Sodium Β-alumina Research Articles

Low-frequency excitations in sodium beta -alumina: An NMR study.

The temperature and frequency dependence of 27 Al and 23 Na nuclear-spin relaxation (NSR) has been studied in Na β-alumina between about 1.5 and 100 K. Below 50 K the NSR is shown to be caused by low-frequency excitations of disorder modes, which are intrinsic to the glasslike state of β aluminas. The data can be interpreted consistently in the framework of thermally activated excitations of modes that are represented by asymmetric double-well-potential configurations with suitably chosen densities of states. Further, the observed NSR rates are shown to be related to corresponding ac-conductivity data via the fluctuation-dissipation theorem

Characterization of sodium β-alumina prepared by sol-gel method

Electrical properties of both the transparent gel and sodium β-alumina with the composition of Na 2O·6Al 2O 3 prepared by the sol-gel method were studied. The gel showed a relatively high electrical conductivity. The increase rate in the conductivity was changed at both temperatures, where shrinkage and transformation to sodium β-alumina started to occur. The β-alumina crystallized by the heat treatment at 1150°C was a good sodium ion conductor, though the structure detected by X-ray analysis was highly defective.

Surface conditioning of a sodium β-alumina based lithium sensor for lithium-lead blends

The impedance of a sodium β-alumina based electrochemical sensor, developed for the determination of the lithium activity in liquid metal blends, was investigated as a function of temperature. The impedance spectra could be rationalised in terms of purely resistive bulk conduction, dispersive grain boundary conduction and a diffusion-limited electrode conduction. Values of log ( T/ R), as derived from the interior grain and grain boundary resistivities were found to follow an Arrhenius behaviour. Corresponding activation energies of respectively 30 and 58 kcal/mol were calculated.

Thermodynamic properties of liquid Na-polyvalent metal alloys under high pressure

The EMF of liquid NaIn and NaBi systems has been measured a as function of temperature and pressure in detail by using an electrochemical cell with sodium β-alumina as the electrolyte. We have obtained the associated thermodynamic quantities of mixing such as free energy δG, entropy ΔS, enthalpy ΔH, excess specific heat ΔC p, excess volume ΔV, compressibility χ T and concentration-concentration fluctuation in the long wavelength limit S cc(O).

RAMAN SCATTERING STUDY OF Ag+-IMPLANTED β-ALUMINA CRYSTALS

We have measured Raman spectra of sample of sodium β-alumina and samples with silver ion implantation where one kind of sample was not annealed after the implantation, and the others were Ag +-implanted with annealing for 30 minutes at 300, 400, 500 and 700°C, respectively. Raman spectra show that silver ion implantation disturbs sublattice of Na + in β-alumina with some disorder features and some resonance modes appeared in the Raman spectra. We have found the critical annealing temperature near 400°C, above which the silver ion, typically with small activation energy 0.18 eV, gains enough energy to diffuse rapidly through the vacancy clusters formed by silver ion implantation and also through the conduction plane of crystal between clusters in β-alumina with high ionic conductivity.

Frontiers in β″-Alumina Research

Beta and β″-alumina are remarkable solid electrolytes whose study has been a principal theme in the field of solid state ionics ever since Yao and Kummer first reported the exceptionally high conductivity of sodium β-alumina in 1967. The unusual properties of sodium β-alumina stimulated widespread interest in both the science and technology of high conductivity solid electrolytes. Since Yao and Kummer's seminal work, many solid electrolytes have been explored, but interest in the β and β″-aluminas has endured, principally because of their virtually unique ability to undergo ion exchange in which the sodium ions are replaced by a wide variety of monovalent, divalent, and trivalent cations as well as various protonic species. The β and β″-aluminas thus are not single compounds, but a family of solid electrolytes with diverse properties and potential technological applications.Kummer first pointed out the ion exchange properties of sodium β-alumina by showing that the sodium ions in the structure can be replaced by different monovalent cations, such as Li+, K+, as well as H(H2O)x+ and NH4+. More recent studies have shown that the ion exchange chemistry of sodium β″-alumina, is far richer than that of sodium β″-alumina. In fact, the sodium ion content of β″-alumina can be exchanged for virtually any +1, +2, or +3 cation in the periodic chart.

Spectroscopy of sodium β″-alumina:Cr3+

Time-resolved luminescence spectra of Mg 2+ -stabilized β″-alumina crystals, containing some traces of chromium at aluminium sites in the spinel block, have been taken at several temperatures between 4.2 and 300 K. The spectra are attributed to the 2 E- 4 A 2 transition of Cr 3+ ions in octahedral sites of the host matrix. The emissions from centres characterized by different lifetimes have been isolated. The results are discussed in terms of static disorder and compared with recent results obtained in sodium β-alumina: Cr 3+ . The luminescence spectra are in agreement with the assumption of a random distribution of Mg 2+ ions over the tetrahedral sites within the spinel block.

Solid electrolytes for the study of thermodynamic properties of silicates

The applicability of fully stabilized zirconia, magnesium fluoride dispersed in a silicate phase, and ion exchanged sodium β-alumina as electrolytes in solid galvanic cells for the study of the thermodynamics of silicates at high temperatures is discussed. Experimental results for AOSiO 2 (with A=Mg, Mn, Fe, Co, Ni) are presented. The obtained Gibbs free energy of formation values are compared to the data given by other authors.

Analysis of Transport Phenomena Occurring in Electron-Irradiated Sodium β-Alumina Crystals

AbstractAuger analysis of sodium β-alumina crystals provides relevant insights concerning Na- transport along the c-axis. In particular, it allows to prove that sodium ion migration is stimulated by electron-irradiation, a result connected to electron-induced relaxation of lattice. Some aspects of the electric fields induced into a dielectric during Auger analysis are discussed.

Influence de la non stoechiométrie sur les modes de vibration, les amplitudes vibrationnelles, et les intensités infra-rouge dans les alumines β

The normal coordinate analysis of nonstoichiometric β-alumina containing cations such as Na +, K +, Tl +, or Ag + with different nonstoichiometric models (Frenkel defect associated with various cation distribution in the conducting plane) is reported. Mean square amplitudes and infrared intensities have been estimated on the basis of normal mode calculations for sodium β-alumina (stoichiometric and nonstoichiometric). The results obtained are in good agreement with experimental data. The spectral modifications that have been observed can be explained by splitting of degenerate levels and modification of selection rules due to the loss of local symmetry induced by the Frenkel defect. The good agreement between the in-plane ( a, b) vibrational calculated frequencies of interstitial oxygen (O i) and Raman or neutron scattering data shows that the compensating oxygen is tightly bounded to the spinel blocks. The difference between the calculated and crystallographic mean square amplitudes can be explained in terms of the static deviation of the bridging oxygen (O(5)) of 0.1 Å (at 300 K) from its ideal site.

Possible uses of sensors in the aluminium industry

AbstractThere is a requirement for on-line monitoring of elements in molten aluminium and one possible approach is to use electrochemical sensors which produce a voltage that is related to the concentration of an element in the melt. The principle of electrochemical sensors is outlined and examples given for hydrogen, lithium, and sodium determination in molten aluminium using sensors based upon calcium hydride, lithium silicate–phosphate, and sodium β-alumina, respectively: sensors could also be used to control the Hall–Heroult cell and possible examples are the measurement of the NaF/AlF3 ratio in the cryolite bath, the evolved CO/CO2 ratio from the anode, as well as the concentration of sodium and lithium in the aluminium tapped from the cell.MST/571

Thermal expansion and optical length variation of sodium β-alumina along the c-axis

Thermal expansion and the optical length variation have been measured for a single crystal sodium β-alumina, along the c-axis. A laser interferometer has been used in the measurement. The measurements of the length and optical length variation do not indicate any anomalous behavior. The thermal expansion and thermooptic coefficients vary smoothly like the ionic conductivity and do not display any indication of cooperative effects.

The determination of the thermodynamics of the NaF-AIF3-AI2O3 system with a solid electrolyte cell

The activity of sodium in cryolite mixtures in equilibrium with superpurity aluminum has been determined with a sodium Β-alumina probe. The activities are in agreement with other determinations. From the sodium activities, the activities of NaF and A1F3 were calculated and these were similar to the values obtained by other workers. Measurements in an operating Hall-Heroult cell showed that equilibrium was not reached between the cryolite electrolyte and the molten aluminum pool.

Electron beam writing on a 20-Å scale in metal β-aluminas

Holes 20 Å diameter and lines 20 Å wide can be cut in sodium β-alumina, sodium β″ alumina, lithium β-alumina, potassium β-alumina, lead β-alumina, and silver β-alumina using a small focused electron probe (approximately 104 A/cm2). Drilling can proceed along any crystal direction in the specimen and can penetrate material more than 1000 Å thick maintaining the small (20 Å) diameter. The holes and lines remain stable and the specimen can be removed from the electron microscope and replaced later for further drilling or examination at reduced beam intensities.

Far-infrared absorption and its relevance to structure and ionic conduction of sodium β-alumina

Infrared absorption studies have been carried out on the vibrations of sodium ions in the mirror plane of both non-stoichiometric and nearly stoichiometric sodium beta -alumina. Absorption by the mixed crystals Nax(NH4)1-x beta -alumina has also been measured for 0<x<1. Measurements were made between 2 and 300K and covered the region 20-200 cm-1. The results are related to configurations of sodium ions in the mirror plane and they are used in conjunction with recent calculations by Walker and Catlow (1982) to discuss models for ionic conduction.

ESR study of Mn 2+ in sodium β″-alumina

ESR measurements are reported on Mn 2+ diffused into sodium β″-alumina. The spin Hamiltonian parameters obtained are g 11 = 2.0000(2), g ⫫ = 2.0019(2) , A 11 = −8.39(3) mT, A ⫫ = −8.33(3) mT and D = −7.20(5) mT. The site position of the Mn 2+ ions is discussed and compared to published results for Mn 2+ in sodium β-alumina.

Surface layers in polycrystalline sodium \-alumina

X-ray powder diffractometry using CrKα radiation has revealed the existence of surface layers 5Μm deep in samples of high-density polycrystalline sodium Β-alumina. The surface layer is prominent by virtue of its crystal lattice distortion in which the c-axis is elongated by 1%. Its presence is due to ion exchange between Na+ and H3O+ that occurs on exposure to air.

The measurement of carbon activity with the sodium β-alumina electrolyte cell

The direct measurement of carbon activity has been studied using a solid galvanic cell, with an electrolyte of sodium β-alumina and Al 4C 3. The cells were set up as follows, (a) Al/β-alumina/Al 4C 3, C and (b) MeC, Al 4C 3/β-alumina/Al 4 C 3,C. It was confirmed that at equilibrium Al activity in cell (a) and C activity in cell (b) would cause a Na activity gradient in the β-alumina electrolyte. The values of Δ G 0(Al 4C 3) measured with cell (a) from 670 to 803 K agreed well with the published data, which confirmed the reliability of the cell. Cell (b) was used for the measurement of C activity in non-stoichiometric uranium carbide. The results showed reasonable dependence of C activities on temperature as well as on composition.

A proton NMR study of water in hydrated sodium β-alumina

Proton nmr study of hydrated sodium β-alumina reveals a Pake doublet spectrum due to stationary water molecules. As the temperature is increased from -196C to 192C, linewidth changes indicate that the stationary protons sense the increasingly rapid motion of nearby sodium nuclei and other protons. The interproton spacing computed from the spectra was found to be 1.7A.

23Na NMR study of ionic mesophases in molten sodium carboxylates

The 23Na NMR lineshapes are reported for the ionic mesophase and isotropic phase of the melts of sodium n-butyrate and sodium isovalerate. The powder pattern for the central transition typical for the second-order quadrupole effect observed in the mesophase melts is of particular interest. Some analogies to 23Na behavior in sodium β-alumina are pointed out.