Kwang Bo Shim

Thermal stability evaluation of diamond-like nanocomposite coatings

Abstract Diamond-like nanocomposite (DLN) is a class of modified diamond-like carbon (DLC) coatings. The DLN films consist of amorphous networks of carbon and silicon, which are made by incorporating the Si–O structures into the DLC films. The thermal stability of DLN films was investigated by annealing the films deposited on Si (1xa00xa00) substrates by plasma enhanced chemical vapor deposition. The films were heated in ambient air for 1 h at the annealing temperatures ranging from 200 to 600 °C. The mechanical properties as well as structural modification of the annealed films were investigated as a function of annealing temperature. The chemical structure, composition, thickness and hardness values of the annealed films were measured by Raman spectrometer, electron probe micro analyzer, scanning electron microscope and ultra micro-indentation system, respectively. The comparison between DLN and DLC films on their thermal stability was made in order to understand the characteristics of DLN films. It is assumed that the DLN films show a good thermal stability for the protective coating applications at high temperature environment in air atmosphere in comparison with DLC films.

Low temperature synthesis of BaCeO3 nano powders by the citrate process

Abstract Nanosized BaCeO 3 powders with the homogeneous composition were synthesized at the relatively low temperature of 900 °C by a citrate process based on the Pechini method. A polymeric precursor was formed by use of citric acid and ethylene glycol as a chelating agent of metal ions and reaction medium, respectively. The orthorhombic BaCeO 3 powders, about 100-nm sized and uniformly shaped, were obtained through the calcination of the polymeric precursor at 900 °C for 4 h. It was found that the small quantities of the remanent carbonate ions (CO 3 2− ) were completely decomposed at over 1100 °C.

Crystallographic orientation of ZrB2‐ZrC composites manufactured by the spark plasma sintering method

The crystallographic grain orientation of ZrB2‐ZrC composites manufactured using a spark plasma sintering (SPS) method, a new sintering technique in development for poorly sinterable ceramic materials, was analysed by the scanning electron microscopy‐electron backscattered diffraction (SEM‐EBSD) method. Their crystallographic features have been compared with those of a conventionally sintered specimen using a pressureless sintering (PLS) method. In the composite sintered by PLS, (0001) planes of ZrB2 were orientated in the direction parallel to the specimen surface (RD) but (1010) and (2110) planes randomly orientated. In the case of SPS, (0001) planes of ZrB2 were orientated normal to the specimen surface (ND) and weakly to the RD. In both cases of PLS and SPS, ZrC grains had a randomly orientated grain structure. The distribution of grain boundary misorientation of PLS and SPS‐processed composites showed the same tendency that high‐angle boundaries were more prevalent than low‐angle boundaries. But in the case of ZrC grains in the SPS sample, the proportion of CSL boundaries with low sigma value (3, 5, 7, 9, 11) was relatively larger.

Defect generation in multi-stacked InAs quantum dot/GaAs structures

Abstract InAs self-assembled quantum dots (SAQDs) were grown on GaAs(1xa00xa00) substrates using a molecular beam epitaxy (MBE) technique. The InAs QDs were multi-stacked with 6,10 and 15 layers in which 2 monolayer (ML) thick InAs QD layers and 20 ML thick GaAs spacers were alternately positioned. The nanostructural features of the QD multi-stacked layers were characterized by scanning transmission electron microscopy (STEM). The QDs were well formed vertically up to 6–7 layers in each multilayered structure. Additionally, stacking faults, “volcano-like” defects and defects of an asymmetrical down triangular shape (ADTS) were observed in the 10- and 15-layer InAs QD/GaAs samples. The generations of these defects are discussed mainly in terms of the interdiffusion of the III-group atoms, Ga and In. The subsequent strain relaxation and gradient of surface chemical potential are suggested to be the cause of these defects.

Pulsed-Laser-Induced Simple Synthetic Route for Tb3Al5O12:Ce3+ Colloidal Nanocrystals and Their Luminescent Properties

Cerium-doped Tb3Al5O12(TAG:Ce3+) colloidal nanocrystals were synthesized by pulsed laser ablation (PLA) in de-ionized water and lauryl dimethylaminoacetic acid betain (LDA) aqueous solution for luminescent bio-labeling application. The influence of LDA molecules on the crystallinity, crystal morphology, crystallite size, and luminescent properties of the prepared TAG:Ce3+colloidal nanocrystals was investigated in detail. When the LDA solution was used, smaller average crystallite size, narrower size distribution, and enhanced luminescence were observed. These characteristics were explained by the effective role of occupying the oxygen defects on the surface of TAG:Ce3+colloidal nanocrystal because the amphoteric LDA molecules were attached by positively charged TAG:Ce3+colloidal nanocrystals. The blue-shifted phenomena found in luminescent spectra of the TAG:Ce3+colloidal nanocrystals could not be explained by previous crystal field theory. We discuss the 5d energy level of Ce3+with decreased crystal size with a phenomenological model that explains the relationship between bond distance with 5d energy level of Ce3+based on the concept of crystal field theory modified by covalency contribution.

Structural characteristics of diamond-like nanocomposite films grown by PECVD

Abstract Diamond-like nanocomposite (DLN) films have been deposited on Si substrates using CH4/(C2H5O)4Si/H2/Ar gas mixtures as source gases by conventional plasma-enhanced chemical vapor deposition (PECVD). The film structure was investigated by transmission electron microscope (TEM), Fourier transform infrared spectrometer (FT-IR) and Raman spectrometry. The DLN films deposited mainly consisted of diamond-like a-C:H and quartz-like a-Si:O networks. The mechanical and tribological properties as well as microstructural modifications of the grown films were investigated. In order to understand the characteristics of DLN films, the diamond-like carbon (DLC) films were also prepared using CH4/H2 gases by the same deposition process.

Silver diffusion and microstructure in LTCC multilayer couplers for high frequency applications

Low temperature cofired glass ceramic-Ag metal electrode (LTCC) systems were investigated in relation to Ag diffusion and microstructural development in the fabrication of high frequency couplers. Sintering temperature was in a range of 825°C–975°C. Ag diffusion was not observed below 875°C but densification of the electrode was greatly improved. At higher temperature above 900°C, Ag ions were diffused through glass phases containing Pb and alkali ions. Crystalline phases behaved as a barrier for the Ag diffusion. With increasing sintering temperature, glass infiltration to the electrode also occurred due to increased fluidity of the glass phase and the electrode line were severely deformed and damaged.

Microwave-Assisted Synthesis of MWO4 and MMoO4 (M = Ca, Ni) Nano-Powders Using Citrate Complex Precursor

Nano-sized MWO4 and MMoO4 (M = Ca, Ni) powders, which have scheelite and wolframite type structure, were successfully synthesized at low temperatures by a modified citrate complex method assisted by microwave irradiation. The citrate complex precursors were heattreated at temperatures from 300 to 600 °C for 3 h. Crystallization of the MWO4 and MMoO4 precursors was detected at 400 °C and completed at a temperature of 500 °C. Most of the MWO4 and MMoO4 powders heat-treated between 300 and 600 °C showed primarily spherical and homogeneous morphology. The average crystallite sizes of MWO4 (M = Ca, Ni) were between 22 and 39 nm, and those for MMoO4 (M = Ca, Ni) were between 19 and 35 nm respectively, showing an ordinary tendency to grow with temperature.

Influence of starting melt composition on La3(TaxGa1−x) Ga5O14 crystals grown by Czochralski method

Abstract Solid solution range of La3(TaxGa1−x) Ga5O14 (LTG) was examined. On the basis of these data, LTG single crystals have been grown by the Czochralski method. In order to investigate the influence of starting melt composition on growth characteristics, LTG single crystals were grown at different starting melt compositions. From the grown crystals, chemical composition and lattice constants along the growth axis of grown crystals were investigated.

Luminescence of Nanocrystalline Tb3Al5O12 : Ce3 + Phosphors Synthesized by Nitrate-Citrate Gel Combustion Method

Ce 3+ -doped nanocrystalline terbium aluminum garnet (Tb 3 Al 5 O 12 :Ce 3+ , TAG:Ce 3+ ) phosphors were successfully synthesized in low temperature by nitrate-citrate gel combustion method using microwave irradiation as a heating source. The pure TAG phase was obtained after calcining at 800°C for 6 h. The prepared nanocrystalline phosphors showed a broad peak at 551 nm, and the maximum doping concentration of Ce 3+ for the highest emission intensity was 0.8 mol % at 551 nm. The temperature dependence on photoluminescence properties of the prepared TAG:Ce 3+ phosphors was investigated from 25 to 275°C, and the activation energies (ΔE) for thermal quenching was determined by Arrhenius fitting. The experimental results clearly indicate that prepared nanocryatalline TAG: Ce 3+ has great potentials for a down-conversion yellow phosphor in white light emitting diodes.