Pyeong Seok Cho

Change in electrical resistance and thermal stability of nitrogen incorporated Ge2Sb2Te5 films

Changes in the electrical resistance of nitrogen incorporated Ge2Sb2Te5 (NGST) films were investigated as a function of nitrogen content by four-point probe and ac two-point probe methods. Some nitrogen is initially located inside the cubic structure, resulting in a significant increase in crystalline temperature and electrical resistance. As the amount of incorporated nitrogen increases, excess nitrogen accumulates in the grain boundaries, which does not contribute substantially to the increase in electrical resistance and the crystallization temperature. The supersaturated nitrogen distorts the Ge2Sb2Te5 structure, resulting in a NGST film with a structure that is different from the metastable fcc structure. X-ray absorption spectroscopy revealed that Ge–N and N2 molecular states were present in the film and gradually increased in proportion to the amount of incorporated nitrogen. Moreover, the nitrogen states were very stably maintained even during the phase transition process.

Improvement of Grain-Boundary Conduction in Gadolinia-Doped Ceria by the Addition of CaO

This study examined the effect of CaO addition on grain-boundary conductivity in 10 mol % Gd 2 O 3 -doped CeO 2 containing 500 ppm SiO 2 as an impurity. The addition of CaO increased the grain-boundary conductivity ∼ 50 times without affecting the grain-interior conductivity significantly. CaO was incorporated into the CeO 2 lattice completely, which means that a scavenging reaction between a large CaO-related second phase and siliceous impurity was not the reason for the increased conductivity. After adding CaO, grain boundary structure changed from random to faceted. Configuration change in the grain-boundary segregation in relation to the boundary-structure variation is suggested as a possible reason.

Grain-Boundary Conduction in Gadolinia-Doped Ceria: The Effect of SrO Addition

This study examined the effect of adding SrO on the grain-boundary conduction of Ce0.9Gd0.1O1.95 gadolinia-doped ceria containing 500 ppm SiO2. The apparent grain-boundary resistivity at 300°C decreased drastically from 746.7 to 0.90–1.97 k cm upon doping with 1 mol % SrO, while the grain-interior resistivity increased gradually from 3.1 to 11.6 k cm as the SrO concentration was increased up to 5 mol %. Therefore, doping with 1 mol % SrO resulted in the minimum total resistivity. The electron probe X-ray microanalysis and the analysis of the lattice parameters suggest that the 140–500-fold enhancement in the grain-boundary conduction is attributed to the scavenging of the highly resistive siliceous phase by the SrO-containing phase.

Mitigation of highly resistive grain-boundary phase in gadolinia-doped ceria by the addition of SrO

The effect of SrO in improving the grain-boundary conduction in gadolinia-doped ceria (GDC) was examined by measuring the spatially resolved local impedance of a SrO-in-diffused GDC specimen containing 500 ppm of SiO2. The addition of SrO increased the grain-boundary conduction by up to similar to 120 times, indicating that SrO effectively mitigates the highly resistive intergranular phase. This paper discusses the possible reasons in terms of the grain-boundary structure, the incorporation of SrO into the GDC lattice, the formation of a second phase, and a scavenging reaction. (c) 2007 The Electrochemical Society.

Improvement of Grain-Boundary Conduction in SiO2-Doped GDC by BaO Addition

Barium oxide (BaO) was suggested as a scavenger material to mitigate the harmful siliceous intergranular phase in gadoliniadoped ceria (GDC). The addition of 1-5 mol % BaO to GDC specimens containing 500 ppm SiO 2 enhanced the grain-boundary conduction by~250 times. Transmission electron microscopy and electron probe microanalysis attributed this enhancement to the scavenging of the siliceous phase by the Ba-rich phase. In contrast, the grain-boundary conduction was significantly deteriorated when doped with 0.1-0.2 mol % BaO, which was attributed to the dissolution of BaO into the intergranular phase and the consequent increase in the ion-blocking effect at the grain boundary.

Preparation of Ni-GDC Powders by the Solution Reduction Method Using Hydrazine and Its Electrical Properties

Ni-GDC (gadolinia-doped ceria) composite powders, the anode material for the application of solid oxide fuel cells, were prepared by a solution reduction method using hydrazine. The distribution of Ni particles in the composite powders was homogeneous. The Ni-GDC powders were sintered at 1400˚C for 2 h and then reduced at 800˚C for 24 h in 3% H2. The percolation limit of Ni of the sintered composite was 20 vol%, which was significantly lower than these values in the literature (30-35 vol%). The marked decrease of percolation limit is attributed to the small size of the Ni particles and the high degree of dispersion. The hydrazine method suggests a facile chemical route to prepare well-dispersed Ni-GDC composite powders.

Gas Sensing Characteristics of WO 3 -Doped SnO 2 Thin Films Prepared by Solution Deposition Method

WO3-doped SnO2 thin films were prepared in a solution-deposition method and their gas-sensing characteristics were investigated. The doping of WO3 to SnO2 increased the response (Ra/Rg, Ra: resistance in air, Rg: resistance in gas) to H2 substantially. Moreover, the Ra/Rg value of 10 ppm CO increased to 5.65, whereas that of NO2 did not change by a significant amount. The enhanced response to H2 and the selective detection of CO in the presence of NO2 were explained in relation to the change in the surface reaction by the addition of WO3. The WO3-doped SnO2 sensor can be used with the application of a H2 sensor for vehicles that utilize fuel cells and as an air quality sensor to detect CO-containing exhaust gases emitted from gasoline engines.

SnO2 Gas Sensors Using LTCC (Low Temperature Co-fired Ceramics)

A sensor element array for combinatorial solution deposition research was fabricated using LTCC (Low-temperature Co-fired Ceramics). The designed LTCC was co-fired at 800 C for 1 hour after lamination at 70 C under 3000 psi for 30 minutes. SnO sol was prepared by a hydrothermal method at 200 C for 3 hours. Tin chloride and ammonium carbonate were used as raw materials and the ammonia solution was added to a Teflon jar. 20 droplets of SnO sol were deposited onto a LTCC sensor element and this was heat treated at 600 C for 5 hours. The gas sensitivity (S = R /R ) values of the SnO sensor and 0.04 wt% Pd-added SnO sensor were measured. The 0.04 wt% Pd-added SnO sensor showed higher sensitivity (S = 8.1) compared to the SnO sensor (S = 5.95) to 200 ppm CH COCH at 400 C.