An Analysis of the Impedance of Polycrystalline Beta‐Alumina

Abstract

A procedure is described in this paper for distinguishing in the measured electrical properties of polycrystalline ~-alumina, the separate contributions of the grain boundaries and of the crystal, i.e., the grain interiors. This separation is brought about through the use of a model for these properties. Certain quantitative consequences of the model are developed and compared with experimental results. For most sintered ~-alumina ceramic, the electrical properties are determined more by the characteristics of the grain boundaries than by those of the interior of the grains.Polycrystalline E-alumina is used in electrochemical devices to circumvent the deleterious high anisotropy in the electrical and mechanical properties of single crystals. Use of the polycrystalline ceramic does require, however, consideration of the effects of grain boundaries. A procedure is described here for distinguishing in the measured electrical properties of the ceramic, the separate contributions of the grain boundaries and of the crystal, i.e., the interior of the grains. This separation is brought about through the use of a model for the electrical properties of p-alumina. Certain quantitative consequences of this model are developed and compared with experimental results. Previous WorkThe conductivity both of /%alumina :single crystals and of polycrystalline ceramic has already received very considerable attention (1-15). Weber and Kummer reported that E-alumina exhibits high ionic conductivity, but no electronic conductivity (1). The ionic conductivity is due to the high mobility of sodium ions in planes perpendicular to the c-axis of the hexagonal structure. However, there is no conductivity parallel to the c-axis. They reported singlecrystal specific resistivity values of 30 and 3.5 ohm-cm at room temperature and 300~ respectively. Comparable values for polycrystalline ceramic prepared from single-crystal material was 25,0 and 18 ohm-cm. They attributed the difference to interface resistivity between the crystals of the polycrystalline material. The activation energy associated with the resistivity of single crystals was given as 3.8 kcal/mole (2).Imai and Harata, on observing that the activation energy associated with conduction in sintered /~-alumina was considerably larger than for single crystals, concluded that the conductivity of ceramic is governed by grain boundary conduction (4).Jones and Miles found that the Arrhenius ~olot curved significantly below 200~ (5). Another activated process with a higher activation energy controlled the conductivity at lower temperatures. These authors speculated that this process was grain boundary contact resistance.Whittingham and Huggins measured the conductivity of a single crys,tal from --150~ to 820~ (8).They found plots of log ~T to be linear in 1/T over this entire temperature interval. The conductivity was found to be 72 ohm-cm at 25~ The activation energy was 3.79 kcal/mole. They reported the conductivity to be sensitive to the presence of moisture below 5,0~ Imai and Harata repo...

The grain boundary effect in heavily doped cerium oxide

“…In the 1980s, Wang and Nowick [3] reported that grain boundaries in polycrystalline cerium oxide significantly reduce the ionic conductivity. The same effect had been observed earlier in zirconia and other materials [4,5]. This grain boundary effect was associated with the presence of silicon impurities, which tend to form glassy films along the grain boundaries and which effectively block the ionic current.…”

Microstructural study of thin films of 5 mol% gadolinia-doped ceria prepared by pulsed laser ablation

“…Wang and Nowick [10] reported that grain boundaries in polycrystalline cerium oxide significantly reduce the ionic conductivity. The same effect has been observed earlier in zirconia and other materials [11,12]. The electrical conductivity of nanocrystalline Gd +3 -doped CeO 2 thin films has been studied as a function of temperature and oxygen activity, and correlated with the grain size [13].…”

Microstructural study of thin films of 5 mol% gadolinia doped ceria prepared by pulsed laser ablation

“…Wang and Nowick [10] reported that grain boundaries in polycrystalline cerium oxide significantly reduce the ionic conductivity. The same effect has been observed earlier in zirconia and other materials [11,12]. The electrical conductivity of nanocrystalline Gd +3 doped CeO 2 thin films has been studied as a function of temperature and oxygen activity and correlated with the grain size [13].…”