Backing Material for the Ultrasonic Transducer

1. Technical Field

The invention relates to a backing material for use in an ultrasonic transducer.

2. Background of the Invention

There are various technical requirements of a backing material for an ultrasonic transducer. According to one requirement, the mating surface between the backing material which serves as an attenuation element and a piezoelectric crystal or piezoelectric film of the ultrasonic transducer must have the same or approximately the same magnitude of acoustic impedance as that of the piezoelectric crystal or piezoelectric film. If this requirement is satisfied, reflection of acoustic energy from the mating surface will be prevented. Another requirement concerns the dissipation of acoustic energy which otherwise would enter the backing material from the piezoelectric crystal or piezoelectric film. The acoustic energy may be dissipated away in the impedance element. In this manner it is possible to avoid any reflection of acoustic energy by the back surface of the backing material. And, under circumstances that the ultrasonic transducer is fabricated with a casting material as the backing material, there is the further requirement that the backing material demonstrate the property of high resistance to voltage. Thus, the backing material will not act as a conductor of voltage between the electrodes connected to the piezoelectric crystal or piezoelectric film. This is a particularly important consideration in a transmitting mode with a phase control array transducer.

The prior art recognizes that the backing material may comprise an insulating cement composite of tungsten. It is also known that the backing material, particularly the backing material of the phase control array transducer for transmitting purposes, may be provided with an insulating film of aluminum oxide.

A patent representative of the prior art is U.S. Pat. No. 4,382,201 to Trazaskos. Trazaskos describes an ultrasonic transducer having a backing material of a tungsten-polyvinyl chloride composite capable of operation at a frequency of 4.5 MHz or higher. Trazaskos also describes the process of enhancing the acoustic attenuation of the backing material by steps, including others, of pressurizing powders of tungsten having a particle size less than 10 microns and polyvinyl chloride, degassing the powders, heating the powders under pressure, and further pressurizing the powders during cooling of the composite until it is in a state of elastic compression capable of spontaneously expanding when the pressure is released.

While certain requirements in properties can be achieved by fabricating a tungsten-polyvinyl chloride composite according to Trazaskos, or using the tungsten-insulating cement composite as a backing material, certain problems and deficiencies have also been discerned. For example, the cast backing material, in use in a general transducer capable of both transmitting and receiving ultrasound, frequently does not possess the property of resistance to both high damping and voltage. It has also been found that the cast backing material frequently permits reflection of acoustic energy which gives rise to spurious signals. In addition, it has been found that the noise level increases at frequencies of operation of the transducer of 4.5 MHz and above. Further still, it has been found that the procedure of coating an insulating film of aluminum oxide which should be controlled strictly within a few microns precision is quite complicated. The insulating film, as previously averted to, may and most likely should be used to coat the insulating cement composite of tungsten used with a phase control array transducer for transmitting purposes in order to overcome a defect in the backing material of low resistance to voltage.

The present invention is in a new backing material for an ultrasonic transducer which successfully overcomes the problems and disadvantages of the prior art. The invention, also, concerns the process of fabrication of the backing material having the property of resistance to voltage, as well as the property of high acoustic attenuation. Accordingly, under the conditions of construction and the procedure of the ultrasonic transducer the backing material of the invention can be fabricated in conformance with testing requirements to provide necessary acoustic impedance in improving the performance of the ultrasonic transducer.

The backing material, as will be discussed, is a tungsten-base composite including a tungsten powder, a powder of an oxide of metal from the lanthanum group and an insulating cement present in the tungsten-metallic oxide in an amount of 4:1 to 50:1.

In a more preferred form of the invention the metallic oxide is cerium oxide present in the composite in an amount of from 1.0 to 4.5% by weight, and the insulating cement is an epoxy having a weight proportion to the tungsten-cerium oxide powder of 5:1.

The FIGURE is an elevational view, partly in section, of an ultrasonic transducer including the backing material of the invention.

The ultrasonic transducer as seen in the FIGURE includes a housing 2 having a side wall 2a extending to an open end and a rear wall 2b. A pair of electrodes 1 is supported by the rear wall. A piezoelectric crystal 6 is supported at the open end of the housing. A film 5 of a conductive material and a like film 7 are applied to opposite inner and outer surfaces, respectively, of the piezoelectric crystal. A conductor is connected between an electrode and a respective film layer. One conductor 3 is illustrated in the FIGURE. A film 8 is coated on the outer surface of film 7 to provide the conductive film with a protective barrier. Finally, a backing material 4 is located within the housing juxtaposed the inner surface of film 5. Any conventional manner of support on the housing for both the electrodes and the piezoelectric crystal may be employed.

The backing material 4 is in the form of a composite of tungsten powder, metallic oxide, and an insulating cement. The metallic oxide may be an oxide of an element from the lanthanum group, and preferably, cerium oxide present in the composite in an amount of 1.0 to 4.5% by weight. More particularly, the cerium oxide will comprise an amount of 1.8 to 2.2% by weight of the composite. The maximum grain size of the tungsten-cerium powder is 7 microns. The insulating cement preferably is an epoxy, and the weight percentage of epoxy to the tungsten-cerium powder will be dependent upon operating requirements. The range of epoxy present in the tungsten-cerium powder may be from 4:1 to 50:1. The composite must be made so that the acoustic impedance will be matched to the acoustic impedance of the piezoelectric crystal or piezoelectric film.

Cerium oxide is a nonconductive material, and while tungsten is conductive the resistance of tungsten powder is very low. Thus the tungsten-cerium powder will have very high resistance. A comparative test between tungsten-cerium powder on the one hand and tungsten powder on the other hand, carried out under identical test conditions, yielded the result that the tungsten-cerium powder had a resistance a third power higher than that of tungsten powder. Therefore, a backing material formed by a composite of tungsten-cerium-epoxy, each present in a defined percentage by weight, when compared with a backing material formed by a composite of tungsten-epoxy, having a substantially identical weight property of tungsten-epoxy in the composite, will have a resistance to voltage increased many times. The test results are shown in the following Table. These test results were obtained using a tungsten-cerium-epoxy composite backing material, fabricated by a casting process. The cerium oxide content in the tungsten powder is 2% by weight, and the epoxy content in the tungsten-cerium powder is 8:1. The tungsten-epoxy backing material composite is made of substantially identical weight percentages of tungsten-epoxy, and similarly fabricated.

The improved properties of the tungsten-cerium-epoxy composite, in use as a backing material are evident and satisfy one of the requirements for a backing material for an ultrasonic transducer of high voltage usage as previously mentioned. On the other hand, the adhesive retarding of the tungsten-cerium-epoxy composite medium is quite different from that of the tungsten-epoxy composite medium. To this end, the tungsten-cerium-epoxy composite possesses comparatively greater acoustic attenuation, and it is also suitable for use in ultrasonic transducers of high impedance.

The process for fabricating the backing material may be either a casting or pressing process. A casting process may be preferred in the fabrication of the backing material from a small proportion of tungsten-cerium powder. Large proportions of tungsten-cerium powder are better suited for fabrication to a backing material by the pressing process. In either process, the acoustic impedance of the composite forming the backing material is matched to that of the piezoelectric crystal or piezoelectric film.

The tungsten-cerium-epoxy composite fabricated to a backing material and used in an ultrasonic detecting instrument, such as an ultrasonic thickness measuring instrument or phase control array transducer, has been found to increase the performance of operation, as well as to satisfy needs of the ultrasonic transducer. High frequency ultrasonic instruments having an ultrasonic transducer with tungsten-cerium-epoxy composite, may be used with operating frequencies above 5 MHz, with a detectable range equal to or greater than 0.2 mm. The backing material or tungsten-cerium-epoxy composite, compared with the backing material of tungsten-epoxy when tested compared as follows:

Sensitivity, residual amount increased about 10 dB (28% approximately)

Resolution, power enhancement about 5 dB (24% approximately)

Path length, width decrease about 5 mm (37% approximately)

Tests have also been carried out during use of an ultrasonic transducer in an underwater ultrasonic receiving figure system. The backing material of tungsten-cerium-epoxy composite, fabricated by a pressing process, included a cerium oxide content in the tungsten powder of 2% weight proportion and a weight proportion of the tungsten-cerium powder to epoxy of 5:1. This tungsten-cerium-epoxy composite was tested and compared with a tungsten-epoxy composite formed by the same fabricating process, having the same mixing ratios and located adjacent the piezoelectric crystal or piezoelectric film with the following results:

It has also been found that the noise level of the tungsten-cerium-epoxy composite is lower, by about 5 times, than that of the tungsten-epoxy composite, and it has a comparatively ideal electric exciting function. Further, the tungsten-cerium-epoxy composite backing material is capable of use with an ultrasonic transducer and ultrasonic detection at both high and low frequencies.