A BACKING MATERIAL FOR AN ULTRASONIC TRANSDUCER

Abstract: A new backing material for an ultrasonic transducer is fabricated by the composite of the tungsten powder, contain­ ing 1,0 - 4.5 % cerium oxide, and insulating cement, which can either be casted or pressed into the required form. The present backing material is composed of merely tungsten and insulating cement. The problems existing in the trans­ ducer utilizing the said composite to be the backing material are: firstly, it cannot both possess the capability of resistance to high damping and voltage at the meantime, because they are contradictory to each other; secondly, the reflection from the interior of the backing gives rise to spurious signals; and finally, the noise level increases considerably at the high frequency ultrasonic stage (above 4.5 MHz). This invention relages to the use of tungsten-cerium-­ insulating cement composite as a substitute for the above said composite to be the backing material of the ultrasonic transducer. Under the conditions of the unchanged, original construction and manufacturing procedure of the transduc­ er, the substituted backing material would now possess the following outstanding and beneficial advantages: A general transducer for both transmitting and receiv­ ing, using the mentioned casting backing material, will improve its resistance to voltage and acoustic attenuation simultaneously, so that the matching of the required acoustic impedance can easily be realized. Moreover, the noise level caused by the reflection of the backing can be minimized if the backing material of the transducer is made of casting new material. Furthermore, the noise level can be reduced many times, when the transducer is put into use above 4.5 MHz high frequency ultrasonic stage. This invented backing material is also suitable for use both in low and high frequency ultrasonic transducer.

Description: The present invention relates to a backing material used for ultrasonic transducers. The technical requirements of backing materials for ultrasonic transducers are, firstly, the mating surface between the backing attenuation element and piezoelectric crystal or piezoelectric film must have the same magnitude (or approximately) of acoustic impedance of the piezoelectric crystal or piezoelectric film, so that a reflection of the acoustic energy from the surface will not happen; secondly, the acoustic energy, entering into the backing attenuation element from the piezoelectric crystal or film, must be dissipated away in the impedance element so as to avoid the cause of the reflection by the back surface of the backing attenuation element; moreover, when the transducer is fabricated with the casting material as the backing attenuation element, then it is required for said element to possess the property of high resistance to voltage so that the backing material may be protected from being conducted through it between the two electrodes, for this is particularly important for the phase control array transducer for transmitting purposes. Under the present existing technique a tungsten-insulating cement composite has been used to make the backing material. The casting backing material used in the transducer for transmitting (particularly the phase control array transducer for transmitting) is needed to add an insulating film of aluminium oxide. In the U.S Patent 4,382,201 it is proposed to use a tungsten-polyvinyle chloride composite as backing material for a high frequency ultrasonic transducer. The process of making the tungsten powder and polyvinyle chloride composite includes degasing, followed by heating and compressing. Then it is cooled under pressure so that the composite is in a state of elastic compression and spontaneously expands when it is released, giving rise to the high level of acoustic attenuation. Certain required properties can be achieved by using the above mentioned method to fabricate a tungsten-polyvinyle chloride composite or using the tungsten-insulating cement composite as backing material. Nevertheless the following problems would arise. A general transducer for transmitting and receiving, using the casting backing material often does not possess the properties of the resistance both to the high damping and voltage meantime, because they are conflicting with each other; reflection often will occur from the back of the backing material, thus resulting in the rising of spurious signals: When the frequency is higher than 4.5 MHz, the noise level will increase. Referring to the transducer for transmitting (especially the phase control array transducer for transmitting) a very thin insulating film of aluminium oxide must be coated in order to overcome the defect existing in the backing material of its low resistance to voltage. Since the thickness of this coating should be controlled strictly within a few micron precision, the coating procedure is quite complicated. The GB-A-2 097 630 (D1) describes a backing material for ultrasonic transducers on the basis of a tungsten-based composite, comprising: a tungsten powder, and a certain weight proportion of insulating cement, and which is fabricated as any such material either by casting or pressing. Instead of tungsten, also aluminium and the oxides of these metals can be used in D1. The object of this invention is to solve the above mentioned problems, which have been existing so far in the backing material. The invention proposes a process of fabricating a new backing material, the properties of which cannot only be a resistance to voltage, but also to the high acoustic attenuation so that under the conditions of unchanged original construction and fabricating procedure of the transducer the substituted tungsten-insulating cement backing material can be fabricated in conformance with the testing requirements and can have various kinds of necessary acoustic impedances to improve the performance of the ultrasonic transducer. Moreover, the fabricating technique may thus be facilitated. Object of the invention is a backing material for ultrasonic transducers on the basis of a tungsten-based composite, including insulating cement, and which is fabricated either by casting or pressing, characterized in that there is present cerium oxide in an amount of from 1.0 to 4.5% by weight of the backing material. The preferred range for cerium oxide is from 1.8 to 2.2%. The referred insulating cement is preferably an epoxy resin. Since the cerium oxide is a non-conductive material, the tungsten cerium powder presents a very high resistance. Though the tungsten is a conductive metal, the resistance of the tungsten powder is very low. A comparative test between the tungsten-cerium powder and tungsten powder had been carried out under the identical testing condition; the tested results indicated that the resistance of the tungsten-cerium powder was 3rd power higher than that of tungsten powder. Hence a backing material using a composite made up of a certain amount of weight proportion of tungsten cerium epoxy when compared with another backing material using a composite made up of an identical amount of weight proportion of tungsten epoxy, the compared results indicate that the resistance to voltage will be increased many times. So it is suitable for fabricating ultrasonic transducers of high voltage transmitting usage, because the conduction passing through the backing material between the two electrodes would not be possible to happen. On the other hand, the adhesive retarding of the tungsten-cerium-epoxy composite material is quite different from that of the tungsten-epoxy composite material. It possesses comparatively greater acoustic attenuation, so it is also suitable for fabricating transducers of high impedance. The above mentioned backing material on the basis of tungsten-cerium-epoxy composite is made of: the cerium oxide, containing in tungsten powder, is by weight proportion 1.0 - 4.5 %; the maximum grain size of the tungsten-cerium powder is 7 micron; the weight proportion of the tungsten-cerium powder to the epoxy resin depends on its using requirements; the range of the proportion is 4:1 - 50:1. The composite must be made so that the acoustic impedance will be matched with the acoustic impedance of the piezoelectric crystal of the piezoelectric film. For a small proportion of tungsten-cerium powder it is suitable to use casting for making the backing impedance element. For a large proportion of tungsten-cerium powder it is necessary to use the pressing method to fabricate the backing impedance element. In the ultrasonic technique, when the tungsten-cerium-epoxy resin composite is used as a backing material of the ultrasonic detecting instruments, the performance of the instruments can certainly be improved, and the various needs of the ultrasonic transducer can thus be satisfied. The mentioned feature is also applicable to the phase control array transducers. Fig. 1 shows a section drawing of the ultrasonic depth measuring instrument. Description of the Preferred Embodiments The tungsten-cerium-epoxy composite is used to make a backing damping element of an ultrasonic thickness measuring instrument. The details are shown in Fig. 1. Its indicated numbers are identified as follows: 1. electrode 2. case 3. conductor wire 4. backing material 5. and 7. conducting film 6. piezoelectric crystal film 8. protecting film. The mixing proportion and fabricating method of the backing material are respectively; the cerium-oxide content in the tungsten powder is 2 % weight proportion and the weight proportion of the tungsten-cerium powder to epoxy is 8:1; the said composite material is fabricated by casting. The probe made of the said composite as backing material and same type probe made of identical weight proportion of tungsten-epoxy composite both with the same fabricating method had been tested. There tested results were compared as follows: The above tested results had proved that the properties of the tungsten-cerium-epoxy resin backing material were better than those of the tungsten-epoxy resin backing material. High frequency ultrasonic instruments with the transducers made of tungsten-cerium-epoxy resin composites show over 5 MHz high frequency and their detectable range can be ≧ 0.2 mm. The backing damping element made of tungsten-cerium-epoxy resin composite and the backing damping element made of tungsten-epoxy resin, both as the ultrasonic instrument transducer, were tested and the results were compared as follows: When transducers are utilized for under-water ultrasonic receiving figure system, using the tungsten-cerium-epoxy resin composite and fabricating method for the backing element the cerium oxide content in the tungsten powder is 2 % weight proportion, and the weight proportion of the tungsten-cerium powder to epoxy resin is 5:1. The fabricating method should be done by pressing into the necessary element form and let it in contact with the piezoelectric film and at the back of the rigid blocking plate. It had been tested and compared with the backing damping element of a transducer, which was made by tungsten-epoxy composite under the same mixing ratio and fabricating method. The results were as follows: The proposed backing material for the ultrasonic transducer of this invention is suitable both for the low frequency and high frequency ultrasonic detection and figure forming system etc.

Claims: A backing material for ultrasonic transducers on the basis of a tungsten-based composite, including insulating cement, and which is fabricated either by casting or pressing, characterized in that there is present cerium oxide in an amount of from 1.0 to 4.5% by weight of the backing material. A backing material as claimed in Claim 1, wherein said cerium oxide, by weight proportion, is 1.8 to 2.2% of the said backing material. A backing material as claimed in one of Claims 1 or 2, wherein said insulating material is an epoxy resin. A backing material as claimed in Claim 3, wherein said tungsten-cerium powder and epoxy weight proportion is 4:1 to 50:1.

Inventors: JUZHEN, WANG
Applicants: SHANGHAI LAMP FACTORY
Application year: 1986
Main classification: G10K11
IPC codes: G10K11, H04R17