WO2013104538A2

WO2013104538A2 - Sound transducer, in particular ultrasonic transducer, and method for the production thereof

Info

Publication number: WO2013104538A2
Application number: PCT/EP2013/000054
Authority: WO
Inventors: Edwin Muz
Original assignee: Nicolay Verwaltung Gmbh
Priority date: 2012-01-10
Filing date: 2013-01-10
Publication date: 2013-07-18
Also published as: DE102012000275A1; WO2013104538A3

Abstract

The invention relates to a sound transducer, in particular an ultrasonic transducer, comprising a transducer element (10) for transducing an electric signal into sound waves and/or from sound waves into an electric signal, and comprising an adapter element (20) for adapting the acoustic impedance of the transducer element (10) to the acoustic impedance of the medium surrounding the transducer (1), the adapter element (20) having hollow bodies (34) that are embedded into a cured resin, characterised in that the adapter element (20) has an at least partially cured bismaleimide resin (36) and in that the hollow bodies (34) are embedded into the bismaleimide resin (36) and, together with the bismaleimide resin (36), form a bismaleimide resin-containing syntactic foam. The invention further relates to a method for producing a transducer of this kind.

Description

Sound transducers, in particular ultrasonic transducers,

and method for its production

The invention relates to a sound transducer, in particular a

Ultrasonic transducer, with a transducer element, such as a piezoceramic disc, and an adaptation layer, such as a syntactic foam, for operation as a transmitter or receiver of ultrasonic pulses, for example in a gaseous medium, and a method for producing such a sound transducer.

A piezoceramic transducer element can emit only a small part of the ultrasound power to the surrounding air, since the interface ceramic / air is less permeable thereto, the permeability is the lower, the greater the difference of the acoustic impedance resulting from the product of the density and the speed of sound in the respective material or medium is calculated. From the prior art, the use of one or more matching layers is known, wherein theoretically the matching layer is optimal when the

Layer thickness is an integer multiple of a quarter of the wavelength of the sound waves in the matching layer and the acoustic

Resistance in the matching layer is equal to the root of the product of the acoustic impedances in the adjacent media, for example, the piezoceramic disk on the one hand and the air on the other hand. From DE 25 37 788 A1 an ultrasonic transducer for use in a rangefinder is known with a transducer element for

Convert an electrical signal into sound waves. At least one scarf labstrahlfläche has a λ / 4-layer whose acoustic impedance has a value which is between the acoustic impedance of the

Transducer element and the acoustic impedance of the

Surrounding the ultrasonic transducer surrounding medium, where λ is the wavelength of the ultrasound generated in the λ / 4 layer. The λ / 4 layer is prepared by mixing glass hollow spheres with a diameter of about 0.1 mm with polystyrene varnish. After drying the paint, a foam having a pore size of about 0.1 mm and a density of about 0.3 g / cm ^{3 is obtained} . From DE 10 2008 055 126 A1 an ultrasonic transducer for use in a fluid medium is known in which a matching body is made of a pressed or porous sintered polyimide.

From DE 695 05 430 T2 or EP 0 667 646 B1 a piezoelectric or electrostrictive thin-film element is known, which can cause or detect deformations or force in the form of distortions or bending or bending.

DE 101 35 414 C1 discloses the production of a non-flat membrane for electroacoustic transducers which has a core layer comprising polymethacrylimide foam and at least one covering layer.

The invention has for its object to provide a sound transducer and an associated manufacturing method, with the simple way an even better adaptation of the transducer element to the

Sound transducer surrounding medium can be achieved.

This object is achieved by the sound transducer defined in claim 1 and by the specific method of manufacture in the independent claim. Particular embodiments of the invention are in the

Subclaims determined.

In one embodiment, the object is achieved by a sound transducer in which the matching element at least partially comprises a hardened bismaleimide resin, preferably the resin used consists entirely of a bismaleimide resin.

Bismaleimide have a high temperature resistance after curing. The hollow bodies used for the adaptation element are embedded in the bismaleimide resin and form together with the

Bismaleimide resin a bismaleimide resin-containing syntactic foam, which provides excellent adjustment of the acoustic impedance of the

Transducer element, such as a piezoelectric lead zirconium titanate (PZT) ceramic having a typical acoustic impedance of 3 x 10 ⁷ kg / (m ² s), to the acoustic impedance of air at about 4 x 10 ² kg / (m ² · s) offers. The adaptation element produced in this way has significantly better properties than the previously known matching elements or λ / 4 layers.

In one embodiment, the syntactic foam has a density of greater than 110 and less than 220 kg / m ³ , more preferably greater than 130 and less than 200 kg / m ^3, and more preferably greater than 150 and less than 180 kg m ³ . As a result, the adaptation of the acoustic impedances is further improved.

In one embodiment, more than 80% of the hollow bodies, in particular more than 90%, and preferably more than 95%, of the hollow bodies are covered with a layer of the bismaleimide resin in the syntactic foam. The layer thickness may be less than 20 μm, in particular less than 12 μm and preferably less than 8 μm. In one embodiment, the packing density of the hollow body in the foam is less than 20%, in particular less than 1 5% and preferably less than 10% lower than in the starting material of the hollow body.

For example, the packing density of the hollow body in

Starting material 60%, in the foam, however, more than 48%, in particular more than 51% and preferably more than 54%.

In one embodiment, the thickness of the matching element is more than three times and less than twenty times the thickness of the

Transducer element, in particular more than five times and less than fifteen times and preferably more than eight times and less than twelve times. In one embodiment, the thickness of the

Transducer element less than 0.6 mm, in particular less than 0.4 mm and preferably less than 0.25 mm. A syntactic foam with one or more of the aforementioned parameters leads to a very good sound conduction with a comparatively low specific weight and overall to a very good adaptation of the acoustic impedance. In addition, the foam and thus the adjustment element is very good heat resistance to at least 200 ° C and therefore also steam sterilizable, as well as chemical resistant and has a high rigidity.

In one embodiment, the adjoining the transducer element first sound coupling surface of the matching element and / or to the the

Sound transducer surrounding medium adjacent second sound coupling surface of the adjustment element unprocessed, in particular mechanically unprocessed, for example, not ground or milled. Investigations have shown that this further improves the adaptation of the acoustic impedance.

In one embodiment, a arranged between the two sound coupling surfaces, in particular one of the two sound coupling surfaces

connecting surface of the adjustment element convex shaped.

This also further improves the acoustic adaptation.

The sound transducer according to the invention can in particular for measuring the flow velocity of a fluid, for example a

Breathing gas mixtures are used. For this purpose, ultrasonic pulses are sent both in the direction of the flow and in the opposite direction, and the time that is necessary in each case to pass through a defined path is measured. From the duration difference, the

Flow rate can be calculated. With operating frequencies between 100 and 500 kHz, a very high accuracy can be achieved. Ultrasound of this frequency can be generated in a simple way with the aid of a slice of piezoceramic, which is provided on the two parallel surfaces with a metallization. The transducer element is operated in the region of its resonant frequency, which is essentially determined by the geometric dimensions, in the case of a disc through the diameter and the thickness. If an alternating voltage is applied to the electrodes of the transducer element, this is excited to oscillate and emits sound waves. Conversely, a voltage proportional to the sound pressure can be tapped at the two electrodes when sound waves impinge on the transducer element. The sound transducer according to the invention not only has an optimal

Adjustment of the acoustic impedance, but also a sufficiently high temperature resistance for the application of the sterilization method used in particular in medical technology. It is also particularly advantageous that through the use of a bismaleimide resin in conjunction with the preferably spherical hollow bodies,

especially hollow glass bodies, a low attenuation of the ultrasound can be achieved. On the other hand, the optimum adaptation of the acoustic impedance results in a rapid decay of the oscillation after switching off the electrical excitation signal, so that overlays are avoided and a high pulse sequence can be achieved. On the in the prior art for this purpose often used back damping of the transducer element, the so-called "backing", can at

Scarf Iwandlern invention are dispensed with. The invention also relates to a process for the preparation of a

Sound transducer element with a transducer element and a

Adaptation element comprising hollow bodies embedded in a bismaleimide resin. The bismaleimide resin may be in the form of a powder, a paste or a liquid. To reduce the viscosity is slurried the bismaleimide resin with a solvent and then mixed with the hollow bodies. Thereafter, the solvent is largely evaporated and the resulting paste is cured or crosslinked at temperatures of for example between 150 and 180 ° C according to the respective regulations of the resin supplier. It is advantageous if as little additional air in the foam

is incorporated. In one embodiment, the mixture is off

Bismaleimidharz and hollow bodies already brought during and / or after evaporation of the solvent in a vacuum chamber to accelerate outgassing of air and / or evaporation of the solvent.

In one embodiment, an as yet uncured green compact of the matching element may be manufactured separately in a mold,

optionally also cured in a mold, and then connected to the transducer element. Alternatively, a green compact of the matching element can be placed on the transducer element and cured on this and in particular crosslinked and are simultaneously firmly connected to the transducer element.

Thereafter, the transducer element without further post-processing

operational.

In one embodiment, when slurried, the ratio of the mass of the bismaleimide resin (excluding the proportion of the solvent) to the mass of the hollow bodies is more than 0.8 and less than 1.9, in particular more than 1.0, and less than 1.7 and preferably more than 1, 2 and less than 1, 5. In one embodiment, the ratio of the mass of

Bismaleimidharz to the mass of the hollow body, which consist of hollow glass spheres, between 1, 0 and 1, 5, in particular between 1, 3 and 1, 4. As glass hollow spheres, it is possible, for example, to use those of the type K1 or K1 1, which are offered by the company 3M as "glass bubbles." The "true density" of these spheres is, for example, 125 or 110 g / cm ³ . The packing density is 60%, which is the

Bulk density between 60 and 75 g / cm ³ . The density of the bismaleimide resin is about 1,000 g / cm ³ . To the glass balls completely and without

To embed air pockets in the resin, the mass ratio of resin mass to glass ball mass would theoretically have to be about 5.

Investigations have shown, however, that very good results are achieved with a density of the foam of 1 50 to 180 g / cm ³ , although the acoustic impedance in this case is higher than the theoretically optimal acoustic impedance. The hollow glass spheres are covered with a resin layer of a few μπ \ thickness, in particular less than 10 μηη, and the packing density of about 60% is maintained, in particular, the packing density is reduced by less than 10% compared to the starting material, preferably less than 5%. If the resin layer is very thin, the attenuation of the sound in the matching element increases; with thicker resin layers, the density of the foam increases.

During curing or crosslinking, the green compact is in contact with a mold element on at least two opposing surfaces. If the green compact is cured directly on the transducer element, the transducer element, in particular its surface facing the adapter element, can form one of the two mold elements. In this way, the thickness of the adjustment element can be reproducibly made very accurate reproducible in a simple manner. In one embodiment, the composite of transducer element and adapter element is fixed by means of an elastomer in a housing of the sound transducer. The housing may for example consist of a polymer plastic, such as polyvinylidene fluoride (PVDF), and / or cup-shaped. The adjustment element can be flush with the

Complete the edge of the housing. As an elastomer, silicone rubber can be used, for example, via an opening in the bottom surface of the housing in a cavity between the bottom surface and the

Transducer element is injected and during curing and the

Connecting lines fixed to the transducer element within the housing.

Further advantages, features and details of the invention will become apparent from the subclaims and the following description in which several embodiments of the invention are described in detail with reference to the drawings. It can in the

Claims and features mentioned in the description in each case be essential to the invention individually or in any combination. Fig. 1 shows a schematic representation of a section through a

2 shows a state in the course of the manufacture of the sound transducer, in which the adaptation element is cured and firmly connected to the transducer element,

Fig. 3 shows a section through the inserted into a housing

Sound transducers, and

Fig. 4 shows an enlarged view of a section IV of the

Adaptation element of FIG. 3. Fig. 1 shows a schematic representation of a section through an embodiment of a transducer 1 according to the invention in a not yet completely produced state. The sound transducer 1 has a transducer element 10, which in the embodiment by a

piezoelectric ceramic disc is formed with a diameter of 10 mm and a thickness of 0.2 mm. The essentially of

Diameter specific oscillation frequency of the transducer element 10 is about 200 kHz. On the opposite plane-parallel end faces, an electrically conductive and preferably metallic contact layer 12, 14 is applied to the transducer element 10 in each case. Before connecting the matching element 20 to the transducer element 10, the first contact layer 12 is electrically contacted by means of a first connecting line 16, in particular a first connecting wire is soldered. At the same time or at a later time, a second connecting line 18 can be attached to the opposite second contact layer 14, in particular soldered.

The matching element 20 is formed of a bismaleimide resin having hollow bodies embedded therein, in particular hollow glass spheres, in the form of a syntactic foam. In the exemplary embodiment that is

Adjustment element 20 in the state of Figure 1 is still a green, d. H. the bismaleimide resin is not yet crosslinked, so that the adjustment element 20 is not yet cured. For the bismaleimide resin, a bismaleimide adhesive of the type PX-305 from Polymerics GmbH Berlin was used

(www.polymerics.de) whose viscosity is reduced by adding a solvent, for example dichloromethane. The solution thus produced can also be used as an intermediate layer between the second contact layer 12 and the green compact of the adaptation element 20

be applied to the connection between the transducer element 10 and the adjustment element 20 to improve. Glass bubbles, for example "3M type K1 Glass Bubbles", are stirred into the solution, and this slurry can be poured out onto a flat dish, and after evaporation of the solvent, a layer with a thickness of, for example, 2.5 to 3 mm remains , which already has sufficient inherent stability for further processing.

From this layer can in the desired shape green compacts of

Adjustment element 20 are made or cut out and then brought into abutment with the transducer element 10.

FIG. 2 shows a state during the production of the

Sound transducer 1, in which the adaptation element 20 is already networked and firmly connected to the transducer element 10. The adjustment element 20 has two sound coupling surfaces 21, 22, wherein a sound coupling surface 21 is in close contact with the transducer element 10 and the

opposite sound coupling surface 22 the scarf labstrahl fold or sound input surface of the transducer 1 forms. Even in the event that the green compact of the adaptation element 20 is already cured before being attached to the transducer element 10, the two remain

Sound coupling surfaces 21, 22 preferably unprocessed. A surface 24 connecting the two sound coupling surfaces 21, 22

Adjustment element 20, in particular the lateral surface of the im

Substantially cylindrical adapter 20 is convex. An electrically insulating hose is laid over the first connection line 16 in order to prevent an electrical short circuit, in particular when installing the sound transducer 1 in a housing 30.

3 shows a section through the sound transducer inserted into a housing 1. The housing 30 preferably consists of a Plastic. The two connection lines 16, 18 are led to outside of the housing 30 and there electrically contacted. The housing 30 is substantially pot-shaped, wherein the sound transducer 1 is inserted so far into the housing that the sound coupling surface 22 of the

Adjustment element 20 is substantially flush with the edge of the housing 30. On one of the sound coupling surface 22nd

opposite bottom surface, the housing 30 openings for the passage of the two connecting lines 16, 18. Either via one of these openings or via an additional opening 28, a filling compound 32 can be introduced into the cavity remaining between the transducer element 10 and the bottom surface of the housing 30, by means of which the transducer element 1 can be fixed mechanically and preferably also fluid-tightly in the housing 30. As filler 32, for example, a silicone rubber can be used.

The transducer element 10 is spaced from the housing 30. in the

Embodiment, the arrangement for this purpose an annular gap between the transducer element 10 and the housing 30, which is filled by the filling compound 32. The dimension of the annular gap is not to scale in FIG. 3; in particular, the dimension of the annular gap may be smaller than shown in FIG. 3. The filling compound 32 is flush with the transducer element 10. Alternatively or additionally, between the adaptation element 20 and the housing 30 at least

an annular gap may be provided in sections, which is preferably also filled by the filling compound 32. In one embodiment, the filling compound 32 at the sound coupling surface 22 flush with the

Complete fitting element 20. In this case, by the

Annular gap between the adjustment element 20 and housing 30 and a fitting of a deviating from the cylindrical shape adjustment element 20, in particular a tone shown in FIG. 2 nenförmförm

Adjustment element 20, carried in the housing 30 with its cylindrical opening. 4 shows an enlarged view of a section IV of the adjustment element 20 in the networked state of FIG. 3. The hollow body 34, in the embodiment glass hollow body, are embedded in the Bismaleimidharz 36e. The wall 38 of the hollow body 34 is so thin that the density of the adaptation element 20 is sufficiently low in order to ensure optimum acoustic coupling of the transducer element 10 to the medium surrounding the sound transducer 1. The

Outer surface 40 of the wall 38 is covered with a preferably few μνη thick layer of Bismaleimidharzes 36, even in the areas in which adjacent hollow body 34 is very close or even abut each other.

Claims

P a n t a n s p r e c h e

Sound transducer (1), in particular ultrasonic transducer, with a

Transducer element (10) for converting an electrical signal in sound waves and / or Schallwel len in an electrical signal and with a matching element (20) for adjusting the acoustic impedance of the transducer element (10) to the acoustic impedance of the sound transducer (1) surrounding medium , where the

Adaptation element (20) in a cured resin embedded hollow body (34), characterized in that the

Adapter element (20) at least partially comprises a hardened bismaleimide resin (36), and in that the hollow bodies (34) are embedded in the bismaleimide resin (36) and together with the

Bismaleimidharz (36) bi lden a bismaleimide resin-containing syntactic foam.

Sound transducer (1) according to claim 1, characterized in that the syntactic foam has a density of more than 1 10 and less than 220 kg / m ³ , in particular of more than 1 30 and less than 200 kg / m ^3, and preferably more as 1 50 and less than 180 kg / m ³ .

Sound transducer (1) according to claim 1 or one of the preceding claims, characterized in that more than 80% of the

Hollow body (34), in particular more than 90% and preferably more than 95% of the hollow body (34) are covered with a layer of Bismaleimidharzes (36), wherein the layer thickness less than 20 μνη, in particular less than 1 2 μηη and preferably less than 8 μτη is. Sound transducer (1) according to claim 1 or one of the preceding claims, characterized in that the thickness of the

Adjustment element (20) is more than three times and less than twenty times the thickness of the transducer element (10), in particular more than five times and less than that

Fifteen times and preferably more than eight times and less than twelvefold.

Sound transducer (1) according to claim 1 or one of the preceding claims, characterized in that the thickness of the

Transducer element (10) is less than 0.6 mm, in particular less than 0.4 mm and preferably less than 0.25 mm.

Acoustic transducer (1) according to claim 1 or one of the preceding claims, characterized in that the first sound coupling surface (21) adjoining the transducer element (10) and / or the second sound coupling surface (22) of the medium surrounding the sound transducer (1) adjoin the second sound coupling surface Adjustment element (20) is unprocessed, in particular mechanically unprocessed.

Sound transducer (1) according to Claim 1 or one of the preceding claims, characterized in that a surface (24) of the surface arranged between the two sound coupling surfaces (21, 22)

Adjustment element (20), in particular one the two

Sound coupling surfaces (21, 22) connecting surface (24) of the

Adjustment element (20) is convex. Method for producing a sound transducer (1), in particular an ultrasound transducer, having a transducer element (10) for converting an electrical signal into sound waves and / or from sound waves into an electrical signal and with a transducer

Adjustment element (20) for adjusting the acoustic impedance of the transducer element (10) to the acoustic impedance of the

Sound transducer (1) surrounding medium, wherein the

Adaptation element (20) hollow body (34) embedded in a cured resin, characterized in that the hollow body (34) are slurried in a solution of a Bismaleimidharz (36) and a solvent, and that by evaporation of the solvent, a green body the adaptation element (20) is formed, which is cured to form a Bismaleimidharz-containing foam and connected during curing or subsequent to curing with the transducer element (10).

A method according to claim 8, characterized in that the ratio of the mass of the bismaleimide resin (36) to the mass of the hollow body (34) during slurrying more than 0.8 and less than 1, 9, in particular more than 1, 0 and less than 1, 7 and preferably more than 1, 2 and less than 1, 5.

A method according to claim 8 or 9, characterized in that the green body of the adaptation element (20) during curing

is at least two opposing surfaces in contact with a mold element. Method according to claim 8 or one of claims 8 to 10, characterized in that the composite of transducer element (10) and adjustment element (20) is fixed by means of an elastomer housing (30).