ACOUSTIC MATCHING MATERIAL, ULTRASONIC VIBRATOR, AND ULTRASONIC FLOWMETER

Abstract

 An acoustic matching body of the present invention includes a dried gel which is formed by polymerizing monomers containing a polymerizable organic silicon compound wherein a nonhydrolyzable organic group is directly bonded to silicon. This acoustic matching body can be used as an acoustic matching layer 2 of an ultrasonic oscillator 1 and can extend or contract following the expansion or contraction of a piezoelectric material 3 or the case connected to the acoustic matching layer 2 even if the piezoelectric material or the case expands or contracts, and thereby the acoustic matching body can be operated stably over a wide range of temperatures and can be used over a long period of time.

Description

Technical Field

[0001] The present invention is related to an acoustic matching body for transmitting an ultrasonic wave in a fluid (particularly gas) and receiving the ultrasonic wave propagating in the fluid, and an ultrasonic oscillator using the acoustic matching body, and an ultrasonic flowmeter using the ultrasonic oscillator.

Background Art

[0002] An ultrasonic oscillator 50 generally has a constitution shown in Fig. 16. In Fig. 16, a piezoelectric material 51 is bonded to a case 53 with a bonding means 55. The case is formed of a metallic material such as stainless. An acoustic matching layer 52 is further bonded to the case 53 with a bonding means 54.

[0003] The piezoelectric material 51 translates an electric signal into mechanical oscillation. This mechanical oscillation is transmitted as an ultrasonic wave into a gas through the acoustic matching layer 55. In order that the oscillation generated in the piezoelectric material 51 is efficiently transmitted into the gas, an acoustic impedance is required to be considered.

[0004] The acoustic impedance Z of a material is defined by a sonic velocity (C) and a density (p), as a formula (10).


The acoustic impedance Z_P of the piezoelectric material 51 as an oscillation-generating means is 30×10⁶ kg/(m²s). On the other hand, the acoustic impedance of the gas, for example air, as the radiation medium of the ultrasonic wave is about 400 kg/(m²s). As described above, the acoustic impedances of the piezoelectric material 51 and the air are substantially different.

[0005] When the acoustic impedances of the piezoelectric material and the radiation medium are different, the ultrasonic wave is liable to reflect at an interface therebetween. When the ultrasonic wave reflects, since an intense of the ultrasonic wave propagating the air is weak, there is a problem that the ultrasonic wave is not efficiently transmitted to an intended object.

[0006] The acoustic matching layer (which is also called as an acoustic matching member) is used in order to solve this problem, and is formed between the piezoelectric material and the radiation medium. The acoustic matching member is formed so that the acoustic impedance Z_M of the member satisfies the following formula (11), whereby the efficient propagation of the ultrasonic wave into the radiation medium is ensured.

[0007] The optimal value is about 11×10⁴ kg/(m²s). As understood from the formula (11), the acoustic matching layer is required to be a solid, and has a low density and a low sonic velocity. A dried gel has been often used as the material satisfying these requirements. A method for producing this dried gel is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2005-8424. Japanese Patent Laid-Open Publication No. 2002-262394 discloses an ultrasonic oscillator wherein a dried silica gel is employed.

Disclosure of Invention

Problems to Be Solved by Invention

[0008] In the ultrasonic oscillator of the constitution shown in Fig. 11, the case 53 is formed of a metallic material such as stainless. A coefficient of thermal expansion of the stainless is, for example, about 17.8 ppm/°C and relatively high. On the other hand, the coefficient of thermal expansion of the dried gel used as the acoustic matching layer 52 is several ppm/°C or less. Therefore, when the dried gel is bonded to the case 53 and the case 53 expands or contracts depending on the change of an environmental temperature, the dried gel does not expand or contract in the same manner as the case 53 since the dried gel is not liable to expand or contract. This results in break of the dried gel. For this reason, the operation of the ultrasonic oscillator is not stabilized. Further, since the coefficient of thermal expansion of the piezoelectric material is, for example, 7.5 ppm/°C, the same problem is seen when the dried gel is bonded to the piezoelectric material 51.

[0009] The object of the present invention is to solve the problem and to provide the acoustic matching body formed of the dried gel, which operates stably against the temperature change of the use environment, the ultrasonic oscillator using the acoustic matching body, and the ultrasonic flowmeter.

Means to Solve the Problems

[0010] In order to solve the problems, the present invention provides an acoustic matching body which includes a dried gel which is formed by polymerizing monomers containing a polymerizable organic silicon compound wherein a nonhydrolyzable organic group is directly bonded to a silicon. The dried gel which is formed by polymerizing the monomers containing this particular organic silicon compound is softer than the conventional dried gel proposed as the acoustic matching body.

[0011] Herein, the term "acoustic matching body" is used for referring to an independent member before being incorporated, as the acoustic matching layer, into the ultrasonic oscillator and so on, and this may be called as an "acoustic matching layer" after being incorporated into the ultrasonic oscillator. That is, the "acoustic matching body" and the "acoustic matching layer" are not different in function, and they are different in whether or not they are incorporated in the ultrasonic oscillator or the like.

[0012] The acoustic matching body of the present invention may be of a single-layer structure which is formed of only the dried gel, or may be a stacked structure which is formed of the dried gel and another porous body. The acoustic matching body of the stacked structure is preferably formed of the dried gel and a ceramic porous body.

[0013] The present invention further provides an ultrasonic oscillator which includes a piezoelectric material and an acoustic matching layer, wherein the acoustic matching layer is the acoustic matching body of the present invention.

[0014] Furthermore, the present invention provides an ultrasonic flowmeter which includes an ultrasonic oscillator which includes a piezoelectric material and an acoustic matching layer, in which oscillator the acoustic matching layer is the acoustic matching body of the present invention.

Effect of Invention

[0015] The acoustic matching body of the present invention is characterized in that it is formed by the dried gel which is formed by the monomers which contain the particular organosilane compound and has more softness. This characteristic prevents the acoustic matching body from breaking if the acoustic matching body is used in the ultrasonic oscillator and the case expands or contracts due to the temperature change of the use environment since the acoustic matching body can follow the expansion or the contraction of the case. Therefore, the ultrasonic oscillator wherein the acoustic matching body of the present invention is used, and the acoustic flowmeter including the ultrasonic oscillator can operate stably over a wide range of temperatures and can transmit and receive the ultrasonic wave surely and well.

Brief Description of Drawings

[0016] 

Fig. 1 is a cross-sectional view schematically showing an embodiment of an ultrasonic oscillator of the present invention.

Fig. 2 is a flow sheet showing a first method for producing an acoustic matching body of the present invention.

Fig. 3 is a schematic view showing a molecular of an organosilane compound which is used in the production of the acoustic matching body of the present invention.

Fig. 4 is a schematic view showing a molecular of a silane compound which is used in the production of the acoustic matching body of the present invention.

Figs. 5(a) and 5(b) are schematic views showing gel materials in a material-preparation step and a gelation step respectively in the first method for producing the acoustic matching body of the present invention.

Figs. 6(a), 6(b) and 6(c) are schematic views showing the gel materials in the material-preparation step, the gelation step and a reconstruction step respectively, in a second method for producing the acoustic matching body of the present invention.

Fig. 7 is a cross-sectional view schematically showing a jig for measuring a deflecting strength of a dried gel which constitutes the acoustic matching body of the present invention.

Fig. 8 is a graph showing a measurement result of the deflecting strength of the dried gel which constitutes the acoustic matching body of the present invention.

Fig. 9 is a cross-sectional view of another embodiment of the ultrasonic oscillator of the present invention.

Fig. 10 is a flow sheet showing a method for producing the ultrasonic oscillator shown in Fig. 9.

Fig. 11 is a cross-sectional view showing an embodiment of the acoustic matching body of the present invention.

Fig. 12 is a flow sheet showing a method for producing a ceramic porous body which constitutes the acoustic matching body of the present invention.

Fig. 13 is a schematic view showing a third method for producing the acoustic matching body of the present invention.

Fig. 14 is a cross-sectional view schematically showing a part of the acoustic matching body of the present invention which is produced in accordance with the method shown in Fig. 13.

Fig. 15 is a block diagram showing a construction of an ultrasonic flowmeter of the present invention.

Fig. 16 is a cross-sectional view schematically showing a prior art ultrasonic oscillator.

Explanation of Letters or Numerals

[0017] 1, 50...ultrasonic oscillator, 2, 52...acoustic matching layer, 3, 51...piezoelectric material, 4, 4a, 4b, 54...bonding member, 5, 6, 55...electrode, 14...organosilane compound, 15... nonhydrolyzable organic group, 16...silicon, 17...hydrolyzable organic group, 18...tetraalkoxysilane, 19...gel material solution, 20...wet gel, 21...gel material solution, 22...wet gel, 23...reconstruction solution, 24...wet gel, 25...jig for measuring a deflecting strength, 26...sample, 27...measuring stand, 28..pushing bar, 31...ultrasonic oscillator, 32, 53...case, 33, 34...electrode, 35...conducting means, 86...terminal plate, 37, 38...electrode terminal, 39...insulation portion of terminal plate, 40...closed space, 41...ceramic matrix, 42...fist porous body, 43...second porous body, 44...acoustic matching body, 45...pore, 61...mold, 62...container, 63...concavity, 64...starting material solution, 65...ceramic matrix, 66...pore, 81...passage, 82...ultrasonic oscillator A, 83...ultrasonic oscillator B, 84...transmitting device, 85...receiving device, 86...timer device, 87...stitching device, 88...ultrasonic flowmeter, 89... arithmetical unit.

Embodiments for Carrying Out the Invention

[0018] Hereinafter the embodiments of the present invention are described, optionally with reference to the drawings. It should be noted that the present invention is not limited by the embodiments of the present invention.

[0019] An example of an ultrasonic oscillator which is constructed by using an acoustic matching body of the present invention is shown in Fig. 1. In Fig. 1, the acoustic matching body is denoted by the reference numeral 2 and is bonded to a piezoelectric material as a member to be bonded 3 with an adhesive as a bonding body 4, whereby the ultrasonic oscillator 1 is made. Electrodes 5 and 6 which are opposed to each other are formed on the piezoelectric material 4. The electrode is formed by heating a conductive paste of silver or gold or the like and baking it. The electrodes 5 and 6 are electrically connected to electrode leads 7 and 8 with solder or silver solder. In the following, a dried gel which constitutes the acoustic matching body is described, and further the acoustic matching body of a composite structure which is formed of the dried gel and another porous body is described.

(Dried gel)

[0020] The acoustic matching body of the present invention includes the dried gel which is formed by polymerizing monomers which contain a polymerizable organic silicon compound wherein a nonhydrolyzable group is directly bonded to silicon (hereinafter, this organic silicon compound is called as a "first organic silicon compound" for convenience). The polymerizable organic silicon compound is a compound which gives a polymer containing repeated Si-0 bonds by a hydrolysis and/or a dehydration reaction. The polymerizable organic silicon compounds are, for example, a silane compound wherein a hydrolyzable organic group is bonded to silicon, a silanol, a siloxane compound having an OH group and so on.

[0021] The acoustic matching body of the present invention preferably contains a dried gel which is formed by polymerizing monomers which contain organosilane compound wherein the hydrolyzable organic group and the nonhydrolyzable compound are directly bonded to one and the same silicon. The dried gel is obtained by polymerizing the organosilane compounds and/or polymerizing the organosilane compound with another compound, followed by drying. In this dried gel, the nonhydrolyzable organic group is bonded to the silicon that forms a framework of the gel. It is considered that this ensures softness.

[0022] The organosilane compound preferable contains two or more hydrolyzable organic groups. This is because two or more polymerizable portions are required in order to hydrolyze the organosilane compounds to polymerize them so that the framework which is formed of Si-O bonds is formed. However, the organosilane compound having one hydrolyzable organic group may be used together with another organosilane compound having two or more hydrolyzable organic groups and/or another polymerizable compound.

[0023] The organosilane compound has, for example, a construction schematically shown in Fig. 3. In the compound 14 shown in Fig. 3, one nonhydrolyzable organic group 15 directly bonds to silicon 16, and three hydrolyzable organic groups 17 directly bond to silicon 16. The nonhydrolyzable organic group is not polymerized with another organosilane compound by hydrolysis. The hydrolyzable organic group is polymerized with another organosilane compound by hydrolysis followed by a dehydration reaction. The example of the compound shown in this figure is organotrialkoxysilane described below as a formula (2).

[0024] The hydrolyzable organic groups include an alkoxyl group with a carbon number of 1 to 4, an acetoxy group, an oxime group represented by -O-N=C-R(R'), an enoxy group represented by -O-C(R)=C(R')R, an amino group, an aminoxy group represented by -O-N(R)R' and an amide group represented by -N(R)-C(=O)R' (in these formulas, R, R' and R" are independently halogen atom or a monovalent hydrocarbon, and a halogen atom.

[0025] As the nonhydrolyzable organic group, a substituted or non-substituted monovalent hydrocarbon group with a carbon number of 1 to 8 can be exemplified. Specifically, an alkyl group such as a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, or an octyl group; a cycloalkyl group such as a cyclopentyl group or a cyclohexyl group; an aralkyl group such as a 2-phenylethyl group or 3-phenylpropyl group; an aryl group such as a phenyl group or a tolyl group; an alkenyl group such as a vinyl group or an allyl group; a halogen-substituted hydrocarbon group such as a chloromethyl group, γ-chloropropyl group or 3,3,3-trifluoropropyl group; a substituted hydrocarbon group such as a γ-methacryloxypropyl group, a γ-glycidoxypropyl group, a 3,4-epoxycyclohexylethyl group or a γ-mercaptopropyl can be exemplified.

[0026] The nonhydrolyzable organic group is preferably the alkyl group with a carbon number of 1 to 4 or a phenyl group considering ease of synthesis and availability of the organosilane compound. Such nonhydrolyzable organic group is preferable from the viewpoint that it prevents the hardness of the resultant dried gel from being reduced excessively.

[0027] More specifically, it is preferable that the organosilane compound is at least one compound selected from the organotrialkoxysilane represented by the following formula (2) and the organodialkoxysilane represented by the following formula (3).

        R³Si(OR₂)₃     (2)

        (R⁵)₂Si(OR⁴)₂     (3)

[0028] In the formulas (2) and (3), each of R² and R⁴ is an alkyl group with a carbon number of 1 to 4. In the formula (2), all R²s are usually the same, but they may be different from each other. The same is applicable to R⁴ in the formula (3).

[0029] Each of R³ and R⁵ is a substituted or non-substituted monovalent hydrocarbon group with a carbon number of 1 to 8. Specifically, as the nonhydrolyzable organic group, the monovalent hydrocarbon with a carbon number of 1 to 8 can be exemplified. Specifically, an alkyl group such as a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, or an octyl group; a cycloalkyl group such as a cyclopentyl group or a cyclohexyl group; an aralkyl group such as a 2-phenylethyl group or 3-phenylpropyl group; an aryl group such as a phenyl group or a tolyl group; an alkenyl group such as a vinyl group or an allyl group; a halogen-substituted hydrocarbon group such as a chloromethyl group, a γ-chloropropyl group or 3,3,3-trifluoropropyl group; a substituted hydrocarbon group such as a γ-methacryloxypropyl group, a γ-glycidoxypropyl group, a 3,4-epoxycyclohexylethyl group or a γ-mercaptopropyl can be exemplified. R³, as well as R⁵, is preferably an alkyl group with a carbon number of 1 to 4 and a phenyl group. The reason therefor is described as above. In the formula (3), two R⁵ are usually the same, but they may be different from each other.

[0030] As the trialkoxysilane of the formula (2), methyltrimethoxysilane, methyltriethoxysilane, methyltriisopropoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, and 3,3,3-trifluoropropyl trimethoxysilane can be exemplified.

[0031] As the dialkoxysilane of the formula (3), dimethyldimethoxysilane, dimethyldiethoxysilane, diphyenyldimethoxysilane, diphenyldiethoxysilane and methylphenyldimethoxysilane can be exemplified.

[0032] When the compound represented by the formula (2) or (3) is polymerized, a molecular weight after polymerization is preferably 200 or more. When the molecular weight is 200 or more, the resultant gel can be more surely and more effectively softened. The molecular weight is generally 10000 or less, and preferably from 200 to 5000. If the molecular weight is less than 200, it may be difficult to maintain the shape of the gel. If the molecular weight is more than 5000, the strength of the resultant dried gel may be reduced.

[0033] Alternatively, the acoustic matching body of the present invention may include a dried gel which is formed by polymerizing monomers which contains, as the first organic silicon compound, a polysiloxane diol represented by the following formula (4). This compound gives remarkable effect that the softness is conferred to the dried gel which is obtained by polymerizing this compound. Therefore, the dried gel formed by polymerizing the monomers containing this compound has softness and exert the same effect as the dried gel formed by polymerizing the above-mentioned organosilane compound when the dried gel is used as the acoustic matching body.

        HO((R⁶)₂SiO)_nH     (4)

[0034] The formula (4) is a siloxane compound having OH groups at the both ends and including the repeated - R⁶)₂SiO-. This compound can be polymerised by a dehydration and condensation reaction at the OH groups. In the formula (4), R⁶ is an alkyl group with a carbon number of 1 to 4. In the formula (4), all R⁶s are usually the same, but they may be different from each other. When R⁶ is methyl, the compound represented by the formula (4) is polydimethylsiloxane diol.

[0035] The "n" is an integer of 2 or greater and preferably from 5 to 100. When "n" is greater than 100, compatibility with other dried gel-forming materials tends to deteriorate, whereby disadvantage such as unevenness in the gel may be caused. When "n" is less than 2, the resultant gel may not be sufficiently softened. Further, when the compound represented by the formula (4) is used together with the compound represented by the formula (1), a reaction velocity is easily controlled since they tend to copolymerized (cross-linked).

[0036] The dried gel contained in the acoustic matching body of the present invention may contain a dried gel formed by polymerizing monomers containing, in addition to the first organic silicon compound, a compound represented by the following formula (1) or a partially-hydrolyzed polymer of the compound represented by (1).

        Si(OR¹)₄     (1)

[0037] The compound represented by the formula (1) is a tetraalkoxysilane. Each of R¹s is a substituted or nonsubstituted monovalent hydrocarbon group with a carbon number of 1 to 8. As the examples of R¹, the groups exemplified above as R³ and R⁵ can be exemplified. Therefore, the examples of R¹ are omitted here. Preferable R¹ is an alkyl group with a carbon number of 1 to 4 or a phenyl group, similarly to preferable R³ and R⁵. In the formula (1), four R¹s are usually the same, but they are different from each other.

[0038] The compound represented by the formula (1) has a construction as schematically shown in Fig. 4. As illustrated in Fig. 4, in the tetraalkoxysilane 18, only the hydrolyzable organic groups 17 are bonded to silicon 15. More specifically, examples of the compound represented by the formula (1) include tetramethoxysilane and tetraethoxysilane.

[0039] The dried gel given by the monomers containing the tetraalkoxysilane represented by the formula (1) in addition to the first organic silicon compound has the strength and the softness at the same time. Therefore, the acoustic matching body wherein this dried gel is used shows durability and is less liable to break since the matching body can follow the expansion or contraction of the member to be bonded (for example, the case) due to the change of the temperature of the use environment and the strength of the body itself is high.

[0040] In this case, the dried gel may be obtained by subjecting the first organic silicon compound and the tetraalkoxysilane or the partially-hydrolyzed polymer thereof to a polymerization process at the same time. Alternatively, the dried gel may be one produced by polymerizing the first organic silicon compound under a wet gel formed by polymerizing or the partially-hydrolyzed polymer of the tetraalkoxysilane or the tetraalkoxysilane. Such a gel as formed in this manner has the strength due to the tetraalkoxysilane and the softness due to the first organic silicon compound, and gives the ultrasonic oscillator and the ultrasonic wave transmitting and receiving apparatus which stably operates over a wide range of temperatures.

[0041] The dried gel contained in the acoustic matching body of the present invention is preferably one formed by monomers wherein an amount of the first organic silicon compound is from 0.01wt% to 10wt% of the total amount of the tetraalkoxysilane and the partially-hydrolyzed polymer of the tetraalkoxysilane. Such monomers give the dried gel which shows excellent characteristics due to the first organic silicon compound and the tetraalkoxysilane respectively. This ratio of the first organic silicon compound is preferably employed when the tetraalkoxysilane is previously polymerized and then the first organic silicon compound is polymerized.

[0042] The dried gel contained in the acoustic matching body of the present invention may contain a dried gel which is formed by polymerizing monomers which contains, in addition to the first organic silicon compound, a copolymer of at least one of the compounds represented by the following formulas (5) and (6) and the compound represented by the following formula (7).

        CH₂=CR¹⁰(COOR¹¹)     (5)

        CH₂=CR¹²(COOR¹³)     (6)

        CH₂=CR¹⁴(COOR¹⁵)     (7)

[0043] In the formulas (5) to (7), each of R¹⁰, R¹² and R¹⁴ is hydrogen atom or a methyl group. In the formula (5), R¹¹ is a substituted or non-substituted monovalent hydrocarbon with a carbon number of 1 to 9. In the formula (6), R¹³ is a group selected from a group consisting of an epoxy group, a glycidyl group, and a hydrocarbon group containing at least one of these groups (for example, a γ-glycidoxypropyl group).

[0044] In the formula (7), R¹⁵ is a hydrocarbon group containing an alkoxysilyl group or a halogenated silyl group. More specifically, R¹⁵ is selected from a trimethoxysilylpropyl group, a dimethoxymethylsilylpropyl group, a monomethoxydimethylsilylpropyl group, a triethoxysilylpropyl group, a diethoxymethylsilylpropyl group, an ethoxydimethylsilylpropyl group, a trichlorosilylpropyl group, a dichloromethylsilylpropyl group, a chlorodimethylsilylpropyl group, a chlorodimethoxysilylpropyl group, and a dichloromethoxysilylpropyl group.

[0045] The compound represented by the formula (5) or (6) and the compound represented by the formula (7) give a copolymer by polymerization of CH₂=C. This copolymer is polymerized with the copolymer or the first organic silicon compound at the silyl group, to give the polymer containing the repetition of Si-O.

[0046] Two or more types compounds may be employed for each of the compounds represented by the formulas (5) to (7). In other words, it may be possible to employ copolymers (four types) formed of two types of the compounds represented by the formula (5) and two types of the compounds represented by the formula (7).

[0047] When the monomers which contains the copolymer of at least one of the compounds represented by the formulas (5) and (6), and the compound represented by the formula (7) are polymerized, the dried gel having a higher softness can be obtained. Therefore the dried gel more easily follows the expansion or contraction of the member to be bonded caused by the change of the temperature of the use environment. The compound represented by the formula (5) is particularly more effective for softening the gel. Further, the dried gel formed by polymerizing the monomers containing the copolymer of these compounds has a higher adhesiveness to a different substrate, and therefore the acoustic matching body wherein the dried gel is employed is hard to fall away from the member to be bonded and can operate stably. The compound represented by the formula (6) is particularly more effective for improving the adhesiveness to the substrate.

[0048] The dried gel formed by polymerizing the monomers containing the first organic silicon compound preferably has a density in a range of from 0.15g/cm³ to 1.00g/cm³ and a mean pore diameter in a range of from 2mm to 40nm which is determined by a BJH method using a nitrogen adsorption method. Further, a size and shape of the acoustic matching body of the present invention is selected depending on usage. For example, when the acoustic matching body is used as the acoustic matching layer in the ultrasonic oscillator, it may be a disc shape having a diameter of from about 5mm to about 20mm and a thickness of from about 0.4mm to about 1.5mm.

[0049] The dried gel described in the above may constitute the acoustic matching body alone, or may constitute a composite body together with another porous material so that the composite body constitutes the acoustic matching body. Since the end portion of the dried gel tends to fracture, the dried gel is preferably to be made a composite structure together with another porous material so that the end portion of the gel is protected. The porous material which constitutes the composite body together with this dried gel is preferably a ceramic porous body. Further, the composite body has a construction wherein a concavity is formed in the ceramic porous body and the dried gel is disposed within this concavity so that peripheral portion of the dried gel is protected.

[0050] For example, the present invention provides an acoustic matching body including a first porous body and a second porous body, wherein:

the fist porous body is a ceramic porous body which includes ceramic particles constituting a ceramic matrix,
wherein

the ceramic matrix defines a plurality of pores,

inter-ceramic particle voids are formed in the ceramic matrix, and

the second porous body is the dried gel formed by polymerizing the monomers containing the first organic silicon compound.

[0051] In this acoustic matching body, the second porous body improves matching of the acoustic impedance thereof with that of the gas and the first porous body serves to protect the second porous body. Further, the first porous body has the inter-ceramic particle voids formed in the ceramic matrix, in addition to the pores defined by the ceramic matrix. In other word, this ceramic porous body has a structure having many voids with a high strength as a whole, and therefore, a density thereof is low. Further, a skeleton of the ceramic matrix does not extend linearly and gives a tortuous passage for an ultrasonic wave since the pores and the inter-ceramic particle voids exist. This reduces a propagation speed of the ultrasonic wave. Therefore, this first porous body has a characteristic of a low density and a low sonic velocity, and an ultrasonic oscillator wherein this is used as an acoustic matching layer has a significantly improved ultrasonic propagation characteristic.

[0052] Here, the "pore" means a portion which is observed as a vacancy when the ceramic matrix composed of ceramic particles is observed macroscopically (for example, by means of a microscope at about 20x magnification). The "inter-ceramic particle void" means a minute space formed between the particles which construct the ceramic matrix, and it is specifically a small hole having a diameter of not greater than 10µm. Alternatively, it can be said that the "pore" is a vacancy formed by foaming a ceramic slurry according to a below-mentioned method, and the "inter-ceramic particle void" is formed in the ceramics regardless with or without the foaming.

[0053] In the first porous body of the acoustic matching body of this composite structure, the pore preferable has a size such that a center value of pore diameter (pore size) distribution is in a range of 10µm to 500µm. When the first porous body has the pores with such size and the second porous body is integrated with the first porous body to give the composite by impregnation with a starting material solution for the second porous body, the impregnation with the material solution is facilitated.

[0054] This ceramic matrix preferably contains hardly-sinterable ceramic, but may not contain hardly-sinterable ceramic. The hardly-sinterable ceramic preferably occupies 80vol% of the ceramic matrix, and more preferably 90vol% of the ceramic matrix, and still more preferably 100vol% of the ceramic matrix.

[0055] Alternatively, the ceramic porous body may be produced by mixing a material for forming the ceramic matrix (for example, alumina) and a pore-forming material and then pressing a mixture of the pore-forming material and the material for forming the ceramic matrix, followed by a sintering process for combining the material for forming the ceramic matrix and removing the pore-forming material. The pore-forming material is formed of a material which melts during the sintering process or a material which dissolves in a particular solvent. The pore-forming material is, for example, an acrylic sphere (which melts during the sintering process) or an iron sphere (which dissolves in a sulfuric acid). The material for forming the ceramic matrix may be formed of a main material which forms the skeleton and an auxiliary material which has a size different from that of the main material and hardens the main material. The auxiliary material is, for example, glass.

[0056] In this acoustic matching body of the composite structure, an outer circumference portion of the second porous body is preferably surrounded by the first porous body. That is, a contour in a surface direction of the second porous body (that is, a contour which defines a surface area of the second porous body) preferably contacts with the first porous body. By this construction, the outside edge of the second porous body is prevented from fracturing, and therefore it is possible to control a thickness of the acoustic matching body of this composite structure by polishing a surface where the second porous body is placed.

[0057] In this acoustic matching body of this composite structure, the second porous body preferably fills a part or all of the pores and the inter-ceramic particle voids in the first porous body. This allows the second porous body to be bonded to the first porous body strongly by an anchoring effect.

[0058] When the coefficients of thermal expansion of the two different materials are different in the acoustic matching body formed of the two different materials, one of the materials may break due to the change of the temperature of the use environment. Since the dried gel has softness in the acoustic matching body of the present invention and easily follows the expansion or contraction of a surrounding member (the porous body), the break is not liable to occur, and therefore the acoustic matching body can be used conveniently over a wide range of temperatures.

[0059] Next, a method for producing the acoustic matching body of the present invention is described with reference to drawings.

(First production method)

[0060] An embodiment of a method for producing the dried gel which constitutes the acoustic matching body of the present invention is described. Fig. 2 illustrates a flow of the dried gel production steps. In Fig. 2, the dried gel production steps include a material-preparation step (a step a), a gelation step (a step b), a reconstruction step (a step c), a hydrophobicizing step (a step d) and a drying step (a step e). The operations specifically carried out in the respective steps are shown as a flow chart in the middle of the drawing. Further, materials used in the respective steps are shown in the right row in the drawing. In the following, the respective steps a to e are described in detail.

Material-preparation step (Step a)

[0061] This step is a step for preparing a mixture solution which is a starting material, in which step the first organic silicon compound that is a material for forming a framework of the dried gel (gel material), water for hydrolyzing the compound, a reaction solvent and a catalyst are added. In Fig. 2, an example is illustrated wherein an organosilane compound is used as the first organic silicon compound. The organosilane compound is as exemplified above. Further, a tetraalkoxysilane which is exemplified above may be used as the gel material. Furthermore, a copolymer of at least one of the compounds represented by the formulas (5) and (6), and the compound represented by the formula (7) may be added, as the gel material (a monomer component), together with the organosilane compound. Alternatively, the polysiloxane diol compound represented by the formula (4) may be used instead of the organosilane compound.

[0062] The proportion of the sum of the first organic silicon compound and the optionally contained polymer and copolymer derived from the compounds of the formulas (5), (6) and (7) and unreacted materials of these compounds, to the whole before polymerization (that is the ratio of these compound to the charged amount which is the total of theses compound and the monomers derived from the compound represented by the formula (1)) is not limited, but preferably from 0.01wt% to 20wt%. When a concentration of the components derived from these compounds is too low, the break of the dried gel may not be prevented, and the dried gel may fall away from a substrate if the dried gel is formed on the substrate. On the other hand, if the concentration of the components derived from these compounds is too high, the strength is reduced and the contraction occurs during the drying of the wet gel. For these reasons, the concentration of the components derived from the compound is preferably from 0.1wt% to 10wt% before the polymerization. Therefore, in the material-preparation step (the step a), tetraalkoxysilane is optionally used so that the concentration of the components derived from these monomers is within this concentration range before the polymerization in the resultant dried gel.

[0063] As the catalyst, a general organic acid, a general inorganic acid, a general organic base or a general inorganic base is used. As the organic acids, acetic acid and citric acid are exemplified. As the inorganic acids, sulfuric acid, hydrochloric acid, and nitric acid are exemplified. As the organic base, piperidine is exemplified. As the inroganic base, ammonia is exemplified. Further, an imine-based catalyst such as piperidine increases the diameter of the pore in the dried gel. Therefore, such a catalyst is preferable from the viewpoint of reduction in a capillary force of the resultant dried gel.

[0064] As the solvent, a water-soluble organic solvent such as a lower alcohol such as methanol, ethanol, propanol, butanol, ethylene glycol or diethylene glycol, a mono- or di-ether of ethylene glycol or diethylene glycol, a lower ketone such as acetone, and a lower ether such as tetrahydrofuran and 1,3-dioxalane. Further, when the gel is formed by hydrolysis and condensation polymerization of the gel material, water necessary for hydrolysis is added in the material-preparation step.

[0065] The solution is preferably prepared so that it contains, for example, the organic silicon compound having the hydrolyzable organic group (the organosilane compound or the polysiloxane diol compound) in an amount of 10wt% to 40 wt%, and the silane compound having only the nonhydrolyzable organic group (a tetraalkoxysilane) in an amount of 0.01wt% to 4.0wt% if such compound is contained, and the solvent in an amount of 60wt% to 90wt%.

[0066] The components as described above are mixed and agitated to conduct the material preparation. Figs. 5 (a) and 5(b) are schematic views showing the state before and after polymerization when the organosilane compound is used as the first organic silicon compound. In Figs. 5(a) and 5(b), a triangle (Δ) denotes the organosilane compound 14 with the nonhydrolyzable organic group 15 shown in Fig. 3 and a square (□) denotes the silane compound 18 having only the hydrolyzable groups 17 shown in Fig. 4. Fig. 5(a) shows the gel material solution 19 wherein these silane compounds, the catalyst (not shown) and the solvent are dispersed. These silane compounds are hydrolyzed by the catalyst and polymerized. Fig. 5(b) shows a schematic view of the polymerized silane compounds. As shown in this figure, the polymerization proceeds in the gel material solution 19 to form the wet gel 20 which is then dried to give the dried gel.

Gelation step (Step b)

[0067] In this step, a solid framework of the gel is formed by promoting the hydrolytic polymerization by addition of the catalyst to the prepared mixture solution so that the wet gel containing the solvent is produced. In order to accelerate the gelation, the catalyst is optionally added and the temperature of the solution is raised. The examples of the catalyst are as described in connection with the material preparation step, and therefore omitted here. The catalyst may be added in both of the material-preparation step (the step a) and the gelation step (the step b). Alternatively, the material-preparation step (the step a) and the gelation step (the step b) may be carried out at the same time. The gelation step may be carried out at, for example, 20°C to 70°C.

Reconstruction step (Step c)

[0068] In this step, a part of the solid framework of the wet gel formed in the gelation step (the step b) is decomposed while a new solid framework is formed. Specifically, a reconstruction material solution is prepared by adding and mixing a material for reconstruction, a catalyst for reconstruction, water, and solvent if necessary. The wet gel formed in the gelation step is immersed in this solution. The density of the dried gel can be adjusted by a treating time and a treating temperature for this step. The treating time is, for example, 2 hours to 50 hours, and the treating temperature is, for example, 40°C to 80°C.

[0069] The gel material and the catalyst which are described in connection with the gelation step (the step b) can be used as the reconstruction material and the reconstruction catalyst which are used in the reconstruction step. However, the reconstruction material and the reconstruction catalyst are not necessarily required to be the same as the gel material and the catalyst used in the gelation step. The reconstruction material solution is preferably prepared to contain the organic silicon compound having the hydrolyzable organic group (the organosilane compound or the polysiloxane diol compound) in an amount of 35wt% to 80wt%, the silane compound having only the nonhydrolyzable organic group (the tetraalkoxysilane) in an amount of 0.04wt% to 8.0wt% if it is contained, and the solvent in an amount of 20wt% to 60wt%. After reinforcing the gel framework, the reaction is stopped by substituting the reconstruction material solution with, for example, isopropyl alcohol for the purpose of stopping the reaction.

Hydrophobicizing step (Step d)

[0070] In this step, a hydrophobic group is introduced in a surface of the wet gel obtained until the reconstruction step (the step c) by reacting a solution for hydrophobicization wherein the hydrophobicization agent is dissolved in a solvent, with the surface.

[0071] As the hydrophobicization agent, a silylation agent is preferably used since its reactivity is high. As the silylation agents, a silazane compound, a chlorosilane compound, an alkylsilanol compound and an alkylalkoxysilane compound are exemplified.

[0072] Of these silylation agent, the silazane compound, the chlorosilane compound and the alkylalkoxysilane compound are changed into the corresponding alkyl silanol directly or by hydrolysis and the reacts with a silanol group on the gel surface. When the alkylsilanol is used as the silyataion agent, it as is reacts with the silanol groups on the surface.

[0073] Of these, the chrolosilane compound and the silazane compound are particularly preferable considering high reactivity in the hydrophobicizing step and ready availability. The alkylalkoxysilane is particularly suitable for use considering the fact that it is readily available and a gas such as hydrogen chloride or ammonia does not generate.

[0074] More specifically, as the typical examples of the silylation agent, the chlorosilane compounds such as trimethylchlorosilane, methyltrichlorosilane and dimethyldichlorosilane, the silazane compound such as hexamethyldisilazane, the alkylalkoxysilane compounds such as methoxytrimethylsilane, ethoxytrimethylsilane, dimethoxydimethylsilane, dimethoxytrimethylsilane and diethoxydimethylsilane, and the silanol compounds such as trimethylsilanol and triethylsilanol. When these compounds are used, the alkyl silyl group such as trimethyl silyl is introduced in the surface of the wet gel, whereby the gel can be hydrophobicized.

[0075] Alternatively, a fluorinated silylation agent can be used as the hydrophobicization agent. When this is used, the hydrophobicity of the gel is increased and the hydrophobicization can be carried out very effectively.

[0076] Alternatively, an alcohol such as ethanol, propanol, butanol, hexanol, heptanal, octanol, ethylene glycol or glycerol and carboxylic acids such as a formic acid, an acetic acid, a propionic acid, and a succinic acid can be used as the hydrophobicization agent. These are reacted with a hyroxy group on the gel surface to form an ether or an ester, whereby the gel is hydrophobicized. The reaction using any of these hydrophobicization agents is relatively slow. For this reason, the treatment with any of these hydrophobicization agents is required to be carried out at a high temperature (for example, from 60°C to 80°C).

[0077] The hydrophobicizing step is a treatment to prevent absorption of moisture by the resultant dried gel. Specifically, the hydrophobicizing step is carried out by putting the gel into a silane coupling treatment liquid. The hydrophobicization reaction (silane treatment reaction) is stopped by substituting the solution with isopropyl alcohol.

Drying step (Step e)

[0078] In this step, the solvent is removed from the wet gel which is obtained by carrying out the steps up to the hydrophobicizing step, whereby the dried gel is obtained. There are (1) natural drying method and (2) special drying method as the drying method.

(1) The natural drying method is most general and convenient drying method. This method is a method wherein the wet gel containing the solvent is left stand to remove the solvent in a liquid state by vaporization thereof. This drying method is most preferable from the viewpoint of cost. Drying by heating wherein the wet gel is heated from the viewpoint of productivity, and drying under reduced pressure wherein the wet gel is in a reduced pressure below an atmospheric pressure, shall be included in the natural drying. The heating temperature for the drying by heating is not limited to a particular one as long as the solvent vaporizes at the temperature.
When the density of the gel is low, the gel during the drying temporarily contracts and may breaks because the capillary force which is proportional to a surface tension of the solvent in the gel. For this reason, the solvent to be removed in the drying step is preferably a hydrocarbon-based solvent of which surface tension is low at a boiling point, and hexane, pentane or a mixture thereof is particularly preferable because of their inexpensiveness. On the other hand, an alcohol such as isopropanol, ethanol or butanol, or a mixture solvent of water and an organic solvent is preferably removed from the gel from the viewpoint of safety. For these reasons, it is preferable to substitute the solvent used for forming the wet gel with the solvent exemplified above and then subject the wet gel to the drying treatment. This makes it possible to reduce a stress during the drying and makes the wet gel resistant to fracture during the drying.

(2) There are two methods - a supercritical drying method and a freeze-drying method - as the special drying method. Since the solvent is removed in a supercritical state, not through the liquid state in the supercritical drying method, the capillary force which causes gas-liquid interface is not generated. For this reason, the wet gel is hardly to break during the drying. As the supercritical fluids used for the drying, water, alcohol and carbon dioxide are exemplified. Carbon dioxide which reaches the supercritical state at the lowest temperature and is harmless, is used in many cases.

[0079] Specifically, a liquefied carbon dioxide is charged into a pressure-resistant container and the solvent in the wet gel placed in the container is substituted with the liquefied carbon dioxide. Next, the pressure and the temperature are raised above a supercritical point and so that the supercritical state is achieved. The carbon dioxide is expelled little by little while maintaining the temperature, to complete the drying.

[0080] The freeze-drying method is a drying method wherein the solvent in the wet gel is frozen and the solvent is removed by sublimation. In these methods, the solvent does not go through the liquid state and the gas-liquid interface is not generated in the gel, and therefore the capillary force is not exerted. For this reason, the contraction of the gel during the drying can be suppressed.

[0081] The solvent used for the freeze-drying method is preferably a solvent of which vapor pressure is high at a solidification point. The examples of the solvent include tertiary butyl alcohol, glycerin, cyclohexane, cyclohexanol, para-xylene, benzen and phenol. Of these, tertiary butyl alcohol and cyclohexane are particularly preferable since the vapor pressures thereof at the melting point are high.

[0082] The freeze-drying method is effectively conducted by previously substituting the solvent in the wet gel with the solvent of which vapor pressure is high at the solidification point. Further, the solvent itself, which is used at the gelation has a high vapor pressure at the solidification point is more preferable. This is because the efficient production is made possible by omitting the solvent substitution.

[0083] The drying may be carried out after the hydrophobicizing step or may be carried out before the hydrophobicizing step. When the hydrophobicizing step is carried out after the drying step, the hydrophobic group is introduced in the surface of the dried gel by exposing the dried gel to a vapor of the hydrophobicization agent, not the solution. Therefore, this method makes it possible to reduce an amount of the solvent used.

[0084] The hydrophobicization agent described above can be used as the hydrophobicization agent which is used in the hydrophobicizing step carried out after the drying step. The chlorosilane compound such as trimethylchlorosilane or dimethyldichlorosilane is most preferable because of high reactivity. Further, when the hydrophobicization agent other than the chlorosilane compound is used, it is effective to use a catalyst such as ammonia or hydrogen chloride which can be introduced in a state of gas.

[0085] Further, when the hydrophobicization is carried out in the gas phase, the temperature for the hydrophobicization can be raised without being limited by the boiling point of the solvent and the hydrophobicization agent. Therefore, the hydrophobicization in the gas phase is effective for accelerating the reaction. Furthermore, when the wet gel is a thin film or powder, the vapor of the hydrophobicization easily penetrates. When the gel is the thin film, an amount of the solvent can be advantageously reduced.

[0086] Softness is conferred to the acoustic matching body containing the dried gel produced in the manner as described above. For this reason, even if the piezoelectric material as the member to be bonded expands or contracts due to the temperature change, the acoustic matching body with softness expands or contracts following the expansion or the contraction of the piezoelectric material, whereby the acoustic matching body itself can be prevented from breaking. This makes it possible for the ultrasonic oscillator of the present invention as shown in Fig. 1 to operate stably over a wide range of temperatures.

(Second production method)

[0087] Another embodiment of a method for producing the dried gel which constitutes the acoustic matching body of the present invention is described. Figs. 6 (a) to 6 (c) are schematic views showing a material-preparation step, a gelation step and a reconstruction step respectively, when the organosilane compound is used as the first organic silicon compound in this embodiment. The meaning of Δ and □ and the meaning of each reference numeral are as described in connection with Fig. 5. Fig. 6(a) shows a gel material solution 21 wherein the silane compounds 18 in each of which only hydrolyzable organic groups are bonded to silicon are dispersed, in the material-preparation step (the step a). This solution is subjected to the gelation step (the step b) and the gelation is conducted.

[0088] As shown in Fig. 6(b), the reconstruction solution 23 wherein the organosilane compound 14 with the nonhydrolyzable organic group 15 and the organosilane compound 18 with only the hydrolyzable organic groups 17 are mixed and dispersed are added to the wet gel 22 formed in the gelation step (the step b) and the hydrolysis is conducted. As a result, the wet gel 24 is obtained by the hydrolysis and the polymerization (cross-linking). The material-preparation step (the step a) and the organosilane compound, the catalyst and the solvent used in the reconstruction step (c) are as described in connection with the first production method, and therefore the descriptions of them are omitted. Further, the hydrophobicizing step and the drying step are carried out in the same manner as described in the first production method.

[0089] Softness is conferred to the acoustic matching body containing the dried gel formed in accordance with this production method. Further, compared to the first production method, this production method wherein the organosilane compound is used in the reconstruction step makes it possible to produce the dried gel stably within a shorter time and makes it possible to give the resultant dried gel a construction which has a relatively high modulus of elasticity and softness.

(Third production method)

[0090] An embodiment for producing an acoustic matching body of a composite structure which is formed of the particular dried gel as described above and a ceramic porous body is described.

[0091] In Fig. 11, the acoustic matching body of composite structure which is produced according to this embodiment is shown. The acoustic matching body 44 shown in Fig. 11 is of a disc shape having, for example, a diameter of 10.8mm and a thickness of 1.8mm, wherein the ceramic porous body is included as the first porous body 42 and the dried gel is filled, as the second porous body 43, in a concavity formed in the first porous body 42.

[0092] In Fig. 11, the first porous body 42 is a structure which includes ceramic matrix 41 which serves to be framework and pores 45 are defined by the ceramic matrix 41. As described above, inter-ceramic particle voids are formed in the ceramic matrix 41. An embodiment of a method for producing this ceramic matrix is described. Fig. 12 illustrates a flow of the ceramic matrix production steps.

[0093] The production steps are classified roughly into preparation of a mixed slurry (a step A), preparation of a bubble-containing slurry (a step B), a mold step (a step C), a drying step (a step D), a degreasing and sintering step (a step E) and a cutting step (a step F). The operations specifically carried out in the respective steps are shown as a flow chart in the right side of the drawing. Further, materials used in the respective steps are shown in the middle row in the drawing.

[0094] The step A includes a mixing/crushing step in which ceramic powder (for example, silicon carbide and glass) and water (in which an organic solvent is mixed, if necessary) as feed materials are mixed and crushed by, for example, a ball mill to give a mixed slurry, and a defoaming step in which the resultant mixed slurry is defoamed. The ceramic powder contains at least one kind of hardly-sinterable ceramic powder. The hardly-sinterable ceramic powder may be, for example, silicon carbide. Defoaming may be carried out in a glove box filled with nitrogen. For this purpose, a degassing and nitrogen-substituting step is carried out before the defoaming step.

[0095] The step B is a foaming step in which a surfactant (a foaming agent) and a gelatinizer are added to the mixed slurry and they are mixed with an agitator under an nitrogen atmosphere. In this step, the kind of the surfactant, the kind of the ceramic powder, a speed of the agitator, an agitation time, and a temperature are parameters which determine the size and the distribution of the contained bubble (that is, the pore defined by the ceramic matrix in the ceramic porous body). Therefore, these parameters are required to be appropriately selected so that a desired pore is obtained. This step is an important step which determines the porous structure.

[0096] The step C is a step of forming a gel porous molded body by transferring the resultant bubble-containing ceramic slurry in a mold of any shape followed by gelation. The gelation proceeds by leaving the slurry in the closed mold for several tens of minutes.

[0097] The step D is carried out for taking out the gel porous molded body from the mold and removing moisture and a part of organic component. Since the gel porous molded body is firm (solidified) to be held by hands, it is easily handled. Alternatively, the step D may be carried out by sliding a part of mold walls of the mold to expose at least one surface of an upper surface, a lower surface and a side surface of the gel porous molded body. Thereby, the possibility of damaging the gel porous molded body is reduced since there is no need to take out the gel porous molded body from the mold.

[0098] The drying is preferably carried out, preventing the bubbles contained in the gel porous molded body from decomposing, moving and aggregating. For example, the gel porous molded body is preferably dried slowly at a temperature of not less than 20°c and not greater than 30°C for 48 hours or longer time.

[0099] The step E includes a degreasing step in which the dried porous molded body is heated to a temperature necessary to remove excess organic component contained in the body, and a sintering step in which sintering (or firing or calcination) is carried out at a high temperature so that the ceramic powder are bonded to form the matrix. Specifically, the temperature and the time period of degreasing are determined depending on the kind and the amount of the used organic components. For example, the sintering may be carried out at from 400°C to 700°C for from 24 hours to 48 hours so as to burn out the gelatinizer. The sintering temperature is determined depending on the ceramic powder employed (that is, the glass or the hardly-sinterable ceramic powder).

[0100] For example, when silicon carbide and glass having a melting point lower than that of silicon carbide are employed as the ceramic powder, the sintering is carried out at, for example, 800°C. The sintering time period may be, for example, from 12 hours to 48 hours. When the ceramic powder containing silicon carbide and glass is employed, it is considered that a part of the silicon carbide particles are bonded to each other by the glass and most of the silicon carbide particles are bonded to each other by oxygen in this sintering process. Alternatively, only silicon carbide may be used as the ceramic powder. In that case, the sintering temperature may be from 900°C to 1350°C, and the sintering time period may be, for example, from 12 hours to 48 hours.

[0101] The step F is a step for cutting the resultant sintered body (a ceramic porous body) into a size necessary for the body to function as the acoustic matching body.

[0102] The ceramic porous body produced in this manner is a structure which has the pores whose center value of pore diameter distribution is in a range of 10µm to 500µm, the porosity of at least 60vol%, and an apparent density of from about 0.4g/cm³ to about 0.8g/cm³. Further, in the ceramic porous body, a plurality of pores is connected to form continued pores.

[0103] As shown in Fig. 11, the composite body wherein the second porous body 43 (the dried gel) is disposed within the concavity formed in the first porous body 42 (the ceramic matrix) may be produced by a method shown in Fig. 13. Fig. 13 shows a state wherein the first porous bodies 42 with concavities 63 for disposing the second porous bodies therein are placed on a mold 61 with the concavities 63 down and they are put in a container 62. The container 62 is filled with a starting material solution 64 which is prepared in the step a shown in Fig. 2 which step is described in connection with the first production method. When the mold 61 is immersed in this solution 64, the solution 64 penetrates the continued pores in the first porous body 42 which is the ceramic porous body. As a result, the concavity 63 is filled with the solution 64. Next, the step b shown in Fig. 2 is conducted keeping this state. Herein, the continued pores in the first porous body 42 are open cells which are formed by connection between the pores defined by the ceramic matrix, connection between the pore and the inter-particle void and connection between the inter-particle voids.

[0104] Fig. 14 is an enlarged view of a part of the first porous body 42 placed on the mold 61 shown in Fig. 13. Since the concavity 63 formed in the first porous body 42 is filled with the solution penetrating the continued pores in the body 42, the gel is also formed within the continued pores of the first porous body (this gel finally serves to form the second porous body (dried gel 43)). The gel formed within the concavity 63 also contacts with the mold 61. In Fig. 14, only the pores 66 defined by the ceramic matrix 65 are shown and the continued pores formed by these pores are shown, but it should be noted that the continued pores are formed between the pore 66 and the inter-ceramic particle void, and between the inter-ceramic particle voids.

[0105] Since the reconstruction step as the step c is carried out keeping this configuration, also the newly added mixture solution of the organosilane compound, water, ammonia, and ethanol passes through the gel formed in the pore 66 in the first porous body 43 and then reaches the gel formed within the concavity 63 to strengthen the framework of the gel within the concavity 63 and within the pores 66. Then, the steps up to the step d are carried out keeping this configuration.

[0106] The solution and the solvent used in each step reach the concavities 63 by passing through the gel formed within the pore 66 in the first porous body 42. In other words, the solution and so on which have passed through the gel formed within the pore 66 strengthen the gel framework formed within the concavity 63 or stop the reaction proceeding in the gel. Therefore, the pore 66 in the first porous body 42 is too small, there is disadvantage that the solution insufficiently penetrates to hardly reach the gel 43 formed within the concavity 63. When the pore 66 in the first porous body 42 is too large, the ultrasonic propagation is hindered. For this reason, the first porous body 42 is preferably formed so that the center value of the pore diameter distribution for the pore 66 defined by the ceramic matrix is in a range of 100µm to 500µm. The size of the pore 66 in the first porous body 42 is adjusted in the preparation step of the bubble-containing slurry which step is the step C shown in Fig. 12.

[0107] According to the method wherein the dried gel as the second porous body is formed by impregnation of the necessary materials so that the dried gel passes through the continued pores formed by the pores and so on defined by the matrix of the ceramic porous body as the first porous body, the steps up to the drying step for obtaining the second porous body may be carried out with a closed space formed by the concavity formed in the first porous body and the surface of the mold. As a result, a crack is less liable to be formed in the gel during the formation of the second porous body. In other words, when the dried gel is produced according to the production method shown in Fig. 2, there is disadvantage that the crack is liable to be formed in the gel since the surface of the gel is exposed. This production method, however, can prevent the dried gel from cracking, since the portion which is to become the second porous body is protected by the first porous body to be less susceptible to stress. Further, since the second porous body is formed in contact with the mold as shown in Fig. 14, its surface becomes very smooth when the surface of the mold is flat and smooth.

[0108] The edge of the second porous body 43 is protected with the first porous body 42, by disposing the second porous body 43 so that the outer edge of the second porous body contacts with the first porous body 42. Therefore, since the employment of this composite structure effectively prevents the edge of the second porous body 43 from fracturing, the thickness D of the second porous body 43 can be easily made a desired one by, for example, polishing the surface of the second porous body 43. For example, the second porous body 43 is formed into a shape having a diameter of about 8mm, and a thickness of 0.15mm to 0.4mm in the first porous body 42 having a diameter of about 10.8 mm.

(Measurement of deflecting strength)

[0109] Fig. 7 shows a cross-sectional view of a jig for measuring a deflecting strength of the dried gel. The dried gel was produced according to the first production method, using the materials, the catalyst, and the drying method shown in Table 1. Further, a dried gel was produced using only the tetraalkoxysilane compound (that is, tetraethoxysilane) for the purpose of comparison. The dried gel was made a cylindrical shape having a diameter of 10mm and a thickness of 1.5mm. A plurality of gels having different densities was produced for each of the samples, by adjusting the treating time of the reconstruction step.

Table 1

	Sample 1	Sample 2
Organosilane compound	Dimethyldimethoxysilane olygomer	-
Tetraalkoxysilane compound	Tetraethoxysilane	Tetraethoxysilane
Solvent	Ethanol	Ethanol
Catalyst	Ammonia water	Ammonia water
Hydrophobicization agent	Dimethoxydimethylsilane	Dimethoxydimethylsilane
Drying method	Natural drying	Natural drying

[0110] The resultant dried gel 26 was placed on a measuring stand 27 of the jig for measuring a deflecting strength 25 and a pushing bar 28 was moved vertically at a constant speed, and then the deflecting strength was measured. Fig. 8 shows the measurement results of deflecting strength of the dried gel.

[0111] The axis of abscissas in Fig. 8 shows the density of the dried gel sample, and the axis of ordinates shows a modulus of elasticity calculated from the deflecting strength of the dried gel sample. In Fig. 8, a broken line 29 shows the dried gel sample produced using only the tetraalkoxysilane compound (Sample 2) and a solid line 30 shows the dried gel sample to which softness is conferred by using the organic silicon compound wherein the non-hydrolyzable organic group is directly bonded to silicon (Sample 1). Sample 1 has a lower modulus of elasticity than that of Sample 2 and a higher softness compared to Sample 2 at the same density. This softness enables the silica dried gel to follow the expansion or contraction of the member to be bonded, if the member to be bonded expands or contracts due to the temperature change. Therefore, the ultrasonic oscillator wherein the acoustic matching body is the dried gel of Sample 1 has an effect of preventing the breakage of the dried gel itself and can operate over a wide range of temperatures.

[0112] The dried gel included in the acoustic matching body of the present invention, which gel is represented by Sample 1, can be formed so that the modulus of elasticity satisfies the following formula (1) although it depends on the production conditions.


wherein E is the modulus of elasticity, K is a constant (or a coefficient) of from 20 to 55, A is a constant (or a coefficient) of from 4 to 5.5, and D is a density of the dried gel.

[0113] On the other hand, the modulus of elasticity of the dried gel, represented by Sample 2, satisfies the following formula (2), in which gel the organic silicon compound in which only the hydrolyzable organic groups are bonded to silicon is polymerized as monomer. Therefore, such dried gel has the modulus of elasticity larger than that of the dried gel contained in the acoustic matching body of the present invention at any density, and has the low softness.


wherein E is the modulus of elasticity, K' is a constant (or a coefficient) of from 60 to 100, A' is a constant (or a coefficient) of from 5.5 to 7, and D is a density of the dried gel.

(Ultrasonic oscillator)

[0114] Fig. 9 shows a cross-sectional view of an embodiment of an ultrasonic oscillator 31 of the present invention having a case and Fig. 10 shows a production process chart of the ultrasonic oscillator 31. The production process of the ultrasonic oscillator 31 is described below. A step (i) is a step of forming the acoustic matching layer 2 and is as described as the first to the third production methods, and therefore the description thereof is omitted. A step (ii) is a step of printing an adhesive as bonding members 4a and 4b on a surface of an electrode 33 which surface is formed by baked silver and on a ceiling outer surface of a cylindrical metal case with a ceiling 32 which is a member to be bonded. The printing method is not limited to a particular one as long as the adhesive is printed into a predetermined thickness by the method, and it may be screen printing, gravure printing, or transfer printing. Further, the cylindrical metal case with ceiling may be formed by, for example, iron, brass, copper, aluminum, stainless, or an alloy thereof, or a metal wherein of a surface of one of these metals is plated.

[0115] The acoustic matching member 2 is attached to the outer surface of the top portion of the cylindrical metal case with ceiling. The piezoelectric material 3 is attached to the inner surface of the top portion of the case. The acoustic matching member 2 and the piezoelectric material 3 are disposed so that they are opposed to each other. In this state, the adhesive is hardened by pressing. The adhesive is not limited to a particular one, and it may be an epoxy resin, a phenol resin or a cyanoacylate resin.

[0116] In a step (iii), the cylindrical metal case with ceiling to which the piezoelectric material 3 and the acoustic matching member 2 are attached, and a terminal plate 36 in which a conducting means 35 is inserted are connected by welding. The terminal plate 36 is provided with electrode terminals 37 and 38. These terminals are insulated from each other by an insulation portion of terminal plate 39. The conducting means 35 is constructed by an elastic body such as a silicone rubber, a butadiene rubber or an elastomer and a conductive body, and electrically connects the electrode 34 and the electrode terminal 38. The electrode 33, the cylindrical case with ceiling as the member to be bonded, and the electrode terminal 37 are electrically connected.

[0117] An inert gas is injected in the inside of the closed space 40 formed by the cylindrical metal case with ceiling and the terminal plate 36 when welding the terminal plate 36 to the cylindrical metal case, whereby the ultrasonic oscillator 31 is completed. The inert gas is, for example, a helium gas or a nitrogen gas, and not limited to a particular one as long as it does not react with a silver electrode. The injection of the inert gas in the cylindrical case with ceiling at the time of weld allows the piezoelectric material 3 provided with the silver electrode to be isolated from outer environment, whereby the electrical connection is stabilized for a long period of time and long-term reliability is ensured.

[0118] The acoustic matching layer contains the dried gel formed by polymerizing the particular organic silicon compound (particularly the organosilane compound or the polysiloxane diol compound) and has softness in the ultrasonic oscillator 31 which is constructed as described above. Therefore, if the case 32 expands or contracts because of the temperature change, the acoustic matching layer is prevented from being destroyed since it can expand or contract following this expansion or contraction. Since the ultrasonic oscillator 31 of the present invention having such acoustic matching layer can operate stably over a wide range of temperatures, it can be used over a long period of time.

(Ultrasonic transmitting and receiving apparatus)

[0119] An ultrasonic transmitting and receiving apparatus including the ultrasonic oscillator which includes the acoustic matching body of the present invention as the acoustic matching layer, is described with reference to Fig. 14. Fig. 15 is a circuit block diagram showing an ultrasonic transmitting and receiving system of the present invention which is incorporated in a flowmeter 88 for measuring a fluid flow rate. An ultrasonic oscillator "A" 82 and an ultrasonic oscillator "B" 83 are disposed in a fluid passage 81. The ultrasonic oscillators "A" and "B" are placed such that the ultrasonic propagation makes an angle ϕ with the fluid passage. A transmitting signal is sent to the ultrasonic oscillator "A" 81 and the ultrasonic oscillator "B" 83 by a transmitting means 84. Further, signals received by the ultrasonic oscillators are transmitted to a receiving means 85. Transmitting or receiving is selected by a switching means 87. When the switching means 87 selects the connection of the ultrasonic oscillator "A" 82 to the transmitting means 84, the ultrasonic oscillator "B" 83 is connected to the receiving means 85.

[0120] As shown in Fig. 14, when the fluid flows from the left to the right in the drawing, the ultrasonic wave transmitted by the ultrasonic oscillator "A" 82 reaches the ultrasonic oscillator "B" 83 after a propagation time T1. On the contrary, the ultrasonic wave transmitted by the ultrasonic oscillator "B" 83 reaches the ultrasonic oscillator "A" 82 after a propagation time T2. Herein, T1<T2 is established since the fluid flow direction is left-to-right one. These times T1 and T2 are measured by a timer means 86. This time relates to a flow speed of the fluid. Since the flow rate of the fluid is calculated from the flow speed and a sectional area of the passage, the flow rate can be determined by determining the flow speed from T1 and T2. An arithmetical unit 89 determines the flow rate based on the data from the timer means 86.

[0121] As described above, the present invention provides the following acoustic matching body, ultrasonic oscillator and ultrasonic flowmeter.

[0122] The present invention, as a first mode, an acoustic matching body which includes a dried gel which is formed by polymerizing monomers containing a polymerizable organic silicon compound wherein a first nonhydrolyzable organic group is directly bonded to silicon.

[0123] The present invention provides, as a second mode, the acoustic matching body according to the first mode, which is composed of the dried gel and another porous body.

[0124] The present invention provides, as a third mode, the acoustic matching body according to the first or the second mode, wherein the another porous body is a ceramic porous body.

[0125] The present invention provides, as a fourth mode, the acoustic matching body according to any one of the first to the third modes, wherein the polymerizable organic silicon compound is an organosilane compound wherein a hydrolyzable organic group and the non-hydrolyzable organic group are directly bonded to one and the same silicon.

[0126] The present invention provides, as a fifth mode, the acoustic matching body according to any one of the first to the fourth modes wherein the monomer further contains a compound represented by a formula (1) or a partially-hydrolyzed polymer of the compound represented by formula (1).

        Si(OR¹)₄     (1)

wherein each of R¹ is a substituted or nonsubstituted monovalent hydrocarbon group with a carbon number of 1 to 8.

[0127] The present invention provides, as a sixth mode, the acoustic matching body according to the fifth mode, wherein the dried gel is formed by polymerizing the compound represented by the formula (1) or the partially-hydrolyzed polymer of the compound represented by the formula (1) to give a wet gel, and then polymerizing the polymerizable organic silicon compound under the wet gel.

[0128] The present invention provides, as a seventh mode, the acoustic matching body according to the fifth or the sixth mode, wherein the dried gel is formed by the monomers in which an amount of the polymerizable organic silicon compound is 0.1wt% to 10wt% relative to a sum of the compound represented by the formula (1) and the partially-hydrolyzed polymer of the compound represented by the formula (1).

[0129] The present invention provides, as an eight mode, the acoustic matching body according to any one of the first to the seventh modes, wherein the polymerizable organic silicon compound is at least one of the compounds represented by the formulas (2) and (3).

        R³Si(OR²)₃     (2)

        (R⁵)₂Si(OR⁴)₂     (3)

wherein each of R² and R⁴ is an alkyl group with a carbon number of 1 to 4 and each of R³ and R⁵ is a substituted or non-substituted monovalent hydrocarbon group with a carbon number of 1 to 8.

[0130] The present invention provides, as a ninth mode, the acoustic matching body according to the eight mode, wherein the dried gel is formed by polymerizing at least one of the compounds represented by the formulas (2) and (3) so that a molecular weight is 200 or more.

[0131] The present invention provides, as a tenth mode, the acoustic matching body according to any one of the first to the eighth modes, wherein the dried gel is formed by polymerizing monomers containing a compound represented by a formula (4) as the polymerizable organic silicon compound.

        HO((R⁶)₂SiO)_nH     (4)

wherein R⁶ is an alkyl group with a carbon number of 1 to 4 and n is an integer of 2 or greater.

[0132] The present invention provides, as an eleventh mode, the acoustic matching body according to the first to the tenth modes, wherein the monomers further includes a copolymer of at least one of compounds represented by the formulas (5) and (6) and a compound represented by the formula (7).

        CH₂=CR¹⁰(COOR¹¹)     (5)

        CH₂=CR¹²(COOR¹³)     (6)

        CH₂=CR¹⁴(COOR¹⁵)     (7)

wherein each of R¹⁰, R¹² and R¹⁴ is hydrogen atom or a methyl group, R¹¹ is a substituted or non-substituted monovalent hydrocarbon with a carbon number of 1 to 9, R¹³ is a group selected from a group consisting of an epoxy group, a glycidyl group, and a hydrocarbon group containing at least one of these groups (for example, a γ-glycidoxypropyl group), and R¹⁵ is a hydrocarbon group containing an alkoxysilyl group or a halogenated silyl group.

[0133] The present invention provides, as a twelfth mode, the acoustic matching body according to any one of the first to the eleventh modes, wherein the dried gel has a modulus of elasticity which satisfies the following formula (1).


wherein E is the modulus of elasticity, K is a constant (or a coefficient) of from 20 to 55, A is a constant (or a coefficient) of from 4 to 5.5, and D is a density of the dried gel.

[0134] The present invention provides, as a thirteenth mode, an ultrasonic oscillator which includes an piezoelectric material and an acoustic matching layer, wherein the acoustic matching layer is formed of the acoustic matching body according to any one of the first to the twelfth modes.

[0135] The present invention provides, as a fourteenth mode, an ultrasonic flowmeter which includes:

a passage where a fluid to be measured flows;

a pair of the ultrasonic oscillators according to the thirteenth mode; and

a timer device for measuring an ultrasonic-wave propagation time between the ultrasonic oscillators.

Industrial Applicability

[0136] As described above, the acoustic matching body of the present invention includes the dried gel having softness and the ultrasonic oscillator having the dried gel as the acoustic matching layer is less liable to break since the dried gel follows the expansion or contraction of the case even if the case expands or contracts. Therefore, the ultrasonic oscillator having the acoustic matching body of the present invention can stably operate over a wide range of temperatures and suitable for being used as an industrial or a household ultrasonic gas flowmeter for measuring a flow rate of a natural gas or a liquefied petroleum gas (for example, a gas meter) and an water works flowmeter.

Claims

1. An acoustic matching body which comprises a dried gel which is formed by polymerizing monomers comprising a polymerizable organic silicon compound wherein a nonhydrolyzable organic group is directly bonded to silicon.

2. The acoustic matching body according to claim 1, which is composed of the dried gel and another porous body.

3. The acoustic matching body according to claim 2, wherein the another porous body is a ceramic porous body.

4. The acoustic matching body according to claim 1, wherein the polymerizable organic silicon compound is an organosilane compound wherein a hydrolyzable organic group and the non-hydrolyzable organic group are directly bonded to one and the same silicon.

5. The acoustic matching body according to claim 1 wherein the monomer further comprises a compound represented by a formula (1) or a partially-hydrolyzed polymer of the compound represented by formula (1),

        Si(OR¹)₄     (1)

wherein each of R¹ is a substituted or nonsubstituted monovalent hydrocarbon group with a carbon number of 1 to 8.

6. The acoustic matching body according to claim 5, wherein the dried gel is formed by polymerizing the compound represented by the formula (1) or the partially-hydrolyzed polymer of the compound represented by the formula (1) to give a wet gel, and then polymerizing the polymerizable organic silicon compound under the wet gel.

7. The acoustic matching body according to claim 5, wherein the dried gel is formed by the monomers in which an amount of the polymerizable organic silicon compound is 0.1wt% to 10wt% relative to a sum of the compound represented by the formula (1) and the partially-hydrolyzed polymer of the compound represented by the formula (1).

8. The acoustic matching body according to claim 4, wherein the organosilane compound is at least one of the compounds represented by the formulas (2) and (3),

        R³Si(OR²)₃     (2)

        (R⁵)₂Si(OR⁴)₂     (3)

wherein each of R² and R⁴ is an alkyl group with a carbon number of 1 to 4 and each of R³ and R⁵ is a substituted or non-substituted monovalent hydrocarbon group with a carbon number of 1 to 8.

9. The acoustic matching body according to claim 8, wherein the dried gel is formed by polymerizing at least one of the compounds represented by the formulas (2) and (3) so that a molecular weight is 200 or more.

10. The acoustic matching body according to claim 1, wherein the dried gel is formed by polymerizing monomers comprising a compound represented by a formula (4) as the polymerizable organic silicon compound:

        HO((R⁶)₂SiO)_nH     (4)

wherein R⁶ is an alkyl group with a carbon number of 1 to 4 and n is an integer of 2 or greater.

11. The acoustic matching body according to claim 1, wherein the monomers further comprises a copolymer of at least one of compounds represented by the formulas (5) and (6) and a compound represented by the formula (7),

        CH₂=CR¹⁰(COOR¹¹)     (5)

        CH₂=CR¹²(COOR¹³)     (6)

        CH₂=CR¹⁴(COOR¹⁵)     (7)

wherein each of R¹⁰, R¹² and R¹⁴ is hydrogen atom or a methyl group, R¹¹ is a substituted or non-substituted monovalent hydrocarbon with a carbon number of 1 to 9, R¹³ is a group selected from a group consisting of an epoxy group, a glycidyl group, and a hydrocarbon group containing at least one of these groups (for example, a γ-glycidoxypropyl group), and R¹⁵ is a hydrocarbon group containing an alkoxysilyl group or a halogenated silyl group.

12. The acoustic matching body according to claim 1, wherein the dried gel has a modulus of elasticity which satisfies the following formula (1),


wherein E is the modulus of elasticity, K is a constant of from 20 to 55, A is a constant of from 4 to 5.5, and D is a density of the dried gel.

13. A method for producing an acoustic matching body which comprises forming a dried gel by a method which comprises:

forming a wet gel by polymerizing monomers which comprise a polymerizable organic silicon compound wherein a nonhydrolyzable organic group is directly bonded to silicon; and

drying the wet gel.

14. A method for producing an acoustic matching body which comprises forming a wet gel by a method which comprises:

forming a first wet gel by polymerizing monomers which comprises a compound represented by a formula (1) or a partially-hydrolyzed polymer of the compound represented by formula (1),

        Si(OR¹)₄     (1)

wherein each of R¹ is a substituted or nonsubstituted monovalent hydrocarbon group with a carbon number of 1 to 8.

forming a second wet gel by polymerizing, in the first wet gel, monomers comprising a polymerizable organic silicon compound wherein a nonhydrolyzable organic group is directly bonded to silicon; and

drying the second wet gel.

15. A method for producing an acoustic matching body which comprises:

(1) forming a first porous body by a method comprising:

gelating a bubble-containing ceramic slurry comprising at least one kind of ceramic powder within a first mold to give a gel porous molded body that has one or more concavities;

drying and degreasing the gel porous molded body; and

sintering the gel porous molded body; and

(2) forming a second porous body within the one or more concavities by a method comprising
placing the first porous body in another mold;
charging a starting material solution which comprise monomers comprising a polymerizable organic silicon compound wherein a nonhydrolyzable organic group is directly bonded to silicon, in the another mold to impregnate the first porous body with the staring material solution; and
polymerizing the monomers to give a wet gel, and
drying the wet gel.

16. An ultrasonic oscillator which comprises an piezoelectric material and an acoustic matching layer, wherein the acoustic matching layer is formed of the acoustic matching body according to any one of claims 1 to 12.

17. An ultrasonic flowmeter which comprises:

a passage where a fluid to be measured flows;

a pair of the ultrasonic oscillators according to the claim 16; and

a timer device for measuring an ultrasonic-wave propagation time between the ultrasonic oscillators.

Drawing