The . present invention relates to piezoelectric composites, and more particularly to a novel method of manufacturing composite material formed of piezoelectric rods embedded in a polymcic matrix.

Composite piezoelectric materials have been conventionally employed widely for the generation and detection of sound waves. One class of these materials, known generally as 1-3 piezocomposites, is formed of a.plurality of parallel, long, thin rods of electrically active ceramic disposed in a continuous passive matrix that completely surrounds the rods, the rods being normal to a substantially plane surface of the composite. Such materials can operate as true composite materials, where the acoustic wavelengths involved are so much longer than the detailed structure of the piezocomposite mat the material can be considered homogeneous.

The more important mechanical properties of piezocomposites are the effectiveness of electromechanical conversion, and the acoustic and electrical impedances. These properties are generally improved in 1-3 composites as has been described in a number of papers published in the late 1970's and early 1980's. Such composites have found extensive use in medical transducers where the rods are spaced very closely together and the resin matrix is a high modulus material that responds substantially in phase with the rods.

^P iezocomposites of this type are exemplified by the PZT/polymer material described in U.S. Patent No. 4,613,784 issued to M. K - et al. Assembling such piezocomposites involves arranging the rods in a uniform array, with or without lateral reinforcements, the rods being precisely located in a relatively expensive holding alignment rack or tooling. The matrix material, typically a resin, is introduced into the interspaces between the aligned ceramic rods, as by injection or transfer molding. The composite is then machined to final dimensions, removing excess resin and creating a composite in which all of the rods are of the same length, the composite having parallel, smooth end faces.

Machining the composite creates not only a problem wim-the disposal of the machine waste, but a problem arising because the pressure of the grinding or cutting

head compresses the elastic resin during removal of the excess material. The rigid piezoelectric rods are incompressible and therefore can be machined to specifications. The resin, however, after the pressure of the machining head is removed, tends to slowly expand beyond the machined ends of the rods. After silver electrodes are coated onto the ends of the rods and the composite is compressed during testing, subsequent relaxation of the pressure tends to debond the electrodes, leading to transducer failure. Attempts to overcome this problem by slow material removal and machining a part cooled to subzero temperatures does not materially alleviate the problem.

A principal object of the present invention is to provide a method of manufacturing a piezocomposite, which method overcomes the problems noted above hitherto experienced in manufacturing such piezocomposites; and specifically to provide such a method which permits one to utilize the entire length of the expensive rod and eliminates or substantially reduces machining operations. Specifically, the present invention provides active composites in which the placement of the ends of the piezoelectric rods is precisely controlled to be at or just above the surface of the resin matrix, thereby solving the machining problem that has hitherto plagued the manufacture of piezoelectric composites.

Yet other objects of the present invention will in part appear obvious and will in part appear hereinafter.

The invention accordingly comprises the processes and the several steps and relation of one or more of such steps with respect to each of the others as are exemplified in the following detailed disclosure, the apparatus possessing the construction, combination of elements and arrangement of parts, and the product possessing the features and the relation of components, all of which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

Generally, to effect the foregoing and other objects, the present invention provides a method of fabricating novel piezocomposites, such as a 1-3 material, in which the ends of the ceramic piezoelectric rods extend beyond the resin matrix by a uniform precise amount. The method of the present invention comprises the steps

of forming a flat, preferably metallic, bottom jig of predetermined thickness and having a specific pattern of a plurality of spaced-apart, elongated parallel holes of substantially the same diameter as the rods. Respective first ends of a plurality of piezoelectric rods of equal or nearly equal length are inserted within those holes through the thickness of the bottom, and those ends are then positioned adjacent a base plate of a mold. The other ends of the rods are inserted in corresponding holes in a similar top jig which is then adhered to a top plate of the mold. The mold is closed and resin is infiltrated into the mold in the interspaces between the rods bounded by the jigs. The resin is then cured and the mold removed. The resulting article composite now comprises a plurality of rods captured in a matrix with jigs covering the ends of the matrix. The rod ends may be machined, if necessary, to assure uniform lengths. The jigs are removed mechanically if reasonably possible, or etched away with an appropriate etchant. It will be seen that the length of the rods then extending beyond the resin surfaces is controlled by the thickness of the jigs-

For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein

Fig. 1 is a flow chart showing a typical process for forming a piezocomposite in accordance with the present invention; and

Fig. 2 is an exploded isometric schematic of apparatus comprising a mold and jig embodying principles of the present invention;

Fig. 3 is an enlarged cross-section of a piezocomposite made in accordance the present invention with the jigs still in place; and

Fig. 4 is an illustration of the piezocomposite of Fig. 3 with the jigs removed. Typically, as shown in Fig. 1, the present invention involves a series of steps shown generally in Fig. 1 wherein step 20 of preparation of tooling. Such tooling, shown in Fig. 2, includes mold 22 shown schematically as comprising first end plate 24 and second end plate 26. Both end plates 24 and 26 are preferably made of metal or other heat-resistant, strong, rigid material, have substantially flat surfaces facing one another in parallel and preferably have identical cross-sectional

configurations, e.g square, rectangular, circular and the like. The sides of the mold are shown formed as a single unit in the form of metal frame 28, the cross-sectional configuration of which is congruent with or matched to the cross-sectional configuration of end plates 24 and 26, but frame 28 can be assembled of several parts or sections if desired. Frame 28 has a depth or thickness typically of about 5.9 mm.

Means, in the form of inlet aperture 30, for furnishing access to the interior of mold 22 are provided, typically extending through first end plate 24 which is also perforated with one or more outlet apertures or vents 31 typically connectable to vacuum pump 33. It will be appreciated then that mold 22 can be any of a large number of prior art molds.

The tooling shown in Fig. 2 also includes first jig 34 and second jig 36, each in the form of a relatively rigid, strong, flat plate, typically made of metal. AS will be seen, these jigs are removed from the final product, and where the removal is to be effected by etching, it is preferred that the jigs be made of a readily etchable metal such as copper, aluminum and the like. Each of jigs 34 and 36 exhibit a cross-sectional configuration of lesser dimensions than the cross-sectional configuration of the interior of frame 28, and a thickness selected according to the desired extent to which the ends of piezoelectric rods will extend beyond the surface of the piezocomposite to be formed, e.g. 0.2mm. Each of jigs 34 and 36 are provided with a plurality of apertures or holes 38 through at least one of the flat surfaces thereof and disposed in identical specific patterns, as by drilling or punching. Holes 38 are shaped and dimensioned in cross-section to provide a tight sliding fit for piezoelectric rods to be inserted therein. Typically, for example, holes 38 are drilled in a hexagonal array perpendicular to the plane of the jig, each hole having a diameter of 0.15 cm, the center-to-center spacing being 0.24 cm, with all of the axes of elongation of the holes being substantially parallel to one another. In one embodiment, holes 38 can extend completely through the jig. In other embodiments, one surface of the jig is covered or capped with a very thin (e.g. 0.1 mm) metal sheet or foil so that holes 38 are blind, or holes 38 are blind because they simply are not drilled all the way through the jig.

Piezoceramic rods 40, typically barium titanate, lead zirconate titanate (PZT), or the like, of diameter substantially the same as that of holes 38 and a length (e.g. 6.3 mm) preferably slightly greater than the desired thickness of the final composite, are provided. It will be appreciated that substantially any piezoceramic material in rolled form may be used to form such rods.

As shown in Fig. 1, the process of the present invention includes step 41 of precleaning the rods, coating the ends thereof with a coupling agent or adhesive. In step 42, the ends of the precleaned, coated rods are fitted snugly within holes 38 in jig 34. Only a few rods 40 are shown emplaced in Fig. 2. Jig 34, either before or after an assembly of rods 40 is made therein, is placed in contact with plate 26 which is intended, for example, to serve as the bottom of the mold, the rods being so pressed into holes 38 as to insure that ends of each of the rods are closely adjacent or butt against plate 26. It will be z predated that the coupling agent applied to the rods serves to retain the latter witnin the holes in the jig.

Jig 36 is then placed over the other ends of rods 40 to provide the desired array of spaced-apart, parallel piezoceramic elements. Jig 36 is brought into facing contact with a surface of end plate 24, again with those ends of rods 40 extended into holes 38 in jig 36 so as to be adjacent or butt against the plate depending on whether the holes are blind or not. Jig 36 is then temporarily bonded or adhered to plate 24, as with any suitable coupling agent or adhesive.

Step 43 of Fig. 1 indicates that either prior to or after completing the assembly of the tooling comprising the end plates, rods and jigs, reinforcing fibers may be emplaced in the interspaces between the rods in the assembly, if desired. The reinforcing fibers, preferably continuous, may be any of a large number of materials such as carbon fiber, glass, alumina, polyester, rayon and alumina-boria- silica and the like, distributed substantially randomly or with any desired orientation to provide between about 5 and 20 volume percent of the volume of the interspaces.

Mold 22 is then finally assembled by emplacing frame 28 about end plates 24 and 26 to completely enclose the assembly of rods 40 and jigs 34 and 36, the assembled mold being so designed as to effectively seal the interior thereof except for the inlet aperture 30 and vents 31. After placement of rods 40 and assembly of

mold 22, the interspaces among the array of rods in the interior of the latter is impregnated, shown as step 44 of Fig. 1, for example, with an epoxy, or with any of a variety of other resins, monomers or polymers, in liquid form or as finely comminuted particles suspended in an appropriate vehicle. The impregnant is introduced by pumping the latter from a supply source or reservoir (not shown) through inlet aperture 30 and drawing a vacuum on vents 31 with pump 33 until the impregnant is thoroughly distributed throughout the interstices of the rod array. In an alternative method, the impregnant liquid is introduced -into mold 22 through inlet aperture 30 under high pressure e.g. of 500 psi or above. After impregnation, the impregnant is cured or solidified in step 45, typically by initiating polymerization by irradiation, chemically or by heating to achieve polymerization or drive off a binder vehicle or the like.

The solidified assembly is allowed to cure, and in step 46, the mold is disassembled, care being taken to remove both end plates from the resulting composite as shown in Fig. 3. If, for example, the coupling agent used to adhere the rods ends to the jigs is temperature degradable, it will be understood that the heat of the polymerization or curing processes can provide the energy necessary to degrade the bond sufficiently to permit easy removal of the jigs. It will be apparent that the resulting composite now comprises an array of rods 40 captured within a solid matrix of solidified resin with or without reinforcing fibers embedded therein as desired, but with both jigs capping the ends of the composite. To the extent that one or more rod ends at opposite ends of the composite do not lie in a respective common lane, or are not per se planar, then the jigs and captured rod ends may be machined in step 47 as by milling to insure the desired coplanarity of the rod ends. In step 48, the jigs are thereafter removed mechanically or by etching away the metal with an appropriate known etchant, thereby to form a composite formed, as shown in Fig. 4, of solidified resin 50 having embedded therein rods 40, the ends of the latter extending from opposite surfaces of resin 50. To for a transducer from such composite, electrodes 52 are applied to the opposite surfaces of the composite from which the rod ends extend. Such application can be achieved by any of several known techniques, e.g. by applying an epoxy loaded with a conductive metal such

as copper or silver.

The transducers thus formed are poled by, for example, attaching the transducer to a high voltage source with one electrode at the top and in contact with all rods and the substrate. The other electrode is bonded to the bottom conductive electrode. The composite is immersed in an oil bath and heated to between 75°C and 160°C. A voltage of about 20,000 volts is applied for several minutes and then slowly reduced while the transducer is still immersed in the constant temperature bath.

An alternative and preferred method of poling the composite is to expose the \ ^~ T to a corona discharge field. In such case, the specimen is electroded on one side and placed, electrode side down, on a conductive ground plate. The plate is placed on a heat source capable of being heated to 150°C and capable of being cooled rapidly as by passing water through holes provided in the heat source. An upper electrode is provided consisting of one or more needles suspended above and aimed at the center of the composite. The entire system is placed within a grounded conductive screen cage and a potential of 20,000 volts per centimeter is applied for about 10 minutes to the heated composite. That temperature is maintained until the composite cools to about 35°C.

The effectiveness of the poling is confirmed by measuring the d ₃₃ piezoelectric coefficient. The coupling coefficient is determined in accordance with the impedance level measured as a function of frequency. The resonance (f _r ) and antiresonance (f frequencies are measured and the thickness coupling coefficient is calculated according to the formula: k.2 = (II/2)(f _r /f _a )cot[(II/2)(f _r /fJ

For some measurements, a special cell is used to determine the impedance curves from liquid nitrogen temperatures, at about -50°C, the impedance analyzer being used with that cold cell. The measurements are made in air.

Since certain changes may be made in the above process, apparatus and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense.