TANG Zhong, HAO Hua, WAN Baoquan, CAO Minghe,YAO Zhonghua, LIU Hanxing

(1.State Key Laboratory of Power Grid Environmental Protection, Wuhan 430070, China; 2.School of Materials Science and Engineering,Wuhan University of Technology, Wuhan 430070, China))

Abstract: The epoxy resin (E-51) was used as polymer matrix, conductive carbon black (CB) as conductive filler, and PZT was used to prepare a composite by curing. The effects of PZT and CB content on the properties of PZT/ CB/ EP piezoelectric composite were studied. When the PZT content reaches 40 wt%,the optimized vibration attenuation properties of PZT/CB/EP materials could be achieved with a loss factor of 0.9 from room temperature to 60 ℃. With the increase of PZT content, the bending strength of PZT/CB/EP piezoelectric composite vibration reduction material firstly increased from 45 MPa to 65 MPa and then decreased to 38 MPa. At room temperature, the dielectric constant increased from 7 to 50, and the dielectric loss increased from 0.1 to 0.5.

Key words: epoxy resin; PZT; piezoelectric vibration attenuation composites; vibration reduction property

1 Introduction

As one of the critical parts of instrument in power system, the power transformer plays an important role in the safety and reliability of power grid. Transformers have different levels of noise and performance during the working process. Nearly 80% of accidents and equipment damages in the machine manufacturing industry are caused by resonance. Vibration will influence not only the service life of materials, but also make a lot of noise, which disturbs people’s life and affects human health. The effect of transformer vibration on the environment of office building or residential building cannot be ignored. Therefore, necessary measures should be taken to reduce the vibration of the transformer. Over the years, a variety of technical measures to solve the problem of vibration and noise of equipment have been studied and developed. The principal one is using vibration reduction materials which is an effective method to control the vibration and noise of structure[1-3].

Transformer body noise depends on the magnetostrictive vibration of the core. The magnetostrictive vibration of the core is transmitted to the oil tank by solid and liquid transfer paths[4-6]. The vibration energy transmitted by these two ways makes the box wall (including magnetic shielding,etc) vibrate and produce body noise. The results show that the energy of vibration is almost between the solid path and the liquid path. Therefore, even if the vibration transmitted by either way which is completely absorbed or attenuated,the body noise of the transformer can only be reduced by 3 dB.It can be seen from the above analysis that when the transformer structure, selection of materials and production and assembly process are restricted by various technical and economic conditions, the transformer oil tank can be selected as the research object.By avoiding resonance of the oil tank, the vibration of the transformer oil tank can be suppressed to reduce the radiation noise of its vibration[7,8].

The existing simple single-layer vibration isolation device is only effective for the isolation of low-frequency vibration. For the high frequency vibration interference above 100 Hz, vibration isolation effect is not ideal because the effect of vibration isolation transmissibility formula can only be used in low frequency[9-13]. The piezoelectric composite vibration reduction materials[14-20]can control the structure vibration and noise.

In this work, a type of epoxy-matrix composites with the conductive black (CB) and the piezoelectric lead zirconate titanate (PZT) was developed. The PZT is usually used as piezoelectric ceramic to build the piezoelectric units in the polymer matrix. By adding the PZT particles into the epoxy matrix, the mechanical vibration energy transformed into electric energy due to its piezoelectric effect and the electric energy transforms heat energy through internal conductive network[21].

2 Experimental

The epoxy resin (diglycidyl ether of bisphenol-A type) with an epoxide equivalent weight of 185-200 g/eq, polyamide resin used as curing agent were supplied by Hangzhou Wuhuigang Adhesive Co. Ltd. China.Lead zireonate titanate ceramics (PZT-5H) (produced in the lab) were used as a piezoelectric ceramic filler；Conductive carbon (CB) (from TIMCAL) was used as all electrical conductive filler.

The PZT ceramics were electrically polarized along the thickness in silicone oil at 4 kV/mm and 80 ℃for 20 min, and then were milled to the particles with the mean particle size of about 5-10 μm. The composites were produced by casting molding as follows: The CB 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 were added to the mixture for another 30 min under ultrasonic agitation. The suspensions were stirred for 1 h at 2 000 rpm. During these stage,the temperature of the resin was kept at 60 ℃ 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 ℃ for 1 h. A three-stage thermal curing procedure was carried out at 100 ℃ for 3 h.

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.Electrical conductivities were measured by using an Agilent-4294A impedance phase analyzer with a voltage-amplitude of 100 V. Dielectric measurements are tested on an impedance analyzer (HP4192A). The experiment is performed in the frequency of l 000 Hz and in the temperature range from 20-120 ℃ with a heating rate of 2 ℃/min．

The distribution of PZT and CB in the samples and morphologies of the impact-fractured surfaces of the composites were determined by scanning electron microscopy (SEM, Hitachi S-4700). Dynamic mechanical measurement was carried out with a dynamic mechanical analyzer(DMA).The experiment was performed in tension mode at 0.1% dynamic strain amplitude for a frequency of 100 Hz and a varied temperature from 30-120 ℃ with a heating rate of 3 ℃/min.The parameters of stored modulus of elasticityE’, 1oss modulus of elasticityE”, and loss factor (tanδ=E”/E’)were measured. Bending strength was measured with Instron 5967 machine tester according to a Chinese national standard of GB/T1449. The sample size of the specimen was 10 mm×10 mm×40 mm. All the tests were done at room temperature and five measurements were carried out for each data point.

3 Results and discussion

3.1 Electrical conductivity

The electrical conductivities of all samples as a function of CB content are showed in Fig.1. The results indicate that the epoxy/CB composites show a seepage phenomenon in electrical conductivity with increasing CB content. The electrical conductivities of these binary composites increase rapidly over the content from 2 wt% to 3 wt%, indicating a percolation threshold of about 2 wt%. When the CB content is 3 wt%,the conductivity of the composite material is around 10−8S·m−1, which is 105times higher than that of the pure epoxy (about 10−13S·m−1), indicating that the epoxy-matrix composites with a CB content have much more conductive path of continuous conduct network that allows electrons to pass through.

Fig.1 Electrical conductivity as a function of CB content

It is also observed (Fig.1) that the electrical conductivities of epoxy/CB/PZT composites bring out a similar trend with CB content increasing. However,in comparison with the binary epoxy/CB composites with the same CB content, their electrical conductivity decreases, and the percolation thresholds also move to higher CB content direction with the PZT content increasing. It is well known that, for the epoxy /CB composites, there is a contact resistance between the CB and epoxy matrix, which decreases the conductivity of the CB. After mixing the PZT particles into the epoxy/CB composites, adjacent conductive groups are separated not only by the insulating epoxy resin but also by the PZT particles. This brings about much lower electrical conductivities of the binary composites than those of the ternary ones with the same CB content.

3.2 Morphology

The SEM images for the fractured surface of the epoxy/CB binary and epoxy/CB/PZT ternary composites are shown in Fig.2. PZT piezoelectric ceramic powder and CB have good dispersion and solubility in piezoelectric composite materials. Fig.2(a) is a cross-sectional view of a binary composite material with a CB content of 2 wt%. When the content of conductive carbon black is low, the CB particles in the piezoelectric composite are dispersed. In addition,when the CB content is higher than 3 wt%, CB particles are connected to each other and aggregate to form a conductive network, as shown in Fig.2(b). It can be observed that the particles of PZT and CB in the matrix are well dispersed, as shown in Fig.2(c) and (d). When CB content is less than 2 wt%, there is less contact between carbon and no conductive network is formed.When the CB content reaches 3 wt%, the CB in the piezoelectric composite material adheres to the surface of PZT piezoelectric ceramic particles, and the conductive network has formed (Fig.2(d)).

Fig.2 SEM images of the composites containing (a) 2 wt%CB,(b) 3 wt%CB, (c) 2 wt%CB and 20 wt% PZT, (d) 3 wt%CB and 40 wt% PZT

3.3 Dynamic mechanical behavior

Fig.3 illustrates the dynamic mechanical properties of the composites in the aspect of the storage and the loss moduli as a function of temperature. The interfusion of the CB and PZT to epoxy resin has a positive impact on the storage modulus both in the glassy region and near the glass transition temperature (Tg). The addition of these fillers improves its viscoelastic properties of the epoxy system. Furthermore, the particles reduce the flow ability of the epoxy matrix around the fillers and result in enhancing the thermal stability. This effect will become stronger when temperature is aboveTg,due to the limited movement of the polymeric matrix.The storage modulus also shifts to higher temperature and declines with the increase of the PZT content. It also illustrates that the loss modulus has significantly improved with the PZT content increasing. The dispersed fillers not only consume energy due to the viscoelastic deformation of the surrounding epoxy matrix,but the piezoelectric vibration reduction effect also begins to work in the system as discussed in the following paragraph, leading to much more energy dissipated in the composites.

Fig.3 Temperature dependences of storage and loss moduli for the composites

Loss factor (tanδ) is an important parameter characterizing macromolecular viscoelasticity. It also represents vibration attenuation capacity of the materials, indicating the ability of transforming the mechanical energy into heat energy when the materials are stressed. It is generally defined as tanδ=E′′/E′, where tanδis the phase angle between stress and strain,E′is the storage modulus andE′′ is the loss modulus.Fig.4 shows the temperature dependence of loss factor for the composites. It can be found that the loss factor of the pure epoxy is relatively low when the temperature is below 40 ℃, indicating lower vibration attenuation capacity. It is found that when the CB content is low (1 wt%) and not enough to improve the electrical property of epoxy composite, loss factor of epoxy/CB/PZT (100/1/20) composites gets lower when temperature is higher than about 50 ℃. However, when the CB content rises up to 2.5 wt%, distinctly improved loss factor is obtained and the peak value of the loss factor of the piezoelectric composite vibration reduction material gradually increases from 0.4 to 0.9, indicating that the increase of CB content leads to the increase of the conductivity of the piezoelectric composite vibration reduction material, and a conductive network is formed in it. When affected by the external vibration energy, small PZT piezoelectric particles can independently convert mechanical energy into electrical energy, and convert electrical energy into thermal energy through the conductive network formed by CB powder,so as to achieve the purpose of vibration reduction.

The loss factor of piezoelectric composite vibration reduction materials in the region of the glass transition temperature about 80 ℃, is the biggest (about 0.9). When the temperature falls below 60 ℃, loss factor is lower than 0.3. This is because that the strength of the epoxy resin is larger when the temperature is lower than 60 ℃, the freedom movement of molecular chain is not easy, resulting less friction between molecular chains, and the less stress could be transferred to piezoelectric composite vibration reduction material of PZT piezoelectric ceramics. The epoxy resin molecular chain can move relatively freely when the temperature reaches 80 ℃, the mechanical energy resulted by vibration can be passed to the PZT piezoelectric ceramics and be transferred into electricity, and then electricity is converted into heat energy through the conductive network. Thus vibration reduction of composite material could be effective.

3.4 Dielectric behavior

Fig.5 shows the dependence of the dielectric constant on the CB and PZT content. With the increase of PZT content from 20 wt% to 80 wt%, the dielectric constant increases from 5 to 12. When the temperature is below 70 ℃, the dielectric constant of composite changed little. When the temperature rises to 120 ℃,the dielectric constant increases quickly to about 25.The dielectric constant of PZT used in this paper is 3 200.When piezoelectric particle with high dielectric constant is added to polymer matrix with low dielectric constant (only 4.3), the composite material will be polarized under the action of external electric field. Due to the directional arrangement of ceramic phase electric domain with high dielectric constant, the accumulated charge of piezoelectric composite vibration reduction material will increase. Moreover, the contribution of directional polarization to the dielectric constant is proportional to the content of PZT. The results show that the dielectric constant of the composite depends mainly on the piezoelectric ceramic phase.

With the increase of CB content from 1 wt%to 3 wt%, the dielectric constant of the piezoelectric composite vibration reduction material also shows an increasing trend. When CB contents is lower than 2.5 wt%, dielectric constant is less than 6, and when the CB content reaches 3 wt%, dielectric constant increases sharply to 50. When the temperature is less than 80 ℃,the change of dielectric constant with temperature only increases about 2. Further raising the temperature to 120 ℃, the dielectric constant changes a lot, which is because conductive network has formed in piezoelectric composite vibration reduction materials.

Fig.4 Temperature dependence of loss factor for the composites

Fig.5 Dielectric constant as a function of PZT and CB content for the composites

When CB content is low (< 3 wt%), and the temperature is less than 80 ℃, the dielectric loss is about 0.1. When further raising the temperature to 120 ℃,the loss increases to 3.0 when CB content is 3 wt%(Fig.5(b)). When the CB content is close to but still less than the percolation threshold, the dielectric loss temperature spectrum has a similar change rule as CB at low content, but the loss greatly increases to about 0.5, which is mainly caused by the conductance loss of leakage current.The higher the temperature, the higher the carrier activity, and the higher the leakage conductance.

When the PZT content is less than 50% and the temperature is less than 100 ℃, the dielectric loss of the composite is about 0.1. When the content of PZT piezoelectric ceramics is more than 50%, the dielectric loss increases rapidly from 0.1 to 2.5. There are two main reasons for the generation of dielectric loss. The electric energy loss overcomes the internal viscous resistance of the material and is converted into heat. The increase of PZT in the composite increases the internal viscous resistance and causes more relaxation loss.The conductive carriers contained in the composite, such as the CB powder, generate an electric current under the action of an external electric field, which consumes part of the electric energy and turns it into heat energy.

3.5 Mechanical properties

The bending strength of epoxy/CB/PZT composites as a function of the PZT content are displayed in Fig.6. It is indicated that the mixing of this packing into epoxy resin plays an important role on the mechanical properties. When the fillers content is low, resin and filler particles combine closely. In the experimental process, when the crack extends along the fillers, the direction of crack propagation can be improved by filler particles. So when the filler content is low, the bending strength increases with filler increasing. When powder mass fraction is 45%, the bending strength of composites reaches the maximum. Whereas the PZT powder content is greater than 45%, the bending strength of composites decreases. With the further increase of the content, the bending performance decreases sharply because of the interface bonding condition becomes worse. Therefore, in the actual preparation process of composites, not only the influence of the addition amount on the vibration reduction properties of the material but also the mechanical properties of the material should be considered.

4 Conclusions

On the basis of the piezoelectric effect and conductivity mechanism, the conductivity, dynamic mechanical and dielectric behaviors of EP/PZT/CB composites have been studied in this paper. With the increase of the PZT content, loss factor of EP/PZT/CB composite material increases gradually. When the PZT content reaches 40 wt%, the optimized vibration attenuation properties of PZT/CB/EP materials can be achieved with loss factor 0.9 from room temperature to 60 ℃. A complete conductive network could form when the CB content is 3 wt%, converting electrical energy into heat energy and dissipating. With the increase of PZT content, the bending strength of PZT/CB/EP composite material increases from 45 MPa to 65 MPa and then decreases to 38 MPa.

Fig.6 Bending strength of the composites as a function of PZT content