Piezoelectric element for an ultrasonic measuring device

Abstract

Piezoelectric element (10) for an ultrasonic measuring device, wherein the piezoelectric element (10) is mainly designed as a porous plate-shaped body (11) made of alkali niobate-based piezoceramic, wherein the piezoelectric element (10) has a porosity in the range of 15% or more and 31% or less and a mean specific acoustic impedance of 20 MN·s / m 3 or less, , characterized by the fact that the porosity is in a range of 23% or more and 31% or less, and the mean value of the specific acoustic impedances 15 MN·s / m 3 or less.

Description

Technical area

[0001] The present invention relates to a piezoelectric element for an ultrasonic measuring device, which is used, for example, in an ultrasonic sensor or an ultrasonic transducer. State of the art

[0002] Piezoelectric elements are known for applications such as ultrasonic sensors and ultrasonic transducers. Piezoelectric elements containing lead compounds, such as lead zirconate titanate (PZT), are currently being used in practice as piezoelectric elements with excellent piezoelectric properties. However, there is increasing concern that piezoelectric elements containing lead compounds may have negative environmental impacts. For this reason, piezoelectric elements free of lead compounds have received increasing attention in recent years, and research and development of such piezoelectric elements are underway. For example, a piezoelectric element made of alkali niobate-based piezoceramic has been proposed as a lead-free piezoelectric element (see, for example, Patent Documents 1 and 2). Citation listPatent specifications Patent Document 1: Publication of the unexamined Japanese patent application (translation of the PCT application) JP 2018 - 514 493 A (e.g., claim 1, paragraphs

[0035] and

[0041] ) Patent Document 2: Publication of Unexamined Japanese Patent Application JP 2011 - 249 533 A (e.g., paragraphs

[0028] to

[0030] and

[0054] )

[0003] RYBYANETS, Andrey N.: "Porous piezoceramics: theory, technology, and properties. IEEE transactions on ultrasonics, ferroelectrics, and frequency control," 2011, Vol. 58, No. 7, pp. 1492-1507. This work describes the development and characterization of porous piezoceramics and piezocomposites. A technological approach is presented to improve the physical, electrical, and functional properties of these materials. The goal is to enable more stable, efficient, and technically sophisticated applications, including the optimization of transducer designs for piezoelectric devices.

[0004] REZNICHENKO, AN [et al.]: "Lead-free porous and composite materials for ultrasonic transducers applications. In: Ferroelectrics", Vol. 539, 2019, No. 1, pp. 93-100. - ISSN 0015-0193 describes the results of an experimental investigation of the properties of piezoceramics, ceramic-matrix composites "ceramic / crystal," and porous ceramics based on lead-free sodium lithium niobate (Na, Li)NbO3 and sodium potassium niobate (Na, K)NbO3 compositions. Measurements of elastic, dielectric, and piezoelectric parameters, as well as microstructural investigations of test samples of lead-free porous piezoelectric ceramics and composites, were performed. It was shown that the use of alkali metal niobates in the form of ceramic composites and porous ceramics makes it possible to increase the main functional parameters of these materials and make them comparable to piezoceramics of the PZT system. Description of the inventionTask of the invention

[0005] Currently, however, a piezoelectric element made of alkali niobate-based piezoceramic is inferior to a piezoelectric element made of lead zirconate titanate (PZT) in terms of piezoelectric properties such as piezoelectric strain constant and electromechanical coupling coefficient. For this reason, an ultrasonic sensor with a piezoelectric element made of alkali niobate-based piezoceramic is considered to have inferior transmission and reception sensitivity. Consequently, such ultrasonic sensors are not able to replace those made of PZT. Furthermore, according to the RoHS directive, lead in piezoceramics is excluded from the list of types of lead considered environmentally harmful, and thus their use must be restricted.The reason for this is that the performance of piezoelectric elements free of lead compound is considered inferior to that of piezoelectric elements containing PZT.

[0006] Furthermore, advances in digital signal processing technology in recent years are expected to change the demand for ultrasonic sensors. For example, frequency analysis using CHIRP transmission is known to improve the signal-to-noise ratio and resolution. However, performing such frequency analysis requires a wide-bandwidth ultrasonic sensor. Therefore, increasing demand is expected in the future for ultrasonic sensors that place greater emphasis on broadband performance than on transmit and receive sensitivity.

[0007] The present invention was conceived to solve the problems described above. The present invention is based on the object of providing a piezoelectric element for an ultrasonic measuring device, wherein the piezoelectric element itself contains alkali niobate-based piezoceramic that is free of lead, can exhibit equivalent or higher transmission and reception sensitivity, and a wider frequency band than a piezoelectric element using PZT. Furthermore, the present invention is based on the object of providing a piezoelectric element that is both mechanically and acoustically homogeneous and thus has a quality and performance suitable for use in the ultrasonic measuring device. Solution to the task

[0008] To achieve the above-described objects, the invention is as claimed in claims 1 to 4 and 7.

[0009] Accordingly, according to the invention according to claims 1 to 4 and 7, the piezoelectric element made of alkali niobate-based piezoceramic, which is free of environmentally harmful lead, is primarily formed as a "porous" plate-shaped body and has a porosity of 15% or more. This piezoelectric element can achieve an equivalent or higher signal-to-noise ratio and an equivalent or higher transmission and reception sensitivity when transmitting and receiving ultrasound with an ultrasonic measuring device than a piezoelectric element made of a non-porous plate-shaped body made of lead zirconate titanate (PZT). The piezoelectric element also has a porosity of 31% or less, which enables high transmission and reception sensitivity without compromising mechanical strength. Furthermore, the piezoelectric element has a low average specific acoustic impedance of 20 MN s / m. 3or less. Ultrasound propagates easily within the piezoelectric element, which can further increase the transmission and reception sensitivity and expand the frequency band when transmitting and receiving ultrasound with an ultrasonic measuring device. The piezoelectric element is made of alkali niobate-based piezoceramic, which is softer than PZT, and is mainly formed as a porous plate-shaped body. Consequently, the piezoelectric element has lower rigidity than a piezoelectric element formed mainly as a non-porous plate-shaped body with PZT. As a result, the vibrations of the piezoelectric element tend to converge after the transmission of ultrasound, which can reduce the reverberation of ultrasound during transmission.

[0010] The piezoelectric element for an ultrasonic sensor is primarily formed as a porous plate-shaped body made of alkali niobate-based piezoceramic. Suitable examples of alkali niobate-based piezoceramic include potassium-sodium niobate (KNN)-based piezoceramic, which is a solid solution of potassium niobate and sodium niobate with a perovskite structure. The term "KNN-based piezoceramic" refers to piezoceramic that contains at least potassium (K), sodium (Na), and niobium (Nb) as its main metal components, while containing little or no other harmful and toxic elements such as lead (Pb). In addition to K and Na, KNN-based piezoceramic can contain an alkali metal such as lithium (Li). Furthermore, KNN-based piezoceramic can contain an alkaline earth metal such as calcium (Ca), strontium (Sr), barium (Ba), tantalum (Ta), or antimony (Sb) in addition to Nb.In addition, the ANN-based piezoceramic may contain small amounts of bismuth (Bi), iron (Fe), aluminum (Al), manganese (Mn), cobalt (Co), nickel (Ni) and other elements.

[0011] The invention according to claim 1 is also directed to a piezoelectric element having a porosity in a range of 23% or more and 31% or less, and having an average value of the specific acoustic impedances which is 15 MN·s / m 3 or less.

[0012] According to the invention of claim 1, since the lower limit of the porosity of the piezoelectric element is higher (23%), this piezoelectric element has a higher signal-to-noise ratio and higher transmission and reception sensitivity when transmitting and receiving ultrasound with an ultrasonic measuring device. Furthermore, the upper limit of the average value of the specific acoustic impedance of the piezoelectric element is lower (15 MN s / m).3 ). Therefore, the following is provided: Ultrasound propagates more reliably within the piezoelectric element, which can further increase the transmitting and receiving sensitivity and safely expand the frequency band when transmitting and receiving ultrasound with an ultrasonic measuring device.

[0013] The invention according to claim 2 is also directed to a piezoelectric element in which twenty or more measuring points are set which are spaced apart from each other on a straight line passing through the center of the main surface of the porous plate-shaped body, wherein the average value of sound velocities measured at the respective measuring points is 5200 m / s or less, and wherein the value of the sound velocity at each of the measuring points is within ±5% of the average value.

[0014] According to the invention of claim 2, the variation in the sound velocity values ​​measured at the respective, spaced-apart measurement points is minimized, so that the sound velocity value at each of the measurement points is within ±5% of the average value. That is, the piezoelectric element is designed so that the porous plate-shaped body becomes acoustically homogeneous in the direction from the center to the edge. Furthermore, the average value of the sound velocities measured at the respective measurement points is 5200 m / s or less, resulting in a lower specific acoustic impedance and higher transmission and reception sensitivity. Therefore, a piezoelectric element with suitable quality and performance for use as an ultrasonic measuring device can be obtained.

[0015] The invention according to claim 3 is also directed to a piezoelectric element in which twenty or more micro-measuring areas are set, which are spaced apart from each other in the direction of the straight line on a sectional plane cut along a straight line passing through the center of the main surface of the porous plate-shaped body, wherein the mean value of pore cross-sectional areas measured in the respective micro-measuring areas is in a range of 33 µm 2 or more and 65 µm 2 or less, and where the standard deviation σ is in the range of 25 µm 2 or more and 95 µm 2 or less, and the variance CV is in the range of 80% or more and 150% or less.

[0016] According to the invention according to claim 3, since the respective micro-measuring areas are arranged with uniformly arranged pores with equivalent pore cross-sectional areas in the direction from the center to the edge of the porous plate-shaped body, the piezoelectric element becomes mechanically homogeneous. Since ultrasound can propagate at the same speed in each micro-measuring area during transmission and reception with an ultrasonic measuring device, the piezoelectric element also becomes acoustically homogeneous. Therefore, a piezoelectric element with suitable quality and performance for use as an ultrasonic measuring device can be obtained.

[0017] The invention according to claim 4 is also directed to a piezoelectric element in which twenty or more micro-measuring areas are set, which are spaced apart from each other in the direction of the straight line on a sectional plane cut along a straight line passing through the center of the main surface of the porous plate-shaped body, wherein the mean value in the measurement of the Voronoi area in the respective micro-measuring areas is in a range of 270 µm 2 or more and 370 µm 2 or less, with the standard deviation σ in the range of 170 µm 2 and 200 µm 2 and where the variance CV is in the range of 50% or more and 65% or less.

[0018] According to the invention according to claim 4, since the respective micro-measuring areas are arranged with uniformly spaced Voronoi regions with equivalent Voronoi surfaces in the direction from the center to the edge of the porous plate-shaped body, the piezoelectric element becomes mechanically homogeneous. Since ultrasound can propagate at the same speed in each micro-measuring area during transmission and reception with an ultrasonic measuring device, the piezoelectric element also becomes acoustically homogeneous. Therefore, a piezoelectric element with suitable quality and performance for use as an ultrasonic measuring device can be obtained.

[0019] The invention according to claim 5 is directed to the piezoelectric element according to one of claims 1 to 4, which has a density ρ of 3.7 g / cm 3 or less.

[0020] According to the invention of claim 5, the density ρ of the piezoelectric element is lower than that of a lead zirconate titanate piezoelectric element. Furthermore, the piezoelectric element has pores, which further lowers the density ρ. Consequently, the specific acoustic impedance, which is the product of the speed of sound and the density, is also low. As a result, ultrasound propagates easily within the piezoelectric element, which can increase the transmission and reception sensitivity when transmitting and receiving ultrasound with an ultrasonic measuring device.

[0021] The invention according to claim 6 is directed to the piezoelectric element according to one of claims 1 to 5, which has a mechanical quality factor Qm of 40 or less.

[0022] According to the invention of claim 6, since the mechanical quality factor Qm of the piezoelectric element is as low as 40 or less, the vibrations of the piezoelectric element converge quickly after the transmission of ultrasound. That is, the reverberation of ultrasound during transmission is reduced.

[0023] The invention according to claim 7 is also directed to a piezoelectric element according to one of claims 1 to 7, in which the normalized bandwidth Δf / fm, which is the ratio of the frequency bandwidth Δf to the center frequency fm, is 20% or more when transmitting and receiving ultrasound with the ultrasonic measuring device, wherein the frequency band is the frequency range at the transmission and reception sensitivity obtained by subtracting 6 dB from the maximum transmission and reception sensitivity, the lower limit frequency is the lower frequency of the frequency band, the upper limit frequency is the upper frequency of the frequency band, the center frequency fm is the intermediate value between the lower and upper limit frequencies, and the frequency bandwidth Δf is the difference between the lower limit frequency and the upper limit frequency.

[0024] According to the invention according to claim 7, the piezoelectric element has a standardized bandwidth Δf / fm of 20% or more, which is significantly larger than the specific bandwidth Δf / fm (6%) of a lead zirconate titanate piezoelectric element. This means that the frequency band for transmitting and receiving ultrasound with the ultrasonic flowmeter can be expanded. Advantageous effects of the invention

[0025] As described above, the inventions of claims 1 to 7 can provide a piezoelectric element for an ultrasonic measuring device, wherein the piezoelectric element itself containing alkali niobate-based piezoceramic free of lead can have equivalent or higher transmission and reception sensitivity and a wider frequency band than a piezoelectric element using PZT. Short description of the drawings Fig.1 is a perspective view showing a piezoelectric element according to an embodiment. Fig. Figure 2 is a plan view showing the piezoelectric element. Fig. Figure 3 is a diagram of the configuration of a spray dryer. Fig. Figure 4 is a graph showing the relationship of porosity to transmit and receive sensitivity in samples A to C, respectively. Fig. 5 includes: a graph (a) showing the reception waveform of ultrasound in sample A; a graph (b) showing the reception waveform of ultrasound in sample B; and a graph (c) showing the reception waveform of ultrasound in sample C. Fig.6 includes: a graph (a) showing a reception waveform processed by Fourier transform in Sample A; a graph (b) showing a reception waveform processed by Fourier transform in Sample B; and a graph (c) showing a reception waveform processed by Fourier transform in Sample C. Fig. Figure 7 is an SEM image showing the outer surface of a piezoelectric element according to Example 1. Fig. Figure 8 is an SEM image showing the outer surface of a piezoelectric element according to Example 2. Fig. Figure 9 is an SEM image showing the outer surface of a piezoelectric element according to Example 3. Fig. 10 is an SEM image showing the outer surface of a piezoelectric element according to Example 4. Fig.11 is a graph showing the relationship of the amount of resin beads added to the porosity in each of Examples 1 to 4 and Comparative Examples 1 to 5. Fig. Figure 12 is a graph showing the porosity-to-density (sintered density) ρ ratio of a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig. Figure 13 is a graph showing the relationship of porosity to sound velocity for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig.Figure 14 is a graph showing the relationship of porosity to specific acoustic impedance for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig. Figure 15 is a graph showing the relationship of porosity to dielectric constant ε for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig. 16 is a graph showing the relationship of porosity to frequency constant Nt for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig.Figure 17 is a graph showing the relationship of porosity to frequency constant Np for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig. Figure 18 is a graph showing the relationship of porosity to electromechanical coupling coefficient Kt for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig. Figure 19 is a graph showing the relationship of porosity to electromechanical coupling coefficient Kp for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and a piezoelectric element made of PZT-based piezoceramic. Fig.Figure 20 is a graph showing the relationship between porosity and piezoelectric strain constant d 33 for a piezoelectric element made of KNN-based piezoceramic (Examples 1 to 4 and Comparative Examples 1 to 4) and for a piezoelectric element made of PZT-based piezoceramic. Fig. Figure 21 is an SEM image showing a cross-sectional view of the piezoelectric element. Fig. Figure 22 is an SEM image showing a binarized micro-measurement area. Fig. Figure 23 is an image showing the binarized micro-measurement area with pore numbers. Fig. Figure 24 is a histogram showing the pore cross-sectional area distribution. Fig. Figure 25 is a picture showing the micro-measuring area with Voronoi lines and Voronoi numbers. Fig. Figure 26 is a histogram showing the Voronoi area distribution. Ways to implement the invention

[0026] The present invention will be described in more detail below with reference to an embodiment in which the present invention is embodied as a piezoelectric element, with reference to the drawings.

[0027] Fig. Fig. 1 is a perspective view showing a piezoelectric element 10 and Fig. Figure 2 is a plan view showing the piezoelectric element 10. The piezoelectric element 10 has a front and a back surface, is disk-shaped, and has an outer diameter of 15 mm and a thickness of 1 mm. The piezoelectric element 10 according to this embodiment is used as an ultrasonic sensor in an ultrasonic flowmeter as an ultrasonic measuring device.

[0028] The piezoelectric element 10 is mainly formed as a porous plate-shaped body 11 made of alkali niobate-based piezoceramic. The alkali niobate-based piezoceramic according to this embodiment is potassium sodium niobate (KNN)-based piezoceramic represented by the following composition formula (1) and contains a small amount of bismuth (Bi) and iron (Fe) as metal elements as additives. {Li x (K 1-y N / a y ) 1-x}(Nb 1-z Sb z )O3 (1)

[0029] However, the alkali niobate-based piezoceramic has a composition in which the numerical ranges 0.03 ≤ x ≤ 0.045, 0.5 ≤ y ≤ 0.58 and 0.03 ≤ z ≤ 0.045 are satisfied.

[0030] The piezoelectric element 10 according to this embodiment has a porosity of 15% or more and 31% or less, preferably 23% or more and 31% or less. Furthermore, the piezoelectric element 10 has an average specific acoustic impedance of 20 MN s / m 3 or less, preferably 15 MN s / m 3 or less. In addition, the piezoelectric element 10 has a density ρ of 3.7 g / cm 3 or less and a mechanical quality factor Qm of 40 or less.

[0031] The piezoelectric element 10 has a frequency constant Nt in the thickness direction in a range of 1600 Hz·m or more and 2100 Hz·m or less, and a frequency constant Np in the radial direction in a range of 2300 Hz·m or more and 2800 Hz·m or less. Furthermore, the piezoelectric element 10 has a piezoelectric strain constant d 33in a range of 250 pC / N or more and 260 pC / N or less.

[0032] In the case where ultrasound is transmitted and received by an ultrasonic sensor of an ultrasonic flowmeter, the piezoelectric element 10 according to this embodiment has a normalized bandwidth Δf / fm of 20% or more. Note that the normalized bandwidth Δf / fm represents the relationship between the frequency bandwidth Δf and the center frequency fm. When the frequency range at the transmission and reception sensitivity obtained by subtracting 6 dB from the maximum transmission and reception sensitivity is determined as the frequency band of the ultrasonic sensor, the frequency bandwidth Δf represents the difference between the lower and upper cutoff frequencies, where the lower cutoff frequency is the lower frequency of the frequency band, and the upper cutoff frequency is the upper frequency of the frequency band. The center frequency fm represents the intermediate value between the lower cutoff frequency and the upper cutoff frequency.

[0033] A method for manufacturing the piezoelectric element 10 will be described in detail below.

[0034] First, raw powders (with purity of 99% or more) of K2CO3, Na2CO3, Li2CO3, Nb2O5, Sb2O3, Bi2O3, and Fe2O3 are prepared. Resin beads made of polymethyl methacrylate (PMMA) are also prepared. The raw powders containing the respective metal elements and the resin beads are weighed and mixed in alcohol for 24 hours through a ball mill to obtain a mixed slurry. The types of raw powders (compounds) containing the respective metal elements are not particularly limited, but oxides, carbonates, etc. of the respective metal elements can be used preferably.

[0035] Next, the resulting mixed slurry is dried, calcined at 900°C for 3 hours, and then ground in a ball mill for 24 hours. Further, an aqueous solution of polyvinyl alcohol is added thereto as a binder. Then, granulation is performed. More specifically, the ground mixed slurry, to which a binder has been added, is granulated by spray drying in a drying chamber 21. Subsequently, the powders obtained after granulation are collected by trap collection (body collection) in the drying chamber 21 or by collection (cyclone collection) in the cyclone chamber 23 arranged in the intermediate position of the exhaust path 22 (see Fig. 3).

[0036] The collected granulated powders are then molded into a round disc with an outer diameter of 18 mm and a thickness of 2 mm under a pressure of 200 MPa. The compact is sintered at 1000°C to 1200°C for 2.5 hours to obtain a sintered body. Along with the sintering of the powders, the resin beads undergo thermal decomposition and disappear. Since the subsequently disappeared parts become pores, a porous sintered body is obtained. Next, the sintered body is subjected to parallel polishing of the two surfaces and then polishing of the outer peripheral surface, so that the sintered body is processed into a disc shape with an outer diameter of 15 mm and a thickness of 1 mm, thereby obtaining a porous plate-shaped body 11.

[0037] Next, electrodes are formed on both the front and back surfaces of the porous plate-shaped body 11. Specifically, silver paste is applied to both the front and back surfaces of the porous plate-shaped body 11. The applied silver paste is then sintered at 700°C to form the electrodes. Subsequently, a DC voltage of 3 kV / mm is applied between the two electrodes in silicone oil at a temperature of 130°C for 20 minutes to polarize the porous plate-shaped body 11 in the thickness direction. At this time, the piezoelectric properties appear, thus completing the piezoelectric element 10.

[0038] Then, a method for evaluating the piezoelectric element and its results are described.

[0039] First, measurement samples were prepared as follows: Two piezoelectric elements (made of bulk PZT) made of PZT-based piezoceramic with a porosity of 0% were prepared and used as sample A. Two piezoelectric elements (made of bulk KNN) made of KNN-based piezoceramic with a porosity of 0% were prepared and used as sample B. Two piezoelectric elements (made of porous KNN) made of KNN-based piezoceramic with a porosity of 30.5% were prepared and used as sample C.

[0040] A comparative test was then conducted using the test samples (samples A to C). Specifically, one of the samples A to C was first selected. The two piezoelectric elements of the selected sample were attached to a test gauge simulating an ultrasonic flowmeter. A voltage was then applied, for example, to the first of the two attached piezoelectric elements (ultrasonic sensors). Ultrasound was then radiated (transmitted) from the first piezoelectric element to the second.

[0041] After a predetermined time has elapsed after transmission, ultrasound is received by the second piezoelectric element. Ultrasound then generates voltage signals at both ends of the second piezoelectric element. The voltage (transmission voltage V1) when ultrasound is transmitted from the first piezoelectric element and the voltage (reception voltage V2) when ultrasound is received from the second piezoelectric element were measured with an oscilloscope. The transmission and reception sensitivity were calculated based on the measurement results. The transmission and reception sensitivity is the ratio of the reception voltage V2 to the transmission voltage V1 and is obtained using the mathematical formula 20 × log (V2 / V1). The measurement of the transmission voltages V1 and V2 and the calculation of the transmission and reception sensitivity were performed for each of the samples A to C.The relationship between porosity and transmit and receive sensitivity for samples A to C is shown in the graph. Fig. 4 shown.

[0042] As a result, it was confirmed that the transmission and reception sensitivity of Sample B (made of bulk ANN) was lower than that of Sample A (made of bulk PZT). However, it was also confirmed that the transmission and reception sensitivity of Sample C (made of porous ANN) was higher than that of Sample A. Thus, it is confirmed that even if the transmission and reception sensitivity of the piezoelectric element made of a lead-free material (ANN) may become low, by using Sample C with the piezoelectric element made of a lead-free porous material, the transmission and reception sensitivity can be increased compared to the case with the piezoelectric element made of a lead-containing material (PZT).

[0043] The received waveforms of ultrasound in samples A to C were displayed on an oscilloscope (see graphs (a) to (c) of Fig. 5). In addition, a fast Fourier transform (FFT) of the digitized received waveforms was performed to calculate the power spectra of the frequency components (see graphs (a) to (c) of Fig. 6).

[0044] From the comparison between samples A to C in terms of receiving waveforms, the following was confirmed: The oscillations of the receiving waveform of sample C consisting of a material with the porosity of 30.5% (see the graph (c) of Fig. 5) converge faster (up to 0V) than those of samples A and B each consisting of a material with a porosity of 0% (see graphs (a) and (b) of Fig.5). In other words, it was confirmed that in Sample C, the reverberation of ultrasound transmitted from a piezoelectric element is small, which can shorten the time until subsequent ultrasound is transmitted. It was also confirmed that in the power spectra of Samples A to C, when the frequency range at the transmitting and receiving sensitivity obtained by subtracting equal to or less than 6 dB from the maximum value of the transmitting and receiving sensitivity (from the maximum transmitting and receiving sensitivity) is found to be a frequency band, the frequency band of the receiving waveform of Sample C is wider than those of Samples A and B (see Fig. 6).

[0045] In addition, the measurement samples were prepared as follows. A mixed slurry obtained by adding 10.0 wt% resin beads to KNN-based piezoceramic was granulated by spray drying. Afterward, the powders obtained after granulation were collected once by body collection and once by cyclone collection. Then, the piezoelectric elements 10 were manufactured using the powders collected by body collection and the powders collected by cyclone collection, respectively. The piezoelectric element containing the powders from the cyclone collection was referred to as Example 1 (see Fig. 7), while the piezoelectric element containing the powders after body collection as Example 3 (see Fig.9). In solid collection, relatively heavy particles are collected, while in cyclone collection, relatively light particles are collected. Accordingly, the case of solid collection and the case of cyclone collection differ in the proportion of resin beads. As a result, the piezoelectric element produced by solid collection alone and the piezoelectric element produced by cyclone collection alone differ in porosity. A mixed slurry obtained by adding 20.0 wt% resin beads to KNN-based piezoceramic was granulated by spray drying. After that, the powders obtained after granulation were collected once by solid collection and once by cyclone collection. Then, the piezoelectric elements 10 were each manufactured using the collected powders. The piezoelectric element containing the cyclone collection powders was referred to as Example 2 (see Fig. 8), while the piezoelectric element containing the powder of the body collection as Example 4 (see Fig. 10).

[0046] On the other hand, granulated powders were also prepared by a method (semi-moist granulation). In this method, preliminarily calcined powders of KNN-based piezoceramics were mixed with resin beads, after which a binder solution was dropped into the resulting intermediate product and kneaded. The intermediate product was then dried, ground, and granulated. Specifically, a slurry obtained by adding 0.0 wt% of resin beads (without adding resin beads) to KNN-based piezoceramics was granulated by semi-moist treatment. A piezoelectric element, designated Comparative Example 1, was fabricated from the granulated powders. A mixed slurry obtained by adding 2.9 wt% of resin beads to KNN-based piezoceramics was granulated by semi-moist treatment.A piezoelectric element was prepared from the granulated powders and designated as Comparative Example 2. A mixed slurry obtained by adding 4.4 wt% of resin beads to KNN-based piezoceramics was granulated using a semi-wet treatment. A piezoelectric element was prepared from the granulated powders and designated as Comparative Example 3. A mixed slurry obtained by adding 5.9 wt% of resin beads to KNN-based piezoceramics was granulated using a semi-wet treatment. A piezoelectric element was prepared from the granulated powders and designated as Comparative Example 4. A mixed slurry obtained by adding 30.0 wt% of resin beads to KNN-based piezoceramics was granulated using a semi-wet treatment.A piezoelectric element was fabricated from the granulated powders, which was designated as Comparative Example 5. The material properties of Examples 1 to 4 and Comparative Examples 1 to 5 are shown in Table 1. Note: In the semi-moist treatment, the resin beads can be mixed according to the mixing ratio, unlike granulation by spray drying, where the mixing state tends to be undesirably uneven. [Table 1] Example 1 Example 2 Example 3 Example 4 Comparison example 1 Comparison example 2 Comparison example 3 Comparison example 4 Comparison example 5 Resin beads (wt%) 10,0 20,0 10,0 20,0 0,0 2,9 4,4 5,9 30,0 Density ρ (g / cm 3 ) 3,67 3,33 3,29 3,03 4,36 4,10 3,91 3,81 1,64 Porosity (%) 15,3% 23,6% 23,9% 30,5% 0,0% 6,6% 9,8% 12,1% 61,6% Kt (%) 43,2 41,5 41,5 43,3 41,8 43,3 39,8 41,1 ε 1143 933 951 775 1521 1352 1146 1089 91 Kp (%) 35,7 31,6 30,7 27,6 48,1 44,1 38,4 38,1 23,6 sqm 38,2 34,6 33,7 34,9 40,2 40,6 45,0 43,6 25,4 Np (H·zm) 2717,0 2476,2 2495,2 2357,4 3034,2 2942,5 2903,0 2811,9 7113,8 d 33 (pC / N)) 253,5 255,0 257,0 254,0 295,5 295,0 235,0 244,0 Speed ​​of sound (m / s) 5151 4453 4388 3875 6196 5365 5402 5141 - specific acoustic impedance 19,0 14,8 14,5 11,7 26,9 21,8 21,2 19,6 -

[0047] Subsequently, the electrical properties of the piezoelectric elements of the respective test samples (Examples 1 to 4 and Comparative Examples 1 to 5) were measured. The measurement results are shown in Table 1 and the graphs of Fig.11 to 20. The electrical properties of the piezoelectric elements made of PZT-based piezoceramic with porosities of 0%, 15%, 21%, and 25% were also measured in the same way as the test samples. The measurement results are shown in Fig. 12 to 20. It should be noted that due to the extremely high porosity (porosity of 61.6%), Comparative Example 5 had only a lower strength and was thus damaged, resulting in not all electrical properties being measured.

[0048] Specifically, first, the porosities of the piezoelectric elements in Examples 1 to 4 and Comparative Examples 1 to 5 were measured. More specifically, the respective porosities were measured using previously known methods such as the Archimedes principle or gravimetric porosity measurement. As a result, it was confirmed that the porosities in Examples 1 to 4 were in a range of 15% or more and 31% or less. In particular, it was confirmed that the porosities in Examples 2 to 4 were in a range of 23% or more and 31% or less. The porosities of Examples 1 to 4 and Comparative Examples 1 to 5 are shown in Table 1 and Fig. 11. The graph of Fig.Figure 11 shows the relationship between the amount of resin beads added and the porosity in the respective samples after semi-moist treatment (Comparative Examples 1 to 5), after spray granulation (solid collection) (Examples 3 and 4), and after spray granulation (cyclone collection) (Examples 1 and 2). As a result, it was confirmed that the porosities increased consistently with increasing addition of resin beads, regardless of the granulation methods used.

[0049] Subsequently, the densities ρ of the piezoelectric elements in Examples 1 to 4 and Comparative Examples 1 to 5 were measured. Specifically, the outer diameters and thicknesses of the respective porous plate-shaped bodies (piezoelectric elements) were measured. The volumes of the respective porous plate-shaped bodies were calculated from the measured outer diameters and thicknesses. Furthermore, the masses of the respective porous plate-shaped bodies were also measured. The respective densities ρ were then calculated from the obtained volumes and the measured masses. As a result, it was confirmed that the samples in Examples 1 to 4 and Comparative Example 5, which have a porosity of 15% or more, each had a density ρ of 3.7 g / m³. 3 or less, which is smaller than the samples in Comparative Examples 1 to 4, which have a porosity of less than 15% (see Table 1 and Fig.12). Furthermore, it was confirmed that the densities ρ of the piezoelectric elements made of KNN-based piezoceramics (hereinafter referred to as “KNN”) in Examples 1 to 4 and Comparative Examples 1 to 4 are generally lower than the densities ρ of the piezoelectric elements made of PZT-based piezoceramics (hereinafter referred to as “PZT”) (see Fig. 12).

[0050] Then, the sound velocities of the longitudinal wave (ultrasound) propagating in the piezoelectric element of Examples 1 to 4 and Comparative Examples 1 to 4 were measured. More specifically, as in Fig.2, a straight line L1 passing through the center C1 of the main surface 12 of the porous plate-shaped body 11 was defined. Twenty spaced-apart measurement points P1 were then defined on the straight line L1. The sound velocity was measured at each of the measurement points P1. The measurement points P1 are arranged at equal distances from each other on the straight line L1. Then, the average value of the sound velocities measured at the respective measurement points P1 was calculated. As a result, it was confirmed that the samples in Examples 1 to 4 and Comparative Example 4, which have a porosity of 10% or more, each have an average sound velocity of 5200 m / s or less, which is lower than the samples in Comparative Examples 1 to 3, which have a porosity of less than 10% (see Table 1 and Fig.13). In Example 1, it was confirmed that the maximum sound speed at the respective measurement points P1 was 5250 m / s and the minimum sound speed at the respective measurement points P1 was 5120 m / s. Likewise, in Example 2, it was confirmed that the maximum sound speed was 4570 m / s and the minimum sound speed was 4100 m / s. In Example 3, it was confirmed that the maximum sound speed was 4580 m / s and the minimum sound speed was 4120 m / s. In Example 4, it was confirmed that the maximum sound speed was 3970 m / s and the minimum sound speed was 3800 m / s. In other words, in Examples 1 to 4, it was confirmed that the sound speed value at the respective measurement points P1 was within ±5% of the mean value. Furthermore, it was also confirmed that the mean value of the sound speeds of ANN was generally higher than the mean value of the sound speeds of PZT (see Fig. 13).

[0051] Next, the specific acoustic impedances of the piezoelectric elements in Examples 1 to 4 and Comparative Examples 1 to 4 were measured. Specifically, the product of the density ρ of each piezoelectric element and the average of the sound velocities of ultrasound was calculated. The calculated product was referred to as the specific acoustic impedance. As a result, it was confirmed that the respective samples in Examples 1 to 4 and Comparative Example 4 had a specific acoustic impedance of 20 MN s / m. 3 or less. Especially in Examples 2 to 4, it was confirmed that the specific acoustic impedance was 15 MN s / m 3 or less. The specific acoustic impedances of Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 1 and Fig.14. Furthermore, it was also confirmed that the specific acoustic impedance of ANN is generally lower than the specific acoustic impedance of PZT (see Fig. 14). In summary, using Examples 1 to 4 and Comparative Example 4, especially Examples 2 to 4 with relatively low specific acoustic impedances, ultrasound can easily propagate in the piezoelectric element, resulting in higher transmission and reception sensitivity. Furthermore, it was also confirmed that the frequency band for transmitting and receiving ultrasound can be broadened, and that the reverberation of ultrasound during transmission can be reduced.

[0052] The dielectric constants ε of the respective piezoelectric elements (Examples 1 to 4 and Comparative Examples 1 to 5) were measured. It follows that the dielectric constant ε tends to decrease with increasing porosity (see Table 1 and Fig. 15). Furthermore, it was also confirmed that the dielectric constant ε of ANN tends to be lower than the dielectric constant ε of PZT, but ANN with a porosity of 25% or more has an equivalent dielectric constant ε to the dielectric constant ε of PZT (see Fig. 15).

[0053] The mechanical quality factors Qm of the respective piezoelectric elements in Examples 1 to 4 and Comparative Examples 1 to 5 were measured. As a result, it was confirmed that the samples in Examples 1 to 4 and Comparative Example 5, which have a porosity of 15% or more, each have a mechanical quality factor Qm of 40 or less, which is lower than the samples in Comparative Examples 1 to 4, which have a porosity of less than 15% (see Table 1). In summary, when Examples 1 to 4 and Comparative Example 5 with the relatively low mechanical quality factors Qm, especially Examples 1 to 4 with a porosity of 31% or less, are used, the vibrations of the piezoelectric elements can be slightly damped, resulting in higher transmission and reception sensitivity.

[0054] The frequency constants Nt of the piezoelectric elements in the thickness direction in Examples 1 to 4 and Comparative Examples 1 to 4 were measured. As a result, it was confirmed that the samples in Examples 1 to 4 with a porosity of 15% or more had a frequency constant Nt in a range of 1600 Hz·m or more and 2100 Hz·m or less, which was smaller than the frequency constant in Comparative Examples 1 to 4 with a porosity of less than 15% (see Fig. 16). Furthermore, it was also confirmed that the frequency constant Nt of ANN is generally larger than the frequency constant of PZT (see Fig. 16).

[0055] The frequency constants Np of the piezoelectric elements in the radial direction in Examples 1 to 4 and Comparative Examples 1 to 5 were measured. As a result, it was confirmed that the samples in Examples 1 to 4 with the porosities in the range of 15% or more and 31% or less had a frequency constant Np in a range of 2300 Hz·m or more and 2800 Hz·m or less, which was smaller than the frequency constant in Comparative Examples 1 to 4 with a porosity of less than 15% and in Comparative Example 5 with a porosity of more than 31% (see Table 1 and Fig. 17). Furthermore, it was also confirmed that the frequency constant Np of ANN is generally larger than the frequency constant of PZT (see Fig. 17).

[0056] The electromechanical coupling coefficients Kt of the piezoelectric elements in the thickness direction were measured at a temperature of 25°C using an impedance analyzer. As a result, it was confirmed that the samples in Examples 1 to 4 with a porosity of 15% or more had an electromechanical coupling coefficient Kt in a range of 41% or more and 44% or less, which is equivalent to the electromechanical coupling coefficient in Comparative Examples 1 to 4 with a porosity of less than 15% (see Table 1 and Fig. 18). Furthermore, it was also confirmed that the electromechanical coupling coefficient Kt of ANN is generally lower than the electromechanical coupling coefficient Kt of PZT (see Fig. 18).

[0057] The electromechanical coupling coefficients Kp of the piezoelectric elements in the radial direction were measured at a temperature of 25°C using an impedance analyzer. As a result, it was confirmed that the samples in Examples 1 to 4 with a porosity in the range of 15% or more and 31% or less had an electromechanical coupling coefficient Kp in a range of 27% or more and 36% or less, which was lower than the electromechanical coupling coefficient Kp in Comparative Examples 1 to 4 with a porosity of less than 15% and higher than the electromechanical coupling coefficient Kp in Comparative Example 5 with a porosity of more than 31% (see Table 1 and Fig.19). Furthermore, it was also confirmed that the electromechanical coupling coefficient Kp of KNN with a porosity of 20% or less is lower than the electromechanical coupling coefficient Kp of PZT, while the electromechanical coupling coefficient Kp of KNN with a porosity of more than 20% is higher than the electromechanical coupling coefficient Kp of PZT (see Fig. 19).

[0058] The piezoelectric strain constant d 33 The piezoelectric elements were tested at a temperature of 25°C with a d 33 / d 31 meters. As a result, it was confirmed that the samples in Examples 1 to 4 each had a piezoelectric strain constant d 33in a range of 250 pC / N or more and 260 pC / N or less, which is lower than the piezoelectric strain constant in Comparative Examples 1 and 2 and higher than the piezoelectric strain constant in Comparative Examples 3 and 4 (see Table 1 and Fig. 20). Furthermore, it was also confirmed that the piezoelectric strain constant d 33 of KNN is generally lower than the piezoelectric strain constant d 33 of PZT (see Fig. 20).

[0059] Subsequently, the images of Examples 1 to 4 and Comparative Examples 1 to 5 were analyzed using image analysis software. Specifically, SEM images of Examples 1 to 4 and Comparative Examples 1 to 5 were first read. The SEM images used here are the 200x magnified images of the section plane cut along the straight line L1 passing through the center C1 of the main surface 12 of the porous plate-shaped body 11 (the piezoelectric element 10).

[0060] Subsequently, twenty or more rectangular micro-measuring areas A1 (see Fig.21) at spaced-apart locations on the section plane in the direction of the straight line L1. Then, one of the twenty or more micro-measuring areas A1 was cut out of it. Furthermore, the cut-out micro-measuring area A1 was binarized so that the pores were displayed in black, while the other areas were displayed in white (see Fig. 22). Then, the number of pores with a size of 10 µm 2 or more were counted. The respective counted pores were assigned a pore number (see Fig. 23).

[0061] The process described above, in which cutting, binarization, pore counting, and pore number assignment were performed in this order, was performed for each micro-measurement area A1. For each micro-measurement area A1, the number of pores (Count), the minimum pore cross-sectional area (Min), the mean pore cross-sectional area (Mean), the maximum pore cross-sectional area (Max), the standard deviation σ (StdDev), and the variance CV were then obtained. In addition, a histogram showing the cross-sectional area distribution of the pores was created (see Fig. 24). As a result, it was confirmed that in the respective Examples 1 to 4, for example, the average pore cross-sectional area was in a range of 33 µm 2 or more and 65 µm 2 or less, the standard deviation σ is in a range of 25 µm 2 or more and 95 µm 2or less, and the variance CV in a range of 80% or more and 150% or less. The number of pores, the minimum pore cross-sectional area, the average pore cross-sectional area, the maximum pore cross-sectional area, the standard deviation σ, and the variance CV in each of Examples 1 to 4 and Comparative Examples 1 to 5 are shown in Table 2. Note that the smaller the average pore cross-sectional area, the more finely distributed the pores become. The smaller the standard deviation σ, the more evenly distributed the pores become. The smaller the variance CV, the smaller the proportion of the pore cross-sectional area that deviates far from the average pore cross-sectional area becomes.On the other hand, it was confirmed that in the case where the variance CV of the pore cross-sectional areas is as large as 305%, as in Comparative Example 5, the uneven distribution of pores causes uneven mechanical strength, resulting in lower manufacturing yield, lower reliability in ultrasonic driving, etc. [Table 2] Example 1 Example 2 Example 3 Example 4 Comparison example 1 Comparison example 2 Comparison example 3 Comparison example 4 Comparison example 5 Count 685 871 665 790 14 309 498 1274 258 Min (µm 2 ) 10,0 10,2 10,4 9,8 10, 7 10,0 10,0 10,0 10,0 Mean (µm 2 ) 33,6 43,0 45,5 64,5 29,5 35,5 35,4 62,4 220,5 Max (µm 2 ) 233,0 733,5 564,4 1448,8 116,8 219,2 459,5 1142,0 5654,8 StdDev (µm 2 ) 28,2 50,8 50,3 94,4 29,5 31,4 37,4 103,1 672,1 CV (%) 84% 118% 111% 146% 100% 89% 106% 165% 305%

[0062] Subsequently, the degree of pore distribution was analyzed. First, a pair of neighboring pores was selected from the numerous pores in the cut-out micro-measurement area A1. Next, a point was set for each of the selected, paired pores. These points are set at locations equidistant from the opening edge of the respective selected pores. An image of a line passing through the set points was created. Then, additional lines were drawn, each surrounding a pore. The region surrounded by one of these lines (contours) was called the Voronoi region A2. Here, the Voronoi regions A2 were set for all pores in the micro-measurement area A1. Subsequently, the Voronoi regions A2 were binarized such that the contours were displayed in black and the other areas in white (see Fig.25). Alternatively, the Voronoi region A2 was binarized so that the contours could be displayed in white and the other regions in black. Then, the number of Voronoi regions A2 was counted. Each of the counted Voronoi regions A2 was assigned a Voronoi number (see Fig. 25).

[0063] The process described above, in which the Voronoi regions A2 were set up, binarized, counted, and assigned Voronoi numbers in this order, was performed for each micro-measurement area A1. For each micro-measurement area A1, the number (Count) of Voronoi regions A2, the minimum area (Min) of the Voronoi region (or simply Voronoi area), the mean Voronoi area (Mean), the maximum Voronoi area (Max), the standard deviation σ (StdDev), and the variance CV were calculated. Furthermore, a histogram showing the area distribution of the Voronoi regions A2 was created (see Fig. 26). As a result, it was confirmed that in the respective examples 1 to 4, the mean Voronoi surface was in a range of 270 µm 2 or more and 560 µm 2 or less, the standard deviation σ is in a range of 170 µm 2 or more and 700 µm 2or less, and the variance CV in a range of 50% or more and 125% or less. In particular, it was confirmed that in the respective Examples 1 to 3, the mean Voronoi area was in a range of 270 µm 2 or more and 370 µm 2 or less, the standard deviation σ is in a range of 170 µm 2 or more and 200 µm 2or less, and the variance CV is in a range of 50% or more and 65% or less. The number of Voronoi regions A2, the minimum Voronoi area, the mean Voronoi area, the maximum Voronoi area, the standard deviation σ, and the variance CV in the respective Examples 1 to 4 and the respective Comparative Examples 1 to 5 are given in Table 3. It should be noted that the smaller the mean Voronoi area, the more finely distributed the pores become. The smaller the standard deviation σ, the more evenly distributed the pores become. The smaller the variance CV, the smaller the proportion of the Voronoi area that deviates far from the mean Voronoi area becomes.On the other hand, it was confirmed that in the case where the variance CV of the Voronoi areas is as large as 156%, as in Comparative Example 5, the uneven distribution of pores causes uneven mechanical strength, resulting in lower manufacturing yield, lower reliability in ultrasonic driving, etc. [Table 3] Example 1 Example 2 Example 3 Example 4 Comparison example 1 Comparison example 2 Comparison example 3 Comparison example 4 Comparison example 5 Count 602 784 579 1723 6 250 428 1151 205 Min (µm 2 ) 54 35 47 14 394 110 43 17 51 Mean (µm 2 ) 351 272 363 559 9469 807 493 839 856 Max (µm 2 ) 1104 1950 1280 13543 21094 2259 1798 5126 13289 StdDev (µm 2 ) 181 174 192 691 7849 388 265 645 1336 CV (%) 52% 64% 53% 124% 83% 48% 54% 77% 156%

[0064] Accordingly, according to the present embodiment, the following effect can be achieved.

[0065] (1) According to this embodiment, the piezoelectric element 10 made of KNN-based piezoceramic, which is free of environmentally harmful lead, is formed primarily as a more "porous" plate-shaped body with a porosity of 15% or more. Consequently, this piezoelectric element can have an equivalent or higher signal-to-noise ratio and an equivalent or higher transmission and reception sensitivity when transmitting and receiving ultrasound with an ultrasonic flowmeter compared to a piezoelectric element formed primarily as a non-porous plate-shaped body made of lead zirconate titanate (PZT). The piezoelectric element 10 has a porosity of 31% or less, which enables high transmission and reception sensitivity without compromising mechanical strength. In addition, the piezoelectric element 10 has a low average value of the specific acoustic impedances, i.e., an average specific acoustic impedance of 20 MN·s / m. 3 or less. Ultrasound propagates easily within the piezoelectric element 10, which can further increase the transmission and reception sensitivity and expand the frequency band when transmitting and receiving ultrasound with an ultrasonic flowmeter. Furthermore, the piezoelectric element 10 is made of KNN-based piezoceramic, which is softer than PZT, and is mainly formed as a porous plate-shaped body. Consequently, the piezoelectric element 10 has lower rigidity than a piezoelectric element formed mainly as a non-porous plate-shaped body made of PZT. As a result, the vibrations of the piezoelectric element 10 tend to converge more easily after the transmission of ultrasound, thereby reducing the reverberation of ultrasound during transmission.

[0066] (2) According to this embodiment, the mechanical quality factor Qm of the piezoelectric element 10 is as low as 40 or less. Therefore, the vibrations of the piezoelectric element 10 can converge quickly after the transmission of ultrasound. In other words, the reverberation is reduced, so that the frequency band of transmitting and receiving ultrasound with the ultrasonic flowmeter can be expanded. The vibrations of the piezoelectric element 10 after the transmission of ultrasound are converged quickly, resulting in the vibrations of the piezoelectric element 10 having already decayed upon receiving ultrasound. Consequently, the incident ultrasound can be received with accuracy. Furthermore, the time until the next ultrasound is transmitted can be shortened.

[0067] (3) The piezoelectric element 10 according to this embodiment is made of KNN-based piezoceramic, which is softer than PZT piezoceramic and, on top of that, porous. Therefore, the piezoelectric element in question is not only fragile but may also be unable to produce a homogeneous output of ultrasound when transmitting and receiving ultrasound with ultrasonic flowmeters. To overcome this problem, the micro-measuring areas A1 (see Fig. 23 and Fig.25) with uniformly arranged pores with equivalent pore cross-sectional areas and uniformly arranged Voronoi regions A2 with equivalent Voronoi areas are arranged such that the micro-measuring regions are arranged in the direction from the center to the edge of the porous plate-shaped body 11. Consequently, the piezoelectric element 10 is mechanically homogeneous, so that ultrasound can be transmitted at the same speed from the respective micro-measuring regions A1 and received in the same way in the respective micro-measuring regions A1. Therefore, the piezoelectric element 10 is acoustically homogeneous, so that ultrasound can be transmitted and received with high accuracy. Therefore, a piezoelectric element 10 with quality and performance suitable for use as an ultrasonic flowmeter can be obtained.

[0068] (4) The piezoelectric element 10 according to this embodiment has a frequency constant Np in the radial direction in a range of 2300 Hz·m or more and 2800 Hz·m or less, which is larger than the frequency constant Np (in a range of 1700 Hz·m or more and 2000 Hz·m or less) of a piezoelectric element using PZT in the case of the same porosity as the piezoelectric element 10 (see Fig.17). To achieve equivalent frequencies to those achieved with a PZT piezoelectric element when transmitting and receiving ultrasound with the above-mentioned radial vibrations, the outer diameter of the piezoelectric element 10 must be set larger than the outer diameter of the PZT piezoelectric element. This increases the area of ​​the main surface 12 of the piezoelectric element 10 (the porous plate-shaped body 11), resulting in higher transmission and reception sensitivity. The piezoelectric element 10 has a frequency constant Nt in the thickness direction in a range of 1600 Hz·m or more and 2100 Hz·m or less, which is larger than the frequency constant Nt (in a range of 1000 Hz·m to 1550 Hz·m) of a piezoelectric element using PZT in the case of the same porosity as the piezoelectric element 10 (see Fig.16). In order to achieve the equivalent frequencies of a PZT piezoelectric element when transmitting and receiving ultrasound with the above-mentioned vibrations in the thickness direction, the thickness of the piezoelectric element 10 must be set larger than that of the PZT piezoelectric element. However, this provides the following advantages: the strength of the piezoelectric element 10 can be ensured, the dimensional tolerance for machining accuracy in the thickness direction can be loosened, and the withstand voltage of the piezoelectric element can be increased.

[0069] (5) The piezoelectric elements 10 of Examples 1 to 4 according to this embodiment do not contain tantalum (Ta) as an additional metal element. It is known that a potassium sodium niobate-based piezoelectric element exhibits good piezoelectric properties when it contains Ta as an additional metal element. However, Ta is a more expensive material compared to other elements (Nb and Sb). Therefore, according to this embodiment, the piezoelectric element 10 with good piezoelectric properties can be manufactured even if the piezoelectric element does not contain Ta. Consequently, the manufacturing cost can be reduced.

[0070] The embodiment described above can be modified as follows.

[0071] According to the embodiment described above, the piezoelectric element 10 is made of KNN-based ceramic; however, the piezoelectric element 10 may be made of ceramics other than alkali niobate-based piezoceramics. For example, the piezoelectric element 10 may be a piezoelectric element containing a perovskite-structured crystal phase represented by the following composition formula (2). {Li x (K 1-y N / a y ) 1-x} a (Nb 1-z-w Ta z Sb w )O3 (2)

[0072] However, in the above composition formula (2), a piezoelectric element 10 with a composition range of 0.90 ≤ a ≤ 1.2, 0.02 ≤ x ≤ 0.2, 0.2 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.5, and 0 ≤ w ≤ 0.2 is used. By adding a metal element such as Bi or Fe as additives in the above composition formula (2), the piezoelectric element 10 can be manufactured.

[0073] The piezoelectric element 10 according to the above-described embodiment is used as an ultrasonic sensor in an ultrasonic flowmeter; however, the application of the piezoelectric element is not limited to this. The piezoelectric element 10 can be used, for example, in the following ultrasonic measuring devices: in bubble detection sensors for measuring the attenuation rate of ultrasound propagating in a liquid and determining the presence or absence of bubbles based on the attenuation rate of ultrasound; in an ultrasonic densitometer for detecting the concentration of a liquid based on the attenuation rate; in air sensors for measuring distances in the air; and in obstacle detection sensors for determining the presence or absence of obstacles based on the receptivity of ultrasound.In addition to the above-mentioned applications, the piezoelectric element 10 can be used in an ultrasonic measuring device such as an engine knock sensor. The piezoelectric element 10 according to this embodiment is disc-shaped, but the shape and size of the element can be changed depending on the application; for example, the shape can be rectangular.

[0074] In addition to the technical ideas described in the claims, the following are listed the other technical ideas resulting from the aforementioned embodiment. (1) A piezoelectric element for an ultrasonic measuring device according to any one of claims 1 to 8, characterized in that the frequency constant Nt in the thickness direction is in a range of 1600 Hz·m to 2100 Hz·m. (2) Piezoelectric element for an ultrasonic measuring device according to one of claims 1 to 8, characterized in that the frequency constant Np in the radial direction is in a range of 2300 Hz·m to 2800 Hz·m. (3) Piezoelectric element for an ultrasonic measuring device according to one of claims 1 to 8, having the feature that the piezoelectric strain constant d 33 in a range of 250 pC / N to 260 pC / N. (4) Piezoelectric element according to one of claims 1 to 8, having the feature that the alkali niobate-based piezoceramic is KNN-based piezoceramic. List of reference symbols 10 Piezoelectric element 11 porous plate-shaped body 12 Main surface of the porous plate-shaped body A1 Micro measuring range C1 Center of the main area Fm center frequency Δf frequency bandwidth Δf / fm normalized bandwidth L1 straight line P1 measuring point

Claims

[1] Piezoelectric element (10) for an ultrasonic measuring device, wherein the piezoelectric element (10) is mainly formed as a porous plate-shaped body (11) made of alkali niobate-based piezoceramic, wherein the piezoelectric element (10) has a porosity in a range of 15% or more and 31% or less and an average value of specific acoustic impedances of 20 MN·s / m 3 or less, characterized by , that the porosity is in a range of 23% or more and 31% or less, and the mean value of the specific acoustic impedances 15 MN·s / m 3 or less. [2] Piezoelectric element (10) for an ultrasonic measuring device, wherein the piezoelectric element (10) is mainly formed as a porous plate-shaped body (11) made of alkali niobate-based piezoceramic, wherein the piezoelectric element (10) has a porosity in a range of 15% or more and 31% or less and an average value of specific acoustic impedances of 20 MN·s / m 3 or less, characterized by , that twenty or more measuring points are set which are spaced apart on a straight line passing through the center of the main surface of the porous plate-shaped body, the mean value of sound velocities measured at the respective measuring points is 5200 m / s or less, and the value of the sound velocity at the respective measuring points is within ±5% of the mean value. [3] Piezoelectric element (10) for an ultrasonic measuring device, wherein the piezoelectric element (10) is mainly formed as a porous plate-shaped body (11) made of alkali niobate-based piezoceramic, wherein the piezoelectric element (10) has a porosity in a range of 15% or more and 31% or less and an average value of specific acoustic impedances of 20 MN·s / m 3 or less, characterized by , that twenty or more micro-measuring ranges are set, which are spaced apart from each other in the direction of the straight line on a section plane cut along a straight line passing through the center of the main surface of the porous plate-shaped body, the mean value of pore cross-sectional areas measured in the respective micro-measuring ranges in a range of 33 µm 2 or more and 65 µm 2 or less, the standard deviation σ is in the range of 25 µm 2 or more and 95 µm 2 or less, and the variance CV is in the range of 80% or more and 150% or less. [4] Piezoelectric element (10) for an ultrasonic measuring device, wherein the piezoelectric element (10) is mainly formed as a porous plate-shaped body (11) made of alkali niobate-based piezoceramic, wherein the piezoelectric element (10) has a porosity in a range of 15% or more and 31% or less and an average value of specific acoustic impedances of 20 MN·s / m 3 or less, characterized by , that twenty or more micro-measuring areas are set, which are spaced apart from one another in the direction of the straight line on a cutting plane cut along a straight line through the center of the main surface of the porous plate-shaped body (11), the mean value of Voronoi areas measured in the respective micro-measuring areas in a range of 270 µm 2 or more and 370 µm 2 or less, the standard deviation σ is in the range of 170 µm2 or more and 200 µm 2 or less, and the variance CV is in the range of 50% or more and 65% or less. [5] Piezoelectric element (10) according to one of claims 1 to 4, characterized by a density ρ of 3.7 g / cm 3 or less. [6] Piezoelectric element (10) according to one of claims 1 to 5, characterized by a mechanical quality factor Qm of 40 or less. [7] Piezoelectric element (10) for an ultrasonic measuring device, wherein the piezoelectric element (10) is mainly formed as a porous plate-shaped body (11) made of alkali niobate-based piezoceramic, wherein the piezoelectric element (10) has a porosity in a range of 15% or more and 31% or less and an average value of specific acoustic impedances of 20 MN·s / m 3 or less, characterized by , that when transmitting and receiving ultrasound with the ultrasonic measuring device, a normalized bandwidth Δf / fm, which is a ratio of a frequency bandwidth Δf to a center frequency fm, is 20% or more, wherein a frequency band is specified as a frequency range at a transmission and reception sensitivity obtained by subtracting 6 dB from a maximum transmission and reception sensitivity, a lower limit frequency is specified as the lower frequency of the frequency band, an upper limit frequency is specified as the upper frequency of the frequency band, the center frequency fm is specified as an intermediate value between the lower limit frequency and the upper limit frequency, and a frequency bandwidth Δf is specified as the difference between the lower limit frequency and the upper limit frequency.

Images