Carbon nanomaterials were synthesized in situ on bulk 316L stainless steel, pure cobalt, and pure nickel by hybrid surface mechanical attrition treatment (SMAT). The microstructures of the treated samples and the resulted carbon nanomaterials were investigated by SEM and TEM characterizations. Different substrates resulted in different morphologies of products. The diameter of carbon nanomaterials is related to the size of the nanograins on the surface layer of substrates. The possible growth mechanism was discussed. Effects of the main parameters of the synthesis, including the carbon source and gas reactant composition, hydrogen, and the reaction temperature, were studied. Using hybrid SMAT is proved to be an effective way to synthesize carbon nanomaterials in situ on surfaces of metallic materials.
Carbon nanofibers (CNFs) and nanotubes (CNTs) have attracted extensive attention in the scientific field because of their remarkable properties [
Up to now, several methods have been studied to synthesize CNFs/CNTs on various substrates. In many cases, the hydrocarbons decomposed on dispersed catalytic metal particles on a support [
Surface mechanical attrition treatment (SMAT) has been proved to be an effective way to achieve surface nanocrystallization on various metallic materials [
Samples were stainless steel (AISI 316L), pure cobalt (purity 99.9%), and pure nickel (purity 99.9%) plates. The plates were 20 mm in diameter and 1 mm in thickness and were firstly subjected to the SMAT process. Details of SMAT can be found in the previous work [
Samples treated by SMAT (SMAT samples) were then subjected to a CVD process using a furnace with a quartz tube. The typical parameters were as follows. The reaction was maintained in 550°C for 30 min. The reactant gas composition is C2H2 : H2 = 50 SCCM (mL/min): 100 SCCM in a N2 carrier of 300 SCCM. The main parameters of CVD, including carbon source, gas reactant composition, and temperature, were studied.
Transmission electron microscopy (TEM) observations were performed to characterize SMAT samples and the resulted carbon products using a Philips CM30 microscope working under 300 kV accelerated voltage.
Scanning electron microscopy (SEM) was also used to investigate the microstructures of the carbon products, using a Sirion, FEI working at 3 kV accelerated voltage.
TEM characterizations indicate the microstructural features of the very top surface of SMAT transition metallic materials, as can be seen in Figure
Bright-field (a, c, e) and dark-field (b, d, f) TEM images showing typical microstructures of the top surface layer in (a, b) SMAT 316L stainless steel; (c, d) SMAT Co; and (e, f) SMAT Ni. The insets are the corresponding SAED patterns.
Although the mechanisms of the nanocrystallization of three metallic materials are not the same [
After CVD process, thin black films were fabricated on SMAT 316L stainless steel and SMAT Co, while soft black products with a certain thickness were synthesized on SMAT Ni. No product was found in the untreated zone of samples. The carbon deposit yields were measured experimentally. SMAT Ni has the largest carbon deposit yield, up to 62.29 mg/cm2, followed in order by SMAT Co (16.4 mg/cm2) and SMAT 316L stainless steel (9.26 mg/cm2).
We compared the morphologies of the carbon nanomaterials synthesized on different SMAT metals. Figure
Comparison of the morphology of carbon products synthesized on different SMAT metals: (a) on SMAT 316L, ×5 K; (b) on 316L, ×20 K; (c) on 316L, ×90 K; (d) on SMAT Co, ×5 K; (e) on SMAT Co, ×20 K; (f) on SMAT Co, ×90 K; (g) on SMAT Ni, ×5 K; (h) on SMAT Ni, ×20 K; and (i) on SMAT Ni, ×90 K.
Besides the dimension, the morphologies are totally different. CNFs synthesized on SMAT 316L stainless steel are neither uniform nor dense, distributed among the metal clusters, with a broad diameter distribution ranging from tens of nanometers to several micrometers. Some of them appear to undergo a further partial and simultaneous change in shape, rotating on an axis perpendicular to the direction of fiber growth, and thus forcing the filament to a helical form (Figure
TEM characterizations were performed to establish the nature of the fibers observed in SEM. Figure
TEM images of the CNFs synthesized on different SMAT metals (a) on SMAT 316L stainless steel; (c) on SMAT Co; (e) on SMAT Ni and the related HREM images of the CNFs synthesized (b) on SMAT 316L stainless steel; (d) on SMAT Co; (f) on SMAT Ni.
As shown in Figure
TEM images of carbon products synthesized on SMAT Ni (a) image at low magnification showing CNTs as well as CNFs; (b) image at high magnification showing details of CNTs; and (c) image showing the presence of the catalyst particles as well as CNFs.
The curly fibers are entangled with each other. The inset image is the corresponding SAED pattern. The higher magnification (Figure
Corresponding to the grain size distributions of SMAT transition metals, the diameter distributions of CNFs are illustrated in Figure
Average grain size of different SMAT metals and the average diameters of corresponding CNFs.
Here we discuss a possible mechanism of the CNFs growth on the surface of SMAT metallic materials. The catalyst particles wrapped with graphite (Figure
Using hybrid SMAT is proved to be an effective way to synthesize carbon nanomaterials in situ on surfaces of metallic materials. However, the whole process and mechanism are rather complicated. Further studies are needed for understanding the principles so that we could control the diameter and the morphology of the CNFs by changing the parameters of hybrid SMAT process.
Effects of the main parameters of the synthesis, including the carbon source, gas reactant composition, hydrogen, and the reaction temperature, were studied using SMAT Co. Similar phenomena were found in SMAT Ni and SMAT 316L stainless steel.
Carbon source is considered to contribute to the special structure and the yield. A series of experiments has been done on SMAT Co using CH4 and C2H2 as carbon source.
When using CH4 as carbon source, no evidence for the formation of filamentous carbon was observed upon SMAT Co in the temperature range 550–900°C, regardless of the CH4/H2/N2 ratio. This phenomenon can be explained; that is, CH4 is a most kinetically stable hydrocarbon and is hard to pyrolyze. CH4 does not decompose at lower temperature (e.g., 550°C), while at higher temperature (e.g., 900°C), the nanocrystalline grains on the SMAT metal surface grow up and exhibit no catalytic activity. On the other hand, C2H2 is much easier to decompose at lower temperature and therefore can be regarded as good carbon source in this work.
Synthesis of carbon nanostructures is dependent on the reactant gas composition. The effect of the reactant composition on the carbon product is listed in Table
Carbon products related to different gas composition.
Gas composition |
Related products |
---|---|
30 : 100 : 300 | Medium yield of CNFs |
50 : 100 : 300 | Abundant CNFs |
50 : 50 : 300 | CNFs with amorphous carbon |
100 : 100 : 300 | CNFs with encapsulated metal particles |
300 : 300 : 300 | Encapsulated metal particles with few CNFs |
Hydrogen is also proved to be critical in creating CNFs/CNTs. The influence of absence of H2 is evident. Without H2, the growth becomes sparse and irregular structures are prominent. However, extra H2 is proved similarly detrimental towards CNFs/CNTs growth. Figure
SEM images of the carbon products synthesized on SMAT Co after CVD process (a) without hydrogen and (b) with hydrogen.
As indicated in the literature [
The reaction temperature is proved to be a crucial parameter for the synthesis of CNFs/CNTs by CVD method. Since the temperature affects the activity of the catalytic metallic materials and the decomposition of the carbon source gases, the microstructure and the yield of CNFs/CNTs vary with the growth temperature [
Carbon nanomaterials have been successfully synthesized in situ on various bulk metallic materials by hybrid SMAT. CNFs were formed on SMAT 316L stainless steel and SMAT Co while CNFs and CNTs were formed on SMAT Ni. SMAT Ni is the most active one compared with SMAT Co and SMAT 316L stainless steel and has the largest carbon deposit yield. Different SMAT metals resulted in different morphologies of products. The diameter of CNFs is related to the size of the nanograins on the surface layer of SMAT metals. The possible growth mechanism was discussed. The atomic diffusivities were greatly enhanced after SMAT and the nanocrystalline grains act as catalyst particles, being nuclei for the carbon nanomaterials. Effects of the main parameters including the carbon sources, gas reactant compositions, hydrogen, and reactant temperature were also investigated. The optimized gas reactant composition is C2H2 : H2 : N2 = 50 SCCM : 100 SCCM : 300 SCCM. The appropriate reaction temperature is 550°C.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The work was supported by National Natural Science Foundation of China (Grant no. 51201190), Natural Science Foundation of Chongqing, China (CSTC. 2011BB4080), and Fundamental Research Funds for the Central Universities (no. CDJZR12130043). The authors also gratefully acknowledge sharing fund of Chongqing University’s large-scale equipment (no. 2012121519).