Composite ultrasonic transducers and methods for making same

A ultrasonic transducer using such a piezoelectric composite that a number of piezoelectric poles made of piezoelectric ceramics are arranged in a plate-like polymer matrix perpendicular to the plate surface. The volume ratio of the piezoelectric poles is set in a range of 0.15-0.75 and a spacing between every adjacent piezoelectric poles is set smaller than the thickness of the polymer plate, thereby resulting in a transducer which has higher sensitivity than the conventional one using a homogeneous piezoelectric ceramic plate.

FIELD OF THE INVENTION 
The present invention relates to a ultrasonic transducer for use in 
ultrasonic diagnostic apparatus, etc. 
BACKGROUND OF THE INVENTION 
Heretofore, in many cases there have been used zircon lead titanate (PZT) 
ceramics as materials for a piezoelectric vibrator of ultrasonic 
transducers. Those piezoelectric ceramics are, however, disadvantageous 
in: (i) an acoustic matching layer requires an ingenious design when used 
for diagnostic purposes, because acoustic impedance is significantly 
larger than that of the human body, (ii) a dielectric constant is 
significantly large and hence a piezoelectric voltage constant g is so 
small that high voltage can not be produced upon receiving ultrasonic 
waves, and (iii) it is difficult for those ceramics to have a curvature 
fit for the shape of the human body. To solve such problems, there have 
been proposed so-called piezoelectric composites in which polymers are 
compounded with piezoelectric substances. As one example, Newnham, et. al. 
in the United States reported that such a composite structure is effective 
where a number of PZT poles 12 are buried in a polymer 11 as shown in FIG. 
1 (see "Material Research Briden", Vol. 13, pp. 525-536 (1978)). In fact, 
the composite structure of PZT and polymers, such as silicon rubber or 
epoxy, results in a material having low acoustic impedance and a large 
piezoelectric voltage constant g. 
In those piezoelectric composites, their piezoelectric characteristics are 
greatly varied depending on the volume ratio of the piezoelectric 
substance to the polymer. This is described in detail in the above 
reference. But it is expected that the piezoelectric characteristics also 
varied depending on the size and arrangement of piezoelectric poles even 
with the same volume ratio of the piezoelectric substance. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a composite ultrasonic 
transducer which is superior in the transmitting and receiving overall 
sensitivity to a conventional transducer using a PZT ceramic plate. 
Another object of the present invention is to provide methods for 
manufacturing high-reliable piezoelectric composites capable of mass 
production. 
The present invention is featured in a ultrasonic transducer made of a 
piezoelectric composite of such structure that a number of ceramic 
piezoelectric poles are buried in a plate-like polymer matrix 
perpendicular to the plate surface, wherein the volume ratio of the 
piezoelectric poles is in a range of 0.15-0.75, and the height of each 
piezoelectric pole is larger than a spacing between every adjacent 
piezoelectric pole. 
Other features of the present invention will be apparent from the following 
detailed description.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustrates a structure of one embodiment of the present invention. 
A piezoelectric composite 100 fabricated by a later-described 
manufacturing process is so structured that a number of ceramic 
piezoelectric poles are arranged in a polymer matrix 102 with constant 
spacings d. Electrodes 103 and 104 are formed on both the upper and lower 
surfaces of the piezoelectric composite 100 to thereby constitute a 
composite transducer. 
PZT (Pb(TiZr)O.sub.3) ceramics or lead titanate (PbTiO.sub.3) ceramics, 
which are polarized in the lengthwise direction, are preferable as the 
piezoelectric poles 101. Silicone rubber, polyurethane or epoxy resin is 
preferable as the polymer 102. The electrodes are preferably formed of 
chrome - gold films, but they may be of course formed of other suitable 
electroconductive films. 
FIG. 2 shows the measured result of changes in sensitivity relative to 
varying spacings d between the piezoelectric poles 101 for the composite 
transducer of FIG. 1 which was manufactured using PZT ceramics and silicon 
rubber. This measurement was conducted with four types of transducers made 
of piezoelectric composites which are 10 mm square, 0.3 mm in thickness h, 
and 0.15, 0.2, 0.3 and 0.4 mm in spacing d between the piezoelectric 
poles, respectively. The volume ratio of the piezoelectric poles 101 to 
the entire piezoelectric composite was set to 25% for any of the 
transducers. Each transducer had about 4.5 MHz resonance frequency in 
depthwise longitudinal vibrations. 
For the purpose of comparison, FIG. 2 also shows the data (in broken line) 
relating to a conventional ultrasonic transducer which was manufactured 
using homogeneous PZT ceramics with the same aperture and the same 
resonance frequency. As will be apparent from FIG. 2, the transmitting and 
receiving sensitivity of the present transducer is higher than that of the 
conventional transducer when the interpole spacing d is smaller than the 
thickness h of the ceramics, but is is rapidly reduced when d exceeds h. 
This is attributable to the fact that, in case of d&lt;h, the filled polymer 
transmits the received pressure to the piezoelectric poles effectively so 
that the polymer and the piezoelectric poles are both vibrated together in 
the depthwise direction, whereas, in case of d&gt;h, the pressure is not 
transmitted effectively so that the polymer and the piezoelectric poles 
will not vibrate together. 
FIG. 3 shows the relationship between the volume ratio of PZT ceramics and 
the transmitting and receiving sensitivity. For the purpose of comparison, 
FIG. 3 also shows the data (in broken line) relating to a conventional 
ultrasonic transducer which was manufactured using a homogeneous PZT 
ceramic plate with the same aperture. As will be apparent from FIG. 3, the 
transmitting and receiving overall sensitivity of the present transducer 
is higher than that of the conventional ultrasonic transducer using a 
homogeneous PZT ceramic plate in a range of the volume ratio between 0.15 
and 0.75. Thus, even when the condition of d&lt;h is satisfied, there can not 
be obtained high sensitivity necessarily, if the volume ratio of PZT 
ceramics is smaller than 0.15 or larger than 0.75. 
As described above, as to the composite ultrasonic transducer shown in FIG. 
1, a high-sensitive transducer is obtained on conditions that the volume 
ratio of piezoelectric poles to the entire piezoelectric composite is in a 
range of 0.15-0.75 and the arrangement spacing d between the piezoelectric 
poles is smaller than the height h thereof. 
Furthermore, as shown in FIG. 1 with broken lines, it is also possible that 
a backing member 105 made of, e.g., epoxy resin, is attached to either one 
surface of the piezoelectric composite having the electrodes formed on 
both surfaces, and the other surface is used for transmitting and 
receiving ultrasonic waves. 
The manufacturing process of the piezoelectric composite 100 in the 
above-mentioned embodiment will now be described with reference to FIGS. 
4A-4C. 
In the step of FIG. 4A, a piezoelectric ceramic plate 201 in the flat form 
is tentatively bonded to a cutting base 203 by making use of an adhesive 
202 such as wax, for example, which is softened under heating. As shown in 
FIG. 4B, a number of grooves are formed to cut the piezoelectric ceramic 
plate longitudinally and transversely, thereby fabricating a number of 
elements 205. Next, polymer 206 is filled and solidified in each cut 
groove and, thereafter, it is torn off the cutting base so as to obtain 
the piezoelectric composite 100 of FIG. 1. 
The above manufacturing process is advantageous in the reduced number of 
steps, but has the following disadvantages: 
(1) The elements 205 tend to be chipped off, because the piezoelectric 
ceramic plate is deeply cut; and 
(2) The grooves may also often be cut in the base 203 in the step of 
cutting, so that the polymer 206 is secured to the base 203. In this case, 
it becomes difficult to tear off the piezoelectric composite from the base 
203 and some of the elements 205 tend to be broken during tearing-off. It 
becomes also difficult to remove the adhesive 202 after the step of 
tearing-off. 
The alternative manufacturing process improved to eliminate such 
disadvantages is illustrated in FIGS. 5A-5H. First, as shown in FIG. 5A, a 
piezoelectric ceramic plate is tentatively bonded to a cutting base 303 
using wax 302. Next, as shown in FIG. 5B, grooves 304 of depth nearly 
equal to a half of the thickness h of the plate 301 are formed therein to 
cut the plate 301 longitudinally and transversely without penetrating 
therethrough. In the step of this cutting, reference lines 305 and 306 are 
prepared on the plate 301. FIG. 5C is a top plan view of FIG. 5B as looked 
from above. Then, as shown in FIG. 5D, a polymer 307 such as polyurethane 
or epoxy is filled and solidified in the grooves 304. Next, the wax 302 is 
melted causing a vibrator to be turned over and again bonded to the 
cutting base 303 using wax or the like 308, as shown in FIG. 5E. 
Subsequently, as shown in FIG. 5F, grooves 309 are cut to reach the 
polymer 307 with the lines 305 and 306 as references. In the step of FIG. 
5G, a polymer is filled and solidified in the grooves 309 to form the 
polymer portion 310 on the reverse side of the transducer. After melting 
the wax 308, it is torn off from the base 303 to thereby obtain the 
piezoelectric composite 100 of FIG. 1. On this occasion, the polymer 307 
is required to have such quality as not degrading machinability at the 
time when the grooves 309 are cut. If the filler introduced in the grooves 
304 is of a soft material such as silicone rubber, there occurs a problem 
in machinability. In such a case, wax or the like is filled in the grooves 
304 in the step of FIG. 5B, and the resultant piezoelectric plate is 
turned over and again bonded to the base 303 (the state of FIG. 5E). After 
cutting the grooves 309 as shown in FIG. 5F, silicon rubber is filled and 
solidified therein. In the state of FIG. 5G in this case, 307 designates 
wax and 310 designates silicone rubber. Next, the vibrator is removed from 
the base 303 (the individual elements are bonded to one another with 
silicon rubber at this time) and the wax in the grooves 304 is washed out, 
thus resulting in the state of FIG. 5H. Finally, silicone rubber or the 
like is filled and solidified in cut grooves 311 now deprived of wax to 
thereby provide the piezoelectric composite 100 of FIG. 1. 
It is to be noted that the polymers 307 and 310 are not always required to 
be made of the same material. For example, it is possible that 307 is 
formed of polyurethane and 310 is formed of silicone rubber. 
As an alternative, the piezoelectric composite 100 can be also manufactured 
in such a manner that the piezoelectric ceramic plate in the state of FIG. 
5D is removed from the cutting base as shown in FIG. 6A, and then the 
resultant piezoelectric ceramic plate is ground from the bottom up to a 
plane 312 as shown in FIG. 6B. In place of grinding, it may be cut at the 
plane 312. 
According to the manufacturing process shown above in FIGS. 5A-5H or FIGS. 
6A and 6B, the grooves in which the polymer is to be filled will not reach 
the cutting base, thus resulting in the advantage that it is easy to tear 
off the piezoelectric ceramic plate filled with the polymer from the 
cutting base. 
In any of the manufacturing processes shown in FIGS. 4A-4C, FIGS. 5A-5H and 
FIGS. 6A-6B, another polymer which can be easily removed by washing, is 
preferably coated in advance on the upper and lower surfaces of the 
piezoelectric ceramic plate 201 or 301, for the purpose of preventing the 
polymer from securing to the upper and lower surfaces of the piezoelectric 
poles. 
FIGS. 7A-7G illustrate the alternative manufacturing process for obtaining 
the piezoelectric composite 100 of FIGS. 1. A piezoelectric ceramic plate 
501 is tentatively bonded to a cutting base 503 using wax or the like 502 
(FIG. 7A), and the plate, i.e., a vibrator, is cut at 504 thoroughly to 
form a plurality of vibrator pieces 505 each having an appropriate width 
(FIG. 2B). Next, the vibrator pieces 505 are removed and then again 
tentatively bonded to a cutting base 506 with intervals using wax or the 
like 507, as shown in FIG. 7C. Grooves 508 each having a width d are cut 
in each vibrator piece 505. Next, a polymer 509 is filled in the 
respective grooves as shown in FIG. 7D. Subsequently, each vibrator piece 
505 is removed from the base, thus resulting in a piece 510 as shown in 
FIG. 7E. At this time, individual elements 511 are bonded to each other 
with the polymer 509. Next, the pieces 510 are arranged on a base 512 with 
a spacing d therebetween, as shown in FIG. 7F. A polymer 514 is filled in 
each space 513 as shown in FIG. 7G and the base 512 is then removed 
therefrom, thereby providing the piezoelectric composite. On this 
occasion, the polymers 509 and 510 may be formed of different materials. 
In the above manufacturing process, shallow grooves for arrangement are 
preferably formed in the upper surface of the base 512 in advance, in 
order to effectively arrange the plurality of the pieces 510 in the step 
of FIG. 7F. 
The manufacturing process shown in FIGS. 7A-7G is advantageous in that, 
since there is no need of cutting grooves in which a polymer is to be 
filled, the possibility is reduced that the piezoelectric ceramics may be 
broken in the step of cutting grooves. 
In the piezoelectric composite thus obtained, if a flexible polymer is used 
as the polymer, the composite itself becomes flexible. Therefore, it 
becomes possible to easily form a transducer with an arbitrary shape such 
as a concave surface. FIGS. 8A-8B illustrate the process for manufacturing 
a transducer with a circular concave surface by way of example. 
First, as shown in FIG. 8A, a circular piezoelectric composite 401 is 
prepared. This circular composite is obtained by cutting the piezoelectric 
composite resulted from the process shown in FIGS. 4A-4C, FIGS. 5A-5H or 
FIGS. 6A and 6B into the circular form. Alternatively, if a circular 
piezoelectric ceramic plate is employed as 301 in FIG. 4A, the circular 
piezoelectric composite can be naturally obtained. It is to be noted that 
402 designates a polymer matrix and 403 designates a piezoelectric pole. 
Next, as shown in a sectional view of FIG. 8B, the piezoelectric composite 
401 is bonded to the surface of a sphere 404 using resin (wax or the like) 
which is softened under heating, the sphere 404 having the same curvature 
as the desired concave surface. 
Next, an electrode 406 is formed on the upper surface of the piezoelectric 
composite 401 by screen printing, evaporation or so. At this time, to 
prevent the electrode from being formed also on the side faces of 401, it 
is more preferable to coat the side faces thereof with wax. Subsequently, 
a signal line 407 is connected to the sphere 404 with an electroconductive 
paste and, as shown in a sectional view of FIG. 8C, a backing member 408 
is formed on the electrode 406. As an alternative, the backing member 408 
shaped into the desired form may be fixed to the electrode 406 using an 
adhesive. Furthermore, if an electroconductive paste with adhesiveness is 
used as the electrode 406, the electrode itself can be employed also as an 
adhesive. Next, the sphere 404 is removed away under heating and a front 
surface of the piezoelectric composite 401, thus resulting in the state of 
FIG. 8D. Thereafter, as shown in a sectional view of FIG. 8E, another 
electrode 410 is formed on the front surface by screen printing, 
evaporation or so. In this example, 410 serves as an earth side electrode. 
Next, an earth wire 411 is connected to the electrode 410. However, if it 
remains as it is, there is a large possibility that the electrode 410 
tends to tear off. For this reason, a film 412 is formed on the front 
surface which film has the effect of protecting the electrode 410. As a 
result, a concave transducer as shown in FIG. 8F is fabricated. 
In the process of manufacturing the circular transducer as mentioned above, 
it is preferable to pay due consideration so that the transducer becomes 
bisymmetrical. More specifically, the piezoelectric composite is 
preferably cut into the form of a circle with its center located at such a 
point as the center of the piezoelectric pole represented by A in FIG. 9 
or the point equally spaced from the four surrounding piezoelectric poles 
represented by B therein. 
Meanwhile, in case the process shown in FIGS. 5A-5H is employed to obtain 
the piezoelectric composite using a piezoelectric ceramic plate in the 
form of a disc from the beginning, it is preferable to adopt the 
manufacturing process as shown in FIGS. 10A and 10B. More specifically, as 
shown in FIG. 10A, an auxiliary member 703 such as epoxy resin is formed 
in the circumference of a disc-like piezoelectric ceramic plate 701. 
Subsequently, the auxiliary member 703 is cut at lines 704 and 705 as 
shown in FIG. 10B, the lines 704, 705 corresponding to the reference lines 
305, 306 shown in FIG. 5C. Thus, the desired piezoelectric composite can 
be obtained through the steps of cutting and filling in a similar manner 
to those shown in FIGS. 5A-5H.