JP2014137254A - Acoustic matching member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reconcile a low acoustic impedance value applicable to an aerial radiation ultrasonic wave transducer with the mechanical strength and durability, in an acoustic matching member obtained by integrating two or more layers of acoustic matching members having different acoustic impedances in order to improve the sensibility of an ultrasonic wave transducer.SOLUTION: The acoustic matching member of the present invention transmits and discharges ultrasonic wave. The acoustic matching member includes: at least a first matching layer 11 mainly made of glass; and a second matching layer 12 that is integrally bonded to the first matching layer 11 and is mainly made of glass. The first matching layer 11 and second matching layer 12 have a plurality of pores inside them. The volume of the pores per unit volume of the second matching layer 12 is larger than the volume of the pores per unit volume of the first matching layer 11.

Description

  The present invention relates to an ultrasonic transducer for use in a flow rate measurement device that measures the flow rate of a fluid such as gas or liquid using ultrasonic waves, a distance measurement device that measures the distance from an object, and the like. The present invention relates to an acoustic matching member that matches acoustic impedance between a transducer that transmits and receives a sound wave and a propagation medium such as air.

  2. Description of the Related Art Conventionally, an ultrasonic sensor including a piezoelectric vibrator and an acoustic matching member is known as a sensor for detecting a flow rate of a fluid and measuring a distance from an object using ultrasonic waves.

  Here, a conventional acoustic matching member will be described below.

  In Patent Document 1, in an acoustic matching member in which acoustic matching members having different acoustic impedance values are multilayered by a screen printing method, an acoustic matching member having different addition amounts of various metal fine powders is integrally formed on an epoxy resin. An ultrasonic probe with enhanced sensitivity for sound transmission is disclosed.

  However, since the acoustic matching member of Patent Document 1 is made of a resin material, the strength is low and the durability is poor. In addition, since the volume expansion coefficient of the resin material is relatively high, there is a problem that the acoustic impedance fluctuates due to a change in actual use temperature. Moreover, since the manufacturing method repeats screen printing and resin solidification, the process becomes complicated, and the acoustic matching member has to be expensive.

  Patent Document 2 discloses an acoustic matching member containing foamed glass as a main component as an acoustic matching member that has excellent mechanical strength and can maintain stable sensor characteristics over a long period of time. However, the acoustic impedance is presumed to be very high as an acoustic matching member for air radiation use, and there is a problem that the sensitivity of sound wave transmission is low because it is a single acoustic matching member.

JP 58-197251 A JP 2004-88626 A

  The acoustic matching member of the present invention solves the above-mentioned problems, and is applied not only to improvement in mechanical strength and durability as an acoustic matching member, but also to ultrasonic transmission sensitivity, as well as to an aerial radiation ultrasonic transducer. This realizes a possible low acoustic impedance value.

  In order to solve the above-mentioned problems, the present invention is an acoustic matching member for transmitting and emitting ultrasonic waves, and is joined integrally with a first matching layer mainly composed of glass and the first matching layer. And at least a second matching layer mainly composed of glass, the first matching layer and the second matching layer having a plurality of pores therein, and the second matching layer. The pore volume per unit volume of the first matching layer is larger than the pore volume per unit volume of the first matching layer, and ultrasonic waves are emitted from the second matching layer.

  With the above configuration, the acoustic matching member of the present invention has excellent mechanical strength and durability, and also has excellent ultrasonic transmission sensitivity, and is particularly suitable for an ultrasonic transmitter / receiver for aerial radiation. It becomes a member.

Sectional drawing which showed the structure of the acoustic matching member in one embodiment of this invention

  First, functions required for the acoustic matching member will be described.

There are three major functions required as an acoustic matching member, and the first is acoustic impedance matching. The acoustic impedance of the object is obtained by density × speed of sound, the acoustic impedance Z _{AIR of} air is about 428 kg / m ² · s, and the acoustic impedance Z _PZT of the piezoelectric vibrator that is a means for generating ultrasonic waves is about 29 × 10 ⁶ kg. / M ² · s. When an ultrasonic wave is radiated from the piezoelectric vibrator into the air, sound reflection occurs due to the difference in acoustic impedance between the two, and the sound radiation efficiency decreases. An acoustic matching member is used to reduce reflection of sound due to the difference in acoustic impedance. The ideal value of the acoustic impedance Z _M of this acoustic matching member is calculated,
Z _M = √ (Z _PZT × Z _AIR )
This value is an ideal value that maximizes the sound propagation efficiency. When the above-described values of Z _PZT and Z _AIR are used, this value becomes Z _M ≈0.11 × 10 ⁶ kg / m ² · s.

  The second point is optimization of the thickness of each layer. The thickness of the acoustic matching member needs to be set in the vicinity of λ / 4 with respect to the wavelength λ of the frequency used.

  The third point is a good bonding state of each layer. Since the sound wave transmitted through the solid is an elastic dense wave, it is important that the bonding state between each layer of the acoustic matching member and the sensor structure such as the piezoelectric vibrator is good. In addition, when layers having different acoustic impedances are simply joined with an adhesive, problems such as peeling and damage to the acoustic matching member are likely to occur due to factors such as inconsistent thermal expansion coefficients and poor adhesion.

  In view of the above contents, the acoustic matching member 1 in the embodiment of the present invention shown in FIG. 1 will be described in detail.

  The acoustic matching member 1 of FIG. 1 is mainly made of glass, and the first matching layer 11 having a plurality of pores inside the glass is also mainly made of glass, and a plurality of pores are provided inside the glass. The second matching layer 12 having a pore volume per unit volume larger than that of the matching layer 11 is integrated without an intermediate layer, and the exposed surface of the second matching layer 12 (ultrasonic radiation) Sound is emitted from the surface 12A).

  By using the second matching layer 12 having a pore volume per unit volume larger than that of the first matching layer 11 as the acoustic matching member 1, the transmission efficiency of sound waves is improved. The acoustic matching member 1 shown in FIG. 1 includes the first matching layer 11 and the second matching layer 12, but is not particularly limited, and includes a plurality of matching layers and an ultrasonic radiation surface 12A. It is sufficient that the pore volume per unit volume increases as the value approaches.

  In the acoustic matching member 1 of the present invention, the pores provided in the first matching layer 11 and the second matching layer 12 are mixed with an inorganic foaming agent that decomposes thermally and generates gas in the glass powder as a raw material. Then, it is formed by firing this to generate gas inside. Due to the pores generated by this, the first matching layer 11 and the second matching layer 12 have a low density, and the acoustic impedance of the first matching layer 11 and the second matching layer 12 can be made extremely low.

  By increasing the amount of the inorganic foaming agent added at the time of producing the second matching layer 12 as compared with the amount of the inorganic foaming agent added at the time of producing the first matching layer 11, the second matching layer 12. The pore volume per unit volume inside can be made larger than the pore volume per unit volume inside the first matching layer 11. Preferably, by setting the porosity of the second matching layer 12 to 50% to 90%, it is possible to appropriately reduce acoustic impedance while maintaining high mechanical strength.

  The first matching layer 11 and the second matching layer 12 are both made of a glass material having a plurality of pores therein, and thus can be integrally fired at the time of firing. Since the structure is integrated without an intermediate layer, the sensitivity of sound wave transmission is improved.

  As a method for producing the first matching layer 11 and the second matching layer 12 having pores, a glass powder as a raw material and an inorganic foaming agent that generates a gas by pyrolysis in a firing temperature range are used. It is desirable to mix.

  In general, as a method for forming pores in glass or ceramics, a method of adding burned and burned particles (resin particles such as polyethylene) or hollow particles to a raw material powder is widely used.

  However, in the pore formation method using resin particles as burnt-off particles, the resin particles disappear at a temperature lower than about 500 ° C. Therefore, when trying to increase the pore volume per unit volume, continuous open pores are generated. It is easy to cause a characteristic variation due to moisture entering from the pores. If sintering is advanced so as not to form the open pores, the porosity is lowered. Moreover, in the pore formation method using hollow particles, since expensive hollow particles are generally used, the manufacturing cost must be increased.

  In the method of adding the inorganic foaming agent in the present invention, the firing shrinkage proceeds to some extent in the firing temperature range, and after the glass melt wets the inorganic foaming agent, the foaming agent is thermally decomposed to generate gas. The gas is preferably trapped inside the glass. Therefore, independent closed pores can be generated at a high density, the pore volume per unit volume can be increased, and only independent pores are generally formed, so that characteristic fluctuations due to moisture absorption and the like hardly occur. .

  Here, the open pores refer to pores that partially communicate with the outside of the glass material, and the closed pores refer to pores inside the glass material that do not communicate with the outside of the glass-based material. . Further, the continuous air hole refers to a pore having a form in which a plurality of pores are connected, and the independent pore refers to a pore that exists independently in the glass material.

As the inorganic foaming agent, CaCO ₃ or SrCO ₃ is particularly preferably used. When these alkaline earth carbonates are used, the carbonate group contributes to the generation of carbon dioxide gas during the firing process, and the alkaline earth metal element dissolves in the glass to lower the viscosity of the glass. Foaming can occur. As described above, CaCO ₃ or SrCO ₃ is preferable as the inorganic foaming agent, but various carbonates, nitrates, sulfates and the like can be used. For example, BaCO ₃ , Al ₂ (SO ₄ ) ₃ , and Ce _2. (SO _{4) 3} and the like.

  In addition, the addition amount of the inorganic foaming agent is preferably 0.5 part by weight to 2 parts by weight with respect to 100 parts by weight of the glass powder. Therefore, the water absorption of the first matching layer 11 and the second matching layer 12 can be 5% or less. Thereby, the characteristic fluctuation | variation by a water | moisture content permeating from a pore can be suppressed.

  The composition of the glass that is the main component of the first matching layer 11 and the second matching layer 12 is not particularly limited, and examples thereof include silicate glass, borosilicate glass, borate glass, and phosphate glass. Borosilicate glass is preferable in order to achieve both good foaming and porosity and chemical durability. Further, considering the adverse effect on the environment, it is desirable that PbO is not substantially contained.

  Next, the manufacturing method of the acoustic matching member 1 in one embodiment of the present invention will be described in detail.

First, as a first sheet constituting the first matching layer 11, 100 parts by weight of a borosilicate glass powder (bending point 600 ° C., density 2.15 g / cm ³ ), and 1 weight of SrCO ₃ powder as an inorganic foaming agent. Part was added and mixed to prepare a mixed powder. Then, this mixed powder, PVB (butyral resin) as an organic binder, and plasticizer BBP (benzylbutyl phthalate) were mixed and dispersed to prepare a slurry.

Next, using this slurry, a first sheet was formed on a PET film by a doctor blade method. Further, the second sheet constituting the second matching layer 12 is formed by adding 2 parts by weight of SrCO ₃ powder as an inorganic foaming agent to 100 parts by weight of the borosilicate glass powder, mixing, and molding the same as the first sheet. did.

  A small amount of a ceramic filler may be added to the first and second sheets as appropriate for the purpose of controlling the shrinkage and expansion behavior during firing, but the filler is preferably used for foaming and lowering the density. The addition amount is preferably 20 parts by weight or less with respect to 100 parts by weight of the glass powder.

  Then, the 2nd sheet | seat was laminated | stacked on the 1st sheet | seat, and the sheet | seat laminated body of diameter 13mm was obtained by punching with a cylindrical metal mold | die.

  Next, this sheet laminate is placed on a porous firing sheath so that the first sheet is on the lower surface, and a porous alumina substrate is placed on the sheet laminate and fired at 840 ° C. for 30 minutes. Thus, the acoustic matching member 1 in which the two layers were integrated was obtained.

In this acoustic matching member 1, the porosity of the first matching layer 11 is 78%, the acoustic impedance is 1.22 × 10 ⁶ kg / m ² · s, the porosity of the second matching layer 12 is 86%, and the acoustic impedance Is 0.66 × 10 ⁶ kg / m ² · s, and these acoustic impedance values are 0.11 which is an ideal value desired for an acoustic matching member when ultrasonic waves are emitted from a piezoelectric vibrator into the air. Although it is larger than × 10 ⁶ kg / m ² · s, it is currently widely used in airborne ultrasonic transducers, and is an acoustic matching member in which glass balloons are dispersed in epoxy resin (acoustic impedance is generally 1.3 × 10 ⁶ kg / m ² · s), which is closer to the ideal value, and it can be said that each individual is an acoustic matching member effective in improving the sensitivity of the aerial radiation ultrasonic transducer. The acoustic impedance (1.22 × 10 ⁶ kg / m ² · s) of the first matching layer 11 is the acoustic impedance (0.66 × 10 ⁶ kg / m ² · s) of the second matching layer 12. And the acoustic impedance Z _PZT (about 29 × 10 ⁶ kg / m ² · s) of the piezoelectric vibrator, the transmission efficiency of sound waves can be improved.

  Furthermore, since the acoustic matching member 1 is formed by integrating the first matching layer 11 and the second matching layer 12, the sensitivity of the aerial radiation ultrasonic transducer is further increased by the multilayer effect as the acoustic matching member. It becomes possible to raise.

  The acoustic matching member 1 of the present invention obtained through the above steps will be described in more detail.

  In the present embodiment, an acoustic matching member consisting only of the first matching layer 11 and an acoustic matching member consisting only of the second matching layer 12 are produced by surface grinding one side of the acoustic matching member 1, By sticking them together with a one-component epoxy adhesive, an acoustic matching member composed of two layers having different porosity and acoustic impedance was obtained.

  Both the acoustic matching member 1 of the present invention in which two layers are integrally fired and the acoustic matching member in which the two layers are bonded together with an adhesive are left for 100 hours in a high-temperature environment of 150 ° C. as a severe test, Thereafter, all samples were dropped onto a vinyl tile floor from a height of 1 m, and the number of NG samples in which peeling, cracking, breakage, etc. were confirmed visually was counted. As a result, the NG number of the acoustic matching member bonded with the adhesive was 93, whereas the NG number of the acoustic matching member 1 obtained by integrally firing the two layers was only two, showing a high survival rate.

  In the acoustic matching member 1 according to the embodiment of the present invention described above, the first matching layer 11 and the second matching layer 12 are formed into a sheet and laminated, but the second matching layer 12 is described. May be formed by pasting a mixed powder obtained by adding and mixing an inorganic foaming agent to a borosilicate glass powder, and screen-printing it on the first sheet constituting the first matching layer 11. In the acoustic matching member 1 according to the embodiment of the present invention described above, the first matching layer 11 and the second matching layer 12 are integrated with each other without an intermediate layer. However, the number of layers of the acoustic matching members having different porosities to be stacked is not limited to two, and may be an acoustic matching member integrated with three or more layers.

  The present invention relates to an acoustic matching member in which two or more acoustic matching members having different acoustic impedances are integrated in order to improve the sensitivity of an ultrasonic transducer, and has a low acoustic impedance value and a high mechanical impedance. Both strength and durability can be achieved, and in particular, it can be suitably applied to an aerial radiation ultrasonic transducer.

DESCRIPTION OF SYMBOLS 1 Acoustic matching member 11 1st matching layer 12 2nd matching layer 12A Ultrasonic radiation surface

Claims (4)

An acoustic matching member for transmitting and emitting ultrasonic waves,
A first matching layer based on glass;
The first matching layer is integrally bonded to the first matching layer, and includes at least a second matching layer mainly composed of glass,
The first matching layer and the second matching layer have a plurality of pores therein,
The pore volume per unit volume of the second matching layer is
An acoustic matching member that radiates ultrasonic waves from the second matching layer, the volume of the pores per unit volume of the first matching layer being larger than the pore volume. The acoustic matching member according to claim 1, wherein the first matching layer and the second matching layer contain an alkaline earth metal element. The acoustic matching member according to claim 1, wherein the first matching layer and the second matching layer are made of the same material composition system. The acoustic matching member according to claim 1, wherein the first matching layer and the second matching layer include borosilicate glass.

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