Fabrication of Carbon Nanotubes/Phenolic Nano-Composite and Its Electrical and Thermal Conductivity Measurements

Transcription

1 Fabrication of Carbon Nanotubes/Phenolic Nano-Composite and Its Electrical and Thermal Conductivity Measurements Chie Gau 1 *, Shuo-Ying Chen 1, Hsin-Luen Tsai 2, Syh-Tsang Jenq 1, Cheng-Ching Lee 3,Yu-Der Chen 3 and Ting-Hua Chien 3 1 Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, Taiwan 2 Department of Electronic Engineering, Kao Yuan University, Kaohsiung county, Taiwan 3 Aeronautical Systems Research Division, Chung-Shan Institute of Science & Technology, Taichung, Taiwan Abstract The carbon nanotubes (CNTs) were reinforced with phenolic resins to improve the electrical, thermal and mechanical properties of CNTs/phenolic resin composites in different contents. The results indicate that by adding MWNTs up to 10% in weight, the thermal conductivity of the nanocomposites can increase from (W/m*K) to (W/m*K) while the electric conductivity increase significantly from 0 (ms) to (ms). In order to further disperse the multi wall carbon nanotubes (MWNTs) in phenolic resin (PR), two different methods to modify MWNTs, such as carboxyl MWNTs (c- MWNTs) and surfactant (sodium dodecyl sulfate, or SDS) modified MWNTs (SDS-MWNTs), were adopted. The TEM images revealed that the length of c-mwnt was become much shorter, while the length of SDS-MWNT has no significant change. Measurements by UV-vis spectrophotometer indicate that for MWNTs dispersion in alcohol the absorption spectrum of c-mwnts dispersion is much higher than both the SDS-MWNTs and the as grown MWNTs dispersion. In addition, with better dispersion of SDS-MWNTs in phenolic resin, both the electrical and the thermal conductivity of SDS- MWNT/phenolic resin nanocomposites is found higher. However, the lower thermal conductivity of c-mwnts mixed in phenolic resins may be attributed to shortening of the MWNTs by the attack of acidic treatment. Keywords nanocomposite; CNTs; functionalized CNTs; phenolic resin(pr); electrical resistivity; thermal conductivity 1. Introduction Polymers with addition of small amount of CNTs to become conducting polymers have combined many advantages of plastics, e.g. flexibility, low temperature process, with advantages of conductivity either in metallic or semiconducting regimes. This has significantly widened the area of application for a polymer [1, 2]. Adding carbon black, however, carbon fiber or metal wire in polymer to produce conducting polymer has been developed [3] long time ago. However, the percentage of adding these kinds of materials is relatively high, and is about 20% to 40% of the polymer. Therefore, the mechanical properties of polymer have been significantly decayed. For example, the material lacks ductility that it becomes difficult to be drawn into fiber. For nanomaterials, however, the percentage of adding these materials into polymer can be small. Not only the electrical property, but also the mechanical property is expected to be significantly improved. One class of these polymer materials that can be added with CNTs is phenolic resin. The phenolic resin has many applications in various areas, such as in electronics, automotive, adhesive and additive for the improvement of thermoplastics [4]. However, the degree of dispersion of CNTs in polymer becomes a major concern which has a detrimental effect on the resulting property of composite materials. Current work attempts to use different functionalized CNTs to improve degree of dispersion of CNTs in phenolic resin. The resulting electrical and thermal conductivity of nanocomposites which combined phenolic resin with different functionalized CNTs are measured and presented. 2. Experiments 2.1 Material Preparation The phenolic resin powder synthesized by polymerizing process of the phenol and formaline is supplied from Kuen Bong Chemical Industry Corp., Taiwan. It has specific gravity of and melting point of 84 o C. The carbon nanotubes used are the multi-wall carbon nantoubes which were supplied from Conyuan Biochemical Co., Ltd. Taiwan. The MWNTs have the diameter of nm, the length of 5-15μm and the degree of purity greater than 95-98%. The MWNTs were functionalized with different methods. The MWNTs can be soaked in ethanol consisting of 1wt% sodium dodecyl benzene sulfonate (SDS) with sonication and becomes SDS-MWNTs. Functionalization of MWNTs with surfactant, SDS, is a nondestructive, noncovalent exohedral binding process. The nanotubes can be transferred to the aqueous phase in the presence of surface-active molecules of SDS. The MWNTs dissolved in methanol at 1mg/10ml is added with 1% SDS surfactant under sonication for 30 mins. After centrifugation, the supernatant decanted and the remaining solid was dried in oven to form SDS-MWNTs. The MWNTs can also be acidic treated, which is boiled in sulfuric acid with nitric in the molar of 1: g of untreated MWNTs were suspended in 40 ml of *Corresponding author, Professor, gauc@mail.ncku.edu.tw, FAX:

2 acid mixture (sulfuric acid and nitric in the molar of 1:3). The reaction was carried out at 50 and magnetic stir for 12 hr. The sample was filtered using a 0.47μm filter membrane and was washed until the PH value was around 7.0. The nanotubes were filtered from the water, and dried in oven at 100 o C under vacuum to form carboxylated MWNTs (designated as c-mwnts). 2.2 Characterization techniques The surface morphology of the functionalized MWNTs and is studied with Field Emission Gun Transmission Electron Microscope (model: Philips Tecnai G2 F20 FEG- TEM). The Raman spectrometer (Jobin Yvon/Labram HR) is used to study the defects and structure change of functionalized MWNTs. In addition, FTIR (model: Integrated Scientific Services Group, Ltd. /Thermo/ Nicloet) is used to studied the attached functional groups on the MWNT surfaces. The capability of MWNTs dispersion in polymer can also be studied by adding MWNTs in organic solvent. After sonication, the light absorption of the solvent added with MWNTs is measured by UV-vis spectrophotometer (U-3010) and was used to indicate the degree of MWNTs dispersion in polymer [4]. The pressed composite was characterized by high resolution thermal field emission scanning electron microscopy (FE-SEM) using a JEOL-7000F. The electrical resistance of the PR composites is measured by four point probe (model CM-308 made from ELCHEMA). Seven different locations on the sample surface were selected for measurements. The averaged value was used for presentation. From the size of the sample, the resistivity of the composite can be calculated. The thermal conductivity of the nanocomposites is measured by thermal conductivity meter (model: Laser Flash LFA-447 Modify ASTM E1461). The system using Xe laser to emit a light beam on the surface and cause a temperature rise of the sample. The diffusivity of the nanocomposite is first calculated from the heat conduction equation [5] in the solid by the temperature rise of the sample surface with the given light heat flux imposed on the surface. From further measurements of density and specific heat of the nanocomposite, the conductivity of the nanocomposite can be calculated and obtained. The TEM images revealed, as shown in Fig. 1, that the length of c-mwnts was become much shorter and the ends of the c-mwnts are opened, while the length of SDS-MWNT has no significant change and the ends of SDS-MWNTs are still closed by the catalysts. It appears that wrapping of the MWNTs by SDS does not damage the surface and entire structure of the nanotubes. In addition, the temperature and duration of the acidic treatment for the case of 50 o C c-mwnts has been controlled in such a way that damage on the MWNTs has been minimized. The Raman spectrum, as shown in Fig. 2, indicated that Raman shifts for the pristine MWNTs are almost the same as that for the SDS-MWNTs and c-mwnts. This suggestes that functionalization of MWNTs by SDS or acidic treatment at 50 o C does not damage the structure of MWNTs. The degree of dispersion of the functionalized MWNTs in ethanol, which is a blending solvent during mixing with phenolic resins, was tested by absorption of UV-vis, as shown in Fig. 3. The greater absorption of c- MWNTs in the solvent suggests a best dispersion of c- MWNTs in this solvent. It is expected that dispersion of c- MWNTs in the phenolic resin will be very good and better electrical and thermal conductivity can be obtained. The FTIR spectrum for the various kinds of nanotubes, as shown in Fig. 4, is used to check and see if the functional groups have attached the nanotube surface. The peaks appearing in the range of cm -1 are attributed to the C=C bond of the nanotube structure. Despite of the damage, all the nanotubes still maintain their carbon network structure. For the 50 o C c-mwnts, the peaks appearing in the vicinity of 1710cm -1 and 1050cm -1 account for C=O and C-O stretching mode of the carboxylic groups, respectively. It appears that the COOH group has attached on the surface of the MWNTs. For SDS-MWNTs, the peaks appearing at 2844cm -1 and 2912cm -1 are the unsymmetric and symmetric stretching mode of the alkyl (-CH 2 -) group [6]. 2.2 Synthesis of MWNTs/phenolic resin nanocomposite The pristine or fuctionalized MWNTs is first added into ethanol and sonicated for two and half hours. Then, the powder material of phenolic resin (PR) is then blended into the mixed solvent and stirred for 12 hours until they are well mixed. The MWNTs/phenolic mixture is heated to 75 o C to remove the solvent. The sample is placed into a mold, as shown in Fig. 5 for each component of the mold, preheated up to 125 o C for hot impress. The arrangement for each component of the mold is shown in the right hand side of the figure. The pressure imposed is 1000 psi. The ramp speed is 10 o C per 20 mins. until the sample reaches 170 o C. After pressure is maintained there for 20 mins, the temperature of the sample is allowed to drop gradually into the ambient condition. The size of the sample can be 8mm x 8mm x 0.5mm or 20mm x 8mm x 0.5mm, as shown in Fig. 6, for both electric resistivity and thermal conductivity measurements. The SEM images of the phenolic resin blended with MWNTs show that the material is composed of fibril distributed homogeneously, as shown in Fig.7 (a). After hot-press, the fibril is distributed homogeneously in the material, no clustering or agglomeration of CNTs was found on the surface. It appears that the hot-pressing process does not cause agglomeration of CNTs. For comparison, the sample without MWCNT is also photographed as shown in Fig. 7 (b). The surface of the sample without MWCNT is very smooth and flat, and does not have fibril structure. In addition, the surface is very shiny that the SEM picture is not readily taken. 3. Electrical and Thermal Aspect The electrical resistivity of pure PR is around ~10 12 ohm-cm for surface resistivity. After adding small amount of MWNTs into the PR matrix the electrical resistivity decreases rapidly, as shown in Fig. 8. The rate of decrease in resistivity becomes mild until the content of MWNTs is

3 greater than 0.75%. It appears that the sharp decrease in the resistivity is attributed to the formation of an interconnected structure of MWNTs. This simply means that a very high percentage of electrons are permitted to transfer through the specimen due to the creation of an interconnecting conductive pathway. For SDS-MWNTs/PR composite, the electrical resistivity can become lower when the amount of the MWNTs becomes greater due to the better dispersion of the nanotubes. However, the wrapping of SDS outside the nanotubes can decrease the electrical conductivity and increase the resistivity. This explains that adding small amount of the SDS-MWNTs into phenolic resin may not increase the electrical conductivity so much as that of adding the pristine MWNTs. Figure 9 shows comparison for the thermal conductivity of the pristine MWNTs, the SDS-MWNTs and the c-mwnts composites. For nanotubes at 0.25wt% contents, the thermal conductivity for both SDS-MWNTs composite and c-mwnts decreases slightly, and increases greatly for nanotubes at 0.5wt%. For nanotubes greater than 1wt%, the thermal conductivity of the SDS-MWNTs composite is greater than that of the pristine MWNTs composite. 4. Conclusions The results indicate that by adding MWNTs up to 10% in weight, the electric conductivity can increase significantly from 0 (ms) to (ms) while the thermal conductivity of the nanocomposites increase from (W/m*K) to (W/m*K). In addition, with better dispersion of SDS-MWNTs in phenolic resin, both the electrical and the thermal conductivity of SDS-MWNT/phenolic resin nanocomposites are found higher. However, the lower thermal conductivity of c-mwnts mixed in phenolic resins may be attributed to shortening of the MWNTs by the attack of acidic treatment. More data measurements will be presented in the conference. Acknowledgements The authors are grateful to the Department of Industrial Technology, Ministry of Economic Affairs of Taiwan, Republic of China for their support of the research under the Contract No. 97-EC-17-A-99-R References [1] W. Ni, B. Wang, H. Wang, Y. Zhang, J. Macromolecular Science, Part B: Physics, Vol. 45, No. 4, 2006, pp [2] Man Wu, Leon Shaw, J. Applied Polymer Science, Vol. 99, No. 2, pp , [3] S. Kumar, H.Doshi, Jung O. Park, Fibers from polypropylene/nano carbon fiber composites, Polymer, Vol. 43, pp , [4] Joseph H. Koo, Polymer Nanocomposites: Processing, Characteristics and Application, McGraw-Hill, New York, [5] Frank P. Incropera David P. DeWitt, Theodore L. Bergman, and Adrienne S. Lavine, Fundamentals of Heat and Mass, 6 th edition, John Wiley & Sons, New York, [6] L. Jiang, L. Gao, and J. Sun, Production of aqueous colloidal dispersions of carbon nanotubes, Journal of Colloid and Interface Science, vol260, pp , (a) (b) (c) Fig. 1. TEM images of the (a) MWNTs (b) c-mwnts (c) SDS-MWNTs. Fig. 2. Raman spectrum of MWNTs, SDS-MWNTs and c-mwnts.

4 Fig. 3. UV-vis spectrum of the MWNTs, SDS-MWNTs and c-mwnts disperse in alcohol. Fig. 6. Completed sample of the hot impressed MWNTs/PR composite. (a) Fig. 4. FT-IR spectra of the nanotubes. (b) Fig. 5. Schematic diagram of the mold for hot-pressing. Fig. 7. SEM picture of the hot-pressed sample for (a) PR- MWNTs and (b) neat PR.

5 Fig. 8. Resistivity variation with MWNT contents for PR- MWNTs composite. Fig. 9. Thermal Conductivity variation with MWNT contents for PR-MWNTs composite.