JP2008193291A - Acoustic matching layer containing dry gel and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the following problem: it is difficult to obtain acoustic matching layers containing dry gel of high density. <P>SOLUTION: Wet gel is created by tetramethoxysilane/ethanol/water = 1/1/4 (molar ratio) inside a cylinder of reinforced porous body with ammonia used as a catalyst. Gelation is advanced by similarly pouring mixed solution containing the gel raw materials of tetramethoxysilane/ethanol/water = 1/3/4 (molar ratio) onto the wet gel. Moreover, the same process is repeated once to form wet gel of three consecutive layers. After this is washed by ethanol and supercritical drying is performed by using carbon dioxide as supercritical fluid, an acoustic matching layer which is compounded with the reinforced porous body and contains the dry gel where the density increases from an upper part to a lower part stepwise in three steps can be obtained. The dry gel on the bottom part becomes very strong and its strength cannot be obtained by onetime gelation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a porous body used for an acoustic matching layer of a transducer such as an ultrasonic wave and a method for manufacturing the same.

  The low-density porous body is used as an acoustic matching layer of an ultrasonic transducer because of its low density and low acoustic impedance due to its low sound velocity.

  A dry gel as a low-density porous body is obtained by mixing a gel raw material, water, a solvent, and a gelation catalyst to advance gelation to produce a wet gel and then removing the solvent. However, during drying, the gel contracts due to the capillary force generated in the pores. Since the hydroxyl groups exist in the pores of the gel, the contracted state is fixed by bonding the hydroxyl groups together. In order to prevent this, a method has been disclosed in which the surface of a wet gel is modified with an organic silyl group to make it hydrophobic, thereby reducing the capillary force during drying and suppressing shrinkage.

  Two examples are shown below.

  In the first example, an alkoxysilane that is a metal alkoxide is used as a raw material. From this raw material, a wet gel containing an alcohol solvent is formed by using the sol-gel method. Furthermore, the water remaining in the wet gel is removed by substitution with alcohol. Next, in supercritical carbon dioxide, a silylating agent as a hydrophobizing agent is allowed to act on the wet gel surface to introduce an organic silyl group, and a hydrophobizing treatment is performed. Finally, supercritical drying is performed by removing carbon dioxide above the critical temperature. The dried gel thus obtained has a skeleton made of silicon oxide and a porous body having an organic silyl group on the surface (see, for example, Patent Document 1).

  In the second example, a wet gel is formed as in the first example, and water remaining in the wet gel is removed by alcohol substitution. Hydrophobization is performed by introducing an organic silyl group by allowing a silylating agent to act on the surface of the wet gel in a hydrocarbon solvent. Drying is performed by removing the solvent in the wet gel by heating. Thus, a porous body having a skeleton made of silicon oxide and having an organic silyl group on the surface is obtained (for example, see Non-Patent Document 1).

  Also by the above two methods, the capillary force generated at the time of drying is reduced, but also the shrinkage temporarily proceeds at the time of drying. However, since the hydroxyl group is reduced, the contracted state is not fixed, and when the capillary force is lost after drying, a phenomenon of returning to the original size occurs.

  A porous body composed of a dry gel having a certain shape is obtained by pouring a solution containing a gel raw material into a certain mold to cause gelation. However, for the reasons already described, temporary shrinkage occurs during drying, and cracks occur due to the difference in size between the mold and the dried gel during drying.

  In order to avoid this, supercritical drying is applied (for example, refer nonpatent literature 2). In supercritical drying, the solvent present in the wet gel at the time of drying is a fluid in a supercritical state, so that no surface tension is generated and no capillary force proportional to the surface tension is generated. For this reason, contraction is suppressed.

Further, in order to increase the density of the obtained dried gel, the concentration of the gel raw material in the solution may be increased during gelation. However, when the concentration of the gel raw material is high, a uniform solution cannot be obtained, and thus a uniform gel cannot be obtained. Therefore, it is difficult to obtain a porous body made of a dried gel having a uniform density of a certain level or higher.

For example, in the case of tetramethoxysilane, which is a typical gel material, tetramethoxysilane, ethanol, and aqueous ammonia are mixed to produce a gel. When the gel is prepared, the concentration of tetramethoxysilane is increased by reducing ethanol as the solvent. However, since uniform gelation does not proceed, it is difficult to adjust to a uniform density at 300 kg / m ³ or more. Similarly, when tetraethoxysilane is used as the gel material, it is difficult to obtain a dry gel with a uniform density at 200 kg / m ³ or more.

When these gels are not hydrophobized or not dried by supercritical drying, the density may increase due to shrinkage during drying. However, cracking occurs due to the stress due to the shrinkage and it cannot be used as an acoustic matching layer. For this reason, it becomes difficult to obtain an acoustic matching layer having a density higher than the density determined by the upper limit concentration of the gel raw material in the solution used for the gelation.
JP-A-7-138375 Journal of Crystalline Solid 186, 104-112, 1995 Journal of Non-Crystaline Solid 186, 23, 1996

  However, in the conventional configuration, since the acoustic matching layer made of the dried gel has a limit in the density range that can be adjusted, when the acoustic matching layer is incorporated in an ultrasonic transducer, the transmission / reception efficiency of ultrasonic waves is low. There was a problem of not becoming high.

  In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a method for manufacturing an acoustic matching layer including a dry gel whose density can be adjusted over a wide range, and a corresponding acoustic matching layer.

  In order to solve the above-mentioned problems, the acoustic matching layer manufacturing method including the dried gel according to the present invention includes a gelling step of forming a wet gel by a sol-gel method using a gel raw material, and removing the solvent included in the wet gel. A method for producing an acoustic matching layer comprising a dry gel comprising a drying step of obtaining a dry gel, wherein the gelation step forms a solid skeleton of the wet gel from a gel raw material, and the The first gelation step further comprises at least one second gelation step in which a new solid skeleton is formed on the solid skeleton after the first gelation step, and the second gelation step is the first gelation step. It is performed by immersing the wet gel formed in (2) in the second gelling mixed solution containing the second gel raw material. Thus, by repeating the gelation a plurality of times, it is possible to adjust the density, which was difficult with one gelation.

  The manufacturing method of the acoustic matching layer containing the dry gel of the present invention is characterized by including a second gelation step after the gelation step. As a result, an acoustic matching layer containing a dry gel whose density can be adjusted in a wide range can be obtained, and when incorporated in an ultrasonic transducer, an effect of increasing ultrasonic transmission / reception efficiency can be obtained.

  Embodiments of the present invention will be described below.

  The manufacturing method of the acoustic matching layer containing the dried gel of the present invention is different from the conventional manufacturing method in which the drying process is performed from the gelation process as shown in FIG. 1 (c). It is a feature that a second gelation step is provided after the gelation step. In the second gelation step, a new solid skeleton is formed while polymerizing the gel raw material on the solid skeleton of the wet gel formed in the first gelation step. By forming a new gel skeleton in this way, the density of the skeleton can be increased.

(Embodiment 1)
The production method of the present embodiment is characterized in that the second gelation is advanced so that the second gelation mixture does not gel outside the wet gel. As described below, this makes it possible to increase the density of the wet gel obtained in the first gelation step as it is.

  Each step will be described in turn below.

(First gelation step)
In the present invention, first, a wet gel is prepared by a so-called sol-gel method. In that case, gelation is advanced using the 1st gelation liquid mixture obtained by mixing a gelation catalyst, a solvent, etc. with a gel raw material. At the time of gelation, the first liquid mixture for gelation is poured into a mold having a shape necessary for the matching layer, and the gel is formed by covering with a lid from the top to form the shape necessary for the matching layer. Since the mold used here is usually finally removed and used as an acoustic matching layer, a material that does not react with the gel is preferable so that the formed gel can be easily removed.

  For example, a fluororesin such as Teflon (registered trademark) is most preferable, but a metal coated with fluorine can also be suitably used. However, when the thickness of the mold is small and the acoustic effect is small, it is possible to use it as an acoustic matching layer as it is without removing the lid from the mold.

  As the gel material used in the production method of the present invention, general materials used in the sol-gel method are used. For example, there are oxide fine particles such as silicon, aluminum, zirconium and titanium, and corresponding metal alkoxides. Among these, a compound containing silicon as a metal is preferable from the viewpoint of availability. In the case of the metal alkoxide, as already described, silicon is preferable from the viewpoint of easy reaction control.

  For example, tetraalkoxysilanes such as tetramethoxysilane and tetraethoxysilane, and silicon alkoxides such as trialkoxysilane and dialkoxysilane are used. Here, since dialkoxysilane is difficult to form a gel by itself, it is used by mixing with other gel raw materials.

  Furthermore, when these oligomers are used as the gel raw material, the boiling point of the gel raw material is increased, so that the safety at the time of production is enhanced. In addition, colloidal silica, water glass, and an aqueous silicic acid solution obtained by electrodialysis from water glass are preferably used because of their low price.

  As the gelation catalyst, a general organic acid, inorganic acid, organic base, or inorganic base is used. Examples of organic acids include acetic acid and citric acid, inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, organic bases such as piperidine, and inorganic bases such as ammonia. In addition, use of an imine type such as piperidine is more preferable from the viewpoint of reducing capillary force because of the effect of increasing the pore diameter.

  When the gel raw material is a metal alkoxide, a water-soluble solvent that dissolves the metal alkoxide and water, particularly alcohols such as ethanol and methanol are preferably used as the solvent used for gelation. When the gel raw material is water glass or the like, the solvent is water.

(Second gelation step)
Subsequent to the first gelation step, a second gelation step is performed.

  Specifically, as shown in FIG. 2A, the second gelation is performed by immersing the wet gel 1a obtained in the gelation step in the second gelation mixture 2 containing the gel raw material. The process proceeds. The wet gel 1a in FIG. 2 (a) is a state in which the wet gel molded in the mold is removed from the mold in the first gelation step. At this time, a new wet gel skeleton is formed on the wet gel solid skeleton inside the wet gel 1a, and the wet gel 1b having an increased solid skeleton density is formed.

  At this time, it is a feature of the present embodiment that the second gelation mixed solution 2 outside the wet gel 1b having an increased solid skeleton density is not gelled. For this reason, it is possible to adjust the density by maintaining the shape of the wet gel 1a formed in the gelation step and selectively increasing the density of the solid skeleton.

  The density can be adjusted by changing the time during which the wet gel is immersed in the second gelling mixture. It can also be adjusted by changing the concentration of the second gel raw material described later, the amount of the second gelation catalyst, and the like.

In this way, by adjusting the density, the acoustic impedance represented by the product of the sound speed is adjusted to an optimum value, and the effect of increasing the transmission and reception efficiency when used as a matching layer of an ultrasonic transducer Is obtained. In particular, it is easy to adjust to a density of 300 kg / m ³ or more, which is difficult with normal gelation. Furthermore, it is also possible to adjust the 400kg ^{/ m 3} ~900kg ^/ m 3 if necessary. In this case, the strength is increased, and the effect of handling is obtained.

  As the second liquid mixture for gelation, a solution obtained by adding a solvent to the second gel raw material and the second gelation catalyst as necessary is used. As the second gelation raw material, a gel raw material that can be used in the first gelation step is used. For example, there is a metal alkoxide, and in this case, water necessary for hydrolysis of the metal alkoxide is also added. A larger amount of water is preferable because the speed of the second gelation is faster, but if it is too much, gelation proceeds even outside the wet gel 1a, so adjust the amount while confirming the presence or absence of gelation. is required. As the second gelation catalyst, a gelation catalyst that can be used in the first gelation step is used. Similarly, it is necessary to accelerate the second gelation, but it is necessary to adjust the amount so that the gelation does not proceed.

  Even if it does not normally gel by itself, the gel skeleton can be formed on the gel skeleton that has already been formed. Therefore, the raw material used for the gel raw material, its concentration, the range of the solvent type, etc. can be expanded.

  In addition, the second gelation step is accelerated by the addition concentration of the second gelation catalyst and water, and the second gelation can sufficiently proceed even at room temperature or lower. It is preferable when it is necessary to immerse in the second gelled mixed solution for processing.

  In addition, as the second gelation catalyst used in the second gelation step, those in the gelation catalyst group used in the first gelation step can be used, particularly during the first gelation step. It is not necessary to be the same as the gelation catalyst.

Moreover, as a 2nd gel raw material used at a 2nd gelation process, the gel raw material used at a 1st gelation process is used, and the relationship with the gelation raw material at the time of a 1st and 2nd gelation process is especially There are no restrictions. For example, when tetraethoxysilane, which is an alkoxysilane, is used as the gelation material in the first gelation step, tetraethoxysilane can be used as the second gel material during the second gelation. If a solvent in which the raw material is dissolved is selected, other metal alkoxides, silicic acid aqueous solutions, and other phenol-based and urethane-based organic gel raw materials can also be used.

  The solvent used in this step is not particularly limited as long as the second gel raw material and the second gelation catalyst are dissolved as described above.

(Hydrophobicization process)
Hydrophobic process is not always necessary, but when used in acoustic matching layers of transducers such as heat insulating materials, sound absorbing materials, and ultrasonic waves, the performance may be reduced by moisture absorption. preferable. This step can be performed before or after the drying step of FIG.

  When performed before the drying step, the hydrophobic group is introduced into the wet gel by reacting the hydroxyl group on the wet gel surface obtained up to the second gelation step with a hydrophobizing agent dissolved in the solvent. In the case of chlorosilane or the like, which will be described later, the hydrophobizing agent reacts with water and decreases the reactivity with the wet gel surface. Therefore, the hydrophobizing agent can be washed with a water-soluble solvent before hydrophobization or with water. It is preferable to remove water by distilling off using a boiling solvent.

  The hydrophobizing agent used in the present invention is preferably a silylating agent from the viewpoint of high reactivity, and examples thereof include silazane compounds, chlorosilane compounds, alkylsilanol compounds, and alkylalkoxysilane compounds.

  In the case of a silazane compound, a chlorosilane compound, or an alkylalkoxysilane compound, these silylating agents react with silanol groups on the gel surface after being directly or hydrolyzed to the corresponding alkylsilanol. Further, when alkylsilanol is used as a silylating agent, it reacts with the silanol group on the surface as it is.

  Among these, chlorosilane compounds and silazane compounds are particularly preferred because of their high reactivity during hydrophobization and availability, and because they are easily available and do not generate gases such as hydrogen chloride and ammonia during hydrophobization. Alkyl alkoxysilanes are particularly preferably used.

  Specifically, chlorosilane compounds such as trimethylchlorosilane, methyltrichlorosilane and dimethyldichlorosilane, silazane compounds such as hexamethyldisilazane, methoxytrimethylsilane, ethoxytrimethylsilane, dimethoxydimethylsilane, dimethoxydiethylsilane and diethoxydimethylsilane There are silylating agents represented by silanol compounds such as alkylalkoxysilane compounds, trimethylsilanol and triethylsilanol. If these are used, hydrophobization can be promoted by introducing an alkylsilyl group such as a trimethylsilyl group into the wet gel or dry gel surface.

  In addition, when a hydrophobizing step is incorporated after the drying step, the dried gel can be reacted with hydroxyl groups remaining on the gel surface by exposure to hydrophobizing agent vapor to introduce hydrophobic groups. As the hydrophobizing agent used at this time, the hydrophobizing agents described below can be used, but chlorosilane compounds such as trimethylchlorosilane and dimethyldichlorosilane are most preferable because of their high reactivity. When using a hydrophobizing agent other than the chlorosilane compound, it is also effective to use a catalyst that can be introduced in a gaseous state, such as ammonia or hydrogen chloride.

  Further, when hydrophobizing in the gas phase, the temperature during hydrophobizing can be increased without being limited by the boiling point of the solvent or hydrophobizing agent. Therefore, hydrophobization in the gas phase is effective for speeding up the reaction. Moreover, if the wet gel is a thin film or powder, the penetration of the hydrophobizing agent vapor is easy, and the thin film is more preferable because the effect of reducing the amount of solvent is great.

(Drying process)
Following the hydrophobization step, a drying step is performed. In this step, a dry gel is obtained by removing the solvent from the wet gel obtained up to the hydrophobization step.

  There are the following three methods for drying the solvent from the wet gel.

(1) Heat drying method (2) Supercritical drying method (3) Freeze drying method The heat drying method is the most common drying method, and the solvent is vaporized and removed by heating the wet gel containing the solvent. To do. During drying, a capillary force proportional to the surface tension of the solvent in the gel may act and the gel may shrink. For this reason, the solvent for drying is preferably a hydrocarbon-based solvent having a low surface tension at the boiling point, and particularly inexpensive hexane, pentane or a mixture thereof is preferable.

  The supercritical drying method uses a supercritical fluid having a surface tension of 0 in order to reduce the capillary force generated by lowering the surface tension of the solvent when the solvent is removed. As the supercritical fluid used for drying, there are supercritical states such as water, alcohol, carbon dioxide, etc., but carbon dioxide that can be obtained at the lowest temperature and is harmless is preferably used. Specifically, by first introducing liquefied carbon dioxide into the pressure vessel, the solvent in the wet gel in the pressure vessel is replaced with liquefied carbon dioxide. Next, the pressure and temperature are raised to a critical point or higher to achieve a supercritical state, and carbon dioxide is gradually released while maintaining the temperature to complete drying.

  The lyophilization method is a drying method in which after the solvent in the wet gel is frozen, the solvent is removed by sublimation. Since it does not go through a liquid state, a gas-liquid interface does not occur in the gel. As a result, capillary force does not work and gel shrinkage during drying can be suppressed.

  The solvent used in the freeze-drying method is preferably a solvent having a high vapor pressure at the freezing point, and examples thereof include tert-butanol, glycerin, cyclohexane, cyclohexanol, para-xylene, benzene, phenol and the like. Among these, tert-butanol and cyclohexane are particularly preferable because of high vapor pressure at the melting point.

  At the time of lyophilization, it is effective to replace the solvent in the wet gel with a solvent having a high vapor pressure at the above-mentioned freezing point. In addition, it is more preferable to use a solvent having a high vapor pressure at the freezing point as the solvent used for the gelation because the solvent replacement can be omitted and efficient production becomes possible.

  The drying can be performed either before or after the hydrophobizing step. When performed before the hydrophobization step, the hydrophobization is carried out by introducing hydrophobic groups on the surface of the dry gel by exposing the dry gel to the hydrophobizing agent vapor instead of in solution. Therefore, there is an effect that the amount of solvent to be used is reduced.

  On the other hand, the second gelation step and the hydrophobization step can be performed simultaneously. In this case, since the two steps are simultaneously performed, an effect that a porous body made of a dry gel can be obtained in a short time is obtained.

  In the second gelling and hydrophobizing step, specifically, the wet gel obtained in the first gelling step was mixed with the second gelation mixture used in the second gelation step and the hydrophobizing agent. It is carried out by immersing in the treatment liquid. By doing so, the second gelation and the hydrophobization proceed simultaneously. As the second gel raw material during the second gelation and the hydrophobizing agent during the hydrophobization, the same ones as described above can be used. For example, after performing first gelation from an aqueous silicic acid solution obtained by electrodialysis from water glass, the resulting wet gel is converted into an alkoxysilane as a gel raw material and a hydrophobizing agent by using a water-soluble solvent. The second gelation and hydrophobization step is performed in a solution in which the silazane compound and the like are dissolved.

  When the second gelation step and the hydrophobization step are performed simultaneously as described above, the combination of these gel raw materials and hydrophobizing agents is not particularly limited as long as solubility is satisfied. In addition, when a polyfunctional alkylalkoxysilane, chlorosilane, or alkylsilanol is used as a hydrophobizing agent, an alkyl group is introduced into the gel skeleton, so that flexibility is imparted to the gel and brittleness is improved. There is also an effect.

(Embodiment 2)
The manufacturing method of the present embodiment is characterized in that the second gelation is advanced so that the second gelation mixture is gelled outside the wet gel. As described below, this can provide an acoustic matching layer that includes a dry gel having a density distribution.

  Specifically, as shown in FIG. 2 (b), as in the first embodiment, the wet gel 1c obtained in the first gelation step is mixed with the second gelation mixture containing the gel raw material. 2nd gelation process advances by being immersed in 2. Here, the wet gel 1c is put in the mold 3 before the second gelation, and after being immersed in the second gelation mixture 2, the gelation is completed after being covered with the mold 3. To do. At the time of gelation, a new wet gel skeleton is formed on the wet gel solid skeleton inside the wet gel 1c, resulting in a wet gel 1d having an increased solid skeleton density. This is the same as in the first embodiment.

  Further, at this time, the second gelation mixed solution 2 outside the wet gel 1c is also gelled to form a wet gel 1e having a solid skeleton having a relatively low density compared to the wet gel 1d. This is a feature of the present embodiment. For this reason, the acoustic matching layer containing the dried gel obtained by drying and removing it from the mold has a density distribution that decreases in the vertical direction. The density adjustment of the acoustic matching layer is performed by adjusting the concentration of the second gelation raw material in the second gelation liquid mixture.

  That is, by increasing the concentration of the second gelation mixed solution 2, the density of the wet gel 1e formed outside the wet gel 1c formed in the first gelation step can be increased. A density increase corresponding to the density is also achieved in the wet gel 1c obtained in the first gelation step.

  In the present embodiment, since the second gelling mixed solution gels outside the wet gel 1c formed in the first gelling step, the second gelling mixed solution is contained in the wet gel 1c. Without sufficient diffusion and penetration, the density of the wet gel 1c does not increase sufficiently. Therefore, it is necessary to sufficiently lengthen the time for the second gelling liquid mixture to gel outside the wet gel 1c. This is possible by adjusting the catalyst concentration to be low.

  The second gelation mixture 2 used in the present embodiment needs to form a gel even outside the wet gel 1c, and the gel raw material, gelation catalyst, and the like used in the conventional gelation step, A mixed solution of a solvent, water and the like can be used.

The other steps are the same as those in the first embodiment. For example, the drying is performed using supercritical drying using carbon dioxide as a supercritical fluid. If the solvent at the time of the second gelation is poorly compatible with liquefied carbon dioxide such as water, replace the water in the wet gel with a water-soluble solvent such as alcohol, Replace and perform supercritical drying.

  Further, the shape of the gel formed in the first gelation step and the shape of the gel formed in the second gelation step can be determined depending on the mold used, and the corresponding shape can be arbitrarily selected. Therefore, it is possible to obtain an acoustic matching layer including a dry gel having a high density portion corresponding to an arbitrary shape corresponding to the mold and having a two-stage density distribution.

  In particular, as shown in FIG. 2B, if the acoustic matching layer is arranged so that the lower density portion is located on the piezoelectric body side that emits the sound wave, and an ultrasonic transducer is configured, the ultrasonic wave has an acoustic impedance. Is transmitted from the higher side to the lower side, the effect of increasing the efficiency of ultrasonic wave transmission / reception is obtained.

(Embodiment 3)
The manufacturing method of the present embodiment is characterized in that the second gelation step is repeated a plurality of times as shown in FIG. By repeating a plurality of times, a porous body made of a dry gel having a portion with a higher density and further having a density distribution of several steps is obtained.

  This embodiment will be described based on FIG.

  In the upper left part of FIG. 3, the second gelation mixed solution 2 is added to the upper part of the wet gel 1f obtained in the first gelation step.

  Here, the reinforcing porous body 5 is the outer periphery when used as the acoustic matching layer, and the portion surrounded by the reinforcing porous body 5 finally becomes the acoustic matching layer. Further, the reinforced porous body 5 is a porous body having communication holes, and the size of the holes is about 1 μm or more. Therefore, the gelled mixed solution enters the holes and the gelation proceeds. Thus, a composite body of the reinforcing porous body 5 and the wet gel 1f is formed. As the reinforced porous body, a silica fine particle formed by bonding with a low melting point glass or a porous body mainly composed of SiC is preferably used, but it should be used if it does not deform by contact with an organic solvent. Can do. The size of the communication hole of the auxiliary porous body is preferably about 10 μm to about several hundred μm so that the mixed solution for gelation can more easily permeate.

  First, the first second gelation step is performed. At this time, the second gelling mixed solution forms a solid skeleton of the wet gel inside and outside the wet gel 1f while diffusing and penetrating into the wet gel 1f, so that the wet gel 1g and the wet gel 1h having different densities are formed. The In the present embodiment shown in FIG. 3, the second gelation step is advanced once more. Specifically, the second gelling mixed solution is added on 1 g of the wet gel and 1 h of the wet gel, and the wet gel 1i, the wet gel 1j, and the wet gel 1k are formed in the same manner. When this is dried, an acoustic matching layer 4 containing a dried gel whose density gradually decreases from the bottom to the top as shown in FIG. 3 is obtained.

  In FIG. 3, the number of times of the second gelation is 2 times, but it is possible to carry out it 3 times or more. By doing so, a porous body having a density distribution of four or more stages can be obtained.

  For example, the acoustic matching layer 4 made of a dry gel having a density change that decreases in the thickness direction as shown in FIG. 3 is suitably used for an acoustic matching layer of a transducer such as an ultrasonic wave. This is because when the acoustic matching layer vibrates the piezoelectric body and emits an ultrasonic wave, a material having an acoustic resistance intermediate between the high acoustic resistance of the piezoelectric body and the low acoustic resistance of air is used to transmit and receive sound waves. This is because the efficiency of transmission and reception is higher when the density changes stepwise and the acoustic resistance also changes stepwise.

  In addition, due to the density distribution, the wavelength dispersion of ultrasonic waves that are easily transmitted increases and, on the contrary, the time dispersion decreases. This is advantageous because the time resolution is increased when measuring time using an ultrasonic transducer.

  Moreover, since the outer periphery of the acoustic matching layer 4 is a composite of the reinforcing porous body 5 and the dried gel, the strength of the outer periphery is increased, and the effect that the handling becomes easy is also obtained. Furthermore, since the density of the bottom part of the acoustic matching layer 4 is high, strength can be ensured. As described above, the strength of the outer peripheral portion and the bottom portion, which is a problem during the normal handling and the incorporation of the adhesive to the ultrasonic transducer, etc., is secured, so that the ultrasonic transducer can be easily manufactured. An effect is obtained. Such an effect can be obtained by using a porous body in which the outer periphery and the bottom are connected. However, the method of the present embodiment is simpler and more effective because it requires molding and post-processing of the reinforced porous body. Is easy.

  The steps other than the second gelation step are the same as those in the first embodiment, and the same methods, raw materials, and the like can be used.

  Hereinafter, the present invention will be described based on specific examples. However, the present invention is not necessarily limited to contents such as the type and amount of raw materials such as gel raw materials and gelation catalysts used below. .

(Example 1)
In this example, after the first gelation step, the second gelation step was repeated twice, and then the hydrophobization step and the drying step were performed to produce an acoustic matching layer containing the dried gel. At the time of the second gelation step, adjustment was performed so that the second gelation mixed solution gelled outside the wet gel obtained in the first gelation step.

As the reinforcing porous body, a porous body having a density of 700 kg / m ^{3 made} of silica fine particles and low melting point glass and having communication holes with a pore diameter of about 40 μm was used. The reinforced porous body used was a cylindrical one having a diameter of 40 mm, a height of 9 mm, and a thickness of 10 mm.

1) First Gelation Step First, a solution mixed with tetramethoxysilane / ethanol / water = 1/4 (molar ratio) is placed in a container in which the reinforcing porous body 5 is placed at the bottom, and 1 at room temperature. The wet gel 1f (FIG. 3) was formed so as to have a thickness of 2 mm by being placed on a day.

  Water was used in the form of aqueous ammonia (0.1 N), and ammonia was used as the gelation catalyst. During this time, the container in which the wet gel was formed was sealed to prevent drying of the wet gel.

2) Second gelation step Tetramethoxysilane same as the first gelation step was used as the second gelation raw material, and ammonia water was used as the second gelation catalyst.

  Before entering the second gelation step, 10 times the volume of ethanol is added to the wet gel 1f (FIG. 3) combined with the reinforced porous body 5 (FIG. 3) obtained in the first gelation step. After standing for a period of time, the solvent outside the wet gel 1f was discharged out of the container. This was further performed once to wash the wet gel 1f with ethanol.

  Next, tetramethoxysilane / ethanol / water = 1/3/4 (molar ratio) (water is 0.02 N ammonia water) is used as the second liquid mixture for gelation, and the thickness of the liquid mixture is 2 mm. As shown in FIG. 3, gelation is promoted by adding onto the wet gel 1 f (FIG. 3) to form wet gels 1 g and 1 h (FIG. 3). After gelation, the container is sealed at room temperature for 1 day. Left alone.

  Subsequently, after washing with the above ethanol, the second gelation step was performed again. The second gelling liquid mixture used in the second gelation was added to a thickness of 5 mm to promote gelation, and wet gels 1i, 1j, 1k (FIG. 3) were prepared. The gel was left at room temperature for one day after gelation.

3) Hydrophobization process Wet gels 1i, 1j, and 1k (FIG. 3) combined with reinforcing porous body 5 obtained in the second gelation process are immersed twice in ethanol having a volume five times the gel volume. Washing was performed. The composite wet gel is hydrophobized by being immersed in a hydrophobizing solution obtained by mixing dimethyldimethoxysilane / ethanol / 10 wt% aqueous ammonia = 45/45/10 (weight ratio) at 40 ° C. for 1 day. Processed. As the hydrophobizing solution, 10 times the combined wet gel volume was used.

4) Drying step The wet gel combined with the hydrophobic reinforced porous body 5 was washed twice with 10 times the volume of ethanol of the gel volume. Next, the gel was placed in a pressure-resistant container, and liquefied carbon dioxide was circulated in the container, whereby ethanol in the gel was replaced with liquefied carbon dioxide. Next, the liquefied carbon dioxide was further pumped into the container to raise the pressure in the container to 10 MPa. Then, the inside of a container was made into the supercritical state by heating up to 50 degreeC. Next, drying was completed by slowly releasing the pressure while maintaining the temperature at 50 ° C. Thus, an acoustic matching layer 4 (FIG. 3) made of a dry gel compounded with the reinforcing porous body was obtained, but the dry gel portion was not contracted during drying and no cracks were observed.

A part of the 0-2 mm, 2-4 mm, and 4-9 mm portions were cut from the bottom surface of the acoustic matching layer 4 (FIG. 3) made of the dried gel combined with the obtained reinforcing porous body, and the density was measured. The density was measured by measuring the weight of the porous piece and measuring the volume by substitution in water. The measured density, 460kg ^/ m 3 (parts of 0-2 mm), (part of 2~4mm) 190kg ^/ m 3, was 120 kg ^/ m 3 (part of 4~9mm).

  Thus, the porous body which consists of a dry gel which has density distribution was able to be obtained by implementing the gelatinization process in multiple times. Further, the strength of the outer peripheral portion of the acoustic matching layer was increased by the reinforcing porous body, and the density of the bottom portion was increased because of the increased density, and no gel was lost during normal handling.

  As described above, the acoustic matching layer according to the present invention and the method for producing the acoustic matching layer according to the present invention can be obtained as an acoustic matching layer containing a dry gel that can be adjusted in density over a wide range. Since the transmission / reception efficiency of ultrasonic waves is increased, the present invention can be applied to a flow rate measuring device that measures the flow rate of gas using ultrasonic waves, an ultrasonic transducer that is used in a distance measurement device that measures the distance to an object, and the like.

(A) Process diagram showing flow of manufacturing method of acoustic matching layer including dry gel of the present invention (b) Process diagram showing flow of manufacturing method of acoustic matching layer including dry gel of the present invention (c) Conventional Process diagram showing the flow of the manufacturing method of the acoustic matching layer containing the dried gel (A) Schematic diagram explaining the second gelation of the manufacturing method of the acoustic matching layer including the dried gel according to the first embodiment of the present invention. (B) The acoustic matching layer including the dried gel according to the second embodiment of the present invention. Schematic diagram explaining the second gelation of the manufacturing method The schematic diagram explaining the 2nd gelatinization of the manufacturing method of the acoustic matching layer containing the dry gel of Embodiment 3 of this invention (A) Sectional view of acoustic matching layer including dry gel of embodiment 3 of the present invention (b) Schematic diagram showing density distribution of acoustic matching layer including dry gel of embodiment 3 of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wet gel 2 2nd gelling liquid mixture 3 type | mold 4 Acoustic matching layer 5 Reinforcement porous body

Claims (6)

A method for producing an acoustic matching layer including a dry gel comprising a gelation step of forming a wet gel by a sol-gel method using a gel raw material and a drying step of obtaining a dry gel by removing a solvent contained in the wet gel. The gelation step is a first gelation step for forming a solid skeleton of the wet gel from a gel raw material, and a new solid skeleton is further formed on the solid skeleton after the first gelation step. The second gelation step comprises immersing the wet gel formed in the first gelation step in a second gelation mixture containing a second gel raw material. A method for producing an acoustic matching layer containing a dry gel, characterized in that the density is adjusted by: The method for producing an acoustic matching layer including a dry gel according to claim 1, further comprising a hydrophobizing step of imparting hydrophobicity to the gel by introducing a hydrophobic group on the surface of the wet gel or the dry gel. The method for producing an acoustic matching layer containing a dry gel according to claim 1 or 2, wherein the second gelation mixture outside the wet gel is not gelled during the second gelation step. 4. The production of an acoustic matching layer containing a dry gel according to claim 1, wherein the second gelation mixture outside the wet gel is gelled during the second gelation step. 5. Method. The method for producing an acoustic matching layer containing a dry gel according to any one of claims 1 to 4, wherein the second gelation step is repeated a plurality of times. The acoustic matching layer containing the dry gel obtained by the manufacturing method according to any one of claims 1 to 5, wherein an outer peripheral portion of the acoustic matching layer is a composite of a dry gel and a reinforcing porous body.

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