Synthesis, mechanism and property of titanium-based nano ceramics (Ti3SiC2 and TiN)

• Main Authors: Tsung-Yen Huang, 黃聰彥
• Other Authors: Chien-Chong Chen
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/59531391062174780453

博士 === 國立中正大學 === 化學工程所 === 97 === This dissertation contains two parts. The first part is a study of a ternary ceramics titanium silicon carbide (Ti3SiC2 TSC), including a synthesis of nano-laminated Ti3SiC2, a proposition of Ti3SiC2 formation mechanism by an electron microscopic study and a investigation of bending deformation induced by atomic force microscopy (AFM) tip in micro- and nano-scale. The second part is a study of titanium nitride (TiN) nanosheets, containing the synthesis, chemical reaction and formation mechanism. In part one, we successfully prepared the nano-laminated Ti3SiC2 powders and a dense bulk by heating a mixture of Ti, Ti5Si3 and carbon nanotube powders in argon at 1330 oC, the eutectic temperature of Ti and Ti5Si3. The scanning electron microscope (SEM) and transmission electron microscope (TEM) show the characterization of nano-laminated Ti3SiC2 in detail. The high-resolution TEM image of cross-sectioned Ti3SiC2 sample shows a layer-interface-layer structure, where layer and interface contained pure Ti3SiC2 and TiC, respectively. According to experimental results, a reaction mechanism was proposed containing three key stages: formation of Ti-Ti5Si3 eutectic liquid, emergence of TiC/CNT from reaction between carbon nanotube and eutectic liquid, and formation of Ti3SiC2 from TiC/CNT and eutectic liquid. As-synthesized Ti3SiC2 powders were pressure-less sintered to produce a compact with 93.3% of theroretical density and the nano layers were preserved in the sintered sample. The measured Vickers hardness of sintered Ti3SiC2 sample was in the range of 3.1–4.5 GPa. After indentation, nano-scale deformations such as buckling, kink band, delamination, and cleavage fracture were observed. The nano-laminated morphology is believed to increase the damage tolerance and machinability. In chapter 3, a thin TEM sample containing a developing Ti3SiC2 crystal was carefully prepared from the product powders. A formation mechanism is proposed for the formation of Ti3SiC2 from the TiC lattice based on the experimentally-observed HRTEM images where various lattice patterns coexisted, including Ti3SiC2, TiC, twin TiC, under-developed lattice, etc. An event containing two steps, atom substitution and atom relocation, is the transformation unit of the formation mechanism. Atom substitution includes carbon vacancy formation and insertion of Si atoms into the TiC lattice, and atom relocation involves position adjustments of C, Si and Ti atoms initiated by the atom substitution. Several occurrences of events result in a lattice pattern distribution similar to those observed in the HRTEM images. If the event continues to take place, it will eventually result in a fully-developed Ti3SiC2 crystal. Lattice spacing values on the HRTEM images were also measured and analyzed, where the reduction of the standard deviation of the lattice spacing along a spatial direction confirmed the chemical reaction/crystal growth path of TSC. In chapter 4, we used AFM tip to damage a very thin Ti3SiC2 specimen and then examined the deformation behavior by SEM and TEM. Deformations like bending, fracture, and kink boundary were observed in the sample. The bended basal plane lattices of Ti3SiC2 with continuity curvature were visible around bended axis region and have a range of 17o–8o bended angle. Both the HRTEM image and the inverse FFT image show the occurrence of dislocation along and directions of Ti3SiC2 at bended axis region. The dislocations with opposite direction occurring along [ ] gathered together in two groups as walls to form kink boundary at bended axis area. On the basis of results here and the knowledge of Ti3SiC2 structure, we proposed that it is possible to have partial Si and Ti layer composing of one basal plane when dislocation occurred perpendicular to the basal plane of Ti3SiC2. In part two, we successfully synthesized the ultra-thin and high-aspect-ratio titanium nitride (TiN) nanosheets by a chemical vapor deposition method using TiCl4 and N2 as source materials reacting at 1200 oC without catalyst assistance. Nanosheet thickness was around 3.2 nm determined from AFM and electron energy-loss spectrometer (EELS) analyses. According to field emission SEM images, length of nanosheet ranged from 5 to 20 µm, indicating the aspect ratio of nanosheet ranged from 1500 to 5500. Because of the extremely thin thickness, TiN nanosheets were translucent with characteristic like bending, folding, and rolling. We conducted a concept of epitaxial growth considering both lattice misfit (f1) and domain misfit (fd) in this reaction process to propose a formation mechanism of TiN nanosheets. Calculations of the lattice misfit and domain misfit show that the most likely TiN plane to grow on Si (100) is TiN (422). The two growth directions of the TiN (422) plane exhibit different growth rates. This uneven growth rate results in mutually perpendicular nanosheets standing on the Si (100) substrate, which is consistent with a FESEM image of a TiN nanosheet sample. The lattice structure of the TiN nanosheet resulting from the proposed mechanism is nearly identical to the HRTEM image of the nanosheet sample. This study also can be applied to understand the formation mechanism on other nanostructured materials grown on a single crystal substrate.