# Carbon Microstructures Synthesis in Low Temperature Plasma Generated by Microdischarges
## 3 Results
### Carbon Deposit
For a 3% cyclohexane concentration (Figure 16c), the structure of the carbon deposit has a tendency to become hybrid: one part of the deposit was grainy, and another part was smooth. For a 4 and 5% cyclohexane concentration, the surface of the carbon fiber was smooth (Figure 16d,e). For these concentrations of cyclohexane, soot particles were often stacked to the surface of the carbon deposit. The anodic carbon deposits have a diameter in the range of 30 to 100 \(\upmu\)m, depending on the diameter of the plasma column. The growth rate of the carbon deposit was dependent on the discharge current (i.e., the power density delivered to the plasma column for cyclohexane decomposition) and the cyclohexane concentration. The volume growth rate of the carbon fibers deposited on the needle tip is shown in Figure 17. The growth rate for 1% cyclohexane was 3-4 times smaller than for 3 and 5% cyclohexane. For higher cyclohexane concentrations, the growth rate was similar, and was nearly a linear function of the discharge current. Figure 16: SEM pictures of carbon fibers obtained during corona discharge for a constant discharge current (1.8 mA) but for different C\({}_{6}\)H\({}_{12}\) concentrations: (**a**) 1%, (**b**) 2%, (**c**) 3%, (**d**) 4%, (**e**) 5% by positive polarity of the needle electrode. Electrode distance: 15 mm. SEM images of the carbon deposit obtained on the cathode (needle electrode) during a negative polarity discharge in a mixture of argon Ar+C\({}_{6}\)H\({}_{12}\) (95:5), by a discharge current of 1.8 mA are shown in Figure 18. For a negative polarity, the surface of the carbon deposit consisted of nodular dendrites, which have a diameter in the range of 5 to 10 \(\upmu\)m, with large gaps between them. At a higher magnification, it is visible that each dendrite has cauliflower structures and consists of smaller nodules. Fragments of broken carbon deposits taken from the needle at a positive and negative polarity are shown in Figure 19. The deposits synthesized for low hydrocarbon concentrations and for a positive polarity of the discharge electrode are similar to those obtained for a negative polarity for 5% cyclohexane. In both cases, the carbon deposits have a columnar morphology with nodular dendrites, and the growth process is determined by the carbon particles' diffusion toward the electrodes (Peclet number >>1). [PERSON] et al. [42] suggested that the columnar morphology with voids between the column results from the process of columnar growth that occurs when the temperature of the deposit's surface is low and the energy of the ions bombarding the anodic deposit is low. The temperature determined from the thermal spectrum was about 1300 K, for the positive and negative polarities of the discharge electrode. The morphology obtained for a negative polarity, by 5% cyclohexane, could result from the diffusion-limited aggregation by low temperatures, due to the reduced surface mobility of adatoms and the high sticking probability of radicals and ions [43; 44]. When a needle is used as a cathode, the main builders of the deposit  
Figure 17: Growth rate of carbon deposit synthesized on the needle tip during electrical discharges versus discharge current, for three different cyclohexane concentrations: 1, 3 and 5%. Figure 15:0 SEM pictures of cathodic carbon deposit obtained during discharge in a mixture of argon Ar+C\({}_{6}\)H\({}_{12}\) (95:5) for negative polarity of the discharge electrode, with a discharge current of 1.8 mA. could be positive ions, the main products of cyclohexane dissociation [45]. Additionally, the cathode sheath could influence morphology (the voltage drop of a cathode should be higher than for an anode). The carbon deposit obtained for 5% cyclohexane is synthesized layer-by-layer with smooth morphology. This structure could be named a carbon fiber according to IUPAC nomenclature [46].