Patent Application
20220120718
The present invention relates to a layered structure, the method for obtaining it and its use as part of a piezoelectric ultrasonic transducer to operate in broadband pulse-echo mode and with high sensitivity and axial resolution in the presence of a pressurised gas at a pressure between 14 bar and 103 bar. Furthermore, the present invention relates to the transducer comprising said layered or stratified structure.
Therefore, the present invention can be framed in the area of materials with applications such as sensors in ultrasonic systems.
The so-called pulse-echo ultrasonic technique for detecting internal discontinuities in solids or the rear surface thereof is well-known and has numerous industrial applications. This technique always uses liquid couplings (water or gel) or even soft solid couplings (dry coupling) between the ultrasound emitter/receiver (transducer) and the solid to be inspected (piece) and it is not particularly complex since this type of transducer/piece coupling is an effective way of ensuring efficient transmission of ultrasound from the transducer to the solid under inspection. This enables either the detection of internal discontinuities (detection of cracks, cavities or inclusions) in the solid or the opposite surface (thickness measurement).
When ultrasonic transducers operate by using air or gas coupling, the detection of echoes from gas/air interfaces with a solid, internal discontinuities of said solid, or the rear surface is much more complex since there is a strong acoustic impedance decoupling at the transducer-gas and gas-solid interfaces. This means that:
Furthermore, i) and ii) give rise to the fact that the transducers used for gas-coupling have a very long ringing, which produces a dead zone that is also very long, making it impossible to discern the presence of echoes close in time, that is, the axial resolution is very poor.
For very high gas pressures, the above problems can be reduced in such a way that it becomes possible to solve the problem using conventional transducers for immersion in water. This is due to the fact that in this case of very high pressures, the increase in gas density contributes to the acoustic impedance of the gas increasing in the same proportion. For a sufficiently high pressure, the impedance mismatch between the transducer and gas and between the solid and gas may decrease so much that it may be possible to use a transducer designed to operate by means of liquid coupling. Although the result is not optimal, if the pressure is high enough and the impedance of the gas increases as necessary, it is possible to obtain an at least acceptable performance. In practice it has been seen that it is possible to apply this approach for pressures greater than 69-103 bar (1000-1500 psi (US005587534A). Unfortunately, these pressures are too high for most of the applications that occur in the industry so this approach is useless.
Indeed, the pressures used in the transport and distribution of gas are in the range of 14-103 bar (200-1500 psi), so this approach to the problem based on the use of conventional transducers for immersion in water is not useful, since at these pressures, transducers designed for coupling by means of liquids do not offer acceptable performance. As the pressure in the gas decreases (below 103 bar), the ultrasound attenuation increases and the impedance decreases, which reduces the sensitivity of the immersion transducers and produces a deterioration in the signal-to-noise ratio; in addition, it lengthens the response signal, which causes a loss of axial resolution.
Moreover, the existing transducers for coupling by means of air are not useful for this application either, since they are designed to operate at reduced pressures and because they are designed to work in transmission mode and not in pulse-echo mode, presenting a poor axial resolution that is not acceptable in these applications.
Therefore, it is necessary to develop new transducers resistant to high pressures (>14 bar), with high sensitivity (>−35 dB) and good axial resolution (<5 μs) to detect echoes generated by a solid body (and its internal discontinuities) in the presence of a pressurised gas and using said gas as a coupling medium.
The present invention relates to a layered or stratified structure and the method for obtaining it and the use of said layered or stratified structure as part of a piezoelectric ultrasonic transducer and to the transducer comprising said sheet or layered or stratified structure.
The piezoelectric ultrasonic transducer to which the present invention relates operates in broadband pulse-echo mode, with high sensitivity (>−35 dB) and with high axial resolution (<5 μs) in the presence of a pressurised gas at a pressure between 14 bar and 103 bar (200-1500 psi), that is, it operates by generating ultrasonic signals and detecting the echoes generated by a solid body in the presence of a pressurised gas at a pressure between 14 bar and 103 bar (200-1500 psi).
Said piezoelectric ultrasonic transducer has a high sensitivity >−30 dB and a bandwidth >90% and can be used to carry out the following measurements in tanks and pipes that contain or transport gases at pressures between 14 bar and 103 bar (between 200 psi and 1500 psi).
In a first aspect, the present invention relates to a layered or stratified structure (hereinafter “the structure of the present invention”) characterised in that it comprises
f
c=/[4(t4/v4+t5/v5+t6/v6)]
wherein vi and ti are the speed of the ultrasound and the thickness in layer i, where i=3, 4, 5. The thickness of the piezoelectric layer (1) is determined from the resonance frequency of the transducer, being set in such a way that the frequency of the electrical resonance of the layer (1) is equal to fc.
The layered or stratified structure of the present invention can have any cross section: circular, rectangular, square, etc.
In the present invention, “resonant adaptive polymer membrane” is understood as the layer with a porosity greater than 70% and with interconnected pores having an acoustic impedance of between 0.1 MRayl and 0.5 MRayl and quarter-wave resonance conditions at a centre frequency greater than or equal to 1 MHz, between 1 MHz and 4 MHz. Said layer is adaptive because, thanks to its open pore nature, the gas inside the pores is in equilibrium with the external gas and this allows it to modify the acoustic impedance thereof in accordance with the pressure of the external gas. Said layer is resonant because the thickness thereof is equal to a quarter of the ultrasound wavelength at the working frequency of the transducer (fc). As the pressure of the external gas varies, its density also varies and due to this open-pore nature, the effective density of the layer is also modified and does so in the same direction. A change in gas density implies a proportional change in the impedance thereof, in such a way that, to maintain an optimal coupling of the transducer, it would also be necessary to modify the impedance of the material on the external face, which is not possible. This problem is addressed by using this open-pore layer the impedance of which varies as necessary when the pressure of the external gas varies. For this reason it is classified as adaptive.
In the present invention, “pressure-sensitive adhesive layer” is understood as that adhesive tape configured for the permanent bonding of two solid surfaces that does not have any physical support. The term “pressure-sensitive” in the present invention is understood as that which its adhesive is activated under pressure.
In the present invention, “syntactic foam” is understood as that foam that is composed of a polymer matrix to which hollow microspheres of glass or other material with similar characteristics are added, preferably with a size <20 μm, distributed randomly. For example, said syntactic foams are used in flotation systems for deep-sea submarines, for depths of 700 m to 10,000 m.
In the present invention, “anechoic conical finish” is understood as that cone-shaped finish that has an internal anechoic structure designed to fully disperse the reflections produced by acoustic waves on any of the surfaces that comprise it.
In a preferred embodiment of the structure of the present invention, the resonant adaptive polymer membrane (7) is selected from polypropylene, cellulose nitrate, cellulose ester, polyethersulfone and nylon.
In another preferred embodiment of the structure of the present invention, the pressure-sensitive adhesive layer (6) is selected from an acrylic layer or a polyurethane layer.
In another preferred embodiment of the structure of the present invention, the syntactic foam layer (5) is composed of an epoxy resin with hollow spheres with a diameter between 1 μm and 20 μm. The hollow spheres are preferably glass or polyolefin.
The polymer layer (4) is preferably an epoxy resin.
In another preferred embodiment of the structure of the present invention, the conductive layers (2e) and (2i) are independently composed of a conductive material selected from Au, Ag, Cu, Sn and any of the combinations thereof.
In another preferred embodiment of the structure of the present invention, wherein the composition of the piezoelectric layer (1) is selected from a PZT-type ceramic of lead zirconate titanate PbZrO3—PbTiO3, a composite material of piezoelectric ceramic and resin with 1-3-type connectivity and with a volumetric concentration of ceramic between 25% and 80%, and a Pb (Mg1/3Nb2/3)O3—PbTiO3 (PMN-PT)-type piezoelectric single crystal.
Another aspect of the invention relates to the method for obtaining the structure of the present invention (hereinafter “the method of the invention”) characterised in that it comprises the following steps:
The method of the present invention can be verified by measuring the electrical impedance at the terminals (layers (2i) and (2e)) in each step thereof.
In a preferred embodiment of the method of the present invention, step (e) of metallising the first face of the piezoelectric layer (1) to form the conductive layer (2e) that completely covers the piezoelectric element and the edge of the bushing is carried out by means of techniques selected from sputtering, vacuum metallisation, evaporation metallisation, screen-printed metallisation, dip coating and spin coating.
In another preferred embodiment of the method of the present invention, the deposition of step (f) is carried out by means of deposition techniques selected from dip coating, spin coating, and screen printing.
In another preferred embodiment of the method of the present invention, step (g) relating to affixing the syntactic foam layer (5) to the polymer layer (4) is carried out using an adhesive that has the same chemical composition as the polymer layer (4).
Another aspect of the present invention relates to the use of the structure of the present invention as part of a piezoelectric ultrasonic transducer to detect echoes generated by a solid body in the presence of a pressurised gas at a pressure of 14 bar and 103 bar (between 200 psi and 1500 psi), for example to detect echoes from natural gas pipelines.
It has been previously mentioned that the layered or stratified structure of the present invention can have any cross section: circular, rectangular, square, etc. The cross section of said structure will be selected according to the final cross section that is desired for the transducer; the shape of the metal housing or case (8) of step (b) of the method of the present invention will determine the final cross section of the transducer. For example, if a transducer with a circular cross section is to be obtained, then the metal housing or case (8) used in step (b) of the production method for obtaining the structure of the present invention will be cylindrical.
In the present invention, “piezoelectric ultrasonic transducer” is understood as the ultrasound generator and receiver based on the use of a piezoelectric material.
In the present invention, the piezoelectric ultrasonic transducer works in pulse echo mode, that is, between the transducer and the solid to be inspected through a pressurised gas, at a pressure between 14 bar and 103 bar (between 200 psi and 1500 psi).
The last aspect of the present invention relates to a piezoelectric ultrasonic transducer for detecting echoes generated by a solid body in the presence of a pressurised gas at a pressure between 14 bar and 103 bar (between 200 psi and 1500 psi), characterised in that it comprises
Throughout the description and the claims, the word “comprise” and its variants are not intended to exclude other technical features, additions, components or steps. For those skilled in the art, other objects, advantages and features of the invention will be deduced from both the description and the practical use of the invention. The following examples and figures are provided by way of illustration and are not intended to limit the present invention.
The invention shall be illustrated below by way of assays carried out by the inventors, which shows the effectiveness of the product of the invention.
The following exemplary embodiment relates to a piezoelectric ultrasonic transducer with a circular cross section. The following table 1 lists and describes the elements that make up the transducer.
Dice and fill refers to the standard manufacturing method of this material: a ceramic disk is taken, a series of regularly spaced cuts are produced in two orthogonal directions that form the pillars, and the gaps are then filled with resin.
The materials used in the manufacturing are listed below:
Structural elements [(8), (9), (11)]:
Housing or case, rear cover and front crown or rim according to
Other materials:
Materials for the backing [(3)]:
Auxiliary materials:
The integrity and suitability of the piezoelectric disc is checked by measuring the electrical impedance thereof in an impedance analyser or network analyser in the vicinity of the resonance frequency of the thickness mode provided by the manufacturer, in this case 1 MHz. To measure the electrical impedance of the disc, the metallised faces are connected to the output terminals of the impedance analyser. It is verified that the electrical resonance is at 1 MHz (tolerance 5%). This will be the centre frequency of the transducer to be manufactured. The integrity of the piezoelectric resonance is also verified by comparing the measured impedance curve with that which is provided by the manufacturer. Likewise, the almost negligible influence of radial modes of vibration is verified (lower frequency resonances and the harmonics thereof that can overlap with the resonance of the thickness mode located at 1 MHz).
This same electrical impedance measurement taken at the terminals of the piezoelectric disc is repeated at each step of the process to verify the integrity of the piezoelectric response and the correct assembly and sizing of the layers and elements placed at each step by comparing the measured impedance curve with that which is calculated knowing the properties (thickness, impedance and damping) of each deposited layer.
A piece of cable with a length approximately equal to twice the length of the housing or case (60 mm in this case) is taken and both ends are stripped to a length, approximately 3 mm). It is wound in the form of a solenoid (with an approximate diameter of ⅓ the internal diameter of the bushing, 4 mm in this case, and a similar length to that of the housing or case, 28.2 mm in this case) and one of the ends thereof is soldered to one of the faces of the piezoelectric disc using tin and paste for soldering. The other end of the cable is free. See
A demoulding agent is applied to the other face of the piezoelectric disc.
A plastic layer (polycarbonate 200 um, 30×30 mm) is taken and double-sided adhesive tape is affixed on a 40×40 mm surface. This will act as a support for fastening the piezoelectric disc to the housing or case. The piezoelectric disc is affixed to this layer on the face on which the demoulding agent was applied. See
The metal housing or case is affixed to the plastic film concentrically to the piezoelectric disc, ensuring the leak-tightness of the cavity formed by the bushing, piezoelectric disc and polycarbonate layer. See
The edge between the housing or case and the piezoelectric disc (width of 1 mm in the present example) is filled with epoxy resin loaded with tungsten powder and rubber powder, manufactured following the method explained in (3). See
The resin deposited between the housing or case and the piezoelectric disc is left to cure.
Once the resin has cured, the polycarbonate layer affixed to the piezoelectric disc and cylinder is removed. The external surface of the piezoelectric disc and the edge of the metal housing or case are cleaned of any remaining adhesive residue, preserving the metallisation of the piezoelectric disc. The end result in this step is that the piezoelectric disc is fastened to the housing or case by means of a resin ring loaded with tungsten powder See
The surface that has just been cleaned is metallised: piezoelectric disc+resin ring+edge of the housing or case. Any available technique can be used, either sputtering, evaporation or screen printing. See
Finally, the electrical conductivity between the surface that has just been metallised and the metal housing or case of the transducer is verified and the electrical impedance measurement is repeated to verify that the piezoelectric resonance remains unchanged except for the introduction of a slight damping due to the fastening from the piezoelectric disc to the housing or case. The result of the impedance measurement is shown in
Deposition of a 300-350 micron resin layer, using a screen printing, deep coating or any similar technique on the external surface of the piezoelectric disc. For this purpose, both components of the resin are mixed, the trapped air is removed using a vacuum hood and deposited, while in a liquid state, on the metallised surface, covering it completely. It is not necessary for the deposited layer to lie flat or parallel. The amount of resin deposited, equivalent to a 300-350 micron layer, is controlled by means of weighing on a precision scale. The final result can be seen in
The resin thus deposited is left to cure following the resin manufacturer's instructions.
Once cured, its external surface is polished using an automatic polisher to leave a flat finish normal to the axis of the bushing. The final thickness of this layer should be 260 μm, roughness <5 μm. The result after this step can be seen in
At this point, the electrical impedance measurement of the transducer is repeated to verify the correct deposition of the resin layer, as explained in (1). For this, the free end of the cable and the bushing are connected to the terminals of the impedance analyser. The result is shown in
A 1 mm thick layer, with a side >25 mm, is cut from a block of syntactic foam.
The thickness of the previous layer is lowered, either by means of a numerical control lathe or by using a polisher, to a thickness of 330 μm, guaranteeing the flat parallelism of both faces of the layer. A disc is cut from this layer with the same diameter as the external diameter of the housing or case of the transducer (13.3 mm in this case). Said operation can be performed using a punch.
The syntactic foam layer is affixed to the surface of the transducer using the same epoxy resin that was deposited on the surface of the piezoelectric element. The added thickness of epoxy resin to achieve this bond must be 100 μm. The amount of resin added is controlled by weighing on a precision scale. See
Once the previous layer has cured and therefore the syntactic foam layer has been affixed, the electrical impedance measurement is repeated by connecting the free end of the cable and the housing or case to the terminals of the impedance analyser to verify the correct placement of this layer as explained in (1). The result is shown in
Once verified, a layer of pressure-sensitive double-sided adhesive tape (thickness 60 μm) is placed on the external surface of the syntactic foam layer. The adaptive and resonant polymer membrane is placed on this layer of adhesive tape. See
Once this cycle is finished and the transducer has cooled down, the electrical impedance measurement is repeated by connecting the free end of the cable and the bushing to the terminals of the impedance analyser to verify the correct placement of this layer as explained in (1). The result is shown in
Once the last layer of the stack of layers has been deposited, both the front face and the side face are protected by placing the front protection ring, which can be simply fitted or fitted and affixed onto the housing or case. See
Once the front protection crown or rim is placed, the transducer is placed in a vertical position, resting on the front crown. The cavity formed by the housing or case and the surface of the piezoelectric disc is filled with the mixture of epoxy resin, tungsten and rubber powder before the mixture begins to cure, up to a height of ⅔ of the total height of the bushing, ensuring that the free end of the cable soldered to the piezoelectric disc is free. The mixture is left to cure. See
Manufacture of the mould for the conical finish of the backing. A disc is cut from the low-density polyurethane foam (100-300 kg/m3), with a diameter equal to the internal diameter of the metal bushing and a thickness equal to the radius. A conical surface with a generatrix at 45 degrees from the axis is carved on one of the faces of this disc. A small hole (<1 mm diameter) is made at the vertex.
The mould thus manufactured is placed on the backing deposited in the housing or case by passing the free end of the cable soldered to the piezoelectric disc through the hole made at the vertex of the conical surface. See
The conical space between the mould and the backing is filled with the same mixture of resin, tungsten and rubber powder with which the backing was manufactured. It is left to cure. See
Once the backing is placed, the correct operation is verified again by measuring the electrical impedance of the transducer. The result is shown in
The panel SMB coaxial connector is fastened to the rear cover. The grounded connector is connected to the rear cover. The free end of the cable is soldered to the internal pin of the coaxial connector (SMB). See
The rear cover is fastened to the housing or case ensuring the electrical connectivity between the rear cover and the body of the housing or case and between the internal pin of the connector and the cable of the transducer. See
(7) Manufacture of the Material to Make the Backing of the Transducer and of the Material to Fasten the Piezoelectric Disc to the Bushing. Both Materials are Identical.
The material is made up of epoxy resin, tungsten powder (12 microns) and rubber powder (<120 microns). Weight ratio: Resin: 22%, Tungsten powder: 72%, rubber powder: 6%
The additives (tungsten powder and rubber powder) are added to component A of the resin.
The three components are mixed well.
Resin hardener (part B) is added.
The mixture is placed in a vacuum to remove trapped gas.
The mixture is deposited in its final location. The mixture is again degassed in a vacuum hood.
It is subjected to hydrostatic pressure to remove the smallest bubbles (30 min, 6 bar).
The resin is cured following the manufacturer's instructions.
To verify the correct selection of the membrane, the characterisation technique described in the following references is applied
In addition, based on the theoretical analysis of the magnitude and phase of the transmission coefficient, it is also possible to determine the impedance of the material [4].
The spectrum of the transmission coefficient of the selected membrane (magnitude and phase) and the theoretical adjustment that enables the determination of the impedance are shown in
To measure the impulse response and the sensitivity band at ambient pressure and with an air overpressure of 5 bar, of the manufactured transducer, an Olympus transmitter-receiver (5072) is used, a 90 V amplitude excitation pulse, and the received signal is taken directly to the oscilloscope, without any extra amplification. A 22 uH inductance is placed in parallel with the terminals of the transducer. The reflector was a steel block located at 7 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| P201930140 | Feb 2019 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2020/070122 | 2/20/2020 | WO | 00 |
