Sodium Beta Alumina Research Articles

Electron energy-loss spectroscopy studies of nanometre-scale structures in alumina produced by intense electron-beam irradiation

Abstract Holes and lines with dimensions of a few nanometres can be formed in a variety of inorganic materials using a high-brightness field-emission electron gun. In this paper we follow the damage mechanism as holes are drilled in amorphous alumina and sodium beta-alumina using, primarily, electron energy-loss spectroscopy. It is found that drilling proceeds in very different ways in the two substances. In the case of the crystalline beta-alumina, material is removed sequentially from both surfaces, while in the amorphous alumina, holes are formed only after the production of subsurface bubbles of oxygen. We also report the direct production of metallic structures in some materials following electron-beam irradiation.

Activity Measurements of Na<SUB>2</SUB>O in Na<SUB>2</SUB>O–Fe<SUB>2</SUB>O<SUB>3</SUB> System by EMF Method Using Sodium Beta Alumina as a Solid Electrolyte

Activity Measurements of Na<SUB>2</SUB>O in Na<SUB>2</SUB>O–Fe<SUB>2</SUB>O<SUB>3</SUB> System by EMF Method Using Sodium Beta Alumina as a Solid Electrolyte

Beta-alumina electrolyte for use in sodium/sulphur batteries Part 2. Manufacture and use

Part 1 of this two-part review was concerned with the fundamental chemical and physical properties of sodium beta-alumina. In Part 2 we consider the ceramic science and technology involved in the manufacture of electrolyte tubes and their use in sodium/sulphur batteries. The specification set for β-alumina tubes is first outlined, followed by a review of the fabrication procedures which have been employed. The problems of quality control and the elimination of flaws are emphasised. Mechanical and thermal expansion data which have been measured for β- alumina ceramics are discussed. The surface interaction effects between the electrolyte and the electrode materials (sodium and sulphur) are considered, and it is suggested that wetting effects are important in determining both the cell resistance and the life span of the electrolyte in use. The failure modes of β-alumina electrolyte tubes fall into three categories — electrical breakdown, mechanical shock, and thermo-mechanical failure — but in a real situation these may be interactive. Electrical breakdown may be either a result of localised high current densities during recharge or a consequence of too high an imposed voltage at top of charge (dielectric breakdown). Published work on these failure modes is reviewed. Finally, the present state-of-the-art in manufacturing β-alumina electrolyte tubes is summarised, and the need for further research and development on improving their durability is noted. As one component of a system — the Na/S cell — the electrolyte cannot be viewed in isolation and its durability is a function both of its perfection of manufacture and the conditions of use in the cell.

Densification kinetics of vacuum hot-pressed sodium beta-aluminas

Abstract Hot pressing of two sodium beta-alumina powders of nominal composition Na 2 O · 6.9Al 2 O 3 and Na 2 O · 8.5Al 2 O 3 at 1100 – 1600°C and 150 – 350 kg/cm 2 in inductively heated graphite dies under vacuum, proved an easy method for obtaining fully dense materials in very short pressing times. Densification rates and microstructures developed are strongly dependent on the characteristics of the starting powders. The microstructure of the materials obtained from the first powder consisted of approx. equiaxed grains of the β and β″ phases; the intermediate and final stages of densification (ϱ > ∼ 85%) obeyed first-order kinetics and showed stress exponents in the steady state creep equation ( e ∼ σ n ) from 0.74 to 1.76. The apparent volume diffusion coefficients at 90% relative density for which n values close to unity were found, calculated on the assumption of the Nabarro-Herring diffusional creep model were of the order of 10 −9 cm 2 /sec. The apparent activation energy for densification was 55 kcal/mole. A duplex microstructure was observed in the materials hot-pressed from the second powder consisting of nearly equiaxed grains and strongly anisotropic elongated crystals grown perpendicularly to the hot-pressing direction. First-order densification kinetics were observed also for this powder but with much lower densification rates. The apparent activation energy for densification was 130 kcal/mole. A strong texture was observed in the materials hot-pressed from both powders with most of the c axes oriented parallel to the hot-pressing direction.

Crystallite growth of a sodium beta-alumina powder determined by X-ray line broadening

growth of a sodium beta - alumina powder (nominal composition Na2O · 8.5Al2O3; impurity content < 0.3w/0 is reported. The powder was obtained from an aqueous solution of reagent grade NaOH and γ-Al2O3∗ calcined at 1200°C after dewatering at 50°C in a rotary drier and then ground for 8 h in an alumina ball-mill. X - ray diffraction and SEM and TEM observations made on the synthesized powder showed pseudo-amorphous forms and plate-like crystallites of the β and β″ phases having an average diameter of 0.02 μm and a BET surface area of 26 m2/g. The specific gravity of the powder, determined by the liquid displacement technique, was 3.10 g/cm3 against a value of 3.27 g/cm3 measured on a sample of the same powder hot pressed to essentially full density. This could indicate the existence of a pore substructure in the interior of the aggregates of crystallites resembling the hydroxide or carbonate derived MgO porous pseudomorphs observed by Tagal et al.1. Isothermal crystallization heat treatments of loose powders were performed in a Pt crucible rapidly introduced in a laboratory electric kiln pre-heated at the crystallization temperature. No appreciable Na2O losses were observed even at the higher temperatures and longer treating times. Crystallization runs were performed in the temperature range from 800°C to 1400°C and from 30 min. to approx. 50 h. The growth of the crystallites of the β phase was determined by the broadening of the (004) diffraction peak at 2θ = 14° 40′ using a scanning speed of 0.5 Deg./min.

Hydration and Carbonation of Beta Alumina Powders

The absorption of water by powders of sodium, potassium, and lithium beta alumina was observed using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and x‐ray powder diffraction. For samples derived from Union Carbide beta alumina, no water absorption/adsorption was observed for potassium beta alumina, about 2.1 molecules of water per unit cell for sodium beta alumina, and about 4 molecules per unit cell for lithium beta alumina. All of these results are at 25°C with the composition at 400°C as a base line. Some samples also were observed to react with in the presence of water to form complex hydrated carbonates, which underwent multistage decomposition reactions upon heating.

The separability of inter- and intragranular resistivities in sodium beta-alumina type ceramics

The intergranular and intragranular resistivity components in β-alumina ceramics cannot be separated quantitatively by impedance analysis, it was concluded a few years ago in a previous article in this journal. This conclusion was based on use of the so-called parallel multi-element equivalent circuit to model the electrical properties of polycrystalline electrolytes. However, this model is shown to be inconsistent with the observation that the activation energy for the intergranular resistivity is independent of the size of that component for many compositions — both β- as well as β″-alumina. From this finding and others, the author infers that the separation of intra- and intergranular resistivities in sodium beta-alumina type ceramics is clean. Consideration of the separability question is greatly facilitated by an unconventional method of resistivity analysis. This alternative method involves essentially d.c. measurements on a set of specimens of the same composition but with different microstructures and resistivities. The method is described and its use illustrated.

High resolution electron microscopy in association with interactive computing

Although facilities for high resolution electron microscopy (HREM) are available, interpretation of micrographs at high magnifications is far from unambiguous. The interpretation of the micrographs using a computed structure analysis program is therefore necessary. The paper presents the theory behind HREM, together with a computer method for image interpretation. Special reference is made to the JEOL JEM 200CX transmission electron microscope used in the study of sodium beta-alumina.

&amp;beta;-アルミナ固体電解質を用いたNa&lt;SUB&gt;2&lt;/SUB&gt;O-SiO&lt;SUB&gt;2&lt;/SUB&gt;-P&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;5&lt;/SUB&gt;融体中のNa&lt;SUB&gt;2&lt;/SUB&gt;Oの活量の測定

&amp;beta;-アルミナ固体電解質を用いたNa&lt;SUB&gt;2&lt;/SUB&gt;O-SiO&lt;SUB&gt;2&lt;/SUB&gt;-P&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;5&lt;/SUB&gt;融体中のNa&lt;SUB&gt;2&lt;/SUB&gt;Oの活量の測定

Oxygen interstitial transport and chemical coloration in sodium-beta alumina

A reaction rate maximum occurs around 250°C in the bleaching of chemically discolored sodium beta aluminas. It is determined that the bleaching rate depends on the diffusion rate of oxygen interstitials in the conduction planes of sodium beta alumina. The reaction rate maximum indicates that an irreversible transition occurs from a high mobility to a low mobility oxygen interstitial around 300†C. It is proposed that this transition is one in which a free oxygen interstitial is transformed into a Roth-type interstitial.

Determination of the sodium activity in aluminum and aluminum silicon alloys using sodium beta alumina

The construction of a sodium beta alumina probe for the determination of the sodium activity in molten aluminum alloys is described. It was found that the emf at a given sodium concentration was a strong function of the silicon content. Henry’s law was obeyed in super purity aluminum and the activity coefficient of 350 at 998 K agrees with other determinations.

Ionic conductivity of sodium β" alumina in the very far infrared

The authors have carried out measurements of the ionic conductivity of sodium beta " alumina in the region 2-200 cm-1. The temperature dependence of the measurements was studied in the range 2-600K. The results show many configurations of sodium ions in the conduction slab of sodium beta " alumina and are used to draw comparisons between the conduction mechanisms of sodium beta alumina and sodium beta " alumina.

Kinetics of the β″‐to‐β Transformation in the System Na2O‐A12O3

Mixtures of α‐A12O3 and NaAlO2 corresponding to the eutectic composition 65 mot % A12O 3 were heated in sealed platinum crucibles above the eutectic temperature (measured to be 1572°C in this study). Once melting was complete, the liquid was quenched and the resultant solid examined by X‐ray diffraction. Sodium aluminate, beta alumina, and beta″ alumina were detected in all cases. However, the relative amounts of beta and beta″ present were a function of the cooling rate, with the amount of beta″ increasing with a decreasing cooling rate. Subsequent annealing (in platinum envelopes) of the solidified eutectic at 1400° and 1530°C showed that the amount of beta″ present decreased with increasing annealing time. Based on this observation, it is apparent that, in the temperature range studied, beta″‐is metastable. Analysis of the time‐transformation data shows that the reaction kinetics are consistent with the diffusion‐controlled growth of the beta phase at the expense of the beta″‐phase.

Thermodynamic properties of liquid sodium-caesium alloys

A comprehensive description of the thermodynamic properties of liquid Na-Cs is presented, all derived from the EMF, E(c,T,p), of a concentration cell in which sodium beta alumina was the separator. The alloy heat capacity was found to vary linearly between the pure component values, and the small departures of the expansivity and compressibility from ideal behaviour are attributable to the volume contraction on mixing. There is no evidence for the existence of Na2Cs clusters in the liquid in either the heat capacity data or the variation of the correlation function Scc(0) with concentration.

The resistivity of sodium beta-alumina samples removed from cycled sodium-sulphur cells

Conductivity studies over the temperature range 223–573 K have been carried out on sodium beta-alumina samples removed from cycled sodium-sulphur cells. Within the limits of experimental error and batch variation, it has been determined that the cell cycling has had little effect on the conductive properties of the sodium beta-alumina sinters studied. Upon removal of the electrolyte tubes from the sodium-sulphur cells, the ceramic material was observed to have undergone a colour change or “blackening”. The presence of the species causing the colour change, suggested to be atomic sodium, does not result in a highly resistive phase, nor is the electronic conductivity of the electrolyte substantially affected.

Initiation of mode I degradation in sodium-beta alumina electrolytes

The extension of initial surface cracks by the focusing of the ionic current in beta alumina electrolytes (Mode I degradation) is discussed in terms of existing models. Focusing for an ion current impinging on an elliptic-cylindrical flaw is calculated by solving for the electric potential with suitable boundary conditions. The current density distribution along the crack is used to calculate the sodium flow velocity and Poiseuille pressure inside the flaw. Calculated critical current densities using aKlc criterion are several orders of magnitude higher than measured average critical current densities. This implies a lower effectiveKlc for electrolytic degradation than for mechanical testing. Current density enhancement around insulating barriers, such as non-wetted surface areas, is also calculated using elliptic-cylindrical coordinates. Significant current density enhancements are found, but they are localized in very small regions. Crack growth would occur within these regions, but should be arrested once the flaw extends past the high current density zone. A plausible mechanism for decreasingKlc in the electrolytic case is discussed.

Failure modes of Na-beta alumina

Degradation of sodium-beta aluminas can take two forms: Mode I and Mode II. Mode I is the well-documented electro-mechanical propagation of a pre-existing microcrack in the surface of the elevtrolyte. The critical electrode current densities, jcrit, were determined below 100°C using acoustic emission detection during electrolysis. The temperature dependence and magnitudes of j indicate that the applied voltage is an important factor. Mode II, a degradation involving internal sodium formation is briefly reviewed.

Beta-alumina electrolyte for use in sodium/sulphur batteries: Part I. Fundamental properties

This paper is the first of a two part review which aims to summarise recently published material on sodium beta alumina relevant to its use as an electrolyte in sodium/sulphur batteries. An overall objective is to relate the practical aspects of manufacturing and using β-alumina tubes (Part 2) to its basic materials science (Part 1) and to show how solid state chemistry and ceramic technology each play an important role in determining the properties and durability of β-alumina electrolyte. In this first Part we consider the latest structural data on sodium beta alumina and discuss how variable stoichiometry is accommodated. The role of dopants is reviewed and the stability and phase relationships of both the pure binary system and doped β-alumina are summarised. Recent electrical conductivity measurements on single crystal and polycrystalline sodium beta alumina are discussed and it is shown why the non-stoichiometric composition (Na 2O) 1+ x · 11Al 2O 3 has a higher conductivity than the stoichiometric phases (Na 2O) 1.0·11Al 2O 3 and (Na 2O) 2.0· 11Al 2O 3 on either side of it.

Beta-alumina electrolyte for use in sodium/sulphur batteries Part 2. Manufacture and use

Part 1 of this two-part review was concerned with the fundamental chemical and physical properties of sodium beta-alumina. In Part 2 we consider the ceramic science and technology involved in the manufacture of electrolyte tubes and their use in sodium/sulphur batteries. The specification set for β-alumina tubes is first outlined, followed by a review of the fabrication procedures which have been employed. The problems of quality control and the elimination of flaws are emphasised. Mechanical and thermal expansion data which have been measured for β-alumina ceramics are discussed. The surface interaction effects between the electrolyte and the electrode materials (sodium and sulphur) are considered, and it is suggested that wetting effects are important in determining both the cell resistance and the life span of the electrolyte in use. The failure modes of β-alumina electrolyte tubes fall into three categories — electrical breakdown, mechanical shock, and thermo-mechanical failure — but in a real situation these may be interactive. Electrical breakdown may be either a result of localised high current densities during recharge or a consequence of too high an imposed voltage at top of charge (dielectric breakdown). Published work on these failure modes is reviewed. Finally, the present state-of-the-art in manufacturing β-alumina electrolyte tubes is summarised, and the need for further research and development on improving their durability is noted. As one component of a system — the Na/S cell — the electrolyte cannot be viewed in isolation and its durability is a function both of its perfection of manufacture and the conditions of use in the cell.

Near-stoichiometric sodium beta alumina

The preparation of near-stoichiometric sodium beta alumina is described. The activation energy for sodium-ion conduction, 0.62 eV, is significantly larger than for the highly non-stoichiometric starting material.