Large area ultrasonic transducer

The description details a preferred embodiment of an improved large area ultrasonic transducer 70 capable of reducing the generation of adverse "edge effect" waves. The transducer has a thin piezoelectric wafer 72 that has a high area-to-thickness ratio of preferably between 30 and 300. A front electrode coating 84 is deposited on the front surface 74, over the front edge 77, along the side surface 82 and over the back edge 78 and onto a border of the back surface 76 to minimize the application of a voltage potential along the side surface. A voltage modifying layer 92 is placed on the back surface 76 along the back edge 78 for further minimizing the generation of "edge effect" waves. The layer 92 varies in thickness to progressively decrease the voltage applied to the back surface 76 from a large central area 79(a) to the back edge 78. The layer 92 is preferably composed of a non-piezoelectric dielectric material.

TECHNICAL FIELD 
This invention relates to ultrasonic transducers and more particularly to 
large area ultrasonic transducers for generating plane waves with minimal 
edge effect distortion for use in ultrasonic holography. 
BACKGROUND OF THE INVENTION 
Although commercial application of ultrasonic holography has been actively 
pursued by many persons in the scientific and industrial communities for 
many years, only limited results have been obtained even though it was 
once thought that ultrasonic holography held great promise. It was felt 
that the application of ultrasonic holography was particularly applicable 
to the fields of non-destructive testing of materials and medical 
diagnostics of soft tissues that are relatively transparent or translucent 
to ultrasonic radiation. One of the principal problems that has been 
encountered and not effectively resolved is the difficulty of obtaining 
visible results having high resolution content. 
Solutions to this problem have been elusive, in part because of the 
difficulty in identifying the many causes that contribute to the problem. 
One culprit that is believed to materially contribute to the problem has 
been the difficulty of generating undistorted ultrasonic plane waves from 
a large surface piezoelectric transducer. It has been suggested that "edge 
effect" radiation from the side and edges of the piezoelectric wafer 
materially interferes with and adversely affects the ability of the 
transducer to generate undistorted plane waves for insonifying the subject 
object. To illustrate this point, reference is made to a typical prior art 
ultrasonic holography system that is schematically shown in FIGS. 1 and 2. 
Such a typical "real time" ultrasonic holographic system is generally 
identified in FIG. 1 with numeral 10. The system 10 is intended to inspect 
the interior of an object 12. The system 10 generally has a hologram 
generating sub-system 13 and a hologram viewing sub-system (optical 
sub-system) 32. One of the principal components and the main subject of 
the focus of this invention is the provision of ultrasonic transducers, 
generally referred to as the object transducer 14 for generating 
ultrasonic plane waves 16 for insonifying the object 12 and reference 
transducer 22 for generating an off-axis beam. 
The ultrasonic energy transmitted through the object 12 is directed to a 
hologram detection surface 18, which is generally an area of a liquid-gas 
interface or liquid surface, such as a water surface. Generally the 
hologram detection surface 18 is physically isolated in a detection 
container 20 to minimize distortions caused by vibration. The ultrasonic 
reference transducer 22 generates an off-axis ultrasonic beam that is also 
directed to the hologram detection surface 18 to form a standing hologram. 
It is frequently desirable to pulse the transducers 14 and 22 at desired 
intervals to minimize dynamic distortions of the detector surface 18. 
Generally an ultrasonic lens assembly 26 is utilized to provide a focused 
hologram of a desired plane 27 within the object 12. In the example shown, 
the assembly 26 has a stationary lens 28 having a focal length coincident 
with the plane of the hologram detection surface 18. A movable 
complementary lens 30 is provided to be moved to focus on the desired 
object plane 27 of the object 12. 
The optical subsystem 32 includes a source of coherent light, preferably a 
laser 34 for generating a beam of coherent light. The laser light beam is 
directed through a laser lens 36 to achieve a point source that is located 
at or near the focal point of a collimating lens 38 and then onto the 
hologram detector surface to illuminate the hologram. The reflected 
coherent light radiation containing holographic information is directed 
back through the optical lens 38 and separated into precisely defined 
diffracted orders in the focal plane of the collimating lens 38. A filter 
42 is used to block all but a first order pattern 44 for "real time" 
observation by a human eye 46 or an optical recorder, such as a video 
recorder. 
As illustrated in FIG. 1, the prior art ultrasonic transducers, in addition 
to generating plane waves 16, generate edge effect waves 48 that adversely 
interfere with the fidelity of the plane waves 16 which causes a reduction 
in the resolution and clarity of the produced hologram. FIG. 2 illustrates 
the distortions in the plane waves. FIG. 2 illustrates the energy profile 
52 of the plane wave emanating from the front face of the transducer 14. 
The energy profile or curve 52 has dramatic end or edge curve sections 54 
showing the sharp decrease in the power levels at the edges of the 
transducer. The curve 52 also shows an irregular and distorted central 
plateau 60 of the wave form indicating the adverse interference of the 
edge effect waves distorting the plane waves emanating from the front face 
of the transducer. 
A principal objective of this invention to provide an ultrasonic transducer 
that materially reduces the generation of disruptive edge effect sound 
waves. The present invention more nearly operates closer to the more ideal 
condition illustrated in FIG. 3, having a power wave form distribution 
across the face of the transducer with a uniform, undistorted central 
section 60 with gradually decreasing transition segments 64 and 66 toward 
the transducer edges. 
These and other objects and advantages of the present invention will become 
apparent upon reading the following description of the preferred and 
alternate embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This disclosure of the invention is submitted in furtherance of the 
constitutional purposes in U.S. Patent Laws "to promote the progress of 
science and useful arts" (Article 1, Section 8). 
A preferred embodiment of this improved ultrasonic transducer invention is 
illustrated in FIGS. 4 and 7-10. FIGS. 5 and 6 illustrate alternative 
embodiments. The improved ultrasonic transducer is generally identified 
with the numeral 70. 
The ultrasonic transducer 70 has a thin piezoelectric polycrystalline body 
or wafer 72 with large area parallel front and back face surfaces 74 and 
76 respectively (FIG. 7). The front face surface 74 extends outward to a 
back perimeter edge 77. The back face surface 76 extends outward to a back 
perimeter edge 78. The back face surface 74 has a large central area 79(a) 
and a surrounding perimeter area 79(b) that extends from the central area 
79(a) to the back perimeter edge 78. The wafer 72 also includes a narrow 
perimeter side surface 82 that extends about the perimeter of the wafer 72 
between the front and back edges 77 and 78. 
The piezoelectric wafer 72 is preferably composed of a polycrystalline 
ceramic oxide material exhibiting a high degree of piezoelectric activity. 
Preferably the polycrystalline ceramic oxide material comprises lead 
zirconate titanate, generally referred to as PZT piezoelectric material. 
Specific formulations referred to as PZT-7A and PZT-5A have been 
successfully employed. The dielectric constant of such PZT material is 
approximately 425. 
The ultrasonic transducer 70 is designed to generate ultrasonic radiation 
at a frequency of between 2 megHz. and 5 megHz. Preferably the wafer 72 
has a thickness "A" between the front and back face surfaces of between 
0.017 and 0.041 inches. Optimally the thickness "A" is between 0.020 and 
0.030 inches. Good results have been obtained using a wafer 72 having a 
thickness "A" of approximately 0.024-0.025 inches. 
It is quite desirable to provide an ultrasonic transducer 70 having the 
capability of generating large area plane waves to ultrasonically inspect 
rather large objects 12 or large internal areas of an object 12. 
Preferably the transducer 70 is a lager area ceramic piezoelectric 
transducer in which the wafer 72 has large face surfaces 74, 76 with a 
minimum face surface dimension "C" greater than 1.5 inches. Preferably the 
minimum face surface dimension "C" is greater than 3 inches and optimally 
between 3 and 6 inches. The minimum face surface dimension "C" should be 
more than 30 times greater than the thickness "A" and preferably between 
30 and 300 times greater than the thickness "A". 
FIG. 4 illustrates a rectangular shaped large surface transducer 70 having 
minimum and maximum surface dimensions of between 3 and 8 inches. 
Alternatively, the transducer 70 may be constructed having a square shape 
as illustrated in FIG. 5 or a circular shape as illustrated in FIG. 6. 
The ultrasonic transducer 70 has a front electrode coating 84 and a back 
electrode coating 86 applied to the respective front and back surfaces 74, 
76 of the wafer 72 to enable the oscillation voltage to be applied to 
generate the desired large area ultrasonic plane waves. Preferably the 
electrode coatings 84, 86 completely overlay the respective front and back 
surfaces 74, 76 and have a uniform thickness of approximately 
0.0003-0.0005 inches. 
The front electrode coating 84 preferable extends from the front face 
surface 74 over the front edge 77 and along the peripheral side surface 82 
and then over the back edge 78 and onto the back surface forming a 
perimeter front electrode border 88 along the back surface edge 78. Such a 
continuous coating electrically combines the side surface 82 and the edges 
77, 78 to the front surface 74 and minimizes the application of an 
excitation voltage at the side surface 82 to thereby minimize the 
generation of interfering ultrasonic waves from the edges 78, 80 and side 
surface 82. As illustrated in FIG. 4, the border 88 extends along the back 
edge 78 forming smooth radius at the corners. Preferably the border 88 has 
an inside radius of curvature R.sub.1 at the corners that is greater than 
10 times the thickness "A" of the wafer 72. 
The ultrasonic transducer 70 has front electrode connector tabs 90 affixed 
to the front electrode coating 84 for applying a voltage to the front 
surface 74. Preferably the tabs 90 are affixed to the front electrode 
coating 84 along the border 88 as illustrated in FIG. 4. Thus, the tabs 90 
do not interfere with the generation of the plane waves from the front 
surface 74. In a preferred embodiment, the tabs 90 are rather evenly 
spaced to enable an even application of voltage to the entire front face 
electrode coating 84. 
The ultrasonic transducer 70 importantly has a voltage modifying or 
reduction layer 92 interposed between the back face surface 76 and the 
back electrode coating 86 along the back edge 78 to reduce the effective 
voltage applied to the face surface 76 adjacent the side surface 82. Such 
a radiation is illustrated ideally in FIG. 3, to further minimize the 
generation of interfering edge effect ultrasonic waves from the side 
surface 82. Preferably the voltage reduction layer 92 surrounds the large 
central portion 79(a) of the back surface 76 and overlies the perimeter 
portion 79(b). 
The voltage reduction layer 92 (FIGS. 8 and 9) has a width "D" extending 
from the back edge 78 over the perimeter portion 79(b) to the large 
central portion 79(a). Preferably, the width "D" is between 5 and 20 times 
the thickness "A" of the wafer 72. Optimally, the width "D" is between 10 
and 20 times the thickness "A" of the wafer 72. 
The maximum thickness "B" of the layer 92 is substantially less than the 
thickness of the wafer 72 and is preferably between 0.005 and 0.010 
inches. The thickness "B" of the layer 92 varies from a maximum adjacent 
the back edge 78 to a minimum at central portion 79(a) of the back surface 
76. Preferably the thickness "b" varies in a tapered pattern from the back 
edge 78 to the central portion 79(a) and more preferably varies similarly 
to a "bell shaped" Guassian curve illustrated in FIGS. 3, 8 and 9. The 
layer 92 preferably has (1) a gradual thickness decreasing first section 
93, (2) a more rapid thickness decreasing second section 94, and (3) a 
flared thickness decreasing third section 95, extending from the edge 78 
and terminating at the central portion 79(a). It should be noted that the 
layer 92 extends over the electrode border 88 to provide a insulating 
material between the electrode coatings 84 and 86 adjacent the back edge 
78. 
The voltage reduction layer 92 is composed of a material that is 
substantially less conductive than the electrode coating material and 
provides a substantial electrical impedance between the back electrode and 
the back surface adjacent the back edge 78 to reduce the exciting voltage 
at the side surface 82 to less than 50% of that applied at the large 
central area 79(a) and preferably less than 25%. It is important that the 
voltage reduction be rather gradual as illustrated in FIG. 3. 
Preferably the layer 92 comprises a non-piezoelectric dielectric material, 
such as an synthetic epoxy resin. In alternate embodiments, metallic 
particles may be added to the epoxy resin to decrease its resistivity and 
increase the voltage drop across the thickness of the layer 92. The 
composition of the layer 92 may vary considerably to obtain the desired 
results. The voltage reduction layer 92 preferably has an electrical 
dielectric constant of between 3 and 100 and an electrical volume 
resistivity value of between 0.1 ohm-cm. and 2.5.times.10.sup.15 ohm-cm. 
More preferred, the voltage reduction layer 92 comprises a synthetic epoxy 
resin having a dielectric constant between 10 and 20 and an electrical 
volume resistivity of between 1.times.10.sup.15 and 5.times.10.sup.15 
ohm-cm. One useful non-piezoelectric dielectric material is a synthetic 
epoxy resin having a trademark "Stycast HiK" manufactured by Emmerson and 
Cummings Corporation. It appears to have an electrical dielectric constant 
of approximately 15 and a volume resistivity of 2.times.10.sup.15 ohm-cm. 
Titanium oxide particles have been added to the epoxy resin to modify its 
electrical characteristics as desired. 
The back electrode coating 86 has electrode connecting tabs 98 affixed to 
the coating 86 to enable an oscillating voltage to be applied to the back 
surface 76. Preferably, the tabs 98 are evenly spaced similarly to the 
spacing of the tabs 90. 
In compliance with the statute, the invention has been described in 
language more or less specific as to methodical features. It is to be 
understood, however, that the invention is not limited to the specific 
features described, since the means herein disclosed comprise preferred 
forms of putting the invention into effect. The invention is, therefore, 
claimed in any of its forms or modifications within the proper scope of 
the appended claims appropriately interpreted in accordance with the 
doctrine of equivalents.