Study Of Nanocrystalline Diamond And N-Diamond From Fe Catalyzed Carbon Black Or Carbon Nanotubes

The microstructure, mechanical, and electrical properties of nanocrystalline diamond offer many opportunities for application, therefor the study of nanocrystalline diamond was the research forcus recently. Whereas the synthesis of nanocrystalline diamond was very difficult, the yield was limited. In order to understand formation mechanisms of diamond, a unique procedure was designed by Hirai and Kondo in 1991, and found a new kind of possible allotrope of carbon. In their original paper, this new phase was referred to as "n-diamond" (new-diamond). For 77-diamond, despite the successful synthesis of n-diamond using different approaches, the crystal structure and properties of n-diamond is still unclear yet, mainly due to the small amount of n-diamond samples and the tiny crystalline size (typically less than 100 nm) in previous experiments. Hence, only transmission microscopy (TEM) and electron diffraction were feasible for structural analysis of n-diamond in the previous experiments. For theoretical part of nanocrystalline carbon particles, the relative stability of different forms of carbon at nanoscale is the key point to understand the formation mechanism of carbon phase at nanoscale, and then there are many insufficiencies as a result of calculational condition now.In order to solve the aboved-mentioned problems for nanocrystalline diamond and n-diamond, a method to synthesize nanocrystalline diamond and n-diamond by nano-sized Fe catalyzed carbon black or carbon nanotubes was suggested in this paper. Based on the larger output of n-diamond powders from the method of catalyzed carbon black, the formation mechanism of nanocrystalline diamond and n-diamond from catalyzed carbon black, and the crystal structure of n-diamond were studied. In addition, the effect of size and shape of nanocrystalline diamond on the stability, electron structure and phonon structure of nanocrystalline diamond were studied by the first principle. Finally, the electromagnetic wave absorption properties of carbon powder from catalyzed carbon black were studied by experiments and theory.With carbon black as carbon source and nano-sized Fe as catalyst, nanocrystalline diamond and n-diamond particles have been synthesized at atmospheric pressure and at 1100 ℃. X-Ray diffraction (XRD), Raman, TEM, and electron-probe microanalyzer (EPMA) were emploied to analyze the samples. The results indicated that the final product consists of two types of particles: spherical nanocrystalline diamond and graphite coated n-diamond particles,and the average size of the nanocrystalline diamond is about 20nm, and that of the graphite-coated ^-diamond particles is about 100 nm.With admixture of ^-diamond and nanocrystalline diamond from catalyzed carbon black as carbon source, transparent wafers have been synthesized by hydrothermal process at 100°C and atmosphere pressure. Scanning electron microscopy (SEM). XRD, Raman spectroscopy, TEM and EPMA were used to analyze those transparent wafers, and the results indicated that the transparent wafers were amorphous sp3-banding carbon wafer, and it also indicated that the wafers weren't aggregate of nanocrystalline diamond from the carbon source, and that the wafers were a new kind of reaction product by hydrothermal treatment.A series of catalyzed carbon black or carbon nanotubes experiments at different temperatures were designed and the final products were studied by XRD, thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). Based on the results of XRD, TGA and DTA, a formation mechanism was proposed to explain the phase transformation from carbon black or carbon nanotubes to nanocrystalline diamond and ^-diamond. With the increase of temperature and hence the carbon diffusion in iron, the phase sequence is from Fe(OH)3 into Fe2C>3, oc-Fe, y-Fe, then liquid iron. When carbon in the liquid iron is saturated, nanocrystalline diamond or graphite is separated out of the liquid iron. With the decrease of temperature, the carbon in y-Fe is separated out- and ^-diamond nuclei are formed and grow up.The thermal stability of /7-diamond was investigated with XRD, TGA and DTA. The results indicated that the phase transformation of w-diamond begins at 150°C and is complete at 400°C, and this phase transformation of ^-diamond was an exothermic reaction. The crystal structure of the ^-diamond after various aging-treatment times was investigated by XRD. The XRD indicated that the ^-diamond was a metastable phase, and crystal structure of the ^-diamond changed with the aging-treatment times at room temperature.Based on the XRD analysis and simulated XRD pattern, two kinds of crystal model for ^-diamond are suggested, and one is "defeciive diamond" model with fractional occupation site, and the other one is "mislayered diamond" model. With the "defective diamond" model, the zero occupancy corresponds to a face-central cubic (fee) crystal, while full occupation leads to a perfect diamond. In this model, a density functional theory computation further confirms the trend of increasing stability during the evolution from fee to diamond structure. Therefore, it was suggested that ^-diamond is indeed an intermediate state between the fee structure and diamond structure. With the "mislayered diamond" model, the crystal structure model of ^-diamond was considered to be R3 space group with cell angle a=90° and lattice parameter ao=3.58O9 A. An equilibrium lattice constant for this '"mislayered diamond" model was obtained by the first-principles calculations, and that is close to the measured value. Itwas assumed mat n-diamond was a transition state between rhombohedral graphite and diamond. The electronic structures of n-diamond further investigated, and it was indiacted that the n-diamond is a good conductor with substantial density of state at Fermi level.Three kinds of nanocrystalline diamond in different shape and a high symmetrically graphene sheet were modeled at different size. The effect of size and shape of nanocrystalline diamond on the stability, electron structure and phonon structure of nanocrystalline diamond were studied by the first principle, and the following conclusions were obtained. 1. By comparing the heat of formation, it was found that the octahedron nanocrystalline diamond was the steadiest one among the three kind of nanocrystalline diamond with different shape. When the number of carbon atom was less than 330, the nanocrystalline diamond was steadier than graphene sheet. 2. The effect of shape of nanocrystalline diamond on the HOMO-LUMO energy gaps Egap, ionization potential IE and electron affinity EA were lesser, the function between energy gaps Egap, ionization potential IE, electron affinity EA and reciprocal of the size of nanocrystalline diamond d'1 was accord with a linear equation. 3. For the three kinds of nanocrystalline diamond in different shape studied in this paper, all of the values of electron affinity of nanocrystalline diamond were negative. With decreasing size of nanocrystalline diamond, the values of electron affinity of nanocrystalline diamond decrease. The negative electron affinity was directly related with the property of electron emission of nanocrystalline diamond.Using a refection/transmission technique, the complex relative permittivity and permeability of the carbon powder from catalyzed carbon black (CPCCB) were measured in X and Ku band. It was indicated that the real part e' of permittivity for CPCCB decreases monotonously from 26 to 15, and the imaginary part s" of permittivity decreases monotonously from 41 to 23, and both the real part s' and the imaginary part s" of permittivity for CPCCB show significant variation with the frequency and the magnitude of both parts decrease with increasing frequency. The real part n' of permeability for CPCCB was 1.0, whereas the imaginary part fx " of it is about zero and nearly remains constant during the frequency region studied. Those results demonstrate that the CPCCB hardly has diamagnetism and magnetic loss. The reflection loss and percentage absorption of the CPCCB/paraffin wax composite were obtained by the arc reflecting method. For the sample with 3 vol. % lossy fillers content, it has a higher absorption value (absorption percentage larger than 70 %) in the X band (8 to 12.4 GHz). For the sample with 6 vol. % lossy fillers content, it has a higher absorption value (absorption percentage larger than 70 %) in the Ku band (12.4 to 18 GHz).The calculational results indicated that with increasing thickness, the absorption of the composite increases. For the sample with the CPCCB content of 3vol%, a broad absorptionpeak appears when the thicknesses reach 8 mm. For the sample with the CPCCB content of 6vol%, a broad absorption peak appears when the thicknesses reach 4 mm, whereas the frequency position of the broad absorption peak is different for the composites with different thickness.