Gregory C. Farrington

Ionic conductivity in Na+, K+, and Ag+ β″-alumina

Abstract This paper presents measurements of the ionic conductivity in single crystals of β″-alumina (0.84 M2O · 0.67 MgO · 5.2 Al2O3, M = Na, K, Ag). Single crystals of sodium β″-alumina were grown from a melt of Na2O, MgO, and Al2O3 at 1660 to 1730°C. Selected crystals were converted to the other isomorphs by ion exchange. The conductivity of sodium β″-alumina varies from 0.18 to 0.01 (ohm · cm)−1 at 25°C depending upon crystal growth conditions. Potassium β″-alumina has the unusually high room temperature conductivity of 0.13 (ohm · cm)−1. Silver β″-alumina has a slightly lower conductivity, 4 × 10−3 (ohm · cm)−1 at 25°C. The activation energies of sodium and potassium β″-alumina decrease with increasing temperature, while that of silver β″-alumina is constant from −80 to 450°C.

Fast ionic transport in solids

The discovery of inorganic solids with ionic conductivities comparable to those of aqueous electrolytes has revolutionized solid-state electrochemistry. Sodium beta alumina, a Na+ conductor, and LixTiS2, an intercalation compound with simultaneous Li+ and electronic conductivity, are two of the best and most versatile fast ionic conductors. A wide variety of cations can replace Na+ in beta alumina and Li+ in LixTiS2 and change the properties of the materials. Sodium beta alumina and LixTiS2 are currently used in the development of high-energy density batteries for electric vehicles and electrical utility load leveling. Current research in solid ionic conductors is exploring new intercalation compounds, solid polymer electrolytes, and alkali ion and proton transport in crystalline solids.

Hydronium beta″ alumina: A fast proton conductor

Abstract Single crystals of sodium beta″ alumina (0.84Na 2 O·0.84MgO·5 Al 2 O 3 ) undergo rapid ion exchange in concentrated sulfuric acid to produce “hydronium” beta alumina (0.84) H 2 O·0.84MgO·5Al 2 O 3 ·2.8H 2 O). Hydronium beta″ alumina undergoes a partial, reversible dehydration between 250–300°C and irreversibly decomposes into alpha alumina and water above 700°C. The conductivity of hydronium beta″ alumina has been measured with blocking and non-blocking electrodes and is 5 × 10 −3 (ohm cm) −1 at 25°C. The high conductivity is interpreted on the basis of a two dimensional liquid model.

Fast divalent ion conduction in Ba++, Cd++ and Sr++ beta″ aluminas

Abstract Single crystals of Ba ++ , Cd ++ and Sr ++ beta″ alumina were prepared from sodium beta″ alumina by ion exchange. The conductivities of these divalent ion solid electrolytes exceed 10 −3 (ohm-cm) −1 at 300°C. These compounds appear to exhibit fast divalent ion motion at moderate temperatures.

Lithium-Sodium Beta Alumina: First of a Family of Co-ionic Conductors?

Lithium-sodium beta alumina having a lithium/sodium ratio greater than about I appears to be the first generally useful lithium superionic conductor that has been reported. It exhibits strikingly nonlinear ion exchange properties and may presage the discovery of similar co-ionic interactions in other superionic conductors. The properties of lithium-sodium beta alumina are discussed in relation to current concepts of ionic interaction and distribution in the beta alumina conduction plane.

Ionic conductivity in H3O+ beta alumina

Abstract The first detailed conductivity measurements on single crystals of H 3 O + beta alumina are discussed in this paper. The conductivity of single crystal H 3 O + beta alumina at 20°C is 10 −11 (ohm cm) −1 . From 20 to about 200°C it increases in an Arrhenius relationship with an activation energy of 18 kcal/mole. Between 200 and 300°C H 3 O + beta alumina undergoes a partial and reversible dehydration in which approximately 50% of its H 3 O + content is converted to H + . The resulting composition, H + H 3 O + beta alumina has a much lower conductivity at 300° than H 3 O + beta alumina. H + H 3 O + beta alumina has a conductivity of 10 −6 (ohm cm) −1 at 500°C, which varies between 300 and 550° in an Arrhenius expression with an activation energy of 29 kcal/mole.

Production of hydronium beta alumina from sodium beta alumina and characterization of conversion products

Abstract Different techniques were applied to convert samples of sodium beta alumina to hydronium beta alumina. Fully and partially converted samples, obtained in boiling concentrated H 2 SO 4 from crystals of monofrax sodium beta alumina as starting material, were characterized by chemical analysis, thermogravimetric measurements and X-ray diffraction analysis. The fully hydrated form of hydronium beta alumina contained about three molecules of water normalized for the sodium content of the starting material while the partially hydrated form had about two molecules of water in our work. The conversion appears to proceed with a moving boundary from the surface into the bulk.

The existence of non-labile sodium in polycrystalline sodium beta alumina

Abstract Experiments were carried out to measure the rate and extent to which silver and potassium ions exchange with the sodium ions in Monofrax sodium beta alumina (1.32Na 2 O·11Al 2 O 3 ) and high soda polycrystalline sodium beta alumina (nominal stoichiometry of 1.80Na 2 O·11Al 2 O 3 ). Ion exchange in molten nitrate melts at 350°C is complete within about 10 hours for 1–2 mm sized Monofrax beta alumina crystals. For the polycrystalline samples, however, exchanges with both silver and potassium reach limiting values of about 87% for silver and 84% for potassium after 50–80 hours. This is interpreted on the basis of second phases present in the polycrystalline samples and the apparent existence of non-labile sodium within the beta alumina grains.

Stability and dehydration of alumina hydrates with the β-alumina structure

Abstract The stability of alumina hydrates with the β-alumina structure was determined over a wide range of temperature and water pressures by high temperature X-ray diffraction and thermal gravimetric analysis. There are two isomorphs, distinguished by different concentrations of intracrystalline water, which reversibly interconvert by absorption and desorption of water in the conduction planes. Quasi-equilibrium data were obtained for the composition and dehydration temperature as a function of water pressure. The hydrates are chemically stable to 700°C, then irreversibly decompose to α-Al 2 O 3 .