GB2495378A - Ultrasonic sensor comprising a piezoelectric element for use in high temperature environments - Google Patents

Abstract

A method for manufacturing a ultrasonic sensor for use in a high temperature environment, e.g. a nuclear reactor, the method comprises providing a piezoelectric element, providing a metal alloy encasing, providing at least one layer of gold on the surfaces of the piezoelectric element and the alloy casing, bonding the piezoelectric element and the metal alloy encasing using gold diffusion bonding, wherein the bonding comprises providing acoustic energy to at least one gold layer during diffusion bonding. The metal alloy encasing may comprise stainless steel and at least one gold layer may be formed of a gold foil. The encasing may also have a layer of electroplated nickel on the surface which is to be gold diffusion bonded. Also disclosed is the device produced from this method. Further disclosed is a bonding system for performing the above method, the system comprising a first holder, a second holder, a contacting system and an acoustic energy source. The ultrasonic sensor is to be used in nuclear reactors wherein it is submerged in liquid metal coolant, the sensor is therefore required to be high temperature resistant, liquid metal compatible and radiation hard.

Description

Methods and systems for detection in high temperature environments

Field of the invention

The invention relates to the field of detection techniques for detection or inspection in harsh environments like high temperature environments. More particularly, the present invention relates to ultrasonic transducer-based methods and systems for sensing in high temperature environments

Background of the invention

Sensing and inspection of systems can be based on a variety of techniques.

Nevertheless, the question regarding which sensing or inspection technique can be used typically depends on the system or environment wherein the sensing or inspection is to be performed. For example in high temperature environments, sensing and inspection can suffer from a number of difficulties.

For visualization and inspection in harsh environments such as for example in high temperature environments, it has been suggested to use ultrasonic techniques. An example of an ultrasonic transducer that can be used in high temperature environments is described in "Ultrasonic Evaluation of Status of Nuclear Reactors Cooled by Liquid Metal" th European Conference on NDT, ECNDT Berlin 2006, September 25-29 2006, ISBN 3-931381-86-2 by Kazys et al. The ultrasonic transducer makes use of an intermediate high purity gold film used as intermediate layer while the components make contact by being pressed against each other. Nevertheless, using ultrasonic transducers in high temperature environments still suffers from unreliable results, as ultrasonic transducers typically are little or not lesistant to high temperatures. Consequently, there is room for improvement in systems for sensing and inspection in harsh environment as well as methods for manufacturing such systems.

Summary of the invention

It is an object of embodiments of the present invention to provide ultrasonic transducers for operation in high temperature environments.

It is an advantage of some embodiments according to the present invention that ultrasonic transducers submersible in hot liquids are provided.

It is an advantage of embodiments according to the present invention that sensors can be provided that are based on ultrasonic transducers and which are resistant to submerging in liquid metals at high temperature.

It is an advantage of some embodiments according to the present invention that radiation hard ultrasonic transducers are provided.

It is an advantage of embodiments according to the present invention that a strong joint is formed between the piezo element and an encasing element.

It is an advantage of at least some embodiments according to the present invention that ultrasonic transducers can be obtained that are temperature resistant up to 450°C, possibly in a nuclear environment.

It is an advantage of at least some embodiments of the present invention that the ultrasonic transducers can be used in liquid metal cooled nuclear reactors such as reactors based on sodium, lead or lead-bismuth.

It is an advantage of at least some embodiments according to the present invention that ultrasonic transducers are provided that can operate in ionizing radiation.

The above objective is accomplished by a method and device according to the present invention.

The present invention relates to a method for manufacturing a ultrasonic sensor for use in a high temperature environment, the method comprising providing a piezo element, providing a metal alloy encasing and bonding the piezo element and the metal alloy encasing using gold-to-gold diffusion bonding. It is an advantage of embodiments according to the present invention that an ultrasonic sensor can be manufactured that is resistant to high temperature. For the particular example of sensing in liquid metal reactors, it is advantageous that ultrasonic sensors can be manufactured that are resistant to radiation and are liquid metal compatible, so that a reliable ultrasonic sensor can be provided for use in a liquid metal cooled reactor.

Providing a metal alloy encasing may comprise providing a stainless steel encasing. It is an advantage of embodiments according to the present invention that the ultrasonic sensor can be reliably used in a liquid metal Pb-Bi reactor and that the encasing is not corroding in liquid metal Pb-Bi.

Bonding may comprise providing a gold layer to at least one surface of the piezo element and the encasing. It is an advantage of embodiments according to the present invention that the provision of a ductile gold layer allows stable bonding between the two components, even though the difference in thermal expansion coefficient.

Providing a gold layer may comprise providing a gold layer to the piezo element and the encasing. It is an advantage of embodiments according to the present invention that by providing a gold layer to both components, a still better bonding can be achieved.

For gold-to-gold diffusion bonding, the method may comprise positioning a gold foil between the piezo element and the encasing. It is an advantage of embodiments according to the present invention that both ease of bonding is improved and that acoustic coupling is improved.

During gold-to-gold diffusion bonding, acoustic energy may be provided to the bonding area. It is an advantage of embodiments according to the present invention that the required pressure and temperature during bonding can be lowered, resulting in little or no damaging of the piezo element and electrodes thereof.

The bonding process may be monitored for optimizing pressure, temperature, duration and dose of the acoustic energy. It is an advantage of embodiments according to the present invention that monitoring of the bonding can be performed, resulting in low or minimal harsh conditions for the piezo element and its components during the bonding.

Monitoring the bonding may comprise applying during the bonding at least one excitation acoustic pulse to the piezo element and detecting the reply from the piezo element. It is an advantage of embodiments according to the present invention that monitoring of the bonding can make use of the piezo element to be bonded itself, thus resulting in the possibility for good monitoring) with only a limited number of external monitoring components.

Applying during the bonding at least one excitation acoustic pulse may comprise applying an acoustic pulse from the side of the encasing and detecting the reply from the piezo element.

Detecting of the reply of a piezo element may be perfomed at the side of the piezo element with respect to the gold-to-gold bonding.

to Detecting of the reply of a piezo element may be performed at the side of the encasing.

The present invention also relates to an ultrasonic sensor for use in a high temperature environment, the ultrasonic sensor comprising a piezo element, a metal alloy encasing and a gold-to-gold diffusion bonding binding the piezo element and the metal alloy.

The ultrasonic sensor may be used in a nuclear environment.

The ultrasonic sensor may for example be embedded in a liquid-metal nuclear reactor.

The present invention also relates to a bonding system for bonding a piezo element to a metal alloy encasing, the bonding system comprising a first holder for supporting a piezo element, a second holder for supporting a metal alloy encasing) a contacting system for bringing the piezo element and the metal alloy encasing, when positioned on the holders, in contact by displacing the first holder and the second holder, and an acoustic energy source for providing acoustic energy to a region where bonding between the piezo element and the metal alloy encasing will be performed.

The bonding system furthermore may comprise a detector adapted for detecting a reply of the piezo element to an acoustic signal.

A signal generator may be provided on the second holder.

The detector for detecting a reply may be positioned on the second holder.

The detector for detecting a reply may be positioned on the first holder.

The present invention also relates to the use of a ultrasonic transducer as described above for visualization and/or inspection of submerged components in a liquid metal cooled reactor.

The present invention furthermore relates to a method for visualizing and/or inspecting submerged components in a liquid metal cooled reactor, whereby the method comprises detecting an acoustic signal after interaction with the submerged components using an ultrasonic transducer as described above.

It is an advantage of embodiments according to the present invention that an ultrasonic transducer can be constructed such that it is resistant to high temperature, resistant to radiation and liquid metal compatible and thus is reliable for use in a liquid metal cooled reactor.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief description of the drawing

FIG. 1 illustrates the plating of the piezoelectric element, the protector and the backing by additional metallic layers, as can be used for improving bonding, according to an embodiment of the present invention.

FIG. 2 illustrates the coating of a piezoelement and a backing body, as can be performed during a method for manufacturing a piezoelectric transducer according to an embodiment of the present invention FIG. 3 illustrates a piezoelement-to-metal protector thermosonic bonding principle as can be used in a particular embodiment of the present invention.

FIG. 4 illustrates ringing of the piezoelectric element, before bonding of the piezoelectric element to the protector and without applying pressure, as can be obtained during monitoring of bonding according to an embodiment of the present invention.

FIG. S part a and b illustrates the effect of applying pressure (increasing pressure (a) at 2OMPa, (b) at 3OMPa) on the reflection from the protector's back side, as can be obtained during monitoring of bonding according to an embodiment of the present invention.

FIG. 6 part a and b illustrates the effect observed at higher pressures ((a) : 50 MPa, (b) 7OMPa) used during the bonding process, as can be obtained during monitoring of bonding according to an embodiment of the present invention.

FIG. 7 illustrates the ultrasonic vibrations (17KHz) as observed upon bonding, the vibrations being highly damped and wide band multiple reflections inside the protector (f = SM Hz), without 17kHz vibrations (a) and with 17kHz vibrations (b), as can be obtained during monitoring of bonding according to an embodiment of the present invention.

FIG. 8 iliustrates a comparison between a non-bonded piezoelectric element at 6OMPa and a thermosonically bonded piezoelement according to an embodiment of the present invention.

FIG. 9 illustrates the reflected pulse from a quartz reflector for a thermosonically bonded ultrasonic transducer immersed in liquid Pb/Bi at 178°C (part a) and at 290°C (part b), illustrating features and advantages of an embodiment of the present invention.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting the scope.

In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed description of illustrative embodiments

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention and how it may be practiced in particular embodiments. However it will be understood that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures and techniques have not been described in detail, so as not to obscure the present invention. While the present invention will be described with respect to particular embodiments and with reference to certain drawings, the reference is not limited hereto.

Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner.

It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the steps or elements listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising A and B" should not be limited to devices consisting only of components A and B. Where reference is made to the term consists of, the latter implies that no other elements are present.

Where in embodiments of the present invention reference is made to a piezoelectric element, also referred to as piezo element, reference is made to an element made of a material that changes in shape when an electric voltage is applied and vice versa induces an electric voltage when the shape is changed, e.g. by pressure waves.

Where in embodiments of the present invention reference is made to an ultrasonic transducer, reference is made to a system for inducing or sensing high frequency sound waves. Such ultrasonic transducers may advantageously be used in ultrasonic sensors, allowing to sense echo from a high frequency sound wave induced e.g. by an/the ultrasonic transducer.

to Where in embodiments of the present invention reference is made to a thick metal alloy encasing, reference is made to a metal alloy encasing having a thickness of at least 2 times the acoustic wavelength, advantageously at least 3 times the acoustic wavelength. Where in embodiments of the present invention reference is made to a thin metal alloy encasing, reference is made to a metal alloy encasing having a thickness of less than 2 times the acoustic waveiength. If for exampie a puise frequency of 5MHz is used, the thickness may for example be considered as thin if it is less than 2mm.

In a first aspect, the present invention relates to a method for manufacturing an ultrasonic transducer for use in a high temperature environment by submerging. The method is especially suitable for manufacturing piezoelectric transducers for use by submerging in liquid metal reactors, but is for example equally suitable for other high temperature applications such as deep well inspection, level measurement of hot liquids, high temperature flowmeters or NDT of powerplant tubing while in operation.

Sensing and inspection, e.g. visualization, of submerged components in a hot liquid, such as for example in a liquid metal cooled reactor, may be performed using ultrasonic waves generated in ultrasonic transducers manufactured according to methods of embodiments of the present invention. The ultrasonic transducers according to embodiments of the present invention are high temperature resistant, liquid metal compatible and, where required, radiation hard. The method comprises providing a piezoelectric element. The piezoelectric element is the element in the ultrasonic transducers that allows generating the ultrasonic waves. The present invention is not limited with respect to the piezoelectric element used, but advantageously, piezoelectric elements are used that allow operation in high temperature environments and optionally also in radiation environments. Examples of the piezoelectric elements that can be used are Bismuth Titanate (BIT) piezoelectric elements or lead zirconate titanate (PZT) type piezoelectric elements, although embodiments of the present invention are not limited thereto. For example also other ceramics such as lead titanate, lead metaniobate or mono crystalline piezoelectric elements such as for example GaPO4, LiNbO3, Tourmaline or Langatate can be used.

According to embodiments of the present invention, the method also comprises providing a metal alloy encasing for the piezoelectric element. The metal alloy encasing may for example be a stainless steel encasing, but may for example also be aluminum alloys, titanium alloys, etc. The metal alloy encasing allows protecting the fragile piezoelectric eiement. Such a metai alloy encasing thus is required for shielding the fragile piezoelectric element from the harsh environment. The ultrasonic waves need to travel from the piezo through the stainless steel encasing into the environment, e.g. liquid metal. This means that for efficient transmission of the ultrasonic energy, the piezo element needs to be rigidly attached to the stainless steel encasing. Therefore, according to embodiments of the present invention, the method also comprises bonding the piezoelectric element and the metal alloy encasing using gold diffusion bonding. As it is beneficial for the bonding that bonding can be performed between flat surfaces, the different components may be selected to have a flat surface or may be prepared so that they have a flat surface. By way of illustration -embodiments of the present invention not limited thereto -a way of obtaining a flat surface for the different components is described in the exemplary embodiment below. Techniques that may be applied are cleaning, lapping, polishing, etching, providing a coating or underlayer, electroless coating, electroplating, etc. In gold diffusion bonding, gold material is added which is used as "glued' to bond the piezoelectric element to the metal alloy encaging. The low ductility of gold is able to compensate for the difference in thermal expansion between the piezoelectric element and the metal alloy encaging. Gold can be introduced as a gold foil positioned between the different components. Such a gold foil may have a relative small thickness, such as for example 5pm and good purity. In one particular example, a gold film with a purity of 99.99% is used and the gold film is first annealed. A gold coating also typically is applied to the surfaces of the components to be bonded. In some embodiments, an underlayer can be provided, such as for example a silver underlayer. After a gold layer is applied to the different components, the gold layer on the component may undergo a further processing, such as for example a light polishing. In some embodiments, a combination of applying a gold coating to one or more of the components to be bonded and introducing gold material between the components is used. According to a number of different embodiments, gold diffusion bonding can be performed in a plurality of manners.

Bonding may be performed in a protective atmosphere, such as for example an argon atmosphere although embodiments of the present invention are not limited thereto.

In some embodiments, bonding is performed by conventional gold diffusion bonding.

Such diffusion bonding may be performed at relative high pressures and temperatures. The pressure and temperature thereby need to be selected as function of the piezoelectric element and as function of the surface quality of the elements to be bonded. The pressure and the temperature may be optimized in view of the Curie temperature of the piezoelectric element and the resistance to cracking of the piezoelectric element. Temperature requirements may for example also depend on the type of piezoelectric element. For example Bismuth Titanate (BIT) piezoelectric elements are less sensitive to high temperatures whereas (lead zirconate titanate) PZT type piezoelectric elements are more sensitive to high temperatures.

In one set of embodiments, the present invention relates to a method and system whereby thermosonic diffusion bonding is performed. Such embodiments have the advantage that bonding can be performed at lower temperatures and pressures which may avoid damage to the piezoelectric element. A too large pressure can lead to the cracking of the piezoelectric element. A too high temperature may rule out certain types of piezoelectric elements with respect to their Curie temperature. For example PZT type piezoelectric elements may suffer more from high temperature treatments than BiT piezoelectric elements. According to embodiments wherein thermosonic assisted diffusion bonding is performed, ultrasonic or sonic vibrations are introduced into the bonding zone. Such introduction can for example be performed using a Langevin type transducer, although embodiments of the present invention are not limited thereto. If for example a Langevin transducer is used, the frequency applied can for example be in the range 10kHz to 50kHz, such as for example about 17 kHz. Introduction of the vibrations into the bonding zone may for to example be performed using a horn. One advantageous embodiment is based on use of a Titanium horn, as Titanium metal has a low thermal conductivity preventing overheating of the Langevin transducer. Due to the ultrasonic energy supplied for the bonding, pressure and temperature required for bonding can be significantly reduced, e.g. to levels more acceptable for avoiding damage to the piezoelectric elements.

In one embodiment, the present invention relates to a method for bonding a piezoelectric element to a metal protector as described above, wherein during the bonding monitoring of the process is performed. Such monitoring may involve determination of optimal bonding parameters, such as pressure, temperature, duration and/or ultrasonic dose if applied, and may result in determination of the end of the bonding process. The monitoring typically may advantageously be applied to those embodiments wherein the metal protector membrane has a thickness of at least a few times the ultrasonic wavelength K. Using a pulse frequency of 5MHz, typically a thickness of 10mm or more may be advantageous. Monitoring may be performed by monitoring a reflection of an ultrasonic pulse by the back surface protector. In advantageous embodiments, since a piezoelement is bonded, generation of the ultrasonic pulse for which the reflection is monitored, may be performed in the piezoelement that is to be bonded itself. The latter is advantageous as it avoids additional elements to be introduced and monitoring thus can be performed without complicating the setup. Different types of pulses may be used for such monitoring.

In another embodiment, bonding with a thin protector membrane is performed and the ultrasonic monitoring of the process is more difficult, as the back reflection can less easily monitored. In such cases, it may be preferred to use earlier determined bonding parameters -e.g. using trial and error. Alternatively or in addition thereto, monitoring as described above can also be applied using an additional temporary protector, e.g. by providing a temporary protector and optionally an additional material, e.g. a lead foil, for contacting the temporary protector with the membrane.

In still another embodiment, the ultrasonic transducer may comprise more than two elements that are bonded to each other. In one example, a thin stainless steel membrane, a piezoelement and a backing material, e.g. of sintered metal or graphite bronze, may be bonded. The bonding of the three elements may be carried out in a single bonding step. Typically optimal bonding parameters that are previously determined (for example, pressure 4OMPa and temperature 200°C for 0.5h are applied, duration of low frequency powerful ultrasonic vibrations is 10mm) are applied, as monitoring the process is difficult.

Detection of ultrasound velocity using any of the above techniques also may allow for determining the protector temperature during the bonding. The temperature can be precisely measured from time delays of the first and the second reflections in the protector. The ultrasound velocity temperature dependence in the protector material must be determined in advance, so this can be taken into account.

In a second aspect, the present invention relates to an ultrasonic transducer for use in harsh environments. The ultrasonic transducer thereby comprises a piezoelectric element, a meta' alloy encaging and a gold diffusion bonding between the piezoelectric element and the metal alloy encasing. The metal alloy encasing allows protecting the piezoelectric element from the harsh environment it is intended to work in, such as for example a liquid metal nuclear fuel reactor.

The ultrasonic transducer according to embodiments of the present invention may advantageously be made using a method as described above, although embodiments of the present invention are not limited thereto.

In a third aspect, the present invention relates to a system for manufacturing an ultrasonic transducer for use in harsh environments. The system for manufacturing an ultrasonic transducer according to embodiments of the present invention comprises a first holder for supporting a piezoelectric element and a second holder for supporting a metal alloy encasing. The system furthermore comprises a contacting system for bringing the piezoelectric element and the metal alloy encasing, when positioned on the first and second holder, in contact by displacing the first holder and the second holder relative to each other. The contacting system furthermore may be suitable for introducing a pressure between the piezoelectric element and the metal alloy encasing, e.g. introducing pressures, depending on the polishing quality, up to 40 to 70 MPa. When the contacting system is adapted for introducing different pressures, a control system for controlling the amount of pressure when contacting for bonding may be present. The system furthermore may be provided with a sensor for pressure sensor measurement, such as for example but not limited to a water cooled compression load cell.

The manufacturing system furthermore may comprise a heating system for providing heat to the bonding region. The heating system may be any suitable heating system allowing to provide sufficient temperature, e.g. up to 250°C, such as for example but not limited to an electric heater. For determining the obtained temperature, a temperature sensor may be provided and/or use may be made of the delay time of an induced acoustic sound for deriving temperature.

In some embodiments, the manufacturing system furthermore may comprise an acoustic energy source, for providing energy to a region where bonding between the piezoelectric element and the metal alloy encasing will be performed. In one embodiment, the acoustic energy source of the bonding system may be a horn-type vibrator, providing acoustic energy to a region where bonding between the piezo element and the encasing will be performed. The horn-type vibrator may consist of a vibrating horn tip with optically planar upper surface and a titanium horn. It may be positioned on the side of the piezoelectric element and/or on the side of the metal alloy encaging.

The bonding system furthermore may comprise a monitoring means or monitor for monitoring a reply of one or more of the components that are to be bonded. Such a monitoring means or monitor may be a detection element for detecting an acoustic signal. The detection element may be positioned on the first or second holder and may measure a transmitted or a reflected signal. The detection element for detecting a reply may comprise or be read out using a digital oscilloscope. By detecting a reply of the piezo element to an electric excitation pulse, the bonding process may be optimized for pressure, temperature, duration and dose of the acoustic energy. The monitoring signals may be induced by the piezoelectric element. The monitoring means therefore may comprise a pulse generator for inducing monitoring pulses in the piezoelectric element.

The manufacturing system furthermore may comprise a gas supply for supplying an inert gas, such as for example Argon, so that the bonding can be performed in an inert atmosphere.

Further components of the manufacturing system may also be present, such as for example additional components for performing one or more steps, e.g. optional steps, of a method for manufacturing as described above.

In one aspect, the present invention also relates to the use of an ultrasonic transducer according to an embodiment of the second aspect or an ultrasonic transducer obtained using a method according to an embodiment of the first aspect for visualisation and/or inspection of components in a high temperature environment, such as for example in liquid metal cooled nuclear reactor, for deep well inspection, for level measurements of hot liquids, for high temperature flowmeters, for NDT of powerplant tubing while in operation, etc. By way of illustration, embodiments of the present invention not being limited thereto, an example of a method for manufacturing a piezo-electric transducer is described below. The exemplary method is illustrated with reference to FIG. 1 to FIG.9.

In a first step, the different components were prepared for bonding. The components to be bonded by gold diffusion bonding were provided with a Au layer, in the present example by electroplating the gold (Au) on the components. Intermediate layers also may be provided, e.g. for obtaining a further flatness or for providing a more appropriate material for depositing the gold. Whereas in general either flattening tO may be performed or components that are already sufficiently flat may be used, in the present particular example, sufficiently flat surfaces were obtained by performing the following preparation steps for the different components -embodiments of the present invention not being limited thereby. The results are shown by way of illustration in FIG. 1 indicating the prepared piezoelectric element (a), the prepared metal alloy encasing (b) and the prepared backing (c) The piezoelectric element, in the present example being a piezoelectric disc being a bismuth titanate BiT disc with a typical frequency of 5MHz, has received following preparation. As the present piezoelectric disc contained original electrodes which did not possess sufficient adhesion, these were removed. A lapped, flat and fine surface was prepared. Electroless plating by Ni (thickness about iRm) was perlomed. About lORm of silver (Ag) was electroplated on the Ni to eliminate the remaining surface roughness. The surface was again lapped, polished and optically tested (using an optical method based on Newton rings) to obtain a roughness A smaller than iRm.

Finally about 2jim gold was electroplated on the surface and a light polishing was performed.

By way of illustration an example of the preparation process is described in more detail below, embodiments of the present invention not particularly restricted thereto.

The [lectroless plating process used is a chemical oxidation-reduction reaction which is initiated by noble catalytic metal particles and then proceeded by autocatalysis. In electroless plating, there are at least complex metallic ions and a reducing agent. A chemical reducing agent can provide the source for electrons, thus no outer current power is needed. Electroless nickel plating is a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution containing a chemical reducing agent. The use of electroless plating systems is advantageous as it results in good hole filling and leveling characteristics, which make them ideal as a first layer for other electroplating processes.

In a first step, the piezoelement was cleaned, e.g. by the organic solvents such acetone, and optionally rinsed well by distilled water. The next step was chemical to etching, e.g. using a mixture of different agents for example based on HF, NH4F and H20. Etching was advantageous for better coating adhesion, but care was taken not to over etch. After that, the sensibilization, activation, reduction and chemical composition of Ni-P coating were carried out. The Ni/P ratio was in one example selected as 96/4. One example of a composition of the pre-treatment solutions and processing conditions is described below Bath Composition Concentration Condition Sensibilization Tin(ll)Chloride x 2H20 20-25 g/L Time: 3Osec, solution Hydrochloride Acid 40/60 g/L 20-25°C Activation Palladium Chloride 0.5-1.0 gIL Time: 2 mm.

solution Hydrochloride Acid 1.0-10 g/L 20-25°C Reduction Sodium Hypophosphite 10-20 g/L Time: 3Osec, solution 20-25°C During the immersion into sensibilization solution, the cavities that have been generated in the etching step can absorb 5n2+ ions. Subsequently the 5n2÷ ions can be replaced via ion exchange by immersing the pre-treated piezoelement into an activation solution that contains Pd2+ ions.

By adding HCI (in the present example being 1 milliliter of concentrated acid per gram of PdCl2x2H2O), chloropalladic acid was formed according to the reaction: 2 HCI + PdCl2 = H2PcJCI2, which affects favorably the activation process.

A part of the palladium ions, not reduced by sensitization-activation: Sn2 + Pd2 4 Sn4 + Pd, can be partially reduced at subsequent interaction with hypophosphite in the solution of electroless deposition according to the reaction: PdCl2 + H2P02' + H20 = Pd + H2P03' + 2H1 + 4C11.

Thus the palladium on the surface of materials acts as an active center for nickel deposition at the initial stage of the process. Thereafter, Ni itself would act as an active center to catch Ni from the solution. Advantageously the surface was rinsed, e.g. about 60 sec, alter each pre-treatment step to avoid introducing any impurities or to drag-in ions or parlicles which might be poisonous lo the reaction and even result in instability of reaction solution. But overtime rising was avoided, otherwise, the pre-treated substrate might become even non-catalysts when all ions or particles have been rinsed and dispelled with distilled water. In this process, ultrasonic cleaning was avoided. It was used only for bare piezoceramics [or with thin gold electrodes] before the immersion into sensibilisation solution. The third step of the pre-treatment was the immersion of the piezoelement into a reducing solution, e.g. for about 30 sec. Upon completing all the pre-treatments above, the element is introduced in the electroless nickel EN plating bath. The composition of the bath can be as follows Chemical reagents Formula Concentration Nickel (II) chloride hexahydrate NiCI2x6H2O 40-50 gIL Ammonium Chloride NH4CI 45-55 gIL Sodium Citrate Na3C6H5O7xS.5H20 40-50 gIL Sodium hypophosphate NaH2PO2 x H20 10-20 giL Ammonia solution NH4OH is slowly added to the bath of prepared electrolees Ni-P plating solution until color changed from green to blue. The autocatalytic reaction for nickel deposition is initiated by catalytic dehydrogenation of the reducing agent with the release of hydride ion, which then supplies electrons for the reduction of nickel ions; so, sodium hypophosphate serves as reducing agent: H2PO/1 + H20 4 H4 + HPOI + 2H Ni2 + 2H -3 Ni° + 2W H2P02' + H -3 H20 + OH + P H2P021 + H20 4 W + HP032 + H2 The plating rate generally increases with temperature. The reaction occurs only when the solution temperature is above 60°C, but when the temperature of the solution exceeds 95°C, the solution can be unstable. The temperature for the reaction process of plating must be in the interval 80 to 88°C. Stirring of the solution was accomplished by means of a magnetic stirrer. Time of deposition could be varied in order to obtain Ni coating of desired thickness, for example, Smin/ilam. Thicker than 1 pm nickel coatings were avoided, because adhesion worsens if the thickness increases. This coating served only as under layer for the next main coatings. After plating, the coated details were cleaned in an ultrasonic bath and allowed to dry at room temperature in air. If the thermosonic bonding is made soon after the electroless and electroplating processes, a thermal treatment takes place along with the bonding process.

After the Ni coating electrodes surface remain rough. Piezoelement electrodes must be polished and finally gold coated. In order to "hide" all ceramic surface irregularities it was in the present example decided to use silver electroplating. For better adhesion to Ni, a special under-layer was applied before electroplating with Ag. This process lasted 1.5mm, current 40 mA, thickness"i pm (theoretically).

The main Ag layer electroplating lasted 1 h, current 27 mA. Theoretically thickness was expected to be 37 pm/lh at 1A/dm2; practically in the present example, a 35-40 pm thickness of Ag layer was obtained. The surface of the layer obtained was not flat.

It was made at room temperature. Cyanide silver plating solution was employed. The Ag layers were grinded (-10 pm) and polished. For grinding wet sandpaper No 2000- 3000 and a special equipment were used in the present example. Fine polishing was also performed. Flatness and the surface quality are checked with a classical Newton's rings method in order to make sure that a uniform surface was obtained in the whole area of the piezoelement.

The last procedure was gold electroplating. In the present example, neutral cyanide-based for high purity gold plating solution was applied, with about 10 grams metallic gold per liter. In the present example, the solution was operated at the temperature of 60°c, at pH values in the range 6.0-7.0 with a cathode current density ik=O.25O.S A/dm2, pure platinum anode. Gold thickness was about 2 pm. Such thick gold coatings, which were very suitable for bonding purposes, are a little rough. Thus they were lightly polished till they became bright and shining. The stainless steel protector (AISI-316) was prepared as follows: a geometrical shape-flat membrane was used as a starting point. Thereafter lapping polishing and testing of the surface using an optical method based on Newton rings, was performed for both flat surfaces. Electroplating with Ni (between 1 and 2 pm) on the surface which will be bonded to the piezoelectric element was then performed.

In one embodiment, the present invention not being limited thereto, the following process may be used. Only one surface was electroplated, the rest was isolated. In the process for plating nickel upon a surface of the protector it at first was immersed for activation in the water solution of HCI (1:1) at the temperature 30 °C -40 °C for about 5 minutes until bubbles of H2 appeared on the surface. A pure nickel anode was used. The current density was about 5 A/dm2. After this operation the sample was entered without rinse into the nickel pre-electroplating bath. The composition of the nickel pre-electroplating bath was as follows Nickel (II) chloride hexahydrate NiCl2x6H2O 200-250 gIL Hydrochloric acid HCI 50-100 gIL In the second step of preparing the protector is electroplated with the main Ni layer.

The surface of the protector is therefore immersed in a bath with following composition NiSO4x7H2O 200 to 240 gIL H3B03 3OgJL NaCI 5 to 15 gIL NaF 5 g/L(H value 5.8 to 6.3) C1OH7SO2Na 2 to 4 g/L Thereafter, a gold layer was electroplated (2 to 3 jim) on top thereof and a light polish was performed until the surface became glossy. The electroplated gold layer results in a better adhesion to the Nickel layer.

The backing body, also referred to as the damping body, has been prepared as follows: Starting from a geometrical shape-cylinder of sintered metal such as stainless steel or graphite bronze, the surface to be bonded to the piezoelectric element is lapped for obtaining one flat surface, while for the opposite surface an irregular surface is chosen or created to prevent possible back reflection of ultrasonic waves.

Further, flat surface coatings are applied to the lapped flat surface, whereby a Ni layer of 1 to 2 jim is applied electroless or via electroplating and a Ag layer is applied using e.g. electroplating to eliminate roughness whereby the thickness may e.g. be 10pm depending on the surface quality. The surface is then lapped, polished and tested whereafter a gold (Au) layer is electroplated (typically about 2pm thick) and light polishing is applied.

in all elements, where Ag electroplating is used, the Ag may be omitted, but in this case a thicker Au layer will be necessary.

By way of illustration, embodiments of the present invention not being limited thereto, a prepared piezoelectric element or backing is illustrated in FIG. 2, indicating the element 1 to be prepared in the present example being the piezoelement or backing body with a typically irregular surface, a Ni (1pm) coating 2 made through electroless plating, repeating the surface irregularity, a Ag coating 3 (10pm), that is lapped and polished and that hides the suface irregularity and a electroplated Au layer 4 (2pm). The silver and gold layer consist in the present example of an underlayer and a main layer.

After preparing the components, in one step the ultrasonic energy providing system is positioned with respect to the piezoelectric element. In the present particular example, use is made of a Langevin type transducer and a Titanium horn. For additionally protecting the piezoelectric element to be bonded from cracking e.g. at high pressures, in the current example a Teflon film 33 (FIG. 3), having a thickness of 10 pm, is placed between the horn cylindrical tip and the piezoelectric element which is to be bonded. The latter may be especially advantageous in case a thin piezoelectric element is to be bonded.

In another optional step, additional gold material is applied between the piezoelectric element 34 (FIG. 3) and the protector to be bonded. For this, a gold foil 35 (FIG. 3)can be used, in the present example having a thickness of 5jim. In the present example a gold foil with a purity of 99.99% is used, and the gold film is first annealed at a high temperature for softening it in an alcohol lamp flame. Use of an additional gold material may be made as it reduces the high requirements to the quality of the contacting layers.

to Then, thermosonic bonding is applied. In the present example, the protector is a thick protector, having a thickness of at least some wavelengths or more. By using such thickness, monitoring of the process can be easily realized and optimal parameters of the process can be determined (pressure, temperature, duration, ultrasonic dose).

During the monitoring, the piezoelement which is bonded is excited by a short electrical pulse, and the reflection of the ultrasonic signal from the protector back side is observed. The thickness of the protector advantageously is more than 3 A. Alternatively -not used in the present example -the optimal bonding parameters could be determined earlier and a thin membrane (with a thickness smaller than 3 wavelengths) can be used. For thin membranes monitoring is much more complex, thus requiring pre-determination of the parameters or the use of further elements.

The thermosonic bonding is in the current example combined with a monitoring process for determining the appropriate bonding conditions. Therefore, in the present example, a stepwise process is applied. As will be described below. FIG. 3 illustrates the principle of the Au diffusion bonding of the piezoelectric element and the metal protector, whereby a piezoelectric disc 34 with polished Au electrodes is bonded to a polished and Au electroplated metal alloy protector 36. An annealed gold foil 35, in the present example having a thickness between 10 and 20 m, is applied between the piezoelectric disc 34 and the protector 36. The gold film was annealed in an alcohol lamp flame, avoiding its burn. As thermosonic bonding is applied, a wave generator having a flat horn tip 32 and a titanium horn or Langevin transducer 41 are provided, whereby the flat horn tip 32 is brought into contact with the piezoelectric disc through a Teflon film 33 (in the present example having a thickness in the range 3Ojim to 5oiim). The Teflon film is placed between the piezoelement back side and the flat horn tip 32 (being a vibrating copper disc) and protects the piezoelement from fracture at high pressure, but allows heat and vibrations to pass freely as these are necessary for the bonding process.

All major elements were inserted in a centering Teflon ring (not shown). A wire, e.g. a Teflon-isolated wire, for monitoring was connected to the protector body. This was done by using a nickel plated copper film inserted between the protector and a buffer to rod cylinder component, the buffer rod cylinder component having a mica layer attached to the side facing the nickel plated copper film.

Heat can be provided (indicated by arrows 31) as well as uniform pressure 39, which can be measured using a pressure measurement unit 38. The bonding is performed in a inert Ar atmosphere 37, although operation in clean air also can be applied.

Ultrasonic monitoring of the bonding process can be applied through the gold foil strip (2-5km) 40 connected to the piezoelectric element.

In the present experiment, initially there was dry acoustic contact between the piezoelement and the protector, and no bonding was present yet. IL at the initial conditions of 200 C and 0 MPa pressure, the piezoelement in the bonding equipment is excited by a short rectangular pulse (e.g. for 5 MHz piezoelement, r=100 ns), the piezoelement was ringing, as illustrated in FIG. 4.

The pressure was then gradually increased, resulting in the reflection from the protector back side becoming more and more visible, as shown in FIG. 5a and FIG. 5b.

In FIG. 5a the situation is shown for 2OMPa whereas in FIG. 5b, the situation is shown for 3OMPa. The pressure was increased further till ringing disappears and the back reflection becomes clear, of maximal amplitude and minimal duration, as illustrated in FIG. 6 part a and FIG. 6 part b. FIG. 6 part a illustrates the situation at 5OMPa and FIG. 6 part b the situation at 7OMPa is shown. If additional increasing of the pressure has no influence on the reflection quality, the optimal previous pressure is fixed. It indicates that the optimal acoustic coupling between piezoelement and protector has been achieved. Depending on the quality of contacting surfaces, the necessary pressure is 40-60 MPa.

At this pressure, the temperature was gradually increased, resulting in that the Au foil was softening additionally. At 100° C ultrasonic vibrations in the bonding zone were introduced from the Langevin transducer for 1 mm. The temperature was increased further up to about 2000 C. The temperature was determined from ultrasound velocity in the protector. In the interval 100 to 200° C, ultrasonic vibrations were introduced for some cycles of 1 mm. Ultrasonic vibrations (17 kHz) could be observed with the same piezoelenient which was bonded (e.g. 5 MHz), as it was highly damped and was wide band. The latter can be seen in FIG. 7. The process was performed in Ar atmosphere, but clean open air is suitable as well.

By way of illustration, the present invention not being limited thereto, one particular example of a bonding process is described below.

Before the bonding, the different components of the setup are installed and the different connections are made. In the present example, e.g. a pulse generator! oscilloscope is installed for process monitoring, a bimetallic thermometer is used for temperature measurement, a compression Load Cell is installed, a cooling system is installed, e.g. based on flowing cold water, a protective atmosphere inducing means, is installed, a connection between the ultrasonic generator and the Langevin transducer is installed whereby optionally frequency adjustment is performed. The ultrasonic generator advantageously can be tuned to the Langevin transducer resonance. To prepare for bonding, the pressure may be increased up to e.g. 40 MPa or 50 MPa, vibrations may be applied for e.g. 2 mm, and after that pulse response can be observed in an oscilloscope. Pulse response may be maximized by adjusting the exitation pulse durationt.

The bonding process may in one example be performed using the following parameters and conditions The electric stove was swiched-on, and the temperature increase was observed.

Heating was performed during 60 mm. Bimetallic thermometer registered the temperature increase up to 230°C of the table on which the bonding system was mounted. The temperature of the bonding zone was less. Every two minutes up to the end of heating vibrations were applied for 1 mm and the pressure, if necessary, was corrected, as it changed a little due to the different temperature expansion of the equipment elements.

Pulse response amplitude we observed and measured. After 60 mm the pressure was reduced to zero. Evaluation of the bonding could be seen as follows: If the bonding process was finished, signal amplitude remained large or even increased; if the signal amplitude decreased, the heating process was to be continued with the same high pressure and periodically applied vibrations. As a rule, 60 mm was enough, if the quality elements was good and they were assembled properly.

Bonding could also be provided without any monitoring, if appropriate parameters were applied.

Checking of the quality of the bonding can be performed at high temperature by decreasing the pressure. If the decrease of pressure leads to the worsening of the pulse response (appears ringing, amplitude decreases), the bonding process must be continued: the pressure is increased again up to the optimum according to the pulse response shape and ultrasonic vibrations are introduced. Normally, the whole heating process from 20°C to 200°C takes about 0.5 h, and the bonding procedures can be finished.

After decreasing of the pressure to 0 MPa, still maximal pulse response was observed, the same as at high pressure (40 -60 MPa) before bonding, as illustrated in FIG. 8.

FIG. 8 illustrates in the upper graph (a) the signal for a non-bonded (only pressed to the protector) piezoelectric element and in the lower graph (b) the signal for the element thermosonically bonded to the protector. The heating therefore could be switched-off so that the bonding unit can be cooled and the optimal bonding parameters were registered. In the present example, the optimal bonding parameters were the pressure, the temperature, the duration and the ultrasonic dose. In the present example a piezelement bonded to a thick protector of 15mm was obtained.

Furthermore, by way of illustration, testing of the quality of the bonding was performed for a piezoelectric transducer with a thick protector made as described in the above example. The thermosonically bonded ultrasonic transducer was immersed in liquid Pb/Bi and the pulse reflected from the quartz reflector placed in the Pb/Bi liquid is evaluated at 178°C (FlG.9, part a) and at 290°C (FlG.9, part b). It could be seen that the reflection is similar to the pulse response obtained without immersing in the liquid Pb/Bi, thus indicating that the ultrasonic transducer operates appropriately.

Also further experiments illustrated that accurate bonding and good adhesion was obtained For polished and gold coated BiT Pz46 piezoelements, prepared as described above and bonded to a 15 mm stainless steel protector which was also polished and gold coated, monitoring of the bonding process was applied. This bonded ultrasonic transducer was compared with the other transducer) in which a piezoelement was accurately soldered [Trneit1OO oC) to the identical protector. Whereas the soldering process typically provides very good acoustic coupling between the piezoelement and the protector) it was found that the signal amplitude for the bonded transducer was larger than for the soldered transducer, signal shapes being identical. The latter illustrates that bonding is good.

For adhesion [electrode to piezoceramics and gold-to-gold] tests a special specimen was made. Two steel cylinders were bonded to the opposite sides of piezoelement in one bonding procedure. The cylinders were of different length with the purpose of not coinciding of both back reflections in time. Good adhesion could be seen, when the structure was subject to tensile strength tests.

Claims (1)

1. <claim-text>Claims 1. A method for manufacturing a ultrasonic sensor suitable for use in a high temperature environment by introduction of the ultrasonic sensor in the high temperature environment, the method for manufacturing comprising -providing a piezo element, -providing a metal alloy encasing, -providing a gold layer to at least one surface of the piezo element and the encasing and, -bonding the piezo element and the metal alloy encasing using gold-to-gold diffusion bonding, wherein said bonding comprises providing acoustic energy to at least one gold layer during the gold-to-gold diffusion bonding.</claim-text> <claim-text>2. A method according to claim 1, wherein providing a metal alloy encasing comprises providing a stainless steel encasing.</claim-text> <claim-text>3. A method according to any of the previous claims, wherein for goid-to-goid diffusion bonding, a gold foil is positioned between the piezo element and the encasing.</claim-text> <claim-text>4. A method according to any of the previous claims, wherein, prior to providing a gold layer to at least one surface of the encasing, the surface of the encasing is electroplated with nickel.</claim-text> <claim-text>5. A method according to any of the previous claims, wherein, prior to providing a gold layer to at least one surface of the piezo element, the piezo element is subject to electroless nickel plating.</claim-text> <claim-text>6. A method according to the previous claim, wherein the piezo element is polished after the electroless nickel plating and prior to the provision of the gold layer.</claim-text> <claim-text>7. A method according to claim 6, wherein the bonding process is monitored for optimizing pressure, temperature, duration and dose of the acoustic energy.</claim-text> <claim-text>8. A method according to claim 7, wherein monitoring the bonding comprises applying during the bonding at least one excitation acoustic pulse to the piezo element and detecting the reply from the piezo element.</claim-text> <claim-text>9. A method according to claim 8 wherein applying during the bonding at least one excitation acoustic pulse comprises applying an acoustic pulse from the side of the encasing and detecting the reply from the piezo element.</claim-text> <claim-text>10. A method according to claim 9, wherein detecting of the reply of a piezo element is perfomed at the side of the piezo element with respect to the gold bonding or wherein detecting of the reply of a piezo element is performed at the side of the encasing.</claim-text> <claim-text>11. An ultrasonic sensor for use in a nuclear environment, the ultrasonic sensor comprising a piezo element, a metal alloy encasing and a gold bonding binding the piezo element and the metal alloy.</claim-text> <claim-text>12. A bonding system for bonding a piezo element to a metal alloy encasing, the bonding system comprising a first holder for supporting a piezo element, a second holder for supporting a metal alloy encasing, a contacting system for bringing the piezo element and the metal alloy encasing, when positioned on the holders, in contact by displacing the first holder and the second holder, and an acoustic energy source for providing acoustic energy to a region where bonding between the piezo element and the metal alloy encasing will be performed.</claim-text> <claim-text>13. A bonding system according to claim 12, wherein the bonding system furthermore is adapted for detecting a reply of the piezo element to an acoustic signal and/or wherein an acoustic signal generator is provided on the second holder.</claim-text> <claim-text>14. A bonding system according to claim 13, wherein the detector for detecting a reply is positioned on the second holder or wherein the detector for detecting a reply is positioned on the first holder.</claim-text> <claim-text>15. Use of a ultrasonic sensor according to claim 11 for visualization and/or inspection of submerged components in a liquid metal cooled reactor.</claim-text>