Name: Characterization of Full Set Material Constants and Their Temperature Dependence for Piezoelectric Materials Using Resonant Ultrasound Spectroscopy (Video) | JoVE Journal
Uploaded: 2016-04-27T13:00:00.000+00:00
Duration: 7 min 44 s
Description: Discover resonant ultrasound spectroscopy for measuring full tensor material constants and temperature dependence in piezoelectric materials using a single sample. This method improves simulation accuracy for electromechanical device performance.

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This work was supported by the National Natural Science Foundation of China (Grant No. 11374245), the NIH under Grant No. P41-EB2182, the Natural Science Foundation of Fujian Province, China (Grant No. 2013J01163), and the Open Research Fund of the State Key Laboratory of Acoustics, Chinese Academy of Science (Grant No. SKLA201306).

<ol><li>Jaffe, B., Cook, W. R., Jaffe, H. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aJaffe%2C+B+%22Piezoelectric+Ceramics%22">Piezoelectric Ceramics</a>. , Academic Press. (1971).</li>
<li>Chaussy, C., Thuroff, S., Rebillard, X., Gelet, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Technology+insight%3A+High-intensity+focused+ultrasound+for+urologic+cancers%5BTitle%5D)+AND+%22Nat.+Clin.+Pract.+Urol%22%5BJournal%5D)">Technology insight: High-intensity focused ultrasound for urologic cancers.</a> Nat. Clin. Pract. Urol. 2, 191-198 (2005).</li>
<li>Haar, G. T., Coussios, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High+intensity+focused+ultrasound%3A+physical+principles+and+devices%5BTitle%5D)+AND+%22Int.+J.+Hyperthermia%22%5BJournal%5D)">High intensity focused ultrasound: physical principles and devices.</a> Int. J. Hyperthermia. 23, 89-104 (2007).</li>
<li>Topolov, V. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Comment+on+%22Complete+sets+of+elastic%2C+dielectric%2C+and+piezoelectric+properties+of+flux-grown+%5B011%5D-poled+Pb%28Mg1%2F3Nb2%2F3%29O3-%2828-32%29%25+PbTiO3+single+crystals%22%5BTitle%5D)+AND+%22Appl.+Phys.+Lett%22%5BJournal%5D)">Comment on "Complete sets of elastic, dielectric, and piezoelectric properties of flux-grown [011]-poled Pb(Mg1/3Nb2/3)O3-(28-32)% PbTiO3 single crystals".</a> Appl. Phys. Lett. 96, 196101(2010).</li>
<li>Li, S. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Characterization+of+full+set+material+constants+of+piezoelectric+materials+based+on+ultrasonic+method+and+inverse+impedance+spectroscopy+using+only+one+sample%5BTitle%5D)+AND+%22J.+Appl.+Phys%22%5BJournal%5D)">Characterization of full set material constants of piezoelectric materials based on ultrasonic method and inverse impedance spectroscopy using only one sample.</a> J. Appl. Phys. 114, 104505(2013).</li>
<li>Ohno, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Rectangular+parallellepiped+resonance+method+for+piezoelectric+crystals+and+elastic+constants+of+alpha-quartz%5BTitle%5D)+AND+%22Phys.+Chem.+Miner%22%5BJournal%5D)">Rectangular parallellepiped resonance method for piezoelectric crystals and elastic constants of alpha-quartz.</a> Phys. Chem. Miner. 17, 371-378 (1990).</li>
<li>Ogi, H., Kawasaki, Y., Hirao, M., Ledbetter, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Acoustic+spectroscopy+of+lithium+niobate%3A+Elastic+and+piezoelectric+coefficients%5BTitle%5D)+AND+%22J.+Appl.+Phys%22%5BJournal%5D)">Acoustic spectroscopy of lithium niobate: Elastic and piezoelectric coefficients.</a> J. Appl. Phys. 92, 2451(2002).</li>
<li>Pujol, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+solution+of+nonlinear+inverse+problems+and+the+Levenberg-Manquardt+method%5BTitle%5D)+AND+%22Geophysics%22%5BJournal%5D)">The solution of nonlinear inverse problems and the Levenberg-Manquardt method.</a> Geophysics. 72, 1-16 (2007).</li>
<li>Moré, J. J., Garbow, B. S., Hillstrom, K. E. User Guide for MINPACK-1. Argonne National Laboratories Report ANL-80-74. , (1980).</li>
<li>Tang, L. G., Cao, W. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Temperature+dependence+of+self-consistent+full+matrix+material+constants+of+lead+zirconate+titanate+ceramics%5BTitle%5D)+AND+%22Appl.+Phys.+Lett%22%5BJournal%5D)">Temperature dependence of self-consistent full matrix material constants of lead zirconate titanate ceramics.</a> Appl. Phys. Lett. 106, 052902(2015).</li>
<li>Topolov, V. Y., Bowen, C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Inconsistencies+of+the+complete+sets+of+electromechanical+constants+of+relaxor-ferroelectric+single+crystals%5BTitle%5D)+AND+%22J.+Appl.+Phys%22%5BJournal%5D)">Inconsistencies of the complete sets of electromechanical constants of relaxor-ferroelectric single crystals.</a> J. Appl. Phys. 109, 094107(2011).</li>
<li>Berlincourt, D., Krueger, H. H. A. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aBerlincourt%2C+D+%22Properties+of+Morgan+Electroceramic+ceramics.+Technique+publication+TP-226%22">Properties of Morgan Electroceramic ceramics. Technique publication TP-226</a>. , Morgan Electroceramics. (2000).</li>
<li>Migliori, A., Sarrao, J. L. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aMigliori%2C+A+%22Resonant+ultrasound+spectroscopy%22">Resonant ultrasound spectroscopy</a>. , Wiley Press. (1997).</li>
<li>Zadler, B. J., Le Rousseau, J. H. L., Scales, J. A., Smith, M. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Resonant+ultrasound+spectroscopy%3A+Theory+and+application%5BTitle%5D)+AND+%22Geophys.+J.+Int%22%5BJournal%5D)">Resonant ultrasound spectroscopy: Theory and application.</a> Geophys. J. Int. 156, 154-169 (2004).</li>
</ol>

Authors have nothing to disclose.

The RUS technique described here can measure the full set material constants using only one sample, which eliminates errors caused by property variation from sample to sample so that self-consistency can be guaranteed. The method can be used for any solid material with a high quality factor Q, no matter if they are piezoelectric or not. All other standard characterization techniques require several samples to get the full set data and are difficult to achieve self-consistent data.
It is import...

Lead zirconate titanate (PZT) piezoelectric ceramics, (1-x)PbZrO3-xPbTiO3, and its derivatives have been widely used in ultrasonic transducers, sensors and actuators since the 1950s1. Many of these electromechanical devices are used at high temperature ranges, such as for space vehicles and underground well logging. Moreover, high power devices, such as therapeutic ultrasonic transducers, piezoelectric transformers and sonar projectors, often heat-up during operation. Such temperature rises will change the resonance frequencies and the focal point of transducers, causing severe performance degradation. High intensity focused ultrasound (HIFU) technology, already used in clinical practice for the treatment of tumors, uses ultrasonic transducers made of PZT ceramics. During operation, the temperature of these transducers will increase, causing a change of the material constants of the PZT resonator, which in turn will change the HIFU focal point as well as the output power2,3. The shift of focal point may lead to serious unwanted results, i.e., healthy tissues being destroyed instead of cancer tissues. On the other hand, if the focal point shift can be predicted, one could use electronic designs to correct such shift. Therefore, measuring the temperature dependence of the full set material properties of piezoelectric materials is very important for the design and evaluation of many electromechanical devices, particularly high power devices.
Poled ferroelectric materials are the best piezoelectric materials known today. In fact, nearly all piezoelectric materials currently in use are ferroelectric materials, including solid solution PZT ceramics and (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystals. The IEEE (Institute of Electrical and Electronics Engineers) impedance resonance method requires 5-7 samples with drastically different geometries in order to characterize the full set material constants4. It is nearly impossible to obtain self-consistent full set matrix data using the IEEE impedance resonance method for ferroelectric materials because the degree of poling depends on the sample geometry (boundary conditions), while sample properties depend on the level of poling. To avoid problems caused by sample to sample variations, all constants should be measured from one sample. Li et al. reported the successful measurement of all constants from one sample at room temperature by using a combination of pulse-echo ultrasound and inverse impedance spectroscopy5. Unfortunately, this technique is hard to perform at elevated temperatures because it is not possible to perform ultrasonic measurements directly inside the furnace. There are also no commercially available shear transducers that can work at high temperatures. In addition, the coupling grease that bound the transducer and the sample cannot work at high temperatures.
In principle, the RUS technique has the capability to determine the full set material constants of piezoelectric materials and their temperature dependence using only one sample6,7. But there are several critical steps for proper implementation of the RUS technique. First, the full set of tensor properties at room temperature should be accurately determined using a combination of pulse-echo and RUS techniques. Second, this room temperature data set can be used to predict the resonance frequencies and to match the measured ones in order to identify the corresponding modes. Third, for each small increment of temperature from room temperature up, one needs to perform spectrum reconstruction against the measured resonance spectrum in order to retrieve the full set constants at this new temperature from the measured resonance spectrum. Then, using the new data set as the new starting point, we can increase the temperature by another small temperature step to get the full set constants at the next temperature. Continuing this process will allow us to obtain the temperature dependence of the full set material constants.
Here, a PZT-4 piezoceramic sample is used to illustrate the measurement procedure of the RUS technique. The poled PZT-4 ceramic has ∞m symmetry with 10 independent material constants: 5 elastic constants, 3 piezoelectric constants and 2 dielectric constants. Because the dielectric constants are insensitive to the change of resonance frequencies, they were measured separately using the same sample. The temperature dependence of clamped dielectric constants <img alt="Static equilibrium equation ΣF=0, ε11 symbol, mathematical analysis diagram for research studies." src="/files/ftp_upload/53461/image003.jpg" data-altupdatedat="1787630408000" data-alt="Equation 3"> and <img alt="Static equilibrium; strain symbol ε₃₃; physics diagram; stress-strain analysis." src="/files/ftp_upload/53461/image004.jpg" data-altupdatedat="1787630408000" data-alt="Equation 4"> were measured directly from the capacitance measurements, while the free dielectric constants  <img alt="Static equilibrium equation ε₁₁ raised to the T power; educational formula study." src="/files/ftp_upload/53461/image005.jpg" data-altupdatedat="1787630408000" data-alt="Equation 5"> and <img alt="Static equilibrium equation ΣFx=0, diagram of force vectors in balance analysis." src="/files/ftp_upload/53461/image002.jpg" data-altupdatedat="1787630408000" data-alt="Equation 2"> measured at the same time were used as data consistency checks. The temperature dependence of elastic stiffness constants at a constant electric field <img alt="Compressibility factor equation C11^E; thermodynamics chart; phase behavior study." src="/files/ftp_upload/53461/image006.jpg" data-altupdatedat="1787630408000" data-alt="Equation 6">, <img alt="Chemical equilibrium equation, symbol C&lt;sub&gt;E&lt;/sub&gt;&lt;sup&gt;12&lt;/sup&gt;, diagram, educational content." src="/files/ftp_upload/53461/image007.jpg" data-altupdatedat="1787630408000" data-alt="Equation 7">, <img alt="Static equilibrium; ΣFx=0; equilibrium diagram; force balance analysis; mechanics study." src="/files/ftp_upload/53461/image008.jpg" data-altupdatedat="1787630408000" data-alt="Equation 8">, <img alt="Elastic modulus equation \( C_{33}^{E} \), symbol used in material science diagram." src="/files/ftp_upload/53461/image009.jpg" data-altupdatedat="1787630408000" data-alt="Equation 9"> and <img alt="Elastic modulus equation \( C_{44}^E \); formula for material property analysis." src="/files/ftp_upload/53461/image010.jpg" data-altupdatedat="1787630408000" data-alt="Equation 10">, and piezoelectric stress constants e15, e31 and e33 were determined by the RUS technique using the same sample.

<table><tbody><tr><td>PZT-4</td><td>TRS</td><td></td><td></td></tr><tr><td>paraffin</td><td>MTI Corporation</td><td>8002-74-2</td><td></td></tr><tr><td>conductive silver paint</td><td>MG Chemicals</td><td>842-20G</td><td></td></tr><tr><td>Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Powder</td><td>MTI Corporation</td><td></td><td></td></tr><tr><td>coupling grease</td><td>Panametrics</td><td></td><td></td></tr></tbody></table>

characterization of full set material constants and their temperature dependence for piezoelectric materials using resonant ultrasound spectroscopy

Characterization of Full Set Material Constants and Their Temperature Dependence for Piezoelectric Materials Using Resonant Ultrasound Spectroscopy

During the operation of high power electromechanical devices, a temperature rise is unavoidable due to mechanical and electrical losses, causing the degradation of device performance. In order to evaluate such degradations using computer simulations, full matrix material properties at elevated temperatures are needed as inputs. It is extremely difficult to measure such data for ferroelectric materials due to their strong anisotropic nature and property variation among samples of different geometries. Because the degree of depolarization is boundary condition dependent, data obtained by the IEEE (Institute of Electrical and Electronics Engineers) impedance resonance technique, which requires several samples with drastically different geometries, usually lack self-consistency. The resonant ultrasound spectroscopy (RUS) technique allows the full set material constants to be measured using only one sample, which can eliminate errors caused by sample to sample variation. A detailed RUS procedure is demonstrated here using a lead zirconate titanate (PZT-4) piezoceramic sample. In the example, the complete set of material constants was measured from room temperature to 120 °C. Measured free dielectric constants <img alt="Static equilibrium equation diagram, ΣFx=0; illustrates balancing forces in physics study." src="/files/ftp_upload/53461/image001.jpg" data-altupdatedat="1787630408000" data-alt="Equation 1"> and <img alt="Static equilibrium equation ΣFx=0, diagram of force vectors in balance analysis." src="/files/ftp_upload/53461/image002.jpg" data-altupdatedat="1787630408000" data-alt="Equation 2"> were compared with calculated ones based on the measured full set data, and piezoelectric constants d15 and d33 were also calculated using different formulas. Excellent agreement was found in the entire range of temperatures, which confirmed the self-consistency of the data set obtained by the RUS.

The LM algorism used in the inversion is a local minimum finder. Therefore, the initial values of elastic stiffness constants <img alt="Compressibility factor equation C11^E; thermodynamics chart; phase behavior study." src="/files/ftp_upload/53461/image006.jpg" data-altupdatedat="1787630408000" data-alt="Equation 6">, <img alt="Chemical equilibrium equation, symbol C&lt;sub&gt;E&lt;/sub&gt;&lt;sup&gt;12&lt;/sup&gt;, diagram, educational content." src="/files/ftp_upload/53461/image007.jpg...

This protocol describes the procedure of measuring the temperature dependence of the full set material constants of piezoelectric materials using resonant ultrasound spectroscopy (RUS).

During the operation of high power electromechanical devices, a temperature rise is unavoidable due to mechanical and electrical losses, causing the ...

The overall goal of this resonance ultrasonic spectroscopy method, is to measure a complete set of material constants and their temperature dependence for a piezoelectric material using only one sample. The Impedance Method defined in the piezoelectric standards of the institute of electrical and electronic engineers, require 5 to 7 samples of different geometries in order to measure the complete set of material constant for piezoelectric material. The main advantage of the resonance ultrasonic spectroscopy technique is that the full tensor properties can be obtained from one sample, avoiding inconsistency caused by sample to sample variations.Data acquired from this method, enable people to simulate the performance of electromechanical devices and to quantify the performance degradation at higher temperatures using finer element method. First, glue a rectangular parallel pipe to PZT-4 ceramic sample to the bottom surface of a metal rod using a very thin layer of wax by heating the rod and sample to about 60 degrees Celsius. After cooling to room temperature, tightly fit the rod into a metal cylinder with a larger outer diameter so that the bottom surface of the cylinder and sample can be polished together to guarantee the flatness of the sample surface Wet a plexiglass plate with tap water, and sprinkle 6 micron aluminum oxide powder onto the wet surface.Place the sample holder with the sample glued to it onto the plexiglass plate, and make a circular motion to grind the sample service flat. Then wash the plexiglass plate, and sample holder, thoroughly with tap water. Following this, sprinkle 3 Micron aluminum oxide powder onto the wet plexiglass plate and repeat the grinding to smoothen the sample surface.Wash the glass plate and sample holder clean with tap water. Lift the sample off of the holder by heating the assembly to about 60 degrees Celsius to melt the wax. When finished, remove the remaining wax from the sample surface with acetone.Connect a 15 megahertz longitudinal wave transducer and a digital oscilloscope to a pulcer reciever. Next, place the transducer onto the sample surface along the X direction with some coupling grease in between. Press the curser key on the control panel of the digital oscilloscope.Then, press the side menu button V bars, and rotate the general purpose knob to move one cursor line to the highest peak of the first echo signal. At this point, press the Select key, and rotate the general purpose knob to move the other cursor line to the corresponding peak in the second echo signal. Read the numerical value at the place marked with an up triangle on the screen, which is the roundtrip time of flight of the longitudinal wave pulse along the X-axis.Connect an impedance analyzer to a control computer, and turn both on. Then, insert the sample in the fixture connected to the analyzer and place the whole assembly into a temperature chamber. After closing the temperature chamber, press the key Meas on the impedance analyzer panel, and select CP-D.Next, set the chamber to 20 degrees Celsius, using the control computer. Open the spreadsheet software and read the capacitance data. Then, save the results into a file.Following this, change the chamber temperature by pressing the up key on the impedance analyzer panel. Repeat the previous step for each temperature increment, after the chamber temperature becomes stable. At this point, place the sample in between the transmitting and receiving transducers of the resonance ultrasound spectroscopy system, with contacts only at the opposite corners of the sample.Run the control interface of the dynamic resonance system by double clicking on the software file DRS.exe. Set the start frequency, the stop frequency, and the total number of data points to be collected. Measure the resonance spectrum of the sample in this frequency range at room temperature, and save the spectrum into a file.Place the sample inbetween the transmitting and receiving transducers that are already in the furnace, with contacts only at the opposite corners of the sample. Following this, run the resonance ultrasound spectroscopy system measuring software and measure the resonance frequencies of the sample. Then, save the results into a file.Increase the temperature of the sample with a temperature step of 5 degrees Celsius. Repeat the previous step, until the desired temperature is reached. For the PZT-4 ceramic sample the elastic constants C11E, C33E, and C44E, increase with the temperature.While the elastic constant C12E and C13E are nearly independent of temperature in the range of 20 to 120 degrees Celsius. On the other hand, the piezoelectric constants E33, E31, and E15 are strongly temperature dependent. The measured dialectric constants and the predicted ones calculated based on the full set of material constants obtained by this method show excellent agreement.The piezoelectric constants D15 and D33 calculated using one set of formulas and the values calculated using another set of formulas also show good agreement. These results confirm that the full set material constants obtained for the PZT-4 ceramic sample, are highly self-consistent for the temperature range from 20 to 120 degrees Celsius. This RUS technique allows us to measure the full tensor properties at elevated temperatures with self-consistency, which paved the way for researchers in the field of device simulation to explore the possibility of predicting the real performance of electromechanical devices particularly to predict the performance degradation with heater generation during operation.After watching this video, you should have a good understanding of how to perform resonance ultrasonic spectroscopy measurements at elevated temperatures. The key is to acquire a reliable set of constants at room temperature and then deroute the full tensor property at the high temperatures based on the room temperature data.

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Discover resonant ultrasound spectroscopy for measuring full tensor material constants and temperature dependence in piezoelectric materials using a single sample. This method improves simulation accuracy for electromechanical device performance.

Characterization of Full Set Material Constants and Their Temperature Dependence for Piezoelectric Materials Using Resonant Ultrasound Spectroscopy (Video) | JoVE Journal

Characterization of Full Set Material Constants and Their Temperature Dependence for Piezoelectric Materials Using Resonant Ultrasound Spectroscopy

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