Nano-structured Silicon Carbide And Carbon Materials: Preparation And Properties

In this dissertation, the structures, properties, applications and fabrication methods are summarized at first; the research developments of nano-structured SiC and carbon materials are mainly discussed. After investigating the synthesis routes and applications of SiC and carbon materials thoroughly, we prepared crystalline cubic SiC nanowires via a sulfur-assisted approach at low temperature; the growth mechanism of the nanowires accord with the vapor-liquid-solid (VLS) growth process. Cubic SiC hexagonal platelets and nanowires were also fabricated by waste plastics and Si powder via sulfur-assisted; this method could make the best of the waste plastics and provide a possible way to solve the plastic pollution. In addition, carbon materials with various morphologies were prepared on the copper and magnesium substrate by pyrolysis of dichloromethane and ferrocene in stainless steel autoclave. In this process, the morphology was controlled to transform from 1-D nanowires to 2-D leaf-like carbon sheets then to 3-D nanospheres and lilac shaped carbon-coated iron oxide hierarchical structures. The dissertation is summarized as follows:1. Highly crystalline 3C-SiC nanowires were fabricated in an autoclave at 130℃by silicon powder as Si source, tetrachlorethylene as carbon source, metallic Na as reductant and sulfur powder as assistant agent. The average diameter of the 3C-SiC nanowires was about 30 nm and the length was up to tens of micrometers. High-resolution TEM photos indicate that preferential growth of the nanowires was along [111] crystal direction. The cubic SiC nanowires with the length of 40-120 nm could also be prepared at 270℃ without sulfur powder additive; however, the ratio of the nanowires was much lower than the typical reaction with sulfur powder. Increasing the reaction time could improve the yield of the nanowires in a certain extent; excessively high temperature could not enhance the content of the nanowires, and the size distribution would broaden instead; either increasing or decreasing the dosage of sulfur powder would reduce the yield of SiC nanowires. In addition, based on the experiment results and related reports, we can draw a conclusion that the possible growth mechanism of SiC nanowires was vapor-liquid-solid(VLS) growth process.2. The cubic SiC nanomaterials were synthesized in the autoclave at 350-500℃by silicon powder and waste plastics as reagents, metallic Na and Mg powder as reductants, sulfur powder as assistant agent. When the waste plastic was high-density polyethylene(HDPE:detergent bottles or beverage bottles), hexagonal platelets of 3C-SiC (about 40%) could be observed; the length of SiC hexagonal platelet-like crystals was 300-500 nm, and the thickness of these platelets was about 50 nm. If the low-density polyethylene (all kinds of waste plastic bags, agricultural film, etc.) was applied as carbon source instead,3C-SiC nanowires (about 35%) were found in the final product; the average diameter of the nanowires was about 50nm and the length was up to tens of micrometers. When the waste plastic was polyethylene terephthalate(PET:beverage, food and other liquid containers), most of the product were two-dimensional crapy lamellas with the thickness of about 100 nm and about 20% hexagonal platelet-like crystals were observed. When the carbon source was HDPE and LDPE, the as-obtained SiC was cubic; however, when the waste plastic was PET, the as-obtained SiC was a mixture of cubic and hexagonal with the phenomenon of stacking fault. Based on the observation of the morphologies of SiC fabricated in different time by HDPE, we supposed that the growth process of cubic SiC hexagonal platelets was self-assembled oriented aggregation mechanism.3. Leaf-like carbon sheets and carbon nanowires array were synthesized through a pyrolysis route on copper and magnesium substrates by ferrocene as catalyzer and dichloromethane as carbon source; the thickness of the 2-D nanosheets could be controlled from 50 nm to 200 nm by adjusting the experimental parameters. When the carbon source was replaced by other halogenated alkanes, monodispersed or chained-like solid carbon spheres could be obtained; if methanol or ethanol was applied instead of dichloromethane and the dosage of ferrocene was increased appropriately, the cactus-like and lilac shaped carbon-coated iron oxide hierarchical structure material could be obtained. We could easily achieved the controlled synthesis from 1-D nanowires to 2-D leaf-like carbon sheets then to 3-D nanospheres and lilac shaped carbon-coated iron oxide hierarchical structures by choosing different reaction system. The growth process of the 2D carbon materials was carefully observed by SEM, and it is considered that Diffusion-Limited-Aggregation mechanism could be responsible for the growth.In addition, we cut the copper attached by carbon materials into desired electrode shape, applied lithium metal piece as the counter electrode, assembled into a simulation lithium-ion battery. The voltage range was the 0.01-3 V, and current density was 100 mA /g for the charge and discharge test. The first discharge capacity was 730 mAh/g, while the charge capacity can be to 321 mAh/g. After several charge and discharge cycles, its specific capacity stabilized at 340 mAh/g or more; when the current density was 300 mA/g, after 50 cycles, its capacity also stabilized at about 220 mAh/g.