Process for producing a piezoelectric sensor and piezoelectric sensor obtained by means of such a process

Abstract

A process for producing a piezoelectric sensor includes the following steps: a step of providing a housing made of stainless steel; a step of producing a solution of a compound comprising a metal or metalloid element; a step of depositing a layer of the solution over at least one inner surface of the housing; a step of oxidizing the deposited layer of solution; a step of placing a piezoelectric element inside the housing; a step of closing the housing. A piezoelectric sensor obtained by such a process and comprising a closed steel housing, a piezoelectric element arranged inside the housing and a layer of a solution of a compound comprising a metal or metalloid element that is arranged over at least one inner surface of the housing.

Claims

1. A process for producing a piezoelectric sensor comprising the following steps: a step of providing a housing made of stainless steel; a step of producing a solution of a compound comprising a metal or metalloid element; a step of depositing a layer of said solution over at least one inner surface of the housing; a step of oxidizing the deposited layer of solution; a step of placing a piezoelectric element inside said housing; a step of hermetically closing the housing, subsequent to all of the previous steps.

2. The process as claimed in claim 1, the solution being a rare earth solution.

3. The process as claimed in claim 2, the rare earth solution comprising a compound based on lanthanum, yttrium, cerium or a combination of said compound.

4. The process as claimed in claim 2, the rare earth solution comprising a compound chosen from a lanthanum oxide, a lanthanum hydroxide, a lanthanum carbonate, a lanthanum acetate, a lanthanum oxalate, an yttrium oxide, an yttrium hydroxide, an yttrium oxalate, a cerium oxide or a combination of said compound.

5. The process as claimed in claim 1, the solution comprising a compound based on polysilazane.

6. The process as claimed in claim 1, the solution comprising a compound based on zirconium.

7. The process as claimed in claim 1, the step of depositing being carried out by dipping the housing into the solution.

8. The process as claimed in claim 1, the step of depositing being carried out by spin-coating the solution onto the housing.

9. The process as claimed in claim 1, the step of depositing being carried out by spraying the solution onto the housing.

10. The process as claimed in claim 1, the step of depositing being carried out by applying the solution to the housing with a paintbrush, a pad or a brush.

11. The process as claimed in claim 1, the steps of producing the solution and of depositing the layer of said solution over at least one inner surface of the housing being carried out by a sol-gel process.

12. The process as claimed in claim 11, the sol-gel process comprising the step of condensing the layer of solution at a temperature lower than or equal to 100° C., said step of condensing being subsequent to the step of depositing, and prior to the step of oxidizing.

13. The process as claimed in claim 1, the step of oxidizing being carried out at a temperature higher than a temperature of use of the piezoelectric sensor.

14. The process as claimed in claim 1, the step of oxidizing being carried out at a temperature higher than 500° C.

15. The process as claimed in claim 6, the compound based on zirconium being an zirconium oxide.

16. The process as claimed in claim 14, the step of oxidizing being carried out at a temperature higher than or equal to 600° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will become apparent through the description which follows by way of non-limiting illustration, given with reference to the appended figures, in which:

(2) FIGS. 1A and 1B illustrate a high-temperature ultrasonic transducer of the prior art;

(3) FIG. 2 illustrates a piezoelectric sensor according to the invention;

(4) FIG. 3 illustrates the mass variation curves of the stainless steel of a housing not treated according to the invention and of a housing treated according to the invention.

DETAILED DESCRIPTION

(5) FIGS. 1A and 1B have already been described and will not be returned to here.

(6) FIG. 2 illustrates a piezoelectric sensor 1 according to the invention, comprising a housing 20 in which at least one piezoelectric active element 10 is arranged.

(7) Additionally, the piezoelectric sensor 1 comprises a layer 30 of a solution of a compound comprising metal or metalloid elements, for example rare earths. Said layer is deposited on inner surfaces of said housing. Said deposited layer is then oxidized, preferably at a temperature higher than the temperature of use of the piezoelectric sensor 1.

(8) In the case of a high-temperature ultrasonic transducer, the piezoelectric active element 10 may be a converter made of piezoelectric material, and the sensor may further comprise, in the housing 20, an upper electrode 12 made of steel or metal, a support 11 made of steel or metal providing the interface between the converter and the acoustic wave propagation medium, a first junction J.sub.11 between the support and the piezoelectric material, a second junction J12 between the converter and the upper electrode. The first junction J.sub.11 may consist of a solid joint comprising gold and indium.

(9) FIG. 3 shows mass variation curves of a 304L stainless steel over 70 h for a housing not treated according to the invention and for a housing treated according to the invention.

(10) In the graph of FIG. 3, it can be seen that the untreated 304L stainless steel (curves a and b) is oxidized very significantly after about 20 hours. This is due to the formation of iron-containing oxides such as FeCr.sub.2O.sub.4 and Fe.sub.2O.sub.3. The oxide layer tends to come away on cooling.

(11) The deposition and oxidation of a lanthanum layer according to the process of the invention (curves c and d) decrease the rate of oxidation by a factor of at least ten, to such an extent that time has practically no effect on oxidation. The appearance of the mass gain curves (expressed in mg.Math.cm.sup.−2) indicates that the metal is well protected. Lanthanum is found in the form of lanthanum chromite LaCrO.sub.3 and the doping of the chromia Cr.sub.2O.sub.3 layer promotes internal anionic diffusion, a slow rate and good bonding of the layer.

(12) The deposition and oxidation of the lanthanum layer results in a bonding oxide layer which acts as a diffusion barrier and ensures a very slow rate of oxidation of the stainless steel.

(13) In order to carry out the steps of producing a solution and of depositing a layer of said solution over at least one surface inside the housing, a sol-gel method is preferably used, prior to the oxidation step.

(14) A sol-gel method implements a first step of producing a rare earth hydroxide solution, the first step also being called the hydrolysis reaction. The solution thus obtained is called a “sol”.

(15) According to one particular embodiment, a solution of lanthanum hydroxide is produced.

(16) By way of example, a solution of lanthanum hydroxide is produced from lanthanum nitrates. Water, lanthanum nitrate, and ammonia are mixed to form a lanthanum hydroxide precipitate, according to the reaction:
La.sub.(aq).sup.3++30H.sub.(aq).sup.−.Math.La(OH).sub.3(S)

(17) The lanthanum hydroxide precipitate is then dissolved in acid until the pH stabilizes. The final concentration is adjusted with water. An ionic solution of lanthanum hydroxide is obtained.

(18) The sol-gel process then comprises a second step of depositing a thin layer of the rare earth hydroxide solution (for example lanthanum hydroxide solution) over one or more inner surfaces of the housing.

(19) The deposition step may be carried out by dipping the housing to be coated into the solution produced, for example into the lanthanum hydroxide solution produced from lanthanum nitrates.

(20) Alternatively, the deposition step may be carried out by applying the solution produced over one or more surfaces of the housing using a paintbrush, or alternatively by spraying the solution produced over one or more surfaces of the housing.

(21) The sol-gel process then comprises a third step of densifying of the thin layer (also called the condensation step), at a temperature lower than or equal to 100° C., or even at room temperature. A “gel” is thus obtained.

(22) The condensation step forms a “gel” layer a few microns thick.

(23) The condensation step is followed by a step of high-temperature oxidation of the layer of solution. During the oxidation step, said layer reacts with the metal constituents of the stainless steel, forming other oxides which are more protective than the layer of chromia present naturally.

(24) The duration of the oxidation step is preferably several hours. The duration is preferably longer than 24 hours, preferably longer than 48 hours and even more preferably longer than 72 hours.

(25) In addition, the oxidation is preferably carried out at a temperature higher than the temperature of use of the piezoelectric sensor, i.e. higher than 500° C., and preferably higher than or equal to 600° C. This makes it possible to form a protective layer which greatly decreases the oxygen depletion of the piezoelectric elements at high temperature under specific conditions of use.

(26) This straightforward-to-implement and highly economical process makes it possible to increase the service life of the piezoelectric sensor and to provide said sensor with better resistance to oxidation.

(27) The different embodiments may be combined with one another.

(28) In addition, the present invention is not limited to the embodiments described above but rather extends to any embodiment that comes within the scope of the claims.

(29) The piezoelectric sensors may be accelerometers, HTUSTs or even other sensors, and in particular sensors which are liable to release oxygen at temperatures higher than or equal to 500° C.