Materials Letters 62 (2008) 3859–3861

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Materials Letters

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New type of piezo-damping epoxy-matrix composites with multi-walled carbonnanotubes and lead zirconate titanate

Sheng Tian, Fangjin Cui, Xiaodong Wang ⁎Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, School of Materials Science and Engineering, Beijing University of Chemical Technology,Beijing 100029, China

⁎ Corresponding author. Tel.: +86 10 6441 0145; fax:E-mail address: wangxdfox@yahoo.com.cn (X. Wang

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.05.003

A B S T R A C T

A R T I C L E I N F O

Article history:

A new type of rigid piezo- Received 11 December 2007Accepted 2 May 2008Available online 10 May 2008

Keywords:Piezo-damping materialsCarbon nanotubesPiezoelectric ceramicElectrical conductivityLoss factor

damping epoxy-matrix composites containing multi-walled carbon nanotubes(CNT) and piezoelectric lead zirconate titanate (PZT) was prepared, and the electrical and the dampingproperties were investigated. Dynamic mechanical thermal analysis reveals that the loss factors of thecomposites were improved by incorporation of PZT and CNT under the concentration above a criticalelectrical percolation. Based on this piezo-damping material, the PZT contributes to the transformation ofmechanical noise and vibration energies into electric energy, while the CNT serve in the shorting of thegenerated electric current to the external circuit. An optimum formulation for the piezo-damping epoxy-based materials can be designed on the basis of the results of this study.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Damping materials have good ability to dissipate elastic strainenergy when subjected to vibratory loads and have been widely usedin the fields of high performance structural applications such asaerospace, marine, construction, etc [1]. Polymers are the most usefuldamping materials; however, only the ones having great loss factorand broad distribution of the molecular relaxation can be employed.In most case, rubbers and rubber-based composites can match thisrequirement, resulting in limitation of the damping polymericmaterials used in the structural fields [2]. Furthermore, the rubberymaterials usually cannot be used alone for high damping structuralfields due to their limited properties such as poor mechanicalproperties, high shrinkage ratio and limited acclimatization [3].

In recent years, a new damping material has been developed onthe basis of a new dampingmechanism, which is described as follows:for some specialized composites, mechanical vibrating energy is firsttransmitted to piezoelectric ceramic powders and converted intoalternating electrical potential energy by the piezoelectric effect.Then, the electrical potential energy is further converted into heat onethrough the networks of electro-conductive particles in polymericmatrix [4,5]. For simplicity, such an energy transferring effect is ratherhereafter referred to as the piezo-damping effect hereby. The PZT isusually used as piezoelectric ceramic fillers to construct the piezo-electric units in the polymer matrix [6]. In order to obtain an electro-conductive polymeric matrix, carbon black or copper wire are

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typically used as fillers are introduced into the polymers; howeverthe high loadings required often have a negative effect on themechanical properties of the matrix [7]. High aspect ratio and highlyconductive single- and multi-walled carbon nanotubes (CNT) areregarded as promising fillers, since they can provide electricalpercolation at very low concentrations [8]. Many studies reveals that

Fig. 1. Dependences of electrical conductivities on the CNT and the PZT loadings for thecomposites.

Fig. 2. SEM images of the composites containing (a) 0.5 g CNT and 20 g PZT per 100 g epoxy, and (b) 1.5 g CNT and 40 g PZT per 100 g epoxy.

3860 S. Tian et al. / Materials Letters 62 (2008) 3859–3861

the multi-walled CNT rather than the single-walled ones have beenpredominantly used as electro-conductive fillers due to their lowercost, better availability and easier dispersibility. In this article, wedeveloped and reported a novel type of the piezo-damping epoxy-matrix composites with the multi-walled CNT and the PZT.

2. Experimental

The epoxy resin (diglycidyl ether of bisphenol-A type) with an EEWof 184−194 g/eq. was kindly supplied by Wuxi Dic Epoxy Co., Ltd. ThePZT ceramic, supplied by the Institute of Acoustics, Chinese Academyof Sciences, was milled to the particles with the mean particle size ofabout 5−10 μm, and electrically polarized before use. Themulti-walledCNT with diameters and average length respectively in the ranges of60−100 nm and 5−10 μm were purchased from Shenzhen NanotechPort Co., Ltd., and were surface-treated with H2O2/NH4OH solutionbefore use. The composite specimens with 12 mm in diameter and1.2mm in thickness were prepared bymixing the epoxy resinwith theCNT, the PZT and 4,4'-Diaminodiphenylmethane as curing agent, andfollowing a thermal curing procedure at 80 °C for 2 h, 120 °C for 2 hand 150 °C for 3 h.

Electrical conductivities were measured with an Agilent-4294AImpedance/Gain phase analyzer. Morphologies of the fracturesurfaces of the composites were performed on a Hitachi S-4700scanning electron microscope (SEM). Damping property wasevaluated by dynamic mechanical thermal analysis using aRheometric Scientific V dynamic mechanical analyzer over thetemperature range from 10 to 220 °C at a heating rate of 5 °C /minwith a frequency of 1 Hz.

Fig. 3. Dependences of loss factor on (a) the temperature and (b) the PZ

3. Results and discussion

As expected, the epoxy/CNT composites show a percolation behavior in electricalconductivitywith increasing CNTcontent as shown in Fig.1. The electrical conductivitiesincrease sharply over the range from 0.2 to 1.0 g CNT per 100 g epoxy and indicate apercolation threshold of around 0.8 g CNT per 100 g epoxy. The composite with the CNTloading of 1.5 g per 100 g epoxy has an electrical conductivity of around 10−3 S/m, whichis 1010 times higher than that of the pure epoxy (around 10−13 S/m). It is understandablethat the epoxy/CNT composites with the concentration above the critical electricalpercolation have continuous nanotube pathways that allow electrons to travel. Anelectron hoppingmechanism has been adopted to describe the electrical conductivity ofthe nanotube/polymer nanocomposites [9]. This mechanism requires close proximity(b5 nm) of the carbon nanotubes or nanotube bundles in the composites and directnanotube contact is unnecessary. It is also noticeable (see Fig. 1) that the electricalconductivities of the epoxy/CNT/PZT composites exhibit a similar variational trend withincreasing CNT loading. However, their values reduce in comparison with the binarycomposites with the same CNT loading, and the percolation thresholds also shift tohigher CNT loading with increasing the PZT loading. It is well known that, for thepolymer/CNT composites, there is a contact resistance that exists between the CNT orclusters of the CNT in polymer matrix, which reduces the effective conductivity of theCNT themselves. After incorporating the PZT particles into the epoxy/CNT composites,twoneighboring conductive clusters are separated notonly by the insulating epoxy resinbut also by the insulating PZT particles. This results in much lower electricalconductivities of the binary composites than those of the ternary ones with the sameCNT loading.

Fig. 2 displays the SEM images for the fracture surface of the epoxy/CNT/PZTcomposites. The importance of a good dispersion can be deduced from the pattern ofthe fracture surfaces of two samples, in which the original traces of the imbedded CNTcan be clearly distinguished. The fracture process did not follow the CNT or the PZTparticles pullout pattern, and the cracks propagated along the plane of the nanotubemesh, indicating a good interfacial adhesion between the CNT (or the PZT) and theepoxy matrix. It is also noticed that there are fewer contacts between adjacentnanotubes, and less CNT loadingmakes less contribution to formation of the conductingnetwork in Fig. 2(a). However, the conductive networks have formed when the CNTloading reaches 1.5 g per 100 g epoxy [see Fig. 2(b)].

T loading at room temperature for the epoxy/CNT/PZT composites.

3861S. Tian et al. / Materials Letters 62 (2008) 3859–3861

Loss factor (tan δ) is an important parameter characterizing macromolecularviscoelasticity, and also represents damping capacity of the materials that expresses anability of converting the mechanical energy into heat energy when the material issubjected to an external loading [4]. Fig. 3 shows the dependences of the loss factors ofthe composites on the temperature and the PZT loading at room temperature,respectively. It can be found that the pure epoxy presents a low value of the loss factorat working temperature of around 25 °C. This suggests its low damping capacity. Wecannot find any improvement in loss factor for the epoxy/CNT/PZT (100/0.5/2)composites at room temperature, indicating that the lower CNT loading makes theepoxy an electrical insulator, and here the electric energy generated from the PZT is notfully dissipated. However, when the CNT loading rises up to 1.5 g per 100 g epoxy, theloss factor exhibits a fair increment. Apparently, the higher CNT loading causesformation of electro-conductive network in the matrix, and the generated electriccurrent flows throughout the composite. As a result, the electric energy is welldissipated, and the piezo-damping effect begins to work in the systems. Moreover, theloss factor is improved with increasing the PZT loading (see Fig. 3(b)), indicating thatthe higher PZT loading can make more mechanical energy be converted into electricalenergy, and the electrical energy is dissipated as heat in a load resistance. It is also foundthat, when the CNT loading ranges within 1−1.5 g per 100 g epoxy, the generatedelectrical energy is well dissipated. However, when their loadings exceed thepercolation threshold significantly, the CNT fully contact each other and may act ratheras electric conductors. The CNTcontribute to the increase of the loss factor only throughthe electric shorting of the electric energy generated by the PZT particles to the externalcircuit. As a result, the electric energy is not fully dissipated, and the piezo-dampingeffect becomes weak.

From Fig. 3(a), it is observed that the loss factors of the epoxy and its composites aregreatly affected by the operating temperature. The loss factor varies with increasingtemperature and achieves a maximum value corresponding to glass transitiontemperature (Tg). For the polymers and their composites, damping capacity is quitesensitive to operating temperature, because their intrinsic damping is matrixviscoelasticity, which is a typical behavior of polymers [10]. At around Tg of the givenpolymer, the peak intensity of loss factor is directly related to the coordinated chainmolecular friction, which dissipates mechanical energy as heat. Moreover, the frictionscaused by the particle boundary sliding (filler–filler) and the interfacial sliding (filler–matrix) are also thermally activated [11]. At this temperature region, the piezo-dampingeffect is not obvious, as the mechanical vibration energy converted into thermal energyis higher due to the frictions within the epoxy matrix. Therefore, the loss factor only

reaches peak values at the Tg of such composites. For the temperature dependence ofthe loss factor on the temperature much higher than Tg, the contribution of the matrixviscoelasticity to the damping property is regnant, so the piezo-damping effect is alsonot obvious.

4. Conclusion

A new type of the piezo-damping epoxy-matrix composites withthe CNT and the PZT was developed in this article. The piezo-dampingeffect is obvious for the epoxy-based composites with the high PZTloading and the CNT above the critical electrical percolation con-centration. The present work provides a useful route to design a goodpiezoelectric damping material, in which the PZT contributes to thetransformation of mechanical noise and vibration energies into elec-tric energy, while the carbon nanotubes serve in the shorting of thegenerated electric current to the external circuit. Based on the resultsof this study, an optimum formulation for the piezo-damping epoxy-based materials is designed.

References

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