Method for producing porous ceramics for acoustic transducers

Abstract

A porous ceramic body is provided that has good structural integrity and can be used to form an acoustic transducer using a simple process. [Solution] The method for producing a porous ceramic body includes forming a green compact using a mixture of ceramic powder, binder material, and pore-forming particles. The method further includes a step of gradually increasing the temperature of pre-firing to remove the binder, decompose the pore-forming particles, and remove residual organic matter from the green compact. After these three steps, the green compact is sintered at an even higher temperature to form a porous ceramic body. This results in a porous ceramic body that is thicker than conventional porous ceramic bodies and has greater structural integrity. During sintering, a substantially non-porous surface is formed on the surface of the porous ceramic body, allowing electrodes to be formed directly on this surface. Advantageously, the final green compact can be formed by injection molding the mixture containing the pore-forming particles.

Description

Related Applications

[0001] This application claims priority to U.S. Provisional Application No. 62 / 992,115, filed March 19, 2020, the entire teachings of which are incorporated herein by reference. [Background technology]

[0002] There are several known advantages to using piezoelectric or electrostrictive ceramics in acoustic transducers, actuators, flow meters, and other ultrasonic devices. The porous ceramics used in these transducers may be compact ceramics with pores uniformly distributed throughout. Porous ceramics have lower densities than non-porous ceramics, which can provide a better acoustic match to the propagation medium (most often water) through which the acoustic transducer propagates. The porosity also provides a lower acoustic impedance and a lower Q factor, improving the performance of ceramics for acoustic transducers. Summary of the Invention Problems that the invention aims to solve

[0003] Existing technologies for producing porous piezoelectric or electrostrictive ceramics for acoustic transducers are constrained by complex and labor-intensive manufacturing processes, limiting the design of the transducer and the quality of the resulting product. Pore-forming particles can be introduced into the compact mixture to increase the porosity of the ceramic body formed after firing the compact. However, the introduction of pore-forming particles can exacerbate potential structural problems in the sintered ceramic body. These structural problems include, for example, cracking and deformation of the ceramic, which can disrupt the flatness of the ceramic body.

[0004] Furthermore, the structural problems described above tend to worsen with increasing ceramic thickness. Perhaps this is one of the reasons why existing porous ceramic bodies using pore-forming particles have a thickness of only about 13 mm. Furthermore, due to the above-mentioned deformation problem, typical porous ceramic bodies are polished to their final thickness and flatness after firing (also referred to herein as "sintering") and before electrode attachment. Furthermore, the firing process naturally forms a skin, i.e., a relatively smooth, non-porous surface structure with a high porosity, whereas the bulk of the sintered ceramic body has a high porosity. Therefore, a smooth glass surface is typically applied to the polished surface of the sintered ceramic body before electrode attachment.

[0005] The addition of pore formers to injection molding mixtures poses several technical challenges. The added material increases the viscosity of the mixture. To lower the viscosity, the solids loading (amount of ceramic relative to the volume of the mixture) must be reduced. This can reduce the strength of the green precursor and can even cause shrinkage. The presence of folds, cold flow regions, and macroscopically induced stressed areas are consistent with the properties of injection molded ceramic preforms (precursors).

[0006] Removing the pore former after molding requires a burn-out step at a higher temperature than that required to remove the organic binder, which adds time and increases the risk of cracking and breakage. [Means for solving the problem]

[0007] Disclosed herein are methods that can be used to overcome the above-mentioned challenges. Embodiments provide methods for treating ceramic material powders mixed with pore-forming particles. The treatment improves the structural integrity of pre-fired compacts and compacts undergoing firing for ceramic production, and further improves the resulting porous ceramic body for improved processability and performance in subsequent processes.

[0008] The method of the embodiment may include an intermediate pre-firing period, which prevents cracking and maintains flatness without polishing the sintered ceramic surface. This has the significant advantage of eliminating the need to apply glass to the ceramic surface prior to electrode formation. Thus, electrode material can be applied directly to the sintered, substantially non-porous skin of the porous ceramic body. Furthermore, the addition of this pre-firing period allows the porous ceramic body to be formed at significantly greater thicknesses than existing methods. Furthermore, the disclosed method is compatible with injection molding, allowing for flexible transducer design and significantly simplifying the manufacturing process, thereby reducing costs. This significantly simplifies the manufacturing process and improves product quality.

[0009] In particular, the method of the embodiment may include decomposing pore-forming particles in the green compact during an intermediate pre-firing period. The intermediate pre-firing period follows a pre-firing period in which the binder is removed from the green compact and precedes a final pre-firing period and sintering. Specifically, the final pre-firing period is characterized by removing residual organic matter from the green compact in which the pore-forming particles have been substantially decomposed. This staged pre-firing process maintains structural integrity and flatness.

[0010] The disclosed method includes a net-shape process for fabricating porous ceramic preforms from piezoelectric or electrostrictive materials by injection molding. The disclosed method may include preparing a homogeneous, granular thermoplastic powder / binder mixture from a powder of a high-strength piezoelectric ceramic material and a thermoplastic organic binder, and may include a selected sacrificial pore former that is removed from the mixture by non-destructive application of heat to the ceramic preform.

[0011] In various embodiments, the mixture may be injection molded to form self-supporting green compacts of various shapes. These include plates, disks, and composite preforms with 1-3 or 2-2 connectivity, which may consist of a planar ceramic substrate and parallel ceramic elements extending perpendicularly from the substrate in a preselected arrangement. Each element may have a rod-like (e.g., 1-3) or lamellar (e.g., 2-2) shape. The green compact may be ejected from the preform mold, heated in stages at temperatures between 70°C and about 700°C for times sufficient to substantially completely remove the binder, pore former, and residual organics, respectively, and sintered to between 65% and 95% of its calculated density.

[0012] The method of the embodiments allows for the production of transducers from a final shape of a disk, plate, rod, hemisphere, wedge, or other moldable shape. An advantage of injection molding 1-3 or 2-2 structures is that it generates less ceramic waste compared to conventional dicing or machining of ceramic preforms. To produce piezoelectric or electrostrictive composite transducers exhibiting 1-3 or 2-2 connectivity and containing a porous piezoelectric or electrostrictive ceramic phase, the sintered porous ceramic body formed by the method of the embodiments may be encapsulated in a polymer material to form a two-phase ceramic / polymer composite. The ceramic substrate may or may not be removed from the lower planar surface of the composite. Elements may be exposed on each of the upper and lower planar surfaces. The composite may be electroded to establish electrical contact with the elements. The elements may be poled, for example, to produce piezoelectric composite transducers exhibiting 1-3 or 2-2 connectivity. Composite geometries can further enhance the performance of porous ceramics by lowering the acoustic impedance, Q factor, and increasing the effective coupling coefficient and piezoelectric coefficient.

[0013] In one embodiment, a method for making a porous ceramic body includes forming a green compact comprising a mixture of a ceramic powder, a binder material, and pore-forming particles. The method also includes removing (extracting) the binder material from the green compact by heating the green compact at a first pre-fire temperature. The method further includes heating the green compact at an intermediate pre-fire temperature higher than the first pre-fire temperature to decompose the pore-forming particles. The method further includes heating the green compact at a final pre-fire temperature higher than the intermediate pre-fire temperature to remove residual organic matter from the green compact. The method further includes sintering the green compact at a sintering temperature higher than the final pre-fire temperature to form the porous ceramic body. Advantageously, this method of making a porous ceramic body allows for injection molding of the mixture to form the green compact. Furthermore, injection molding allows for net-shape processing, such that the ceramic body formed upon sintering assumes its final shape, eliminating the need for post-processing of the surface of the sintered porous ceramic body. The elimination of post-sintering machining significantly streamlines and simplifies the manufacturing process.

[0014] In another embodiment, the acoustic transducer is made from a porous ceramic body made according to the method described above, or any of the alternative methods described herein.

[0015] In a further embodiment, a piezoelectric or electrostrictive acoustic transducer can be made by first preparing a porous ceramic body according to the method described above, or according to an alternative method within the scope of this specification, which method further includes machining electrodes on both sides of the sintered porous ceramic body.

[0016] In yet another embodiment, the process for producing a porous ceramic body includes a step of injection molding a mixture of a ceramic powder, a binder material, and pore-forming particles to form a compact, the compact being heated to decompose the pore-forming particles from the compact, the binder material being substantially extracted from the injection-molded compact before decomposing the pore-forming particles, and the process includes a step of sintering the compact to form a porous ceramic body after decomposing the pore-forming particles from the compact.

[0017] In one embodiment, a method for making a porous ceramic body includes injection molding a mixture of powdered ceramic material, binder material, and pore-forming particles to form a green compact. The method further includes extracting the binder material, decomposing the pore-forming particles, and sintering the green compact to form the porous ceramic body. Additionally, in another embodiment, an acoustic transducer or transducer array is formed using this method to form a porous ceramic body (or multiple such porous ceramic bodies). Injection molding can be net-shape, such that the ceramic body formed upon sintering assumes its final shape and no post-processing is required. [Brief explanation of the drawings]

[0018] The foregoing will become apparent from the following more detailed description of exemplary embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the embodiments. [Figure 1] FIG. 1 is a flow diagram illustrating a method for fabricating a porous ceramic body according to one embodiment. [Figure 2] FIG. 2 is a flow diagram illustrating a method for fabricating a porous ceramic body using the method of FIG. 1 and then forming electrodes on the porous ceramic body to fabricate a piezoelectric or electrostrictive acoustic transducer in one embodiment. [Figure 3]FIG. 1 is a flow diagram illustrating a method for making a porous ceramic body according to another embodiment, the method including injection molding a mixture of ceramic material powders. [Figure 4] 1 is a graph illustrating a temperature history for a method of making a porous ceramic body, in one embodiment. [Figure 5] 1 is a graph illustrating another temperature history for a method of making a porous ceramic body, in accordance with one embodiment, and in particular illustrating the flexibility of the temperature profile during the method. [Figure 6] 1(a)-(d) are a series of schematic phase diagrams illustrating particles of a green compact mixture or a porous ceramic body formed therefrom at various processing stages in one embodiment of the method. [Figure 7] FIG. 2 is a cross-sectional view illustrating a powder compact used in manufacturing a porous ceramic body, showing a state in which the powder compact is processed using a base and a sand bath. [Figure 8] 1 is a cross-sectional view of an existing acoustic transducer manufactured by an existing method (prior art). [Figure 9] 1 is a cross-sectional view of an acoustic transducer manufactured according to a method of one embodiment. [Figure 10] FIG. 1 is a perspective view illustrating an acoustic transducer array with 1-3 connectivity that can be manufactured using a method of one embodiment. [Figure 11] 11 is a perspective view of an acoustic transducer forming one of the acoustic transducer elements illustrated in the array of FIG. 10. [Figure 12A] 10 is a flow chart illustrating additional features of a method in which poling is performed after encapsulation in a composite, in one embodiment. [Figure 12B] 10 is a flow chart illustrating a method according to another embodiment, in which the composite process includes poling, followed by composite casting, followed by electrode fabrication. [Figure 13] FIG. 1 is a schematic cross-sectional front view of an exemplary mold that may be used in one implementation method. [Figure 14] 1 is a photograph of a 1-3 type preform that can be made by one embodiment of the method. [Figure 15] 1 is a photograph of a 1-3 type composite preform that can be made according to yet another embodiment of the method. [Figure 16] FIG. 10 is a schematic perspective view of a 2-2 type preform that can be made according to a method of yet another embodiment. [Figure 17] 17 is a photograph of a fine 2-2 type composite preform that can be produced according to the preform of FIG. 16. [Figure 18] 1 is a graph illustrating a primary pre-baking temperature profile that can be used in a method according to one embodiment. [Figure 19] 19 is a graph showing a final pre-bake temperature profile following the pre-bake temperature profile of FIG. 18 as an example that may be used in some specific implementations. [Figure 20] 1 is a graph illustrating a temperature profile that may be used in one embodiment, including a combination of a first pre-bake temperature period, an intermediate temperature period, and a final pre-bake temperature period, each pre-bake period separated from the other by a temperature decrease to room temperature. [Figure 21] 1 is a graph showing an eight-stage temperature profile that can be used in an embodiment, in which the temperature is increased without any temperature decrease during the first pre-baking period, intermediate pre-baking period, and final pre-baking period. [Figure 22] 1 is a schematic diagram of a forced air convection oven that may be advantageously employed in a method according to an embodiment; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0019] Exemplary embodiments are described below.

[0020] DETAILS OF THE PREFERRED SPECIFIC EMBODIMENTS As discussed above in the Summary section of this specification, the methods described herein provide a much easier and simpler method for forming porous ceramic bodies and transducer arrays. This method is cleaner, less expensive, and can offer a wider range of design parameters and better performance. In particular, the methods described herein can form a green compact by injection molding that is subsequently processed into a porous ceramic body. The resulting ceramic body can be, for example, thicker than about 15 mm (e.g., between about 15 mm and about 30 mm) and does not require surface polishing before application of electrodes to the surface.

[0021] FIG. 1 is a flow diagram illustrating a method 100 that can be used to achieve the above objectives, particularly for forming a porous ceramic body. A green compact is formed (102) containing a mixture of ceramic powder, binder material, and pore-forming particles. The green compact is then heated at a pre-fire temperature to remove the binder material from the green compact (104). The green compact is then heated at an intermediate pre-fire temperature higher than the pre-fire temperature to decompose the pore-forming particles (106). The green compact is then heated at a final pre-fire temperature to remove residual organics from the green compact (108). The final pre-fire temperature is higher than the intermediate pre-fire temperature. The green compact is then sintered (110) at a sintering temperature higher than the final pre-fire temperature to form a porous ceramic body.

[0022] A significant advantage of method 100 is that it allows for the injection molding of a green compact comprised of a mixture of ceramic powder, binder material, and pore-forming particles. Method 100 also provides a way to maintain the structural integrity of the porous ceramic body formed thereby, eliminating additional processing steps prior to ceramic surface polishing and electrode machining. Thus, method 100 may be modified to include the step of injection molding a mixture of ceramic powder, binder material, and pore-forming particles to form a green compact.

[0023] The porous ceramic body formed by method 100 may be a piezoelectric composite. In various embodiments including the process elements of FIG. 1, the ceramic material powder may be, for example, a PZT powder. The binder material may advantageously be a wax material, and the pore-forming particles may be, for example, PMMA particles. The pore-forming particles may advantageously have an average diameter between about 5 μm and about 15 μm. More specifically, the average particle diameter may be between about 7.5 μm and about 12.5 μm. In a preferred embodiment, the pore-forming particles have an average diameter of about 10 μm. As used herein, in the context of diameter, "about" means within 1 micron. For example, "about 10 μm" means 10±1 μm. If the pore-forming particles described herein are not spherical or are not perfectly spherical, the diameter described may be the average diameter. As used herein, "average diameter" may be the arithmetic mean of the diameters of the pore-forming particles measured for rotation of the particles about three mutually orthogonal axes of rotation or known by other means.

[0024] In various embodiments, the primary pre-fire temperature, intermediate pre-fire temperature, and final pre-fire temperature, as well as the sintering temperature, may have various temperature ranges, for example, as described in connection with Figures 4-5. Additionally, any of the various methods and physical factors described in connection with Figures 1-7 and 9-11, and elsewhere herein, may be used to modify method 100 and the various porous ceramic bodies and transducers that may be formed thereby.

[0025] Figure 2 is a flow diagram illustrating a method 200 for fabricating a piezoelectric or electrostrictive acoustic transducer according to one embodiment. A sintered porous ceramic body is first prepared (202) according to the method 100 illustrated in Figure 1. Electrodes are then formed on opposing surfaces of the sintered porous ceramic body to form a piezoelectric or electrostrictive acoustic transducer (204).

[0026] Method 200 may be modified to include any element or feature noted in connection with other embodiments of the method, porous ceramic body, or acoustic transducer assembly within the scope of this disclosure. Of note, injection molding may be used to prepare a green compact for processing to form a sintered porous ceramic body, as described in connection with method 100 of FIG.

[0027] FIG. 3 is also a flow diagram illustrating a method 300 for fabricating a porous ceramic body according to one embodiment. A compact previously formed by injection molding a mixture of ceramic powder, binder material, and pore-forming particles is heated to dissolve the pore-forming particles from the compact (302). When the pore-forming particles are dissolved in step 302, the binder material is substantially removed from the previously injection-molded compact. The compact is then sintered to form the porous ceramic body (304). The sintering in step 304 occurs after the pore-forming particles have been substantially dissolved from the compact.

[0028] Method 300 may further include any methodological or physical elements described herein with respect to other embodiments. For example, method 300 may be modified to include forming electrodes on a porous ceramic body to form an acoustic transducer. Exemplary acoustic transducers that may be formed in a manner consistent with this method include those illustrated in Figures 9-11 and described below. Also of note, method 300 may be modified to include injection molding a mixture of ceramic material powder, binder material, and pore-forming particles to form a green compact.

[0029] Additionally, method 300 may be modified as follows: decomposition of the pore-forming particles may be performed by heating the compact at an intermediate pre-fire temperature; sintering may be performed by heating the compact at a sintering temperature higher than the intermediate pre-fire temperature; and the method may further include removing residual organic matter from the compact after decomposition and before sintering by heating the compact at a final pre-fire temperature higher than the intermediate pre-fire temperature but lower than the sintering temperature.

[0030] FIG. 4 is a graph of a temperature history 400 illustrating various exemplary temperature steps consistent with the method 100 of FIG. 1 during conversion of a powder compact to a porous ceramic body. In particular, the temperature history 400 illustrates temperature (arbitrary units) as a function of time (arbitrary units). After forming the powder compact (e.g., by injection molding), the powder compact may be at room temperature 412 as shown in FIG. 4. During a heat-up period 414, the temperature of the powder compact may be increased to a primary pre-fire temperature 416. The primary pre-fire temperature 416 may optionally be held for a primary pre-fire hold period 418.

[0031] The primary pre-fire hold period 418 is characterized by the removal of binder material from the compact. This is because the primary pre-fire temperature during this period is sufficiently high to remove (e.g., by melting) the binder material (e.g., wax) from the compact, while remaining low enough to allow the pore-forming particles to remain without significant decomposition. This period at the primary pre-fire temperature is particularly advantageous because it allows the binder material to be removed without interfering with the decomposition and removal of the pore-forming particles at a later stage. Removing the binder before the decomposition of the pore-forming particles leaves a porous structure, which provides a path for decomposition products to escape without causing blistering or cracking. This process helps preserve the structural integrity of the compact, preventing, for example, cracking, especially in relatively thick compacts.

[0032] After the primary pre-fire hold period 418, the temperature of the compact is ramped down over a ramp-down period 420, in this case until the temperature again reaches room temperature 412. The compact then enters an optional rest period 422 during which the temperature is held at room temperature.

[0033] After the rest period 422, the temperature is increased again, this time reaching an intermediate pre-fire temperature 424. This temperature is optionally held for an intermediate pre-fire hold period 426. This period 426 is characterized, among other things, by the decomposition of pore-forming particles. Due to the prior removal of the binder during the first pre-fire, the decomposing pore-forming particles are more freely removable from the compact, helping to maintain the structural integrity of the compact by avoiding warping or cracking.

[0034] The temperature is then lowered to room temperature, and optionally held at room temperature for a period of time, after which the temperature is raised again to a final pre-fire temperature 428. This temperature may be held constant for an optional period of time 430 (sometimes referred to as a final pre-fire hold period). Period 430 is characterized, among other things, by the removal of residual organic matter from the compact. Due to the prior removal and decomposition of the binder material and pore-forming particles, respectively, residual organic material can be more easily removed from the compact during this period. The final pre-fire temperature 428 is high enough to effectively remove residual organic material from the compact, but not so high as to substantially sinter the compact. In this way, the compact can be free of residual organic matter.

[0035] After ramping down from the final pre-prefire temperature 428, the temperature is then optionally held at room temperature for a dwell period and then ramped up to the sintering temperature 432. Optionally, the compact may be held at the sintering temperature 432 for a sintering hold period 434 before the temperature is ramped down again.

[0036] It should be understood that the temperature history 400 of FIG. 4 is consistent with the method 100 of FIG. 1 and the methods 200 and 300 of FIGS. 2 and 3, respectively, but is illustrative. Accordingly, in other examples, there may be multiple primary, intermediate, and final pre-fire periods or multiple sintering periods, optionally separated by temperature ramp-down or rest periods. Furthermore, the temperature ramp-up and ramp-down periods need not be linear. The primary, intermediate, and final pre-fire and sintering holds need not be characterized by strictly constant temperatures. The temperature may exhibit variations other than monotonically increasing and decreasing during the temperature ramp-up and ramp-down periods, respectively. Furthermore, it is not necessary for all stages to reach room temperature 412 after each stage. Some of the variability of these parameters is discussed below in connection with FIG. 5.

[0037] FIG. 5 is a graph illustrating an exemplary temperature history 500 consistent with methods 100, 200, and 300 of FIGS. 1, 2, and 3, respectively. As an example of the variability described in connection with FIG. 4, temperature history 500 shows that primary pre-fire temperature 416 is maintained within a primary pre-fire temperature tolerance 536 during primary pre-fire hold period 518. Temperature tolerance 536 may be, for example, ±3°C, ±5°C, ±10°C, ±15°C, ±25°C, or ±50°C with respect to primary pre-fire temperature 416 in methods according to various specific embodiments. Similarly, intermediate pre-fire temperatures 424, final pre-fire temperatures 428, and sintering temperatures 432 may be maintained within specific tolerances, such as the exemplary tolerances noted with respect to primary pre-fire temperature 416.

[0038] Thus, removing the binder from the compact by heating the compact "at the primary pre-fire temperature," as used herein, should be understood to broadly indicate that the primary pre-fire temperature 416 is maintained within an acceptable range of nominal temperature values ​​throughout the hold period, and that the actual temperature during the primary pre-fire hold period always remains below the intermediate pre-fire temperature 424 and any lower acceptable ranges thereof. Similarly, decomposing the pore-forming particles by heating the compact "at an intermediate pre-fire temperature higher than the primary pre-fire temperature," as used herein, indicates that the actual temperature during the intermediate pre-fire hold period 426 is maintained within an acceptable range of the nominal intermediate pre-fire temperature 424. The actual temperature during the hold period 426 is always maintained above the primary pre-fire temperature and any upper acceptable ranges thereof, and below the final pre-fire temperature 428 and any lower acceptable ranges thereof.

[0039] Similarly, as used herein, removing residual organics from a compact by heating the compact "at a final pre-fire temperature higher than the intermediate pre-fire temperature" indicates that the temperature is maintained within an acceptable range of the nominal final pre-fire temperature 428 during the final pre-fire hold period 430, during which the actual temperature remains above the intermediate pre-fire temperature 424 and any upper tolerance limits thereof. Similarly, as used herein, sintering a compact "at a sintering temperature higher than the final pre-fire temperature" indicates that the temperature is maintained within an acceptable range at the sintering temperature 432 and above the final pre-fire temperature 428 and any upper tolerance limits thereof during the sintering hold period 434.

[0040] Temperature history 500 also illustrates that in the second stage, the intermediate pre-bake, the temperature ramp to the intermediate pre-bake temperature 424 is not linear. Similarly, the temperature ramp from the intermediate pre-bake temperature 424 is not linear. Temperature history 500 also illustrates that after the final pre-bake temperature 428, the final pre-bake temperature step ramps down to a temperature between room temperature and the final pre-bake temperature 428, rather than to room temperature 412.

[0041] Temperature history 500 further illustrates that during a non-monotonic temperature ramp-up period 514, the temperature is ramped up to sintering temperature 432. After reaching sintering temperature 432, the temperature is ramped back down to room temperature during a non-monotonic temperature ramp-down period 520. Non-monotonic temperature ramp-up period 514 includes ramp-downs and, generally, may include any number of temperature ramp-downs. Similarly, non-monotonic temperature ramp-down period 520 includes two localized temperature increases during the ramp down from sintering temperature 432, which may be intentional or may reflect characteristics of a particular heating device or process, but are less critical to the methods described herein for forming porous ceramic bodies. In other embodiments, there may be localized increases in temperature during the sintering or pre-firing period, for example, the temperature may reach a value higher than the nominal pre-firing or sintering temperature.

[0042] In some embodiments, the primary pre-fire temperature 416, characterized specifically by the removal of binder material, may be between 70°C and 390°C. More preferably, the primary pre-fire temperature 416 may be between 290°C and 390°C. In some embodiments, the primary pre-fire temperature 416 may be between 310°C and 370°C, with a nominal value of 340°C. In some embodiments, the intermediate pre-fire temperature 424, characterized by the decomposition of pore-forming particles, may be between 450°C and 550°C, for example. More specifically, in certain embodiments, the intermediate pre-fire temperature 424 is between 475°C and 525°C, with a nominal value of 500°C.

[0043] In certain embodiments, the final pre-fire temperature 428, characterized by removing residual organics from the compact, may be between 600°C and 800°C. In certain embodiments, the final pre-fire temperature may be between 675°C and 725°C, with a nominal value of, for example, 700°C. Like the primary, intermediate, and final pre-fire temperatures, the sintering temperature may depend on the specific composition of the compact, including the specific ceramic material powder used. These temperature values ​​may also depend on the size and shape of the compact. In certain embodiments, the sintering temperature may be between 1025°C and 1485°C. In other more specific embodiments, the sintering temperature 432 is between 1225°C and 1285°C, e.g., a nominal sintering temperature of 1255°C.

[0044] Further variability in pre-bake temperature profiles that may be applied in various methods according to embodiments is described in connection with FIGS.

[0045] 6(a)-(d) are a series of schematic phase diagrams illustrating particles of a compacted mixture or a porous ceramic body formed therefrom at various processing stages of a method according to embodiments. FIG. 6(a) illustrates a compacted mixture 638 during a primary pre-firing and pre-firing. The compacted mixture 638 is a mixture of ceramic material powder 640 (e.g., PZT powder), pore-forming particles 642 (e.g., PMMA microbeads), and a binder material 644 (e.g., wax). The binder material 644 can hold the mixture into a preform, for example, after injection molding. The mold used in the injection molding process can then be removed, and the compacted mixture can retain its desired shape for further processing. As the temperature rises to the primary pre-firing temperature, the binder 644 begins to be extracted, as indicated by arrow 646.

[0046] FIG. 6(b) illustrates the compact (designated compact 648 at this stage) before and during the intermediate pre-fire period 426 illustrated in FIG. 4. During the intermediate pre-fire period, the ceramic material powder particles 640 remain substantially intact and unchanged, while the pore-forming particles 642 are decomposed, as illustrated by arrows 650, while the compact 648 is heated to the intermediate pre-fire temperature and held for the holding period. The decomposed pore-forming particles (water and CO) are desirably separated from the matrix. Fresh air may be circulated through the oven to aid this process. While the decomposed pore-forming particles may join residual organic materials that may be removed in a final pre-fire stage, it is preferred that the decomposed pore-forming particles are substantially completely removed in the intermediate pre-fire stage. Circulating airflow can advantageously facilitate this process.

[0047] Figure 6(c) illustrates the compact (at this stage designated as compact 652) before and during the final pre-firing stage 430 illustrated in Figure 4. During this time, the ceramic material powder particles 640 are still largely unchanged, but residual organic material is further extracted from compact 652, as illustrated by arrows 654.

[0048] Figure 6(d) illustrates the grains of a porous ceramic body 656 formed from the compact 652 after the sintering period 434 illustrated in Figure 4. Exemplary pores 658 of the porous ceramic body 656 remain in place of the pore-forming particles 642, and the ceramic material powder 640 has transformed into largely consolidated, consolidated grains of sintered ceramic material 640' with the pores 658 still remaining.

[0049] FIG. 7 is a cross-sectional schematic diagram of a powder compact 638, 648, or 652 at each stage of processing as illustrated in the method 100 of FIG. 1, the temperature history 400 of FIG. 4, and the schematic phase diagrams of FIGS. 6(a)-(d). FIG. 7 optionally illustrates the powder compact 638, 648, or 652 placed on a base (setter) 760. In some embodiments, the extraction of binder material, decomposition of pore-forming particles, or removal of residual organic material, as illustrated in FIGS. 6(a), 6(b), and 6(c), respectively, may be performed with the powder compact placed on the base 760. In some embodiments, the powder compact may be placed on an Al2O3 base for sintering (i.e., the base 760 may be an Al2O3 base).

[0050] 7, which may aid in the extraction 646 of binder material, the decomposition 650 of pore-forming particles, or the removal 652 of residual organic matter from the compact 638, 648, or 652, respectively. The compact may be immersed in a sand bath during any of these heating / processing steps. The sand bath may include, for example, stabilized zirconia or zircon (ZrSiO4) sand, or other suitable materials that will be apparent to those skilled in the art in light of this specification.

[0051] FIG. 8 (Prior Art) is a cross-sectional view of a prior art acoustic transducer 800. The transducer 800 is formed from a porous ceramic body 856 that is not formed by injection molding or prepared according to methods 100, 200, or 300 of FIGS. 1, 2, or 3. Therefore, the ceramic body 856 is constrained in thickness 870, e.g., approximately 13 mm. Furthermore, the ceramic body 856 is ground at its upper and lower surfaces 864 to its final thickness 870. Because the ground surface 864 is porous at least after this surface grinding, a glass layer 866 is applied to the ground surface for smoothing purposes prior to forming the transducer electrodes with electrode layer 868.

[0052] The glass layer can be applied by screen printing, pad printing, or painting a glass material suspended in an organic matrix. The organic material is then fired off, and the glass is heated above its softening point, allowing it to flow and coat the surface. In contrast to FIG. 8, FIG. 9 illustrates an acoustic transducer 900 fabricated in accordance with methods 100, 200, and 300 of FIGS. 1, 2, and 3, respectively. The acoustic transducer 900 includes a sintered porous ceramic body 656 prepared according to the method illustrated in FIGS. 6(a)-(d). Due to the multi-step process involving extraction, decomposition, and removal of residual organics at each temperature, followed by sintering, the thickness 970 of the porous ceramic body 656 fabricated in accordance with the embodiment can be much greater than the thickness 870 of the prior art porous ceramic body 856 of FIG. 8.

[0053] Furthermore, as a natural by-product of the firing process (sintering) of the green compact 652 to form the porous ceramic body 656, the green compact 652 of FIG. 6(c) develops substantially smooth, substantially nonporous, opposing top and bottom surfaces 964 as it transforms into the porous ceramic body 656 of FIGS. 6(d) and 9 through sintering. As used herein, "substantially nonporous" means a surface free of spaces or holes greater than 1 micron in diameter through which liquids or gases can pass. These smooth surfaces 964 may be referred to herein as "skins" formed by the sintering process. Because polishing of the smooth surfaces 964 is not required when forming the porous ceramic body 656 according to the method described in connection with FIGS. 1-6, electrodes 868 can be formed directly on the smooth top and bottom surfaces 964 of the acoustic transducer 900 without applying the glass layer 866 shown in FIG. 8. This results in a cleaner and simpler manufacturing process. Forming methods such as pressing cannot produce parts with the precise uniform thickness that injection molding can produce. Workpieces formed by forming methods such as pressing must be ground flat and to final thickness.

[0054] In some embodiments, the thickness 970 of the porous ceramic body 656 is at least about 15 mm after fabrication according to the method of FIGS. 6(a)-6(d). In other embodiments, the thickness is at least about 20 mm after the sintering process. As used herein in the context of ceramic body thickness, "about" refers to a tolerance of ±1 mm in thickness. Thus, for example, "about 20 mm" means 20 ±1 mm. In some embodiments, the thickness is between about 15 mm and about 40 mm. In other embodiments, the thickness is between about 15 mm and about 30 mm. In yet other embodiments, the thickness is between about 20 mm and about 30 mm. It should be understood that the acoustic transducer 900 may be a piezoelectric or electrostrictive acoustic transducer. The process described in connection with FIGS. 1-7 is suitable for both types of acoustic transducers. The piezoelectric or electrostrictive properties can be selected by including the appropriate ceramic material powder in forming the green compact mixture, as will be understood by those skilled in the art in light of this specification.

[0055] FIG. 10 is a perspective view of an acoustic transducer array 1001 fabricated in accordance with method 200 of FIG. 2. The acoustic transducer 1001 exhibits 1-3 piezoelectric connectivity. Additionally, piezoelectric or electrostrictive acoustic transducers exhibiting 2-2 piezoelectric composite connectivity or other known connectivity may be fabricated using the disclosed method. The methods described herein are particularly advantageous when used with complex transducer arrays such as the array 1001. In the prior art, grooves 1074 had to be machined (i.e., ground) into a ceramic block. However, in methods according to embodiments, the porous ceramic body 656 of the acoustic transducer array 1001 can be formed from a green compact mixture including ceramic material powder, pore-forming particles, and a binder, which is injection molded to form a green compact that may be further processed according to methods according to embodiments described herein. This method significantly facilitates the fabrication of transducer arrays such as the array 1001.

[0056] The acoustic transducer array 1001 further includes a backing layer material 1072. The individual acoustic transducers 1000 are disposed between grooves 1074 extending from the backing material 1072, with the entire array of acoustic transducers 1000 being formed using an injection molding process. The grooves 1074 are also referred to herein as "trenches." The grooves 1074 may also be referred to as "voids" or "void spaces" before being filled with a polymeric or other matrix material to form the composite layer 1057. The backing layer material 1072 may be directly or indirectly bonded to the electroded transducers 1000. In the case of indirect bonding, for example, there may be one or more layers of other material(s) disposed between the electrode layer and the backing.

[0057] FIG. 11 is a cross-sectional view illustrating one acoustic transducer 1000 included in the array 1001 of FIG. 10. As shown in FIG. 11, the acoustic transducer 1000 includes one or more matching layers 1176 in addition to the backing layer material 1072. The one or more matching layers 1176 may be applied directly or indirectly to the electrode layer 868 after the porous ceramic body 656 has formed thereon the electrodes. If formed indirectly, one or more layers of other material(s) may be disposed between the matching layer and the electrode layer. It should be understood that the acoustic transducer 1000, or the array 1001 of FIG. 10, may be encapsulated in a polymeric material according to methods known in the art. The polymeric material used for encapsulation forms an insulating and protective barrier between the acoustic transducer 1000 and the environment.

[0058] When the transducer or array comprises a piezoelectric ceramic body, methods according to embodiments can further include poling the sintered porous ceramic body by applying an electric field, as will be understood by those skilled in the art in light of this specification. With appropriate formation of an acoustic transducer array, such as array 1001, electrode formation on porous ceramic body 656 can result in piezoelectric composite connectivity, such as 1-3 or 2-2 connectivity.

[0059] Further details applicable to specific embodiments Electromechanical transducers are used to convert electrical and mechanical energy into and from acoustical applications. Such applications include, but are not limited to, resonators, acoustic signal detectors, acoustic projectors, nondestructive testing, and ultrasonic imaging. Typically, the acoustically active elements of such transducers are made of piezoelectric ceramic materials such as lead zirconate titanate (PZT), lead magnesium niobate (PMN), or similar high-strength piezoelectric materials. (See, e.g., B. Jaffe et al., Piezoelectric Ceramics, Academic Press, London, NY, 1971, especially chapters 5, 7, and 8.)

[0060] For some applications, composites, consisting of piezoelectric ceramic fibers or sheets in a polymer or plastic matrix, can be more effective transducers than solid ceramic materials. Such piezoelectric ceramic-polymer combinations have been widely described in the literature, and some have found commercial applications. In particular, fine piezoelectric ceramic fibers or strips separated by an organic polymer phase have been used in medical ultrasound and acoustic nondestructive evaluation. Other applications include hydrophones, which are lightweight, large-area actuators used in underwater environments, where composite materials can provide improved acoustic coupling to water and body fluids compared to solid ceramic transducers. (See R.E. Newnham et al., "Composite Piezoelectric Transducers," Materials in Engineering 2, 93-106 (December 1980); J.A. Hossack et al., "Finite Element Analysis of 1-3 Composite Transducers," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 38[6]618-629 (November 1991). Newnham et al. are incorporated herein by reference.)

[0061] Many composite materials have been fabricated and evaluated in the laboratory. One successful combination involves a one-dimensionally connected ceramic phase (e.g., PZT fibers) within a matrix that is three-dimensionally connected by an organic polymer phase. This combination is known in the art as a 1-3 composite (Newnham et al., op. cit., p. 95). The hydrophone properties of this composite (Newnham et al., op. cit., p. 93) can be more than 10,000 times those of solid PZT ceramic by appropriate selection of the phase properties and composite structure. Another important configuration involves two-dimensionally connected bands of PZT ceramic separated by parallel bands of two-dimensionally connected polymer, known as a 2-2 composite (Newnham et al., op. cit., p. 95). This configuration is widely used in phased-array type transducers.

[0062] For high-frequency applications such as ultrasound imaging, the piezoelectric ceramic elements in the composite must have extremely small dimensions (e.g., 20-200 μm diameter) to achieve sufficiently high frequencies. This miniaturization of the composite array elements makes the composite extremely difficult to fabricate.

[0063] Early composites were fabricated by manually aligning extruded PZT ceramic rods in a jig, encapsulating them with epoxy resin, and then slicing them to the appropriate thickness and poling the ceramic rods (Newnham et al., op. cit., pp. 98-99). While this method demonstrated the performance advantages of the material, it also highlighted the difficulties associated with fabricating composites 1-3 on a large scale, even for prototyping purposes. These difficulties included (1) the need to align and support a large number of PZT fibers during polymer encapsulation, and (2) the high probability of encountering one or more defective fibers in typical large arrays, which increased the likelihood of dielectric breakdown during poling.

[0064] Several attempts have been made to demonstrate a more practical fabrication method. Automated fiber placement and assembly methods have been developed to assemble composites from extruded PZT fibers. This method works well for coarse composites with fibers approximately 0.5–1 mm in diameter, which are strong enough to be machined. For finer composites, a typical approach is to dice the ceramic composite structure from the solid ceramic using a wafer dicing saw. In this case, a portion of the solid ceramic piece is left intact to serve as support for the PZT fiber array (Hossack et al., op. cit., pp. 621–625). While this technique can be used to fabricate microstructures with ceramic elements smaller than 50 μm, the presence of defects in the ceramic and limitations of the sawing method itself result in low process yields as the element dimensions decrease. Furthermore, the limited linear cut makes it difficult to fabricate ceramic elements with unusual shapes.

[0065] Subsequent attempts to fabricate such composite transducers using a net-shape slip-casting process have been reported (U. Bast et al., "A new Technique for the Production of Piezoelectric Composites with 1-3 Connectivity," Ceramics Today-Tomorrow's Ceramics, Proc. 7th CIMTEC--World Ceramics Congress, Terme, Italy, June 24-30, 1990, Materials Science Monographs, 66C, P. Vincenzini, Ed., Elsevier Science Press, NY, 1991). In this slip-casting method, a disposable plastic tool containing holes is fabricated to serve as a template or support for forming a liquid slurry of PZT powder into the desired preform shape. The "preform" is also referred to herein as the "green compact." During the ceramic firing process, the plastic tool is burned out, leaving behind a porous PZT fiber array, which is sintered to obtain a dense product. However, this process has the disadvantage that a new plastic tool is required each time a ceramic part is produced, resulting in high mold costs.

[0066] Subsequently, a net-shape process was developed for producing fully dense ceramic preforms (also referred to herein as "ceramic bodies") for piezoelectric or electrostrictive composite transducers exhibiting 1-3 or 2-2 connectivity. The net-shape process involves preparing a homogeneous, granular thermoplastic powder / binder mixture containing a ceramic powder selected from a high-strength piezoelectric or electrostrictive ceramic material and a thermoplastic organic binder sufficient to render the mixture formable by injection molding. The binder can be selected so that its solids content renders the mixture thermoplastic during injection molding and allows it to be nondestructively removed from the mixture by heating. A preform green compact may be formed by injection molding the mixture in an injection molding apparatus containing a preform mold that is the negative of the preform green compact, and allowing the green compact in the preform mold to cool sufficiently to permit removal from the preform mold as a free-standing green compact.

[0067] The green compact may include a planar substrate and a plurality of parallel elements integral with the substrate, extending perpendicularly from the substrate in a preselected arrangement and fixedly supported by the substrate to maintain the arrangement. The planar substrate and the plurality of parallel elements are composed of a mixture of ceramic powder and binder material. Each element has a rod-like or lamellar shape. The self-supporting green compact is ejected from the preform mold and heated to a temperature of 300°C to 700°C for a time sufficient to completely remove the binder from the green compact. The debindered body can be sintered at a temperature and for a time sufficient to densify the body to at least about 95% of theoretical density, producing a fully dense preform.

[0068] In another aspect, the fully dense preform can be used to fabricate a piezoelectric or electrostrictive composite transducer exhibiting 1-3 or 2-2 connectivity and including a fully dense piezoelectric or electrostrictive ceramic phase. The method may further include encapsulating the fully dense elements of the preform in a polymeric material to form a two-phase ceramic / polymer composite having upper and lower planar surfaces. The lower planar surface may include a ceramic substrate. The ceramic substrate can be removed from the lower planar surface, exposing the fully dense elements at each of the upper and lower planar surfaces. The composite can be electroplated on the upper and lower surfaces so that electrodes establish electrical contact with opposing faces of each fully dense element. In a narrower aspect, the fully dense elements can be poled at a high electric field at a temperature just below the Curie temperature of the ceramic material for a time sufficient to produce a piezoelectric composite transducer exhibiting 1-3 or 2-2 connectivity.

[0069] In one aspect applicable to some embodiments, ceramic fiber or strip (lamella) preforms (for 1-3 or 2-2 composites, respectively) can be produced by injection molding a near-net-shape body using a reusable mold. After binder removal, the body can be sintered to produce the desired net-shape preform. To make piezoelectric or electrostrictive elements, the preform is filled into a polymer matrix and machined to remove excess material and the base. Electrodes are then formed using known methods. Poling of the ceramic phase can be performed using known methods to produce piezoelectric transducers.

[0070] In this exemplary method, injection molding is used as a means of forming PZT powder into the shape of a composite preform. Injection molding has been widely used in the plastics industry as a means of producing complex shapes in large quantities at low cost. The production of ceramics by injection molding techniques was subsequently developed and has been most successful for small cross-section features, such as screw guides, and large complex shapes that do not require sintering to high density, such as cast inserts for turbine blades.

[0071] Complex shapes can be fabricated with the ease and speed of conventional plastic molding by pressurizing a hot thermoplastic mixture of PZT ceramic powder and an organic binder into a cooled mold. PZT powders can be doped, usually with a few percent of donor or acceptor elements, to enhance their piezoelectric properties or tailor their properties to the intended application, but this is not required. Typical dopants include iron, niobium, lanthanum, and other rare earth elements. The powders are typically added to the mixture at high solids contents, typically greater than 50% by volume. The organic binder is nondestructively removed from the compact by slow debinding over a period of approximately 10–100 hours. Nondestructive binder removal is aided by the high solids loading in the injection-molded mixture. Sintering, poling, and polymer encapsulation after binder removal are performed in the same manner as for conventional PZT / polymer composites.

[0072] One method for producing such PZT ceramic preforms uses a molding die, allowing part design changes without incurring excessive rework costs. The elements are molded onto a monolithic ceramic substrate, which can be removed after the sintering and polymer encapsulation steps are complete. Individual preforms can be assembled together as needed (e.g., the base can be machined end-to-end to form larger arrays of PZT fibers or strips).

[0073] Figure 12A is a flowchart illustrating the process. The PZT powder may be milled, if desired, to achieve the desired particle size and distribution. The powder is then homogeneously mixed with a thermoplastic organic binder at a solids content of at least about 50% by volume in a heated mixer. The mixer must be heated to at least the binder's melting point. The binder is selected to deliver a hot thermoplastic mixture suitable for injection molding along with the powder, leave no residue harmful to the manufacturing process, and be removed by slow heating. Examples of suitable binders (also referred to herein as "binder materials") include waxes such as paraffin wax, polypropylene, and polyethylene. The mixture may be cooled and granulated prior to molding. The granulated mixture may be injection molded using conventional equipment equipped with specially designed molds. Typically, injection molding pressures are about 50 to 20,000 psi. Preferably, all surfaces in contact with the hot mixture are selected to be non-reactive with it.

[0074] Figure 12B is a flow chart illustrating a method according to an alternative embodiment in which the composite method includes poling, followed by composite casting, followed by electrode fabrication, in contrast to Figure 12A in which poling is performed after composite formation by encapsulation.

[0075] FIG. 13 is a schematic diagram illustrating an exemplary mold assembly 10. Tool body 12 includes a cavity portion 14 that surrounds cavity 16. Tool body 12 also includes a sprue portion 18 through which a sprue 20 enables filling of cavity 16 during the injection molding process. Removably positioned within cavity 16 is an interchangeable insert 22, which, together with cavity 16, defines a mold 24. Mold 24 is a negative of a desired preform, as described below. Conveniently, insert 22 may be one of a set of interchangeable inserts (not shown) of different shapes to mate with cavity 16 to provide molds of different designs.

[0076] The injection molding process may heat the mixture sufficiently to remelt the binder and allow the mixture to completely fill cavity 16 and conform to mold 24. Mold 24 may then be cooled sufficiently to produce a shape-retaining green compact having a shape similar to, but slightly larger than, the shape of the preform. Typical preform dimensions are approximately 50 x 50 mm. Some mold shrinkage is usually expected. In some cases, the size of the preform may be limited to minimize the possibility of shedding of the outermost elements during the cooling portion of the molding cycle. Such size limitations can be determined empirically. The production of larger transducers is discussed below. Of course, mold 24 is also designed to facilitate removal of the molded preform, producing preforms with straight-sided or tapered elements without interfering with removal of the preform from the mold. After the preform is demolded, mold 24 can be reused, with the same or different inserts, to produce another preform using the same injection molding process.

[0077] An injection-molded compact is shown in FIG. 14. This compact contains rod-shaped ceramic mixture elements arranged in a regular array on one flat surface of a ceramic mixture support substrate and extending perpendicularly outward therefrom. The individual elements are separated from one another by air gaps. The distance between the elements is the so-called "groove width." Extending outward from the opposite flat surface of the substrate are stems, not shown in FIG. 14, that conform to the shape of the mold sprue. The stems are typically removed immediately after molding. The size and shape of each element, the trench width, and the design of the arrangement of elements on the substrate in the injection-molded compact can be empirically determined to allow for shrinkage during the binder burnout (also referred to herein as "extraction") and densification steps, producing a sintered preform (ceramic body) of a shape selected according to known principles to obtain the desired transducer characteristics.

[0078] Two sintered preforms are also shown in Figure 14. Typical sintered preform cross-sectional shapes for rod-shaped elements include cylindrical, square, rectangular, triangular, and hexagonal, and the elements may be straight-sided or tapered. Typical element sizes are diameters, or equivalent diameters, of about 20-200 μm and aspect ratios (length-to-diameter ratios) of at least about 3:1 to about 20:1. The trench width is typically 20-5000 μm. The substrate thickness is selected to provide rigid support for the element array during processing, and the substrate typically extends beyond the outermost elements on each edge by about half the trench width.

[0079] The compact may be heat-treated at a temperature slightly above the decomposition temperature of the binder for a time sufficient to completely remove the binder. For example, for a paraffin wax binder, a typical burnout (extraction of the binder material) process involves temperatures of about 300°C to 700°C and a time period of about 10 to 100 hours. It is advantageous to successful preform production if the binder burnout is non-destructive (i.e., the binder removal is performed in a manner that does not cause fracture of the sintered preform or leave cracks or voids in the sintered body large enough to interfere with the operation of the transducer manufactured therefrom). As previously mentioned, such non-destructive removal depends on factors such as the choice of binder and the parameters selected for the binder burnout heating process.

[0080] The debindered preform can then be densified to full density, i.e., at least about 95%, preferably at least about 97%, of theoretical density, by conventional methods for producing fully dense ceramics. (See, e.g., Newnham et al., op. cit., p. 98; Bast et al., op. cit., 2012, p. 98; L.J. Bowen et al., ISAF92, 8th Int. Symposium on Applns. of Ferroelectrics, Clemson, SC, Sep. 1, 1992, Published March 1993; Bowen et al., incorporated herein by reference.) A typical heating cycle suitable for sintering the preform includes sintering at 1100°C to 1400°C for 0.1 to 4 hours in a PbO-rich atmosphere. Optionally, the preform may be further densified after sintering by hot isostatic pressing (HIPing), for example, at about 1100°C-1400°C and about 200-30,000 psi for about 0.1-4 hours. Conventional sinter-HIP processing using similar parameters is also a suitable method for densifying the preform.

[0081] The fully dense preform, containing the array of elements supported by the integral substrate, can then be encapsulated by known methods to form a second-phase matrix of hard or soft polymer that surrounds the elements and fills the voids between them. A typical encapsulation process involves pouring a freshly formulated epoxy resin mixture under vacuum over an inverted preform in a mold slightly larger and deeper than the preform, allowing the epoxy resin to solidify, and removing the resulting composite structure from the mold. The composite is then machined to expose the element ends, typically by removing the preform's supporting ceramic substrate. Such a 1-3 composite is shown in Figure 15.

[0082] The photograph in Figure 15 shows that the rod- or fiber-shaped piezoelectric elements from the preform are embedded in a parallel relationship to one another in the polymer matrix to form the 1-3 composite, which can then be electroded in a known manner by depositing conventional electrode materials, such as silver, gold, palladium, or conductive polymers, onto the planar surface to establish electrical contact with the ceramic elements.

[0083] To make the device highly piezoelectric, it may be poled by known methods, i.e., by applying a strong electric field to the element at a temperature slightly below the Curie temperature to form a polar axis (see, e.g., Newnham et al., op. cit., p. 102). Alternatively, the ceramic preform may be poled before being encapsulated in a polymer matrix and the bottom surface polished.

[0084] Alternatively, the preform mold 24 may be configured to produce a preform for a 2-2 composite. A typical ceramic preform for a 2-2 composite is shown schematically in Figure 16, which shows a preform 50 comprising an array of parallel planar ceramic elements 2 separated by parallel voids, or trenches 54. The planar elements 52 are supported by a planar ceramic substrate 56.

[0085] Figure 17 shows a photograph of a fine 2-2 composite preform, suitable for molding, sintering, and polymer encapsulation as described above for the 1-3 composite. PZT / polymer lamellar 2-2 composites fabricated in this manner have been observed to have ceramic layers as thin as 30 μm. The 2-2 composite can then be machined to remove the ceramic substrate, exposing the outer flat surfaces of the outermost elements, on which electrodes can be formed in a manner similar to that described above for the 1-3 composite. This composite may then be poled as described herein.

[0086] The individual preforms described above provide element arrays approximately 50 × 50 mm in size. However, larger arrays can be fabricated. In one approach, the outer edges of the ceramic substrates of multiple densified preforms are machined as needed to remove excess substrate material, allowing the substrates to be abutted in a coplanar arrangement, e.g., with adjacent edges butted, and then packed into an encapsulation mold of the required size. In another approach, the preforms are designed to tile seamlessly. After encapsulation, the substrates can be machined to leave the desired large array of encapsulated elements. The 1-3 composite shown in Figure 15 was fabricated in this manner using four ceramic preforms, each approximately 50 × 50 mm in size.

[0087] While the ceramic material described above is lead zirconate titanate (PZT), other materials such as lead magnesium niobate (PMN) and similar high-strength piezoelectric or electrostrictive materials, such as barium titanate, lead titanate, strontium titanate, bismuth titanate, and their derivatives, are also suitable for fabrication using the injection molding methods described herein. The appropriate matrix material depends on the intended application of the fabricated device and its ability to provide a monolithic matrix using the encapsulation process described above. Such materials include both rigid and flexible polymeric materials, such as epoxy resins, polyurethanes, and elastomers. [Example]

[0088] The following examples are presented to enable those skilled in the art to more clearly understand and to practice the disclosed embodiments, and are not to be construed as limitations on the scope of the disclosure, but merely as illustrative and representative thereof.

[0089] Piezoelectric composites (configurations 1-3) consisting of parallel-oriented PZT fibers in a polymer matrix were fabricated by injection molding using the method described. PZT powder (Morgan Matroc, Inc., Bedford, Ohio, designation PZT-5H) with a rare-earth donor was mixed with paraffin wax in a heated mixer at a temperature above the melting point of the wax for a sufficient time (approximately 1-8 hours) to form a homogeneous mixture. After cooling, the mixture was granulated into small pieces and placed in an injection molding machine equipped with a mold. The mold had an open-bottom cavity approximately 50 mm square, and housed an exchangeable tool containing 361 fibrous open cavities approximately 1 mm in diameter and 10 mm in length. This resulted in a cavity with the same shape as the negative of the preform to be molded. During injection molding, the PZT and wax mixture was remelted and completely filled into the cavity under pressure. The metal surfaces of the device exposed to the hot PZT mixture were hardfaced to minimize contamination of the mixture.

[0090] Following the molding step, the PZT / wax preform was removed from the mold, the stem formed by the sprue was removed, and the preform was placed in an oven where it was slowly heated over two days until the wax was completely burned off. The resulting porous PZT preform was then placed in a sealed ceramic crucible and heated at temperatures ranging from 1200 to 1300 °C until densification was complete, after which it was cooled and removed as a dense ceramic preform.

[0091] The preforms thus produced were then embedded in epoxy resin or polyurethane polymer, depending on the application, to complete the composite manufacturing process. In the final step, the ceramic substrate plate was polished, and metal electrodes were attached to the exposed ends of the PZT elements. Polarization was then achieved under a high electric field of 20–25 kV / cm at room temperature to 110°C.

[0092] Using this procedure, PZT ceramic preforms of shapes 1-3 and 2-2 were fabricated with PZT element dimensions of a minimum of 25 μm and a maximum width of 1.5 mm. The piezoelectric and dielectric properties of the injection-molded PZT piezoelectric ceramic 1-3 composite were compared with those of the die-pressed bulk material, as shown in the table below. As can be seen from the table, this method minimizes iron contamination while maintaining excellent piezoelectric and dielectric properties.

[0093] [Table 1]

[0094] This method uses reusable tool inserts that can be easily replaced to accommodate variations in preform design. The free-standing preform green compact is separated from the tool prior to binder removal and sintering. Full density can be easily achieved in the preform body using commercially available sintering methods with existing sintering equipment. Compared to other manufacturing methods, this method offers several advantages: complex near-net-shape capabilities that allow for many fibers to be processed simultaneously; high-speed processing (typically minutes per part); flexibility regarding transducer design (variations in PZT element spacing and shape); and cost savings for medium-volume production by reusing the tooling used to form the part, spreading the cost across many parts.

[0095] 18 is a graph illustrating a primary pre-fire temperature profile that may be used in a method according to an exemplary embodiment. The graph shows temperature (°C) as a function of time (arbitrary units). After a ramp-up period 1814 from room temperature, the temperature of the compact is held at approximately 335°C for a primary pre-fire hold period 1818. After the hold period 1818, the temperature of the compact is again lowered to room temperature during a ramp-down period 1820.

[0096] Figure 19 is a graph illustrating a temperature profile that may form part of the temperature portion of a pre-firing stage in an embodiment of a method for preparing a porous ceramic body. In Figure 19, a pre-firing hold period 1818 is illustrated, in which, following the temperature ramp-down period 1820 shown in Figure 18, the temperature of the compact is increased to about 700°C for a final pre-firing step, where the final pre-firing temperature is held for a final pre-firing hold period 1930. The temperature is then ramped down to room temperature.

[0097] Figure 20 is a graph showing all three pre-fire periods that may be used in an embodiment method for preparing a porous ceramic body for an acoustic transducer. Following the pre-fire temperature profile shown in Figure 18, the temperature is increased from room temperature to an intermediate pre-fire temperature, in this case about 500°C, and held for an intermediate pre-fire hold period 2026. After hold period 2026, the temperature is decreased again to room temperature. Following this decrease, the temperature is increased again, this time to a final pre-fire temperature of about 700°C, and held at the final pre-fire temperature for a final pre-fire hold period 1930, before decreasing to room temperature, all in preparation for the sintering (firing) process.

[0098] The various pre-firing periods need not be separated by a temperature decrease between them. Advantageously, in some embodiments, the green compact may be pre-fired according to all pre-firing periods described herein in the same oven. Thus, there need not be a temperature decrease to room temperature or other temperature between successive pre-firing periods.

[0099] Figure 21 is a graph illustrating this option, showing the temperature history for the pre-fire, intermediate, and final firing periods of a compact, each performed in the same oven without a temperature drop between pre-fire periods. In Figure 21, after the temperature is increased to the first pre-fire temperature and held there for the first pre-fire hold period 1818, the temperature is increased directly to the intermediate pre-fire temperature, approximately 500°C, and held there for the intermediate pre-fire hold period 2026. After the hold period 2026, the temperature of the oven environment in which the compact is held is increased directly to the final pre-fire temperature, approximately 700°C, and held there for the final pre-fire hold period 1930. In this manner, the method is easier to implement.

[0100] FIG. 22 is a schematic diagram illustrating a built-in forced-air convection oven 2288 that may be advantageously employed in methods according to embodiments. In particular, extraction of binder material, decomposition of pore-forming particles, removal of residual organic material, and sintering of the green compact may all occur in the same built-in oven 2288, thus significantly facilitating implementation of methods according to embodiments described herein. In particular, FIG. 22 illustrates an existing forced-air convection oven 2280 in which a blower fan 2282 induces circulation 2284 within the oven. As described herein, air circulation may be advantageously employed to promote decomposition of pore-forming particles and the resulting removal of water and carbon dioxide from the green body matrix, for example, by circulating fresh air within the oven. FIG. 22 also illustrates an existing oven 2286 that may be used to heat the green compact for sintering, for example, for the sintering step to form a porous ceramic body.

[0101] When the functions of the forced air convection oven 2280 and the furnace 2286 are combined in a forced air convection furnace 2288, in part by including a blower fan 2282 and resulting circulation 2284, the pre-firing and sintering steps can be performed in the same furnace. This minimizes or eliminates cooling and other intermediate steps in the process, from forming a green compact, e.g., by injection molding, through the sintering step, according to embodiments. In this manner, the temperature profile illustrated in FIG. 21 can be extended so that there is no cooling period between the final pre-firing hold 1930 and the sintering step, not illustrated in FIG. 21 but illustrated in, e.g., FIGS. 4-5.

[0102] While we have shown and described what are presently considered to be the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the appended claims.

[0103] The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

[0104] Although exemplary embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the scope of the embodiments encompassed by the appended claims. The present invention includes the following embodiments. [Aspect 1] 1. A method of making a porous ceramic body, comprising: forming a powder compact by mixing a ceramic material powder, a binder material, and pore-forming particles; heating the powder compact at a primary pre-firing temperature to remove the binder material from the powder compact; heating the powder compact at an intermediate pre-firing temperature higher than the primary pre-firing temperature to decompose the pore-forming particles; removing residual organics from the compact by heating the compact at a final pre-firing temperature that is higher than the intermediate pre-firing temperature; sintering the powder compact at a sintering temperature greater than the final pre-fire temperature to form a porous ceramic body; A method comprising: [Aspect 2] In the method of embodiment 1, The step of forming the powder compact comprises: injection molding said mixture. Aspect 3 In the method according to aspect 2, The method for forming into a net shape by injection molding. Aspect 4 A method according to any one of aspects 1 to 3, comprising: The method wherein the porous ceramic body is a piezoelectric composite. Aspect 5 A method according to any one of aspects 1 to 4, comprising: The method wherein the ceramic material powder is a PZT powder. Aspect 6 A method according to any one of aspects 1 to 5, comprising: The method wherein the binder material is a wax. Aspect 7 10. The method according to any one of aspects 1 to 6, The method wherein the pore-forming particles are PMMA particles. Aspect 8 A method according to any one of aspects 1 to 7, comprising: The method wherein the pore-forming particles have an average diameter of 5 μm or more and 15 μm or less. A method according to any one of aspects 1 to 8, comprising: The method, wherein the primary pre-baking temperature is 290°C or higher and 390°C or lower. Aspect 10 A method according to any one of aspects 1 to 9, comprising: The method, wherein the primary pre-baking temperature is 310°C or higher and 370°C or lower. Aspect 11 11. The method according to any one of aspects 1 to 10, The method, wherein the intermediate pre-baking temperature is 450°C or higher and 550°C or lower. Aspect 12 12. The method according to any one of embodiments 1 to 11, The method wherein the intermediate pre-baking temperature is 475°C or higher and 525°C or lower. Aspect 13 13. The method according to any one of aspects 1 to 12, The method, wherein the final pre-baking temperature is 600°C or higher and 800°C or lower. Aspect 14 14. The method according to any one of embodiments 1 to 13, The method wherein the final pre-baking temperature is 675°C or higher and 725°C or lower. Aspect 15 15. The method according to any one of embodiments 1 to 14, The sintering temperature is 1025°C or higher and 1485°C or lower. Aspect 16 16. The method according to any one of embodiments 1 to 15, The sintering temperature is 1225°C or higher and 1285°C or lower. Aspect 17 16. The method according to any one of embodiments 1 to 15, The method further comprises sealing the green compact in a Pb atmosphere after the final pre-firing step and before the sintering step. Aspect 18 18. The method according to any one of embodiments 1 to 17, At least one of removing binder material, decomposing pore-forming particles, and removing residual organic matter is performed by treating the compact with stabilized zirconia or zircon (ZrSiO 4 ) a method in which the material is immersed in a sand bath. Aspect 19 19. The method according to any one of the preceding aspects, The sintering of the green compact is carried out by subjecting the green compact to Al 2 O 3 A method carried out on a base. Aspect 20 An acoustic transducer comprising a porous ceramic body made by the method of any one of embodiments 1 to 19. Aspect 21 the porous ceramic body, after being made according to the method of embodiment 1, has a thickness of at least about 15 mm. 21. The acoustic transducer of claim 20. Aspect 22 The porous ceramic body, after being made according to the method of embodiment 1, has a thickness of at least about 20 mm. 22. The acoustic transducer of claim 21. Aspect 23 The compact is sintered to 65% to 95% of the theoretical density. The acoustic transducer according to any one of aspects 20 to 22. Aspect 24 A method of making a piezoelectric or electrostrictive acoustic transducer, comprising: providing a porous ceramic body according to the method of any one of aspects 1 to 19; forming electrodes on opposing surfaces of the sintered porous ceramic body to form a piezoelectric or electrostrictive acoustic transducer. Aspect 25 25. The method of embodiment 24, wherein forming a green compact comprises injection molding the mixture. Aspect 26 26. The method of embodiment 25, wherein the injection molding is performed to a net shape. Aspect 27 The method of any one of embodiments 24-26, comprising forming electrodes on both substantially non-porous surfaces of the sintered porous ceramic body. Aspect 28 In the method according to any one of aspects 24 to 27, The method further comprising attaching at least one backing or matching layer to the acoustic transducer. Aspect 29 In the method according to any one of aspects 24 to 28, The method further comprising encapsulating the acoustic transducer in a polymeric material. Aspect 30 In the method according to any one of aspects 24 to 29, the sintered porous ceramic body is a piezoelectric ceramic body, The method further comprising poling the sintered porous ceramic body by applying an electric field. Aspect 31 In the method according to any one of aspects 24 to 30, By forming the electrode, The method, wherein the piezoelectric or electrostrictive acoustic transducer exhibits a piezoelectric composite connectivity of 1-3 or 2-2. Aspect 32 A piezoelectric or electrostrictive acoustic transducer produced by the method according to any one of aspects 24 to 31. Aspect 33 1. A method of making a porous ceramic body, comprising: heating a green compact formed by injection molding a mixture of a ceramic material powder, a binder material, and pore-forming particles, from which the binder material has been substantially removed after injection molding, to decompose the pore-forming particles from the green compact; after dissolving the pore-forming particles from the green compact, sintering the green compact to form a porous ceramic body. Aspect 34 decomposing the pore-forming particles by heating the green compact at an intermediate pre-fire temperature, and sintering by heating the green compact at a sintering temperature higher than the intermediate pre-fire temperature; 34. The method of embodiment 33, further comprising, after decomposition and prior to sintering, heating the compact at a final pre-fire temperature that is greater than the intermediate pre-fire temperature and less than the sintering temperature to remove residual organic material from the compact. Aspect 35 35. An acoustic transducer comprising a porous ceramic body made by the method of embodiment 33 or embodiment 34. [Explanation of symbols]

[0105] 50 preforms 52 Flat element 54 Trench 56 Flat substrate 412 Room temperature 416 Primary pre-firing temperature 424 Intermediate Pre-baking Temperature 428 Final pre-baking temperature 432 Sintering temperature 638 mixture 640 Ceramic material powder 642 Pore-forming particles 644 Binder Materials 638, 648, 652 Compacted Powder 656 Porous ceramic body 658 Stoma 760 Stand 762 Sand bath 868 Electrode 900, 1000 Acoustic Transducer 1001 Acoustic Transducer Array 1072 Backing layer material 1176 Matching layer material

Claims

1. 1. A method of making a porous ceramic body, comprising: forming a powder compact by mixing a ceramic material powder, a binder material, and pore-forming particles; heating the powder compact at a primary pre-fire temperature to remove the binder material from the powder compact; heating the powder compact at an intermediate pre-firing temperature higher than the primary pre-firing temperature to decompose the pore-forming particles; removing residual organics from the powder compact by heating the powder compact at a final pre-fire temperature that is higher than the intermediate pre-fire temperature; sintering the powder compact at a sintering temperature greater than the final pre-fire temperature to form a porous ceramic body; forming electrodes on opposing surfaces of the sintered porous ceramic body to form a piezoelectric or electrostrictive acoustic transducer; A method comprising:

2. 10. The method of claim 1, The step of forming the powder compact comprises: injection molding said mixture.

3. 3. The method of claim 2, The method for forming into a net shape by injection molding.

4. 4. The method according to claim 1, wherein The method wherein the porous ceramic body is a piezoelectric composite.

5. 5. The method according to claim 1, wherein The method wherein the ceramic material powder is a PZT powder.

6. 6. The method according to any one of claims 1 to 5, The method wherein the binder material is a wax.

7. 7. The method according to any one of claims 1 to 6, The method wherein said pore-forming particles are PMMA particles.

8. 8. The method according to any one of claims 1 to 7, The method wherein the pore-forming particles have an average diameter of 5 μm or more and 15 μm or less.

9. 9. The method according to any one of claims 1 to 8, The method, wherein the primary pre-baking temperature is 290°C or higher and 390°C or lower.

10. 10. The method according to any one of claims 1 to 9, The method, wherein the primary pre-baking temperature is 310°C or higher and 370°C or lower.

11. 11. The method according to any one of claims 1 to 10, The method, wherein the intermediate pre-baking temperature is 450°C or higher and 550°C or lower.

12. 12. The method according to any one of claims 1 to 11, The method wherein the intermediate pre-baking temperature is 475°C or higher and 525°C or lower.

13. 13. The method according to any one of claims 1 to 12, The method, wherein the final pre-baking temperature is 600°C or higher and 800°C or lower.

14. 14. The method according to any one of claims 1 to 13, The method wherein the final pre-baking temperature is 675°C or higher and 725°C or lower.

15. 15. The method according to any one of claims 1 to 14, The method, wherein the sintering temperature is 1025°C or higher and 1485°C or lower.

16. 16. The method of any one of claims 1 to 15, The method, wherein the sintering temperature is 1225°C or higher and 1285°C or lower.

17. 17. The method of any one of claims 1 to 16, The method further comprises sealing the green compact in a Pb atmosphere after the final pre-firing step and before the sintering step.

18. 18. The method of any one of claims 1 to 17, At least one of removing binder material, decomposing pore-forming particles, and removing residual organic matter may be performed by treating the compact with stabilized zirconia or zircon (ZrSiO ４ ) a method in which the material is immersed in a sand bath.

19. 19. The method of any one of claims 1 to 18, The sintering of the green compact is carried out by subjecting the green compact to Al ２ O ３ A method carried out on a base.

20. 20. The method of claim 1, further comprising the step of: the porous ceramic body after sintering has a thickness of at least about 15 mm; A method for manufacturing an acoustic transducer.

21. The porous ceramic body after sintering has a thickness of at least about 20 mm. A method for manufacturing an acoustic transducer according to claim 20.

22. The compact is sintered to 65% or more and 95% or less of the theoretical density. A method for manufacturing an acoustic transducer according to claim 20 or 21.

23. A method of making a piezoelectric or electrostrictive acoustic transducer, comprising:

20. A method comprising providing a porous ceramic body according to any one of claims 1 to 19.

24. 24. The method of claim 23, wherein forming a green compact comprises injection molding the mixture.

25. 25. The method of claim 24, wherein the injection molding is to a net shape.

26. 26. The method of any one of claims 23 to 25, comprising forming electrodes on both substantially non-porous surfaces of the sintered porous ceramic body.

27. 27. The method of any one of claims 23 to 26, The method further comprising attaching at least one backing or matching layer to the acoustic transducer.

28. 28. The method of any one of claims 23 to 27, The method further comprising encapsulating the acoustic transducer in a polymeric material.

29. 29. The method of any one of claims 23 to 28, the sintered porous ceramic body is a piezoelectric ceramic body, The method further comprising poling the sintered porous ceramic body by applying an electric field.

30. 30. The method of any one of claims 23 to 29, By forming the electrode, The method wherein the piezoelectric or electrostrictive acoustic transducer exhibits a piezoelectric composite connectivity of 1-3 or 2-2.

31. 1. A method of making a porous ceramic body, comprising: heating a green compact formed by injection molding a mixture of ceramic powder, binder material, and pore-forming particles to dissolve the pore-forming particles from the green compact; removing residual organic matter from the compact after decomposition of the pore-forming particles; a step of decomposing the pore-forming particles from the green compact, removing residual organic matter, and then sintering the green compact to form a porous ceramic body; forming electrodes on opposing surfaces of the sintered porous ceramic body to form a piezoelectric or electrostrictive acoustic transducer; Including, the binder material is substantially removed from the injection molded green compact prior to the step of decomposing the pore-forming particles; method.

32. decomposing the pore-forming particles by heating the green compact at an intermediate pre-firing temperature, and sintering by heating the green compact at a sintering temperature higher than the intermediate pre-firing temperature; 32. The method of claim 31 , wherein removing residual organics from the compact is accomplished after decomposition of pore-forming particles and prior to sintering by heating the compact to a final pre-fire temperature that is higher than an intermediate pre-fire temperature and lower than the sintering temperature.

33. 33. A method of manufacturing an acoustic transducer, comprising the step of making a porous ceramic body by the method of claim 31 or claim 32.

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