# Carbon Microstructures Synthesis in Low Temperature Plasma Generated by Microdischarges
## 3 Results
### Carbon Deposit  
Various forms of carbon deposit were obtained on the needle and plate electrodes as an effect of hydrocarbon decomposition in electric-discharge plasma. The morphology and growth rate of these deposits depended on the discharge current, the distance between the electrodes and the concentration of cyclohexane in the gas mixture. In the case of onset streamers and glow corona discharges, the deposit obtained on the needle was in the form of irregular structures (Figure 15). The plate was coated by oiled substances during these irregular discharges, and some small carbon deposits in the form of a mound can also be obtained (photograph not shown). These kinds of structures were observed for an inter-electrode distance in the range of 2.55 to 25 mm. Corona discharge is characteristic for a discharge where one of the electrodes is of high curvature. The magnitude of the electric field close to the electrode tip is large enough to lead to collisional ionization processes. Primary electrons released as a result of collisional ionization and secondary electrons released from the electrode by the bombardment of its  
Figure 14: Temperature of needle deposit for positive polarity for 1 and 5% cyclohexane, and for negative polarity for 5% cyclohexane. Figure 15: Deposit formed during glow discharge for discharge current of 0.1 mA, in a mixture of Ar+C\({}_{6}\)H\({}_{12}\) (95:5 vol.). surface by gaseous ions cause avalanche ionization in the presence of a strong electric field. These processes only occur in a thin layer near the electrode because there is a sufficiently strong electric field in this region (so-called ionization zone). The electric field in this area is of the order of magnitude of 106 V/m. With such a high electric field, the electrons present in this region are accelerated to a high kinetic energy sufficient for collisional ionization. The main building blocks of carbon deposits obtained in the electric discharge microplasma in a mixture of argon and cyclohexane are radicals and neutral products of the decomposition of cyclohexane. These products are obtained due to the collision of electrons of sufficiently high energy with cyclohexane molecules; one example is the following:  
\[\mathrm{c-C_{6}H_{12}+e\to R_{m}+R_{n}+e,} \tag{11}\]  
where \(R_{m}\) and \(R_{n}\) are radicals. A similar situation occurs when an argon atom is in the first excited state (metastable state) after collision with an energetic electron and transfers its internal energy to a cyclohexane molecule after an impact, as follows:  
\[\mathrm{c-C_{6}H_{12}+Ar_{m}\to Ar(^{1}S_{0})+R_{m}+R_{n}.} \tag{12}\]  
As the energies of argon metastable states are relatively high, 11.55 eV and 11.72 eV, the cyclohexane and all products of cyclohexane decomposition can be ionized or dissociated to other products after collision. Radicals could interact with each other forming neutral molecules and/or other new radicals. The formation of an \"oil\" form of deposit remaining on the plate electrode can proceed due to the interaction of radicals in the following reaction:  
\[R_{x}+R_{y}\to P_{x+y}, \tag{13}\]  
where \(P_{x+y}\) is a neutral product (hydrocarbon) of higher molecular weight than cyclohexane. The gas temperature in this zone is low, and the interaction with charged or high energetic particles is weak. In the case of glow discharge, the interaction of atoms and molecules with electrons and charged particles occurs for all inter-electrode distances. Figure 16 shows typical SEM images of carbon deposits grown under different cyclohexane concentrations by a discharge current of 1.8 mA. The change of the cyclohexane concentration influenced the morphology of carbon fibers. For 1% cyclohexane, the surface of the deposited carbon fibers was grainy, with large spaces between the grains (Figure 16a). With an increasing concentration of cyclohexane, the edge of the grains appeared smoother and the space between the grains became much smaller than those deposited under a lower concentration of cyclohexane (Figure 16b).