Author name used in this publication: K. H. LamAuthor name used in this publication: D. M. LinAuthor name used in this publication: H. L. W. Chan2007-2008 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Lam, KH

Lin, D

Review of Scientific Instruments

Author name used in this publication: K. H. LamAuthor name used in this publication: D. M. LinAuthor name used in this publication: H. L. W. Chan2007-2008 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Lead-free acoustic emission sensorsK. H. Lam,a D. M. Lin, and H. L. W. ChanDepartment of Applied Physics, The Hong Kong Polytechnic University, Hunghom, Hong Kong, Chinaand Materials Research Centre, The Hong Kong Polytechnic University, Hunghom, Hong Kong, ChinaReceived 2 September 2007; accepted 28 October 2007; published online 28 November 2007Acoustic emission AE sensors have been fabricated using both soft- and hard-type lead-freeNa0.5K0.5NbO3-based ceramics. The acoustic and electromechanical properties of the ceramicshave been determined using the resonance technique. The lead-free AE sensors were calibratedusing a laser source and compared to a commercial sensor. A lead zirconate titanate PZT 5Hceramics AE sensor has also been fabricated and calibrated for comparison. It was found that thesensitivity of lead-free AE sensors is comparable to that of the lead-based PZT sensor. To evaluatethe sensors for potential application, they have been used in the detection of AE in an impact test.The lead-free sensors can reproduce AE signals accurately without giving artifacts and havepotential use in commercial AE systems. © 2007 American Institute of Physics.DOI: 10.1063/1.2814024I. INTRODUCTIONAcoustic emission AE is a transient elastic wave gen-erated by sudden deformation taking place in materials andstructures under stress. It is a powerful technology that canbe deployed within a wide range of usable applications ofnondestructive testing: e.g., for piping systems, reactors,civil structures, etc.1–3In general, AE sensors are used to detect the dynamicmotion of waves resulting from acoustic emission and thenconvert the detected motion into an electrical signal. Previ-ously, copolymers, shape memory alloys, and ceramic/polymer composites have been proposed as the sensingelements of AE sensors.4–6 Nevertheless, most AE measure-ments were studied with piezoelectric ceramic sensors. Itwas proven that the piezoelectric ceramics are appropriatematerials for AE testing because of their relatively low costand high sensitivity.The most widely used piezoelectric ceramics are lead-based ceramics, especially PbZr,TiO3 PZT, because oftheir superior piezoelectric properties. Recently, with con-cern in the environmental pollution of PbO evaporation, it isdesirable to use lead-free materials for environmental protec-tion. Lead-free piezoelectric ceramics have attracted consid-erable interest to replace the lead-based material systems.7–9Bi0.5Na0.5TiO3 BNT is considered to be one of the goodcandidates of lead-free piezoelectric ceramics because ofits large remnant polarization.10 However, due to their highcoercive field, pure BNT ceramics are difficult to pole andpossess lower piezoelectric properties compared with PZTceramics. In order to improve the properties of BNT ceram-ics, new Bi0.5Na1−x−yKxLiy0.5TiO3 lead-free piezoelectricceramics were proposed.11,12 The partial substitution ofNa+ by K+ and Li+ effectively decreases the coercive field ofthe ceramics, while the strong ferroelectricity is still main-tained with good piezoelectric properties. With differentdopants, K,NaNbO3 KNN-based ceramics can be modi-fied to form soft- and hard-type ceramics with reasonablygood performance.12,13 Since the KNN-based ceramics haveno low depolarization temperature but only the high Curietemperature, it would be useful for stable performance inapplications. In this study, both soft-type and hard-typeK,NaNbO3-based ceramics were fabricated and used assensing elements of the AE sensors.II. MATERIALSThere are two types of KNN-based ceramics usedas the sensing elements of the AE sensors:Na0.5K0.5NbO3–0.06BaTi0.95Zr0.05O3+1 mol % MnO2KNN-BZT and Na0.5K0.5NbO3+1 mol % CuOKNN-Cu.14 Both ceramics were prepared by the conven-tional mixed oxide technique using analytical-grade metaloxides or carbonate powders as raw materials. The powdersin the stoichiometric ratio of K0.5Na0.5NbO3 were first mixedand calcined at 880 °C for 6 h by solid-state reaction. Then,the dopants were weighed according to the molar ratios inthe above formula and then ball milled for 8 h. The resultingpowders were mixed thoroughly with a polyvinyl acetatebinder solution and then pressed into disks of 13 mm diam-eter and 1.3 mm thickness. After removal of the binder, theKNN-BZT and KNN-Cu samples were sintered at 1140–1160 and 1100–1120 °C for 4 h in air, respectively.Fired-on silver slurry was used as electrodes. The samplewas poled across the thickness under a dc electric field of6 kV /mm for 30 min in silicone oil at 200 °C. After poling,the samples were short-circuit annealed at room temperaturefor 8 h to remove the injected charges. For comparison, PZT5H ceramic disks with similar dimensions have also beenfabricated.The density  of the samples was measured using amethod based on the Archimedes principle. The piezoelectriccoefficient d33 of the ceramic disk was measured by a d33aFAX: 852-2333-7629. Electronic mail: kokokh.lam@polyu.edu.hk.REVIEW OF SCIENTIFIC INSTRUMENTS 78, 115109 20070034-6748/2007/7811/115109/4/$23.00 © 2007 American Institute of Physics78, 115109-1meter ZJ-3B supplied by the Beijing Institute of Acoustics,Academia Sinica. The electromechanical coupling coeffi-cients kt and kp, mechanical quality factor QM, frequencyconstants Np and Nt, and acoustic impedance Za of thesamples were determined at room temperature following theIEEE standard on piezoelectricity.15 The dielectric propertiesof the ceramics were measured using an impedance/gainphase analyzer Agilent 4294A. The properties of the ce-ramics are shown in Table I.As expected, the piezoelectric d coefficient of the PZTsample is higher than that of the lead-free ceramics. Never-theless, the density and relative permittivity of the lead-freeceramics are much lower than those of PZT ceramics andthey have reasonably good electromechanical coupling coef-ficients. With low relative permittivity, the lead-free sampleshave relatively high piezoelectric voltage coefficient g33which is a key parameter for being an appropriate sensingmaterial. Among the lead-free ceramics, the KNN-BZT ce-ramics is the soft type, which has relative high d33 and T33with low QM. On the other hand, the KNN-Cu sample showsa relatively high QM with low d33 and T33 which is a hard-type ceramics.III. AE SENSORSA schematic diagram of the AE sensor is shown in Fig.1. The ceramic disk was attached inside the aluminum hous-ing a diameter of 16 mm and a thickness of 6 mm using asilver epoxy. A coaxial cable was used as an electrical con-nection to bring the signal out. Figure 2 shows a photographof the AE sensor.A commercial broadband AE sensor supplied by Physi-cal Acoustics model no. S9220 was used to calibrate thehomemade sensors. The calibration yields the frequencyresponse of a sensor to waves generated by a laser impacton a metal surface. The voltage response of the sensor isdetermined at discrete frequency intervals of approximatelyup to 1.25 MHz by comparing the signal with that receivedby a commercial sensor with known sensitivity given by thecompany.Figure 3 shows the experimental setup of the calibrationof the AE sensors with a laser source neodymium dopedyttrium aluminum garnet laser, Spectra-Physics GCR 16.The lens in front of the metal block is used to focus the laseron the metal block surface. During the calibration, the AEsensor was placed 5 cm from the laser impact point on themetal block. To ensure a good acoustic coupling, a thin layerof wax was applied between the sensor and the metal block.Once the laser impinges on the metal block, the wave formDt generated from the sensor was amplified using a low-noise preamplifier Stanford Research Systems, SR560 andthen captured using a digital storage oscilloscope HP In-finium, as shown in Fig. 4. A preamplifier gain of 20 wasused throughout the experiments. For calibration, similar ex-periments have been done with the commercial and home-made AE sensors. The wave forms were then fast Fouriertransformed to determine the frequency spectra Df. Theresponse of the homemade sensors, Muf, with respect tothat of the standard commercial sensor Msf, is as follows:TABLE I. Properties of various sensing elements of the AE sensor.PZT 5H KNN-BZT KNN-CuDensity kg /m3 7600 4400 4500Poling field kV/mm 3 6 6Poling temperature °C 120 200 200Poling time min 30 30 30d33 10−12 C /N 550 200 75T33 at 1 kHz 3400 1100 250kt 0.50 0.47 0.46kp 0.62 0.48 0.42g33 10−3 V m /N 18.3 20.5 33.9QM 70 50 800Np Hz m 1980 3085 3718Nt Hz m 2200 2750 3400Za MRayl 35.1 34.2 33.8FIG. 1. Color online Schematic diagram of the AE sensor.FIG. 2. Color online Photograph of the AE sensor in aluminum housing.FIG. 3. Color online Experimental setup of the calibration of the AEsensor.115109-2 Lam, Lin, and Chan Rev. Sci. Instrum. 78, 115109 2007Muf =DufDsf Msf , 1where the subscripts u and s stand for the unknown andstandard sensors, respectively. Figure 5 shows the sensitivi-ties of the AE sensors fabricated using KNN-BZT, KNN-Cu,and PZT. It can be seen that the lead-free sensors have thesensitivity comparable to the PZT sensor. This may be due toa high voltage coefficient g of the lead-free ceramics. In thefrequency range of 150–400 kHz where most of the AEevents are likely to occur, the sensitivity of the sensors ap-proaches 55, even the sensing element consists of lead-freeceramics.IV. EVALUATION OF AE SENSORSBesides calibrated using the laser source, the AE sensorswere also evaluated for potential application in the detectionof impact signals produced by a hammer test. During themeasurements, the AE sensor under test was mounted at adesignated position on the steel block. The experimentalsetup is similar to that shown in Fig. 3, and the only dif-ference is the source of impact which was produced by ahammer, as shown in Fig. 6. Since the hammer test was alow-frequency measurement, a low-frequency commercialsensor model: PICO was chosen for comparison. Figure 7compares the detected signals in time domain from KNN-BZT and the PICO sensors inset. It is shown that the peakamplitude of the lead-free sensor is much higher than that ofthe commercial sensor with similar signal pattern. It may bedue to the relative large area and low electrical impedance ofthe homemade sensing elements. The low-frequency re-sponse of the KNN-BZT, KNN-Cu, and PZT AE sensors iscompared in Fig. 8. The lead-free sensors still have sensitiv-ity comparable to that of PZT sensor.V. CONCLUSIONSAcoustic emission sensors fabricated using lead-free ce-ramics have been fabricated and characterized. They showcomparable sensitivity with the lead-based sensor in both thebroadband laser calibration and low-frequency impact test.FIG. 4. Voltage vs time wave form of the commercial S9220 AE sensor.FIG. 5. Color online The sensitivity curves of the KNN-BZT, KNN-Cu,and PZT AE sensors as a function of frequency.FIG. 6. Color online Experimental setup of the evaluation of the AEsensor.FIG. 7. Voltage vs time wave form of the KNN-BZT AE sensor inset: awave form of the commercial PICO AE sensor.115109-3 Lead-free acoustic emission sensors Rev. Sci. Instrum. 78, 115109 2007Both the soft- and hard-type KNN-based lead-free ceramicswith high g coefficients have the potential to be used incommercial AE systems.ACKNOWLEDGMENTSThis work was supported by a grant from the ResearchGrants Council of the Hong Kong Special AdministrativeRegion Project No. PolyU 5317/04E and the Centre forSmart Materials of the Hong Kong Polytechnic University.Supports from the PolyU internal grants 1-BBZ3 and1-BB95 are also acknowledged.1 E. Haque, A. B. Darus, M. M. Yaacob, and F. Ahmed, Proceedings ofthe 1997 International Conference on Information, Communications andSignal Processing, 1997 unpublished, Vol. 2, p. 745.2 C. U. Grosse, H. W. Reinhardt, and F. Finck, Coll. Math. J. 15, 2742003.3 H. Chan, E. Han, J. Q. Wang, and W. Ke, J. Mater. Sci. 40, 5669 2005.4 R. Stiffler and E. G. Henneke, Mater. Eval. 41, 956 1983.5 R. Oishi, and H. Nagai, Sens. Actuators, A 22, 39 2005.6 W. K. Sakamoto, J. A. Malmonge, L. F. Malmonge, and A. F. G. da Silva,IEEE Trans. Dielectr. Electr. Insul. 13, 1177 2006.7 H. Nagata, M. Yoshida, Y. Makiuchi, and T. Takenaka, Jpn. J. Appl. Phys.,Part 1 42, 7401 2003.8 Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T.Nagaya, and M. Nakamura, Nature London 432, 84 2004.9 M. D. Maeder, D. Damjanovic, and N. Setter, J. Electroceram. 13, 3852004.10 T. Takenaka, K. I. Maruyama, and K. Sakata, Jpn. J. Appl. Phys., Part 130, 2236 1991.11 C. W. Ahn, H. C. Song, S. Nahm, S. H. Park, K. Uchino, S. Priya, H. G.Lee, and N. K. Kang, Jpn. J. Appl. Phys., Part 1 44, L1361 2005.12 S. J. Zhang, R. Xia, T. R. Shrout, G. Z. Zang, and J. F. Wang, J. Appl.Phys. 100, 104108 2006.13 M. Matsubara, K. Kikuta, and S. Hirano, J. Appl. Phys. 97, 1141052005.14 D. M. Lin, K. W. Kwok, and H. L. W. Chan, Appl. Phys. Lett. 90, 2329032007.15 IEEE standard on piezoelectricity, ANSI/IEEE Std. 176 1987.FIG. 8. Color online The sensitivity curves of the KNN-BZT, KNN-Cu,PZT AE, and the commercial PICO sensors as a function of frequency.115109-4 Lam, Lin, and Chan Rev. Sci. Instrum. 78, 115109 2007

Lead-free acoustic emission sensors

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