Seong-Hyeon Hong

Gas sensing properties of defect-controlled ZnO-nanowire gas sensor

The effect of oxygen-vacancy-related defects on gas-sensing properties of ZnO-nanowire gas sensors was investigated. Gas sensors were fabricated by growing ZnO nanowires bridging the gap between two prepatterned Au catalysts. The sensor displayed fast response and recovery behavior with a maximum sensitivity to NO2 gas at 225 °C. Gas sensitivity was found to be linearly proportional to the photoluminescence intensity of oxygen-vacancy-related defects in both as-fabricated and defect-controlled gas sensors by postannealing in Ar and H2 atmosphere. This result agrees well with previous theoretical prediction that oxygen vacancies play a role of preferential adsorption sites for NO2 molecules.

Structure and in vitro bioactivity of titania-based films by micro-arc oxidation

Abstract Titania-based films on titanium were formed by micro-arc oxidation in electrolytic solutions containing sodium carbonate, sodium phosphate, acetate monohydrate and β-glycerophosphate disodium salt pentahydrate using a pulse power supply. The morphology, elemental composition and phase components of the films were investigated as a function of the electrolytes composition and the applied voltage (in the range of 200–500 V). In vitro bioactivity of the films was evaluated in a most commonly used simulated body fluid as proposed by Kokubo et al. The results showed that the films were porous with 1–8 μm pores and nano-crystallized, without apparent interface to the titanium substrates. The phase components of the films could be anatase, rutile, CaTiO 3 , β-Ca 2 P 2 O 7 and α-Ca 3 (PO 4 ) 2 , strongly depending on the electrolytes composition and the applied voltage. The pore size and the content of Ca and P tended to increase with the applied voltage. Among the prepared titania-based films, only the film containing CaTiO 3 , β-Ca 2 P 2 O 7 and α-Ca 3 (PO 4 ) 2 could induce an apatite layer on its surface, exhibiting bioactivity. The bioactive response of the micro-arc oxidized films to the structural factors and the apatite-induced mechanism were discussed.

Synthesis and dissolution behavior of β-TCP and HA/β-TCP composite powders

Abstract Calcium phosphate powders, β-TCP and biphasic HA/β-TCP, were synthesized by calcining the powders obtained from the co-precipitation method using Ca(NO3)2·4H2O and (NH4)2HPO4. The effects of the initial Ca/P ratio and pH of the solution on the phase evolution and in vitro dissolution behavior of the powders in a Ringers solution were investigated. The Ca/P ratio of the resulting powders was strongly dependent on the pH of the solution and weakly dependent on the initial Ca/P ratio. Single phase TCP powder was obtained at pH=7.4 and the initial Ca/P ratio had a little effect on the resulting Ca/P ratio. Biphasic composite powders were prepared at pH=8.0 and the Ca/P ratio of resulting powder was controllable by adjusting the initial Ca/P ratio. TCP powder showed the highest dissolution rate in the Ringers solution and biphasic composite powder exhibited an intermediate dissolution behavior between that of HA and TCP.

Fabrication of W–20 wt % Cu composite nanopowder and sintered alloy with high thermal conductivity

Abstract The reduced W–20% Cu powder with W particle sizes of about 30–100 nm and uniform distribution of components was successfully synthesized by mechano-thermochemical process. This process consists of three steps: the producing of oxide powder, the wet ball milling of oxide powder and the final reduction. W/Cu composite oxide clusters with a spherical shell structure were prepared by spray drying of aqueous solution of Cu and W salts with subsequent oxidation at 750 °C for 2 h. These oxide clusters were fragmented to fine oxide powders by wet milling and reduced at 600–800 °C in hydrogen. The sintering behavior of composite powder and thermal conductivity of sintered alloys were also investigated. This composite powder showed higher sinterability comparatively with conventional blended powders. The thermal conductivity of W–20% Cu alloys prepared by this work is 239±5.0 W/mK, which is equal to the theoretical value. Superior properties of the present powder are due to the homogeneous mixing state of nanosized W and Cu particles in powder, homogeneous redistribution of W solid particles in liquid Cu and high purity of powder. This alloy did not contain such impurities as Fe and Co that can be easily introduced in powder in conventional processing.

Antibacterial properties of Ag (or Pt)-containing calcium phosphate coatings formed by micro-arc oxidation

Silver (or platinum)-containing calcium phosphate (hydroxyapatite (HA) and tricalcium phosphate (alpha-TCP)) coatings on titanium substrates were formed by micro-arc oxidation (MAO) and their in vitro antibacterial activity and in vitro cytotoxicity were evaluated. MAO was performed in an electrolytic solution containing beta-glycerophosphate disodium salt pentahydrate (beta-GP) and calcium acetate monohydrate (CA), and Ag and Pt were introduced in the form of AgNO(3) (or CH(3)COOAg) and H(2)PtCl(6), respectively. The MG63 and human osteosarcoma (HOS) cell lines were used to investigate the proliferation and differentiation behavior of the cells, respectively, whereas two strains of bacteria, Staphylococcus aureus and Escherichia coli, were used to evaluate the antibacterial activity of the coatings. The phase, morphology, and Ag content of the coatings were strongly dependent on the applied voltage and Ag precursor concentration. HA and alpha-TCP phases were detected in the coatings oxidized above 400 V and the presence of Ag was confirmed by EDS. While the coatings with a high content of Ag were cytotoxic and those obtained in the Pt-containing electrolyte had no apparent antibacterial activity, the calcium phosphate coatings obtained in the low Ag concentration electrolyte exhibited in vitro antibacterial activity but no cytotoxicity. Thus, biocompatible calcium phosphate coatings on Ti implants with antibacterial activity can be achieved by one-step MAO.

Deposition of ZnO thin films by magnetron sputtering for a film bulk acoustic resonator

Abstract To fabricate lateral-field excitation (LFE)-mode solid mounted resonator (SMR)-type film bulk acoustic resonators (FBARs), piezoelectric ZnO layers were deposited in an RF magnetron sputtering system. Control of the crystallinity, microstructure and electric properties of the piezoelectric layers was essential for fabricating high-quality LFE-mode SMR-type FBARs. In the appropriate deposition condition for FBAR devices, ZnO thin films with highly c-axis-preferred orientation (XRD rocking curve, σ=2.17°), high resistivity of 106 Ω cm and surface roughness of 10.6 A were deposited. Optimal substrate rotation was especially important for improvement of the c-axis-preferred orientation of ZnO films. Plasma properties such as the electron temperature, plasma density and saturated ion current were also analyzed for optimal ZnO deposition conditions using a Langmuir double-probe system. The resonator, for which the active piezoelectric area was 200×200 μm2, consisted of 1.25-μm-thick ZnO film and a 110-nm Au electrode. Its series and parallel resonance frequencies appeared at 1.68 and 1.71 GHz, respectively, and the quality factor was 201.4±7.4.

SnO2@Co3O4 hollow nano-spheres for a Li-ion battery anode with extraordinary performance

AbstractSnO2@Co3O4 hollow nano-spheres have been prepared using the template-based sol-gel coating technique and their electrochemical performance as an anode for lithium-ion battery (LIB) was investigated. The size of synthesized hollow spheres was about 50 nm with the shell thickness of 7–8 nm. The fabricated SnO2@Co3O4 hollow nano-sphere electrode exhibited an extraordinary reversible capacity (962 mAh·g−1 after 100 cycles at 100 mA·g−1), good cyclability, and high rate capability, which was attributed to the Co-enhanced reversibility of the Li2O reduction reaction during cycling.

SnO2 nanotubes fabricated using electrospinning and atomic layer deposition and their gas sensing performance

A novel method is developed to fabricate a SnO(2) nanotube network by utilizing electrospinning and atomic layer deposition (ALD), and the network sensor is proven to exhibit excellent sensitivity to ethanol owing to its hollow, nanostructured character. The electrospun polyacrylonitrile (PAN) nanofibers of 100-200 nm diameter are used as a template after stabilization at 250 degrees C. An uniform and conformal SnO(2) coating on the nanofiber template is achieved by ALD using dibutyltindiacetate (DBTDA) as the Sn source at 100 degrees C and the wall thickness is precisely controlled by adjusting the number of ALD cycles. The calcination at 700 degrees C transforms the amorphous nanofibers into SnO(2) nanotubes composed of several nanometer-sized crystallites. The SnO(2) nanotube network sensor responds to ethanol, H(2), CO, NH(3) and NO(2) gases, but it exhibited an extremely high gas response to ethanol with a short response time (<5 s). The results demonstrate that the combination of electrospinning and ALD is a very effective and promising technique to fabricate long and uniform metal oxide nanotubes with the precise control of wall thickness, which can be applied to various applications such as gas sensors and lithium ion batteries.

The fabrication and biochemical evaluation of alumina reinforced calcium phosphate porous implants.

Alumina reinforced calcium phosphate porous implants were manufactured to improve the mechanical strength while maintaining the bioactivity of calcium phosphate ceramics. The alumina porous bodies, which provided the mechanical strength, were fabricated by a polyurethane sponge method and multiple coating techniques resulted in the porous bodies with a 90-75% porosity and a compressive strength of up to approximately 6MPa. The coating of hydroxyapatite (HAp) or tricalcium phosphate (beta-TCP) was performed by dipping the alumina porous bodies into calcium phosphate ceramic slurries and sintering the specimens. The fairly strong bonding between the HAp or TCP coating layer and the alumina substrate was obtained by repeating the coating and sintering processes. The biochemical evaluations of the porous implants were conducted by in vitro and in vivo tests. For in vitro test, the implants were immersed in Ringers solution and the release of Ca and P ions were detected and compared with those of calcium phosphate powders. For in vivo test, the porous bodies were implanted into mixed breed dogs and bone mineral density measurements and histological studies were conducted. The alumina reinforced HAp porous implants had a higher strength than the HAp porous implants and exhibited a similar bioactivity and osteoconduction property to the HAp porous implants.

Preparation of nanostructured TiO2 ceramics by spark plasma sintering

The effect of spark plasma sintering (SPS) on the densification of TiO2 ceramics was investigated using a nanocrystalline TiO2 powder. A fully-dense TiO2 specimen with an average grain size of ∼200 nm was obtained by SPS at 700 °C for 1 h. In contrast, a theoretical density specimen could only be obtained using conventional sintering above 900 °C for 1 h with an average grain size of 1–2 μm.