Hwack Joo Lee

Low temperature growth and photoluminescence of well-aligned zinc oxide nanowires

Abstract Well-aligned single-crystalline zinc oxide (ZnO) nanowires with high density were successfully synthesized on nickel monoxide (NiO) catalyzed alumina substrate through a simple metal–vapor deposition method at an extremely low temperature (450 °C). The single-crystalline ZnO nanowires had a hexagonal wurzite structure and diameters of about 55 nm, and lengths up to 2.6 μm. The photoluminescence spectra under excitation 325 nm showed a ultra-violet (UV) emission at 3.26 eV and a green emission at 2.44 eV. The UV emission and green emission bands were attributed to near band-edge transition and radial combination of a singly ionized oxygen vacancy with a photo-induced hole, respectively.

Microstructure and piezoelectric properties of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 ceramics

For 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 (0.95NKN-0.05BT) ceramics sintered at 1040–1075°C, abnormal grain growth occurred but the grain size decreased when the sintering temperature exceeded 1075°C. The dielectric constant (ϵ3T∕ϵ3), electromechanical coupling factor (kp), and piezoelectric constant (d33) were considerably increased with increasing relative density and grain size. Evaporation of Na2O deteriorated the piezoelectric properties by decreasing the resistivity. To minimize Na2O evaporation, specimens were muffled with 0.95NKN-0.05BT powders during the sintering. Improved piezoelectric properties of d33=225pC∕N, kp=36%, and ϵ3T∕ϵ3=1058 were obtained for specimen sintered at 1060°C for 2h with muffling.

Effect of carbon nanotube addition on the tribological behavior of carbon/carbon composites

Carbon nanotube composite coatings were applied onto carbon/carbon composites to improve wear properties. Carbon nanotubes have been prepared by catalytic pyrolysis of hydrocarbons. The nanotube slurry was prepared by addition of phenolic resin and solvent to infiltrate into C/C composites. The nanotube added composites were then carbonized in a nitrogen atmosphere. Ball-on-disc type wear tests were performed to evaluate the tribological properties of the carbon nanotube added carbon composites. The result showed that addition of nanotube has the potential to increase the wear resistance of carbon composites. Changes in Raman spectra, morphology and surface damage were studied to explain observed wear behavior.

Temperature-dependent growth of carbon nanotubes by pyrolysis of ferrocene and acetylene in the range between 700 and 1000 °C

Abstract Aligned carbon nanotubes were grown by pyrolysis of ferrocene and acetylene in the temperature range 700–1000 °C. The average diameter is constantly 20 nm for all growth temperatures. As the temperature increases, the growth rate increases by 60 times. The length reaches up to 3 mm at 1000 °C. These long carbon nanotubes exhibit a cylindrical structure. The relative amount of crystalline graphitic sheets increases significantly with the growth temperature. The Arrhenius plot yields the activation energy 35±3 kcal/mol, which is close to the diffusion energy of carbon in bulk γ-Fe. We suggest that the bulk diffusion of carbons would play an important role in the growth of cylindrical structured carbon nanotubes.

Nanoscale InGaAs concave disks fabricated by heterogeneous droplet epitaxy

The detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.

Room‐temperature visible photoluminescence from silicon‐rich oxide layers deposited by an electron cyclotron resonance plasma source

Highly split, visible light emissions at room temperature were observed in the range from 335 to 650 nm in silicon‐rich oxide filmsdeposited in the plasma phase of a mixture of silane and oxygen. The mechanism of the light emissions is classified into two categories. The photoluminescence bands at both 365 and 469 nm are related to the intrinsic defects of the E′ center and the neutral oxygen vacancy, respectively. However, the relatively sharp peaks at 403 and 535 nm are correlated with the development of polycrystalline core of Si‐enriched parts.

Effect of B2O3 and CuO on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics

Sintering temperature of the Ba(Zn1/3Ta2/3)O3 (BZT) ceramic is about 1550°C and it decreased to 950°C when B2O3 was added. The BaB4O7 second phase whose melting temperature is 889°C was found in the B2O3 added BZT ceramics. The BaB4O7 second phase assisted the sintering of the BZT ceramics at 950°C. However, the B2O3 added BZT ceramic was not sintered below 950°C and the microwave dielecric properties were not satisfactory. On the other hand, when the B2O3 and the CuO were added, the BZT ceramic was sintered even at 870°C. The BaCu(B2O5) phase which was found in the CuO and B2O3 added BZT ceramics, existed as the liquid phase during the sintering and helped the densification of the BZT ceramics at temperatures lower than 950°C. Good microwave dielectric properties of Q×f=11,000 GHz, er=26 and τf=0.0 ppm/°C were obtained from the BZT + 5.0 mol% B2O3 + 10.0 mol% CuO ceramic sintered at 870°C for 2 h.

Oxide growth on silicon (100) in the plasma phase of dry oxygen using an electron cyclotron resonance source

Silicon dioxide films were grown using an oxygen plasma generated by an electron cyclotron resonance (ECR) source at several low temperatures. The plasma oxidation rate was investigated by varying the growth parameters. The oxide thickness parabolically increases with microwave power but decreases with increasing pressure or flow rate. A complementary model of the Deal–Grove oxidation theory is suggested for the plasma oxidation, and kinetic parameters are compared with the other plasma and thermal oxidation cases. The diffusion rate constant in O2 plasma oxidation at room temperature is enhanced up to the level of the diffusion rate in thermal oxidation and the reaction rate constant is much larger than the thermal oxidation case. This may imply that, due to oxygen atoms dissociated by the ECR plasma, plasma oxidation is related to the atomic diffusion through oxide layer and the atomic chemical reaction at the Si–SiO2 interface. The high quality of the ultrathin oxide film was characterized with a break...

Microstructure and Microwave Dielectric Properties of Ba(Co1/3Nb2/3)O3 Ceramics

Ba(Co1/3Nb2/3)O3 (BCN) ceramics has a 1:2 ordered hexagonal structure and the degree of the 1:2 ordering slightly decreased when the sintering temperature exceeded 1400°C. A large amount of the liquid phase was found in the BCN ceramics sintered above 1400°C the formation of which is related to the evaporation of CoO. The liquid phase contains high concentrations of Ba and Nb ions. The grain size increased for the specimens sintered above 1400°C due to the presence of the liquid phase during the sintering. The Q-value of BCN increased with increasing sintering temperature and the specimen sintered at 1400°C had the maximum Q-value. When the sintering temperature exceeded 1400°C, however, the Q-value significantly decreased. The presence of a large amount of liquid phase could be responsible for the decrease of the Q-value. BCN ceramics were also sintered for various times at 1400°C and 1450°C and the variations of the microwave dielectric properties were explained in terms of the grain size and the relative density.

Low-Temperature Sintering of B2O3-Added Ba(Mg1/3Nb2/3)O3 Ceramics

The sintering temperature of Ba(Mg1/3Nb2/3)O3 (BMN) ceramic is about 1450°C. When B2O3 was added, BMN sintered at 900°C and it had a 1:2 ordered hexagonal structure. A liquid phase, which could be responsible for the decrease in sintering temperature, was observed in B2O3-added BMN. Mg4Nb2O9 and Ba2B2O5 second phases were found in the specimens and the Ba2B2O5 second phase might be related to the liquid phase. The dielectric constant (er) and Q value increased with increasing sintering temperature and it is due to the increase in bulk density. However, they decreased with increasing B2O3 content, which can be explained by the existence of the second phases. Good microwave dielectric properties of Q ×f = 8500 GHz, er = 27.5 and temperature coefficient of resonance frequency (τf) = 27.0 ppm/°C were obtained for BMN with 2.0 mol% B2O3 sintered at 930°C for 2 h.