X-ray Diffraction and Raman Spectroscopy Characterization of Isotropic Pyrocarbon Obtained by Hot Wall Chemical Vapor Deposition

Varying the flow rate of natural gas from 50 to 80 to 120 l/h, isotropic pyrocarbon produced by hot wall chemical vapor deposition at 1000 ℃ were examined by X-ray diffraction and Raman spectroscopy. The X-ray data were evaluated by Scherrer equation, and the intensity ratio of D to G band derived from Raman data was used to evaluate the lateral extension of isotropic pyrocarbon. The experimental results show that the d002-spacing of isotropic pyroearbon decreases from 0.3499 nm to 0.3451 nm, while the stack height increases from 6.5 to 8.4 nm with the increase of flow rate of natural gas. The intensity ratio of D to G band and lateral extension of isotropie pyrocarbon increases with natural gas flow rate increasing. After heat treatment, all the crystallite parameters （stack height, lateral extension, and d002-spacing） decrease, indicating the improvement of the arrangement of the basic structural units of isotropic pyrocarbon.

Fabrication of Isotropic Pyrocarbon at 1400 ℃ by Thermal Gradient Chemical Vapor Deposition Apparatus

An experiment was designed to prepare isotropic pyrocarbon by thermal gradient chemical vapor deposition apparatus. The deposition was performed under ambient atmosphere at 1400 ℃, with natural gas volume flow of 3.5 m^3/h for 80 h. The results show that the thickness and the bulk density of the deposit are about 1.95 g/cm^3 and 10 mm, respectively. The microstructure of the deposit was examined by polarized light microscopy and scanning electron microscopy, which shows that the deposit is constituted of sphere isotropic pyrocarbon, pebble pyrocarbon and laminar pyrocarbon.

Micro-and Nano-structure Characterization of Isotropic Pyrocarbon Obtained via Chemical Vapor Deposition in Hot Wall Reactor

The hot wall reactor was used to deposit isotropic pyrocarbon. The density of the product was measured by Archimedes method. The fracture morphology and texture of the product were investigated by scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. Raman spectroscopy was also applied to characterizing the structure of the product. The results show that the density of the product is about 1.31 g/cm^3. The pyrocarbon consists of granular particles with the size of about 1.5 μm under SEM images. Under TEM images, the pyrocarbon is composed of irregular grains and onion-like particles are also detected. The orientation angle （180 deg.） of pyrocarbon reveals its isotropic character. The ID/IG ratio in Raman spectrum is about 1.2, indicating the isotropic feature of the product qualitatively.

The Influence of Deposition Parameters on the Microstructure and Deposition Mechanism of Low-temperature Isotropic Pyrocarbon on a Bed of Fluidized Particles

To investigate the microstructure and deposition mechanism of low-temperature isotropic pyrocarbon(LTIC),chemical vapour deposition was conducted in a steady-state fluidized bed using different propane concentrations and deposition temperatures.The microstructure of LTIC obtained at different deposition conditions was characterized using scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results show that LTIC is composed of both globular-like and laminar structures.Increasing the deposition temperature is propitious for decreasing the nucleation barrier-forming pyrocarbon in vapour,causing the quantity of spherical particles with a smaller diameter to increase and the texture of the outer layer around carbon blacks inside the spherical particles to decrease gradually.Increasing the propane concentration also made the globular-like fracture morphology more obvious and caused the laminar structure to gradually disappear.Pyrocarbon formation is dominated by a surficial growth mechanism at lower propane concentrations,while gaseous nucleation mechanism is more dominant at higher propane concentrations.