JPH07232974A - Porous piezoelectric ceramic element and its production - Google Patents

Abstract

PURPOSE:To obtain a porous piezoelectric ceramic element having acoustic impedance reduced to a level equal to that of the acoustic impedance of a living body or water and also having such strength as to make the element withstand practical use. CONSTITUTION:Foamed polystyrene balls as a pore forming material are mixed with an aq. slurry of piezoelectric ceramic powder contg. a hardenable resin and the resulting mixture is hardened and fired to produce the objective porous piezoelectric ceramic element having spherical pores of 0.05-5mm pore diameter at 50-85% porosity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous piezoelectric ceramic element used in a medical diagnostic apparatus, a fish finder, a sonar, etc., and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, various piezoelectric ceramics such as lead zirconate titanate-based ceramics have been used as piezoelectric ceramics element materials for medical diagnostic devices for fish and water, fish finder, sonar, etc. Material is used. However, these materials have an acoustic impedance of about 30 × 10 ⁶ kg / m ² S, which is significantly higher than the acoustic impedance of living organisms and water (about 1.5 × 10 ⁶ kg / m ² S). The sound wave that propagates through is reflected at the element interface,
There is a problem that it hinders transmission and reception. Therefore, by making the piezoelectric ceramic porous, the acoustic impedance is reduced and the compatibility with the living body and water is taken.

The piezoelectric ceramic element made porous is formed by utilizing an organic three-dimensional network structure such as urethane foam (Japanese Patent Laid-Open No. 61-13697).
No. 3, etc.), three-dimensionally assembled rod-shaped or linear piezoelectric ceramics (JP-A-2-57099, etc.), spherical piezoelectric ceramics moldings, and
Examples include baked products. However, although the elements of and have a high porosity, they have low mechanical strength and therefore cannot be used practically. Also, the element of
Since the porosity cannot theoretically exceed about 25% which is the porosity of the close-packed body, it is difficult to reduce the acoustic impedance of the element to the same level as the acoustic impedance of the living body or water. Therefore, under the present circumstances, the conventional element and the manufacturing method thereof cannot provide a porous piezoelectric ceramic element having high compatibility with living bodies and water and high mechanical strength.

[0004]

The present inventor has completed the present invention as a result of intensive studies in view of the above problems, and its purpose is to equalize the acoustic impedance of a living body or water. Has a level of acoustic impedance,
Another object of the present invention is to provide a porous piezoelectric ceramic element having sufficient mechanical strength and a method for manufacturing the same.

[0005]

The above-mentioned element is achieved by a porous piezoelectric ceramic element having spherical pores having a pore diameter of 0.05 to 5 mm and a porosity of 50 to 85%. The method of manufacturing an element for the purpose is to mix foamed polystyrene balls that are pore-forming materials and an aqueous slurry of piezoelectric ceramic powder containing a curable resin, then cure the mixture, and then fire it. And a method for manufacturing a porous piezoelectric ceramic element.

What is important in the present invention is that the acoustic impedance of a living body or a living body is improved by increasing the mechanical strength of the porous material by forming it into spherical pores and by strictly controlling the pore diameter and porosity. This is a reduction to the same level as the acoustic impedance of water.

The pore shape of the present invention is spherical. If spherical pores are not used, the mechanical strength will be low, so that even a small impact force may damage the element itself. In other words, the spherical pores can achieve a high porosity with the same strength, and as a result, the acoustic impedance can be sufficiently reduced.

The pore size in the present invention is 0.05 to 5 mm.
Is. The porosity can be increased by using spherical pores. A pore diameter of less than 0.05 mm is difficult to manufacture due to the manufacturing method. On the other hand, if the pore diameter exceeds 5 mm, the mechanical strength of the obtained element decreases, and it cannot be put to practical use.

The porosity of the device of the present invention is 50 to 85%. If the porosity is less than 50%, the reduction of acoustic impedance is insufficient. On the other hand, when the porosity exceeds 85%, the mechanical strength is lowered and it cannot be put to practical use.

The piezoelectric ceramic material forming the skeleton of the device of the present invention includes lead titanate, lead zirconate,
Lead zirconate titanate, lead magnesium niobate, barium titanate, cobalt niobate, lithium niobate,
Examples thereof include lithium tantalate. Among these, lead zirconate titanate having a high Curie temperature and high piezoelectricity is particularly preferable.

The method of manufacturing the device of the present invention will be described below. The manufacturing method of the present invention prepares an aqueous slurry comprising at least a piezoelectric ceramic powder, a curable resin, and water, and after mixing the slurry with foamed polystyrene balls having a desired pore diameter and a curing agent, curing The mold resin is cured, and then, the steps of drying, degreasing and firing are performed to produce a porous piezoelectric ceramic element.

As the piezoelectric ceramic powder in the present invention, any powder may be used as long as it is a powder of the above-mentioned commonly used piezoelectric ceramic material. The particle size of the piezoelectric ceramic powder is preferably such that it can be easily slurried by mixing with water. Specifically, a powder having a central particle diameter of 0.1 to 10 μm is preferable. If it is less than 0.1 μm, the amount of water required for the slurry is large, and the viscosity thereof is too high, so that there is a problem in working such as trouble in using the slurry, which is not preferable. On the other hand, when it exceeds 10 μm, the powder is precipitated in the slurry and cannot be uniformly dispersed, or the sinterability is deteriorated, which is not preferable.

The pore forming material applicable to the manufacturing method of the present invention is expanded polystyrene spheres. Since the foamed polystyrene balls are composed of a small amount of organic matter in comparison with the volume thereof, there is an advantage that degreasing can be performed more smoothly than other organic matter. Foamed styrene spheres are produced by treating beads in which a foaming gas such as butane is contained in polystyrene spheres in steam at 100 to 140 ° C. or in a warmer for 3 minutes to 20 hours. The expansion ratio is preferably 5 times or more. If the expansion ratio is too low, degreasing becomes difficult and the porosity cannot be increased. In the present invention, the expanded polystyrene balls will be simply referred to as expanded polystyrene balls hereinafter.

Since the Styrofoam spheres are used as a pore-forming material, they do not remain in the final product element and are all removed during the manufacturing process. Examples of the removal method include a method of using a solvent that dissolves Styrofoam spheres, a method of incinerating Styrofoam spheres by heat treatment, and the like.
In order to manufacture the porous piezoelectric ceramics element of the present invention, a paraffinic solvent that dissolves a small amount of styrofoam spheres at 30 to 60 ° C. is coated on the styrofoam sphere surface in advance, and the styrofoam sphere is treated at 30 to 60 ° C. It is preferable to dissolve and completely incinerate and remove in a degreasing step described later.

The Styrofoam spheres are prepared by using beads obtained by allowing a foaming gas such as butane to be contained in polystyrene spheres as described above in steam at 100 to 140 ° C. or in a warmer.
It is produced by treating for 20 minutes to 20 hours. This foaming treatment is preferably carried out until the foaming gas is completely removed. If the foaming gas remains, it is feared that the styrene spheres will be further expanded and the skeleton of the molded body will be damaged during the subsequent heat treatment such as drying of the molded body. Specifically, it is foamed by a heat treatment at 100 to 140 ° C., but with the lapse of the treatment time, the polystyrene once largely foams, and after the foaming gas is completely removed, the polystyrene foaming stops and then gradually shrinks. To do. Therefore, it is preferable to confirm that the shrinking stage has been reached, complete the foaming treatment, and use the styrene balls.

The Styrofoam spheres are then mixed with the slurry to become a unit and cure in a mold at 30-60 ° C.
At that time, stress acts on the Styrofoam sphere. If the molded body is taken out from the mold with this stress applied, the skeleton of the molded body is damaged. Therefore, in order to relieve the stress, it is preferable to coat the surface of the styrofoam spheres with a paraffinic solvent in advance so that the styrofoam spheres dissolve in the cured state in the mold at 30 to 60 ° C. as described above. Styrofoam spheres start to dissolve at a relatively low temperature of 30 to 60 ° C. by coating with a paraffinic solvent.

The particle size of the Styrofoam spheres can be appropriately controlled to a desired particle size depending on the size of the polystyrene spheres, the content of the foaming gas and the foaming conditions. In addition, as described above, after the foaming gas is completely removed, 100 to 100
It is also possible to obtain a desired particle size by utilizing the fact that styrene balls gradually shrink when heated at 140 ° C. still,
Since the particle size of the Styrofoam spheres determines the pore size of the final product, it is preferable to classify the particle sizes in advance by classifying in order to obtain the desired pore size. Further, since the element shrinks by firing along with firing, it is preferable to appropriately select this particle size in consideration of the rate of firing shrinkage in advance.

The aqueous slurry in the present invention comprises at least the above-mentioned piezoelectric ceramic material powder, a curable resin and water as a dispersion medium, and is used to disperse the piezoelectric ceramic powder efficiently and stably as required. Peptizer,
Viscosity adjusting agent for making the workability of the slurry suitable, drying rate adjusting agent such as ethylene glycol or polyethylene glycol, defoaming agent or defoaming agent for reducing foaming property, pH
A regulator etc. can be contained. The piezoelectric ceramic powder slurry is prepared according to a conventional method by using a dispersing device such as a ball mill or an attritor.

The curable resin in the present invention is in a slurry containing piezoelectric ceramic powder, and enhances the skeleton strength by the curing action of the curing agent as it is added. As the curable resin, a crosslinking reaction type resin that forms a three-dimensional network bond by reaction is preferable. Specifically, vinyl resins such as acrylic and vinyl acetate, epoxy, phenol,
Examples include soluble or dispersed resins such as urea, melamine, and urethane. Of these, water-soluble epoxy resins that cause a crosslinking reaction in the alkaline region where the peptizer effectively acts are preferable.

The content of the curable resin is 1 to 20 parts by weight, preferably 3 to 10 parts by weight, based on 100 parts by weight of the piezoelectric ceramic powder. If the content of the resin is too small, the strength of the molded body after curing becomes small, while if the content is too large, it becomes difficult to degrease this resin component,
As a result, in either case, the strength of the final product is reduced.

In the present invention, the aqueous slurry of piezoelectric ceramic powder and the Styrofoam spheres are mixed by simply stirring and mixing the Styrofoam spheres and the slurry in a container and then injecting them into a mold or in advance. There are various methods such as filling expanded polystyrene balls in a container and pouring the slurry into the voids.

The mixture enhances its strength by the curing action of the curable resin in the slurry, and then evaporates water as a dispersion medium. Specifically, the resin is left to cure in a mold at 30 to 60 ° C. for 0.5 to 10 hours so that water as a dispersion solvent does not evaporate. After that, the molded body is taken out from the mold and gradually dried at 60 to 120 ° C. for 10 to 200 hours. In order to slow the evaporation of water as much as possible, it is preferable that the initial humidity of the drying is 80% or higher.

The molded body which has been sufficiently dried by increasing the skeleton strength through the curing reaction is then degreased and fired. Degreasing and firing are carried out by a conventional method. In the degreasing step, the organic substances such as the binder and Styrofoam spheres are removed by raising the temperature to 500 to 600 ° C. at a relatively moderate temperature raising rate. At this time, it is possible to flow fresh air into the furnace to promote it. When using piezoelectric ceramics that evaporate at high temperatures such as lead during firing, fill them in a sealed container to compensate for this, or in powder that generates evaporative substances,
It is preferable to bake at a predetermined temperature (usually 1200 to 1350 ° C.).

[0024]

EXAMPLES The present invention will be specifically described with reference to the following examples. Example 1 First, a styrofoam raw material (Eslen beads, HJ, manufactured by Sekisui Plastics Co., Ltd.) was foamed at 110 ° C. for 7 minutes, and after confirming that the foaming gas was completely removed, a diameter of 0.5 was obtained.
It was classified to mm. Next, foamed Styrole spheres (foaming ratio 10 times) were coated on the surface with liquid paraffin (Wako reagent). Then, after vibrating and filling into a polypropylene female container of 50 × 100 × 100 mm size, 40 × 100 × 100 mm with a male type having an injection port in the center
A styrofoam molded body was prepared by compressing to a size.

Next, a slurry of piezoelectric ceramic powder having the following composition was prepared using a ball mill. [Slurry composition] PZT (lead zirconate titanate) powder 100 parts by weight (PE-650 Fuji Titanium Industry Co., Ltd.) Crosslinking type water-soluble epoxy resin 10 parts by weight (EX-421 Nagase Kasei Co., Ltd.) Water 18 parts by weight Dispersant (polycarboxylic acid ammonium salt) 1 part by weight Curing agent (triethylenetetramine) 1.5 parts by weight ...

The hardener was mixed in a ball mill and then mixed in a state where the slurry was cooled to 5 ° C. or lower. The prepared slurry was quickly injected from the upper injection port of the Styrofoam molded body prepared in advance. Then, it is cured at 50 ° C for 3 hours, the molded body is taken out of the mold, and the temperature is 70 ° C and the humidity is 90%.
10 hours, temperature 90 ° C, humidity 90%, 10 hours, temperature 1
A molded body was obtained by allowing to stand at 10 ° C. and 80% humidity for 20 hours. It was confirmed that the obtained molded product was sufficiently dried, and the styrofoam was also sufficiently shrunk so that the skeleton was not damaged. Then, it was fired at 1230 ° C. for 2 hours to obtain a porous fired body. The temperature rising rate during firing was 1 ° C./min up to 400 ° C. accompanied with degreasing, and 3 ° C./min thereafter. The porosity, pore diameter, acoustic impedance and bending strength of the obtained device were measured by the methods described below. For comparison, the devices proposed hitherto were prototyped using the same powder, and their performances were also measured. The results are shown in Table 1.

[Porosity] The porosity of the sintered body was calculated based on the following formula. Porosity = (1-ρ / ρt) × 100 (%) ρ: Bulk density of sintered body (kg / m ³ ) ρt: Theoretical density of used ceramics (kg / m ³ ) [Pore size] For optical microscope I observed it. [Acoustic Impedance] A test piece was cut into a size of 6 × 6 × 15 mm, a silver electrode was applied to the opposite surface, and baked at 800 ° C. 1 to this in silicone oil at 120 ℃
Polarization treatment was performed at 2 kv / mm for 1 hour to produce a porous piezoelectric ceramic element. The resonance frequency was measured using this element, and the acoustic impedance was calculated based on the following formula. Acoustic impedance = 2 × l × fr × ρ l: Length of vibration propagation direction (m) fr: Resonance frequency (Hz) ρ: Bulk density of sintered body (kg / m ³ ) [Bending strength] 6 × 8 The test piece was cut into a dimension of × 50 mm, and the three-point bending strength was measured under the conditions of a crosshead speed of 0.5 mm / min and a span of 40 mm.

[0028]

[Table 1] 1 *: Comparative example Porous piezoelectric ceramic element manufactured using urethane foam 2 *: Comparative example Porous piezoelectric ceramic element manufactured by three-dimensionally assembling rod-shaped piezoelectric ceramics 3 *: Comparison Example Porous piezoelectric ceramic element prepared by filling spherical piezoelectric ceramics

From the results shown in Table 1, it can be seen that the spherical pores can maintain the high strength even with the same porosity. Furthermore, its acoustic impedance is 1.7 × 1.
It shows 0 ⁶ kg / m ² S, which is almost the same level as the acoustic impedance of living organisms and water.

Example 2 A porous piezoelectric ceramics element was produced in the same manner as in Example 1 except that the particle size of the expanded polystyrene used was changed. The results are shown in Table 2.

[Table 2] * Indicates comparative example From the results in Table 2, the proper range of pore size is 0.05-5m
m. In addition, it was difficult to manufacture the element of No. 5 with an increased porosity.

Example 3 A porous piezoelectric ceramics element was prepared in the same manner as in Example 1 except that the particle diameter of the expanded polystyrene balls was set to 0.15 mm and the amount of expanded polystyrene balls to be filled was changed in order to control the porosity. It was made. The results are shown in Table 3.

[Table 3] From the results of Table 3, it is understood that the appropriate range of the porosity is 50 to 85%.

Comparative Example A porous piezoelectric ceramic element was manufactured in accordance with Example 1 except that unexpanded styrene spherical beads having a particle size of 1 mm which were commercially available were used as the pore-forming material. As a result, the temperature increase rate in the temperature range of 150 to 400 ° C. where styrene decomposes was set as low as 0.1 ° C./min as much as possible, and despite the attempt of degreasing, many cracks were formed in the finished element. Existed. The flexural strength was measured and found to be 2 kg / cm ² .

[0033]

According to the present invention, it is possible to provide a porous piezoelectric ceramics element whose acoustic impedance is reduced to a level equivalent to that of living body and water, and which has mechanical strength enough to be practically used. Further, this element can be manufactured by the manufacturing method of the present invention.

Claims (2)

[Claims] 1. A porous piezoelectric ceramic element having spherical pores with a pore diameter of 0.05 to 5 mm and having a porosity of 50 to 85%. 2. A method of mixing foamed polystyrene balls as a pore-forming material and an aqueous slurry of piezoelectric ceramic powder containing a curable resin, curing the mixture, and then firing the mixture. A method for manufacturing a porous piezoelectric ceramic element.