WO2002013178A1 - Ultrasonic transducer with an acoustic impedance matching layer - Google Patents

Ultrasonic transducer with an acoustic impedance matching layer Download PDF

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Publication number
WO2002013178A1
WO2002013178A1 PCT/GB2001/003526 GB0103526W WO0213178A1 WO 2002013178 A1 WO2002013178 A1 WO 2002013178A1 GB 0103526 W GB0103526 W GB 0103526W WO 0213178 A1 WO0213178 A1 WO 0213178A1
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WO
WIPO (PCT)
Prior art keywords
resin
transducer
matching layer
acoustic impedance
porous material
Prior art date
Application number
PCT/GB2001/003526
Other languages
French (fr)
Inventor
Stephen Patrick Kelly
Gordon Hayward
Original Assignee
University Of Strathclyde
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by University Of Strathclyde filed Critical University Of Strathclyde
Publication of WO2002013178A1 publication Critical patent/WO2002013178A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
    • B06B1/0644—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element
    • B06B1/0662—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element with an electrode on the sensitive surface
    • B06B1/067—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element with an electrode on the sensitive surface which is used as, or combined with, an impedance matching layer

Definitions

  • the present invention relates to ultrasonic transducers and in particular to ultrasonic transducers having impedance matching layers for efficiently coupling ultrasonic energy from an ultrasonic generator or receiver to a relatively low acoustic impedance medium and vice versa.
  • the present invention also relates to impedance matching layers for use with ultrasonic transducers.
  • the coupling of energy between an ultrasonic transducer and some transmission medium is maximised when the medium and transducer have the same specific acoustic impedance. Any mismatch in impedance results, in the case of ultrasound generation, in a proportion of the generated energy being reflected back into the transducer and being lost through absorption and leakage from the back face of the transducer. A similar energy loss in the medium occurs where the transducer acts as a receiver of ultrasound.
  • An ultrasonic matching layer is a passive layer which is fixed to the front face of an ultrasonic transducer in order to improve the coupling of energy to and from the transmission medium. Under narrow-band conditions, coupling is maximised when the thickness of the matching layer is equal to one quarter of the wavelength (or an odd multiple of a quarter wavelength) of the energy being transmitted. It is also known that the ideal acoustic impedance for a matching layer is equal to the geometric mean of the acoustic impedances of the transducer and the transmission medium. This theory can be extended to cover multiple matching layers in series where the optimum impedance for each layer is equal to the geometric mean of the impedances of the layers on either side.
  • acoustic impedance mismatch is particularly serious where piezoelectric ceramic or piezo- composite (ceramic/polymer matrix) transducers are required to transmit or receive ultrasonic energy to or from air (which is a low impedance load media) .
  • homogeneous materials of this type generally possess high attenuation and absorption characteristics for ultrasound and these characteristics tend to cancel out the beneficial matching effect of the matching layer.
  • silicone rubber room temperature vulcanised rubber - RTV
  • ' microballoons' air filled spheres
  • this technique is limited because the introduction of microballoons tends to increase the attenuation of the rubber and a point is reached where the benefit of any further reduction in acoustic impedance is offset by a corresponding increase in attenuation.
  • an ultrasonic transducer in combination with an acoustic impedance matching layer, the matching layer being arranged to efficiently couple ultrasonic energy between the transducer and a load and having first and second opposed sides respectively for contact with the transducer and with said load and having a generally increasing specific acoustic impedance between said sides such that the matching layer has a relatively low specific acoustic impedance in the region adjacent that one of the transducer and the load which has the lower specific acoustic impedance and a relatively high specific acoustic impedance in the region adjacent the other of the transducer and the load.
  • the increasing specific acoustic impedance of the matching layer results from an increase in porosity across the matching layer.
  • the matching layer comprises a porous material impregnated from one side with a layer of resin, subsequently set. This results in a graded change of porosity across the thickness of the matching layer.
  • the resin may overlap the edges of the porous material so that the porous material is surrounded by resin on all but said second side.
  • the porous material comprises a cellulose nitrate membrane and said resin comprises silicone rubber.
  • other porous materials and resins may be used, for example porous PTFE .
  • the matching layer may be fixed to the ultrasonic transducer using adhesive, e.g. low viscosity epoxy.
  • adhesive e.g. low viscosity epoxy.
  • the matching layer may be self-adhered to the transducer.
  • an impedance matching layer for an ultrasonic transducer and for efficiently coupling ultrasonic energy between the transducer and a load
  • the matching layer comprising a porous material impregnated from a first side of the layer with a resin, subsequently set, so that said first side is substantially non-porous whilst said second side of the layer, opposed to said first side, is porous, wherein in use said first side contacts that one of the transducer and the load which has the higher specific acoustic impedance and said second side contacts the other of the transducer and the load.
  • a method of making a matching layer for an ultrasonic transducer and intended to efficiently couple ultrasonic energy between the transducer and a load comprising applying a resin in liquid form to one side of a porous material and allowing the resin to permeate part-way into the porous material prior to the resin becoming permanently set.
  • the method comprises providing a thin sheet or membrane of porous material, arranging the sheet on a substantially planar support, depositing a quantity of liquid resin on the sheet or membrane, and bringing a planar contact surface into contact with the resin, wherein the contact surface is spaced apart from the upper surface of the sheet or membrane so as to disperse the liquid resin over the sheet/or membrane with a substantially uniform thickness .
  • the porous sheet and resin are then maintained within this arrangement until the resin is cured.
  • these surfaces are preferably covered with a release agent.
  • a preferred release agent is cling- film.
  • the resin is a room temperature setting resin. More preferably, the resin is silicone rubber.
  • Figure 1 illustrates in schematic form an ultrasonic transducer provided with an impedance matching layer
  • Figure 2 shows a plan view of an impedance matching layer according to an embodiment of the present invention
  • Figure 3 shows a vertical diametric cross-section of the impedance matching layer of Figure 2 ;
  • FIG 4 illustrates apparatus for fabricating the impedance matching layer of Figures 2 and 3.
  • an ultrasonic transmitting transducer which comprises a transducer housing 1 containing an ultrasonic energy generator 2 which is typically a piezo- ceramic block driven by an electric signal (not shown) to emit ultrasonic energy 3 from a front face thereof .
  • the transducer of Figure 1 is typically a narrowband 600kHz composite comprising generator 2 with a 40mm radius of curvature perspex lens 2a attached so that focussed energy is emitted from face 4.
  • an impedance matching layer 6 is secured to the front face 4 of the lens 2a.
  • FIGS 2 and 3 show in more detail the impedance matching layer 6 which has a generally cylindrical shape.
  • the matching layer 6 comprises a central portion 7 provided by a cellulose nitrate membrane of the type used to sterile filter biological solutions (WhatmanTM - plain white, 50mm diameter, 0.12mm thick, 0.45 ⁇ m pore size) .
  • a cellulose nitrate membrane of the type used to sterile filter biological solutions (WhatmanTM - plain white, 50mm diameter, 0.12mm thick, 0.45 ⁇ m pore size) .
  • silicone rubber 9 BartolineTM multipurpose silicone sealant
  • the overall diameter d of the matching layer 6 is typically a few centimetres whilst the overall thickness t is 0.57mm.
  • the matching layer 6 is fabricated (see below) so that the silicone rubber penetrates part-way into the porous membrane 7 from the upper surface 10 of the membrane
  • the apparatus comprises a pair of glass blocks 12, 13
  • the lower block 13 is provided with spacers 14, 15 (150mm x 10mm x 0.57mm) at respective opposed end edges which spacers project upwards so that, when the upper block 12 is lowered onto the lower block 13, the central portions of the blocks are spaced apart by a uniform thickness t equal to the thickness of the spacers 14, 15.
  • the glass blocks 12, 13 are tightly wrapped in clingfilm (safewrap FoodcareTM) to present a smooth, bubble and wrinkle free surface over the opposed surfaces of the blocks 12, 13.
  • clingfilm safewrap FoodcareTM
  • a cellulose nitrate membrane 7 is first placed in a central region of the lower block 13.
  • the membrane 7 is a Whatman (RTM) disc which has a porosity of 70% and a density of 0.36g.cr ⁇ r 3 .
  • RTM Whatman
  • the resin is left to a cure within this structure for 48 hours at room temperature. At the end of this period, the glass blocks 12, 13 are separated and the cling-film wrap facilitates the easy release of the matching layer from the blocks 12, 13.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Transducers For Ultrasonic Waves (AREA)

Abstract

An ultrasonic transducer (2) is provided with an acoustic impedance matching layer (6). The matching layer (6) comprises a porous material (7) impregnated from one side (10) with a layer of resin (9), subsequently set. The resin (9) overlaps the edges of the porous material (7) so that the porous material is surrounded by resin on all but the second side (8). The porous material (7) comprises a cellulose nitrate membrane and the resin comprises silicone rubber.

Description

ULTRASONIC TRANSDUCER WITH AN ACOUSTIC IMPEDANCE MATCHING LAYER
The present invention relates to ultrasonic transducers and in particular to ultrasonic transducers having impedance matching layers for efficiently coupling ultrasonic energy from an ultrasonic generator or receiver to a relatively low acoustic impedance medium and vice versa. The present invention also relates to impedance matching layers for use with ultrasonic transducers.
The coupling of energy between an ultrasonic transducer and some transmission medium (e.g. air or water) is maximised when the medium and transducer have the same specific acoustic impedance. Any mismatch in impedance results, in the case of ultrasound generation, in a proportion of the generated energy being reflected back into the transducer and being lost through absorption and leakage from the back face of the transducer. A similar energy loss in the medium occurs where the transducer acts as a receiver of ultrasound.
An ultrasonic matching layer is a passive layer which is fixed to the front face of an ultrasonic transducer in order to improve the coupling of energy to and from the transmission medium. Under narrow-band conditions, coupling is maximised when the thickness of the matching layer is equal to one quarter of the wavelength (or an odd multiple of a quarter wavelength) of the energy being transmitted. It is also known that the ideal acoustic impedance for a matching layer is equal to the geometric mean of the acoustic impedances of the transducer and the transmission medium. This theory can be extended to cover multiple matching layers in series where the optimum impedance for each layer is equal to the geometric mean of the impedances of the layers on either side.
The problem of acoustic impedance mismatch is particularly serious where piezoelectric ceramic or piezo- composite (ceramic/polymer matrix) transducers are required to transmit or receive ultrasonic energy to or from air (which is a low impedance load media) . Matching layers formed of a relatively low acoustic impedance material, such as balsa wood and cork, have been used. However, homogeneous materials of this type generally possess high attenuation and absorption characteristics for ultrasound and these characteristics tend to cancel out the beneficial matching effect of the matching layer.
Another material which has been used to provide matching layers is silicone rubber (room temperature vulcanised rubber - RTV) which, whilst having a higher acoustic impedance than balsa wood and cork, generally exhibits lower levels of attenuation and absorption. It has been found advantageous to decrease the acoustic impedance of the silicone rubber still further by introducing air into the silicone rubber, for example by introducing very small air filled spheres, sometimes referred to as ' microballoons' , into the silicone rubber. However, this technique is limited because the introduction of microballoons tends to increase the attenuation of the rubber and a point is reached where the benefit of any further reduction in acoustic impedance is offset by a corresponding increase in attenuation.
It is an object of the present invention to overcome or at least mitigate certain disadvantages of the above described ultrasonic matching layers . According to a first aspect of the present invention there is provided an ultrasonic transducer in combination with an acoustic impedance matching layer, the matching layer being arranged to efficiently couple ultrasonic energy between the transducer and a load and having first and second opposed sides respectively for contact with the transducer and with said load and having a generally increasing specific acoustic impedance between said sides such that the matching layer has a relatively low specific acoustic impedance in the region adjacent that one of the transducer and the load which has the lower specific acoustic impedance and a relatively high specific acoustic impedance in the region adjacent the other of the transducer and the load. Preferably, the increasing specific acoustic impedance of the matching layer results from an increase in porosity across the matching layer. More preferably, the matching layer comprises a porous material impregnated from one side with a layer of resin, subsequently set. This results in a graded change of porosity across the thickness of the matching layer. The resin may overlap the edges of the porous material so that the porous material is surrounded by resin on all but said second side. More preferably the porous material comprises a cellulose nitrate membrane and said resin comprises silicone rubber. However, it will be appreciated that other porous materials and resins may be used, for example porous PTFE .
The matching layer may be fixed to the ultrasonic transducer using adhesive, e.g. low viscosity epoxy. Alternatively, and in particular where said first side of the matching layer comprises resin, the matching layer may be self-adhered to the transducer.
According to a second aspect of the present invention there is provided an impedance matching layer for an ultrasonic transducer and for efficiently coupling ultrasonic energy between the transducer and a load, the matching layer comprising a porous material impregnated from a first side of the layer with a resin, subsequently set, so that said first side is substantially non-porous whilst said second side of the layer, opposed to said first side, is porous, wherein in use said first side contacts that one of the transducer and the load which has the higher specific acoustic impedance and said second side contacts the other of the transducer and the load.
According to a third aspect of the present invention there is provided a method of making a matching layer for an ultrasonic transducer and intended to efficiently couple ultrasonic energy between the transducer and a load, the method comprising applying a resin in liquid form to one side of a porous material and allowing the resin to permeate part-way into the porous material prior to the resin becoming permanently set.
Preferably, the method comprises providing a thin sheet or membrane of porous material, arranging the sheet on a substantially planar support, depositing a quantity of liquid resin on the sheet or membrane, and bringing a planar contact surface into contact with the resin, wherein the contact surface is spaced apart from the upper surface of the sheet or membrane so as to disperse the liquid resin over the sheet/or membrane with a substantially uniform thickness . The porous sheet and resin are then maintained within this arrangement until the resin is cured. In order to ensure that the matching layer may be easily released from the contacting surfaces, following curing of the resin, these surfaces are preferably covered with a release agent. A preferred release agent is cling- film.
Preferably, the resin is a room temperature setting resin. More preferably, the resin is silicone rubber.
For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawings, in which:
Figure 1 illustrates in schematic form an ultrasonic transducer provided with an impedance matching layer;
Figure 2 shows a plan view of an impedance matching layer according to an embodiment of the present invention; Figure 3 shows a vertical diametric cross-section of the impedance matching layer of Figure 2 ; and
Figure 4 illustrates apparatus for fabricating the impedance matching layer of Figures 2 and 3.- There is shown in Figure 1 an ultrasonic transmitting transducer which comprises a transducer housing 1 containing an ultrasonic energy generator 2 which is typically a piezo- ceramic block driven by an electric signal (not shown) to emit ultrasonic energy 3 from a front face thereof . The transducer of Figure 1 is typically a narrowband 600kHz composite comprising generator 2 with a 40mm radius of curvature perspex lens 2a attached so that focussed energy is emitted from face 4.
As described above, in order to increase the efficiency with which the ultrasonic energy 3 can be coupled from the front face of the piezo-ceramic block 2 and lens 2a to a low impedance medium, indicated generally in figure 1 by the reference numeral 5, an impedance matching layer 6 is secured to the front face 4 of the lens 2a.
Figures 2 and 3 show in more detail the impedance matching layer 6 which has a generally cylindrical shape. The matching layer 6 comprises a central portion 7 provided by a cellulose nitrate membrane of the type used to sterile filter biological solutions (Whatman™ - plain white, 50mm diameter, 0.12mm thick, 0.45μm pore size) . Surrounding the cellulose nitrate membrane 7 on all sides except the lower surface 8, is set silicone rubber 9 (Bartoline™ multipurpose silicone sealant) .
The overall diameter d of the matching layer 6 is typically a few centimetres whilst the overall thickness t is 0.57mm. The matching layer 6 is fabricated (see below) so that the silicone rubber penetrates part-way into the porous membrane 7 from the upper surface 10 of the membrane
(the extent of penetration .being illustrated by the dotted line in figure 3) . This structure results in the upper surface 10 • of the membrane being non-porous whilst the lower, central, surface 8 remains porous. The porosity of the membrane 7 may in practice be graded across its thickness, varying from non-porous at its upper surface 10 to porous at its lower surface 8. The matching layer 6 is secured to the emitting surface
4 of the piezo-ceramic block 2 and lens 2a by coating the upper surface 11 of the matching layer 6 with an adhesive and bringing that surface 11 into contact with the block 2 and lens 2a. This leaves the lower surface 8 of the cellulose nitrate membrane exposed to the low impedance transmission medium 5. As a result of the structure of the layer 6 which is substantially continuous the region of the layer 6 adjacent surface 11 has a high specific acoustic impedance whilst the region of the layer 6 adjacent surface 8 has a low specific acoustic impedance.
There will now be described, with reference to Figure 4, a method and apparatus for fabricating matching layers of the type described above with reference to Figures 2 and 3. The apparatus comprises a pair of glass blocks 12, 13
(15cm x 100mm x 15mm) having planar upper and lower surfaces. The lower block 13 is provided with spacers 14, 15 (150mm x 10mm x 0.57mm) at respective opposed end edges which spacers project upwards so that, when the upper block 12 is lowered onto the lower block 13, the central portions of the blocks are spaced apart by a uniform thickness t equal to the thickness of the spacers 14, 15.
The glass blocks 12, 13 are tightly wrapped in clingfilm (safewrap Foodcare™) to present a smooth, bubble and wrinkle free surface over the opposed surfaces of the blocks 12, 13.
In order to fabricate the matching layer 6, a cellulose nitrate membrane 7 is first placed in a central region of the lower block 13. As previously described the membrane 7 is a Whatman (RTM) disc which has a porosity of 70% and a density of 0.36g.crτr3. A portion of silicone rubber resin
(Bartoline) 18 in liquid form is placed onto the upper surface of the cellulose nitrate membrane 17 in the centre of the membrane. Care is taken to ensure that the silicone rubber resin does not contain any air bubbles . The upper glass block 12 is then carefully lowered into contact with the spacers 14, 15 of the lower block 13, resulting in the dispersion of the silicone rubber resin 18 across, and over the edges of, the cellulose nitrate membrane 17. If necessary, pressure is applied to the top block 12 in order to disperse the silicone rubber and bring the upper block 12 into contact with spacers 14, 15. In general, the amount of silicone rubber resin 18 introduced onto the membrane 17 is such that, when the blocks are brought into contact, the silicone rubber resin spreads over the edges of the membrane by a uniform distance of a few millimetres. It will also be apparent that in order to achieve this degree of overlap, the amount of silicone rubber resin introduced will be a function of the thickness t of the spacers.
The resin is left to a cure within this structure for 48 hours at room temperature. At the end of this period, the glass blocks 12, 13 are separated and the cling-film wrap facilitates the easy release of the matching layer from the blocks 12, 13.
It will be appreciated that various modifications may be made to the above described embodiment without departing from the scope of the present invention. For example, whilst the embodiment described above relates to an ultrasonic transmitter, the invention is equally applicable to ultrasonic receivers and to combined transmitter/receivers .
Whilst the embodiment described above achieves a varying specific impedance across the matching layer by varying the porosity of the matching layer material, alternative approaches may be used. For example, by impregnating tungsten powder in epoxy resin, such that the relative proportions of tungsten powder and epoxy vary in a graded manner from one side of the matching layer to the other, a similar variation in specific acoustic impedance may be achieved.

Claims

1. An ultrasonic transducer in combination with an acoustic impedance matching layer, the matching layer being arranged to efficiently couple ultrasonic energy between the transducer and a load and having first and second opposed sides respectively for contact with the transducer and with said load and having a generally increasing specific acoustic impedance between said sides such that the matching layer has a relatively low specific acoustic impedance in the region adjacent that one of the transducer and the load which has the lower specific acoustic impedance and a relatively high specific acoustic impedance in the region adjacent the other of the transducer and the load.
2. An ultrasonic transducer as claimed in claim 1, wherein the increasing specific acoustic impedance of the matching layer results from an increase in porosity across the matching layer.
3. An ultrasonic transducer as claimed in either preceding claim, wherein the matching layer comprises a porous material impregnated from one side with a layer of resin, subsequently set.
4. An ultrasonic transducer as claimed in claim 3 , wherein the resin overlaps the edges of the porous material so that the porous material is surrounded by resin on all but said second side .
5. An ultrasonic transducer as claimed in claim 3 or claim 4, wherein the porous material comprises a cellulose nitrate membrane and said resin comprises silicone rubber.
6. An impedance matching layer for an ultrasonic transducer and for efficiently coupling ultrasonic energy between the transducer and a load, the matching layer comprising a porous material impregnated from a first side of the layer with a resin, subsequently set, so that said first side is substantially non-porous whilst said second side of the layer, opposed to said first side, is porous, wherein in use said first side contacts that one of the transducer and the load which has the higher specific acoustic impedance and said second side contacts the other of the transducer and the load.
7. A method of making a matching layer for an ultrasonic transducer and intended to efficiently couple ultrasonic energy between the transducer and a load, the method comprising applying a resin in liquid form to one side of a porous material and allowing the resin to permeate partway into the porous material prior to the resin becoming permanently set.
8. The method of claim 7, comprising providing a thin sheet or membrane of porous material, arranging the sheet on a substantially planar support, depositing a quantity of liquid resin on the sheet or membrane, and bringing a planar contact surface into contact with the resin, wherein the contact surface is spaced apart from the upper surface of the sheet or membrane so as to disperse the liquid resin over the sheet/or membrane with a substantially uniform thickness .
9. The method of claim 8, wherein the contacting surfaces are covered with a release agent .
PCT/GB2001/003526 2000-08-05 2001-08-06 Ultrasonic transducer with an acoustic impedance matching layer WO2002013178A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0019140A GB0019140D0 (en) 2000-08-05 2000-08-05 Ultrasonic transducers
GB0019140.3 2000-08-05

Publications (1)

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WO2002013178A1 true WO2002013178A1 (en) 2002-02-14

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4686409A (en) * 1984-08-16 1987-08-11 Siemens Aktiengesellschaft Porous adaptation layer in an ultrasonic applicator
US4976150A (en) * 1986-12-30 1990-12-11 Bethlehem Steel Corporation Ultrasonic transducers
EP0640564A1 (en) * 1993-08-19 1995-03-01 Siemens Aktiengesellschaft Process for the preparation of a hydrophobic aerogel

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4686409A (en) * 1984-08-16 1987-08-11 Siemens Aktiengesellschaft Porous adaptation layer in an ultrasonic applicator
US4976150A (en) * 1986-12-30 1990-12-11 Bethlehem Steel Corporation Ultrasonic transducers
EP0640564A1 (en) * 1993-08-19 1995-03-01 Siemens Aktiengesellschaft Process for the preparation of a hydrophobic aerogel

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