50˚C showing more carbon nano tubes with some carbon nano beads.

cm–1 indicating defect in graphitic carbon. Further Gband at 1587 cm–1 is the characteristic peak for graphitic carbon. It is interesting to note that D-band obtained with carbon produced either at 750˚C or 850˚C are almost the same (~1332 - 1335 cm–1), which appears at the expected peak of carbon containing defects (1332 cm–1). However, the G-band peaks obtained with carbon produced at 750˚C is at 1511 cm–1 while for carbon produced at 850˚C is 1587 cm–1. This suggests that while the defect present in both forms of these carbons are almost the same type but graphitization has taken place more at higher temperature.

SEM micrographs of CNM obtained at 550˚C - 750˚C, and those obtained in the temperature range of 850˚C - 1050˚C are shown in Tables 3 and 4 along with their TGA spectra. It interesting to observe that CNM obtained at lower temperature range i.e. 550˚C - 750˚C, most of

Figure 3. TEM micrographs of CNT (obtained by pyrolyzing HDPE in hydrogen atmosphere for 2 h at 1050˚C showing coiled multi walled carbon nano tubes.

(a) (b)

Figure 4. (a) XRD of the CNM produced by HDPE at 750˚C shows peak at 26.35 (002) and 43.56 which are characteristic peak of the graphitic material; (b) Micro Raman spectra of CNM obtained in the temperature range 550˚C -750˚C.

(a) (b)

Figure 5. (a) Graph showing the variation of signal to noise ratio for each level of corresponding parameters. Mean value of signal to noise ratio has been taken as zero and calculation is done with “larger the better” (850˚C - 1050˚C); (b) Showing the % impact of each parameters.

(a) (b)

Figure 6. (a) XRD analysis of the un-purified CNM from LDPE at 850˚C showing characteristic peaks of the graphitic material; (b) Micro Raman of CNM obtained from 850˚C - 1050˚C showing D-band at 1332 cm–1, indicating defect in graphitic carbon.

them are carbon beads with very small amount of carbon nanotubes. But at higher temperature beads seems to have elongated to CNTs among with some remnant carbon nano beads, hence there is a large percentage of carbon nanotubes and very few carbon nano beads. Taguchi optimization methodology has also suggested that yield of carbon nano tubes is affected more by the temperature (Figure 1(b)) or duration of exposure to temperature (Figure 5(b)) during of pyrolysis; as also noted earlier with other precursors [8]. The SEM micrographs of Figure 2 suggest that carbon nano beads and carbon nano tubes are obtained when HDPE is pyrolyzed at 750˚C and 1050˚C respectively.

TGA Results of the derivatives of TGA also showed that CNM obtained at 550˚C - 750˚C, decomposed in the temperature range of 559˚C - 648˚C which is an indication that these materials are not amorphous carbon (Table 3). Derivatives of TGA of CNM obtained at 850˚C - 1050˚C shows that these materials decomposes at 624˚C - 668˚C; suggesting that CNT synthesized at this temperature range is more crystalline than what was obtained at the 550˚C - 750˚C (Tables 3 and 4). Raman spectra also support this observation.

4. Conclusion

Virgin polymers (LDPE, HDPE and LLDPE) on pyrolysis using CVD system can be converted into MWCNT or CNB depending on the temperature range used. Taguchi Optimization methodology helped in selecting the best suitable parameters for high yield of desired CNM. Hydrogen atmosphere and a duration of 2 h pyrolysis was found to be the best condition for both low (550˚C - 750˚C) temperature range that produced CNB; as well as high (850˚C - 1050˚C) temperature range, which produced more MWCNT. Various characterization data viz. SEM, TEM, Micro RAMAN & TGA; all supported the same findings that CNM synthesized were graphitic, crystalline material having MWCNT or CNB morphology. Temperature higher than 600˚C helped in getting rid of amorphous carbon from the CNM produced.

Table 3. CNM Taguchi method (550˚C - 750˚C) analysis by SEM and TGA.

Table 4. CNM Taguchi method (850˚C - 1050˚C) analysis by SEM and TGA.

5. Acknowledgements

Authors acknowledge UGC, New Delhi for financial support to carryout this project.

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NOTES

*Corresponding author.

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