Jin et al. Nanoscale Research Letters 2011, 6:515 http://www.nanoscalereslett.com/content/6/1/515 Page 5 of 7 Figure 4 TEM images of the TiCx particles formed in the Me-Ti-CNT samples. (a) 80 wt.% Cu, (b) 80 wt.% Al, and (c) 80 wt.% Fe. Inset images show the corresponding diffraction rings. stoichiometry increases, the free energy of the (111) sur- faces increases, which leads to the diminishing in the (111) surfaces on the TiCx crystals and the exposure of the (100) surfaces. According to this speculation, the stoichiometry of the TiCx crystals formed in the Cu- and Al-Ti-CNT systems should be low and that in the Fe-Ti-CNT system should be high. Here, we qualita- tively estimate the stoichiometry of the TiCx formed in the combustion stage based on the phenomenon that the TiCx particles grown in the Fe-Ti-CNT samples are the largest while their combustion temperatures are relatively low. As known, carbon has good chemical affi- nity with Fe. Hence, the carbon atoms could dissolve rapidly in the Fe melt, which leads to the formation of the C-rich regions near the CNTs at the initial stage of the SHS. In these C-rich regions, the TiCx particles form and grow rapidly. That is why the sizes of the TiCx particles formed in the Fe-Ti-CNT system are gen- erally large even though their combustion temperatures are quite low. As another consequence of the high C concentration, the stoichiometry of these primitively formed TiCx particles in the Fe melt is relatively high. Then, the (100) surfaces of TiCx are stable and the growth shape is cube. For the Cu- and Al-Ti-CNT sys- tems, the CNT dissolves more slowly because of the poor chemical reactivity between carbon and the Cu (or Al) melt as well as very limited solubility of carbon in molten Cu and Al. In this case, the TiCx forms and grows under a condition of C scarcity. Hence, the TiCx particles grown in these two melts are with relatively