Papers by Author: Cheolmun Yim

Abstract: Boron-doped diamonds were deposited by microwave plasma chemical vapor deposition (MPCVD) method in order to investigate the influence of inlet boron concentration on the film properties. The substrate material of the specimens was pure titanium (99.9 %). Boron source was introduced into the vacuum chamber by bubbling of B2O3, acetone and methanol mixture. Samples were produced with different B2O3 concentrations in mixture (1000 ppm, 5000 ppm, and 10000 ppm). The surface morphology of the samples was observed by scanning electron microscope (SEM). X-ray diffraction was used to identify crystal structures of the films. Secondary ion mass spectroscopy was used to examine the qualitative boron contents in the films. For low B2O3 concentrations in liquid mixture (1000 ppm), the surface morphology of the film showed both micro crystalline diamond and nano crystalline diamond. For medium B2O3 concentrations in liquid mixture (5000 ppm), the surface morphology of the film was also consisted of micro crystalline diamond and nano crystalline diamond. However, the content of micro crystalline diamond decreased in comparison with low B2O3 concentration. For high B2O3 concentration in liquid mixture (10000 ppm), the surface morphology of the film was almost dominated by nano crystalline diamond. Therefore, the crystal size of boron doped diamond decreased with increasing boron concentration. From these results, it appears that boron will restrain the growth of diamond crystal during deposition.

Abstract: To prove the suitability the honeycomb composites structure with VARTM, the mechanical properties of the skin materials and honeycomb composites structure were evaluated with the static strength tests. The mechanical properties of honeycomb composites structure made by VARTM were satisfied with the real using conditions instead of the composites structure made by autoclave process. Accordingly, the honeycomb sandwich composites made by VARTM is available for manufacturing various composites parts. VARTM was very effective method to manufacture the honeycomb sandwich composites. It was possible that the manufacturing process was changed from autoclave process to VARTM to solve the problems on the autoclave process.

Abstract: Diamond film was deposited on the pure titanium substrate by CH4-H2 gas mixture using MPCVD method. In order to carboxylate the surface of the diamond film, it was chemically treated in H2SO4:HNO3 (9:1, case 1) stirred at room temperature or in H2SO4:HNO3 (3:1, case 2) stirred at reflux. The oxidized diamond film was successively treated with 0.1M NaOH for 2hours and 0.1M HCl at 363K for 2hours, and then washed by distilled water. The surface of diamond film was observed by scanning electron microscopy (SEM). The diamond film was characterized using Raman spectroscopy and X-ray diffraction (XRD). Carboxylated diamond film was evaluated by Fourier transform infrared spectrometer (FT-IR), Mini secondary ion mass spectrometer (Mini SIMS) and X-ray photoelectron spectroscopy (XPS). In the FT-IR spectrum, the peak at 1640cm -1 was assigned with C=O stretching vibration of carboxylic acid. In the Mini SIMS profile, the peak intensities of mass number 16 (-O-) and 17 (-OH) were increased after the chemical treatment. The XPS results indicated COO- group and C=O group on the surface of diamond film.

Abstract: Decreasing of wind turbine blade weight by using honeycomb sandwich collides with strength lack of the honeycomb facets at the high loaded places of the blade under heavy wind load. For providing tensile and compressive strength profiling of the variable thickness facets was made. By using response surface model based on design space the facets rational thickness distribution was performed by using optimization. The condition of evenly distributed stresses at narrow range of values for reinforced facets was used like state variables for optimum designing. By combining finite element analysis and sequential programming the response of the thickness within the process integration framework on criteria the rationalization of the facets thickness was performed. The facets thickness variations are assigned by polynomial of fifth degree to provide small difference of stresses in the facets for blade. The angles of the glass fiber stacking relatively of the blade axis for mass minimization were selected on the similar stress reinforcement condition for outside and inside facets of the honeycomb sandwich. The structure of the reinforcing was built to coincide the principal stresses and strains of the facets to longitudinal and transversal direction of the blade. Calculation results were obtained for glass fiber -epoxy resin composite material having and shown that decreasing total mass of the blade 19% compare to shape optimization.