Jin et al. Nanoscale Research Letters 2011, 6:515 http://www.nanoscalereslett.com/content/6/1/515 particles during the SHS in the Me (Cu, Al, and Fe)-Ti- CNT systems with the high contents of the Me incor- poration. The morphologies of the TiCx particles formed in these systems were investigated, and the mechanism for the difference in their morphology was discussed. Experimental methods The raw materials utilized were multi-walled carbon nano- tubes (20 to 30 nm in diameter and approximately 30 μm in length, purity > 95 wt.%, Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences, Chengdu, China), Ti powders (> 99.5% purity, approximately 48 μm, Insti- tute of Nonferrous Metals, Beijing, China), Al powders (> 99.0% purity, approximately 48 μm, Northeast Light Alloy Ltd. Co., Harbin, China), Cu powders (> 99.5% pur- ity, approximately 48 μm, Institute of Nonferrous Metals, Beijing, China) and Fe powders (> 99.5% purity, approxi- mately 48 μm, Institute of Nonferrous Metals, Beijing, China). The Ti and CNT powders with a molar ratio of 1:1 were mixed with the Me (Cu, Al, and Fe) powders in relative quantities of 50, 60, 70, and 80 wt.%, respectively. The reactants were mixed sufficiently by ball milling at a low speed (approximately 35 rpm) for 6 h and then pressed into the cylindrical compacts of approximately 22 mm in diameter and approximately 15 mm in height with green densities of approximately 60 ± 2% of theoreti- cal. The SHS experiments were conducted in a self-made vacuum vessel in an Ar atmosphere using an arc as igni- tion source. During the SHS process, the temperature in the position about 3 mm beneath the center of the com- pact top surface was measured by W5-Re26 thermocou- ples, and the signals were recorded and processed by a data acquisition system using an acquisition speed of 50 ms per point. The phase compositions in the reacted samples were identified by X-ray diffraction (XRD, Rigaku D/Max 2500PC, Rigaku Corporation, Tokyo, Japan) with CuKa radiation using a scanning speed of 4°/min. The reacted Cu-Ti-CNT samples were then dissolved in a saturated FeCl3 water solution, and the reacted Al- and Fe-Ti-CNT samples were dissolved in an 18 vol.% HCl-distilled water solution, to remove the Me coatings on the surfaces of the TiCx particles. The morphologies of the extracted TiCx particles were observed using a field emission scan- ning electron microscope (FESEM, JSM 6700F, JEOL, Tokyo, Japan) and a transmission electron microscope (TEM, JSM 200EX, JEOL). Results and discussion In the Me-Ti-C systems, the Me-Ti liquid forms firstly during the heating. The carbon then diffuses into the Me-Ti liquid, and when a critical concentration is achieved, the TiCx begins to form by reaction between [C] and [Ti]. Accordingly, the diffusion of carbon in the Page 2 of 7 molten metals is a key step to form TiCx, and thus differ- ent carbon sources, i.e., graphite and C black, have great effects on the product morphology and the reaction rate of [Ti] and [C] to form TiCx. Generally speaking, the car- bon source with finer sizes will make the combustion reaction proceed more thoroughly. For example, when C black was used as the carbon source in 50 wt.% Al-Ti-C system, the content of the intermediate phase Al3Ti decreases greatly than that of the graphite being used as the carbon source (Figure 1a). In contrast to the graphite and C black, carbon nano-tube (CNT) has much finer sizes. Furthermore, the defects such as pentagons, hepta- gons and vacancies in the structure of the CNT endow it with more chemical activity [19,20]. Therefore, the CNT will dissolve more rapidly in the liquid Me to provide dis- sociated [C], which promotes the SHS reaction. This speculation was proved as there is no Al3Ti formed in the 50 wt.% Al-Ti-CNT system. Actually, only when the Al content was increased to 80 wt.% in the Al-Ti-CNT system, a little amount of Al3Ti formed. In Cu- and Fe- Ti-CNT systems, within the range of 50 to 80 wt.% for the Me content, no Al3Ti is formed. As known, according to Merzhanov’s empirical criter- ion, for the reaction to be self-sustaining in the absence of preheat, the adiabatic temperature (Tad) should not be less than 1,800 K, corresponding to the maximum addi- tion of 67.12 wt.% Cu, 46.65 wt.% Al [16], and 77.4 wt.% Fe [21] in the Me-Ti-C systems, respectively. However, in our experiments, because of the high activity of the CNT, the samples with 70 wt.% Al and 80 wt.% Cu and Fe can be ignited easily. Figure 1b shows the variation in the maximum combustion temperature with the Me content. Clearly, the maximum combustion temperature in all the systems decreases as the Me content increases, and the sequence is TCu-Ti-CNT >TFe-Ti-CNT >TAl-Ti-CNT. The dif- ference in the combustion temperature in these systems, of course, will have an important influence on the shape and size of the synthesized TiCx particles. As indicated in Figure 2, with increasing the Me content, the TiCx particles formed in the Cu-, Al-, and Fe-Ti-CNT systems show a significant decrease in size. In the sample with 50 wt.% Cu, the sizes of the TiCx particles are about 600 nm (Figure 2a), while when the Cu content increases to 60, 70, and 80 wt.%, the sizes of the TiCx particles decrease to about 400, 100, and 60 nm, respectively (Figure 2b, d, f). Accompanying the decrease in the parti- cle size, the TiCx particles change their shapes from sphere-like to regular octahedron (Figure 2c, e). The same growth shape as octahedron can be also observed in the TiCx particles formed in the samples with 50, 60, and 70 wt.% Al (Figure 2g, h, j), of which the particle sizes are about 200, 150, and 70 nm, respectively. When the Al con- tent is increased to 80 wt.%, the shape of the TiCx particles cannot be observed clearly, and the particle size decreases