www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICAT

Xi Chen , Jinwei Li , Guitao Zhang , and Yong Shi *

PZT Nanoactive Fiber Composites for Acoustic Emission Detection

ION

Active nanowire and nanofi ber based sensors and devices, [ 1 , 2 ]

such as bio-sensors, [ 3 , 4 ] optical sensors, [ 5 , 6 ] mass sensors, [ 7 , 8 ] and chemical sensors, [ 9 ] are among the leading applications of nanotechnology. Recently, the application of carbon fi ber com-posite structure has grown rapidly in the aerospace industry due to its high mechanical strength, high stiffness and min-imal weight. [ 10 ] Being able to analyze and identify damages within composite structures using embedded sensors is highly desired. [ 11 ] Currently, strain gauges, [ 12 , 13 ] fi ber optic sensors, [ 14 , 15 ] microelectromechanical systems (MEMS) accelero meters [ 16 ] and piezoelectric acoustic emission (AE) sensors [ 17 ] are commonly used for structural health monitoring (SHM) applications. Piezoelectric sensors can achieve active sensing (excitation and sensing) [ 18 ] and the three other kinds of sensors are based on passive sensing. However, most of them are unsuitable for integrating into composites due to the small scale of the carbon fi bers and the sensing technology. The active fi ber composites (AFCs), developed fi rst by Bent and Hagood, [ 19 , 20 ] are one of the great candidates for this application. A similar lead zirconate titanate (PZT) fi ber composite, macro fi ber composites (MFCs) developed by NASA, [ 21 ] has also been used for such purposes. The MFCs with micrometer-scale PZT fi bers embedded in a polymer matrix have demonstrated a number of advantages compared to other AE sensors. [ 17 ] Compared with the carbon fi bers involved, the larger diameter and high stiffness of PZT fi ber make it diffi cult to embed into composite structures. A potential solution to this issue is to replace the microsize PZT fi ber with nanoscale fi ber in MFCs.

Piezoelectric AE sensors are designed using a direct piezoe-lectric effect. The sensitivity of these kinds of sensors is mainly determined by the piezoelectric voltage constant ( g 33 ) of the piezoelectric materials. The polyvinylidene fl uoride (PVDF) thin-fi lm-based AE sensors have much less sensitivity than PZT sensors and cannot be used at a high temperature. [ 22 ] PZT nanofi bers prepared by an electrospinning process exhibit an extremely high piezoelectric constant, high bending fl exibility, and high mechanical strength, which have been demonstrated in our previous work. [ 23 ] The g 33 of PZT nanofi ber is roughly 0.079 V m N − 1 , [ 23 ] which is much higher than that of the PZT bulk (0.025 V m N − 1 ) or PZT microfi ber (0.059 V m N − 1 ). [ 24 ] For a given energy input of the elastic waves, PZT nanofi ber

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 3965–3969

X. Chen , J. Li , G. Zhang , Prof. Y. Shi Department of Mechanical Engineering Stevens Institute of Technology Castle Point on Hudson Hoboken, NJ, 07030 USA E-mail: yong.shi@stevens.edu

DOI: 10.1002/adma.201101592

can produce much higher voltage than the PZT microfi ber, which results in a higher sensitivity of the sensor. Unlike the bulk or microsize PZT ceramic materials, the high fl exibility allows PZT nanofi ber to bend freely in a transverse direction. Thus, the PZT nanofi ber-based AE sensor can be easily bonded to curved structures. When implemented in a practical applica-tion, the PZT nanofi ber has a smaller chance of being damaged due to its small size, fl exibility, and high mechanical strength, which can extend the lifetime of nanofi ber-based devices. The latest developments of carbon nanotube (CNT) composite based strain sensors [ 25 , 26 ] provide a way to achieve the SHM of composite structures. However, the passive sensing technique requires a current passing through the sensor, which generates heat and costs more energy. The PZT nanofi ber based nanogen-erator that can covert mechanical energy into electrical energy has been successfully demonstrated in ref. [27] and illustrates the possibility of using the PZT nanofi ber-based AE sensor to achieve the self-powered active sensing for SHM applications.

The fi rst AE sensor based on PZT nanoactive fi ber compos-ites (NAFCs) was demonstrated using interdigitated electrodes where the NAFCs operate in a longitudinal mode during detec-tion. The design of the PZT NAFCs is presented in Figure 1 a. PZT nanofi bers were electrospun [ 28 ] onto a silicon substrate with thermal oxide under an electric fi eld of 0.5 kV cm − 1 . Fol-lowing with an annealing process at 650 ° C for about 30 min, PZT nanofi bers with a diameter of approximately 80 nm had a large concentration of perovskite phase as shown in the X-ray diffraction (XRD) pattern (inset in Figure 1 b). Gold (Au)/chro-mium (Cr) interdigitated electrodes were thermally evaporated and patterned on top of the PZT nanofi bers network as shown in Figure 1 b. The width of the deposited electrode was 3 μ m and the gap between the two adjacent electrodes varied from 3 μ m to 20 μ m (see Figure 1 c,d). The PZT nanofi bers were entirely covered by the Au/Cr interdigitated electrodes to achieve a good connection. The size of a single PZT NAFCs sensor cell was fabricated from millimeter scale to microscale as shown in Figure 1 b,d. A thin polymer (polydimethylsiloxane, PDMS) layer was then applied as the matrix material of NAFCs. Finally, the PZT nanofi bers were polled by applying an electric fi eld of 4 V μ m − 1 across the interdigitated electrodes at a temperature of 140 ° C for about 24 h. The fi nished PZT NAFCs sensor cells are as shown in Figure 1 e. The silicon substrate was used as a rigid mechanical backing to avoid excessive stresses on PZT nanofi bers and minimized the risk of damaging the electrical connection. The PZT NAFCs can be released from the silicon substrate by dry etching for different applications.

The AE detecting mechanism of PZT NAFCs is illustrated in Figure 2 . The acoustic waves transferred through the struc-tures, which induced elastic stress waves along the aligned nanofi bers. The charge was separated in those nanofi bers by

bH & Co. KGaA, Weinheim 3965wileyonlinelibrary.com

396

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

Figure 1 . Concept and fabrication of PZT NAFCs sensor cell. a) Schematic view of the PZT NAFCs. b) A scanning electron microscopy (SEM) image of the millimeter size PZT NAFCs sensor cell before coating the PDMS layer. The inset in (b) shows the XRD pattern for the PZT nanofi bers. c) A SEM image of PZT nanofi bers covered by Au/Cr interdigitated electrodes. The inset in (c) shows the diameter of PZT nanofi ber is approximately 80 nm. d) A SEM image of a micrometer-size PZT NAFCs sensor cell. e) An optical image of PZT NAFCs sensor cell array after coating with a thin PDMS layer on the top surface.

the longitudinal stress waves, which resulted in a piezoelectric potential gradient along the nanofi ber. [ 29 ] The generated pie-zoelectric potential ≈ V out was picked up through the adjacent electrodes and transported to an external detection circuit. The voltage generated between two ajacent electrodes of the PZT NAFCs can be caculated by

V 1

out =∫

g33g33(l )Epdl

(1)

where ε 33 ( l ) is the longitudinal strain along PZT nanofi bers, l is the length of the nanofi bers across the two adjacent electrodes, and E p is the Young’s modulus of PZT nanofi bers. Assuming a perfect bonding between PZT nanofi bers and PDMS polymer matrix, we have

6 © 2011 WILEY-VCH Verlag Gwileyonlinelibrary.com

Figure 2 . The AE detection mechanism of PZT NAFCs. The fi gure shows the side view of the PZT NAFCs sensor mounted on a solid material to be monitored. The acoustic wave with a wavelength of λ illustrates the typical wave that PZT NAFCs can generate for the active sensing application.

gp = gm = gc (2)

where ε p is the strain in PZT nanofi bers and ε m and ε c are the strain in the PDMS matrix and composite. Assuming both the PZT nanofi bers and PDMS matrix are elastic, the PZT NAFCs can be considered as unidirectional continuous fi ber lamina. Therefore, we have

E33 = Epvp + Em(1 − vp) (3)

vp = Ap /Ac (4)

where E 33 is the longitudinal modulus, E m is the Young’s mod-ulus of PDMS polymer, A p is net cross-sectional area for the fi bers, and A c is the cross-sectional area for the PZT NAFC. The major Poisson’s ratio could be calculated as

L31 = L32 = Lpvp + Lm(1 − vp)

(5)

where Lp and Lm are the Poisson’s ratio of PZT nanofi bers and PDMS polymer, respectively. The longitudinal strain in the PZT nanofi bers can be given as

g33(l ) = F33(l ) E33 − F11(l )L31 /E33 − F22(l )L32 /E33/ (6)

where σ 11 ( l ), σ 22 ( l ), and σ 33 ( l ) are the stress functions in three directions. Thus, the output voltage can be rewritten as

V 1out =

∫[F33(l )/E33 − F11(l )L31 /E33

− F22(l L32 /E33]Epg33dl)

(7)

mbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 3965–3969

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Figure 3 . The demonstration of PZT NAFCs used for acoustic emission detection. a) The voltage generation when used for surface elastic wave detec-tion. b) The voltage generation when the PZT NAFCs was embedded in an epoxy structure.

According to Equation (7) , the output voltage from the PZT NAFCs sensor is a function of the strength of the elastic wave. Since the PZT nanofi bers were polled in opposite directions between each adjacent electrode (see Figure 2 ), the output power was enhanced by the parallel connection of those units using an interdigitated pattern of electrodes. For the application of active sensing, the PZT NAFCs can generate elastic waves with the wavelength of λ , which is equal to twice the distance [ 30 ] between the electrodes, as shown in Figure 2 .

A single AE sensor cell, with the width of 400 μ m and length of 800 μ m, was selected to perform the measurement. The experiment was fi rst carried out for the surface elastic wave detection. The PZT NAFCs was reversibly mounted on a grounded steel surface as shown in the upper inset in Figure 3 a. To eliminate the infl uence of the electromagnetic interference, the sensor structure was fully covered using a faraday cage. A grounded steel bar was used to generate the acoustic wave along the parallel direction of the nanofi bers by periodic knocking on the steel surface. As shown in Figure 3 a, the charge separation was recorded when the elastic wave reached the NAFCs. The sensed signal was induced by a piezopotential driven transient fl ow of electrons under the stress of the substrate generated by the waves. [ 31 ] The higher the impact energy applied on the steel surface, the higher the voltage generated from the PZT NAFCs. The measured peaks were around 0.08 V during the test and the signal vanished fast due to the low energy. The PZT NAFCs were then embedded in a square-shaped epoxy structure and fi ne copper wires were utilized as the extraction electrodes to transport electrons to an external circuit, as seen in the upper inset in Figure 3 b. As shown in Figure 3 b, the positive and neg-ative voltage outputs were observed when a grounded steel bar was knocking on the epoxy structure and the voltages reached 0.2 V during the test. The negative voltage distribution was generated by the reversely fl owing carriers when the imbedded

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 3965–3969

NAFCs were subjected to the restoring force of the structure oscillation after the external load was removed. Therefore, the PZT NAFCs AE sensor shows a promising application in heath monitoring by being integrated into structures.

In order to further study the sensing properties of the NAFCs sensor, the signal attenuation curves were measured by changing the location of the AE source on an isotropic PMMA test plate. The signal attenuation test was fi rst performed as a fucntion of distance ( d ) along a direction parallel to the PZT NAFCs, as seen in the inset in Figure 4 a. The PZT NAFCs were permanently bonded on the top surface of the PMMA test plate using epoxy. The AE signal was generated by utilizing the impact energy of a metal ball dropped on the test plate from a certain height. By varying the distance of AE source from 4 cm to 10 cm, the average amplitute of the output voltage was recorded at each location after several tests as shown in Figure 4 a. From the measurement, it was determined that the values also depend on the shape of the PMMA plate and the sensor locations. According to Equation (7) , the piezoelectric potential only indicates the longitudinal deformation of PZT nanofi bers. The elastic waves along the nanofi bers direction produce higher voltage than the waves normal to the fi ber dire-cion. The measurement of angular attenuation was then car-ried out by fi xing the distance at 5 cm and changing the angles between the AE source and the sensor orientation. By varying the polar angle, the angular attenuation curve was measured as shown in Figure 4 b. Therefore, the aligned nanofi bers and interdigitated electrodes can lead to the anisotropic sensitivity of PZT NAFCs sensors. With the distance and angular attenua-tion properties, the AE source information can be easily identi-fi ed using a small number of sensors.

The AE detections of micrometer-size PZT NAFCs sensors were also demonstrated by permanently mounting the sensors on the top surface of the PMMA. For fewer PZT nanofi bers

3967mbH & Co. KGaA, Weinheim wileyonlinelibrary.com

3968

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

Figure 4 . a) Distance attenuation curve of the PZT NAFCs sensor as a function of distance between the AE source and the sensor. b) Angular attenuation curve of the PZT NAFCs as a function of angle between the AE source and the sensor.

Figure 5 . The demonstration of micrometer-size PZT NAFCs used for acoustic emission detection. a) The voltage generation when used for AE detection of sensor A. b) The voltage generation when used for AE detection of sensor B.

and interdigitated eletrodes, the sensitivity of a micrometer-size PZT NAFCs sensor is much lower than that of a millimeter-size sensor. A preamplifi er of SR560 was used to amplify the gen-erated voltage during the AE detection. A band-pass resistor-capacitor (RC) fi lter with cutoff frequencies of 300 Hz and 1 MHz was used to reduce the environmental noise while enhancing the sensitivity of the sensor. Two micrometer-size PZT NAFCs sensors, with a width of 15 μ m and length of 30 μ m, were used to sense the AE signal as shown in Figure 5 . By periodically tapping on the PMMA surface using a steel bar, the voltage output was found to be about 10 mV, which is much lower than that of the larger sensors above. The sensi-tivity can be enlarged by precisely designing the gaps between electrodes for different materials. [ 32 ] During the test, all equip-ment and structures were well grounded in order to eliminate the infl uence of the bioelectric fi eld of the human body and the electromagnetic interference from the testing equipment. The faraday cage was involved and well grounded during the experi-ment. The amplitude of the noise signal was controlled under 2 mV. An interdigitated electrode based silicon substrate

© 2011 WILEY-VCH Verlag wileyonlinelibrary.com

without PZT nanofi bers had also been tested to verify the pie-zoelectric phenomena of NAFCs when subjected to the elastic waves. Although the surface charge of the devices could also create the potential difference, those signals can be eliminated by correct grounding.

In conclusion, a new concept of using PZT NAFCs with interdigitated electrodes as AE sensors for SHM application was demonstrated. The diameter of PZT nanofi bers was controlled at ≈ 80 nm. The acoustic responses of the PZT NAFCs sensor were demonstrated by mounting the sensor on the surface of the steel table and embedding it in the epoxy structure. The peak voltage output reached 0.2 V in response to the acoustic wave generated by the periodic impacts using a grounded steel bar. The signal attenuation curves showed the anisotropic sen-sitivity of the PZT NAFCs sensor. The high sensitivity, micro-scale size, low weight, large fl exibility, low cost, and anisotropic sensitivity cause the PZT NAFCs sensors have a promising application in health monitoring by integrating them into com-posites. The active PZT materials also allow the possibility of the NAFCs sensor to achieve self-powered active sensing for the SHM application.

Experimental Section Fabrication of the PZT NAFCs : PZT nanofi bers were electrospun and

aligned onto a silicon wafer with thermal oxide as a diffusion barrier. 15 kV was applied to make the electric fi eld as 0.5 kV cm − 1 during the spinning process. A pure perovskite phase was obtained after annealing at 650 ° C for about 30 min. Au/Cr interdigitated electrodes were thermally evaporated and patterned by the lift-off process using the photoresist of AZ 5214 on top of the PZT nanofi bers network. A thin PDMS layer was applied by spin coating to protect the sensor structure. Finally, the PZT nanofi bers were polled by applying an electric fi eld of 4 V μ m − 1 across the interdigitated electrodes at a temperature of 140 ° C for ≈ 24 h.

GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 3965–3969

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATIO

N

Acknowledgements This work was supported in part by the National Science Foundation (Award No. CMMI-0826418 & No. ECCS-0802168). The authors would also like to thank Jon Whiten in the Stevens Institute of Technology’s Writing and Communications Center.

Received: April 27, 2011 Revised: June 9, 2011

Published online: July 26, 2011

[ 1 ] C. M. Lieber , Z. L. Wang , MRS Bull. 2007 , 32 , 99 . [ 2 ] F. Patolsky , C. M. Lieber , Mater. Today 2005 , 8 , 20 . [ 3 ] Y. Cui , Q. Wei , H. Park , C. M. Lieber , Science 2001 , 293 , 1289 . [ 4 ] G. F. Zheng , F. Patolsky , Y. Cui , W. U. Wang , C. M. Lieber , Nat. Bio-

technol. 2005 , 23 , 1294 . [ 5 ] Y. Hu , J. Zhou , P. Yeh , Z. Li , T. Wei , Z. L. Wang , Adv. Mater. 2010 , 22 ,

3327 . [ 6 ] D. J. Sirbuly , A. R. Tao , M. Law , R. Fan , P. Yang , Adv. Mater. 2007 , 19 ,

66 . [ 7 ] E. G. Santos , D. Ramos , J. Martínez , M. Fernández-Regúlez ,

R. García , Á. S. Paulo , M. Calleja , J. Tamayo , Nat. Nanotechnol. 2010 , 5 , 641 .

[ 8 ] K. Jensen , K. Kim , A. Zettl , Nat. Nanotechnol. 2008 , 3 , 533 . [ 9 ] H. Liu , J. Kameoka , D. A. Czaplewski , H. G. Craighead , Nano Lett.

2004 , 4 , 671 . [ 10 ] M. Lin , F. K. Chang , Comp. Sci. Technol. 2002 , 62 , 919 . [ 11 ] V. Giugiutiu , J. M. Redmond , D. P. Roach , K. Rackow , SPIE Proc.

2000 , 3985 , 294 . [ 12 ] C. C. Perry , Strain 1987 , 23 , 155 .

© 2011 WILEY-VCH Verlag GAdv. Mater. 2011, 23, 3965–3969

[ 13 ] L. P. Tairova , S. V. Tsvetlov , Mech. Compos. Mater. 1993 , 28 , 489 . [ 14 ] H. Singh , J. S. Sirkis , J. Andrews , R. Pulfrey , J. Lightwave Technol.

1995 , 13 , 1772 . [ 15 ] M. D. Todd , G. A. Johnson , S. T. Vohra , Smart Mater. Struct. 2001 ,

10 , 534 . [ 16 ] D. E. Adams , Health monitoring of structural materials and compo-

nents , John Wiley & Sons , West Sussex 2007 . [ 17 ] M. Barbezat , A. J. Brunner , P. Flueler , C. Huber , X. Kornmann , Sens.

Actuators A 2004 , 114 , 13 . [ 18 ] H. W. Park , H. Sohn , K. H. Law , C. R. Farrar , J. Sound Vib. 2007 , 302 ,

50 . [ 19 ] A. Bent , N. W. Hagood , J. Intell. Mater. Syst. Struct. 1995 . 3 , 338 . [ 20 ] A. Bent , N. W. Hagood , J. Intell. Mater. Syst. Struct. 1997 , 8 , 903 . [ 21 ] W. K. Wilkie , R. G. Bryant , J. W. High , R. L. Fox , R. F. Hellbaum ,

A. Jalink Jr. , B. D. Little , P. H. Mirick , Proc. SPIE 2000 , 3991 , 323 . [ 22 ] R. S. C. Monkhouse , P. W. Wilcox , M. J. S. Lowe , R. P. Dalton ,

P. Cawley , Smart Mater. Struct. 2000 , 9 , 304 . [ 23 ] X. Chen , S. Y. Xu , N. Yao , W. H. Xu , Y. Shi , Appl. Phys. Lett. 2009 , 94 ,

253113 . [ 24 ] L. M. Swallow , J. K. Luo , E. Siores , I. Patel , D. Dodds , Smart Mater.

Struct. 2008 , 17 , 025017 . [ 25 ] E. T. Thostenson , T. W. Chou , Adv. Mater. 2006 , 18 , 2837 . [ 26 ] T. Yamada , Y. Hayamizu , Y. Yamamoto , Y. Yomogida , A. Izadi-

Najafabadi , D. N. Futaba , K. Hata , Nat. Nanotechnol. 2011 , 6 , 296 . [ 27 ] X. Chen , S. Y. Xu , N. Yao , Y. Shi , Nano Lett. 2010 , 10 , 2133 . [ 28 ] S. Y. Xu , Y. Shi , S. G. Kim , Nanotechnology 2006 , 17 , 4497 . [ 29 ] S. Xu , Y. Qing , C. Xu , Y. G. Wei , R. S. Yang , Z. L. Wang , Nat. Nano-

technol. 2010 , 5 , 366 . [ 30 ] R. S. C. Monkhouse , P. D. Wilcox , P. Cawley , Ultrasonics 1997 , 35 ,

489 . [ 31 ] Z. L. Wang , J. Phys. Chem. Lett. 2010 , 1 , 1388 . [ 32 ] W. Zhu , J. L. Rose , IEEE Trans. Ultrason. 1999 , 46 , 654 .

3969mbH & Co. KGaA, Weinheim wileyonlinelibrary.com