US3189767A - Ultrasonic transmitting means and method of producing same - Google Patents

Description

June 15,1965 R. G. GOLDMAN ErAL 3,189,767

ULTRASONIC TRANSMITTIYNG MEANS AND METHOD OF PRODUCING SAME FiledA Jan 28 1963 l l a l l l l @ISG l@ I L United States Patent O f `ce ULTRASUNIC TRANSMIT'IING MEANS AND METHOD F PRODUCING SAME Richard G. Goldman and Wiiliam R. Marklein, Schenectady, NSY., assignors to General Electric Company, a corporation of New York Filed Jan. 28, 1963, Ser. No. 254,328

9 Claims. (Cl. 310--8.2)

This invention relates to an article of manufacture for transmitting ultrasonic energy and method of producing same. The article is particularly applicable as a matchying layer or backing member for an electro-acoustical transducer.

In the generation and reception of ultrasonic waves for many purposes, but particularly in electro-acoustical or piezoelectric ultrasonic transducers yfor flaw detection purposes, it is desirable that the ultrasonic pulse vbe short in order to achieve good definition and to detect flaws near `the surface of the object under test. Despite the application of short electrical pulses to the transducer, short, Well-defined ultrasonic pulses have not been realized due to oscillations or ringing in the transducer itself. To dampen this ringing and to .reduce the reflections to the transducer from its interfaces with other mediums, transducer backings and matching layers have been developed.

An ideal matching 4layer permits a maximum transmission of ultrasonic energy to the object under test with a minimum of interface .reflections by providing an impedance match with the transducer interface and with the inter-face of the object under test. The reduction of reflections as well as the improved transmission increase the damping of the transducer oscillations. An ideal backing likewise matches the transducer impedance exactly and :additionally provides maximum Adamping through high absorption of energy transmitted from the rear face of the transducer.

In the past, it has not been possible to find a single material which provides the above-mentioned ideal characteristics of matching layers or backings for commonly used transducers such .as quartz or ceramic piezoelectric while presenting no discontinuities of acoustic impedance. Consequently, various expedients have been adopted in attempts to fulli-ll the requirements of absorption and impedance matching. 1n the case of matching layers, laminations of different materials having average impedance matches, various filling substances, etc. have been tried. Likewise, transducer `backings utilizing multiple, specifically dimensioned, layers of different substances, `some of which provide an impedance match and others yof whic-h have a high absorption, have been combined in multiple layer-s.

Furthermore, the -methods employed heretofore in .the production of matching layers and backings have precluded the control necessary to achieve a `device having the desired characteristics of accurate impedance matching, absorption and freedom from discontinuity. This lack of control is eremplied by devices utilizing multiple layers of various dimensions having mult-iple bonding `layers therebetween and by various compositions comprising substances having particles, fibers, etc. mixed therein to produce varying densities.

My invention overcomes the failures of .the prior art by providing backings and matching layers having accurately controlled acoustical impedances and freedom from discontinuities either in physical structure or in acoustical impedance. Also, the process used is capable of producing transducer backings having high absorption.

Accordingly, it is an object of this invention to provide improved ultrasonic `energy transmitting means free from discontinuity of acoustic impedance.

3,189,767 Patented June I5, 1965 It is another object of this invention to provide a matching layer of ceramic mate-rial for transmitting ultrasonic energy from a first medium to a second medium in which the ceramic material provides a matching impedance at the interface with the first medium, and a second matching impedance at the interface of said second medium, and is free from discontinuity of acoustic impedance.

It is a farther object of this invention to provide a transducer backing permitting a maximum transmission of ultrasonic energy from the transducer to the backing and having high absorption.

It is yet another object of this invention to provide an improved method of producing a ceramic block for transmitting ultrasonic energy.

Brieiy stated, in accordance with my invention, an ultrasonic transmitting means having the ideal characteristics outlined above is obtained by a gradient tiring process. The gradient firing is applied to a block of ceramic material which has been previously isostatical'ly pressed and pre-fired, in a manner described more lspecifically hereinafter. One end of the block is subjected to a heat source at a greater temperature than that of the pretiring, the gradient firing being achieved yby thermal conductivity of the ceramic material between the heat-ed end and the opposite unheated end of the block. Ceramic material treated in this manner may be produced under controlled conditions of time, temperature and composition to meet varying impedance matching and damping requirements for ultrasonic transmitting devices.

For a complete understanding of my invention, reference may be had to the accompanying drawing, in which:

FIG. 1 shows, in vertical cross-section, the transmitting means of this invention used as a backing for a piezoelectric crystal.

FIG. la is a graph representing the gradient firing temperatures achieved by thermal conductivity throughout the backing means of FIG. l.

FIG. 2 shows, in vertical cross-section, the transmitting means of this invention used as a matching layer between a piezoelectric crystal and an object under test.

FIG. 2a is a graph of the gradient firing temperature achieved by thermal conductivity through the transmitting means of FIG. 2.

FIG. 3 shows, in diagrammatic form, one method of gradient tiring in accordance with the present invention.

The ultrasonic transmitting means of the present invention may be used for many purposes related to the generation and reception of ultrasonic waves. However, for purposes of illustration, my invention is described in this application in connection with the use of backings and matching layers for piezoelectric crystals as used, for instance, in ultrasonic flaw detection apparatus.

Referring now to the drawings, there is shown in FIG. l a piezoelectric element 1t) which may be in the Iform of quartz or ceramic cry-stal to which high-frequency electric oscillations may be applied to effect high-frequency mechanical vibrations of the crystal in a manner well known in the art. To damp the crystal, a backing or ceramic block indicated generally at 15 is joined to the rear surface 11 of the element 1) by means of a thin layer of any suitable bonding agent 13, such as, for instance, an epoxy resin.

In FIG. 2, a piezoelectric element 10 is shown having its front surface 12 attached by a bonding agent 13 to a ceramic block or matching layer indicated at 20. The matching layer 20 transmits ultrasonic energy between the element 10 and a medium or object 21 under test. The matching layer 20 at one end 22 has an impedance which matches that of the element 1d, and at the opposite end 23 has an acoustic impedance which matches that of the object 21. The matching layer 20 may be used not only in cooperation with piezoelectric elements but may ed be used between any two media for the transmission of ultrasonic energy therebetween.

The transmitting means shown respectively in FIGS. 1 and 2 as backing i5 and matching layer 20 comprises a ceramic material which has been gradiently fired in a manner illustrated in FIG. 3. The gradient firing used in accordance with my invention is applied to a ceramic material which has been formed to a solid mass or block by conventional means, such as isostatic pressing, whereby powder is compressed in a flexible mold by the uniform application of pressure through a huid in which the mold is immersed. Conventional steel die pressing or extruding processes may also be utilized. The block is then pre-fired to sufficiently bond the ceramic material, producing a homogeneous, relatively soft, chalk-like block 3d.

The gradient ring process may be accomplished, for example, in a furnace comprising a steel vessel 31 which is capable of being evacuated. A heat source is provided by an annular sleeve or susceptor 32, having a high-frequency coil 32 Wrapped about its outer surface, the susceptor being mounted within vessel 31 in any suitable manner. The pre-fired block 3i) is placed within the vessei 3l. with one end 3e resting on a large cast iron block 3S serving as a heat sink, and the other end 36 disposed within the graphite susceptor 32. Air is then evacuated from the Vessel and a high-frequency current passed through coil 33, heating the block by radiation from the susceptor 32. The firing of the block in a vacuum insures maximum density at the end 36 of the block and freedom from heating, by convection currents of gas in chamber 3i, of the end 3d. The temperature applied to surface 36 by susceptor 32 is greater than the pre-tiring temperature. The opposite end 34tof block 3@ is maintained at a relatively low temperature by means of conduction of Vheat to the sink 35, the block being thereby fired by the temperature gradient established between the ends 34 and 35 due to the thermal conductivity of the ceramic material.

FIGS. la and 2a illustrate graphically the tiring temperature gradient which exists across the thickness of backing member l5 and matching layer 2d respectively. The temperature and resulting density of the ceramic point A in FiG. 2a will be determined by the impedance desired to match that of the object 21 under test. The end of block 29 abutting the object 2] may be red at a higher or lower temperature than the end 22, depending on the acoustic impedance of the object 21. It will of course be understood that the end desired to be fired at the higher temperature will be disposed within the heating device 32, and the lower temperature end will abut thev heat sink 35. The temperature attained at the lower temperature end of the block will of course depend on the material, size, and shape of the block being fired, the temperature at which the hot end is fired, the characteristics of the heat sink 35, the length of the firing cycle, etc.

Using the ceramic materials discussed below in this gradient firing process, it has been found that a backing material having an acoustical impedance matching that of the rear surface of a piezoelectric element can be produced, while at the same time providing high absorption and therefore maximum damping of the ultrasonic energy transferred to the backing. The more dense end lof backin 7 15 which has been heated to the higher temperature, being adjacent the heating element during the gradient firing process, provides a matching impedance for the rear surface l1 of the element 10. The end le of the backing T is less dense, having been fired to a lower temperature, and provides high absorption characteristics. Furthermore, the density of the block uniformly decreases between ends 14 and 16 and provides no discontinuity of acoustical impedance, since a single homogeneous ceramic material is used.

Turning now to the matching layer of FlG. 2 produced by the gradient ring process, the end 22, having been heated to the proper temperature as shown in FIG. 2, provides a matching impedance for the transducer element l?. in this case, the temperature at the end 23 of the matching layer 2G is controlled so as to provide an impedance match with the object 21 under test. As in the case of the backing 15' of FIG. l, the material of matching layer 29 has a density uniformly varying from end 22 to end 23 wmie eing free from discontinuity of acoustical impedance, which would create undesirable reflections from reaching the element il?.

it has been found that very satisfactory results are obtained when the ceramic material of the matching means is formed from a powdered refractory oxide. Certain aluminum oxide powders sold under the trademarks 3S-900 Alundum, by the Norton Company, and Linde A, by Linde Air Products Company, when compressed and fired in the above manner, have produced excellent backings or matching layers for ceramic and quartz piezoelectric crystals. These materials may be cornbl red with small amounts of other materials. The 3S- SCO Alundum comprises a relatively pure (99.9%) fused alumina having an average particle size of 7 microns. The Linde A material consists of a very fine (0.3 micron) and pure (99-{%) aluminum oxide obtained by the controlled calcination of an aluminum sulphate compound.

When fired in the range of l200 C. to 1800 7 C., the 3S-900 Alundum compressed at 10,06() p.s.i. forms a ceramic body or block in which the density and the sonic velocity vary over a wide range with firing temperature. At approximately 1620 C. firing temperature, the irnpedance (Z) of the ceramic formed from the 3S-9G() Alundum, which is the product of p, the density, and v, the sonic velocity, (that is, Z zpv) is found to equal that of many of the commonly used ceramic piezoelectric elements which are formed from proprietary formulas based on barium titanate or lead zirconate titanate, such as that sold under the trade name Gulton HT. When tired at approximately l530 C., the impedance of the alumina ceramic matches that of quartz.

Improved damping results from using a transducer backing, all of which has been initially subjected uniformly to such temperatures. However, maximum damping is provided by initially pre-firing the entire backing at a uniform temperature of ll00 C. and then, in producing a transducer backing from the compressed 3S-960 Alundum, gradient firing the backing at a maximum ternperature of either l620 C. or 1530 C., depending on whether the piezoelectric element with which the backing is to be used is of ceramic or quartz. Gf course, in the case of matching layers, the pre-tiring temperature permits different acoustical impedances to exist at the respective ends of the block.

With respect to the Linde A material, it has been found that an isostatic pressing at 10,000 p.s.i., a uniform pre-tiring at 1100 C. and a gradient tiring at 1465L1 C. provide an excellent backing material for the common piezoelectric ceramic materials. A sample produced by this process was found to substantially shorten the ultrasonic pulse, to increase damping, and to eliminate detectable'reections from the transducer interface when used with a 2.25 megacycle barium titanate transducer having a one-inch diameter.

When using either of the above-mentioned alumina ceramics, the pre-firing process results in a block having a chalk-like consistency which permits machining of the block to any desired configuration or size prior to the gradient firing. Also the pressed material may be machined or ground to conform to wanted dimensions prior to the pre-firing step.

Refractory om'des, including magnesium oxides and the 3 8-900 Alundum and the Linde A alumina ceramics, have a very poor heat transmitting characteristic. For this reason, such materials are ideal for the gradient firing technique, since the end to which the heat source is applied may be highly heated while the opposite end of the block may be maintained at a relatively low ternperature, thereby achieving the desirable varying density while avoiding discontinuity of acoustical impedance. The poor heat transmission characteristic additionally permits backings and matching layers of minimum thickness, as the end of the block away from the heat source will be heated very slowly even when a thin block is used. Although the ceramic block is illustrated in FIGS. l, T2 and 3 as a rectangular prism or cylinder, the invention is not so limited but may take any suitable geometric form.

While the invention has thus been disclosed and the presently preferred embodiment described, it is not intended that the invention be limited to the applications discussed herein. instead, many modifications will occur to those skilled in the art which lie within the spirit and scope of the present invention.

Having described the invention, what is claimed is:

1. For use with a piezoelectric element having two spaced surfaces, a transmitting means comprising a homogeneous ceramic block, said block having one end engaging one of said surfaces, said block having uniformly changing density from ysaid one end to the other end of said bloei(` so that the acoustical impedance of said block varies uniformly whereby said block is rendered free from discontinuity of acoustical impedance.

2. For use with a piezoelectric element having two spaced surfaces, a transmitting means comprising a homogenous ceramic block of refractory oxide, said block having one end engaging one of said surfaces, said block having uniformly changing density from said one end to the other end of said block so that the acoustical impedance of said block varies uniformly whereby said block is rendered free from discontinuity of acoustical impedance.

3. For use with a piezoelectric element having two spaced surfaces, a transmitting means as specied in claim 2 in which the refractory oxide is an alumina ceramic.

4. For use with a piezoelectric element having two spaced surfaces, a backing comprising a gradient red homogeneous ceramic block, said block having one end engaging one of said surfaces and having a density which increases uniformly from said one other end to a maximum at the other end so that the acoustic impedance of said block at said one end is substantially the same as the acoustical impedance of said element and said acoustical impedance of said block increases uniformly from said one end to said other end.

5. A matching layer for transmitting ultrasonic energy from a first medium to a second medium comprising a homogeneous ceramic block, said block having one end engaging said first medium, `said block having a density at said one end providing an acoustic impedance substantially the same as the acoustical impedance of said rst medium and a density at the other end providing an acoustical impedance substantially the same as the acoustical impedance of said second medium, said block having uniformly charging density between said ends whereby said block is rendered free from discontinuity of acoustical impedance.

6. A method for forming a backing or transmitting member for ultrasonic energy comprising the steps of:

(a) forming a homogeneous ceramic block, and

(b) gradient firing said block by applyin7 a heat source to one end thereof to provide uniformly changing density between said one end and the other end of said block so that the acoustical impedance of said block varies uniformly whereby said block is rendered free from discontinuity of acoustical impedance.

7. A method of forming a backing or transmitting member for ultrasonic energy comprising the steps of:

(a) forming a homogeneous block of refractory oxide material,

(b) pre-firing ysaid block uniformly at a rst temperature, and

(c) gradient firing said block by applying a heat source having a second temperature greater than said first temperature to one end of said block to provide uniformly changing density between said one end and the other end of said block so that the acoustical impedance of said block varies uniformly whereby said block is rendered free from discontinuity of acoustical impedance.

S. A method of forming a backing or transmitting member for a piezoelectric element comprising the steps of (a) forming a homogeneous block of refractory oxide material,

(b) pre-firing said block uniformly at approximately 1100 C., and

(c) gradient firing said block by contacting a heat sink member with one end of the block and applying a heat source 'in the range of 1400 C. to l650 C. to the other end thereof to provide uniformly changing density from said one end to the other end of said block so that the acoustical impedance of said block varies uniformly whereby said fired block is rendered free from discontinuity of acoustical impedance.

9. The method of forming a piezoelectric backing or matching member in accordance with claim 8 in which the gradient firing step is carried out in a vacuum to reduce the transfer of heat by convection.

References Cited bythe Examiner UNITED STATES PATENTS 2,430,013 11/47 Hansell S10-8.2 2,707,755 5/55 Hardie et al. 310-8.2 2,972,068 2/61 Howry et al. S10-8.2

MILTON O. HIRSHFIELD, Primary Examiner.

Claims (1)

1. FOR USE WITH A PIEZOELECTRIC ELEMENT HAVING TWO SPACED SURFACES, A TRANSMITTING MEANS COMPRISING A HOMOGENEOUS CERAMIC BLOCK, SAID BLOCK HAVING ONE END ENGAGING ONE OF SAID SURFACES, SAID BLOCK HAVING UNIFORMLY CHANGING DENSITY FROM SAID ONE END TO THE OTHER END OF SAID BLOCK SO THAT THE ACOUSTICAL IMPEDANCE OF SAID BLOCK VARIES UNIFORMLY WHEREBY SAID BLOCK IS RENDERED FREE FROM DISCONTINUITY OF ACOUSTICAL IMPEDANCE.

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