Fabrication and performances of epoxy/multi-walled carbon nanotubes/piezoelectric ceramic composites as rigid piezo-damping materials
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- Tian, S. & Wang, X. J Mater Sci (2008) 43: 4979. doi:10.1007/s10853-008-2734-7
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Piezo-damping epoxy-based composites containing various amounts of multi-walled carbon nanotubes (CNT) and piezoelectric lead zirconate titanate (PZT) were prepared, and their performances were investigated. The composites exhibited a percolation threshold in the range of 1–1.5 g CNT per 100 g epoxy, in which a continuous electro-conductive network formed. Dynamic mechanical thermal analysis reveals that the loss factors of the composites were improved by incorporation of the PZT and the CNT at above critical electrical percolation loading. Based on this new type rigid piezo-damping material, the PZT contributes to the transformation of mechanical noise and vibration energies into electric energy, while the CNT serve in shorting of the generated electric current to the external circuit. Thermal stability and mechanical properties were also improved by incorporating these two fillers. An optimum formulation for the rigid piezo-damping materials can be designed on the basis of the results of this study.
Damping materials have a good ability to dissipate elastic strain energy when subjected to vibratory loads, and they have been widely used in the fields of high-performance structural applications, such as aerospace, marine, aircraft, construction, automobile, skyscrapers, appliance industries, etc. [1, 2]. Polymers are the most useful damping materials; however, only the ones having the great loss factor and broad distribution of the molecular relaxation can be employed. In most cases, rubbers and rubber-based composites can match this requirement, which, to some extent, results in a limitation of the polymeric damping materials used in structural fields [3, 4]. In addition, the damping efficiency for polymeric materials is not very good. It is well known that the width and height of the loss peak for a given polymer cannot be independently adjusted, as the broadening of the loss peak usually results in a decrease in its maximum height. In fact, values of loss factor of good damping materials are higher than 0.3 in the range of temperature more than 60 °C. Moreover, once these materials and structures for damping are decided, their dynamic mechanical properties of the polymers cannot be changed. The interpenetrating polymer network (IPN) was considered as a possible ways of overcoming some of these problems [5, 6]. Owing to such properties of partial compatibility and broad distribution of the molecular relaxation for IPN, a broader temperature range of thermal transition can be obtained. Some IPN like polyurethane (PU)/polymethyl methacrylate, epoxy/acrylics, PU/epoxy, and unsaturated polyester/epoxy systems were reported as the good damping materials [7–10]. On the other hand, polymers usually cannot be used alone for high damping structural fields because of their limited properties such as poor mechanical properties, high shrinkage ratio and limited acclimatization. The polymer-inorganic composites have received great interest for their high specific strength, high modulus and lightweight, as well as good damping properties such as the fly ash filled epoxy composites that reported by Gu et al. [11, 12].
In recent years, a new damping material has been developed on the basis of a new damping mechanism. It can be described as follows: for some specialized composites, mechanical vibrating energy is first transmitted to the piezoelectric ceramic powders and converted into alternating electrical potential energy by the piezoelectric effect. Then, the electrical potential energy is further converted into joules heat through the networks of electro-conductive particles in polymeric matrix [13, 14]. For simplicity, such an energy transferring effect is rather hereafter referred to as the piezo-damping effect hereby. In order to obtain an electro-conductive polymeric matrix, carbon black or copper wires used typically as fillers are introduced into the polymers; however, high loadings required often have a negative effect on the mechanical properties of the matrix [15, 16]. High aspect ratio and highly conductive single- and multi-walled carbon nanotubes (CNT) are regarded as promising fillers, since they can provide electrical percolation at very low concentrations [17–19]. In terms of the experimental date, the multi-walled CNT rather than the single-walled ones have been predominantly used as electro-conductive fillers due to their lower cost, better availability and easier dispersibility . On the other hand, a few literatures recently reported the electrorheological and the magnetrorheological characteristics for the composites containing CNTs, which implies that the CNTs could be applied potentially as smart damping materials under either electric or magnetic fields [21, 22].
In this article, we developed a novel type of epoxy-matrix composites with the multi-walled CNT and the piezoelectric lead zirconate titanate (PZT), which can be used as the rigid piezo-damping materials. Epoxy resins are well established as thermosetting matrices of advanced composites, displaying a series of promising characteristics for a wide range of application. The PZT is usually used as piezoelectric ceramic fillers to construct the piezoelectric units in the polymer matrix . Through the PZT particles embedded in the epoxy matrix, the vibrational energy is changed into electric energy due to its piezoelectric effect and the electric energy is dissipated as thermal energy through an external electric resistance . This may donate a good damping property to the rigid polymers like epoxy resins. The main aim of the present article is to fabricate and investigate the epoxy/CNT/PZT composites, and to discover their damping mechanisms.
The epoxy resin (diglycidyl ether of bisphenol-A type) with an epoxide equivalent weight of 184–194 g/eq. was supplied by Wuxi Dic Epoxy Co., Ltd. China. 4,4′-Diaminodiphenylmethane (DDM) used as curing agent was purchased from Beijing Chemical Reagent Co., Ltd., China. The PZT ceramic was commercially supplied by the Institute of Acoustics, Chinese Academy of Sciences. The multi-walled CNT with diameters and average length respectively in the ranges of 60–100 nm and 5–10 μm was purchased from Shenzhen Nanotech Port Co., Ltd.
Preparation of the composites
The PZT plates were electrically polarized along the thickness direction in silicone oil at 7 kV/mm and 110 °C for 20 min, and then were milled to the particles with the mean particle size of about 5–10 μm. The PZT and the CNT was surface-treated, respectively, with 3-glycidyl-oxypropyl trimethoxysilane and with a mixed alkaline solution containing H2O2/NH4OH (28.5 wt.%) before use, so that the fillers can be functionalized with amino groups. The composites were prepared by thermosetting process as follows: The CNT and PZT particles were first dispersed in acetone together under an ultrasonic agitation at room temperature for 30 min. In addition, a well-mixed epoxy resin and curing agent was added to the mixture for another 30 min under ultrasonic agitation. In succession, the suspensions were stirred for 1 h at 2,000 rpm. During these stage, the temperature of the resin was kept at 60 °C using a silicone bath to maintain a low viscosity of the resin so as to make the fillers dispersed adequately. In order to evaporate the acetone solvent, the mixtures were kept in a vacuum oven at 50 °C for 1 h. A three-stage thermal curing procedure was carried out at 80 °C for 2 h, 120 °C for 2 h and 150 °C for 3 h.
Measurements of electrical conductivity
Electrical conductivities were measured by using an Agilent-4294A impedance/gain phase analyzer with a voltage-amplitude of 0.5 VAC at a frequency of 1 MHz. The testing specimens were prepared with 12 mm in diameter and 1.2 mm in thickness through polishing and electroplated by silver-paint on both sides. Dielectric constants were measured on a WY2851-type LCR bridge meter (manufactured by Shanghai Wuyi Electronics Co., Ltd.) at a frequency of 1 MHz.
Measurements of damping properties
The damping properties were evaluated by dynamic mechanical thermal analysis (DMTA) using a Rheometric Scientific V dynamic mechanical analyzer with three point bending mode over the temperature range from 10 to 220 °C at a heating rate of 5 °C min−1. The frequency and the strain were set to 1 Hz and 0.5%, respectively. The specimen size was 50 × 6 × 2 mm3 in length, width, and thickness, respectively.
Scanning electronic microscopy
Scanning electron microscopy (SEM) was performed on a Hitachi S-4700 scanning electron microscope, and morphologies of the impact-fractured surfaces of the composites were determined from SEM images. The brittle impact-fractured surfaces of the composites were obtained through an impacted fracture in liquid nitrogen and mounted on the sample stud by means of a double-sided adhesive tape for cross-sectional view study. A thin layer of gold was sputtered onto the cross-sectional surface prior to SEM observation.
Measurements of thermal properties
The Perkin-Elmer Pyris-1 differential scanning calorimeter was used to measure the differential scanning calorimetry (DSC). All measurements were made under an N2 atmosphere at a heating rate of 10 °C/min on samples weighing about 10 mg. Thermal gravimetric analysis (TGA) was carried out on a Perkin-Elmer Pyrid-1 thermal gravimetric analyzer at a heating rate of 10 °C min−1 from 50 to 220 °C under an N2 atmosphere.
Measurements of mechanical properties
The impact and tensile test bars were fabricated via a cast molding. Charpy impact strength was measured with a SUMITOMO impact machine tester according to a Chinese national standard of GB/T1043-98. The thickness of the Charpy impact specimen was 4 mm, and impact energy was 4 J. The tensile properties were determined with an Instron-1185 universal testing instrument using a 1000 Newton load transducer according to a Chinese national standard of GB/T1040-98. Small dumb-bell specimens with waist dimensions of 20 × 4 mm2 were used for tensile mechanical tests. All the tests were done at room temperature and five measurements were carried out for each data point.
Results and discussion
An electron hopping mechanism has been adopted to describe the electrical conductivity of nanotube/polymer nanocomposites [24, 25]. This mechanism requires close proximity (<5 nm) of the nanotubes or nanotube bundles in the nanocomposites and direct nanotube contact is unnecessary. It is also noticeable (see Fig. 1) that the electrical conductivities of epoxy/CNT/PZT composites exhibit a similar variational trend with increasing CNT loading. However, their values reduce in comparison with the binary composites with same CNT loading, and the percolation thresholds also shift to higher CNT loading with increasing the PZT loading. It is well known that, for the polymer/CNT composites, there is a contact resistance existing between the CNT (or clusters of the CNT) in polymer matrix, which reduces the effective conductivity of the CNT themselves. After incorporating the PZT particles into the epoxy/CNT composites, two neighboring conductive clusters are separated not only by the insulating epoxy resin but also by the insulating PZT particles. This results in much lower electrical conductivities of the binary composites than those of the ternary ones with the same CNT loading.
A very good dispersion of the PZT particles is also observed in matrix in a large field of vision from the fracture surfaces of the epoxy/CNT/PZT ternary composites as shown in Fig. 3c and e indicating a good interfacial adhesion between the PZT and the epoxy. The much higher magnification SEM images shown in Fig. 3d and f give an insight of the nanoscale morphology, in which the dispersion of the CNT could be clearly distinguished. The fracture process did not follow the CNT or the PZT particles pullout pattern and the cracks propagated along the plane of the nanotube mesh, indicating a good interfacial adhesion between the CNT (or PZT) and the epoxy matrix. It is also noticed that there are fewer contacts between adjacent nanotubes and less CNT loading makes less contribution to formation of the conducting network in Fig. 3d. However, the conductive networks have formed when the CNT loading reaches 1.5 g per 100 g epoxy (see Fig. 3f).
Dynamic mechanical analysis
From Fig. 5, it can also be seen that the loss factor of the composites exhibits the same variation trends as that of the pure epoxy resin are greatly affected by the operating temperature. The loss factor varies with increasing temperature and achieves a maximum value corresponding to the Tg. For polymers and their composites, damping capacity is quite sensitive to the operating temperature because their intrinsic damping is the matrix viscoelasticity, which is one typical behavior of polymeric materials . At around Tg of the given polymer, the peak intensity of loss factor is directly related to the coordinated chain molecular friction, which dissipates mechanical energy as heat. Moreover, the frictions caused by the particle boundary sliding (filler–filler) and the interfacial sliding (filler–matrix) are all thermally activated . It also should be noted that the addition of fillers like the CNT and the PZT has a profound effect on the terminal relaxation time of the composites. With increasing filler loading, the liquidlike relaxation observed for the pure polymer gradually changes to solidlike (or pseudo-solidlike) behavior for the composites . At this temperature region, the piezo-damping effect is not obvious, as the mechanical vibration energy converts into thermal energy is higher, because of the frictions within the epoxy matrix. Therefore, the loss factor can only reach peak values at the Tg of such composites. For the temperature dependence of the loss factor of the composites at the temperature much higher than Tg, and the contribution of the matrix viscoelasticity to composites damping is regnant, so the piezo-damping effect is not obvious.
Thermal analysis experimental data of the epoxy/CTN/PZT composites
Samples epoxy/CTN/PZT (wt.%/wt.%/wt.%)
Temperature at characteristic weight loss (°C)
Temperature at rapid weight loss (°C)
Char ratios at 750 °C (wt.%)
A new type of the piezo-damping epoxy-based composites with CNT and PZT was developed in this article. The piezo-damping effect is obvious for the epoxy-based composites with high PZT loading and CNT at above critical electrical percolation loadings. The thermal stability and the mechanical properties as well as the dielectric constants were also improved by incorporating these two fillers. The present work provided a useful route to design a good piezoelectric damping material, in which PZT contributes to the transformation of mechanical noise and vibration energies into electric energy, while CNT serve in shorting of the generated electric current to the external circuit. Based on the results of this study, an optimum formulation for the piezo-damping epoxy-based materials is designed.
The authors greatly appreciated financial supports from the National Natural Science Foundation of China (Grant No. 50573006).