Ultrasonic transducer

Abstract: An ultrasonic transducer includes a piezoelectric body (1) having a reflector (2) and an acoustic matching layer (4) deposited on the opposing surfaces thereof. The piezoelectric body (1) is constituted by a piezoelectric member exhibiting an acoustic impedence of from 2.5 x 10⁶ to 15 x 10⁶ kg/m² s, and the reflector (2) has a thickness of less than λ₂/4, where λ₂ represents the wavelength of sound in the reflector (2) at a frequency which is one half the free resonance frequency of the piezoelectric body (1). The acoustic matching layer (4) includes a material exhibiting an acoustic impedence of from 1.6 x 10⁶ to 4 x 10⁶ kg/m² s and has a thickness of from λ₃/8 to 28λ₃/120, where λ₃ represents the wavelength of sound in the acoustic matching layer (4) at a frequency which is one half the free resonance frequency of the piezoelectric body (1).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to an ultrasonic transducer and, more particularly, to an ultrasonic transducer well-suited for use underwater or for diagnosis of a living body. 2. Description of the Prior Art: Ultrasonic transducers generally make use of a piezoelectric member which, as is well-known in the art, may be a ceramic piezoelectric member consisting of lead zirconate titanate, barium titanate, lead titanate or the like, a piezoelectric polymer member consisting of polyvinylidene fluoride or the like, or a composite piezoelectric member consisting of a polymer and a ceramic. The piezoelectric polymer member and the composite piezoelectric member have much higher workability and a lower acoustic impedence than the ceramic piezoelectric member and for this reason have come to be widely employed in ultrasonic transducers for use underwater or for diagnosis of a living body. Fig. 2 illustrates an example of the cross-sectional structure of an ultrasonic transducer using a piezoelectric polymer member. The transducer includes a piezoelectric body 1 obtained by providing each of the opposing main surfaces of a flat, plate-shaped piezoelectric polymer member with an electrode, not shown. The piezoelectric body 1 has a reflector 2 deposited on one of its surfaces and is affixed to a support member 3 via the reflector 2. Numeral 5 denotes a specimen undergoing examination. It is known that when an ultrasonic transducer of this construction is driven by a λ/4 driving waveform, namely when a λ/4 drive method is employed, sensitivity is maximized if the thickness of the piezoelectric body 1 is made λ/4. Note that λ₁ represents the sonic wavelength inside the piezoelectric body 1 at a frequency which is one half the free resonance frequency of the piezoelectric body 1. Ordinarily, the thickness of the reflector 2 is set to λ₂/4 (where λ₂ is the sonic wavelength inside the reflector). However, it has been proposed in the specification of Japanese Patent Publication for Opposition No. 59-9000 to reduce the thickness of the reflector to below λ/4 in order to exploit the flexibility possessed by the piezoelectric polymer member and attain a higher efficiency and a wider band. The graph of Fig. 3 shows the results of analyzing the sensitivity and response of an ultrasonic transducer when the thickness of the reflector 2 is varied from 0 to λ₂/3. The analytic method is in line with the principle of analyzing, by a gradualistic method (or sequence definition equation method), the amplitude of a pressure wave produced under the application of a single voltage pulse, as set forth in the specification of Japanese Patent Application Laid-Open No. 60 185 499 (published 20.9.85). The thickness of the reflector 2 is plotted along the horizontal axis, and both the sensitivity (relative values) and response of the ultrasonic transducer are plotted along the vertical axis. The characters S and R indicate the analytical data representative of sensitivity and response, respectively, and f stands for frequency (MHz). The various materials analyzed and the corresponding acoustic impedences are shown in Table 1. It will be understood from Fig. 3 that, for a reflector 2 having a thickness within the range λ₂/30 (=4 λ₂/120) to 11 λ₂/60 (=22 λ₂/120) (A-A' line in Fig. 3), sensitivity and response are higher than for a reflector having a thickness of λ₂/4 (=30λ₂/120), This agrees with the subject matter disclosed in the aforementioned specification of Japanese Patent Publication No. 59 9000 (EP-A 1 14 693). The present invention, however, is directed to an ultrasonic transducer of the type shown in Fig. 1, which is formed from a polymeric or composite piezoelectric member, the surface of the piezoelectric body 1, namely the surface that receives the ultrasonic wave, has an acoustic matching layer 4 deposited thereon to serve as a protective layer for protecting the piezoelectric member and the electrode formed on the abovementioned surface. This ultrasonic transducer ordinarily is used by being brought into contact with the specimen 5 through the intermediary of the acoustic matching layer 4. However, the ultrasonic transducer described in the abovementioned JP-B2-59-9000 does not possess an acoustic matching layer and the specification does not go beyond a description of the polymeric piezoelectric body. In general, the thickness and acoustic impedence of the acoustic matching layer are important factors which influence various characteristics of the ultrasonic transducer. Common technical knowledge in the prior art is that λ₃/4 is the preferred acoustic matching layer thickness (where λ₃/4 is the sonic wavelength inside the acoustic matching layer). However, whether λ₃/4 is indeed the optimum thickness of the acoustic matching layer has not been investigated, and neither has the relationship between the acoustic matching layer thickness and the reflector thickness. From the EP-A1 18 614 an ultrasonic transducer is known using such material as PVDF which can resonant at a frequency less than the free resonance frequency of the piezoelectric material used in the transducer. More specifically it is emphasized in this document that because it is difficult to make a polymer piezoelectrical material very thin, it is difficult to obtain a piezoelectric vibrator which exhibits low free resonant frequency. Therefore it is suggested to provide a transducer made from a material having a thickness of less than 100 µm which transducer exhibits a resonance frequency less than 10 MHz. Additionally a (or two) layer(s) is (are) provided with either (both) of the acoustic radiation side or (and) the opposite side of the transducer. The additional layer(s) exhibit(s) an acoustic impedence Z larger than 2 × Z o , where Z o is the acoustic impedence of the transducer. As is well known, important factors is assessing an ultrasonic transducer are acoustic radiation efficiency and phase characteristic. The radiation efficiency (referred to in the following as sensitivity S) is defined as the ratio of acoustic energy radiated from the piezoelectric member to acoustic energy propagated to the object. Phase characteristic (referred to in the following as response R) for the transducer having a reflector is determined by the phase difference between a wave reflected at the boundary between the piezoelectric body and the reflector and a wave reflected at the boundary between the reflector and the air which is adjacent to the outer face of the reflector. The phase characteristic influences the resolution of the transducer. If the thickness of the reflector is so large that the wave reflected at the boundary between the piezoelectric body and the reflector may not interfere with the wave reflected at the boundary between the reflector and the adjacent air, or if the attenuation coefficient of the reflector is large, it is not necessary to consider the phase characteristic in assessing the characteristic of the transducer. However, it is well known that it is necessary for a transducer to make the reflector thin! The inventors of the present application found out that, in λ/4 wave mode, making the reflector thin, may result in a deteriation due to an interference of the above reflected waves because the wave reflected at the boundary between the reflector and the adjacent air is not negligible. In other words, the influence of the last mentioned reflected wave becomes significant in the same extent as the reflector becomes thinner. SUMMARY OF THE INVENTION Therefore it is the object of the present invention to provide an ultrasonic transducer of quarter wave length type (shown in Fig. 1) having an excellent phase characteristic. This object, in accordance with the claim 1, is attained by presenting optimised ranges of thickness and acoustic impedance of the acoustic matching layer when the thickness of the reflector varies. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing the construction of an ultrasonic transducer having an acoustic matching layer in accordance with the present invention; Fig. 2 is a sectional view showing the construction of an ultrasonic transducer devoid of an acoustic matching layer; Fig. 3 is a graph showing the relationship between the thickness of a reflector of the ultrasonic transducer shown in Fig. 2 and both the sensitivity and response of the ultrasonic transducer; Fig. 4 is a graph indicating the optimum thickness of an acoustic matching layer with respect to the thickness of a reflector for improved response without a loss in sensitivity in the ultrasonic transducer shown in Fig. 1, wherein use is made of a piezoelectric polymer member according to an embodiment of the present invention; Fig. 5 is a graph indicating the optimum thickness of an acoustic matching layer with respect to the thickness of a reflector for improved response without a loss in sensitivity in the ultrasonic transducer shown in Fig. 1, wherein use is made of a composite piezoelectric member according to an embodiment of the present invention; and Figs. 6 and 7 are graphs showing the relationship between the thickness of a reflector and both sensitivity and response for the purpose of making a comparison with the embodiment of Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENT An ultrasonic transducer according to the present invention has the cross-sectional structure illustrated in Fig. 1. The thicknesses of the acoustic matching layer 4 in examples giving optimum analytical results with respect to the thickness of the reflector 2 are illustrated graphically in Figs. 4 and 5. The materials and acoustic impedences used in each of the examples are illustrated in Table 1. 2. The piezoelectric body 1 may consist of a piezoelectric polymer member or a composite obtained by kneading finely divided powder of a ferro-electric ceramic such as lead titanate or lead zirconate titanate with a polymeric material such as polyvinylidene fluoride, polyvinyl fluoride, nylon, polyacetal or polyacrylnitrile. Figs. 4 and 5 differ in that the piezoelectric body 1 of the former is made of the polymer PVF₂, while that of the latter is made of a composite material. The acoustic matching layer 4 consists of a well-known polymer material such as polyester or polyimide film or a polymer composite. In the graphs of Figs. 4 and 5, the thickness of the acoustic matching layer 4 is plotted on the right side along the vertical axis and the analytic data indicative of the optimum thickness of the acoustic matching layer 4 are indicated by the curve T. Fig. 4 is for a case where the ultrasonic transducer shown in Fig. 3 is provided with the acoustic matching layer 4. The optimum acoustic impedence of the acoustic matching layer 4 is decided by the acoustic impedence of the piezoelectric body 1 and the acoustic impedence of the load, namely the specimen 5, and in this case is 2.0 x 106 kg/m² s, as shown in Table 1. The load 5 is water or a living body. The curve T in Fig. 4 indicates examples in which the thickness of the acoustic matching layer 4 is optimum. It will be appreciated from the curve T that the thickness of the acoustic matching layer 4 that improves response R without detracting from sensitivity S differs depending upon the thickness of the reflector 2. It will also be understood from Fig. 4 that the optimum thickness of the acoustic matching layer 4 ranges from 26λ₃/120 to 32λ₃/120 (C-C') where the thickness of the reflector 2 ranges from 4λ₂/120 to 22λ₂/120 (B-B'). Similarly, the curve T in the graph of Fig. 5 gives the optimum thickness of the acoustic matching layer 4 in an embodiment where a composite piezoelectric member constitutes the piezoelectric body 1. It will also be understood from Fig. 5 that the optimum thickness of the acoustic matching layer 4 ranges from 22λ₃/120 to 28λ₃/120 (G-G') where the thickness of the reflector 2 ranges from 4λ₂/120 to 18λ₂/120 (F-F'). For the purpose of comparison, Fig. 6 shows sensitivity and response for an ultrasonic transducer devoid of the acoustic matching layer 4, and Fig. 7 shows sensitivity and response for an ultrasonic transducer in which the acoustic matching layer 4 has a set thickness of λ₃/4. Thus, Fig. 6 corresponds to the ultrasonic transducer having the cross-sectional structure of Fig. 2, and Fig. 7 corresponds to the ultrasonic transducer having the cross-sectional structure of Fig. 1. Other conditions are the same as those of Fig. 5, as shown in Table 1 (sheet 1/7). As will be appreciated from Fig. 6, response R can be improved without detracting from sensitivity S if the thickness of the reflector 2 is selected in the range 8λ₂/120 - 30λ₂/120 (D-D'), which is less than λ₂/4. Fig. 7 demonstrates that by providing the acoustic matching layer 4 of thickness λ₃/4, response R is markedly improved while sensitivity S is improved only marginally. Comparing these with Fig. 5 reveals that a thickness for the acoustic matching layer 4 in the range 22λ₃/120 - 28λ₃/120 (G-G') provides sensitivity and response characteristics superior to those of the conventional ultrasonic transducer having the acoustic matching layer 4 of thickness λ₃/4. The foregoing facts show that the optimum thickness of the acoustic matching layer 4 becomes less than λ₃/4 owing to the presence of the reflector 2. If the thickness of the reflector 2 is made as small as possible, for example zero, the overall ultrasonic transducer shifts to so-called ordinary λ₁/2 drive. If the piezoelectric body 1 in this case is subjected to λ/4 drive, it goes without saying that the optimum thickness of the acoustic matching layer 4 will be λ₃/4 x 1/2 = λ₃/8. This is evident from Fig. 5, which shows that λ₃/8 is the optimum thickness of the acoustic matching layer 4 corresponding to a reflector thickness of zero. ADVANTAGES OF THE INVENTION According to the present invention as described above, an ultrasonic transducer including an acoustic matching layer and having a reflector the thickness whereof is less than λ₂/4 has the thickness of its acoustic matching layer selected in a range (15/120 - 28/120) x λ₃, depending upon the actual thickness of the reflector. An ultrasonic transducer provided with an acoustic matching layer having this optimum thickness exhibits improved response with no decline in sensitivity. This makes it possible for the ultrasonic transducer to send and receive ultrasonic signals over a wide band. As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims: An ultrasonic transducer used for ultrasonic measurement in water or biological tissues comprising: a piezoelectric body (1) having opposing first and second surfaces, a reflector (2) deposited on the first surface and an acoustic matching layer (4) deposited on the second surface, characterized in that said piezoelectric body (1) includes a piezoelectric member exhibiting an acoustic impedance Z T from 2.5 x 10⁶ to 15 x 10⁶ kg/m²s, said reflector (2) having a thickness of less than λ₂/4, where λ₂ represents the sonic wavelength internally of said reflector (2) at a frequency which is one half the free resonant frequency of said piezoelectric body (1), said acoustic matching layer (4) includes a material exhibiting an acoustic impedance Z M of 1.6 x 10⁶ to 4 x 10⁶ kg/m²s, said acoustic matching layer (4) has a thickness of from λ₃/8 to 28 λ₃/120, becoming thinner as said reflector (2) becomes thinner, where λ₃ represents the sonic wavelength internally of said acoustic matching layer (4) at a frequency which is one half the free resonant frequency of said piezoelectric body (1), and that the relation Z O ≦ Z M ≦ Z T holds between Z M , ZT and a Z O which represents the acoustic impedance of an object (5) under measurement. The ultrasonic transducer according to claim 1, wherein the piezoelectric body (1) consists of a piezoelectric polymer member. The ultrasonic transducer according to claim 1, wherein the piezoelectric body (1) consists of a composite obtained by kneading finely divided powder of a ferro-electric ceramic with a polymeric material.

Inventors: FUJII TADASHI, YAGAMI HIROYUKI, FUJII, TADASHI, YAGAMI, HIROYUKI
Applicants: TERUMO CORP, TERUMO KABUSHIKI KAISHA TRADING AS TERUMO CORPORATION
Application year: 1986
Main classification: G01H11
IPC codes: G10K11, H04R1, G01N29, A61B8, G01H11, H04R17