XU Hai-ping, ZHANG Xiao-yi

Effect of BaTiO3on the Network Structure and Electrical Properties of CB/polymer Composites

XU Hai-ping, ZHANG Xiao-yi

(School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University, Shanghai 201209, P. R. China)

Barium titanate ceramic (BaTiO3, BT) is considered as the most important positive temperature coefficient (PTC) ceramic materials, which can easily span across insulating regions and further connect the existing conductive pathways. BT is incorporated into the carbon black (CB) filled high density polyethylene (HDPE) polymer composites to achieve a desired conductivity and further improved PTC effect. It was found that the conductivity and PTC intensity of the specimens were remarkably improved when the concentration of BT reaches a certain level. When compared to the CB/HDPE composites, the negative temperature coefficient effect (NTC) in the CB/(HDPE-BT) composites was also weakened to some extent. These results were explained in microstructure of the CB/(HDPE-BT) composites.

CB/(HDPE-BT) composites; electrical property; PTC effect

0 Introduction

In order to endow insulating polymers with conductive properties, carbon black (CB), carbon fibers (CF) and some metal powders have been utilized as conductive fillers[1-6]. In previous works, the significant positive temperature coefficient (PTC) effect materials were mainly reported for employing a single filler, most of it is CB, into the high density polyethylene (HDPE) to endow CB/HDPE composites conductivity. This is due to the cost, small size and aggregation behavior of CB. To pursue the relatively low loading levels needed to achieve desired conductivities and further improvement on PTC properties, a lot of efforts have been done and then adding the second conductive filler into the CB filled polymer composites is considered as one of the effective ways[7]. The purpose of adding the second filler is to span across insulating regions and further connect existing conductive pathways. In this manner, the second filler serves to bridge the network and enhance the net conductivity of the composite[7-10]. However, the fabrication of micro components made from ceramic materials is becoming more and more important because of their outstanding chemical stability[11-13]. Utilizing ceramic materials as reinforcers in polymer composites is an effective solution to the challenge of developing new polymers for specific sets of properties and applications[14]. With the increasing applications of these potential materials, more and more knowledge is needed to gain a better understanding of their filler matrix interaction, which can give them different physical properties. The most well known PTC ceramic thermistor is based on doped barium titanate (BaTiO3, BT) near the temperature of phase transition from tetragonal ferroelectric to cubic paraelectric phase[14,15]. BT is the most important material for great PTC ceramic materials because of its high melting point, hardness, elastic modulus and electrical conductivity as well as its relatively low coefficient of thermal expansion[11-13]. As far as we know, no experimental work had been reported in the open literature on BT mixed into polymer composites to achieve PTC polymer-ceramic material.

In this work, we introduce BT ceramic as a secondary filler particles into the HDPE matrix. The aim is to investigate the effect of BT ceramic on the network structure, electrical and thermal properties of CB/HDPEcomposites with the purpose of understanding and enhancing the PTC and physical properties for practical applications.

1 Experimental

Semi-crystalline polymer HDPE was obtained from Beijing Yanshan Petrochemical Ltd. The acetylene CB used as conductive filler was provided by Beijing Calcium Carbide Manufactory with the average size of about 50 nm. The BT used as ceramic adulteration filler was provided by Mitsubishi Rayon Co. with the diameter of about 700nm.

The CB was mixed with the HDPE and BT intermixture by a Haake rheometer at 180oC for 10 min. The volume content (vol %) of CB was all 8.0 vol%, which around the percolation threshold value, and the BT content against the composite is varied by 0.0, 1.0, 2.0, 3.0 and 5.0 vol%, respectively. The mixture obtained was further compressed by hot pressing at 18 MPa and 200oC for 10 min to obtain the composite sheets with 12 mm in diameter and 1 mm in thickness.

The surface of specimens was coated with thin silver pastes to ensure good contact with electrodes of the conduction tester. The resistivities were measured along the direction of the thickness. The temperature dependence of resistivity was recorded by employing both a digital multimeter (the resistance was lower than 2×107Ω) and a ZC-36 type megger (the resistance was higher than 2×107Ω). The melting behaviors of the samples were measured on a differential scanning calorimeter (DSC, DuPont TA 2910) under N2flow. Microstructures of the composites were observed by scanning electron microscope (SEM, HITACHI S-4700).

2 Results and Discussion

2.1 Effect of BT content on the electrical conductivity of CB/(HDPE-BT) composites

Figure 1 shows the effect of BT content on the electrical conductivity of CB/(HDPE-BT) composites at room temperature. It was found that the conductivity of CB/(HDPE-BT) composites increased with increasing BT content up to 3.0 vol%, then decreases from 3.0 vol% BT content to 5.0 vol%. Theoretically, the corresponding HDPE phase decreased with increasing loading of BT particles. On the other hand, the BT and CB all loaded in the amorphous phase of HDPE, the conductive CB aggregates get more tightly packed and the gaps between them become very small. Therefore, the conductivity in this region is higher. However, there is an important fact that the high BT content leads it difficult to be mixed by melting mixture method.

Fig. 1 Dependence of electrical conductivity of CB/(HDPE-BT) composites on BT content

2.2 SEM images of CB/(HDPE-BT) composites

Figure 2 shows the SEM micrographs of the CB/(HDPE-BT) composite containing 10 vol% BT，showing a random conglomeration of CB particles in the host. Large BT particles were also observed in the composite sample, and their dispersion was very random and isolated in the HDPE matrix. The high order arrangement of crystal phase of HDPE in the composites could exclude the filler particles into it, so the CB and BT particles should distribute in the amorphous phase of HDPE. By loading BT into the polymer, the better adhesive interaction of BT can make CB more uniformly arrange at the interspace of BT particles, thus forming a relatively homogeneous conducting network to improve the conductivity of the blends. This argument confirms that BT improves the microstructure texturing and network structure of the composites.

Fig. 2 SEM micrographs of freeze fractured surface of the CB/(HDPE-BT)composites with 8%CB and 10%BT

2.3 The melting behavior of HDPE polymers

The influence of BT filler on melting behavior and crystal property of HDPE were examined by DSC measurements. The dynamic DSC curves of the neat HDPE and the composites with different fillers were shown in Fig. 3. The thermal properties of HDPE and its correlative composites derived from DSC data are listed in Table 1. It was found that the intrinsic thermal property of HDPE was hardly affected by the introduction of BT, namely, the composites displayed the same melting temperature of HDPE at about 132oC. It was also discovered that the crystallinity estimated from the DSC curves was diminished by the introduction of BT. It was generally believed that the perfection of crystalline regions of pure HDPE was destroyed because of the interaction between HDPE and BT.

Fig. 3 The DSC curves of neat HDPE, CB/HDPE and CB/(HDPE-BT) composites

Tab.1 Thermal properties of HDPE and its composites Derived from DSC data

2.4 Temperature dependence of conductivity of CB/(HDPE-BT) composites

The temperature dependence of the electrical conductivity of CB/(HDPE-BT) composites with different BT content were shown in Fig. 4. It can be seen that the PTC curves of CB/(HDPE-BT) composites (see Fig. 4a) are approximately similar to that of CB/HDPE composite. A comparison of Fig. 4a and Fig. 3 indicates that the transition critical temperatures, Tc, in the conductivity-temperature curves are basically consistent with that of the corresponding thermal expansion of HDPE. The consistency reveals that there exists a inherent relationship between PTC characteristic and volume expansion of the polymer, but has no relation with the BT. The properties of PTC composites were estimated by the PTC intensity, which was defined as the ratio of maximum resistivity (maxρ) to the room temperature resistivity (RTρ). The results of PTC intensity of composites with different content of BT in this study were shown in Fig. 4b. It could be seen that CB/(HDPE-BT) composite exhibited the higher PTC intensity value in the present composite systems. One of the reasons for arising PTC effect is possible the formation ofpotential barriers at grain boundaries, where semiconducting CB grains and high-resistance BT grain boundaries are formed. The CB/(HDPE-BT) composites exhibited a relative small NTC effect than CB/HDPE composite after Tc, indicating that the reparation of the conductive pathways is more difficult in the existence of BT particles.

Fig. 4 (a)Temperature dependence of the electrical conductivity of CB/(HDPE-BT) composites with 8.0 vol% CB and different BT content, and (b) The corresponding PTC intensity of CB/(HDPE-BT) composites.

3 Conclusion

This work demonstrates that the addition of BT to the CB/HDPE composites improves the PTC effect of composites. The increase in conductivity is more obvious when the BT content reaches a proper level. The PTC effect existed in a wide range of the volume fraction of BT. NTC effect is weakened to some extent because the migration of BT at the melting point of the matrix is more difficult than that of CB. So the BT in the polymer is confirmed to play some useful roles in forming particle networks, and inhibiting the reorganization of conductive chains at high temperature to improve the NTC effect. Therefore, it is proposed that the effect of BT ceramic filler on polymer-based PTC materials is worthy of taking more attention in the next work.

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1001-4543(2010)02-0112-05

2009-12-15;

2010-03-14

徐海萍（1966－），女，博士，主要研究領域為聚合物基功能復合材料，電子郵件：hpxu@eed.sspu.cn

上海市科委基礎研究重點項目基金(No.09JC1406700)，上海市教委科研創(chuàng)新重點項目基金（No.10ZZ132）及2009年度“聯(lián)盟計劃”項目基金（No.09LM10）