| Ultrasonic flowmeter are widely used for trade settlement of liquid flow or gas flow in power plants and some key energy use departments of government supervision. Its working principle is the use of characteristics of ultrasound through the object to measure fluid flow rate in tube by setting send and receive ultrasound device (transducer) outside the pipe. For different flow rates, different media, different conditions, suitable power output of ultrasonic transducer can greatly improve the accuracy of measurement. However, the lack of power detection devices for industrial use, making the acoustic parameters of ultrasonic transmitters and receivers measured are restricted. Currently, much attentions on adverse effects on human and especially the fetus because of the excessive ultrasonic power of transducer were given by the international community. Therefore, the size of the output power should be strict controlled. In order to gear to the international conventions, a complete system of ultrasonic power detection device or directly to use the highest level of accuracy of ultrasonic power reference unit was establish in the State for testing.With the application of ultrasound technology and the rapid development of a variety of ultrasound equipment,the sound power measurement indicators covering the expansion of the frequency. And in China, by the restrictions of various technical and legal, the frequency of the quartz transducer in ultrasonic power standards-based device is only up to 10MHz, span the range of 1mW ~ 20W, 5% measurement accuracy(k=2). It can not adapt and meet the development requirements of frequency and power measurement, and has a certain gap with the current international development. In the transformation process of milliwatts and watts ultrasonic national benchmarks power plant of 2007. Developed using the standard lithium niobate ultrasonic transducer, created the appropriate impedance matching and temperature measurement devices , and made the national benchmark device ultrasonic power measurement accuracy to 3%(k=2), this can reach the international advanced level.In recent years, the international comparison of ultrasonic power value, the standard transducer has reached the highest frequency of 15.8788MHz. To narrow the gap with the international,development of higher frequency standards transducer is particularly pressing.Based on this, the research of development and output power measurement of lithium niobate high-frequency ultrasonic transducer was done. This thesis mainly contain:1. By comparing the study found lithium niobate and quartz have the similar mechanical quality factor and stability, and the thickness extension vibration mode electromechanical coupling coefficient is much higher than the quartz, therefore, choosing lithium niobate as the standard high-frequency transducer piezoelectric material is the ideal choice, Through comparisonl study of lthium niobate single crystals cut, Found that the best way is to use 36°rotated y-cut way inclined cutting strict Czochralski method from the Z-axis lithium niobate piezoelectric crystal to be cut. The relationship between the parallel resonance frequency( fp) and the chip thickness which is solved By h-type piezoelectric equation is used to roughly calculate the needed thickness area of chip. Calculations show that when magnitude of a resonant frequency is 18MHz, the magnitude of chip thickness is as small as 0.2mm.2. Studying layout of the electrode and the electrode thickness and other factors.Required lithium niobate single crystal grow by Czochralski method, and X-ray diffraction shows that single crystal is strictly from the Z-axis. Gold electrode material whose thickness is 43nm measured by interference microscope is deposited by magnetron sputtering method to the chip surface. X-ray diffraction shows that the gold plated layer has been crystallized with (111) preferred orientation, and the other there is (200) and (220), and other planes.3. Finite element analysis software ANSYS is used to simulate the transducer model. By harmonic analysis Calculating the lithium niobate ultrasonic transducer frequency admittance component of G, B change curve with the frequency f.On this basis, Calculating the transducer equivalent circuit parameters, and the results provide guidance for the transducer design, at the same time refer to match circuit for the transducer designed.4. Six different specifications of the covering the frequency range of 0.5 ~ 18MHz are made of lithium niobate piezoelectric. Using the method of Smithadmittancechart, measurement data processed with the least square fitting of circular, series resonance frequency(fs), direct capacitance(C0) and Active resistance(R1) and other electrical parameters of transducers are obtained. On this basis, inductance and transformer parameters match with the circuit are defined. Through the study of more than 16 months, the radiation acoustic stability of the development of lithium niobate acoustic transducer is less than 2%, in line with the standard transducer requirements.5. The results measured on the national benchmark device show that the acoustic radiation value of the lithium niobate standard transducer are mS order of magnitude, and input-output linearity (R2) have reached more than 0.9999. Compared to the commonly used quartz standard transducer, he radiation value of a transducer is up to 33.4mS whose resonant frequency up to 18.5MHz. and simply enter the lower voltage can output higher ultrasound power in the range of high frequency. This can effectively solve the traceability of high-frequency high-pressure international problems,initially achieved the goals of using low voltage in the high frequency outputing considerable ultrasonic power values.In this thesis, a standard ultrasound transducer as the starting point, through the study of relevant work and measuring method of sound parameters, the foundation is laid for us to further explore measuring acoustic parameters of the industrial ultrasonic transducer. Results of this thesis won 2007 3rd“Flourishing AQSIQ by Science and Technology Prize”of AQSIQ in November 2007 and 3rd“2007 Guangdong science and technology award”in May 2008, and the author is the first winner. |
