METHOD AND ARRANGEMENT FOR ADDING A PIEZOELECTRIC MATERIAL FOR A WIDE TEMPERATURE RANGE

Description

SPECIALIZATION

[0001] The invention lies in the field of the development and manufacture of sound transducer systems, in particular an ultrasonic transducer system, for continuous high-temperature use, but also relates generally to the joining of a metal and a piezoelectric material. BACKGROUND

[0002] In ultrasonic flow measurement or structural condition monitoring, transducer systems are used to convert electrical signals into ultrasonic signals and vice versa. These transducer systems consist of a piezoelectric material for converting electrical and acoustic signals and a pre-element for sound transmission between the piezoelectric material and the object being measured or tested. Piezoelectric ceramics or piezoelectric crystals are used as the piezoelectric materials. Known transducer systems are only suitable for continuous use within a relatively narrow temperature range of -40°C to 200°C. This system aims to cover a temperature range of -200°C to 600°C. Such a temperature range is characteristic, for example, of applications in the chemical industry, such as ultrasonic flow measurement, or structural condition monitoring.In the chemical industry, the temperature range from -200°C to 600°C is used, in particular the temperature range from -20°C to 500°C and the temperature range from -200°C to 80°C, and in structural condition monitoring, the temperature range from -80°C to 400°C is used.

[0003] Typically, for applications across such a wide temperature range, multiple transducer systems must be operated in parallel, with these systems utilizing different sensor concepts. This results in disadvantages, particularly due to differing signal-to-noise ratios among the individual transducers and the consequent insufficient measurement accuracy. Combining different transducer systems with overlapping temperature ranges is extremely complex. Furthermore, existing transducer systems capable of covering a wider temperature range are only suitable for short-term operation. However, systems that only allow short-term operation are unsuitable for many applications, such as ultrasonic flow measurement or structural condition monitoring, as both require continuous measurements.

[0004] A sound transducer system that can be used over a wide temperature range, especially under high-temperature conditions up to 600 °C, is subject to special requirements regarding the permanently reliable acoustic coupling of the piezoelectric material. This necessitates a force-fit, i.e., acoustic and / or electrical, connection between the piezoelectric material and the two electrodes used to control the piezoelectric material, and between the piezoelectric material and the pre-element. As is known (su), this is achieved in the prior art, for example, by a liquid coupling layer.

[0005] One of the biggest challenges in developing an ultrasonic transducer for continuous use at extreme temperatures is creating a temperature-resistant coupling between the piezoelectric material and the pre-movement body, which also serves as the electrode. Since most conventional adhesives are typically only usable from approximately -40°C to approximately 150°C, a strong, material-bonded connection between the individual components can only be achieved using high-temperature-resistant joining techniques.

[0006] Similarly, the reliable connection of metal electrodes with a piezoelectric material, for example in a ceramic filter or an intermediate frequency filter, a ceramic resonator, a piezo actuator, or for a ceramic feedthrough of a high temperature sensor, can be exposed to extreme operating temperatures.

[0007] Soldering processes, particularly metal soldering and glass soldering, are suitable for the applications described. However, both techniques have their own disadvantages. Since the high-temperature-resistant piezoelectric material to be joined is usually an oxide-based material, the use of conventional metal soldering techniques is significantly hampered by the typically very poor wetting of the non-metallic material by the molten solder.

[0008] Glass soldering materials, on the other hand, exhibit good wetting properties compared to non-metallic materials, but have several other disadvantages such as: poor electrical conductivity, which leads to a reduction in the effective field strength at the piezoelectric material; a generally relatively low (and in most cases unsuitable) coefficient of thermal expansion, which can lead to cracking in the solder layer and in the piezoelectric material; brittleness, which leads to cracking in the coupling layer when the temperature changes, thus deteriorating the coupling properties.

[0009] All of the aforementioned disadvantages can be overcome by using low-melting-point glass brazing materials. In this case, the softening temperature of the brazing material is significantly below the operating temperature, so the molten glass acts as a liquid coupling. This allows for high-temperature applications, but not for low-temperature applications.

[0010] However, due to the strong corrosive effect of the molten glass at high temperatures, such couplings, which are liquid under operating conditions, exhibit insufficient long-term stability and are therefore only suitable for short-term use.

[0011] In contrast, active soldering is a well-known method for the metallic joining of oxide materials, where the active soldering material contains an actively oxidizing component. The bonding occurs through a chemical reaction in which an active layer forms at the interface between the active solder melt and the oxide material, bonding the oxide material to the solder layer. Unfortunately, the implementation of active soldering technology for coupling a piezoelectric material in a sound transducer, e.g., lithium niobate, to a steel electrode leads to... the use of a protective (gas) atmosphere in the soldering process and an exceptional oxygen activity of the lithium niobate This leads to excessive chemical and physical stress on both the piezoelectric material and the soldering material, resulting in a deterioration of long-term stability and impairment of the piezoelectric properties. Piezoelectric actuators, for example in high-temperature printheads, and ceramic filters or sensor feedthroughs can be exposed to similar challenges.

[0012] DE 11 2015 006135 T5 discloses an ultrasonic transducer comprising two metal blocks, a plurality of piezoelectric elements having rectangular surfaces and stacked between the metal blocks; and bonding materials that connect the metal blocks and the piezoelectric elements. A corresponding joining method is also disclosed.

[0013] DE 10 2015 101878 A1 discloses, inter alia, a method for manufacturing a component of a building element. BRIEF SUMMARY OF THE INVENTION

[0014] Against this background, a joining method according to claim 1 is proposed, which is adapted for use in sound transducer systems in the measurement situations described above, in particular over the specified wide temperature range. The joining connection produced according to the method enables a reliable acoustic coupling and a reliable electrical connection between the piezoelectric material of the sound transducer and the coupled electrodes and / or pre-elements over the entire specified temperature range.

[0015] Surprisingly, it was found that an active solder foil can be used as a joining material in an unconventional manner. In contrast to the conventional use of an active solder foil, according to the invention, it is used at a significantly lower temperature than the melting point of the solder material. Investigations into the dynamics of active layer formation in contact with lithium niobate, which was used here as an example piezoelectric material, have shown that a mechanically and electrically reliable connection can be established even at temperatures below the melting point of the solder material. Advantageously, the reduced process temperature proposed according to the invention prevents melting and subsequent recrystallization of the joining material and also reduces otherwise unavoidable thermal stresses during the cooling phase of the joining process.

[0016] The reduced process temperature proposed according to the invention enables joining without a protective atmosphere, which not only reduces the effort required to produce the joint, but also prevents the negative effect of oxygen reduction, i.e., chemical reduction of the piezoelectric crystal due to oxygen loss in the oxygen-deficient protective atmosphere, and the associated loss of the piezoelectric properties of the lithium niobate used as the piezoelectric crystal (MH Amini, AN Sinclair, TW Coyle (2015) High-temperature ultrasonic transducer for real-time inspection. Physics Procedia 70:343-347).

[0017] Furthermore, it was surprisingly discovered that, according to the invention, a metal foil without the addition of commercially available active components can also be used in the same way. In this context, a metal foil is understood to be a metal rolled into a foil, such as aluminum foil, silver foil, titanium foil, copper foil, etc.

[0018] According to typical embodiments, the use of an adhesive film as a reservoir for an active element is described in Fig. 1 The illustrated embodiment of a layer arrangement for joining a piezoceramic material (crystal or ceramic) and the corresponding connecting electrodes or metallic pre-lead and / or damping elements is proposed. The active element, which is part of the joining film, forms a so-called active layer with a component of the piezoelectric material under the influence of elevated temperature and pressure acting on the joining partners during the joining process. As a separate phase, the active layer establishes a material-bonded connection between the piezoelectric material and the active element reservoir. Likewise, an active layer is formed between the active element reservoir and a component of the adjacent joining partner, in particular a metallic pre-lead or damping element. The force-fit and / or material-bonded connection creates an electrically and / or acoustically (i.e.,mechanically) conductive connection between two adjacent components is established, thus enabling the use of the piezoelectric material in a sound transducer system. BRIEF DESCRIPTION OF THE FIGURES

[0019] The accompanying drawings illustrate embodiments and, together with the description, serve to explain the principles of the invention. Fig. 1 The inventive arrangement for joining the piezoelectric material using joining films is shown. Fig. 2 This shows the temperature profile over the duration of the joining process. Here, TSF denotes a melting point of the joining material or its solidus temperature, if a melting interval exists. Fig. 3 shows the growth of the signal amplitude during the joining process in relation to the temperature profile used. Fig. 4 shows a metallographic image of the produced bonding layer. Fig. 5 shows the course of the signal amplitude and the temperature during the joining process with joining foil made of a silver-based alloy and the piezoelectric material lithium niobate. Fig. 6 shows the course of the signal amplitude and the temperature during the joining process with joining foil made of silver and the piezoelectric material lithium niobate. Fig. 7 shows the course of the signal amplitude and the temperature during the joining process with joining foil made of silver and the piezoelectric material bismuth titanate. EXECUTION FORMS

[0020] According to one embodiment, a joining method for manufacturing a sound transducer system for extreme temperature operation from -200°C to 600°C, in particular from -20°C to 500°C, from -200°C to 80°C and from -80°C to 400°C, is proposed, comprising the following steps: Providing a piezoelectric material 3 and a plurality of components, each component being characterized by a solidus temperature; arranging the piezoelectric material 3 and the plurality of components in the form of a stack 10, such that a front stack part is adjacent to a front side of the piezoelectric material 3 and a back stack part is adjacent to a rear side of the piezoelectric material 3; and consolidating the stack 10 by introducing heat into the material stack and applying pressure to the material stack, the resulting application of heat and pressure being carried out for a predetermined time period. wherein, during consolidation, the piezoelectric material 3 is directly acoustically coupled and / or electrically connected to an immediately adjacent component of the front and / or rear stack part, characterized in that none of the solidus temperatures of the majority of the components is exceeded during consolidation; and the acoustic coupling and / or electrical connection comprises the formation of an active layer by chemical reaction of a first component of the piezoelectric material (3) with a second component of an active element deposit, which comprises a phase that is foreign to the originally inserted piezoelectric material (3) and the unattached front stack part and / or the originally inserted piezoelectric material (3) and the unattached rear stack part, or is only finely dispersed therein.

[0021] In other words, the active layer in question only emerges during consolidation.

[0022] In other words, a piezoelectric material is provided, for example, a suitable titanate, tantalate, niobate, orthophosphate, or nitride; that is, a piezoelectric crystal or piezoceramic. This is stacked with the aforementioned joining partners, which can also be in the form of a film. The joining partners are selected from a damping body, a first electrode, a second electrode, the piezoelectric material, and a pre-forming body. When stacking, i.e., arranging the joining partners to obtain a stack, the following sequence is preferably obtained: first electrode and / or damping body, metallic joining foil, piezoelectric material, metallic joining foil, second electrode and / or pre-flow body.

[0023] The resulting stack is then subjected to a temperature and, optionally, additional contact pressure. This temperature application is also referred to as temperature impregnation, whereby the temperature is specifically below the solidus temperature of all components of the metallic joining foil. Here, a component of the metallic joining foil is understood to be a pure metal—if the joining foil consists of only one metal plus unavoidable accompanying impurities of that metal. Likewise, a component of the metallic joining foil also includes any other metal or any metal-containing chemical compound that contains one or more metals present in the joining foil. This is typically an alloying element, for example, a component of a eutectic alloy, or an intermetallic phase.

[0024] During the aforementioned impact process, an active layer is formed between the damping element and the piezoelectric material, or between the first electrode and the piezoelectric material, on the one hand, and between the piezoelectric material and the second electrode, or between the piezoelectric material and the pre-forming element, on the other. This occurs through a chemical reaction between one or more components of the respective joining partner or one or more components of the metallic joining foils and the piezoelectric material. The active layer is typically formed in the joining partner adjacent to the piezoelectric material, in the contact area with the piezoelectric material.Provided that the joining partners used as electrodes are appropriately controlled with an electrical pulse of suitable frequency and amplitude, this active layer ultimately ensures the acoustic coupling of a sample under investigation, regardless of whether via an additional coupling agent introduced between the resulting sound transducer and the sample or dry / via the surrounding atmosphere.

[0025] Depending on the materials used for the pre-movement element and the optional damping element, separate electrodes may not be necessary, provided that both the pre-movement element and the damping element are electrically conductive. In this case, the metallic connecting foil directly bonds the respective element to the piezoelectric material, so that the damping element and pre-movement element serve as the first and second electrodes, respectively.

[0026] According to one embodiment, the majority of the components provided in addition to the piezoelectric material comprise at least one of the following: a leading element; a damping element, an electrode 2, 4 and an active element depot; wherein the active element depot can be in the form of the leading element, in the form of the damping element, in the form of the electrode 2, 4 or in the form of a separate film 1; wherein the front stacking part and the rear stacking part each comprise a component that is designed as an electrode or functions as an electrode; and wherein the leading element and the damping element can independently function as both electrode 2, 4 and active element depot.

[0027] According to one embodiment, the front stacking part has the leading body and / or an electrode 2 comprising an active element depot, and the rear stacking part has an electrode 4, 2 comprising an active element depot and / or the damping body.

[0028] According to a further development of the above embodiment, the damping element also functions as an electrode, or is designed as an electrode and can be connected to a power source. It should be noted that the damping element can be made of a ceramic material in addition to a metal.

[0029] Examples of electrically conductive (metal-like ceramics) include ZrC, TiC, WC, TiN, ZrN, TiB 2 , ZrB 2 , TiO, TiSi 2 and MoSi 2.

[0030] According to one embodiment, the active layer is formed in a component immediately adjacent to the piezoelectric material 3, selected from a pre-flow body, damping body, electrode, and active element reservoir. In particular, the active layer is formed in a contact area of ​​the component immediately adjacent to the piezoelectric material 3 with the piezoelectric material.

[0031] According to one embodiment, the front side 3.2 and the back side 3.1 of the piezoelectric material 3 are aligned parallel to each other.

[0032] According to one embodiment, the front side 3.2 and / or the back side 3.1 of the piezoelectric material 3 and the immediately adjacent surface of the component of the corresponding front stack part and the rear stack part have a roughness ≤ 1µm.

[0033] This promotes the diffusion processes underlying the formation of the active layer during the process step of "exposure to pressure and / or temperature (heat input)."

[0034] According to one embodiment, the piezoelectric material 3 is selected from: lead zircon titanate; lead lanthanum zircon titanate, barium titanate; gallium orthophosphate; lithium tantalate; lithium niobate; lead titanate; lead niobate; a compound of the langasite group, in particular a lanthanum gallium silicate, aluminium nitride and bismuth titanate.

[0035] According to one embodiment, the active layer accordingly comprises a chemical compound selected from: a copper oxide, a copper titanate, a copper niobate, a silver oxide, a silver titanate, a silver niobate, a titanium oxide and a titanium niobate.

[0036] According to one embodiment, the active layer extends substantially over the entire extent of a cross-sectional area of ​​the stack oriented orthogonally to a stacking direction. In other words, at least one of the substantially opposite surfaces of the piezoelectric material is substantially completely connected to the immediately adjacent joining partner via the active layer.

[0037] According to one embodiment, the active element depot serving to form the active layer is a joining foil 1 and two active layers are formed on the joining foil 1 immediately adjacent to the piezoelectric material 3 when the joining foil 1 is arranged between the piezoelectric material 3 and the pre-flow body.

[0038] According to one embodiment, both of the aforementioned active layers are formed on opposite sides of the joining film 1.

[0039] According to one embodiment, the two active layers differ in thickness and / or chemical composition.

[0040] According to a further development of the two preceding embodiments, the pre-flow body comprises steel and the active element reservoir includes a silver foil. The pre-flow body can also comprise copper.

[0041] According to one embodiment, the feed body comprises silver.

[0042] According to one embodiment, the active element depot is a foil 1 comprising copper, titanium, a copper-based alloy, or a silver-based alloy.

[0043] According to one embodiment, the aforementioned silver-based alloy comprises 63-71% Ag, 26-35% Cu, and 1-5% Ti, as well as unavoidable impurities.

[0044] According to one embodiment, the active metal depot is an active solder foil.

[0045] Advantageously, active solder foils of different specifications are commercially available.

[0046] According to one embodiment, the active solder foil comprises at least one of aluminium, hafnium, magnesium, nickel, niobium, titanium, vanadium, yttrium, and zirconium.

[0047] According to one embodiment, the pressure applied during consolidation is 0.1-5 MPa, preferably 0.2-2 MPa.

[0048] According to one embodiment, the heating rate applied during heat input, or when subjecting the stack to temperature, or during temperature exposure, or during consolidation of the manufactured material stack comprising joining partners and joining film, is less than or equal to 100 K / h.

[0049] This advantageously prevents oxidation of the surface of the pre-molding body before the joining process is completed, i.e., before the active layer is formed.

[0050] According to one embodiment, the temperature of the stack remains ≤ 600 °C during consolidation.

[0051] According to one embodiment, once the temperature of the stack reaches 300°C, the heating rate is set to a value less than 100 K / h.

[0052] According to one embodiment, the joining method described above and below is used to manufacture a sound transducer, a piezo actuator, a ceramic filter, a ceramic resonator, an intermediate frequency filter or a high temperature sensor comprising a ceramic feedthrough.

[0053] According to another embodiment, the joining partners to be connected for the production of a sound transducer system are stacked as follows: first electrode made of silver; piezoelectric material lithium niobate; joining foil, in particular a silver foil; and stainless steel lead-in, which simultaneously serves as the second electrode.

[0054] In this example, silver fulfills the function of both an electrode and a joining foil, i.e., the active element depot for forming the active layer.

[0055] According to one embodiment, the bonding foil comprises a silver-based alloy. Specifically, the silver-based alloy is a eutectic silver-copper alloy. Here, a silver-based alloy is defined as one that—based on total weight—consists predominantly of silver with optional additional alloying elements (e.g., copper and titanium) and unavoidable impurities. Similarly, a copper-based alloy is defined as one that—based on total weight—consists predominantly of copper with optional additional alloying elements and unavoidable impurities.

[0056] According to further embodiments, the alloy contains 70-71% Ag, 26-27% Cu, and 2.5-3.5% Ti, in addition to unavoidable foreign components, i.e., impurities. As already stated, however, active layer formation can also occur on both silver and copper without titanium. This means that even in the eutectic silver-copper alloy described here, titanium does not act as the sole active element. Generally, an active element is understood to be an oxygen-affine element.

[0057] According to exemplary embodiments, the material of an joining foil is a copper-tin alloy, in particular a eutectic copper-tin alloy.

[0058] Copper-based alloys generally exhibit higher oxide formation activity compared to silver-based alloys, which can be advantageous for the speed and effectiveness of active layer formation. However, this can also be disadvantageous in the long-term use of a transducer bonded with active layers, for example, through faster degradation of the active bond.

[0059] According to another embodiment, the joining foil comprises an alloy including an active element, for example titanium, zirconium or hafnium.

[0060] By adding the named active elements to the material of the joining film, the effectiveness and intensity of the active layer formation can be increased even further, which can shorten the process times.

[0061] The process time is essentially determined by the holding time after reaching the joining temperature. Typical holding times range from 10 to 20 hours, for example, between 12 and 18 hours, and particularly around 15 hours ± 1 hour. The holding time could, for example, be 15 hours.

[0062] Different alloys exhibit varying levels of oxygen activity. While a less oxygen-active joining foil – such as a silver-based one – requires a longer joining time, it advantageously results in higher long-term stability of the resulting transducer under high-temperature conditions.

[0063] If the joining time needs to be shortened, a more active alloy, such as a copper-based alloy, can be used according to the method proposed here. However, this may slightly impair the long-term stability and thus the service life of the resulting transducer. Adding the active elements titanium, zirconium, or ferrous ...

[0064] Advantageously, the selected active element can increase the oxygen activity of the joining foil and thus shorten the joining time when using the joining foil in combination with the silver-based or copper-based alloy of the joining foil as proposed here.

[0065] According to one embodiment of a manufacturing process for a sound transducer system comprising a pre-forming body made of steel, in particular stainless steel, the heating rate during consolidation, i.e. when the manufactured stack of material is exposed to a temperature below 300°C, is more than 100 K / h.

[0066] This advantageously allows for an accelerated joining process while simultaneously avoiding oxidation, since stainless steel only begins to oxidize above 300°C.

[0067] According to one embodiment, the piezoelectric material is selected from: lead zircon titanate; barium titanate; gallium orthophosphate; lithium tantalate; lithium niobate; lead titanate; lead niobate; a lanthanum gallium silicate (so-called langasite), aluminum nitride, and bismuth titanate.

[0068] This advantageously allows for adaptation to the thermal expansion coefficients of the feed body used and to the respective desired upper limit of the operating temperature of the transducer system.

[0069] According to one embodiment, a material for the metallic electrode is selected from: Ag, Cu, and Ti alloys.

[0070] Advantageously, this embodiment allows the electrode to be used as a joining foil, i.e., as an active element depot.

[0071] According to a further embodiment, it is proposed to use an joining layer to produce a high-temperature resistant connection between a pre-flow body and a piezoelectric material, wherein the joining layer is selected from: a silver-based alloy, a copper-based alloy and a metallic joining foil, and the alloys each comprise an active metal selected from titanium, zirconium and hafnium.

[0072] This use enables the advantages already described above.

[0073] The embodiments described above can be combined with each other as desired. DETAILED DESCRIPTION OF THE INVENTION USING THE FIGURES

[0074] In Fig. 1 The arrangement 10 used according to the invention for joining the piezoelectric material is shown schematically for one embodiment. It comprises a piezoelectric material 3 which is brought into material-bonded contact with an electrode, an inner electrode 2 and an outer electrode 4, on two opposite sides 3.1, 3.2, via a bonding film 1.

[0075] For example, a joining foil 1 usable according to the invention is based on a eutectic Ag-Cu alloy with the addition of 3% titanium as an active component. Subsequent investigations have shown that the addition of an additional active component such as titanium is not absolutely necessary, so that other alloys, both silver- and copper-based, as well as pure silver and copper foils, are also suitable as joining foils usable according to the invention.

[0076] The individual components of the in Fig. 1 The schematically shown arrangement is pressed together in a clamping device with a defined pressing force to ensure good mechanical contact between the piezoelectric material, active solder foil and electrodes.

[0077] The joining process proposed according to the invention is advantageously carried out in accordance with the one described in Fig. 2 The temperature profile shown is precisely controlled during the joining process. In particular, the temperature increase per unit of time (heating rate) is set so that no oxides form prematurely on the surface of the metallic electrode, which could impair subsequent active layer formation in the contact area.

[0078] The formation of an active layer and thus the permanent acoustic connection (coupling) starts at low speeds from about 400 °C and accelerates with increasing temperature, so that the heat treatment at a final joining temperature of 500 °C ensures the formation of a comprehensive active connection between the piezoelectric material and the joining film, guaranteeing good and long-term stable ultrasound transmission.

[0079] When using iron-containing electrodes or pre-heating materials, it should be noted that a heating rate of 100 °C / h is not exceeded above 300 °C in order to avoid the formation of undesirable iron oxides in the boundary layer between the joining foil 1 and the electrodes 2, 4.

[0080] The in Fig. 2 The specified temperature profile applies to a material stack consisting of steel electrodes, lithium niobate as a piezoelectric crystal, and an active element depot in the form of a joining foil made of a eutectic Ag-Cu alloy with the addition of 3% titanium. In general, the joining process parameters are application-specific and are selected taking into account the properties of both the components of the metallic joining foil and the respective joining partners.

[0081] The joining temperature must, on the one hand, remain within the limits of the thermal stability of the joining partners, but on the other hand, it should be kept as high as possible to enable sufficiently rapid formation of the active layer on the joining material, as this speed determines the minimum holding time and thus the speed of the joining process. A compressive force applied to the material stack, for example by means of a clamping device, can accelerate the joining process somewhat, but must, of course, not exceed the mechanical strength of the joined materials (especially the brittleness of the piezoelectric material).

[0082] The product of the aforementioned chemical reaction forms the so-called active layer between the joining material and the joining partner, both of which are in a solid state without any intermediate liquefaction of components. This active layer ensures a comprehensive sound-transmitting coupling by achieving an almost void-free filling of the space between the adjacent joining partners.

[0083] When subjecting the stack of the aforementioned joining partners and metallic joining foils to a temperature, a certain rate of temperature increase must not be exceeded. A maximum temperature increase rate of 100°C / h has proven advantageous for various combinations of joining partners and metallic joining foils.

[0084] When joining a piezoelectric ceramic, as well as in the subsequent application of the transducer obtained by forming at least one active layer, the temperature used should advantageously not reach the Curie temperature of a piezoceramic. However, when using a transducer whose piezoelectric material has a crystalline structure, for example, lithium niobate, the Curie temperature can certainly be exceeded. Since no melting occurs in the joining process proposed according to the invention, a higher operating temperature of a transducer comprising a piezoelectric crystal may reduce the long-term stability of the transducer, but may in principle also be above the joining temperature.

[0085] The contact pressure exerted when the stack of joining partners is pressurized influences the joining time required for the formation of the active layer. The required joining time is inversely proportional to the applied contact pressure. Thus, increasing the contact pressure can accelerate the process speed or reduce the required joining time: Applying a pressing force of 100 N to a combination of steel electrodes, lithium niobate as a piezoelectric crystal (1.83 mm thick, 20 mm diameter), and joining foils made of a eutectic Ag-Cu alloy with the addition of 3% titanium (foil thickness 100 µm), resulting in a contact pressure of 0.3 MPa, good acoustic coupling can be achieved after 15 hours of joining time at 500 °C. The amplitude of a continuous ultrasonic signal was evaluated to characterize the acoustic coupling. (see Fig. 3 ).For joining lithium niobate as a piezoelectric crystal onto a steel pre-forming body with silver foil on both sides as an electrode, good acoustic coupling was achieved with a contact force of 400 N and a joining time of 12 h (see [reference]). Fig. 6 ). The dimensions of the piezoelectric crystal were 10 x 20 mm, resulting in a contact pressure of 2 MPa. Under identical conditions, good acoustic coupling was also achieved with bismuth titanate as the piezoelectric ceramic on a steel pre-forming body with silver foil electrodes arranged on both sides of the piezoelectric material (see [reference]). Fig. 7 ). In this context, good acoustic coupling is understood to mean a force-fit connection that allows reliable transmission of acoustic vibrations in the range of 10 kHz - 10 MHz, especially from 50 kHz - 5 MHz, for example from 100 kHz - 4 MHz.

[0086] For the reliable formation of an active layer and thus for successful joining, the joining surfaces advantageously have a defined surface finish. Preferably, the joining surfaces of the piezoelectric material are designed to be plane-parallel to each other and polished to a roughness of less than one micrometer.

[0087] The use of an adhesive film is optional, depending on the type of pre-element used (e.g., copper). This means that when using a copper pre-element in the material stack, the copper acts as a reservoir for the active element (essentially as an adhesive film) to form the active layer and also fulfills the function of the electrode. Therefore, if the piezoelectric crystal is placed directly onto the copper pre-element and clamped with a clamping force, an active layer can form between them at 500 °C. This allows for the formation of an active connection (active layer) and acoustic coupling without an adhesive film. If the pre-element material (a) is conductive and (b) forms an active layer in contact with the piezoelectric transducer during assembly, then the use of a separate adhesive film is unnecessary. PRACTICAL EXAMPLE OF IMPLEMENTATION

[0088] Fig. 4This metallographic image shows an example of a bonded layer produced during the joining of a lithium niobate piezoelectric crystal to a steel electrode using a bonding foil made of a eutectic Ag-Cu alloy containing 3 wt% titanium after a 15-hour bonding time at 500 °C. Although this bonding temperature is approximately 180 °C below the solidus temperature of the bonding material, it is clearly visible that an active layer has formed on both sides of the foil, indicating a chemical reaction between adjacent components. According to theory, the active layer consists primarily of titanium oxide. However, subsequent experiments have shown that titanium-free foils made of both pure copper and pure silver also lead to the formation of active layers on both sides and ensure comparable bonding quality.This demonstrates that the elements copper and silver each contribute to the formation of an active layer and are therefore possibly also present in the active layer shown. This is further supported by the phase distribution of the components in the joining foil after the joining process: While the central area of ​​the foil hardly differs from the original phase distribution in the foil used, the areas immediately adjacent to the active layers contain significantly less copper, or even no copper at all.

[0089] The developed joining principle results in significantly higher long-term temperature stability compared to the previously known high-temperature-resistant coupling technique with liquid glass (EP 0 459 431 B1), does not require elaborate protective measures against corrosion attack, which would be necessary in the case of glass melting, and also enables the use of the bonded partners once joined even at low temperatures, whereas coupling via glass melting (glass coupling) can only be used above the softening temperature of the glass from approximately 350 °C.

[0090] Aspects of the present invention can be further described according to the following points: 1. Production of an active joining connection using a joining foil at a temperature significantly below the melting point of the joining foil (solder melting point) or its solidus temperature, if a melting interval exists; 2. Joining method for joining a piezoelectric material to a sound transducer body by means of a joining layer formed during joining, which is referred to here as the active layer; 3. Construction of a material stack according to the description of the invention; 4. Transducer system based on the described joining technique; 5. Transducer system with angled sound transmission based on the described joining technique; 6. Transducer system for flow measurement based on the described joining technique; 7. Multi-transducer system for flow measurement based on the described joining technique; 8. Joining method for producing a transducer system (10) for extreme temperature applications.

[0091] The present invention has been explained by means of general descriptions, aspects, embodiments, and practical examples. These examples should in no way be considered limiting to the present invention. The following claims represent a first, non-binding attempt to define the invention in general terms. REFERENCE MARK

[0092] 1. Foil, joining foil, active solder foil 2. Damping body and / or inner electrode 3. Piezoelectric material 3.1, 3.2 Opposing surfaces of the piezoelectric material 4. Leading body and / or outer electrode 10. Stack, joining arrangement (section)

Claims

1. Joining method for manufacturing a sound transducer system, comprising: - providing a piezoelectric material (3) and a plurality of components, each of the components being characterised by a solidus temperature; - arranging the piezoelectric material (3) and the plurality of components in the form of a stack (10) so that there is a front stack part adjacent to a front side of the piezoelectric material (3) and a rear stack part adjacent to a rear side of the piezoelectric material (3); and - consolidating the stack (10) using heat and pressure for a predetermined period of time, wherein, during consolidation, the piezoelectric material (3) is directly acoustically coupled and / or electrically connected to an immediately adjacent component of the front and / or rear stack part, characterised in that none of the solidus temperatures of the plurality of components is exceeded; and the acoustic coupling and / or electrical connection comprises the formation of an active layer through chemical reaction of a first component of the piezoelectric material (3) with a second component of an active element deposit comprising a phase which is foreign to the piezoelectric material (3) originally used and the unjoined front stack part and / or the piezoelectric material (3) originally used and the unjoined rear stack part or is only present therein in finely dispersed form.

2. Joining method according to claim 1, wherein the plurality of components comprises at least one of the following: a delay line; a damping body, an electrode (2, 4) and the active element deposit; wherein the active element deposit may be in the form of the delay line, in the form of the damping body, in the form of the electrode (2, 4) or in the form of a separate film (1); wherein the front stack part and the rear stack part in each case comprise a component which is in each case designed as an electrode; and wherein the delay line and the damping body can, independently of each other, be functionally both an electrode (2, 4) and an active element deposit.

3. Joining method according to claim 2, wherein the front stack part contains the delay line and / or an electrode (2) comprising the active element deposit; and the rear stack part contains the electrode (4, 2) comprising the active element deposit and / or the damping body.

4. Joining method according to claim 3, wherein the damping body is also designed as an electrode (2, 4).

5. Joining method according to claim 4, wherein the active layer is formed in a component immediately adjacent to the piezoelectric material (3), selected from delay line, damping body, electrode and active element deposit, wherein the active layer is formed in a contact region of the component immediately adjacent to the piezoelectric material (3) with the piezoelectric material.

6. Joining method according to one of the claims 1 to 5, wherein the front side and rear side of the piezoelectric material (3) are aligned plane-parallel to each other.

7. Joining method according to one of the claims 1 to 6, wherein the front and / or rear side of the piezoelectric material (3) and the immediately adjacent surface of the component of the front stack part and the rear stack part have a roughness ≤1 um.

8. Joining method according to one of the claims 1 to 7, wherein the piezoelectric material (3) is selected from: lead zirconium titanate; lead lanthanum zirconium titanate, barium titanate; gallium orthophosphate; lithium tantalate; lithium niobate; lead titanate; lead niobate; a compound of the langasite group, in particular a lanthanum gallium silicate, aluminium nitride and bismuth titanate.

9. Joining method according to one of the claims 1 to 8, wherein the active layer comprises a chemical compound selected from: a copper oxide, a copper titanate, a copper niobate, a silver oxide, a silver titanate, a silver niobate, a titanium oxide and a titanium niobate.

10. Joining method according to claim 9, wherein the active layer substantially comprises an overall extent of a cross-sectional area of the stack aligned orthogonally to a stacking direction.

11. Joining method according to claim 9 or 10, wherein the active element deposit is a joining film (1) and two active layers are formed on the joining film (1) immediately adjacent to the piezoelectric material (3) if the joining film (1) is arranged between the piezoelectric material (3) and the delay line.

12. Joining method according to claim 11, wherein the two active layers are formed on opposite sides of the joining film (1).

13. Joining method according to claim 11 or 12, wherein the two active layers differ in thickness and / or chemical composition.

14. Joining method according to one of the claims 2 to 13, wherein the delay line comprises a steel and the active element deposit comprises a silver film, or the delay line comprises copper.

15. Joining method according to one of the claims 2 to 13, wherein the delay line comprises silver.

16. Joining method according to one of the claims 2 to 13, wherein the active element deposit comprises a film (1) comprising copper, titanium, a copper-based alloy or a silver-based alloy.

17. Joining method according to claim 16, wherein the silver-based alloy comprises 63-71% Ag, 26-35% Cu and 1-5% Ti as well as unavoidable impurities.

18. Joining method according to claims 2 to 13, wherein the active metal deposit is an active brazing film (1).

19. Joining method according to claim 18, wherein the active brazing film (1) comprises at least one of aluminum, hafnium, magnesium, nickel, niobium, titanium, vanadium, yttrium, and zirconium.

20. Joining method according to one of the claims 1 to 19, wherein the pressure applied during consolidation is 0.1-5 MPa, preferably 0.2-2 MPa.

21. Joining method according to one of the claims 1 to 20, wherein a heating rate during consolidation is ≤100 K / h.

22. Joining method according to one of the claims 1 to 21, wherein a temperature of the stack (10) during consolidation is ≤600 °C.

23. Joining method according to claim 22, wherein the heating rate after the stack (10) reaches a temperature of 300°C is less than 100 K / h.

24. Use of the joining method according to one of the claims 1-23 for the manufacture of a sound transducer, a piezo actuator, a ceramic filter, a ceramic resonator, an intermediate frequency filter or a high-temperature sensor comprising a ceramic feedthrough.