Ki Hyun Yoon

Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni–YSZ cermet

Abstract The microstructural and electrical properties of Ni–YSZ composite anode of solid oxide fuel cells (SOFC) were investigated. We measured the electrical conductivity via 4-probe DC technique as a function of Ni content (10–70 vol.%) in order to examine the correlation with the microstructure of Ni–YSZ cermet. Image analysis based on quantitative microscopic theory was performed to quantify the microstructure of Ni–YSZ composite. The size and distribution, contiguity and interfacial area of each phase or between the phases could be obtained from the image analysis. According to image analysis, contiguity between the same phases was simply dependent on the amount of that phase itself while the contiguity between different phases was greatly influenced by the amount of Ni phase because the overall microstructural changes were mainly controlled by the coarsening of Ni phase. These similarly quantified microstructural properties were used for the characterization of the electrical properties of Ni–YSZ cermet.

Microwave Dielectric Properties and Far Infrared Reflectivity Spectra of the (Zr0.8Sn0.2)TiO4 Ceramics with Additives

Microwave dielectric properties and far infrared reflectivity spectra were investigated in the (Zr0.8Sn0.2)TiO4 ceramics with 1 mol% Sb2O5, Sb2O3 and Nd2O3, respectively. Dielectric constant was not significantly changed, while the unloaded Q value was affected with additives. The unloaded Q of the pure (Zr0.8Sn0.2)TiO4 specimens was higher than that of the specimens with Sb2O3 and/or Nd2O3, but lower than that of the specimens with Sb2O5. The high unloaded Q value of 6600 at 7 GHz was obtained for the specimens with 1 mol% Sb2O5 due to elimination of oxygen vacancies. The effects of additives on the intrinsic microwave losses of the specimens were investigated by the infrared reflectivity spectra from 50 to 4000 cm-1, which were evaluated using the Kramers-Kronig analysis and classical oscillator model. The relative tendency of microwave dielectric properties calculated from the reflectivity data was in good agreement with the results by the Hakki and Colemans method.

Molten salt synthesis of lead-based relaxors

The molten salt synthesis (MSS) of lead-based relaxors which have a perovskite structure, A(BIBII)O3 where BI is Mg2+, Fe3+, Zn2+, Ni2+ or Co2+, and BII is Nb5+, has been reviewed with regard to the formation of the perovskites, phase stability and morphology characteristics. Two relaxor materials, Pb(Mg1/3Nb2/3)O3 and Pb(Fe1/2Nb1/2)O3 were found to be successfully synthesized at a low temperature in a very short time by the MSS method. Using the example of Pb(Mg1/3Nb2/3)O3, the phase stability has been discussed on the basis of thermal and chemical analyses. The influences of the processing parameters, such as temperature, time, type and amount of salt, and non-stoichiometry, on the formation and the powder characteristics of the perovskite phase were investigated with possible explanations for the observed differences which were induced by changing the parameters. Finally, densification behaviour and dielectric properties resulting from the MSS powder were examined and compared to those of powders obtained by using the conventional mixed oxides (CMO) method.

Effect of mass and stress on resonant frequency shift of functionalized Pb(Zr0.52Ti0.48)O3 thin film microcantilever for the detection of C-reactive protein

A micromachined PZT (52/48) thin film cantilever composed of SiO2/Ta/Pt/PZT/Pt/SiO2 on a SiNx supporting layer for simultaneous self-exciting and sensing was fabricated. We present the resonant frequency change of piezoelectric microcantilevers due to a combination of mass loading and spring constant variation arisen from antigen-antibody interaction of C-reactive protein (CRP). Experimentally measured resonant frequency shift is larger than that of theoretically calculated resonant frequency by two orders of magnitude due to a compressive stress arising from CRP antigen-antibody interaction. The changes in normalized resonant frequency shift δf expt/f1 expt were found to increase as the value of 40.5, 74.7, and 185×10−4 as the CRP antibody site ratio, increased 10%, 50%, and 100%, respectively.

PHOTOELECTROCHEMICAL PROPERTIES OF COPPER OXIDE THIN FILMS COATED ON AN N-SI SUBSTRATE

Abstract The photoelectrochemical properties of the copper oxide thin film coated on the n-type silicon electrode were investigated as a function of film deposition temperature. The variation in the deposition temperature affected the film morphology and the ratio of copper to oxygen. In case of the films deposited below 200°C, the main phase was found to be CuO while the amount of the Cu2O phase increased with further increases in deposition temperature. The n-silicon photoelectrode showed enhanced photocurrent–potential (I–V) properties by forming a copper oxide/n-silicon heterojunction. In particular, the electrode, which mainly consisted of a CuO phase, showed better photoelectrochemical conversion efficiencies compared to the Cu2O phase. This result was explained in terms of the electrical conductance and transmittance of the copper oxide film.

Characteristics of ZnO thin films deposited onto Al/Si substrates by r.f. magnetron sputtering

Abstract ZnO thin films were deposited onto Al/Si (Al film on Si) substrates by r.f. magnetron sputtering of a ZnO target. The effects of the oxygen content on the structural characteristics and the electrical resistivity were studied as a function of the ratio of the sputtering gas mixture (Ar-to-O 2 ratio). As the Ar-to-O 2 ratio decreased, the preferred orientation also decreased owing to the resputtering effect of the neutral oxygen. The resputtering of the ZnO film enhanced the grain growth. The width of the interface layer between the ZnO film and the Al/Si substrate increased with increasing oxygen content owing to resputtering. The ZnO film composition as a function of thickness and gas mixture was nearly exactly stoichiometric. As the Ar-to-O 2 ratio decreased, the resistivity increased owing to the decrease in preferred orientation.

Growth of highly c-axis textured AlN films on Mo electrodes for film bulk acoustic wave resonators

Highly c-axis textured AlN films on Mo could be obtained using the structural modification of Mo thin films by the reactive rf magnetron sputtering at a low temperature. The correlation of structural properties of Mo and the degree of c-axis texturing in AlN films was studied as a function of sputtering pressure during the dc sputter deposition of Mo. The microstructure and residual stress of Mo films were found to be very dependent on the sputtering pressure. As the pressure decreases and the stress changes from tension to compression, the surface morphology and roughness of Mo films became gradually denser and smoother. It is found that the controlled smooth surface of Mo electrodes plays a key role in the growth of highly c-axis textured AlN films deposited onto them. A full width at half maximum of the x-ray rocking curve of the best AlN film on the surface-controlled Mo electrode was 2.30°. Film bulk acoustic wave resonators with an effective coupling coefficient of 5.6% could be achieved using the i...

Characteristics of (Pb1-xSrx)TiO3 thin film prepared by a chemical solution processing

Abstract (Pb1-xSrx)TiO3 (PST) (0≤x ≤0.7) thin films on Pt/Ti/SiO2/Si(100) substrate were prepared by the modified sol-gel process and their characteristics were investigated as a function of strontium content (x). A stable PST sol was obtained by mixing lead and strontium acetates dissolved in deionized water and ethylene glycol and a titanium alkoxide modified by chelating acetylacetone in 2-methoxyethanol. The multi-layer annealing at 700 °C was adopted to prepare the 4000 °PST thin films with perovskite phase and dense microstructure. With increasing x, the tetragonality (c/a) was slightly decreased and approached to cubic phase above x ≅ 0.5 within XRD detection limit. The dielectric constant at room temperature abnormally increased and showed the maximum one (er = 1377, tan8 = 0.037 at 1 kHz) for the Pb0.5Sr0.5TiO3 thin film. Also strontium substitution suppressed the leakage current density of the films effectively. These properties can be explained in terms of variations of phase transition boundary (tetragonal to cubic), resulting from the substitution of nonvolatile strontium into lead site of PbTiO3 structure.

Influence of electrode configurations on the quality factor and piezoelectric coupling constant of solidly mounted bulk acoustic wave resonators

This article presents an electrode effect on the quality factor and effective coupling coefficient empirically and theoretically. The possible quality factor detractors in film bulk acoustic wave resonator devices are the acoustic wave attenuation in the metal and piezoelectric films, the wave scattering loss due to the surface roughness, and the electrode electrical loss. Due to the small acoustic wave attenuation of the Mo electrode, the composite Mo–AlN–Mo resonator with the thick electrode provides higher Q values than the resonator with the good quality AlN film and the thin Mo electrode. In such Mo electroded resonators, the effective coupling coefficient is kept relatively high. It is found that the Q value of solidly mounted resonators (SMRs) is not determined by only the piezoelectric film quality, but by a weighted average of the electrode and piezoelectric film quality. Of Q value detractors, the scattering loss originating from surface roughness is thought to be very important factor in SMR qu...

Microwave dielectric properties of (1−x)CaTiO3–xLi1/2Sm1/2TiO3 ceramics

The effect of the bond valence on microwave dielectric properties of (1-x)CaTiO 3-x Li 1/2 Sm 1/2 TiO 3 ccramics was investigated in the range of 0.0 ≤ x ≤ 1.0. A complete solid solutions was formed with the orthorhombic perovskite structure in the entire composition range. As the Li 1/2 Sm 1/2 TiO 3 content increased, the deviation of the observed dielectric polarizabilities calculated by the Clausius-Mosotti equation from the theoretical values, calculated by the additivity rule of dielectric polarizahility, decreased with increasing bond valence of the B-site in ABO 3 perovskite compound. The temperature coefficient of the resonant frequency (TCF) of the specimens decreased, depending on the B-site bond valence in the tilted region (t < 1). Typically, the dielectric constant K= 114, Q.f = 3700 GHz, and TCF = 11.5 ppm/C for the x = 0.7 composition sintered at 1300 C for 3 h.