Abstract:
It is an object of the present invention to provide an ultrasonic transducer, which is so configured as to reduce the variations in characteristics, thereby to enable the stabilization of the precision, as well as to enable the improvement of the durability, and the like, a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter. In order to attain this object, in accordance with the present invention, the ultrasonic transducer is so configured as to include a piezoelectric element and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and a solid skeletal part of the dry gel has been rendered hydrophobic. With this configuration, it is possible to obtain the ultrasonic transducer having an acoustic matching layer  3  which is very lightweight and has a small acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, it is also possible to obtain the ultrasonic transducer which shows a narrow range of characteristic variations, and is stable due to the high homogeneity of the dry gel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ultrasonic transducer for transmitting and receiving ultrasonic waves, and a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter using the ultrasonic transducer. 
     2. Description of Prior Art 
     In recent years, an ultrasonic flowmeter which measures the time of flight of an ultrasonic wave across the propagation path, and determines the passing speed of a fluid, thereby to measure the flow rate has come into use for a gas meter or the like. 
       FIG. 1  is a diagram showing the measurement principle of an ultrasonic flowmeter. As shown in  FIG. 1 , a fluid flows in the direction indicated by an arrow at a velocity V in a tube. A pair of ultrasonic transducers  101  and  102  are oppositely mounted on a tube wall  103 . The ultrasonic transducers  101  and  102  are respectively configured with piezoelectric vibrators such as piezoelectric ceramics as electrical energy/mechanical energy conversion elements. Herein, the ultrasonic transducer  101  is used as an ultrasonic transmitter, and the ultrasonic transducer  102  is used as an ultrasonic receiver. 
     The operation is as follows. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer  101  to the piezoelectric vibrator, the ultrasonic transducer  101  emits an ultrasonic wave into an external fluid along a propagation path denoted by L 1  in the diagram. Then, the ultrasonic transducer  102  receives the propagated ultrasonic wave, and converts it into a voltage. Subsequently, the ultrasonic transducer  102  is used as an ultrasonic transmitter, and the ultrasonic transducer  101  is used as an ultrasonic receiver. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer  102  to the piezoelectric vibrator, the ultrasonic transducer  102  emits an ultrasonic wave into the external fluid along a propagation path denoted by L 2  in the diagram. Then, the ultrasonic transducer  101  receives the propagated ultrasonic wave, and converts it into a voltage. 
     Further, with such an ultrasonic transducer, if an alternating voltage is successively applied thereto, ultrasonic waves are successively emitted from the ultrasonic transducer. Accordingly, it becomes difficult to determine the time of flight. For this reason, in general, a burst voltage signal using a pulse signal as a carrier wave is used as a driving voltage. Hereinafter, the measurement principle will be described in more details. Upon application of a burst voltage signal for driving to the ultrasonic transducer  101 , an ultrasonic pulse wave is emitted from the ultrasonic transducer  101 . The ultrasonic pulse wave propagates through the propagation path L 1  with a length L, and reaches the ultrasonic transducer  102  after (time of flight) t hours. With the ultrasonic transducer  102 , the propagated ultrasonic pulse wave can be converted into an electrical pulse wave at a high S/N ratio. By using the electrical pulse wave as a trigger signal, the ultrasonic transducer  101  is driven again to emit an ultrasonic pulse wave. This device is referred to as a sing-around device. The time required for an ultrasonic pulse to be emitted from the ultrasonic transducer  101 , and propagate through the propagation path to reach the ultrasonic transducer  102  is referred to as a sing-around period. The inverse thereof is referred to as a sing-around frequency. 
     In  FIG. 1 , a reference character V denotes the flow velocity of the fluid flowing in the pipe, C denotes the velocity of an ultrasonic wave in the fluid, and θ denotes the angle between the direction of flow of the fluid and the direction of propagation of an ultrasonic pulse. When the ultrasonic transducer  101  is used as a transmitter, and the ultrasonic transducer  102  is used as a receiver, the following equation (1) holds:
 
 f 1=1/ t 1=( C+V  cos θ)/ L    (1)
 
     where t 1  denotes the sing-around period which is the time for an ultrasonic pulse emitted from the ultrasonic transducer  101  to reach the ultrasonic transducer  102 , and f 1  denotes the sing-around frequency. 
     In contrast, when the ultrasonic transducer  102  is used as a transmitter, and the ultrasonic transducer  101  is used as a receiver, the following equation (2) holds:
 
 f 2=1/ t 2=( C−V  cos θ)/ L    (2)
 
     where t 2  denotes the sing-around period therefor, and f 2  denotes the sing-around frequency. 
     Accordingly, the frequency difference Δf between both the sing-around frequencies is expressed as the following equation (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path for the ultrasonic wave, and the frequency difference Δf:
 
Δ f=f 1 −f 2=2  V  cos θ/ L    (3)
 
     Namely, it is possible to determine the flow velocity V of the fluid from the length L of the propagation path for the ultrasonic wave, and the frequency difference Δf. Therefore, it is possible to determine the flow rate from the flow velocity V. 
     Such an ultrasonic flowmeter is required to have a high degree of precision. In order to improve the precision, the acoustic impedance of a matching layer becomes important which is formed on the transmitting and receiving surface of ultrasonic waves in the piezoelectric vibrator configuring the ultrasonic transducer for transmitting ultrasonic waves to a gas, or receiving the ultrasonic waves propagated through the gas. The acoustic impedance of the piezoelectric vibrator for generating the ultrasonic vibrations is about 30×10 6 . The acoustic impedance of air is about 400. The ideal value of the acoustic impedance of the acoustic matching layer is about 0.11×10 6 . Further, the acoustic impedance is defined as the following equation (4):
 
Acoustic impedance=(density)×(sound velocity)
 
     Therefore, a low density material, such as a material obtained by solidifying a glass balloon or a plastic balloon with a resin material, is used for the acoustic matching layer for controlling the acoustic impedance at a low level. Alternatively, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like. The method is disclosed in Japanese Patent Publication No. 2559144, or the like. 
     For the acoustic matching layer used in a conventional ultrasonic transducer used for an ultrasonic flowmeter, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like, as described above. For this reason, there occur the following problems. The medium tends to be heterogeneous due to fracture of the glass ball under pressure, separation under insufficient pressure, foaming of the peeled molten material, or the like. Accordingly, variations occur in characteristics, which then generates variations in device precision. Further, there also occur the following problems. For example, since the acoustic matching layer is exposed to a gas, the surface is collapsed by the moisture, or the layer is easily deteriorated by a chemically active substance, resulting in inferior durability. 
     SUMMARY OF THE INVENTION 
     The present invention has been completed for solving such problems. It is an object of the present invention to provide a high sensitivity ultrasonic transducer, which is so configured as to reduce the variations in characteristics, thereby to enable the stabilization of the precision, as well as to enable the improvement of the durability, and the like, a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter. 
     An ultrasonic transducer of the present invention is so configured as to include a piezoelectric element and an acoustic matching layer, wherein the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part of the dry gel has been rendered hydrophobic. With this configuration, it is possible to obtain the ultrasonic transducer having an acoustic matching layer which has a low acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, the ultrasonic transducer shows a narrow range of characteristic variations due to the high homogeneity of the dry gel. 
     Further, if the ultrasonic transducer of the present invention is embodied in the following manner, it is possible to obtain more preferred ultrasonic transducers. 
     First of all, the ultrasonic transducer is so configured that the piezoelectric element and the acoustic matching layer are chemically bonded with each other. 
     Secondly, the ultrasonic transducer is so configured that the piezoelectric element is mounted on the inner side of a hermetically sealed case, and the acoustic matching layer is mounted on the outer side of the hermetically sealed case opposed to the mounting position of the piezoelectric element. 
     Thirdly, the ultrasonic transducer is so configured that the hermetically sealed case has an acoustic matching layer mounting part in the form of recess with a depth which is a quarter of the ultrasonic oscillation frequency at the position of the outer side opposed to the mounting position of the piezoelectric element, and the acoustic matching layer mounting part is filled with the dry gel of an inorganic oxide or an organic polymer. 
     Fourthly, the ultrasonic transducer is so configured that the hermetically sealed case and the acoustic matching layer are chemically bonded with each other. 
     Fifthly, the ultrasonic transducer is so configured that the hermetically sealed case is made of a conductive material. 
     Sixthly, the ultrasonic transducer is so configured that the conductive material is a metal material. 
     Seventhly, the ultrasonic transducer is so configured that the dry gel constituting the acoustic matching layer has a density of 500 kg/m 3  or less, and a mean pore diameter of 100 nm or less. 
     Eighthly, the ultrasonic transducer is so configured that the solid skeletal part of the dry gel contains at least silicon oxide or aluminium oxide as a component. 
     Ninthly, the ultrasonic transducer is so configured that a protective layer with a density of 800 kg/m 3  or more, and a thickness of 10 μm or less is formed on the surface of the acoustic matching layer. 
     Tenthly, the ultrasonic transducer is so configured that the protective layer is made of any of a metal material, an inorganic material, and a polymer material. 
     Eleventhly, the ultrasonic transducer is so configured that the protective layer is made of any of aluminium, silicon oxide, aluminium oxide, amorphous carbon, and polyparaxylene. 
     In a method for manufacturing an ultrasonic transducer of the present invention, the ultrasonic transducer includes an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, and a piezoelectric element. The method includes a step of brazing (or soldering) the dry gel to the piezoelectric element or a gas shielding case on the inner side of which the piezoelectric element is mounted. With the ultrasonic transducer obtained by using this manufacturing method, it is possible to achieve higher sensitivity and stabilization of the characteristics due to the acoustic matching layer with a low acoustic impedance. 
     In a method for manufacturing an ultrasonic transducer of the present invention, the ultrasonic transducer includes an acoustic matching layer made of a dry gel of an inorganic oxide or an organic polymer, the solid skeletal part of the dry gel having been rendered hydrophobic, and a piezoelectric element. The method includes a step of forming the acoustic matching layer. The acoustic matching layer formation process includes: a deposition step of applying a gel raw material solution to the piezoelectric element or a gas shielding case on the inner side of which the piezoelectric element is mounted; a solidification step of obtaining a wet gel from the gel raw material solution; and a drying step of removing a solvent in the wet gel to obtain a dry gel. With the ultrasonic transducer obtained by using this manufacturing method, it is possible to achieve higher sensitivity and stabilization of the characteristics due to the acoustic matching layer with a low acoustic impedance. 
     Further, if the method for manufacturing an ultrasonic transducer of the present invention is embodied in the following manner, it is possible to obtain more preferred ultrasonic transducers. 
     First of all, in the ultrasonic transducer including the piezoelectric element mounted on the inner side of a hermetically sealed case, the hermetically sealed case has an acoustic matching layer mounting part in the form of recess with a depth which is a quarter of the ultrasonic wave length at the position of the outer side opposed to the mounting position of the piezoelectric element of the hermetically sealed case. In the method for manufacturing the ultrasonic transducer, the gel raw material solution is applied to the acoustic matching layer mounting part. 
     Secondly, in the method for manufacturing the ultrasonic transducer, a protective layer is formed on the surface of the acoustic matching layer by a dry deposition method. 
     Further, an ultrasonic flowmeter of the present invention includes: a flow rate measuring part through which a fluid to be measured flows; a pair of ultrasonic transducers for transmitting and receiving an ultrasonic wave mounted at the flow rate measuring part; a measuring circuit for measuring the time of flight of an ultrasonic wave between the ultrasonic transducers; and a flow rate operation means for calculating the flow rate based on the signal from the measuring circuit, each of the ultrasonic transducers being made up of a hermetically sealed case by which the fluid to be measured and the piezoelectric element are shielded from each other. With this ultrasonic flowmeter, it is possible to achieve the improvement of the stability of the flow rate measurement due to the high sensitivity and the narrow range of variations in characteristics of the ultrasonic transducers. 
     As described above, the present invention provides such a configuration that the acoustic matching layer is made of a dry gel of an inorganic oxide or an organic polymer, and the solid skeletal part of the dry gel has been rendered hydrophobic. Accordingly, it is possible to obtain an ultrasonic transducer having an acoustic matching layer which is very lightweight and has a small acoustic impedance due to the solid skeletal part of the dry gel which has been rendered hydrophobic. Further, it is also possible to obtain the ultrasonic transducer which shows a narrow range of characteristic variations, and is stable due to the high homogeneity of the dry gel. Still further, upon formation of the dry gel of an inorganic oxide or an organic polymer, the OH group on the piezoelectric element surface or the case surface and the component of the raw material react and chemically bonded with each other to ensure the bond therebetween. Therefore, such an excellent effect can also be expected that an adhesion layer-free, or a so-called adhesion layer-less ultrasonic transducer is obtainable. 
     Such objects and advantages of the present invention will become more apparent from the following description of embodiments given by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view showing the measurement principle of a conventional ultrasonic flowmeter; 
         FIG. 2  is a cross sectional view of an ultrasonic transducer shown as a first embodiment of the present invention; 
         FIG. 3  is a cross sectional view of an ultrasonic transducer shown as a second embodiment of the present invention; 
         FIG. 4  is a cross sectional view of an ultrasonic transducer shown as a third embodiment of the present invention; 
         FIG. 5  is a cross sectional view of an ultrasonic transducer shown as a fourth embodiment of the present invention 
         FIG. 6  is a flow chart illustrating a first example of a manufacturing method of the ultrasonic transducer of the present invention; 
         FIG. 7  is a flow chart illustrating a second example of a manufacturing method of the ultrasonic transducer of the present invention; 
         FIG. 8  is a flow chart illustrating a third example of a manufacturing method of the ultrasonic transducer of the present invention; 
         FIG. 9  is a flow chart illustrating a fourth example of a manufacturing method of the ultrasonic transducer of the present invention; and 
         FIG. 10  is a block diagram showing an ultrasonic flowmeter using the ultrasonic transducer of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 2  is a cross sectional diagram of a piezoelectric vibrator of an ultrasonic transducer used for an ultrasonic flowmeter, showing a first embodiment of the present invention. In  FIG. 2 , a piezoelectric vibrator  1  for performing electric-ultrasonic wave interconversion is made up of a piezoelectric element  2  and an acoustic matching layer  3 . The piezoelectric element  2  generates ultrasonic vibrations. It is made up of piezoelectric ceramics, piezoelectric monocrystals, or the like. It is polarized in the direction of thickness, and it has electrodes on the upper and lower sides. The acoustic matching layer  3  is for transmitting an ultrasonic wave to a gas, or receiving the ultrasonic wave propagated through the gas. The acoustic matching layer  3  has functions of causing the mechanical vibrations of the piezoelectric element  2  excited by a driving alternating voltage to efficiently outgo to the external medium as ultrasonic waves, and efficiently converting the incoming ultrasonic waves into a voltage. It is bonded on one side of the piezoelectric element  2  through a chemical bond so as to form the transmitting and receiving surface of ultrasonic waves in the piezoelectric element  2 . Further, the acoustic matching layer  3  is made of a dry gel of an inorganic oxide or an organic polymer. The solid skeletal matrix of the dry gel is rendered hydrophobic. Thus, it is formed as a nanoporous material dry gel (nanoporous dry gel) with a density of 500 kg/m 3  or less, and a mean pore diameter of 100 nm or less. It is noted that the solid skeletal part of the dry gel of an inorganic oxide contains at least silicon dioxide (silica) or aluminium oxide (alumina) as a component. Further, the solid skeletal part of the dry gel of an organic polymer can be made of a general thermosetting resin or thermoplastic resin. For example, polyurethane, polyurea, cured phenolic resin, polyacrylamide, methyl polymethacrylate, or the like can be applied thereto. 
     Second Embodiment 
       FIG. 3  is a cross sectional diagram of an ultrasonic transducer using the piezoelectric vibrator  1  of  FIG. 2 , showing a second embodiment of the present invention. Identical numerals and characters are used to designate elements formed with the identical materials and identical functions in  FIGS. 2 and 3 . A piezoelectric vibrator  1  is bonded to the inside of a case  4  made of a conductive material, for example, made of a material such as a metal capable of ensuring the reliability to the external fluid, generally in tube form. The top opening of the case  4  is covered with a protective layer  5 , along with the top of an acoustic matching layer  3  in the piezoelectric vibrator  1 . In consequence, the inside thereof is blocked off from the external fluid. In addition, the bottom thereof is covered and completely sealed with a cover plate  7  similarly made of a conductive material such as a metal. As a result, the case  4  is formed as a gas shielding case, together with the cover plate  7 . Further, driving terminals  6   a  and  6   b  are attached to the cover plate  7 . Out of the two driving terminals  6   a  and  6   b , one driving terminal  6   a  is electrically connected to the top electrode of the piezoelectric element  2  via the cover plate  7  and the case  4 . The other driving terminal  6   a is electrically insulated by an insulating material  8  from the cover plate  7 , and electrically connected to the bottom electrode of the piezoelectric element  2  in the case  4 . 
     The protective layer  5  is a layer with a density of 800 kg/m 3  or more, and a thickness of 10 μm or less, and made of a metal material, an inorganic material, a polymer material, or the like. Specifically, other than aluminium, silicon oxide, aluminium oxide, low melting glass, amorphous carbon, polymers (polyparaxylene and polyimide), and the like, coating resins, UV (ultraviolet) curable resins, thermosetting resins, and the like are used. Further, when aluminium is used, it is provided through a vapor deposition or sputtering process. When silicon oxide or aluminium oxide is used, it is provided through a vapor deposition process, a sputtering process, a CVD (Chemical Vapor Deposition) process, or the like. When low melting glass is used, it is provided through a coating process. When an amorphous carbon is used, it is provided through a plasma CVD process. When a polymer (polyparaxylene or polyimide) is used, it is provided through a vapor deposition polymerization process, or the like. 
     With the ultrasonic transducer thus configured, when a pulse wave voltage having an AC signal component with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer is applied to the driving terminals  6   a  and  6   b , the piezoelectric vibrator  1  vibrates in thickness vibration mode. Accordingly, it emits burst-like ultrasonic waves into a fluid such as a gas or a liquid. 
     Third Embodiment 
       FIG. 4  is a cross sectional diagram of an ultrasonic transducer so configured that a piezoelectric vibrator  1  obtained by slightly modifying the piezoelectric vibrator  1  of  FIG. 2  is mounted in a case  14 , showing a third embodiment of the present invention. Identical numerals and characters are used in  FIGS. 2 to 4  to designate elements formed with the identical materials and identical functions. Therefore, the overlapping description thereof is omitted, and only the different points are described. The case  14  is configured as follows. A recess  15  as an acoustic matching layer mounting part is provided on the top  14   a  to close the whole of the top  14   a , resulting in a generally tube-like case. In addition, the bottom is closed with a cover plate  7 , also resulting in a gas shielding case. Incidentally, the depth of the recess  15  is formed with a depth which is a quarter of the ultrasonic oscillation frequency. Further, the case  14  is made of a conductive material, for example, a material such as a metal capable of ensuring the reliability to the external fluid in the same manner as with the case  4  of the first embodiment. The piezoelectric vibrator  1  is so configured that a piezoelectric element  2  is bonded to the top  14   a  inside the case  14 , and an acoustic matching layer  3  is bonded into the external recess  15 . Also for the ultrasonic transducer thus configured, when a pulse wave voltage having an AC signal component with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer is applied to the driving terminals  6   a  and  6   b , the piezoelectric vibrator  1  vibrates in thickness vibration mode. Accordingly, it emits burst-like ultrasonic waves into a fluid such as a gas or a liquid. 
     Fourth Embodiment 
       FIG. 5  shows an ultrasonic transducer of a fourth embodiment of the present invention. The ultrasonic transducer of this embodiment is identical in configuration to the ultrasonic transducer of the third embodiment shown in  FIG. 4 , except that a protective layer  5  is provided so as to cover the whole top of the acoustic matching layer  3  in the ultrasonic transducer shown in  FIG. 4 , and the acoustic matching layer  3  is sealed inside the recess  15  serving as an acoustic matching layer mounting part. Also for the ultrasonic transducer thus configured, when a pulse wave voltage having an AC signal component with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer is applied to the driving terminals  6   a  and  6   b , the piezoelectric vibrator  1  vibrates in thickness vibration mode. Accordingly, it emits burst-like ultrasonic waves into a fluid such as a gas or a liquid. 
     Then, a description will be given to examples of a method for manufacturing each of the acoustic matching layers  3  shown in  FIGS. 2 to 5 , and bonding it to the piezoelectric element  2  or the case  14 . 
     FIRST EXAMPLE 
       FIG. 6  is an explanatory diagram illustrating a method for manufacturing the acoustic matching layer  3 , and bonding it to the piezoelectric element  2  step by step in accordance with a first example. The first example will be described in the order of steps 1 to 6. 
     Step 1: electrodialysis of sodium silicate is performed to form a silicic acid solution with a pH of 9 to 10; 
     Step 2: the silicic acid solution is adjusted to a pH of 5.5, and the resulting solution is poured into a mold. Accordingly, the silicic acid solution gels with time to obtain a water base wet gel block; 
     Step 3; the gelled silicic acid solution is rendered hydrophobic by an acetone solution of trimethylchlorosilane (TMCS), followed by a dehydration treatment, to form a wet gel block; 
     Step 4: the wet gel block is subjected to supercritical. drying by using carbon dioxide to form a silica dry gel block; 
     Step 5: the silica dry gel block is cut to a thickness of λ/4 to form a prescribed acoustic matching layer  3 ; and 
     Step 6: the acoustic matching layer  3  cut in the step 5 is bonded to one side of the piezoelectric element  2  or the top  14   a  of the case  14  through silver soldering or an adhesive, resulting in the piezoelectric vibrator  1 . 
     SECOND EXAMPLE 
       FIG. 7  is an explanatory diagram illustrating a method for manufacturing the acoustic matching layer  3 , and bonding it to the metal sealing type of case  14  step by step in accordance with a second example. The second example will be described in the order of steps 1 to 7. 
     Step 1: a sol solution is formed from tetraethoxysilane, aluminium-sec-buthoxide, and ethanol; 
     Step 2: to the resulting sol solution, a gelling catalyst, or an acid or a base is added, to form a coating raw material solution before start of gelation, to be coated in its thickened state to the case  14 ; 
     Step 3: the coating raw material solution is coated to the coating surface of the case  14 , and merged with the case (hermetically sealed case)  14 ; 
     Step 4: when the coating raw material solution is allowed to gel after coating, the OH group on the surface of the case  14  and the alkoxy group of the raw material react and are chemically bonded with each other to form a wet gel film on the surface of the case  14 ; 
     Step 5: the film is washed with ethanol. After washing, the film is supercritically dried. Subsequently, it is subjected to a hydrophobization treatment with the vapor of trimethylchlorosilane to form an aluminosilica dry gel film on the surface of the case  14 ; 
     Step 6: On the aluminosilica dry gel film formed on the surface of the case  14 , a dry protective film (protective layer  5 ) is formed in vacuum. The protective film  5  is formed by vapor deposition, sputtering, CVD, or the like of silicon dioxide; and 
     Step 7: thereafter, to the case  14 , the piezoelectric element  2 , the cover plate  7 , the driving terminals  6   a  and  6   b , and the like are mounted, resulting in the piezoelectric vibrator  1 . 
     THIRD EXAMPLE 
       FIG. 8  is an explanatory diagram illustrating a method for manufacturing the acoustic matching layer  3 , and bonding it to the metal sealing type of case  14 , particularly, when the case  14  is a case made of stainless steel, bonding the acoustic matching layer  3  to the case  14 , step by step in accordance with a third example. The third example will be described in the order of steps 1 to 6. 
     Step 1: electrodialysis of sodium silicate is performed to form a silicic acid solution with a pH of 9 to 10; 
     Step 2: the silicic acid solution is adjusted to a pH of 5.5, and the resulting solution is added dropwise onto the case  14  for merging between the silicic acid solution and the case  14 ; 
     Step 3: when the coating raw material solution is allowed to gel after coating of the coating raw material solution, the OH group on the surface of the case  14  and the alkoxy group of the raw material react and are chemically bonded with each other to form a wet gel film on the surface of the case  14 ; 
     Step 4: the gelled silicic acid solution is rendered hydrophobic by an acetone solution of trimethylchlorosilane (TMCS), followed by a dehydration treatment. Further, after solvent exchange to hexane, the solution is dried in a case held at 100° C. to form a silica dry gel film of silicon dioxide; 
     Step 5: on the silica dry gel film formed on the surface of the case  14 , a protective film (protective layer  5 ) of amorphous carbon (diamond-like carbon) film is formed by high frequency plasma CVD. The protective film  5  is hard, and resistant to scratch, and has a high chemical resistance. Further, it is excellent in gas barrier property, and it has a low sorption ability, and is less likely to sorb chemical substances; and 
     Step 6: thereafter, to the case  14 , the piezoelectric element  2 , the cover plate  7 , the driving terminals  6   a  and  6   b , and the like are mounted, resulting in the piezoelectric vibrator  1 . 
     FOURTH EXAMPLE 
       FIG. 9  is an explanatory diagram illustrating a method for manufacturing the acoustic matching layer  3 , and bonding it to the case  14  of metal sealing type, particularly, when the case  14  is a case made of aluminium, bonding the acoustic matching layer  3  to the case  14 , step by step in accordance with a fourth example. The fourth example will be described in the order of steps 1 to 7. 
     Step 1: tolylene diisocyanate (TDI) and toluene diamine (TDA) are mixed in an aprotic solvent such as acetone or tetrahydrofuran to form an aqueous solution having a polyurea structure. It is important that a solvent which will not react with the raw material is prepared as the solvent in this step; 
     Step 2: the resulting aqueous solution is added dropwise onto the case  14  to be merged with the case  14 ; 
     Step 3: when the coating raw material solution is allowed to gel after coating of the coating raw material solution, the OH group on the surface of the case  14  and the alkoxy group of the raw material react and are chemically bonded with each other to form a wet gel film on the surface of the case  14 ; 
     Step 4: the gelled coating raw material solution is subjected to solvent exchange to tertiary butyl alcohol, and then frozen at 25° C. or less. Subsequently, the frozen solution is heated and dried under reduced pressure of 41 Torr or less to form an organic dry gel film. The resulting organic dry gel film has water repellency; 
     Step 5: on the organic dry gel film formed on the surface of the case  14 , an urethane-based ultraviolet (UV) curable resin is coated, and cured to form a protective film (protective layer  5 ); and 
     Step 6: thereafter, to the case  14 , the piezoelectric element  2 , the cover plate  7 , the driving terminals  6   a  and  6   b , and the like are mounted, resulting in the piezoelectric vibrator  1 . 
       FIG. 10  is a block diagram showing an ultrasonic flowmeter using the ultrasonic transducer of the present invention. The ultrasonic flowmeter in  FIG. 10  is installed so that a fluid to be measured flows in the direction indicated by an arrow of the figure at a velocity of V in a tube which is a flow rate measuring part  51 . On the tube wall of the flow rate measuring part  51 , the ultrasonic transducers  1   a  and  1   b  of the present invention are oppositely mounted. Herein, the ultrasonic transducer  1   a  is used as an ultrasonic transmitter (below, referred to as “ultrasonic transmitter  1   a ”), and the ultrasonic transducer  1   b  is used as an ultrasonic receiver (below, referred to as “ultrasonic receiver  1   b ”). Further, to the ultrasonic transmitter  1   a  and the ultrasonic receiver  1   b , there-are connected, via a switching circuit  55  for switching between transmission and reception thereof, a driving circuit  54  for driving the ultrasonic transducers  1   a  and  1   b , a reception and detecting circuit  56  for detecting the ultrasonic pulse, a timer  57  for determining the time of flight of the ultrasonic pulse, an operation unit  58  for performing operation on the flow rate based on the output from the timer  57 , and a control unit  59  for outputting a control signal to the driving circuit  54  and the timer  57 . 
     The operation of the ultrasonic flowmeter thus configured will now be described below. The fluid to be measured is set to be, for example, a LP gas, and the driving frequency of the ultrasonic transducers  1   a  and  1   b  is set to be about 500 kHz. The control unit  59  outputs a transmission start signal to the driving circuit  54 , and at the same time, starts the time measurement of the timer  57 . The driving circuit  54  receives the transmission start signal, and drives the ultrasonic transducer  1   a  to transmit an ultrasonic pulse. The transmitted ultrasonic pulse propagates in the flow rate measuring part, and is received at the ultrasonic transducer  1   b . The received ultrasonic pulse is converted into an electric signal at the ultrasonic transducer  1   b , and outputted to the reception and detecting circuit  56 . The reception and detecting circuit  56  determines the reception timing of the reception signal, and stops the timer  57 , and the operation unit  58  performs operation on the time of flight t 1 . 
     Subsequently, the switching circuit  55  switches between the ultrasonic transducers  1   a  and  1   b  connected therethrough to the driving unit  54  and the reception and detecting circuit  56 . Then, the control unit  59  again outputs a transmission start signal to the driving circuit  54 , and at the same time, starts the time measurement of the timer  57 . Contrary to the case of measurement of the time of flight t 1 , the ultrasonic transducer  1   b  transmits an ultrasonic pulse, and the ultrasonic transducer  1   a  receives it, and the operation unit  58  performs operation on the time of flight t 2 . 
     Herein, assuming that the distance connecting between the centers of the ultrasonic transducers  1   a  and  1   b  is L, the sound velocity of the LP gas in airless conditions is C, the flow velocity in the flow rate measuring part  51  is V, and the angle between the direction of flow of the fluid to be measured and the line connecting between the centers of the ultrasonic transducers  1   a  and  1   b  is θ, it is possible to determine the times of flight t 1  and t 2 , respectively. Further, since the distance L is known, it is possible to determine the flow velocity V if the times t 1  and t 2  are determined. Accordingly, it becomes possible to check the flow rate from the flow velocity V. 
     Having described the present invention as related to the preferred embodiments shown in the accompanying drawings, it will be obvious to those skilled in the art that changes and modifications may be made with ease to the invention. Such variations are intended to, be within the scope of the present invention.