Key Engineering Materials Vols. 317-318 (2006) pp. 661-664 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland
Preparation and Electrical Properties of Carbon Nanotubes Dispersed Zirconia Nanocomposites T. Ukai1, a, T. Sekino1, b, A. Hirvonen1, c, N. Tanaka1, d, T.Kusunose1, e, T.Nakayama1, f, and K. Niihara1, g 1
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan
a
[email protected],
[email protected],
[email protected] d
[email protected],
[email protected], d
[email protected],
[email protected]
Keywords: Tetragonal zirconia, Carbon nanotubes, Electrical property, Toughning, Nanocomposite
Abstract. Multi Wall Carbon Nanotubes (MWCNTs) with a diameter of 20-30 nm were used as a conductive phase to add electric conductivity to yttria stabilized tetragonal zirconia (3Y-TZP). Almost fully dense 3Y-TZP/MWCNTs nanocomposite was obtained by pressureless sintering under inert atmosphere and Hot Isostatic Pressing (HIP) treatment. The conductivity of the nanocomposites increased with increasing content of MWCNTs. Moreover, the fracture toughness increment of the composite was confirmed at 0.5 wt% addition. Scanning electron microscopy and transmission electron microscopy observation of the microstructures showed that MWCNTs were fairly homogeneously dispersed in the 3Y-TZP matrix. Introduction 3 mol% Y2O3 stabilized tetragonal ZrO2 (3Y-TZP) has received much attention as a structural ceramic due to the phase transformation toughening [1], and it has much higher strength than most of the other ceramics at ambient temperature. Many attempts have been made to develop functionalized 3Y-TZP with improved mechanical, magnetic, and electrical properties by using the nanocomposite technique [2-4]. Carbon nanotubes (CNTs) [5], either single-wall (SWCNTs) or multi-wall (MWCNTs), should be ideal reinforcing fibers for composites due to their light weight, having a hollow core, high aspect ratio and exceptional high axial strength. Additionally, CNTs have high electrical conductivity, so CNTs can convert insulating 3Y-TZP to electric conductive ceramics. SiC/ MWCNTs composites have been prepared by Ma et al. [6], who mixed multiwall carbon nanotubes (30-40 nm in diameter) with SiC powder and hot-pressed the mixture. They reported an improvement of about 10 % over monolithic SiC both in bending strength and fracture toughness. Recently, the preparation of Al2O3/CNTs composites were reported by Zhan et al. [7] who applied SWCNTs in the reinforcement of ceramic composites through spark-plasma-sintering (SPS), resulting in 15 orders of magnitude higher conductivity in the 5.7 vol% SWCNTs-Al2O3 nanocomposite compared to single phase Al2O3. In this study, MWCNTs with a diameter of 20-30 nm were used as a 3-dimensional conductive phase in ceramics matrix to add electrical conductivity to 3Y-TZP. The composites were fabricated by pressureless sintering in inert atmosphere and post-HIP treatment process, which has greater advantage for industrial production than pressure sintering such as SPS or hot-pressing. Experimental Procedure The starting material was 3 mol% Y2O3 stabilized ZrO2 (3Y-TZP) with an average size of 60 nm. MWCNTs with a diameter of 20-30 nm were used as a conductive second phase. These powders were dispersed in ethanol with surfactant, and then mixed by an ultrasonic homogenizer. The
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composite powders were dried and ball-milled for 12 hours using zirconia ball media, and then sieved through 150 µm and subsequently uniaxially pressed to form cylindrical pellets of 15 mmφ × 3mm thick. Using these techniques seven weight fractions (wt%) of MWCNTs of 0, 0.05, 0.1, 0.25, 0.5, 1, and 2 were prepared. The green compacts were pressureless sintered at 1773 K under Ar atmosphere for 4 hrs. In order to obtain fully dense nanocomposites, post-HIP treatment was carried out at 1723 K in a 200 MPa Ar atmosphere for 2 hrs. The sintered specimens (12 mm in diameter and 2 mm thick) for mechanical tests were ground with diamond suspensions. The MWCNTs and composites were analyzed with scanning electron microscopy (FE-SEM at 20 kV; model S-5000, Hitachi Co. Ltd.) and transmission electron microscopy (TEM at 200 kV; H-8100, Hitachi Co. Ltd.). Relative densities were calculated from measurements obtained by the Archimedes’ method, using the density of MWCNTs (= 2.0 g/cm3). The 3Y-TZP/MWCNTs nanocomposite materials were also studied by X-ray diffraction (XRD at 50 kV 150 mA CuKα; RU-200B, Rigaku Co. Ltd.). Large-sized nanocomposites 3.5×4.5×35 mm in size were also fabricated for the mechanical tests followed by the same procedure. The specimens were cut using a diamond saw and polished into 3×4×35 mm pieces for the purpose of testing three-point bending strength. The fracture toughness at room temperature was determined simultaneously by the indentation fracture (IF) method [8]. The electrical resistivity of dense specimens was measured at room temperature with Van Der Pauw method [9]. Result and Discussion Dispersion of multiwall carbon nanotubes. TEM images of the raw MWCNTs, after being dispersed in ethanol, and the composite powders are shown in Fig. 1. As shown in Fig. 1a, the MWCNTs ordinarily exist as agglomerates of several hundred micrometers, which is an obstacle to most applications. To disperse MWCNTs in ethanol, the surfactant was added, and then ultrasonic irradiation was applied. After ultrasonic irradiation, stable homogeneous dispersions of MWCNTs were obtained with the aid of the surfactant. TEM observation showed the absence of large agglomerates, and that the MWCNTs existed as well-dispersed nanotubes in suspensions (Fig. 1b). When mixed with 3Y-TZP powders by ultrasonic irradiation, the MWCNTs are dispersed throughout the 3Y-TZP powders, as can be seen clearly from Fig. 1c.
500nm
1µm (a)
(b)
500nm (c)
Fig. 1 TEM images of (a) raw MWCNTs, (b) MWCNTs dispersed in ethanol, and (c) 3Y-TZP/MWCNTs composite powders
Densification behavior and characterization of the composites. The composite powders were pressureless sintered at 1773 K for 4 hrs. However, the composites were not fully densified with density of around 96 %. After HIP treatment, however, almost fully dense (99~100 %)
3Y-TZP/MWCNTs nanocomposites, were obtained. XRD analysis (Fig. 2) showed that all composites consisted of mainly t-ZrO2, however, small amounts of m-ZrO2 and ZrC formed, starting from 1 to 2 wt% of MWCNTs addition.
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Electrical resistivity of the composites. It is 2wt% well known that pure 3Y-TZP is an insulator with 12 very high electrical resistivity (~10 Ωcm). The 1wt% resistivity of the nanocomposites decreased with increasing content of MWCNTs. With very small 0wt% amounts of MWCNTs, only 0.75 vol% (equal to 0.25 20 30 40 50 60 70 wt%), the resistivity of nanocomposite was reduced, 2θ (deg.) and was 11 orders of magnitude lower compared to single phase3Y-TZP. The variation of electrical Fig. 2 XRD of 3Y-TZP/MWCNTs composites: □ resistivity with the addition of MWCNTs is shown in (t-ZrO2) ○ (m-ZrO2) ∆ (ZrC) Table 1. Although we didn’t analyze the actual value of critical volume fraction φc in this system, it can be clearly stated that 3Y-TZP/ MWCNTs system has a φc less than 0.75 vol%. SEM observation of the fracture surface of the 3Y-TZP/MWCNTs composite shows that the MWCNTs are fairly homogeneously dispersed in the matrix(Fig. 3). The decrease in electrical resistivity is related to the interwined network structure of MWCNTs dispersed in the 3Y-TZP.
40nm Fig. 3
SEM image of 3Y-TZP/MWCNTs with 1 wt% MWCNTs content
Fig. 4 TEM image of 3Y-TZP/MWCNTs with 1 wt% MWCNTs content
Table 1 Mechanical and electrical properties of 3Y-TZP/MWCNTs composites Fracture Strength (MPa)
3Y-TZP monolith
1381 H 110
Fracture Toughness (MPam1/2)
Electrical Resistivity (Ω·cm)
5.56 H 0.21
1.44 × 1012
0.25 wt% MWCNTs addtion
727 H 82.5
5.76 H 0.15
21.0 H 9.79
0.5 wt% MWCNTs addtion
653 H 27.7
6.10 H 0.05
1.71 H 0.21
1 wt% MWCNTs addtion
567 H 46.0
5.69 H 0.09
7.55 H 4.12
Moreover, TEM observation of the 3Y-TZP/MWCNTs composite shows dispersions of MWCNTs both in the grain boundary and intragranular of the zirconia matrix (Fig. 4). Percolation
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of the conductive phase at very low volume fraction (under 0.75 vol%) could be achieved by the formation of a 3D network structure of MWCNTs in the 3Y-TZP matrix. Mechanical properties of the composite. The variation in the fracture strength and toughness with the addition of MWCNTs is shown in Table 1. Although each composite became fully densified, a degradation of strength was found in the 3Y-TZP/MWCNTs system. However, appropriate properties were maintained up to 1 wt%. On the other hand, fracture toughness increment of the composite was confirmed at 0.5 wt% addition. Toughening mechanisms of CNTs dispersed ceramics matrix composites reported by Xia et al. [10] showed that CNTs dispersed nanocomposites exhibit the three hallmarks of toughening found in micron-scale fiber composites: crack deflection at the CNTs/matrix interface; crack bridging by the CNTs; and CNTs pullout on the fracture surfaces. In the present nanocomposite, similar mechanisms such as pull out might be taken, as is partly evident from the fracture surface observation shown in Fig. 3. Conclusions 3Y-TZP/MWCNTs nanocomposites have been fabricated. The fabrication process involved the dispersion of MWCNTs using ultrasonic homogenization. Almost fully dense 3Y-TZP/MWCNT nanocomposites could be obtained by post-HIP treatment. Electrical conductive properties could be added to 3Y-TZP ceramics with balanced mechanical properties, when the addition of MWCNTs was over 0.25 wt%. SEM observation showed that the MWCNTs were fairly homogeneously dispersed in the 3Y-TZP matrix. In this system, the critical volume fraction φc was estimated less than 0.75 vol%. Percolation of the conductive phase at such a low volume fraction could be achieved by the formation of a 3D network structure of MWCNTs in the 3Y-TZP matrix. We have thus succeeded to fabricate multifunctional 3Y-TZP nanocomposites with balanced electrical conductivity and mechanical properties. References [1] M. V. Swain, and L.R.F. Rose: J. Am. Ceram. Soc. Vol. 69 (1986), p. 511-518 [2] K. Niihara: J. Ceram. Soc. Jpn. Vol. 99 (1991), p. 974-982 [3] N. Bamba: J. Eur. Ceram. Soc. Vol. 23 (2003), p. 773-780 [4] H. Kondo: J. Nanosci. Nanotech. Vol. 2 (2002), p. 485-490 [5] S. Iijima: Nature. 354 (1991), p.56-58 [6] R. Z. Ma, J. Wu, B. Q. Wei, J. Liang, and D. H. Wu: J. Mater. Sci. Vol. 33 (1998), p.5243-5246 [7] G.-D. Zhan, J. D. Kuntz, J. E. Garay, and A. K. Mukherjee: Appl. Phys. Lett. Vol. 83(2003), p.1228-1230 [8] K. Niihara, R. Morena, and D. P. H. Hasselman: J. Mater. Sci. Lett. Vol. 1 (1982), p. 13-16 [9] L. J. Van der pauw: Phillips Technical Review Vol. 20 (1958), p.220-224 [10] Z. Xia, L. Riester, W. A. Curtin, H. Li, B. W. Sheldon, J. Liang, B. Chang, and J. M. Xu: Acta Mater. Vol. 52 (2004), p. 931–944
