• Journal of the Korean Ceramic Society
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• v.32 no.3
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• pp.359-365
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• 1995

The grain boundary effect on the ionic conductivity was investigated using a.c. admittance analysis in (Bi2O3)0.715(CaO)0.285 oxygen-ion conducting solid electrolyte. As a separated arc representing grain boundary polarization was not observed in the admittance plane, bulk conductivity was measrued for samples with various grain sizes in the temperature range from 48$0^{\circ}C$ to 72$0^{\circ}C$ and the conductivity distribution between grain interior and grain boundary was determined by the reported analytical methods. In the above temperature range, grain boundary worked as a high conductive path instead of blocking layer and ionic conduction through grain boundary was significant. The activation energy for conduction through grain and grain boundary was 78 and 106 kJ/mol, respectively.

• The Korean Journal of Ceramics
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• v.4 no.2
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• pp.122-127
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• 1998

Conductivities of polycrystalline ceria doped with several rare earth oxides were measured by AC admittance and DC four probe method. The conductions were separated into grain and grain boundary contributions using the complex admittance technique as well as grain size dependence of conductivity. The grain size dependence of polycrystalline conductivity, which can be adequately described by the so-called brick layer model, appears to give a more reliable measure of the grain conductivity compared to the complex admittance method. Polycrystalline resistivity(1/conductivity) increases linearly with the reciprocal of grain size. The intercept of resistivity vs. inverse grain size plot gives a measure of the grain resistivity and the slope gives a measure of the grain boundary resistivity. It was also noted that errors involved in the analysis of experimental data may be different between the complex admittance method and the impedance method. A greater resolution of the spectra was found in the complex admittance method, insofar as the present work is concerned, suggesting that the commonly used equivalent circuit may require re-evaluation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.11 no.12
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• pp.1099-1107
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• 1998

In this study, the microstructure and electric conductivity of 5mol% $Gd_2O_3$-5mol% $Y_2O_3-ZrO_2$ system(5G5YZ) with a variation of sintering time at $1600^{\circ}C$ were investigated. By the result of TEM analysis of 5G5YZ sintered for 12h, a microcrack was observed near grain boundary. The change of the sintering time did not affect the lattice conductivity, but the grain boundary contribution was varied with the sintering time. The grain boundary conductivity of the sample sintered for 1h showed the highest value. Furthermore, the activation energy of the total conductivity was independent upon the sintering time and showed approximately 1.01eV. The highest conductivity measured at $1000^{\circ}C$ was 0.0197S/cm with the sample sintered for 1h. Comparing to 0h’s, the thickness ration of grain boundary as a function of sintering time were 0.88, 1.11 and 1.29 for 1h, 5h and 12h, respectively. In case of the sample sintered for 1h, the thickness of the grain boundary showed the lowest value. The increase of the sintering time over 1h made the decrease of the electric conductivity as well as the increase of the grain growth and the thickness of the grain boundary. As a result, it seemed that the proper sintering time for 5G5YZ composition was 1h.

• Journal of the Korean Ceramic Society
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• v.44 no.2 s.297
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• pp.116-123
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• 2007

Electrical properties of AlN ceramics sintered with 1 wt% $Y_2O_3$ have been investigated. From the impedance spectroscopy, electrical conductivity of grain boundary was found to be much lower than that of grain. DC conductivity measurement showed the electrode polarization effects caused by blocking electrode. The heat-treatment at $1700^{\circ}C$ of the specimen sintered at $1850^{\circ}C$ transformed continuous pain boundary phases along triple boundary junctions into isolated particles in grain comers. The heat-treatment induced decreases both in grain and grain boundary conductivity, and in DC electrical conductivities. From the analysis on the transference number, ionic conductivity was shown to be more dominant than electron conductivity, which was due to ion compensation mechanism during oxygen incorporation into grain.

• Journal of the Korean Ceramic Society
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• v.47 no.1
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• pp.71-74
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• 2010

We have investigated protonic conduction in highly dense (>98%) polycrystalline Y-doepd $BaZrO_3$ (BYZ) ceramic with an average grain size of ~85 nm. It is observed that the protonic conductivity across the grain boundaries in this nano-crystallilne BYZ (n-BYZ) is significantly higher than the microcrystalline counterpart. Such a remarkable enhancement in grain boundary conductivity results in high overall conductivity that may allow this chemically stable protonic conductor to serve as a solid electrolyte for low-temperature solid oxide fuel cell applications.

• Journal of the Korean Ceramic Society
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• v.53 no.3
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• pp.301-305
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• 2016

We investigate proton conduction in a nonstoichiometric ${\Sigma}3$ $BaZrO_3$ (210)[001] tilt grain boundary using density functional theory (DFT). We employ the space charge layer (SCL) and structural disorder (SD) models with the introduction of protons and oxygen vacancies into the system. The segregation energies of proton and oxygen vacancy are determined as -0.70 and -0.54 eV, respectively. Based on this data, we obtain a Schottky barrier height of 0.52 V and defect concentrations at 600K, in agreement with the reported experimental values. We calculate the energy barrier for proton migration across the grain boundary core as 0.61 eV, from which we derive proton mobility. We also obtain the proton conductivity from the knowledge of proton concentration and mobility. We find that the calculated conductivity of the nonstoichiometric grain boundary is similar to those of the stoichiometric ones in the literature.

• Journal of the Korean Ceramic Society
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• v.54 no.5
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• pp.413-421
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• 2017

In this work, we studied a fluorite structure oxides: Yttria stabilized zirconia, (YSZ); Gd doped $CeO_2$ (GDC); erbia stabilized $Bi_2O_3$ (ESB); Zr doped erbia stabilized $Bi_2O_3$ (ZESB); Ca doped erbia stabilized $Bi_2O_3$ (CESB) in the temperature range of 250 to $600^{\circ}C$ using electrochemical impedance spectroscopy (EIS). As is well known, grain boundary blocking effect was observed in YSZ and GDC. However, there is no grain boundary effect on ESB, ZESB, and CESB. The Nyquist plots of these materials exhibit a single arc at low temperature. This means that there is no space charge effect on ${\delta}-Bi_2O_3$. In addition, impedance data were analyzed by using the brick layer model. We indirectly demonstrate that grain boundary ionic conductivity is similar to or even higher than bulk ionic conductivity on cubic bismuth oxide.

• Journal of the Korean Ceramic Society
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• v.44 no.12
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• pp.655-659
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• 2007

Zirconia polycrystals co-doped with x mol% CaO and (10-x) mol% $Y_2O_3$ were prepared by solid state reaction method. The compositions were chosen for nominally the same oxygen vacancy concentration of 5 mol%. X-ray diffraction patterns indicated the formation of cubic zirconia by heat treatment at $1600^{\circ}C$. Impedance spectroscopy was applied to deconvolute the bulk and grain boundary response. Electrical conductivity was measured using the complex impedance technique from 516 to 874 K in air. Maximum conductivity was exhibited by the composition with equal amounts of CaO and $Y_2O_3$, which may be ascribed to the smaller degree of defect-interactions in that composition due to the competition of different ordering schemes between the two systems. When compared to the composition containing $Y_2O_3$ only, co-doping of CaO increases the grain boundary resistance considerably. The activation energy of grain and grain boundary conductivity was 1.1 eV and 1.2 eV, respectively, with no appreciable dependence on dopant compositions.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.15 no.1
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• pp.16-20
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• 2005

In order to improve electrical conductivity and mechanical properties of 8YSZ SOFC electrolyte material, we used Al₂O₃ as an additive and applied the spark plasma sintering (SPS) method. The sintered bodies were densified above 96 ％ of theoretical density at 1200℃ and possessed microstructures composed of homogeneous grains less than 1 ㎛ in size. The addition of Al₂O₃ improved fracture toughness and bending strength by inhibiting grain growth of 8YSZ and increased total ionic conductivity because grain interior conductivity appeared to remain constant and grain boundary conductivity increased. It was assumed that the dissolution of Al₂O₃ into 8YSZ which was inevitable problem at commercial sintering method was effectively prohibited by the SPS technique with a relatively low sintering temperature and the reaction between Al₂O₃ and SiO₂ present at grain boundary to produce the crystalline Al/sub 2-x/Si/sub l-y/O/sub 5/ phase, resulting in the increase of grain boundary conductivity.

• Journal of Korea Foundry Society
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• v.31 no.3
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• pp.130-136
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• 2011

Effects of GBF (gas bubbling filtration) treatment conditions and scrap ratio on the electric conductivity of a commercial pure aluminum for diecasting were investigated using by specific gravity and electrical conductivity measurement system, hydrogen gas analyzer, XRD, and EDS. Electrical conductivities of specimen mixed Al scrap ratio until 60% from 0% were decreased with increasing the precipitates amount and size of AlFeSi ternary intermetallic compound on the grain boundary as well as amount of porosity in the grain. On the other hand, electrical conductivities was reincreased gradually in spite of scrap ratio increase from 80% to 100%. Size of AlFeSi compound formed on the grain boundary were coarsened with the increament of scrap ratios untill 80% and GBF treatment time simultaneously.