Microstructures and mechanical properties of Al2O3-C refractories with addition of multi-walled carbon nanotubes

Microstructures and mechanical properties of Al2O3-C refractories with addition of multi-walled carbon nanotubes

Materials Science and Engineering A 548 (2012) 134–141 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering A journa...

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Materials Science and Engineering A 548 (2012) 134–141

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea

Microstructures and mechanical properties of Al2 O3 -C refractories with addition of multi-walled carbon nanotubes Ming Luo, Yawei Li ∗ , Shengli Jin, Shaobai Sang, Lei Zhao, Yuanbing Li The Key State Laboratory Breeding Base of Refractories and Ceramics, Wuhan University of Science and Technology, Wuhan 430081, PR China

a r t i c l e

i n f o

Article history: Received 24 August 2011 Received in revised form 8 February 2012 Accepted 1 April 2012 Available online 7 April 2012 Keywords: MWCNTs Graphite flake Al2 O3 -C refractories Microstructures Mechanical properties

a b s t r a c t Microstructures and mechanical properties of Al2 O3 -C refractories with Al, Si and SiO2 as the additives fired in the temperature range from 800 to 1400 ◦ C are investigated when multi-walled carbon nanotubes (MWCNTs) were used as the carbon source to partially or totally replace graphite flake in the materials. The results showed that the mechanical properties such as cold modulus of rupture (CMOR), modulus of elasticity (E) of all the specimens increased in the firing temperature range from 800 to 1200 ◦ C, but decreased dramatically at 1400 ◦ C. Compared with specimens with only graphite flake, specimens containing 0.05 wt% MWCNTs possessed better mechanical properties. However, they got deteriorated with the further increase of MWCNTs amount from 0.1 to 1 wt%. The differences in mechanical properties were closely associated with the microstructures of Al2 O3 -C refractories. In comparison with specimens having only graphite flake, the improvement of mechanical properties of specimens containing MWCNTs was attributed to strengthening and toughening mechanism of MWCNTs as well as formation of larger amount of ceramic phases in the matrix at 800 and 1000 ◦ C, respectively. At 1200 and 1400 ◦ C, the improvement was mainly associated with the quantity and morphology of SiC whiskers induced from MWCNTs in the specimens. However, with the increase of MWCNTs amount, the mechanical properties of the materials got deteriorated because of agglomeration of MWCNTs and their influence on the morphology of ceramics phases. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Carbon containing refractories have been widely used in the steelmaking industry due to their unique mechanical and chemical properties. Incorporation of carbon sources into this kind of refractories, not only endows the materials with excellent thermal shock and erosion resistance, but also improves the strength and toughness due to the formation of ceramic phases such as Al4 C3 , SiC by the reactions between carbon sources and additives like Al, Si or SiO2 [1–3]. However, with the increasing demand on clean steel production, traditional carbon containing refractories having a relatively high amount of carbon, cannot meet the requirements because of their recarburization into molten steel [4–6]. However, the mechanical properties of carbon containing refractories are bound to get deteriorated by simply decreasing carbon content. Therefore, it is of great importance to develop low carbon containing refractories that have excellent properties [7,8]. Since MWCNTs were first reported in 1991, they have attracted considerable attention due to their excellent physical, chemical and mechanical properties, which makes them potentially useful as a reinforcement to enhance the strength and toughness of

ceramic matrix composites [9–16]. For example, Zhu et al. [17] reported that addition of 1.5 wt% MWCNTs into Al2 O3 –MWCNTs nanocomposites could result in an increase of 67% and 119% in cold modulus of rupture (CMOR) and fracture toughness (KIC ), respectively. Yamamoto et al. [18] also obtained an enhancement of 25% and 27% in CMOR and KIC with only incorporating 0.9 wt% MWCNTs into Al2 O3 –MWCNTs nanocomposites. In fact, MWCNTs are one of the most promising carbon source replacing graphite flake to develop low carbon containing refractories with high strength, toughness and excellent thermal shock resistance [19]. However, it is to be regretted that introduction of MWCNTs into carbon containing refractories has rarely been reported up to now. In the present work, different amounts of MWCNTs were selected as carbon source to partially or totally replace graphite flake in Al2 O3 -C refractories in order to study the effect of MWCNTs on the microstructures and mechanical properties of this kind of refractories.

2. Experimental 2.1. Raw materials and refractories fabrication

∗ Corresponding author. Tel.: +86 27 68862188; fax: +86 27 68862018. E-mail address: [email protected] (Y. Li).

Tabular alumina (2–1 mm, 1–0.5 mm, 0.6–0.2 mm, 75 ␮m and 20 ␮m, 98.5 wt% Al2 O3 , Almatis), white fused alumina (10 ␮m,

0921-5093/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.001

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Table 1 CMOR and E of specimens with different amounts of MWCNTs fired at various temperatures. Temperature ◦

800 C 1000 ◦ C 1200 ◦ C 1400 ◦ C

Index

N0

N0.05

N0.1

N0.3

N0.5

N1

CMOR (MPa) E (MPa) CMOR (MPa) E (MPa) CMOR (MPa) E (MPa) CMOR (MPa) E (MPa)

4.27 1000.16 9.09 1702.95 14.11 2540.91 9.41 2257.97

5.76 1329.41 12.08 2388.14 23.76 3733.36 14.98 2606.00

4.63 1109.59 10.66 2026.56 22.51 3596.59 10.57 2488.89

4.50 1045.87 10.08 1958.47 21.91 3316.75 9.55 2160.18

4.44 1042.8 9.94 1762.02 18.49 3142.68 9.02 2109.00

3.07 772.96 7.77 1605.1 16.88 2992.48 8.9 2038.4

98.6 wt% Al2 O3 , Almatis), silicon powder (45 ␮m, 98.4 wt% Si, Anyang, China), aluminum powder (45 ␮m, 98.3 wt% Al, Xinxiang, China), microsilica powder (∼0.5 ␮m, 97.0 wt% SiO2 , Elkem, Norway), graphite flake (200 mesh, 97.5 wt% fixed carbon, Qingdao, China) and MWCNTs (diameter, 20–70 nm; lengthen, ∼20 ␮m; >95.0 wt% C, Chengdu, China) were used as raw materials. In addition, thermosetting phenolic resin (liquid, >40 wt% fixed carbon, Wuhan, China) was added as a binder. The batch composition consisted of 83 wt% tabular alumina, 10 wt% white fused alumina, 2 wt% Al powder, 3 wt% Si powder, 1 wt% microsilica powder and 1 wt% graphite flake as a basis (labeled as N0). In order to figure out the influence of MWCNTs, various amounts of MWCNTs such as 0.05 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt% and 1 wt% (labeled as N0.05, N0.1, N0.3, N0.5 and N1, respectively) were used to partially or totally replace graphite flake as the carbon source in the compositions. For trying to obtain the homogenous dispersion of MWCNTs in the mixtures, MWCNTs were firstly mixed with white fused alumina powder in the ball mill using corundum balls as the abrasive media and absolute ethyl alcohol as dispersant at the speed of 250 RPM for 6 h. Then the mixtures were dried at 110 ◦ C for 12 h. Finally, all the raw materials were mixed for 30 min in a mixer with the rotating speed of 80–100 RPM. After kneading, specimens of 25 mm in width, 25 mm in height and 100 mm in length were prepared by cold pressing at a pressure of 150 MPa and then cured at 180 ◦ C for 24 h. Lastly, as-prepared specimens were put into an alumina crucible fed with fine petroleum coke powder and fired from room temperature to 800, 1000, 1200 and 1400 ◦ C using a heating rate of 5 ◦ C/min and a holding time of 3 h, respectively.

2.2. Tests and characterization methods The physical properties such as apparent porosity and bulk density, together with weight changes and linear change of the fired specimens were measured. Mechanical properties including cold modulus of rupture (CMOR) and modulus of elasticity (E) were measured by three-point bending test at ambient temperature with a span of 80 mm and a loading rate of 0.5 mm by means of electronic digital control system (EDC 120, DOLI Company, Germany). The force–displacement curve of each refractory specimen was recorded simultaneously during the test. All the preceding measurements were carried out with three samples from each composition. The phase compositions of the coked specimens were analyzed by X-ray diffraction (XRD, x’Pert Pro, Philips, Netherlands). The microstructures of ruptured surfaces of all the coked Al2 O3 -C refractories were observed by scanning electron microscope (SEM, Quanta 400, FEI Company, USA) equipped with energy dispersive X-ray spectroscope (EDS, Noran 623M-3SUT, Thermo Electron Corporation, Japan). Thermogravimetry-differential scanning calorimetry (TG-DSC, STA499, NETZSCH, Germany) was employed to characterize the reactivity of graphite flake and MWCNTs from room temperature to 1000 ◦ C at a heating rate of 10 ◦ C/min in air atmosphere.

3. Results 3.1. Mechanical properties Mechanical properties including CMOR and E of coked specimens were measured by three-point bending test at room temperature, and the results are depicted in Table 1. It is obvious that CMOR and E of all the specimens increased simultaneously with the increase of firing temperature from 800 to 1200 ◦ C, then decreased dramatically with the temperature up to 1400 ◦ C. For example, CMOR and E of specimen N0 at 800 ◦ C were 4.27 MPa and 1.00 GPa, respectively. Then, they reached a maximum at 1200 ◦ C of 14.11 MPa and 2.54 GPa, but decreased a lot at 1400 ◦ C to 9.41 MPa and 2.26 GPa, respectively. For specimen N0.05, CMOR and E were much higher than those of N0 at the same firing temperature. That is to say, they were 5.76 MPa and 1.33 GPa at 800 ◦ C and came up to 23.76 MPa and 3.73 GPa at 1200 ◦ C, but decreased to 14.98 MPa and 2.61 GPa at 1400 ◦ C, respectively. However, with the increase of MWCNTs amount from 0.1 wt% to 1 wt%, both CMOR and E decreased continuously. Fig. 1 shows the force–displacement curves of specimens coked at 800, 1000, 1200 and 1400 ◦ C. It can be clearly seen that the firing temperature and MWCNTs amount had a big influence on the forces and displacements, while their changes had the same trend with CMOR and E. Namely, forces and displacements increased with the increase of firing temperature and reach a maximum at 1200 ◦ C, then decreased dramatically at 1400 ◦ C. Meanwhile, the specimens with 0.05 wt% MWCNTs had the highest forces and displacements at various firing temperatures. Likewise, forces and displacements decreased with the increase of MWCNTs amount. The differences in mechanical properties were closely associated with the physical properties such as apparent porosity, bulk density, weight change and linear change. Fig. 2 illustrates the evolutions of apparent porosity and bulk density of the specimens with different amounts of MWCNTs fired at various temperatures. It is obvious that the apparent porosity decreased with increasing the firing temperature from 800 to 1200 ◦ C, but it increased dramatically at 1400 ◦ C (Fig. 2a). Correspondingly, the bulk density as a function of firing temperature was opposite to that of apparent porosity (Fig. 2b). In addition, it has to be noticed that either N0.05 or N0.1 had the lowest apparent porosity, while it increased with further increasing the MWCNTs amount accordingly no matter what firing temperatures were. As can be seen from the results above, the variation trends of apparent porosity and bulk density were similar with those of mechanical properties. The higher apparent porosity of specimens at 800 and 1000 ◦ C was due to the release of volatile species of the binder, which also resulted in the weight loss and shrinkage of the materials [21], as can be seen in Fig. 3. With the increase of firing temperature to 1200 ◦ C, the enhancement of the mechanical properties as well as the increase of weight change and linear change were considered as a direct consequence of formation of new ceramics phases in the materials, which was discussed in detail in the previous reports [2,21,22]. However, when the firing temperature reached 1400 ◦ C,

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Fig. 1. Force–displacement curves of specimens fired at different temperatures. (a) 800 ◦ C, (b) 1000 ◦ C, (c) 1200 ◦ C and (d) 1400 ◦ C.

the apparent porosity, bulk density as well as mechanical properties get deteriorated due to the considerable expansion resulting from the larger weight gain of the materials. 3.2. Phase compositions In order to clearly observe the effect of MWCNTs addition and temperature on the phase compositions, XRD patterns of specimens N0 and N1 fired in the temperature range from 800 to 1400 ◦ C are measured and shown in Fig. 4. In the specimen N0 (Fig. 4a), Al2 O3 , Al, Si and graphite flake phases were detected after fired at 800 ◦ C, indicating that no new ceramic phases formed at this temperature.

At 1000 ◦ C, Al phase disappeared whereas AlN, Al4 C3 and critobalite phases formed on the contrary. With further increasing the firing temperature to 1200 ◦ C, AlN and Al4 C3 phases disappeared and SiC phase formed, while the peak intensity of Si phase decreased dramatically. At 1400 ◦ C, the Si phase disappeared completely and the peak intensity of SiC phase decreased a little, together with the formation of mullite phase. For the specimen N1 (Fig. 4b), the same results were also observed at 800 and 1400 ◦ C with N0. However, the differences were that at 1000 ◦ C, the peak intensity of AlN and Al4 C3 was a little higher than that of N0. In addition, SiC phase appeared in N1 while it was not detected in N0 at this temperature. At 1200 ◦ C, it

Fig. 2. Apparent porosity and bulk density of specimens fired at different temperatures. (a) Apparent porosity and (b) bulk porosity.

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Fig. 3. Weight change and linear change of specimen N0 fired at different temperatures.

also deserves attention that Si phase disappeared for specimen N1, while the peak intensity of SiC phase was a little higher than that of N0.

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With regard to specimen N0.05, it can be seen that MWCNTs were homogeneously dispersed in the matrix at 800 ◦ C (Fig. 6a). At 1000 ◦ C, besides formation of larger amount of AlN and Al4 C3 phases, SiC whiskers with micrometer in length formed in the matrix (Fig. 6b). With the increase of firing temperature to 1200 ◦ C, the SiC whiskers were interlocked with each other to form intertextures and well distributed in the matrix of the samples (Fig. 6c). The amount of SiC whiskers was smaller and the diameter of some got finer in the same way like specimen N0 at 1400 ◦ C (Fig. 6d). For specimen N1, agglomeration of MWCNTs became very severe in the matrix at 800 ◦ C (Fig. 7a). At 1000 ◦ C, more SiC whiskers formed but located partly in the matrix compared with specimen N0.05 (Fig. 7b). When the specimen was fired at 1200 ◦ C, differently, SiC whiskers became more curved and less homogenously distributed in the matrix, which was attributed to the agglomeration of MWCNTs (Fig. 7c). Compared with specimen N0, it appeared that the amount of SiC whiskers in N1 was much larger. In addition, the SiC whiskers became coarser and locally formed in the matrix, while flaky mullite formed simultaneously with the increase of firing temperature to 1400 ◦ C (Fig. 7d).

4. Discussions 3.3. Microstructures The SEM micrographs of ruptured surfaces of specimens N0, N0.05 and N1 are shown in Figs. 5–7, respectively. In the specimen N0, graphite flake was intact and no new ceramic phases formed in the matrix when the firing temperature was at 800 ◦ C (Fig. 5a). At 1000 ◦ C, striated AlN and regular columnar or plate-shaped Al4 C3 were observed in the texture as indentified by EDS along with XRD analysis (Fig. 5b). It is well known that Al could form AlN and Al4 C3 in carbon containing refractories [23–25]. In contrast, many whiskers with nanosize in diameter and micrometer size in length formed in the specimens at 1200 ◦ C. According to XRD and EDS analysis, they were confirmed to be SiC whiskers (Fig. 5c). Interestingly, with further increasing the firing temperature to 1400 ◦ C, the amount of SiC whiskers decreased and their length got longer. This was different from the previous reports which suggested that the amount of SiC whiskers increased with increasing the firing temperature [22,26,27], probably due to the usage of different additives and different amount of carbon content in the materials. As well, short columnar-shaped substances formed and based on XRD pattern, only presence of Si, Al and O elements in the EDS analysis confirmed that these substances were mullite (Fig. 5d).

The results from XRD and SEM analysis suggested that, the phase compositions and microstructures were greatly affected by firing temperature and MWCNTs amount. For specimen N0, no new ceramic phases formed at 800 ◦ C. However, AlN and Al4 C3 largely formed at 1000 ◦ C by direct carbonization and nitriding reactions via Eqs. (1)–(3) (Fig. 5b). At 1200 ◦ C, AlN and Al4 C3 disappeared due to their transformation into Al2 O3 phase via Eqs. (4) and (5), as discussed in detail in the previous reports [2,20], while SiC whiskers in large quantities formed by reactions (6)–(10) (Fig. 5c). At 1400 ◦ C, the amount of SiC whiskers decreased while certain amount of mullite formed correspondingly by reactions (11)–(13). For specimens containing MWCNTs, it was deduced that MWCNTs could promote the formation of ceramic phases at a lower temperature, especially for SiC whiskers at 1000 ◦ C. Meanwhile, at 1200 ◦ C, the amount of SiC whiskers was much larger for specimens containing MWCNTs. The differences in phase compositions and microstructures of specimens with or without MWCNTs were attributed to the higher reactivity of MWCNTs compared with graphite flake. As shown in Fig. 8, it can be seen that, the exothermic oxidation peak was much sharper and the peak temperature was about 220 ◦ C lower compared MWCNTs with graphite flake, indicating the much higher reactivity of MWCNTs. Therefore, ceramic

Fig. 4. XRD patterns of specimens N0 and N1 fired at different temperatures. (a) Specimen N0 and (b) specimen N1. : Corundum, : ␤-SiC, 夽: critobalite, : Si, 䊉: Al4 C3 , #: AlN, : Al, : graphite, : mullite.

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Fig. 5. SEM micrographs of specimen N0 fired at different temperatures. (a) 800 ◦ C, (b) 1000 ◦ C, (c) 1200 ◦ C and (d) 1400 ◦ C.

phases more easily formed at a lower temperature and their amount was larger in the specimens containing MWCNTs. 4Al (l, g) + 3C (s) = Al4 C3 (s)

(1)

2Al (l, g) + N2 (g) = 2AlN (s)

(2)

8Al (l, g) + 6CO (g) = 2Al4 C3 (s) + 3O2 (g)

(3)

Al4 C3 (s) + 6CO (g) = 2Al2 O3 (s) + 9C (s)

(4)

2AlN (s) + 3CO (g) = Al2 O3 (s) + 3C (s) + N2 (g)

(5)

Si (s, g) + C (s) = SiC (s)

(6)

2Si (s, g) + O2 (g) = 2SiO (g)

(7)

Si (s, g) + SiO2 (s) = 2SiO (g)

(8)

SiO (g) + 2C (s) = SiC (s) + CO (g)

(9)

SiO (g) + 3CO (g) = SiC (s) + 2CO2 (g)

(10)

SiC (s) + 2CO (g) = SiO2 (s) + 3C (s)

(11)

SiC (s) + 2O2 (g) = SiO2 (s) + CO2 (g)

(12)

2SiO2 (s) + 3Al2 O3 (s) = Al6 Si2 O13 (s)

(13)

The difference in microstructures unquestionably leaded to different physical and mechanical properties of Al2 O3 -C refractories. At 800 ◦ C, compared with specimen N0, the lower apparent porosity and higher bulk density of specimens containing MWCNTs, especially for N0.05 or N0.1, were believed to result from MWCNTs that possessed a small particle size and high specific surface area, which could effectively fill in the pores of the materials as long as they were homogenously dispersed in the matrixes [28]. In addition, MWCNTs with strengthening and toughening mechanisms, including crack deflection, bridging, fracture and pull-out, strengthened the specimens, leading to the better mechanical properties of specimens [29–32]. At 1000 and 1200 ◦ C, the ceramic phases formed in the pores, which promoted the densification of the materials, leading to the decrease in apparent porosity and increase in bulk density. Meanwhile, addition of MWCNTs could supply higher amount and better interlocking

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Fig. 6. SEM micrographs of specimen N0.05 fired at different temperatures. (a) 800 ◦ C, (b) 1000 ◦ C, (c) 1200 ◦ C and (d) 1400 ◦ C.

morphologies of SiC whiskers, providing better mechanical properties of specimens containing MWCNTs. With the increase of firing temperature to 1400 ◦ C, the physical properties get deteriorated due to the volume expansion as discussed earlier. On the other hand, the decrease of SiC whiskers amount also caused great damage to the mechanical properties of the materials. Therefore, based on the results discussed above, it is obvious that 0.05 wt% MWCNTs was more suitable to optimize the microstructures and enhance the mechanical properties of Al2 O3 -C refractories. However, with the further increase of MWCNTs amount, the properties of the specimens got deteriorated due to the aggregation of MWCNTs and their influence on the morphology of ceramic phases. Theoretically, MWCNTs can considerably improve the mechanical properties of Al2 O3 -C refractories and it is also confirmed in this work. However, most or all of them were consumed and transformed into ceramic phases during the firing process, which was of great difference from ceramic–MWCNTs composites due to different compositions and sintering or application conditions. So how

to remain MWCNTs in the refractories is a great challenge deserved consideration. Morisada et al. reported that MWCNTs were coated with a thin SiC layer using SiO vapors in vacuum [33]. Then they introduced the coated MWCNTs into WC-10 wt.%Co composite, leading to a increase of microhardness by 14%, which was caused by the fact that the SiC coating prevented MWCNTs from reacting with the metal [34]. In our previous work, we also had successfully coated MWCNTs with SiCx Oy coating using polycarbosilane as the precursor to improve their oxidation resistance [35]. On the other hand, MWCNTs could easily tangle in the matrixes and cause damage to the properties of the materials. Many methods such as in situ chemical vapor deposition, surface modification, chemical precipitation were used to solve the problem of MWCNTs dispersion in the ceramic matrix composites [36–39]. However, related studies have rarely been reported in carbon containing refractories. Therefore, future work should be carried out by incorporating such coated MWCNTs and seeking the appropriate methods to solve the problem of transformation and dispersion of MWCNTs in carbon containing refractories.

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Fig. 7. SEM micrographs of specimen N1 fired at different temperatures. (a) 800 ◦ C, (b) 1000 ◦ C, (c) 1200 ◦ C and (d) 1400 ◦ C.

properties of Al2 O3 –C refractories fired from 800 to 1400 ◦ C in a coke powder bed.

Fig. 8. DSC curves of MWCNTs and graphite flake from room temperature to 1100 ◦ C at a heating rate of 10 ◦ C/min in air atmosphere.

5. Conclusions The following conclusions can be made on the basis of study of phase compositions, microstructures, physical and mechanical

(1) MWCNTs acting as a carbon source, had a higher reactivity than graphite flake and most of them were consumed during firing process. (2) The firing temperature and MWCNTs amount played a very important role in the microstructures of Al2 O3 -C refractories. At 800 ◦ C, no ceramic phases formed in all specimens. At 1000 ◦ C, specimens containing only graphite flake possessed AlN and Al4 C3 phases, while besides AlN and Al4 C3 , SiC whiskers also formed in the specimens containing MWCNTs. With the increase of firing temperature to 1200 ◦ C, larger amount of SiC whiskers formed in the specimens containing MWCNTs. The amount of SiC whiskers decreased and certain amount of mullite formed when the firing temperature was at 1400 ◦ C. (3) The mechanical properties of specimens were mainly influenced by the microstructures of the materials. The mechanical properties improvement of specimens containing MWCNTs were attributed to strengthening and toughening actions of MWCNTs at 800 ◦ C and their influence on the quantity and morphology of ceramic phases above 1000 ◦ C. However, the mechanical properties got deteriorated with the increase of

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MWCNTs amount from 0.1 to 1 wt% because of their agglomeration and the influence on the morphology of ceramic phases. Acknowledgements This work is financially supported by Natural Science Foundation of Hubei Province (2009CDA050 and 2008CDB258), the New Century Excellent Talents in University (NCET-10-0137) and Natural Science Foundation of China (51072143 and 51002108). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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