having a thickness comprised between 0.05 and 3 mm.

[0094] Transducer+Support

[0095] In the embodiments in FIGS. 2, 3, 5 and 6, the transducer comprises positioning elements 41, 42 and 43, shown in FIG. 3.

[0096] Preferably, these positioning elements 41, 42 and 43 can be made from a material the stiffness of which is less than 200 Mpa, preferably less than 50 Mpa, more preferentially less than 10 Mpa, typically comprising or consisting of silicone or polyurethane.

[0097] Moreover, these positioning elements 41, 42 and 43 can be arranged in order to filter the frequencies of acoustic waves greater than 5 kHz, preferably greater than 1 kHz, more preferentially greater than 100 Hz. In order to obtain such an arrangement for the filtration of acoustic waves, it is possible for example to use three silicone blocks of diameter 1.5 mm and thickness 0.5 mm, inasmuch as the transducer has a weight of the order of 0.3 grams.

[0098] With reference to FIG. 5, the transducer 9 is typically mounted on a bearing element 7 by the positioning elements 41, 42 and 43.

[0099] The bearing element 7 is for example a printed circuit board made from epoxy resin of the FR-4 type.

[0100] Typically, the piezoelectric pellet of the transducer 9 is connected to an impedance matching module 6, typically of 50 Ohms, by wires 81 and 82. Wires 83 and 84 connect this module 6 to fastening points of the circuit board 7.

[0101] Resonant Ultrasound Spectroscopy Device

[0102] FIG. 6 represents a resonant ultrasound spectroscopy device according to the invention.

[0103] A sample E1 to be analyzed is positioned on two piezoelectric transducers 9a and 9b having a cylindrical overall shape, according to the transducer in FIGS. 2 and 3.

[0104] Typically, one of the transducers, 9a, operates as a transmitter and the other, 9b, as a receiver.

[0105] The two transducers 9a and 9b are arranged so that they do not pinch the sample between them, which increases the number of degrees of freedom of the sample E1 by comparison with the devices of the state of the prior art such as those described in Gladden 2003 and Hirano 1984.

[0106] The relative positioning of the two transducers 9a and 9b, at angles and distance, must be adjusted as a function of the size and shape of the sample E1.

[0107] In the case illustrated in FIG. 6, the direction in which one of the transducers, 9a, extends intersects with the direction in which the other transducer, 9b, extends. In other words, the axis of the cylindrical support of the transducer 9a intersects with the axis of the cylindrical support of the transducer 9b.

[0108] In other words, the directions in which the tips of the transducer 9a extend intersect with the directions in which the tips of the transducer 9b extend.

[0109] Moreover, the points of intersection of said directions are situated in the direction in which the tips extend from their base to the contact points.

[0110] Preferably, a weight, respectively 5a and 5b, is mounted on each of the bearing elements 7a and 7b on the opposite side with respect to the respective transducer 9a and 9b.

[0111] The weights 5a and 5b can be made from lead and can be positioned and present a weight and a shape suitable for stabilizing the transducers 9a and 9b by inertia.

[0112] These weights 5a and 5b are also useful for reducing the natural frequencies of the bearing element 7.

[0113] Preferably, the piezoelectric transducers 9a and 9b can be arranged in order to transmit and/or receive acoustic waves having frequencies comprised between 5 kHz and 1 Mhz, preferably comprised between 1 kHz and 10 Mhz, more preferentially comprised between 100 Hz and 100 Mhz.

[0114] Process for the Manufacture of the Contact Piece

[0115] The contact piece 1 is preferably manufactured by a process implementing a step of machining the tips 10 by wire electrical discharge machining, comprising: [0116] a first series of lines cut by electroerosion of the contact piece 1, parallel to a first axis of erosion, [0117] a second series of lines cut by electroerosion of the contact piece 1, parallel to a second axis of erosion,
the first axis of erosion not being parallel to the second axis of erosion.

[0118] In the example in FIG. 1, it is apparent that the contact piece 1 has been machined with a second axis of erosion perpendicular to the first axis of erosion.

[0119] Of course, the invention is not limited to the examples that have just been described, and numerous modifications may be made to these examples without exceeding the scope of the invention. For example: [0120] the contact piece 1 can have a truncation different from that shown in FIGS. 1 to 6, or not have any truncation, in particular if the axisymmetric support and the contact piece are electrically conductive; [0121] the support can have any axisymmetric shape, for example with parabolic generatrices, making it possible to further improve the performance of the first natural frequency.

[0122] In addition, the different characteristics, forms, variants and embodiments of the invention may be combined together in various combinations to the extent that they are not incompatible or mutually exclusive.